MEND/NEDEM

Hatch was commissioned by the Mine Environment Neutral Drainage (MEND) Program to complete a study to identify best available technologies economically achievable (BATEA) to manage and control effluent from metal, diamond, and coal mines in Canada. The objective of the study was to provide reference information for potential forthcoming changes within the Metal Mining Effluent Regulations (MMER) to the types of regulated mining facilities, the list of Schedule 4 parameters, and the authorized limits of Schedule 4 concentrations in effluent discharged to the environment. These potential changes are outlined in the Environment Canada 2012 discussion paper, “10-Year Review of Metal Mining Effluent Regulations”. For metal mining effluent, Environment Canada has proposed the addition of aluminum, iron, selenium, and total ammonia to the list of Schedule 4 substances, and has proposed the reduction of authorized limits for arsenic, copper, cyanide, lead, nickel, and zinc. For diamond mining effluent, which is currently not regulated under MMER, Environment Canada has proposed limits for chloride, phosphorus, ammonia, and total suspended solids (TSS) concentrations in effluent discharged to the environment, as well as limits for pH. For coal mining effluent, which is currently not regulated under MMER, Environment Canada has proposed limits for arsenic, aluminum, iron, manganese, selenium, ammonia, and TSS concentrations in effluent discharged to the environment, as well as limits for pH. Other proposed changes include the addition of a new requirement that effluent be non-acutely lethal to Daphnia magna and changes to Environmental Effects Monitoring requirements. (more…)

Lime addition is a common method for the treatment of acid rock drainage (ARD) whereby neutralization promotes a reduction in acidity and the precipitation of metals as voluminous sludges that may contain gypsum, calcite, Fe-oxides and a spectrum of other phases. Due to the extremely fine-grained and often amorphous (i.e., non-crystalline) character of sludge solids, the composition of these materials has been difficult to elucidate. Traditional methods such as X-Ray Diffraction (XRD) and optical microscopy, for example, have proved largely ineffective. In order to provide further insight into the solid-phase characterization of neutralization sludges, high density sludge (HDS) materials from seven mine sites across Canada were examined by high-resolution microscopy techniques in combination with influent and effluent characterization. The primary objectives of the study were to: 1) define the nature of metal phase associations in sludge materials; 2) define the links between ARD influent/effluent chemistry, treatment process and sludge composition; and 3) provide the basis from which to develop a sludge management framework from the perspective of long-term chemical stability.

High-resolution microscopy methods utilized included Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), and X-ray Absorption Near Edge Structure (XANES). SEM, and in particular STEM, provided the spatial resolution required to resolve the trace-metal associations in the sludge samples. With respect to XANES, there is a general absence of suitable model compounds analogous to many of the sludge phases examined, and as a result, the applicability of XANES to discern metal-associations in sludge samples is currently limited. However, the XANES data acquired for Zn offer insight into the potential stability of Zn through differentiation of potentially labile and non-labile complexes.

Predictably, SO4 was a dominant component in all ARD influents. However, considerable variability was observed with respect to other major parameters. Dominant influent chemistries on a molar basis included:

SO4>>Mg>Al>Fe>>Ca>Cl>F>Mn>Zn (Equity Mine),

SO4>Fe>>Ca=Mg (Geco Mine),

SO4>Ca>Mg>>Al>Cl>Cu =>Zn (Britannia Mine),

Na> SO4>Mg>Ca=Cl>Zn (Brunswick Mine),

Cl>Na>Ca> SO4>Mg>Zn (Chisel North Mine),

SO4>Mg>Ca>Fe>Zn (Samatosum Mine) and

SO4>Mg>>Ca>Fe>Na=Cl (Sullivan Mine).

Outflow compositions showed uniformly circum-neutral pH and greatly reduced values for Fe, Mn, Al and trace elements (e.g., Zn, Cu). In some cases, values for major species also showed pronounced declines through the HDS process, including SO4 (Equity, Geco, Samatosum) and Mg (Equity, Samatosum and Sullivan).

Treatment sludges show variable elemental compositions, but in all cases, the elemental abundances can be linked to ARD influent chemistry. Crystalline materials identified by XRD include calcite or Mg-calcite (Britannia, Brunswick, Chisel, Samatosum and Sullivan Mines) and gypsum (Equity, Geco, Samatosum and Sullivan). However, none of these phases were shown to be significant repositories for precipitated trace metals.

SEM and STEM data demonstrate that the trace metal-bearing phases in the HDS materials are amorphous or poorly crystalline, and variable in composition (relatively pure to highly heterogeneous). The trace metal host phases are invariably fine-grained, often occurring as oval aggregates ranging in size from <5 to 20 μm, and interspersed with other non-metal-bearing material (e.g., gypsum). Compositional zonation, often in concentric layers, is common, with the zones showing contrasting major ion signatures (e.g., Fe, Mg, Al). Such zonation is predicted to result from the recycling of sludge within the HDS process.

The dominant metal-hosting phases were site-specific, and included relatively pure Fe-oxyhydroxide (Geco Mine), amorphous Mg-Al-(Fe) hydroxysulfate (Equity Mine), Zn-Cu oxyhydroxide (Britannia Mine), Zn-Fe-Mn oxyhydroxide (Brunswick and Chisel North Mines), and Fe-Mg oxyhydroxide (Samatosum and Sullivan Mines). For all HDS samples, selected area electron diffraction patterns revealed broad diffuse rings, consistent with poorly crystalline to amorphous phases. Zn K-edge x-ray absorption near edge structure (XANES) spectra revealed a mixture of labile (outer-sphere complexes) and less labile (structurally incorporated) Zn species.

The data indicate that the nature of the dominant metal-hosting phase in the HDS materials is strongly dependent on ARD influent chemistry, with both the concentrations and relative proportions of Fe, Mg, Mn, Al and SO4 being dominant variables. In this manner, the results of the assessment highlight the potential for the development of a sludge management framework, which may permit prediction of “sludge type” from the ARD composition. In order to develop a defensible framework, further sludge characterization would be required to assess both within-mine and between-mine variability. A further requirement would also be a detailed understanding of the chemical stability of the various sludge types in varying depositional environments. Currently, there is not sufficient information available from which to assess the chemical stability of the various trace metal-bearing phases identified in this study. Given the substantial contrasts in the nature of the various metal-hosting phases, significant differences in chemical stability can be expected. In order for a potential framework to be applied successfully, sludge chemical stability as a function of varying pH and redox conditions must be established. This would be best achieved through the in situ collection of sludge porewaters and laboratory studies designed to assess pH- and pE-dependent solubility controls.

A new simulation model, MINTOX, has been developed to provide a useful tool for
predicting the behaviour of kinetic sulphide mineral oxidation within mine tailings
impoundments, and for simulating the subsequent speciation and transport of oxidation
products through the tailings and into downstream aquifers. MINTOX includes the
major reaction sequences known to control the hydrogeochemistry at many base metal
tailings sites. These processes include diffusion of oxygen into the unsaturated zone,
diffusion of oxygen into the sulphide mineral grains, sulphide mineral oxidation, acid
generation and release of iron, sulphate and heavy metals. Furthermore, the model
can simulate the advective-dispersive transport of the mobilized species, accounting
for equilibrium speciation and reactive processes including solid mineral dissolution
and precipitation.

MINTOX has been tested in both one-dimensional and two-dimensional modes against
observed field data from the Nordic Main tailings impoundment near Elliot Lake Ontario
(Wunderly et al. 1995, 1996). Simulated depth profiles of selected species, including
oxygen and pyrite content, agreed well with observed data, with discrepancies in other
phases due primarily to the assumption of local geochemical equilibrium. The
two-dimensional simulations of the Elliot Lake site showed reaction sequences and
concentration levels consistent with observed or inferred behaviour. MINTOX has also
been applied to simulate the geochemical processes occurring at the Nickel Rim
tailings impoundment and has provided new insights into processes governing acid
generation and neutralization.

Methods to control the rate of sulphide mineral oxidation, and the impact of AMD
include reducing the rate of oxygen diffusion into the tailings using a moisture-retaining
surface cover, and adding limestone to increase the buffer capacity. MINTOX has
simulated the beneficial effects of these types of remediation measures at both the
Elliot Lake and Nickel Rim sites. Simulations showed for example, that an increase in
moisture content from background levels to saturation effectively restricted the
oxidation process. Since most oxidation occurs within 10 years of deposition however,
covers appear best suited if emplaced immediately following tailings deposition.

Additional simulations for both the Elliot Lake and Nickel Rim sites were completed to
address the effects of adding limestone to the tailings. At Elliot Lake, the results
showed significant reductions in heavy metal concentrations as higher pH favours the
precipitation of minerals which removes the aqueous species from solution. At Nickel
Rim, higher pH and sulphate concentrations were also observed.

A 1D sensitivity analysis based on the Nickel Rim site showed significant variation with
diffusion coefficients, fraction of sulphide mineral, initial grain size, and carbonate
buffer mineralogy. The simulations suggested a need for determining the influence of
spatial variation of physical and chemical properties on AMD evolution, and
incorporating uncertainty in the interpretation of results.

This report documents the results of the first part of a research
project jointly funded by Noranda Inc. and the Mine Environment
Neutral Drainage (MEND) program. The second part is covered in a
separate report entitled Hydrology and Solute Transport in Oxidised
Waste Rock from Stratmat Site, N.B., published concurrently with
this report (MEND 2.36.2b). The overall objective of the project was
to understand the geochemical and hydrological interactions
between the partially oxidised waste rock and water and to improve
our capabilities and techniques in the prediction of acidic drainage
from waste rock piles.

Partially oxidised waste rock was sampled from the Stratmat pile at
Heath Steele Division of Noranda Inc. by grabbing, trenching, and
bulk excavation techniques. The samples were physically and
geochemically characterised in the laboratory whereas the bulk
density was measured in the field. The trenched samples were
used in column dissolution tests in which 25-kg composite
sub-samples were subjected to repeated washing with water to
observe the water quality evolution over time. The resulting data
were used to predict water quality for a hypothetical scenario where
the waste rock were backfilled in the Stratmat pit. In addition, water
quality profiles were measured in the Stratmat pit.

Results of the column dissolution tests suggest the following mass
balance for the Stratmat pile: Approximately 7% of the original
sulphide sulphur has been oxidised since deposition, releasing a
total acidity of 11 800 t CaCO3 equivalent, of which 56% has been
neutralised in situ. Currently, the acidity inventory is approximately
5200 t CaCO3 equivalent whereas the inventory of soluble zinc is
about 1660 t. The mass balance appears to support preferential
oxidation of sphalerite over pyrite.

The column dissolution experiments further indicate that dumping
the waste rock into the flooded Stratmat pit will cause significant
release of stored metals and sulphate. The long-term pore water
quality in the absence of ground water movement is predicted as
follows: pH 3.32, acidity 12 500 mg CaCO3/L, SO42- 19 500 mg/L,
Zn 4500 mg/L, Cu 180 mg/L and Pb 2.4 mg/L. In the presence of
uncontaminated ground water movement, the water quality would
gradually improve as the initial pore water is displaced or diluted. It
would take nine pore volumes of flushing to reduce the
concentrations of most metals (except Pb) to below 0.1 mg/L. For
Pb, this would take many more pore volumes.

Geochemical modelling suggests various concentration control
mechanisms. Concentrations of Pb and Fe in the pore water are
likely controlled by equilibria of the leachate solution with anglesite
and ferric hydroxide, respectively. On the other hand,
concentrations of SO4, Zn, Ca, Mg, Mn, and Al in the pore water
seem to be limited in the short term by dissolution/diffusion rate
controls. The presence of gypsum is found to inhibit the dissolution
of anglesite. As a result the anglesite stored in the waste rock
would not dissolve appreciably until gypsum storage is exhausted
by dissolution. This implies that decommissioning of the waste rock
by a backfilling-flushing-treatment process would last a long time
before the pore water in the backfilled waste rock becomes
acceptable for discharge to the receiving groundwater.

The purpose of this review was to provide a transfer of recent Norwegian
experience and technology on the remediation of acidic rock drainage (ARD) at four
sulphide mines in Norway to Canada, and to provide an opinion as to the immediate
and long term effectiveness of the remediation measures to the Norwegian State
Pollution Control Authority (SPT).

The sites visited and remediation measures reviewed were:

– The consolidation of mine rock at the Kjøli mine and the placement 0f.a geosynthetic
membrane cover to control infiltration, and hence migration of ARD from the mine rock
pile. The composite geosynthetic cover comprised a sequence of a filter fabric, a 2mm
HDPE liner, geonet drainage layer, a filter fabric and a till cover. Rip rap was placed for
erosion control and the till cover was revegetated. The lower drainage adit was
plugged resulting in the flooding of about 70% of the underground mine workings.

– The excavation of mine rock from the original rock pile site at the Skorovas mine and its
placement in a nearby lake under a permanent water cover of 2 m. The mine rock was
excavated with a back hoe and transported in dump trucks to the new site. The original
site was cleaned by hand to minimise the migration of stored ARD products from
residual mine rock .Placement of the old mine rock in the lake required temporary
lowering of the water level by pumping to allow ‘placement in the dry’, construction of a
retention berm for stability purposes, and placing a till cover over the mine rock.’ During
mine rock placement, the water quality in the lake water was controlled by adding lime.
On completion of mine rock placement, a concrete dam was constructed at the lake
outlet to permanently raise the water level in the lake to 2 m above the mine rock level.

– At the Killingdal mine site, the mine rock was consolidated and covered with a till
cover. A seepage collection ditch was constructed to divert ARD into the underground
workings, with the objective of utilizing the evaporative action of the natural convective
air flow through the mineto evaporate the ARD as it trickled down through the
workings, and thus deposit salts in the mine.

– At the Løkken underground mine dewatering has been discontinued and the mine
allowed to flood. Natural improvement of the water quality in the Wallenberg shaft has
been observed. Sulphate reducing bacteria have been isolated from the deep
anaerobic zones of the water filled shaft. The presence of the sulphate reducing
bacteria, and the observed natural improvement in the water quality observed,
presents an in.situ ARD treatment alternative. It is turned to divert ARD from mine
rock in surface piles into the mine and to rely on the natural processes occurring in the
mine to improve water quality.

The report describes the implementation techniques applied at the respective sites,
and where possible provides a performance evaluation based on the site water quality
monitoring data. Summaries of the cost incurred for the implementation of the various
measures are provided.

The geosynthetic membrane cover was found to be effective in reducing the overall
metal load transported from the Kjøli mine rock pile through the prevention of water
infiltration. Large reductions in the measured oxygen concentrations indicate that the
cover also served to reduce, but not eliminate, the entry of oxygen into the rock pile.
The decreasing temperature profiles of the pile after covering shows that the oxidation
process is considerably reduced.

Since relocating the mine rock to a below water location at Skorovas mine it has been
possible to discontinue lime treatment without an increase in downstream contaminant
discharges. This represents a very large decrease in acidity and contaminant release
from the mine rock. Water quality monitoring in the lake appears to indicate that long
tenn pH values of 4.5 will be achieved. The old mine rock was not treated with lime
prior to placement in the lake and this results in a pH of about 2.5 in the water in the
submerged pile. Stored oxidation products were therefore not neutralized and
precipitated prior to submergence. Migration of this low pH water to the lake could
result in lake water qualities that are less than what may have been achieved if such
neutralisation had been completed.

The costs for all mine rock remediation at each of the Kjøli and Skorovas mine sites,
including placement of mine rock under a permanent water cover or providing a
geosynthetic cover, are comparable at approximately 59 NOK per tonne mine rock
(CAD$l0.00). Costs for individual remediation measures are summarized in the report.

The effectiveness of placement of a till cover only, as at the Killingdal mine, could not
be evaluated from available data. Substantial quantities of low pH seepage was
observed to be occurring from below the collection ditch. It- is not known if this
seepage originates from seepage flow in the bedrock below the ditch or from the ditch
itself. Although the concept of collection and treatment in the mine by evaporation . is
unique, its effectiveness is dependant on the effectiveness of the collection system.
The collection ditch was incomplete at the time of the site visit.

The results of a one year pilot project designed to evaluate the effectiveness of various organic
cover materials at limiting or reducing the impact of acid generation on the environment from
acid generating tailings are summarized within this report. Lakefield Research Limited were
contracted by Falconbridge Limited, with funding support provided by the MEND program, to
perform this work. Three organic cover materials were tested: lime stabilized sewage sludge
(LSSS), municipal solid waste compost and peat. Desulphurized tailings, an inorganic cover
material, was also included in the study, both for comparative purposes and due to the potential
for production of large volumes of this material at operating mining properties.

The test program included three components of study: characterization of tailings and cover
materials, salt migration column tests and pilot scale cell tests. Both the salt migration column
tests and the pilot cell tests enabled cover-tailings system evaluations.

The oxidized tailings, pyrrhotite tailings, and cover materials were characterized chemically,
physically, mineralogically and hydrogeologically. A multi-element scan indicated that the
oxidized tailings were 3-4 times higher in aluminum, calcium, magnesium and sodium than the
pyrrhotite tailings. Similar concentrations of cadmium, cobalt, chromium, copper, manganese
and lead were detected in both the oxidized tailings and pyrrhotite tailings. The pyrrhotite
tailings contained higher concentrations of nickel and zinc than the oxidized tailings. The
oxidized tailings contained 38% iron and 19% total sulphur, whereas the pyrrhotite tailings
contained 53% iron and 31% total sulphur.

The compost, peat and the capillary break material exhibited a coarser grain size distribution
while the oxidized tailings, desulphurized tailings, pyrrhotite tailings and the LSSS exhibited
much finer grain size distributions. The 80% passing size was 3022 μm for the capillary break
material, 2835 μm for compost, 2653 μm for peat, 170 μm for oxidized tailings, 130 μm for
desulphurized tailings, 76 μm for LSSS and 34 μm for pyrrhotite tailings.

The as-received volumetric moisture content was 158% for peat, 32% for LSSS and 13% for
compost. The porosity was calculated to be 71% for LSSS, 59% for compost and 60% for peat.

The mineralogical examination indicated that pyrrhotite was the major sulphide mineral in both
the oxidized tailings (75-80% of sulphides by volume) and pyrrhotite tailings (>90% of
sulphides by volume).

The in-situ hydraulic conductivity measurements conducted at the surface of the covers in the
pilot cells indicated that the hydraulic conductivity did not change after one year of testing for
the oxidized tailings (2.0×10-5 cm/s), desulphurized tailings (3.5×10-5 cm/s) and LSSS (3×10-3
cm/s). However, the hydraulic conductivity was decreased by one order of magnitude at the
surface of the compost cover (from 2.1×10-1 to 2.6×10-2 cm/s) and the peat cover (from
6.8×10-2 to 4.8×10-3 cm/s). Decomposition and compaction are believed to have caused this
observed decrease in hydraulic conductivity values.

The air entry values estimated from the drainage curves were 450 cm H2O for the oxidized
tailings, 525 cm H2O for the desulphurized tailings, 100 cm H2O for LSSS, 25 cm H2O for
compost and 15 cm H2O for peat. These values indicate that the oxidized tailings and
desulphurized tailings can hold water under much higher exerted suction than the LSSS,
compost and peat. The LSSS, compost and peat would be more readily drained than the tailings
materials.

A bench scale evaporative flux test was conducted and it was determined that for the first seven
days, an average ratio of actual evaporation to potential evaporation was 0.75 for the oxidized
tailings, 0.893 for the desulphurized tailings, 0.831 for the LSSS, 0.869 for the compost and
0.843 for the peat. These ratios were used in the calculation of water loss by evaporation in the
pilot cells during cell water balance determinations.

Total organic carbon (TOC) was determined for the three organic cover materials at the
beginning and end of the one year pilot cell test. The initial TOC content was 1.68% for the
LSSS, 20.1% for the compost and 23.1% for the peat. After one year of testing, no changes in
the TOC was found in the LSSS cover layer, while about 69% and 29% of TOC disappeared at
the 15 cm and 45 cm cover depth of the compost, respectively, and about 4% and 16% of TOC
disappeared at the 15 cm and 45 cm cover depth of the peat, respectively.

Short term incubation testing and computer modeling was performed on a similar LSSS
material to that used in the pilot cell tests and a mixture of this LSSS with desulphurized
tailings. The incubation test indicated that there was no evidence of decomposition as carbon
dioxide evolution. The analyses of TOC and soluble organic carbon before and after the test
suggest that the decomposition occurred only in the LSSS, not in the mixture of the LSSS and
desulphurized tailings. Computer modeling of the TOC results predicted an annual 16%
decomposition rate for the LSSS during the first two years under climatic conditions similar to
the Sudbury area. After two years, when all the readily decomposable organic carbon would
have been depleted, the decomposition rate would slow markedly. It is roughly estimated that
after ten years about 80% TOC would be lost as carbon dioxide. Dry LSSS blended with the
desulphurized tailings forms a mixture that is very resistant to decomposition.

In the salt migration column tests, the surface evaporation rate was noted to decrease with
increasing cover depth. Among the cover materials tested, the desulphurized tailings and peat
showed the highest evaporation rate and the LSSS and compost showed the lowest evaporation
rate. The highest percentage increase in electrical conductivity was also found at the surface of
the desulphurized tailings and peat cover, and the lowest percentage increase in the compost
and LSSS, suggesting that there was a direct relationship between the evaporation and the salt
accumulation at the surface. The salt accumulated at the surface consisted primarily of sulphate,
iron and magnesium.

The evaporation rate was influenced by the moisture content near the surface of the covers. As
the cover depth increased, the moisture content at the surface decreased, thereby resulting in a
lower evaporation rate and a corresponding low percentage increase in electrical conductivity.

The high electrical conductivity noted in the LSSS was not considered to be related to the
evaporation rate nor to the upward migration of salts from the underlying oxidized tailings, but
rather to the dissolution of salts present in the LSSS. The salts responsible for the observed
increase in electrical conductivity consisted mainly of calcium, magnesium, sulphate and
possibly bicarbonate and hydroxyl ions.

The use of a capillary barrier to reduce upward salt migration was effective in the 0.15 m cover
depth, but not in the 0.5 m and 1.0 m cover depth. The capillary barrier exhibited no discernible
effect on the evaporation rate. The ineffectiveness of the capillary barrier to reduce the salt
accumulation in the thick cover was due to capillary rise caused by compaction and the use of a
well graded material.

During one year pilot cell testing, positive effects on reducing acid generation were observed
beneath the LSSS cover material. The oxidized tailings beneath the LSSS cover exhibited an
increase in pore water pH and dissolved organic carbon and a decrease in pore water sulphate
and dissolved metals. An increase in leachate pH and a decrease in leachate sulphate and
dissolved metals, with reduced loading of salts from the underlying oxidized tailings into the
environment were also noted in the pilot cell with the LSSS cover. The high alkalinity of the
LSSS contributed to the observed increase in pH in both pore water and leachate. Other
microbially mediated acid consuming processes (e.g. sulphate reduction by SRB’s) also
contributed to the increase in pH. The decrease in sulphate and metals in the underlying
oxidized tailings may have been due to 1) precipitation of iron as ferric hydroxide and sulphate
as hydrated calcium sulphate and 2) precipitation of metal sulphides through sulphate reducing
bacteria. The LSSS also exhibited a high oxygen consumption ability and maintained >90%
degree of saturation throughout the cover depth.

While the LSSS cover provides many benefits as noted above there are, also, some concerns
associated with the LSSS cover. These include: the very high pH (>12) and high electrical
conductivity which creates a harsh environment for plants; the appearance of elevated copper
concentrations in the pore water of the LSSS; high concentrations of total phosphorus in the
LSSS pore water and in the leachates, and high concentrations of phenol in the leachate.

The desulphurized tailings cover was also effective in minimizing loadings to the environment
through maintenance of a high moisture level and inhibition of oxygen and water movement to
the underlying tailings. The desulphurized tailings had a high air entry value (510 cm H2O), a
low hydraulic conductivity (in the range of 10-5 cm/s) and maintained >90% degree of
saturation throughout the cover depth. The combination of these characteristics and the low
salts concentrations in the cover resulted in reduced volumes of leachate from the underlying
oxidized tailings. The desulphurized tailings were considered unsuitable for use as a cover
alone, due to the high evaporation rate and resultant formation of cracks in the cover that
permitted oxygen to migrate directly to the underlying oxidized tailings. This direct oxygen
migration may be responsible for the observed increase in the pore water iron and sulphate
concentration in the underlying oxidized tailings indicative of ongoing oxidation.

The compost exhibited a high oxygen consumption ability and an alkaline pH. However, the
pore water chemistry of the underlying oxidized tailings showed no changes in dissolved
organic carbon and pH, and an increase in sulphate and iron concentrations. This suggested that
the leachate from the cover had not interacted positively with the underlying tailings. The
compost maintained a degree of saturation at about 60%, and was able to capture all of the
influent precipitation, thereby, resulting in a high loading of salt from the underlying oxidized
tailings into the environment.

Of the organic cover materials tested, peat appears to show the least favourable characteristics.
The peat did not exhibit any effect on oxygen migration due to a combined inability to consume
the incident oxygen and maintain a level of saturation sufficient to reduce the oxygen
infiltration rate. Similar to the desulphurized tailings and compost, the pore water sulphate and
iron concentrations in the underlying oxidized tailings remained high and the tailings pore water
pH remained below 4.0. Also, similar to compost, the peat cell exhibited a high loading of salts
from the underlying oxidized tailings into the environment.

Based on the results to date, it is concluded that the lime stabilized sewage sludge and
desulphurized tailings appear to offer the greatest potential for reducing sulphate and metals
loading in water migrating from the underlying oxidized tailings into the environment.

Due to the nature of the oxidized tailings (i.e. presence of oxidants from previous oxidation;
limit of reducible sulphate), it is necessary to evaluate the effectiveness of the cover materials at
reducing acid generation over a longer time period. It has, therefore, been recommended that
monitoring be continued for the LSSS, desulphurized tailings and compost cover cells.

Due to the potential for production of large volumes of desulphurized tailings at mining
operation properties and the favourable cover characteristics noted in this test program, methods
of reducing evaporation loss and resultant cracking from the desulphurized tailings are being
investigated. It is very likely that an evaporation barrier over the desulphurized tailings would
serve to reduce evaporative losses, salt accumulation at the surface, and the formation of cracks.
The evaporation barrier would also provide erosion protection to maintain the physical integrity
of the cover system.

It is recommended that well-designed field scale tests be conducted to provide a more thorough
evaluation of cover performance complementary to the laboratory studies described in this
report.

The Land Directorate of Environment Canada has estimated that by the year 2000 A.D. over
172,000 hectares of land would be disturbed by the mining industry. Of all the land used by the
industry, the disposal areas for the waste from the beneficiation of sulphide ore have the
greatest effect on neighbouring ecosystems. The sulphide tailings disposal areas, estimated at
over 9,000 hectares by the 1984 CANMET report, DSS 8032-8, impact the atmosphere,
hydrosphere and lithosphere if left uncontrolled. To date, the only environmentally sound
measure that has some remedial action on all three of the components of the ecosystem is
reclamation by the use of vegetation. It should, however, be mentioned that the effect of
vegetation on acid generation has not yet been realized.

Even though each sulphide tailings area has its own unique method of approach, seed mixture,
fertilizer analysis, planting schedule, etc., there are a number of components, technical
questions, problems and solutions that are common to all.

This manual provides as comprehensive a methodology as possible describing procedures for
preparation, neutralization, fertilizing, seeding and follow-up maintenance, if required, for
revegetation of reactive sulphide tailings disposal areas in Canada.

The manual introduces some fundamental basic building blocks which will assist the reader in
discussions with agriculturalists and other consultants experienced in the art of reclamation. The
building blocks are:

• The concept of macro and micro-climate and their effects
• Introduction to the nature and physical and chemical properties of soil
• Types of materials (overburden, waste rock and tailings) encountered at a mine
• General effects of lime, fertilizer and other amendments on soils and tailings

From these basic concepts a tailings revegetation program is developed. The site specific
program covers a two to three year time frame covering a minimum of at least two growing
seasons. It is divided into seven segments:

• Evaluation
• Program Planning
• Site Preparation
• Lime and Fertilizer
• Vegetation Selection
• Mulches, Chemical Stabilizers and Other Amendments
• Establishing Vegetation

The manual describes and illustrates some of the more common agricultural equipment and
implements required to prepare, seed and maintain mine waste areas.

A brief discussion of the problems associated with establishing revegetation programs north of
60 degrees latitude is presented. As well, the new avenues of research presently being pursued
in Canada and elsewhere are described.

Case histories are appended, reprinted from the May 1984 CANMET Tailings Management
Study, courtesy of Energy, Mines and Resources Canada. The case histories outlined include
the following:

1. Reclamation at Cominco’s Bluebell and Pinchi Lake Mines
2. Reclamation at Falconbridge Operations
3. Reclamation Program on Inco Limited’s Tailings Areas
4. Reclamation at Noranda’s Operations.

Production of Acid Mine Drainage (AMD) due to the oxidation of sulphidic
minerals in the presence of water is probably the main threat to many natural
ecosystems located close to existing mines. AMD is multi-factor pollutant that
encompasses the effects of acidity, metal toxicity, suspended solids and
salinization. The impact of AMD on the environment is also influenced by the
buffering capacity of the receiving water and available dilution. The overall effect
of AMD is reduction of living species in the affected area, leading to a
simplification of the food chain and a significant loss of ecological stability.

Over the last two decades, various techniques have been studied and used to limit
the environmental impact of AMD. One approach is to collect and chemically treat
the acidic effluent before its discharge to the environment, to produce a final
effluent that meets environmental criteria. The main disadvantage of this technique
is the need to operate the treatment system for decades (or more) following mine
closure. Another approach is to control the production of AMD, which is generated
from tailings or waste rocks, by reducing the availability of one of the three main
components of the oxidation reaction: oxygen, water or sulphide minerals. In a wet
climate, an oxygen barrier is usually considered the best available option.

Oxygen barriers can be created in different ways. For instance, a water cover is
sometimes used to limit the oxygen flux to the reactive materials. Alternate covers
include those made of oxygen consuming materials or geosynthetic materials.
Multi-layered systems can also be used to produce Covers with Capillary Barrier
Effects (CCBE). This type of cover, made with soils and/or other suitable
particulate media, involves the capillary barrier concept, which occurs when a
fine-grained material is placed over a coarser one. The difference in unsaturated
hydraulic characteristics of the two adjacent materials favors a high degree of
saturation in the top, fine material layer. This, in turn, helps to reduce the oxygen
diffusion flux to the lower layer, since a saturated porous media is a much better
barrier than a dry one. However, CCBE and other multi-layered systems can be
fairly expensive to construct.

To reduce the costs of such CCBE, the authors proposed the use of clean tailings,
or tailings with very low sulphide content that do not generate AMD, as fine grain
material in layered cover systems. This type of material is often available in
mining areas close to problematic sites. Such clean tailings usually have favourable
geotechnical properties that can enhance the durability and performance of CCBE.
A preliminary laboratory study had shown that clean tailings, used as a
water-retaining layer in a properly designed CCBE, efficiently limited the
production of AMD from sulphidic tailings (MEND 2.22.2a). Subsequent to this
investigation, experimental cells were constructed in 1995 to evaluate, on a larger
scale and under more realistic field conditions, the performance of CCBE built
with clean tailings.

This report presents the results of a detailed study on the behaviour of CCBE
acting as oxygen barriers to reduce the production of AMD. A key feature of this
cover system was that one of the layers consisted of “clean tailings”. The
availability of such type of fine grained materials in mining areas such as the
Abitibi in northern Québec makes this concept quite interesting and feasible. In
most cases, this can help reduce the costs and improve cover efficiency.

The report contains a general introduction on the project objectives and content,
including a short presentation on the different steps leading to the final design of a
cover system (see Figure 1.1). Chapter 2 includes a summary of the main results
obtained during Phase I of this project, based on the work performed between 1991
and 1995 (MEND Report 2.22.2a). This preliminary study included the
characterization of key hydro-geotechnical properties of the different materials (see
Figure 2.2 to 2.6 and Table 2.2) and the development of an experimental procedure
to evaluate the behaviour of CCBE. The procedure used laboratory columns in
which one of the layers consisted of clean tailings with good water retention
capacities (Figure 2.7 and 2.12). The results have shown that non-reactive
(non-acid generating) tailings, when used in properly designed layered cover
systems, can successfully create capillary barrier effects with the adjacent
coarse-grained materials (e.g. sand), so that the fine material layer remains close to
saturation at all times (Figures 2.8 to 2.11 and 2.14 to 2.22). This reduces the
oxygen diffusion flux and hence the oxidation of the sulphidic minerals in the
waste underlying the cover. The basic principles of a cover system using the
capillary barrier concept are also explained, together with the mechanisms
involved in oxygen flux reduction with increased saturation.

In 1995, Phase II of the project was initiated to better evaluate the performance of
CCBE constructed with clean tailings, using field test plots. Six experimental cells
were constructed on the Norebec-Manitou site (near Val d’Or, Québec), to study
the behaviour of engineered cover systems under more realistic conditions. The
hydrogeological conditions and leachate characteristics were the main parameters
monitored. A laboratory study was run in parallel, based on the work performed
during Phase I. The preliminary results of the laboratory component of Phase II
were presented in MEND Report 2.22.2b, and are updated in this report.

Laboratory work included a characterization of the various materials used in the
field study. A series of column tests were run with these materials to evaluate the
hydraulic behaviour of the different cover systems, and their relative performance
with respect to sulphidic minerals oxidation and acidic leachate production. The
key results are presented in Chapter 3. Additional details can also be obtained from
the various progress reports produced over the course of the project (available from
the MEND Secretariat) and in the Masters Thesis from Monzon (1998) and Joanes
(1999).

The hydro-geotechnical and mineralogical properties of the materials are also
presented in Chapter 3 (Table 3.1 to 3.8 and Figures 3.2 to 3.10). The materials
included the clean tailings taken from the Sigma site located a short distance from
Norebec-Manitou, a natural silty soil from the Val d’Or area, a mixture of tailings
with bentonite, and a well-graded sand. Details of the column experiments are
presented, including layer stratification and column dimensions (Table 3.9 and
Figure 3.11), and testing procedures with wetting and drainage cycles. The data
obtained for water distribution (Figure 3.12 and Table 3.10), unsaturated modelling
(Figures 3.14 to 3.20), leachate characteristics (Figures 3.21 to 3.27), and oxygen
diffusion calculations (Figure 3.30) are shown and discussed.

Five of the seven columns were set-up with the same layered configuration as the
five cells built in situ (the sixth cell was not covered – control). Two other columns
were set-up with loose layers of sand and tailings, to evaluate the behaviour of
hydraulically placed materials. The column tests lasted about two years and
completed over 20 wetting and drainage cycles. The results confirmed the good
performance of the different covers, although the efficiency varied according to the
material characteristics. During the test period, little or no oxidation was observed
from the reactive tailings (Norebec-Manitou) covered by the different systems.

Furthermore, the results also confirmed the validity of the numerical modelling
completed for the prediction of the CCBE during the column tests. Because
different materials and thicknesses were used, it has also been possible to make a
comparison between the different configurations based on the relative effects of
key parameters. The results clearly indicate that the various systems designed and
tested were efficient, although some were more efficient than others.

Construction of a full-scale CCBE on an actual mine waste disposal site can be a
difficult and expensive undertaking. For this project, experimental cells of
intermediate sizes were constructed to validate the concepts and to compare
different design scenarios. In Chapter 4, the six cells (including five with a cover)
built during the summer of 1995 on ITEC Mineral Inc.’s Norebec-Manitou site,
near Val d’Or are described. The cell design is presented in Table 4.1 and Figures
4.1 to 4.6. Each cell was shaped like an inverted truncated pyramid. The sides of
the cells were covered with a geomembrane (COEX 30 mil) to control exfiltration
and lateral water inflow. A water collection system was built at the bottom of each
cell and connected to an underground reservoir installed outside the cell area. The
reservoirs and collecting systems were designed so that the water could easily flow
out of the cell while air could not move inward. Each of the five CCBE cells had 3
layers of material placed on the reactive tailings. The top layer consisted of 0.3 m
of well-graded sand while the bottom coarse material was made from 0.4 m of the
same sand. The fine material layer, placed between the two sand layers, was either
made of clean tailings with different thickness (for 3 cells), a natural silty soil (one
cell), or a mixture of bentonite and clean tailings (one cell). The sixth cell was left
uncovered and served as the control. The sulphidic tailings placed at the bottom of
the cells were taken from the Norebec-Manitou site, located a few hundred meters
from the test plots. The materials used for each cover were placed and densified to
a specific porosity: about 0.44 for the different fine material layers; and
approximately 0.32 for the sand layers. Photographs taken during construction of
the cells are presented at the end of Chapter 4.

The same materials were used in the laboratory and field experiments; their main
properties are given in Chapter 3. The natural silt (till) and the clean tailings used
for the fine-grained, water- retaining layers have similar geotechnical properties.
Their grain size distribution, measured saturated hydraulic conductivity ksat
(between 10-4 and 10-5 cm/s) and AEV (approximately between 15 and 40 kPa) are
typical of silty soils. The clean tailings and the till were not acid generating. The
coarse material was a typical concrete sand with a saturated hydraulic conductivity
between 3 x 10-2 cm/s to 5 x 10-2 cm/s and an air entry value of about 2 kPa. This
created the necessary contrast with the fine-grained material to have a capillary
barrier effect. The sulphidic tailings, on the other hand, was a relatively coarse
material with most particles corresponding in size to the sand fraction. Their
saturated hydraulic conductivity was between 10-3 and 10-4 cm/s and their AEV
was between 7 to 11 kPa. The sulphidic tailings contained about 5% pyrite and
were considered acid generating, with a Net Neutralization Potential (NNP) of -88
kg CaCO3/tonnes (see Mineralogical Analysis Report from Louis Bernier in
Appendix).

The covers on the experimental cells were instrumented (Figure 4.8) and
monitored between August 1995 and November 1998 (at which time the cells were
disassembled). The main parameters monitored included volumetric water content
q , matric suction Y , oxygen flux, and chemical composition of the leachate. The
monitoring equipment used for measuring each parameter was selected after an
extensive literature review and discussions with other organizations that had used
such instrumentation for monitoring purposes. The time domain reflectometry
(TDR) technique was used to measure the volumetric water content in the cover
layers. This technique had been used successfully for the previous laboratory
experiments. Three-wire probes were placed during construction at predefined
locations within the covers (see Figure 4.8). The 25 probes installed were linked to
a control panel and data acquisition system, which allowed regular measurements.

The matric suction was also measured in each of the three layers of the different
covers, using both Watermark sensors and Jet Fill tensiometers. The measurements
made over the years have shown that Watermark sensors gave results quite similar
to the tensiometers. In this study, most of the matric suction results were obtained
with the Watermark sensors because of the simplicity associated with using this
type of equipment. Each of the sensors was located close to a TDR probe to allow
a comparison between the volumetric water content and matric suction
relationships obtained in the laboratory and in situ. A good correlation was
obtained between field and laboratory results.

Stainless steel tubes were installed (not shown in Figure 4.8) to evaluate the
efficiency of the CCBE to limit oxidation. These tubes, installed vertically at the
surface of the cover, can act as gas chambers for measuring oxygen flux through
the cover. This method, called Oxygen Consumption Method, consists of
measuring, within a few hours, the decrease in oxygen concentration in the gas
chamber and to convert the relationship between concentration of oxygen and time
into an oxygen flux.

The design of the cells also allowed collection of the percolated water in the
underground reservoirs. Chemical characteristics of the water were determined
during and after sampling. Parameters measured were pH, conductivity and,
occasionally, sulphates and metals contents.

Detailed progress reports were submitted to MEND on a regular basis throughout
the project; these are summarized and presented in a concise format in this final
report. For instance, the volumetric water content measurements (q vs. depth) for
the five covered test cells are shown for different time periods in Figures 4.9 to
4.33. As expected, the value of q was low in the two sand layers (usually between
0.05 to 0.15) and high in the fine layer (usually above 0.33). Such distributions are
typical for covers using the capillary barrier technology, and the results are
comparable to those obtained from numerical calculations (Section 4.6.3 and
Figures 4.96 to 4.112). For cells 1, 2, 3 and 5, the degree of saturation,
corresponding to the measured volumetric water content, was usually above 85%
to 90%. For cell 4, the degree of saturation was usually between 65 to 80% and
was less than the other cells. This is likely due to the use of a tailings-bentonite
mixture, which prevented the full hydration of the fine layers.

Suction (y ) measurements taken at the same time as the q values are presented in
Figures 4.34 to 4.58. The suction values were usually less than 5 kPa in the bottom
sand layers, between 5 to 15 kPa in the fine grain material layers and varied from 1
to 23 kPa in the top sand layers. The suction values in the fine grained layers were
generally less than the AEV for these materials; this explains why the degree of
saturation remained high in each of these layers. A high suction value was
sometimes obtained in the top sand layer, mainly due to evaporation. However, the
results also indicate that, as expected, the top sand layer efficiently limited the
evaporation from within the fine layer. Thus, it plays a key role in maintaining
cover efficiency to reduce oxygen flux.

Water quality monitoring results are presented in Figures 4.61 to 4.71. The water
collected in the reservoirs from cells 1, 2, 3 and 5 stayed above pH 6 for the entire
experiment. The pH of the water from bottom of cell 4 was above 6 for the first
three years, but dropped to values ranging from 5 to 6 during the last year. The
control cell leachate pH confirmed that the sulphidic tailings were highly acid
generating, with an initial pH of 6 that dropped to values around 2 at the end of the
project. The conductivity measurements are consistent with the pH measurements;
high values were obtained for the seepage water of the control cell (from » 10,000
to 50,000 micro-ohms) while lower values were measured for the covered cells
(between 2,000 to 3,000 micro-ohms).

Sulphates are produced by sulphide minerals oxidation. The cover performance can
also be evaluated from the sulphate concentration in the seepage water. The results
showed that there was a difference of about two orders of magnitude in the
sulphate concentration between the control cell and the covered cells. This means
that, even if the precipitation of secondary minerals (like gypsum and jarosite) in
the control cell are ignored (these minerals were nevertheless observed), the
reduction of the oxidation flux was about two orders of magnitude. The relative
performance of covers can also be compared on the basis of metal concentrations
in the leachate water from the control cell and from the covered cells. For cells 1,
2, 3 and 5, results indicate a reduction in metal concentrations of about 3 to 4
orders of magnitude for zinc and iron and between 2 to 3 orders of magnitude for
copper. The reduction is lower for cell 4, and this also confirms that this cover was
somewhat less efficient than the others.

A recently developed method to determine the rate of sulphide oxidation, the
Oxygen Consumption Method, was investigated for the covered cells. Four series
of tests were completed during the research project. The main results are presented
in Table 4.3. The first series of tests were performed after construction was
completed, in October 1995, by University of Waterloo personnel. These results
were higher than subsequent measurements made by the authors in 1996 and 1997.
The lower values measured in 1996 and1997 can be partially explained by the fact
that the systems needed a certain amount of time after construction to establish the
baseline profiles for moisture and oxygen concentration.

The test results from 1996 and 1997 have shown that the oxygen fluxes through the
covers were usually lower than 15 moles/m2/year and often lower than 3 to 5
moles/m2/year. It is important to note, however, that the accuracy of this technique
is better defined for high consumption rates than for the lower rates, such as those
determined in this project. Nevertheless, these tests, combined with all other
results, confirm that the oxygen fluxes were greatly reduced by the use of a
properly designed CCBE.

The last component of this research project dealt with the financial aspects for the
implementation of the CCBE technology. In Chapter 5, a relatively simple model,
called ECR (for Evaluation of the Cost for Reclamation) is presented. The model
components are introduced and explained, and further illustrated using specific
Tables and Figures taken from the Excel File, which is included with the report
(ECR.1). The model allows a relative comparison of the costs incurred by the
application of various techniques, namely: chemical treatment (lime) of AMD (see
Figure 5.1), the use of a water cover with impervious dams (Figures 5.5 to 5.6), the
construction of a CCBE made of soils and/or clean tailings (Figures 5.5 and 5.8)
and the inclusion of a desulphurization process to produce “non-acid generating
tailings” that could be used as the fine material layer in a cover system (Figure
5.10). Each technique is explained with sufficient detail to allow the interested
reader to apply the model and make preliminary calculations for site specific
applications. To further help understand the model and its application, three sample
case studies are presented (Table 5.2). The model aims at providing the reader with
a simple tool to obtain preliminary estimates of various techniques and to help
select the most promising technology for further site-specific investigations. It is
shown, in the process, that CCBE made of clean tailings can be a very competitive
closure technique to control AMD (Tables 5.4, 5.7, 5.10 and 5.11).

The main conclusions and recommendations for future work are presented in
Chapter 6. Considering the successful results obtained with this project, and the
full-scale application of this concept at Les Terrains Aurifères (MEND Report
2.22.4a – Construction and Implementation of a Multi-Layer Cover – LTA) it is
recommended that further field investigations be pursued to better correlate
calculated results and field observations. The ongoing project at the crown-owned
(Québec) Lorraine site (Temiscamingue, Québec) is another good example of the
in situ work presently underway.

The Canadian mining industry depends on the successful full-scale application of
various closure technologies, including the CCBE, to help in the decommissioning
of new and old mine sites. Results from these studies will provide options and
direction for future applications.

The present study was undertaken to evaluate the impact of flooding of the Solbec-Cupra
oxidized mine tailings on the production of acid mine drainage (AMD). Representative oxidized and
unoxidized mine tailings samples were collected on the site and used in experimental columns and
reservoirs (large lysimeters) to simulate artificial flooding during a minimal period of nine months. The
objectives of this research projet was to verify the efficiency of flooding on the viability and oxidative
activity of microorganisms producing AMD and also to obtain data on the efficiency of the incorporation
of lime into the tailings.

We obtained the following results:

1. Flooding of unoxidized tailings prevented their colonization by known iron-oxidizers such
   as Tlaiobacillus ferrooxidans and Leptospirillum ferrooxidans mainly because of the neutral pH water.
   We did not isolate any of the sulfide oxidizing bacterial strains from the already installed experimental
   large lysimeters on the Solbec-Cupra tailings site; however, the oxidized tailings yielded numerous and
   varied acidophihc and acidifying bacterial species.
2. Artificial flooding of unoxidized tailings mixed with netralizing lime material caused the
   inhibition of inoculated bacteria as well as their oxidative properties towards pyrite in columns without
   eliminating them; their overall viability was preserved during the entire experimentation period.
3. Lime application was estimated to be 118 tonskectare to a depth of 15 cm, this rate of
   application was found to neutralize most of the accumulated acididity in the oxidized tailings, it also
   prevented the iron and calcium migration. A security factor, to be determined, could be used to maintain
   the pH at an acceptable level on a long term basis. Liming material could be added directiy to water if reacidification
   was to occur after flooding.
4. The existing methods to asses lime dosage in soils are not adequate to predict neutralizing
   material needed in mine tailings, mainly because they do not have any buffering capacity.

The Centre de recherches minérales du Québec (CRM) has undertaken an
experimental research project to determine the effect of flooding the existing
Solbec-Cupra tailings site; the influence of flooding on covering water quality and
porewater quality in the oxidized and nonoxidized tailings was particularly examined.

The experimental research project was achieved through financial assistance from the
MEND program, with a contribution from CAMBIOR. The on-site experimentation was
initiated in the summer of 1989. The first two phases of the project were realized by the
CRM. The third phase was contracted out to MONTERVAL INC. in 1991. The fourth
phase of this program, contracted out to ROCHE LTÉE, Groupe-conseil, involved a
two year monitoring period.

The flooding experimentation consisted in simulating different scenarios “in situ”, using
instrumented basins. A total of five (5) inert plastic cylinders were driven down to
different depths in the tailings, each cylinder being long enough to allow a minimum
water depth of one meter above tailings surface. Each basin was instrumented with
inside single level piezometers to sample porewater: in oxidized and non-oxidized
tailings, organic soil (peat) and in the underlying till.

Three single piezometers were also installed outside (but close to) the basins, and six
others were installed in the center part of the tailings site; the objective of these was to
assess tailings and sub-soil conditions prior to flooding.

The 1992-1993 program (phase IV) was intended at determining the effect of
neutralization of the oxidized layer of tailings prior to flooding, and involved the
installation of a new basin.

The key conclusions drawn from the experimental results are:

• Flooding of tailings under a shallow cover of water inhibits oxidation of acidic
  tailings, and prevents the continuation of the process in non-oxidized tailings.
  The inhibitive effect is progressive, and porewater in oxidized tailings remains
  acid for a certain period of time before reaching a neutral level.
• Neutralization of the oxidized layer of tailings by adding and mixing finely crushed
  limestone prior to flooding had the effect of rapidly stopping the oxidation process
  and raising and maintaining porewater pH to neutral level.
• A downward infiltration of acid porewater from oxidized tailings was induced by
  the water column in the basin. This phenomenon is not anticipated to happen,
  though, from flooding the total pond as a whole.
• Results from monitoring the water cover quality indicate that flooding of tailings
  under one meter of water will improve the quality of tailings pond effluent, and
  eventually to a level complying with MENVIQ mining industry environmental
  guidelines.
• Results from monitoring the water cover quality also indicate that flooding of
  tailings under one meter of water will eventually result in a pond having the same
  water quality as the incoming surface water.
• Based on the overall positive experimental results, and from an environmental
  point of view, flooding of the Solbec-Cupra tailings site appears as the most
  acceptable rehabilitation solution.

This report describes the results of laboratory column leaching studies that were conducted as a part
of a broad research program titled “Development of Wet Barriers on Pyritic Uranium Tailings for
Controlling Acid Generation”. The research was undertaken jointly by CANMET, Elliot Lake
Laboratory and Rio Algom Limited, under the Canadian Mine Environmental Neutral Drainage
(MEND) Program.

Purpose:

Column leaching studies were conducted for evaluating the oxidation and leaching characteristics of
pyritic uranium tailings with and without limestone amendments and crushed waste rock, under
unsaturated and submerged conditions. These experiments were designed to determine the acid
generation characteristics and subsequent metal and radionuclide releases of test samples subjected
to submersion underwater and exposed conditions.

Methods:

The leaching experiments were conducted using PVC cylindrical column lysimeters filled with total
mill tailings, coarse tailings, tailings amended with 7.5% (w/w) limestone of varying screen sizes, and
crushed waste rock samples obtained from Quirke Mine. The experiments provided unsaturated
conditions for leaching of test samples as well as a submerged condition for coarse tailings where a
shallow water cover, 0.45 m in depth, was established using distilled water. The submerged coarse
tailings were also rinsed rapidly during the initial leaching period to remove gypsum and other soluble
minerals for studying oxidation and leaching characteristics of such tailings in the absence of gypsum
controlled sulphate solubility.

All tests were conducted at laboratory room temperature (22 ºC) and light conditions, which aided in
enhancing the chemical reactivity of test materials in laboratory simulations compared to field
conditions.

Duration of the Study:

The experiments, conducted at the CANMET, Elliot Lake Laboratory, were started in April 1989 and
concluded in June 1993.

Results:

Coarse Tailings:
Unsaturated Condition:

• The unsaturated coarse tailings without limestone amendment oxidized
  readily and produced highly acidic drainage (pH ~ 1.0 – 2.0 and acidity
  ~ 12,000 – 30,000 mg CaCO3/l);
• The coarse tailings amended with coarse limestone (screen size -6.3 mm)
  also produced highly acidic drainage (pH ~ 2.5 – 3.0 and acidity ~ 4,000 –
  12,000 mg CaCO3/l), but its onset was delayed by approximately one
  year in comparison to coarse tailings without limestone. The acid drainage
  occurred with 90% of the available alkalinity still remaining in the sample;
• The coarse tailings amended with pulverized wet ground limestone did not
  produce any acidic drainage during the entire four year study period.
  Oxidation and acid generation processes were active in these tailings as
  well, but the fine grained limestone provided complete acid neutralization
  and acidic drainage was prevented during the study period.

Submerged Condition:

• The submerged coarse tailings did not produce any porewater acidic
  drainage for the first three years, but in the fourth year a weak acidic
  drainage (pH ~ 5.5 – 7.0 and acidity ~ 10 -20 mg CaCO3/l) was observed
• In comparison to unsaturated coarse tailings, submersion of tailings
  underwater decreased cumulative porewater acidity, sulphate and iron
  loadings by factors of 295, 200 and 950 respectively;
• The submersion of coarse tailings also resulted in increased mobility and
  release of Ra-226 in the porewater when tailings developed acidic
  conditions at the surface after they were depleted of available alkalinity
  and gypsum. There was a 10 fold increase in the cumulative loading of
  Ra-226 in the porewater over that of unsaturated coarse tailings;
• The increased drainage of Ra-226 in the porewater was also
  accompanied by that of iron, both occurring near the end of the fourth
  year, where as gypsum was completely removed from the tailings during
  the first three months of leaching;
• The surface water above submerged tailings contained low concentrations
  of dissolved metals, when the water cover above the tailings was
  continuously maintained in a well oxygenated condition by fresh water
  inflow and where both surface and pore water flows were present in equal
  proportions. The dissolved Ra-226 concentration in the surface water was
  also low, ranging between 30 and 140 mBq/l.

Total Mill Tailings:
Unsaturated Condition:

• The total mill tailings, in all test conditions, drained poorly and retained
  appreciable amounts of moisture which controlled their oxidation and acid
  generation rates;
• The unsaturated total mill tailings without limestone amendment also
  produced acid drainage, but it was moderate and less acidic (pH ~ 2.0 –
  3.0 and acidity ~ 500 – 1500 mg CaCO3/l) than for coarse tailings under
  similar conditions;
• The total mill tailings amended with various screen-sized limestone did not
  produce acid drainages during the entire study period. The tailings were
  oxidizing slowly but the added limestone was actively consumed in
  providing complete acid neutralization in all amended samples.

Waste Rock:
Unsaturated Conditions:

• The unsaturated crushed waste rock having a high acid generation
  potential (NNP = -17.8 kg CaCO3/tonne) also oxidized and started to
  produce highly acidic drainage ( pH ~ 2.0 – 2.5 and acidity ~ 7,000 –
  20,000 mg CaCO3/l) in the fourth year of the study period;
• The unsaturated crushed waste rock having medium (NNP = -10.7 kg
  CaCO3/tonne) and low (NNP = +13.8 kg CaCO3/tonne) acid generation
  potentials did not produce acidic drainage during the entire study period.

This report documents the geochemical and physical characteristics of the Eskay
Creek waste-rock dump, from its origin in underground workings in the early 1990’s, to
its disassembly and placement underwater in a nearby lake in 1995. The dump is a
Type 2 mined-rock pile as defined by the MEND Waste-Rock Literature Review,
meaning its is confined along its axis by valley walls which direct runoff into it. The
objectives of this project, coinciding with the disassembly, were:

• to summarize geochemical characteristics of Eskay Creek waste rock,
• to describe the construction of the waste-rock dump,
• to record visually the disassembly of the waste-rock dump,
• to log visually and collect samples of selected cut faces of exposed waste rock,
• to visibly stain and examine small-scale flowpaths within the dump,
• to document the evolution of water chemistry in the dump area, prior to dump construction until after disassembly, and
• to examine the mass balance of acidity within the dump using two independent approaches.

The chapters and appendices of the report present the findings and the data on which
they are based. Some earlier data were reinterpreted for use in this report.

Static and kinetic geochemical tests on Eskay Creek rock showed that most rock was
reactive and net acid generating, with some dependence on grain size. As a result,
acidic drainage appeared within two years after dump construction began.

During disassembly, most of the dump was found to contain relatively fine-grained
waste rock that limited infiltration, particularly at the surface. Nevertheless, infiltration
did occur, sometimes enhanced by a single pebble embedded in the fine-grained
layers. As is typical of Type 2 dumps, a creek often flowed into and through the dump,
enhancing the flushing and removal of acidity and metals from the dump.

Almost a year after dump disassembly, acidic water around pH 4.7 was still draining
from the watershed. Thus recovery to near-background concentrations will require
more than one year.

Two independent mass-balance calculations for acidity in the dump provided similar
estimates of acid generation: 220 and 530 t/yr. At these rates, all sulphide within the
dump would have been consumed within 11-26 years.

Despite a significant amount of reactive neutralization potential within the dump, acidic
drainage appeared within two years after the first rock was dumped. Mass-balance
calculations indicated that only roughly 10-20% of total NP was consumed before
acidic drainage began. This may be the result of physical factors like channelling or
stratification within the dump, which was noted in another MEND study of an
acid-generating waste-rock dump in British Columbia.

This study was sponsored by Homestake Canada Inc. on behalf of the Canadian
MEND Program. It is another case study in the MEND series on waste-rock dumps.

This final report presents the results of studies conducted at the Waite Amulet tailings
site during the four-year period from 1985 to 1988. These studies were performed
under the national MEND (Mine Environment Neutral Drainage) program, a combined
industry-government consortium committed to the development of practical techniques
to reduce the environmental impact of acid mine drainage.

The influence of various factors (gaseous oxygen and carbon dioxide concentrations,
temperature, bacteria, sulphide content and pH) on acid generation at Waite Amulet
are presented and discussed along with saturated and saturated zone pore water
geochemistry, tailings hydrogeology, and surface hydrology. Results of simulations of
pore water flow by a two-dimensional steady state finite element model are also given.
The evolution of acidic pore waters at the tailings site is discussed.

The study verified that the most important control on the pyrite oxidation process is the
availability of oxygen. In addition to contributing directly to oxidation, oxygen influences
the production of ferric iron and the density of bacterial populations. Oxygen
concentration distribution with depth in the tailings defines the depth of active
oxidation, and show that oxygen movement within the tailings is controlled by diffusion.

The oxidation of sulphides in the deep unsaturated zone occurs at a rate dependent on
pH, and will be very low. Acidic porewater is partly neutralized by mineral dissolution
and precipitation reactions in the deeper unsaturated zone. As sulphides in the shallow
zone are depleted, the oxidation front will move downwards. The remaining acidity and
dissolved metals will flow downwards with the pore water in the unsaturated zone until
it reaches the water table. Acid conditions in the shallow saturated zone become more
neutral with depth due to more buffering.

Pore water in the saturated zone flows in a direction which varies across the area of
the tailings. Flow in the central part of the tailings is vertical and downwards while flow
along the perimeter of the tailings is horizontal. Anisotropy in the hydraulic properties of
the tailings is a major control on the flow of porewater in the tailings. The anisotropy is
produced by the presence of fine-grained, horizontal (“slime”) layers. It has the effect of
promoting horizontal flow over vertical downward flow.

Along the west side of the tailings, a surface drainage ditch collects most of the water
infiltrating the tailings. Analysis of porewater quality and calculation of groundwater
velocity suggested that tailings porewater does not penetrate deep into the clayey
horizon underlying the south section of the tailings. In the northwest section, a small
portion of the porewater (approximately 10%) flows through the bottom of the tailings
into the underlying geological units. Sulphate levels above background values,
observed in the deeper layer of the clay, may be attributed to migration by diffusion.

The Solbec tailings pond was active from 1962 to 1977. Over 2.5M cubic meters of sulfide waste were deposited at the site, and sporadic sampling performed between 1972 and 1980 confirmed the presence of acid mine drainage (AMD). A 1987 characterization report of the tailings pond tabled proposed flooding of the
tailings as the most appropriate solution to the problem. It was at this time that Cambior became involved through its acquisition of the Sullivan Group, owner of the Solbec mine.

A series of experiments and studies followed from 1989 to 1993 to test the efficiency and viability of the flooding as a solution. The work required to achieve flooding was carried out in 1994. Since then, as part of the MEND program, a project has been conducted to monitor the quality of the water cover and groundwater associated with the tailings pond. The monitoring program also includes an evaluation of the oxydizing micro-organisms viability, their activity level activity in the flooded pond and of the hydrology and wave impact on site.

After twelve sampling and analysis campaigns of over 50 samples and ten parameters each, the solution now seems to be effective. The pH is now near neutral and the anomalous concentrations of iron, zinc and copper are resorbing. In addition, the basin retains enough water to keep the tailings submerged and thus prevent harmful wave action consequences. The effectiveness of the solution is supported by a decrease in the oxidizing microbial population, the cessation of its oxidizing activity and especially the appearance of sulfate-reducing bacteria. These bacteria contribute to the inverse oxidation process by reducing sulfate ions to sulfide ions, which reprecipitate metals in the more stable form of metal sulfides.

The Heath Steele Mine site is located approximately 50 km northwest of the City of Miramichi, along Highway 430 in north central New Brunswick.

The tailings basin covers an area of about 215 hectares and is divided into three cells (the North, Upper and Lower Cells), all of which are hydraulically linked. The North Cell discharges through the North dam spillway into the Upper Cell, then to the Lower Cell through the internal dam spillway and finally to the environment via the main dam spillway. The Old Tailings Area is located in the northeast corner of the tailings basin. It contains oxidized tailings from the early operation of the mine (1957). This area is hydraulically connected with the Upper Cell. The old tailings deposit has been partly covered with high density sludge produced by the water treatment plant and partly by soil.

The mine and milling operations ended in 1999. During operations, tailings in the North Cell were deposited underwater, while in the Upper Cell, the deposition of tailings was sub-aerial. Providing a water cover, to form an oxygen barrier designed to prevent/reduce acid generation and metal leaching, was considered to be the preferable option for the tailings basin (Lower, Upper and North Cells). In order to meet one of the cover design criteria that calls for a water cover of no less than one meter over the tailings under normal operating conditions, tailings in the Upper and North Cell were dredged to lower elevations in 2000 and 2001. A large volume of potentially oxidized tailings of the Upper Cell was moved to the North Cell.

The water level in the Old Tailings Area was allowed to equilibrate with the water level in the Upper Cell by hydraulic connection created through a permeable sludge pond dam built on top of old tailings dam in 2001. The Closure Plan had anticipated that some AMD generation in the old tailings deposit was expected to occur in the long term and that the flushing of contaminant may affect the water cover quality of the North and/or Upper Cells.

Heath Steele Mine monitors the performance of the water cover in terms of surface water quality. It has observed a pH variation between the North and Upper Cells. The addition of lime at the internal dam is necessary to raise the pH in order to meet the final effluent requirement. Based on the water quality improvement observed downstream, the water cover had a positive impact on the environment. Only 2 years after the completion of the water cover, a reduction of the metal loadings to the environment was observed. In accordance with the Closure Plan, the tailings water cover is in the midst of a transition period in terms of water quality.

Case Studies of Metal Leaching/ARD Mitigation and Assessment at British Columbia Mine Sites – Sulphurets Project

British Columbia MEND Case Study Project

The objective of MEND Case Studies is to show the practical experience with different aspects of ML/ARD mitigation and assessment practices at well-characterized sites. In the British Columbia MEND Case Study Project, case studies were produced from the extensive work completed at Johnny Mountain Mine, Snip Mine and the Sulphurets project, three of the more than 60 mines in British Columbia with significant metal leaching / acid rock drainage concerns. The MEND Initiative intends to add additional case studies from BC and other sites in Canada. The information provided on prediction and design and the resulting performance should improve the ability of the mining community to estimate mine site liability and prevent future impacts.

The three British Columbia case studies outline site-specific issues and key design and performance parameters, along with constraints, information gaps, errors and their management implications. This information on operational practices should be of interest to educators, regulators and the public, in addition to practitioners directly or indirectly employed by the mining industry.

In each case study, an attempt was made to describe the evolution in understanding and the rationale for decisions, in addition to the results. Raw data are provided so readers can make their own assessments. There are also appendices explaining the theory behind some of the procedures used for the prediction of drainage chemistry. The inclusion of raw data along with maps and photos allows the reader to draw their own conclusions and could enable the case studies to be used as an educational tool in which students could be given the raw data and asked to predict the drainage chemistry, develop a mitigation plan and decide what additional test work and management practices they would recommend.

Conclusions from the case studies include the following.

• Samples should be spatially, temporally, physically and geochemically representative of and include the materials, events or locations of most concern.
• Assumptions regarding sampling, analysis and field performance should be checked. Before proceeding with monitoring, identify the objectives and question(s) that need to be answered and how the answers will be used.
• All plausible outcomes, hypotheses or mechanisms should be presented, not just the most easily managed.
• It is important to show how proposed explanation(s) may be verified if correct, or proven wrong if incorrect.
• Environmental problems may result from a relatively small portion of the mass and therefore variability and distribution of results are often more important than the central tendency.
• Not all properties and processes can be accurately measured and a critical part of management is dealing with this uncertainty.

These conclusions indicate the importance of evaluating the practical details and taking a scientific approach in the prediction and mitigation of sulphidic mine drainage. They also indicate that no amount of QA/QC, statistics, computer modeling and expert opinion will help if the assessment ignores some of the key properties, materials, events or processes, such as maintenance requirements.

This report provides a comprehensive list of information requirements and factors to consider in assessing, and mitigating, acidic drainage. Specific requirements are listed under the two categories:

1. Material Characterization & Acidic Drainage Prediction:
• geology and mineralogy;
• static tests to measure material composition;
• kinetic tests to measure reaction rates and drainage chemistry;
• assessment of waste materials and site components; and
• interpretation of geochemical results.

2. Mitigation:
• flooding of mine wastes and workings;
• measures to reduce infiltration of drainage and/or oxygen entry;
• drainage treatment;
• addition of neutral materials; and
• desulphurization of tailings.

When developing or reviewing mining plans, this list can be used to identify what factors to consider and any outstanding information requirements. While this list will help ensure all relevant issues are addressed, it is only intended as a starting point. For any particular site, many of the listed factors or information requirements may not be relevant. Similarly, there will be instances where additional information requirements and factors must be considered. Users are encouraged to adapt the generic suggestions presented here to their own site and requirements, incorporating appropriate site-specific performance measures.

In collaboration with McGill University’s Dept. of Mining and Metallurgical Engineering, Noranda Technology Centre (NTC) investigated the prospect of recovering valuable metals from acid mine drainage (AMD) while maintaining effluent quality and reducing the amount of sludge generated. Several chemical methods were evaluated to selectively precipitate and recover metal ions, leading to the development of a conceptual flowsheet for a three-step precipitation process. This flowsheet was further evaluated, along with the associated process economics. The research was funded by Noranda and carried out for the MEND program.

The original three-step precipitation process was developed and investigated in detail at the laboratory scale (Part I) by McGill University. This process consisted of: a) Fe precipitation in the presence of surfactant (dodecylamine, DDA, which was expected to alter the surface properties of Fe(OH)3, thus reducing co-precipitation of Zn); b) sulphide precipitation to obtain a Zn rich sulphide precipitate; and c) a final lime treatment to remove residual metals if, necessary to comply with water quality standards.

Several concerns were raised pertaining to this process. Solid/liquid (S/L) separation and materials handling appear difficult. Furthermore, residual DDA in the effluent may be detrimental to aquatic life. Ultimately, process control and cost are the key problems for sulphide precipitation. As a result, alternative processes were sought.

To this end, three alternatives processes were developed and evaluated by NTC (Part II). In one process, the first step consisted of Fe(III) precipitation with CaCO3, followed by precipitation of metals with NaOH/Na2CO3. In another process, Cu was removed by cementation, using Fe powder. Iron (III) was subsequently precipitated as a phosphate, using H3PO4; and Zn was removed as a hydroxide, using Ca(OH)2. The use of Na2S to obtain ZnS/CuS precipitates was further investigated in a reverse version of the three-step process. Following the removal of Zn and Cu as sulphides at pH 3.5 in the first step, lime neutralization in conjunction with aeration at pH 9.5 was applied to simultaneously precipitate iron and the remaining metal ions, and to produce acceptable effluent quality together in one step.

The results obtained from both studies are:

Part I – McGill Study:
1. With the use of lime, the iron present in AMD can be completely removed as ferric hydroxide at pH 3.5, following oxidation with H2O2. The solid content of the settled sludge ranges from 6 to 8%.
2. The use of DDA in the first step to reduce co-precipitation of other metal ions onto ferric hydroxide sludge slightly improved the subsequent Zn recovery. However, settling of precipitates in all stages deteriorated and the content of the leachable metals in the iron sludge increased. As a result, the use of DDA is not recommended in the process.
3. The iron sludge required several washing cycles in order to remove leachable metals.
4. Zn and Cu can be selectively recovered by using either Na2S, H2S or NaHS in the second step. The Zn/Cu selectivity is closely dependent on pH (e.g. 3.5) and the alkaline reagent (e.g. NaOH) used for pH control. However, as the Zn/Cu recovery increases with increasing pH, the Zn/Cu grade of the sludge decreases.
5. More than 90% Zn recovery and greater than 50% Zn grade can be obtained at pH 4.5 when lime and Na2S are used to set the pH and precipitate the Zn.
6. H2O2, O3 and Trapzene (a CaO2 mixture; patent pending FMC Corp.) were evaluated as Fe oxidants. O3 was technically the most effective when it was used in stoichiometric quantities.
7. Lime treatment of the overflow from the second step to pH 9.5 resulted in an effluent quality similar to that from the conventional lime neutralization process.

Part II – NTC Study:
1. The McGill process was further examined in reverse order and as a two-step process. In this two-step process the removal of Zn/Cu as sulphides at pH 3.5 was performed first followed by the oxidation of iron with air and precipitation with lime at pH 9.5. This method required a large quantity of Na2S (e.g. 3-4x stoichiometric requirement) and technical difficulties in the separation of the Zn-rich sludge were encountered.
2. Of the chemical processes examined, the two-step process yielded the least contaminated iron precipitate and the highest Zn recovery. In addition, this process did not produce sludge requiring special disposal. However, the Zn grade of the precipitate was about 30%.
3. Biological oxidation could oxidize iron, but the required retention time was 2-6 days. On a cost basis, biological oxidation seems to be at least one order of magnitude cheaper than chemical methods.
4. Although the use of CaCO3 resulted in the least contaminated Fe(OH)3 precipitate and its cost was one order of magnitude less than other chemicals tested, the iron still needed to be oxidized before precipitation.
5. Process economics for each process investigated and oxidizing reagent used were assessed. The cost for each process was compared to the cost of the lime neutralization treatment plant being operated at Les Mines Gallen, site of the AMD used in the tests. The most expensive method was the three-step ZnS precipitation, and the least expensive was the two-step CaCO3/NaOH process.
6. Use of PO4(-3)to selectively remove ferric iron, following cementation of copper with iron, and precipitation of zinc with lime was also explored (based on initial Dahnke’s master’s thesis, 1985). The process was not technically or chemically feasible.
7. Requirements for dewatering of each precipitate, generated at each step along the processes were determined. ZnS precipitates required flocculation, clarifier settling and good filtering (e.g. via filter press).

The processes investigated suffer from high costs relative to conventional lime treatment. In particular, the costs of chemical sulphide reagents, and H2O2 are not economical. As a result, further research aimed at optimising the existing flowsheets is not recommended, unless new concepts which radically reduce costs or S/L separation steps are involved.

Some alternative options for developing a process flow sheet should be examined. Suggestions are listed below in order of priority:

I. Following the reduction of all iron in the AMD to ferrous iron with SO2, precipitate ZnCO3 in the first step; then oxidize iron with air and precipitate iron and other residual metal ions with lime.
II. Investigate the biological sulphate reduction process and/or a combined biological/chemical processes.
III. Selective leaching of Zn from lime sludges should also be explored as a potential alternative. Particularly, the sludge generated from the two-step NTC process should be looked at, due to its co-absorption property. Zn selectively leached out from the sludge can be subsequently precipitated with Na2CO3.
Any new process must consider the acceptability of end products from each process for recycling to a Zn roaster, Zn concentrator or lead circuit.

The extraction of Sb, Cd, Cu, Ni and Zn from acid mine drainage solutions using various ion exchangers was studied with batch and column tests. The extrac¬tion was studied at low pH, at mildly acidic pH and at neutral pH. The interference caused by the presence of Al, Ca, Mg and Fe was monitored.

It was found that most commercial ion exchangers do not exhibit a marked selectivity for the metals of interest. Consequently, the co-extraction of iron is a major obstacle for the application of ion exchange to acid mine drainage solutions. The selectivity did not improve signi¬fi¬cantly when Fe(III) was reduced to Fe(II). The co-extraction of calcium becomes a problem at higher pH when lime is used to neutralize the solution. The most promising results were obtained with copper and antimony, which were amenable to extraction even at low pH.

While the selective extraction of Cd, Ni or Zn was not feasible under most conditions, the simultaneous extraction of all three metals can be done at neutral pH using either a chela¬ting resin or a synthetic zeolite. This fact could be used to reduce the amount of lime that is normally added to ensure the complete precipitation of metals.

Removal and recycling of metal contaminants and other by-products from acid mine drainage is the subject of this MEND project. Critical literature surveys and investigations in several selected research alternatives such as chemical treatment (that is modifications to lime neutralization), solvent extraction, ion exchange, bioadsorption and electrowinning, are aimed at the development of a viable treatment process. The technical and economic feasibility of the techniques will be evaluated, in search of a process that produces a high quality effluent, secures a satisfactory revenue and minimizes/eliminates hazardous sludge production.

A literature review was carried out by CANMET scientists from four different disciplines in the field of metals and/or by-products recovery from acidic mineral effluents. Existing expertise from the application of these technologies in related fields in mineral processing, as well as preliminary results from investigations with acidic mineral effluents have been utilized to evaluate selected publications and to determine the most promising process approaches. A preliminary compilation of data in acidic mineral effluents in Canada has been made, with particular interest in high metal concentrations.

Discussion provided in this report supports the opinion that the recovery of metals and/or by-products from acid mine drainage is a promising treatment approach from a technical, economical and environmental point of view. Detailed laboratory investigations should follow to study the recovery of various by-products, to develop a treatment process flowsheet and to evaluate the economical and environmental benefits. Samples from two milling operations should be selected to carry out the tests.

With increasingly more stringent environmental regulations, growing concern with water quality and management and a greater focus on water recycling and minimization of water use in the mining industry, conventional treatment methods are being challenged to meet lower concentrations of contaminants in the discharge stream. Economics and costs are important factors in technology selection and have a significant weight in the selection of treatment options, unless other factors such as regulatory requirements are the drivers. In the past decade, membrane separation processes have attracted significant attention and are applied in different sectors of the industry especially in water and wastewater treatment

The applicability of different membrane separation processes for the mitigation of acidic drainage has been demonstrated in the literature. Work by various researchers has demonstrated the successful application of Reverse Osmosis (RO), Nanofiltration (NF) and charged Ultrafiltration (UF) membranes for the treatment of acidic drainage and the removal of contaminants and metal species of interest, such as selenium, from neutral drainage streams.

Membrane separation processes, such as reverse osmosis, can generate a highly concentrated retentate. This could create problems with the precipitation of different metal species and could cause scaling and fouling problems; however, these issues can be managed with the proper mode of operation, the use of additives, the implementation of cleaning cycles and module design. The membrane concentrate, in industrial applications, is usually a highly saline stream that requires treatment before disposal. However, this stream could also have a cost benefit by allowing for the economic recovery of metals or other products of interest and improve efficiency of chemical treatment due to increases in the concentration of contaminants. With proper process design and material selection, high water recoveries are possible with the added benefits of waste minimization and volume reduction.

Commercially available hybrid membrane systems that apply combinations of Microfiltration (MF) or Ultrafiltration with Reverse Osmosis or Nanofiltration are reviewed in this report. This report also addresses other membrane options such as the use of charged UF membranes or other innovative approaches such as the VSEP (Vibratory Shear Enhanced Process) filtration process for acidic drainage (AD) treatment. Although the main focus of the present report was to assess membrane separation technology to AD treatment, the application of membrane separation to other mining applications was also reviewed.

Finally, a number of case studies on the application of membrane separation in mining operations are presented, with an emphasis on the cost impacts and improvements on the environmental performance.

The information in this review shows that membrane separation is an efficient and cost-effective technology for mine water and wastewater treatment and mitigation applications. A comparison with conventional treatment technologies has also shown that membrane separation, if properly designed and operated, can provide superior treatment results and can have lower capital and operating costs. Membranes cannot completely replace conventional treatment technologies and be a stand-alone treatment option. They can be a powerful tool for volume reduction and waste minimization, allowing for the recovery and recycling of water and other potentially valuable by-products such as acid, gypsum (calcium sulphate), heavy metals and sulphur. Because of the volume reduction that membrane separation offers, the footprint and capital costs of the accompanying conventional treatment options such as clarifiers and other chemical treatments could be substantially reduced.

Due to the composition of mining streams and effluents, the most important aspects of membrane separation are membrane fouling and brine disposal. As a result, the main technology development drivers are:
• Membrane fouling – lowering membrane replacement costs, maximizing recoveries;
• Pretreatment as a means of fouling control;
• Maximizing water recoveries; and
• Brine disposal or treatment and the minimization of its associated costs.

One of the major issues facing the Canadian mining industry is the treatment of effluent during and after closure of a mining property. Effluent treatment may be complicated by the presence of acid mine drainage (AMD) which even under the best reclamation scenario may require long term collection and treatment. While chemical treatment or some other form of active effluent treatment has traditionally been conducted in Canada, greater consideration has been given recently to forms of passive treatment.

Passive treatment systems utilize the chemical, biological and physical removal processes that often occur naturally in the environment to modify the influent characteristics. Passive treatment systems were initially considered attractive to treat acid mine drainage due to their lower costs of construction, operation and maintenance, and their ability to operate at remote locations with limited operational requirements. The objective of this project was to review passive treatment systems, and make recommendations on their applicability to treat acid mine drainage in Canada.

Although passive systems have been proven at many locations around the world, the Canadian climate and aquatic environments present considerable challenges to large-scale use in Canada. Biologically driven systems have low activity in cold temperatures and drought, while storm and spring “freshet” events demand flexible, strong systems.

In this review, four major types of passive technologies for the treatment of acid mine drainage have been examined:

• anoxic limestone drains;
• constructed wetlands;
• microbial reactor systems; and,
• biosorption systems.

In accordance with the scope of work, this document provides:

• summary of known passive treatment technologies;
• maintenance and monitoring requirements;
• life expectancy and long term implications (>100 years);
• implications of treatment product disposal;
• estimate of costs for the technologies described based on generic cases;
• ability to meet Canadian Metal Mining Liquid Effluent Regulations, and to control toxicity;
• descriptions of case studies including: range of flow, temperature, water chemistry, and identification of limiting conditions where available in the literature (Appendix A); and,
• general assessment of the applicability of current passive treatment systems to Canadian mine sites.

ANOXIC LIMESTONE DRAINS (ALD’S)

The basic design of an ALD is a trench filled with high quality crushed limestone, sealed under plastic and geotechnical fabric, covered by soil, through which an unaerated, contaminated effluent stream flows by gravity. As it flows through the system, the acid mine drainage gradually dissolves the limestone, releasing calcium as bicarbonate, thus raising the pH.

Based primarily on studies conducted at coal mines in the United States, ALD’s have been shown to be most effective for influent with dissolved oxygen, ferric iron (Fe3+) and aluminum concentrations of less than 1 mg/L, and sulphate concentrations below 2,000 mg/L. At higher concentrations the limestone may become armoured with oxides or gypsum, reducing the rate of limestone dissolution or plugging the system. In either instance, the ability of the ALD to generate alkalinity may be significantly reduced, and failure of the system may occur.

As a result of these strict influent requirements, ALD’s are expected to have only limited application to treatment of acid mine drainage at Canadian metal mines.

CONSTRUCTED WETLANDS

Constructed wetlands are ecological systems designed to optimize a variety of natural physical, chemical, microbial and plant-mediated processes. In a constructed wetland, influent AMD drains by gravity through the wetland, progressively undergoing metal removal and neutralization. Metals are removed by precipitation, chelation and exchange reactions, while neutralization is primarily achieved by the activity of sulphate reducing bacteria (SRB), or the increase in alkalinity from the chemical and microbial reactions including limestone dissolution.

Passive systems for AMD treatment have commonly used combinations of natural or constructed wetlands, Sphagnum peat and open ponds, supplemented by chemical amendments (mostly limestone) and organic substrate to increase alkalinity and reduce acidity. Sequential treatment of AMD to remove iron by oxidation, hydrolysis and settling in the aerobic stage, followed by SRB activity in an anaerobic stage to raise pH, is an effective combination.

For either aerobic or anaerobic cells, the design must maximize contact with the matrix, which can either be aerated water, or anaerobic substrate. It is essential that constructed wetlands are managed in terms of their individual components and their mutual interactions to gain a desired overall efficiency. Many of the metal removing mechanisms in a wetland are temporary and reversible, and can reach saturation; thereby reducing the wetland’s efficiency and decreasing their cost effectiveness. In addition, para-reversibility represents a challenge for treatment product monitoring or disposal.

Constructed wetlands have the potential to address AMD treatment at some Canadian sites where sufficient surface area is available, and can form the preferred alternative in terms of costs, efficiency and environmental safety. The ‘black box’ design approach that has been used in the past and is still being suggested, is not recommended. The design should be based on an understanding of the interactions between the chemical, microbial and plant-mediated components of the system and the engineering, climate and hydrogeological realities of the treatment site.

A well designed, constructed wetland is an efficient accumulator of metals and reaction products. Key to its efficiency, is the continuous physical, chemical and biotic matrix in the wetland. This capacity will be limited during freezing or high flow conditions. As a result, constructed wetlands may be most applicable to Canadian mines having shorter and milder winters, and at sites where a constant rate of flow can be maintained. An alternative treatment method may be required during the winter and during spring runoff conditions, or else large retention ponds are required.

MICROBIAL REACTOR SYSTEMS

Microbial reactor systems or bioreactors may be in an open or closed configuration, referring to whether they are exposed to the atmosphere. In either instance, the microbial reactor shell contains a biodegradable substrate (usually agricultural products such as mushroom compost or straw) which supports the growth of micro-organisms, which in turn treat acid mine drainage.

The cellulose of the agricultural products is degraded by cellulolytic bacteria to generate free sugars and other metabolites, which are further metabolized to provide substrate for the fermentative anaerobes. Under anaerobic conditions these free sugars are fermented to short chain organic acids or short chain fatty acids, which are suitable substrates to support the growth of sulphate reducing bacteria (SRB). SRB reduce sulphate to hydrogen sulphide which precipitates metal ions as low solubility metal sulphides. Concurrently, the sulphate reducing bacteria consume hydrogen ions and produce carbon dioxide during their metabolism, causing an increase in the pH of the solution due to reduced concentration of free hydrogen ions and the buffering effect of the CO2/bicarbonate buffer system.

Pilot plant data available suggests that bioreactors are a feasible technology for treatment of small AMD streams. Open bioreactors are expected to only be applicable at Canadian mines with mild or moderate winters. Closed bioreactors can operate anywhere that a fairly constant temperature can be maintained.

BIOSORPTION SYSTEMS

Micro-organisms, including bacteria, algae, fungi and yeasts, can efficiently accumulate heavy metals and radionuclides from their external environment. Biosorption systems in a wide variety of configurations rely on this ability to treat acidic drainage. Living cells can be used to treat effluent where metal concentrations are below toxic levels. The use of dead biomass in the form of commercial biosorbents eliminates the problems of metal toxicity, adverse climatic conditions, and the costs associated with nutrient supply and culture maintenance.

Only a limited number of studies have been conducted to date in regards to treating AMD with biosorption systems. While it does not appear that biosorption systems are an effective stand-alone treatment system for AMD, with further study they may become an alternative form of treatment of parts of an effluent stream, or as a final polishing step. The success of biosorption systems using living biomass during the winter is expected to be limited. Treatment efficiency will be lowered under poor growth conditions. Systems employing dead biomass are expected to have greater applicability, and may not be compromised by winter conditions as long as flow is maintained.

OVERVIEW

Passive treatment of acid mine drainage has a future in Canada, but is limited to applications where:

• flows are of relatively constant volume
• water temperature is greater than 7C (eg. mine water or embankment seepage)
• water chemistry of low to medium strength acidity and metal concentration
• low concentrations of aluminum and iron
• low sensitivity of the receiving environment to upsets in the passive treatment system.

Further research and field experience is needed to more precisely specify passive treatments for AMD in Canada to ensure that metal mining liquid effluent regulations are met all the time. No “passive treatment” system is truly passive. All systems require monitoring and replacement of consumed alkalinity or organic-based nutrients for bacteria. Also, metal precipitates need to be removed and in some jurisdictions sludge falls under “hazardous waste” regulations and disposal may be a significant challenge.

Several methods to delay the onset of acidic drainage have been tested:

· bactericide application;
· deposition in permafrost;
· phosphate application; and
· irrigation with alkali,

Other methods have been applied as permanent controls but could be designed as delays:

· simple soil covers;
· oxygen-consuming organic covers;
· blending; and
· lime application,

Information about each of the above methods was collected from literature and from interviews with mine operators and researchers. A brief overview of each method is presented, and case histories are described. The extent to which the case histories prove the effectiveness of each method is reviewed.

In general, the review concludes that there is a lack of well-designed and documented studies of methods to delay the onset of acidic drainage. However, several of the reviewed methods are promising enough to warrant further research. A well-designed field study of bactericides is needed to demonstrate whether this method can be effective in metal mine waste. Alkali addition and soil covers are clearly effective for dealing with waste that is already acidic, but their effectiveness as delay methods needs to be demonstrated. Organic oxygen-consuming covers have been shown to be effective for tailings but still need testing on waste rock. Low temperatures have been shown to slow ARD, but further work is needed to quantify the benefits and investigate possible difficulties associated with depositing waste into a permafrost setting.

A list of general requirements for such studies is presented. The review also provides specific recommendations for studies of each method.

As a long term means of preventing the onset of acidic drainage, potentially acid generating and acid consuming waste rock can theoretically be blended or layered to produce a geochemically benign composite. This report presents the general theory behind blending and layering for prevention and control of acid rock drainage (ARD), and presents selected case studies on the use of this technique at mines in Canada, United States and other parts of the world.

The focus of this report is the blending, mixing and layering of waste rock, and, to some extent, the addition of limestone and other alkaline materials. The addition of alkaline materials was initially considered beyond the scope of this MEND sponsored project (Project2.37.1). However, the scope was expanded as very few case studies were found which exclusively examined blended waste rock, especially in the coal fields. Both coal and metal mines are covered in the review.

Coal Mines

Most of the reviewed coal case studies were located in the Appalachian region of the eastern United States with pyrite sulphur values in the range of 0.1 to 1 %. At these sites, the effect of waste rock blending was often confused or masked by additional management measures such as lime and flyash addition, and/or the presence of acidic drainage from adjacent historic sites. Comprehensive reports, providing compilations and assessments of acid-base accounting (ABA) data and kinetic tests, predictions of long-term effluent quality, pre and post mining monitoring of rock characteristics, and pre and post mining monitoring of drainage quality, were not found in the course of this project. However, numerous papers in the published literature reviewed specific and general management practices at individual sites.

Researchers in Appalachia have compiled estimates of volume-weighted ABA data derived from drill core analyses, and compared these values to effluent quality at associated seepages. These compilations (diPretoro and Rauch, 1988; Ericksen and Hedin, 1988; Brady et al, 1994) have resulted in the development of general criteria for overall volume-weighted waste rock blends considered likely to produce net alkaline drainage but not necessarily prevent metal leaching. The commonly cited criteria in the Appalachia (P. Ziemkiewicz, pers. comm., 1997) are:

Net Neutralization Potential (NNP) > 10 kg CaCO3 equivalent/tonne;

Neutralization Potential (NP) > 15 kg CaCO3 equivalent/tonne; and,

Ratio of Neutralization Potential to Acid Potential (NP/AP) ratio > 2.

The reviewed studies on coal wastes tended to examine the addition of alkaline materials rather than the deliberate blending of acid generating and acid consuming waste rock types. These studies indicated that:

a) Fine alkaline material distributed uniformly throughout a waste pile were effective in delaying the onset of acidic effluent;

b) Layering of limestone within test piles was effective in reducing the amount of net acidity produced, and in delaying the onset of acidic effluent, but did not effectively prevent acid conditions from developing within the potentially acid generating (PAG) portions of the piles;

c) Lime kiln dust was more effective in reducing the amount of net acidity produced and in preventing the onset of acidic effluent than limestone, possibly due to the higher surface area and the greater reactivity of CaO;

d) The order in which overburden was placed in relation to acid drainage generating strata had a significant effect on leachate quality; and,

e) Placement of overburden as soon as possible after excavation was beneficial in reducing ARD.

The critical ratio of alkaline addition to acid potential which would permanently prevent the onset of acidic drainage at specific sites was generally not predicted.

Metal Mines

Few metal mine sites were identified that had deliberately examined or applied blending and/or layering as a prevention method. Typically, older sites that provided long term water quality data to support the success or failure of this technique Jacked sufficient documentation on materials in the waste piles and the level of blending employed to allow conclusions to be drawn. Newer sites that characterized their materials and operational procedures have had limited time in which to demonstrate success or failure.

Site data and laboratory test data for metal mines tended to indicate that:

a) The majority of cases evaluated blends with NP/AP ratios less than 2;

b) Blending did not reduce sulphide oxidation rates in the potentially acid generating material unless highly reactive neutralizing material (limestone) was applied and the blending was near ideal. Perfect mixing was generally only possible in column or humidity cell tests. Such ideal blending was not considered feasible at field scale;

c) Layering thicknesses down to 10 cm did not reduce sulphide oxidation rates in potentially acid generating material;

d) Blending and layering were effective in delaying the onset of acidic effluent, and reducing the metal and acidity loadings exiting from the combined material. The presence of alkaline drainage from neutralizing materials appeared to temper the locally produced acidic leachate, resulting in less acidic drainage and lower metals levels due to decreased solubility of hydroxide and carbonate phases at the elevated pH.

e) Prevention of acidic effluent from the blended or layered materials did not necessarily prevent dissolved metal levels from being problematic.

The reviewed metal mine case studies did not identify safe waste rock blends which would prevent ARD and metal leaching at those sites.

From the literature and case studies reviewed, it appears that blending and layering of acid generating and acid consuming waste rock may be a legitimate method of neutralizing acidic drainage with a consequent reduction in metal leaching through precipitation of metals within the pile. However, there remains considerable uncertainty, and further investigation into the application and practice of this method is required. Specifically, more information on large scale, controlled field studies is needed. The identification and detailed documentation of additional sites should also be encouraged, in association with predictive laboratory tests. At issue is what portion of alkalinity is available to neutralize the acidity produced by sulphide oxidation, and the extent that this is influenced by the thoroughness with which alkaline and acid producing waste rock are blended at a given site.

Waste rock is typically stored in a subaerial environment, a setting that may promote the oxidation of sulphide minerals and therefore be conducive to the initiation of acid rock drainage (ARD) and commensurate trace metal release. To mitigate this problem several strategies are currently being employed and tested by the mining industry including the subaqueous disposal of sulphide-rich waste rock. Subaqueous disposal has a number of features that make it attractive as a long-term storage option. However, the secondary mineral assemblages that accumulate during subaerial exposure could have a profound influence on the geochemical behaviour of the waste when submerged, such that deleterious effects on water quality may result. In order to assess adequately the environmental implications of placing oxidized waste rock underwater, techniques must be developed to allow proponents and government agencies to evaluate scientifically, and ultimately predict, the potential water quality impacts of this waste rock management strategy.

The ultimate objective of this project was to design a laboratory test protocol that could be used to quantify the chemical stability of waste rock oxidation products in a range of subaqueous environments. To this objective, the report first examines the mechanisms that control the formation and stability of secondary minerals in waste rock dumps, identifies the minerals that may be present in the dumps and evaluates their subsequent stability in a subaqueous setting. Second, the report examines available laboratory methods and their applicability to assessing metal release from oxidized waste rock.

The extent of oxidation is spatially variable in a subaerial waste rock dump; hence the distribution of weathering products is heterogeneous. The climate and physical characteristics of a waste rock dump indirectly control the extent of these weathering products by regulating the intensity of various physico-chemical conditions such as pH, temperature, dump hydrology and mineral weathering. A general indication of the mineralogical form taken by metal precipitates can be inferred from the site-specific physico-chemical conditions and the elemental constituents of the waste rock. An indication of which elements will be retained as major or minor components of secondary minerals in oxidized waste rock can be obtained by determining the elements associated with various types of ore deposits and their relative mobilities. However, iron and sulphate secondary minerals are common in all oxidized sulphide-rich waste rock dumps and exert a major control on the mobility of other less abundant elements.

The subaqueous stability of a mineral is controlled by the thermodynamic and kinetic properties of the mineral which are in turn influenced by variable characteristics such as surface area, crystallinity, solution chemistry and temperature. Four general mineral dissolution categories have been examined including: water soluble; pH sensitive; reducible; and oxidizable. Water soluble precipitates found in waste rock dumps are typically hydrated sulphate minerals and many of these minerals produce acidity upon their dissolution. Carbonate minerals and adsorbed cations are extremely pH sensitive and are soluble in acidic solutions. Mature oxyhydroxide and sulphate minerals are typically insoluble in oxic waters but are susceptible to reductive dissolution. Organic matter and sulphide minerals, which are susceptible to oxidation have also been examined, although it is acknowledged that subaqueous storage minimizes sulphide oxidation.

A number of test methods were examined to evaluate the subaqueous stability of oxidized waste rock including: partial extractions; sequential extractions; shake flask; column; and tank tests. Sequential extraction tests are best suited for use as an initial screening tool for the evaluation of metal release from subaqueous waste rock. Shake flask, column and tank tests have been used to determine the subaqueous stability of oxidation products. However, they do not discriminate between the geochemical processes or mineral phases responsible for the release of metals to solution. Partial extraction tests are limited to simulating one environmental condition and thus one standard test cannot be applied to examine a range of receiving environments. In contrast, sequential extraction can be used to evaluate metal release for a range of extreme environmental conditions. The application of sequential extraction methods to assess subaqueous stability of oxidized waste rock is comparable to the use of acid-base accounting (ABA) to assess the acid generating characteristics of subaerially-exposed waste rock. Extractions are more powerful, however, because they provide data on the element-phase associations which are lacking in ABA testing.

A four step sequential extraction scheme is proposed to examine metal partitioning in oxidized waste rock by targeting the following four phases:
– F1: water soluble (e.g., hydrated sulphates);
– F2: exchangeable/adsorbed/bound to carbonates;
– F3: total reducible (e.g., oxyhydroxides); and
– F4: total oxidizable (e.g., sulphides and organic matter).

In order to determine the elemental composition of the sample prior and subsequent to the application of the proposed extraction scheme, two analytical techniques have been considered. These are XRF and acid digestion followed by ICP-MS analysis. The applicability of these techniques should be assessed during the verification and standardization phase for the proposed extraction procedure, with the optimum method being chosen at that stage. Determination of whole rock elemental abundances in the initial and residual fractions is critical for the interpretation of results from the proposed extraction procedure.

It is of utmost importance to note that sequential extractions are “operationally-defined”, therefore an extraction procedure that provides an accurate representation of metal partitioning in one type of sample (i.e., soil) may not be effective in another (i.e., waste rock). Thus, prior to utilizing sequential extractions directly for assessing metal release from oxidized waste rock, verification and validation of the proposed procedure is required.

The proposed sequential extraction method can be used as an effective tool to assess metal-phase associations in waste rock when more direct methods (e.g., SEM, XRD, etc.) become to expensive and time consuming due to the fine grained/amorphous nature of many secondary minerals. Although the extraction alone cannot predict quantitative water quality impacts due to kinetic controls on mineral dissolution, when combined with kinetic or in-situ testing it is an effective tool to assess environmental risk associated with the subaqueous disposal of oxidized waste rock.

Open pit mines that have ceased production are increasingly being considered for the permanent and environmentally acceptable disposal of mine waste rock and tailings that are, or have the demonstrated potential to become, sources of acidic drainage. This report addresses key aspects that need to be taken into consideration when evaluating the in-pit disposal of wastes and presents 12 case studies of actual and planned in-pit disposal of mine wastes.

There are four basic concepts for the placement of wastes in pits:

Option 1 – Underwater disposal
Option 2 – Elevated water tables
Option 3 – Dry disposal
Option 4 – Perched water tables

The four options are described in terms of the theoretical concepts and practical aspects; selected examples of research and field applications are provided from the published literature.

Not all pits are suitable for the in-pit disposal of wastes. The success of an application would depend on many technical factors, including:

* the acid generation potential of the wastes and pit walls;
* the geotechnical characteristics and properties of the wastes and the pit walls;
* predicted pore water, pit water, and groundwater quality;
* hydrogeology of the open pit; and
* the hydrology of the open pit.

Mine related constraints must also be taken into consideration. These include: limiting access to remaining mineralization below the pit; wall stability and related safety concerns; available access; and the proximity of underground workings to the open pit.

Consideration must be given to both the short term and long term implications of the in-pit disposal concept; these include ecological and human health protection and closure planning perspectives. Each site is unique and may have special constraints with respect to the quality and use of surface water and groundwater, land use, and sensitive ecological communities.

The potential costs for future in-pit disposal of wastes should be considered in the preliminary economic evaluations that are used to establish final pit limits. Greater than expected waste disposal costs could have an unfavourable impact on the future profitability of some open pit mines.

Volume 2 of this report presents twelve case studies: seven case studies describe in-pit disposal programs that have been implemented; two case studies describe proposed in-pit disposal programs; and three case studies provide technical information from other pits which will likely be of interest to persons evaluating in-pit disposal programs.

The following case studies describe pits that have been used for in-pit disposal of mine wastes:

The Owl Creek pit – To prevent discharge of acidic drainage from waste rock piles, the waste rock was relocated to the pit and flooded.

The Rabbit Lake pit – This pit is likely the first application of an engineered pit disposal concept, which involved the use of a bottom rock drain, an engineered pervious envelope, placement of tailings as a dry filter cake, and closure with a surface lake and a soil/sand diffusion barrier.

The Collins Bay B-Zone pit – Following extensive decommissioning studies, the special waste (which contains elevated arsenic, nickel, sulphur, or uranium content) was placed in the bottom of the pit and the pit was flooded, creating a pit lake.

The Solbec pit – A crown pillar pit was filled with reactive waste as a means of inhibiting further acidic drainage.

The Udden pit – The pit was backfilled with reactive waste rock and allowed to flood.

The Stratabound CNE pit – The in-pit disposal of reactive waste rock was planned at the design stage, and the waste rock has been relocated to the pit and clay capped.

The Cluff Lake “D” Zone pit: The pit flooded after the end of mining operations, and an extensive monitoring program has been carried out to evaluate the physical and chemical changes occurring in the pit lake water column.

In-pit disposal programs have been proposed at the following case study sites:

The Island Copper pit – As part of planned closure, it is proposed that the pit be flooded to create a meromictic lake.

The Deilmann pit – Insufficient capacity exists within an existing surface tailings management facility to accommodate future tailings production therefore tailings disposal options are being reviewed; one option involves the conversion of the Deilmann pit to a full side drain and under-drain tailings management facility.

Additional case studies which may provide useful information to persons evaluating in-pit disposal programs are:

Robinson Mining District – Extensive numerical modelling was carried out to evaluate the hydrogeochemistry and to support the prediction of environmental impacts from several pit lakes.

The Gunnar pit – This pit lake is interesting because of unique and well-documented limnological characteristics.

The Berkeley pit – This mine site is part of a U.S. Superfund site and is a well-known example of a serious acidic drainage problem in a flooding open pit.

The management of waste rock produced from mining of sulphidic ores poses a challenge to the mining industry. Acid generation occurs when sulphide minerals (principally pyrite and pyrrhotite) contained in the rock are exposed to air and water. In the absence of sufficient alkaline or buffering minerals, the resulting leach water becomes acidic, and is characterized by high sulphate, iron and, metal concentrations. This water, sometimes called acid rock drainage (ARD), can contaminate surface water and ground water courses, damaging the health of plants, wild life, fish and, possibly, humans.

A study was initiated by Noranda Technology Centre (NTC) and the Centre de recherches minérales (CRW to evaluate the relative effectiveness of various techniques for controlling ARD in waste mine rock. This study was undertaken at NTC as part of the MEND (Mine Environment Neutral Drainage) Program. The techniques investigated were water cover, soil cover, wood bark cover, and addition of limestone and phosphate rock (apatite).

Potentially acid-generating waste rock samples used in the investigation were obtained from the Stratmat site, located on the Heath Steele Mine property, near Newcastle, New Brunswick, and from Les Mines Selbaie, located near Joutel, Québec. Both types of rock samples were crushed to particle sizes between 25 and 50 mm. The investigation involved outdoor lysimeter tests and indoor or laboratory column experiments. Cover techniques investigated were a 1 m water cover, a soil cover consisting of a 150 mm thick water-saturated clay layer sandwiched between two 75 mm thick sand layers, and a 150 mm thick wood bark layer. Limestone and phosphate were added at 1 and 3% dosages. Control experiments, using waste rock without cover and additive, were also installed for comparison. The outdoor tests were subjected to natural weather conditions (rain, freeze-thaw and evaporation). The laboratory or indoor tests were run at an average temperature of 20°C and subjected to a cycle of 8 weeks of dry conditions and 8 weeks of wet conditions (water addition). Water was added to simulate the average annual precipitation for a nearby municipality, Dorval, Québec. All tests were conducted in triplicate.

Monitoring of the effluent water quality to three years (154 weeks) indicated the control waste rock started producing acid very early in the tests (about the 5th week). The rate of acid production was quantified (mg of CaCO3 per day per kilogram of rock) and found to be higher in the laboratory than outside. Higher laboratory temperatures are most probably responsible for the higher rate. The Stratmat rock generated acid at a higher rate than the Selbaie rock, although the latter has a higher pyrite content. A detailed post-testing investigation of the two rock types was conducted at The University of Western Ontario, using mercury intrusion porosimetry, surface analytical techniques and X-ray fluorescence and diffraction methods. The results indicate the fresh, unoxidized Stratmat and Selbaie rocks have a similar pore structure, but different gangue mineralogy. The Stratmat rock consists of pyrite and minor amounts of metal sulphides held in a matrix of silicate minerals including illite and feldspar. The Selbaie rock, on the other hand, contains mainly pyrite and quartz. Trace amounts of metal sulphides appear to be in solid solution with the pyrite. Results of accelerated leaching tests clearly showed that the gangue composition of the Stratmat rock has a major influence on its acid generation ability and would explain the difference in acid production between the Stratmat and Selbaie rocks.

Water cover was found to be the most effective control technique during the three years of indoor testing, followed by 3% and 1% limestone, soil cover and, finally, 3% and 1% phosphate. The effectiveness of the various techniques observed in the outdoor tests were as follows: water cover, 99%; 1% limestone, 93%; soil cover, 70%; and 1% phosphate, 9%. 10-15% increase in effectiveness was observed (from 83 to 98%) when the amount of limestone added to the rock was increased from 1 to 3%. A similar increase in the amount of phosphate yielded higher effectiveness, from 10 to 70%. All the techniques, with the exception of the water cover, were found to be slightly more effective in the laboratory than outside. The water cover maintained the same effectiveness (> 99%) in both laboratory and outside tests. The soil cover was more effective in the laboratory (98%) than outside (70%). The difference may be explained by the effects of adverse natural climatic conditions (for example, freezing and thawing) which were not resent in the laboratory. It is also believed that oxygen and water enter the soil covered waste rock mostly by the side walls of the lysimeters. The phosphate was found to contain some carbonate mineral (calcite) which probably delayed acid production (at the 3% dosage) for some time. An increase in acidity and a decrease in pH were observed in both the Stratmat and Selbaie rocks when all the calcite was presumably consumed. It should be noted that the relative effectiveness of the different techniques is likely to change with time, due to depletion of alkalinity or phosphate materials.

The wood bark accelerated acid production by about 60% in the laboratory and 500% outside. The role of the iron oxidizing bacteria (Thiobacillus ferrooxidans) was invoked to explain this acceleration. This was confirmed when a bactericide (0.02% thymol solution), added to the wood bark cover, reduced acid generation considerably. The iron oxidizing bacteria are mostly autotrophs (that is, they require inorganic carbon for their metabolism), and would become more active by using CO2 produced from fungal decomposition of the wood bark. Other heterotrophic iron oxidizing bacteria would use organic carbon from the wood bark for metabolism. Thus, a wood bark cover is not considered a good technique for reducing acid generation in sulphide-bearing waste mine rock. The water covered waste rock began to release low concentrations of metals (zinc, iron and lead) after two and a half years of operation. The delay in metal release may be attributed to the presence of trace amounts of alkaline minerals which were probably depleted after the initial two and a half years. The results of the study thus far indicate that, although. a water cover may not completely prevent oxidation, it will reduce acid generation considerably. In fact, when considering both feasibility and efficiency, it is the most promising ARD control technology known to the industry. The rate of oxidation is decreased in two important ways: first, the oxidation will begin much later if fresh rock is covered (two and a half years in this case), and second, the oxidation will continue at a considerably reduced rate, due to the oxygen diffusion barrier the water presents. The delay before oxidation begins is probably proportional to the neutralization potential of the rock. If oxidized waste rock is covered with a layer of water, it is likely that the alkaline materials will be depleted and that the oxidation will begin immediately.

The effectiveness of the water cover may be enhanced by increasing the depth of the water or applying an organic layer on top of the waste. With an organic layer, the oxygen may be consumed by biodegradation before it can reach the sulphides. The practical implementation of a water cover scheme presents some other questions (for example, maintaining the required depth of water and long-term stability of holding structures) which still have to be addressed through hydrological and engineering studies. Laboratory studies such as this one are also necessary to address initial uncertainties prior to implementation.

Since January 1997, Barrick Gold Corporation’s Les Terrains Auriferes (LTA) tailings site has been the subject of a detailed study MEND Project 2.22.4 aimed at assessing the performance of a composite cover placed on the acid generating tailings impoundment. The full scale composite cover was built in the winter of 1996 on the 60 hectare site. Alkaline tailings were used for the fine material layer. In addition to being the first full scale cover of this type (e.g. oxygen barrier) in Canada (and possibly elsewhere), this project offers the added advantage of using mine tailings, a waste product.

The main objective of this report is to describe the various phases of the LTA project, from the initial conceptual design of the cover to the final construction and monitoring. The theory behind composite covers and design methods have already been thoroughly discussed in the literature; the present report will, therefore, be a practical field application example of this technology.

This report will describe initial concerns, the flow charts used in the decision process, the laboratory work performed as well as the fundamental properties of all the materials used for construction. Sequence of events, construction difficulties will be presented subsequently along with monitoring instruments used. Analytical results outlined in this report will allow preliminary comparison of properties observed in laboratory and in situ.

Conclusions will be formulated and discussed to assess the applicability of this technology to other sites. A final monitoring report will be available in 1999.

This manual includes a summary volume (Volume 1) and the following four supporting technical documents:
• Volume 2 – Theory and Background;
• Volume 3 – Site Characterization and Numerical Analyses of Cover Performance;
• Volume 4 – Field Performance Monitoring and Sustainable Performance of Cover Systems; and
• Volume 5 – Case Studies.

This volume presents a compilation of case studies that have previously been presented in publications such as the CETEM manual (CANMET, 2002), the MEND manual (MEND 5.4.2d), and the INAP long-term cover performance monitoring report (INAP, 2003). Depending on the objective of the original report, the case studies in these reports were written to highlight different aspects of the cover design process. The case studies have been re-formatted from their original presentation so that each case study is presented in a similar format. This volume has been divided into four sections based on the reports from which the case studies were taken: CETEM Manual Case Studies, MEND Manual Case Studies (a section each for tailings and waste rock), and INAP Report Case Studies.

The case studies in this manual are presented in terms of the successes and design objectives achieved. However, where appropriate, considerable emphasis is also placed on identifying the “lessons learned” as a result of the work having been undertaken. The objective will be to determine whether the cover system would have been designed, constructed, or monitored differently (i.e. would an amended methodology have been utilized), if the lessons learned had been known at the start of the project.

A potential fatal cover system design flaw is simply applying a successful design from one site to a second site, when in fact material properties, slope angles, slope lengths, and in particular climate conditions are in fact much different at the second site. Hence, the key idea(s) behind presentation of the case studies is that it is the design methodology that is transferable from one site to the next, and not the actual design itself, and that the methodology should be updated constantly as new information is developed.

The manual incorporates and integrates the best available technology for the design and construction of cover systems over mine wastes. Information is garnered from a wide variety of sources, but the document is not intended as a comprehensive design manual. It is meant for use by mining personnel and others interested in cover systems. The control concepts behind the design and construction of cover systems are explained and illustrated, and the types of activities that are required in the design process are described. The manual includes a Summary volume (Volume 1) and the following four supporting technical documents: Theory and Background (Volume 2); Site Characterization and Numerical Analyses of Cover Performance (Volume 3); Field Performance Monitoring, Sustainable Performance of Cover Systems (Volume 4); and Case Studies (Volume 5).

A review of non-traditional materials used in environmental cover applications was conducted to determine which of these materials have potential for use in acid rock/acid mine drainage cover systems. The following non-traditional materials were reviewed:
• asphalt covers,
• cementitious covers (polypropylene fibre reinforced shotcrete),
• cement-stabilized coal fly ash grout (Cefill, fly ash mixtures, and geopolymers),
• synthetic liners and covers (geomembranes, spray-on membranes barriers, and geosynthetic clay liners),
• bentonite modified soil barriers (soil-bentonite mixtures, polymer modified soil, and polymer surfactants),
• mine waste (tailings and waste rock), and
• wax barriers.

The above non-traditional cover materials can all be characterized as possessing low hydraulic conductivity. A majority of the case studies reviewed were focused primarily on the nature of the hydraulic barrier (i.e. measuring saturated hydraulic conductivity). This may be adequate for landfill and other similar waste applications where water infiltration alone needs to be controlled. In AMD applications, it is also necessary to prevent the transfer (ingress) of oxygen to the underlaying waste. Also, it is desirable that the cover systems provide a medium for the development of sustainable vegetation.

Most of the non-traditional cover materials reviewed have the potential to be used as hydraulic barriers to control infiltration into acid generating waste piles. Some of the non-traditional cover materials may also be effective oxygen barriers. These materials would include bentonite modified sands, asphalt, spray-on membranes and wax. Other materials such as Cefill, geosynthetic clay liners and soil modified barriers can also be effective oxygen barriers if they are maintained in a saturated state.

The most attractive non-traditional cover materials for AMD applications are mining wastes (tailings and waste rocks) since these materials are already on site. These materials may need to be modified with bentonite or possibly fly ash to make the tailings and waste rock suitable as cover materials. The use of bentonite modified sand appears to provide one of the best combination of low permeability water barrier and oxygen ingress barrier, however, this material may be more costly than using mining wastes. Many of the remaining non-traditional cover materials have potential uses especially as a redundant hydraulic barrier in multi-layer cover systems using capillary barriers and capillary breaks.

A basic understanding of the theoretical concepts which can be applied to prevent or at least reduce acid mine drainage from tailings through the use of dry covers has been obtained. Recent research has focussed on evaluating potential barrier materials. This has produced a multitude of suggestions with regards to alternate barrier materials and the direction of future dry covers research. This study reviews dry covers theory and current research; evaluates potential materials including several which have not yet been evaluated for use in covers; and after evaluation, provides recommendations regarding further dry covers research.

The theoretical concept of dry covers is reviewed and the barriers are classified as oxygen barriers (moisture retaining), oxygen consuming, or reaction inhibiting. Potential barrier materials are identified through a technical literature search; a literature review focussed on forest product industry wastes; and a search for potential materials that may be obtained from municipal and industrial waste streams. A short list of potential materials is developed applying a two stage screening process which takes technical aspects, cost, and practical application into consideration. Potential barrier materials considered to provide broadly based (country wide) benefit and a good likelihood for success are listed. Recommendations are made regarding the future direction of dry barrier research and specific research work currently underway.

Potential materials given a high priority for future research include natural soils, modified soils, desulphurized tailings, tailings slimes, wood waste, and paper mill sludge. Materials given a lower priority for future research include peat, waxes, ashes, and the PHITO layer. Available barrier materials not requiring additional research are limestone (mixed into the tailings mass), and synthetic liners.

In 1996, the Geotechnical Research Centre at The University of Western Ontario initiated a project to review water cover research and applications. The objectives of the project were to document the design and performance of water covers used to prevent acid generation in sulphide-bearing mine waste and compile results from water cover research. Water cover sites in Canada, Norway and Sweden were reviewed. Research programs reviewed were from Canada, Norway and the United States. They include MEND-sponsored work at the CANMET Elliot Lake Laboratory and Noranda Technology Centre, Pointe-Claire, Québec, and research undertaken by former United States Bureau of Mines, Norwegian Water Research Institute and Norwegian Hydrotechnical Laboratory.

The Province of Ontario, Canada Centre for Mineral and Energy Technology (CANMET) and Brunswick Mining and Smelting Corporation provided funding for the project.

The survey of water cover sites showed that the selection of the minimum depth of water is based on maintaining saturation of tailings in the event of a drought and preventing tailings resuspension. Predictions of resuspension are based on empirical correlations with bed shear stress, bed water velocity, or the wave height to water depth ratio. These correlations were generally obtained from experiments on particles other than tailings. Estimates of bed shear stress, velocity, or wave height are calculated from empirical equations relating these parameters to wind speed and pond fetch length, using linear wave theory. There was no conclusive verification of resuspension prediction at any of the sites we reviewed.

Methods presently used to predict the quality of pond and effluent waters do not consider many biological, geochemical and physical phenomena that may significantly affect water cover performance. In general, not enough long-term field data are available to verify water quality predictions, however, at one site field data have already shown the predictions to be unconservative.

The character of the reviewed sites, in terms of site hydrology, the initial composition of the tailings, the degree of oxidation of the tailings prior to flooding, the physical layout of the site, and the treatment of the tailings before or after flooding, varies considerably. Due to the dissimilarity of the sites examined, no correlation between water depth and design performance could be made. Observations common to many sites include a dramatic increase in pH during the summer, possibly due to microfauna activity, CO2 degassing and increase in alkalinity from surface runoff, and the formation of a thin coating of orange iron oxyhydroxides and organic matter at the tailings-water interface.

In column and lysimeter experiments for both oxidized and unoxidized tailings, flooding produces a thin surface coating of brownish-orange iron oxyhydroxides which in previously unoxidized tailings marks the extent of vertical oxidation into the tailings. The finite thickness of the oxidation zone apparently occurs because of the attainment of steady-state conditions between pyrite oxidation and sulphate reduction to sulphide. With time, the growth of organic deposits at the surface occurs, which further impedes upward metal flux to the water cover. Column experiments on oxidized tailings show that flooding causes the dissolution of oxidation products and the subsequent release of metals into the water cover, often resulting in metal concentrations higher than permissible levels. The dissolution of some oxidation products, or other minerals, may be slow and contribute to high metal concentrations in the water cover for many years. The use of a sand or a peat protective cover at the tailings-water interface reduces metal fluxes into the water cover to negligible levels. However, metal flux into the water cover may resume when the absorptive capacity of the protective cover is reached. The use of a peat layer creates highly anoxic and reducing conditions in the tailings. Peat also produces acidity that can significantly increase the mobility of some metals in the tailings pore water.

The addition of lime to previously unoxidized tailings prior to flooding inhibits the acitivity of iron-oxidizing bacteria by keeping pH high. Lime addition to oxidized tailings increases pH and limits the mobility of most metals in the water cover and pore waters. Mixing the top portion of tailings with lime was found to be more effective than simply adding the lime to the surface. Traditional methods used to estimate the required amount of neutralizing material, such as acid-base accounting, were inaccurate when applied to long-term (37-week) experiments on flooded tailings.
Laboratory test results show that coarser tailings under a water cover initially generate higher amounts of acid drainage than finer tailings. This could be due to the initially faster rate of vertical oxygen transport in coarser tailings.

The effectiveness of a Biologically Supported Water Cover (BSWC) has been demonstrated in the laboratory. A BSWC involves colonizing flooded tailings with plants to facilitate organic buildup for limiting resuspension and metal flux through biological adsorption. Preliminary results suggest that its effectiveness in the field is dependent on the depth of water and wind conditions.

It is apparent from laboratory and field studies that flooding tailings is the most successful method presently known for preventing and controlling ARD. However, in many cases the water cover discharge will require treatment to meet regulatory standards, and it is not currently possible to accurately predict the required amount and length of treatment. State-of-the-art predictive contaminant models ignore many important phenomena in water covers, including secondary mineral dissolution and metal release independent of oxidation and resuspension. Some of the work reviewed hypothesized that flooding will eventually establish a diagenetic environment for some tailings minerals with accumulation of organic matter. It is, however, uncertain how long this will take to develop and whether it will occur under all conditions. Furthermore, some metals may continue to be released to the water cover in such an environment. Although water covers are a promising technology, fundamental understanding of many phenomena influencing their performance is unknown, and the minimum depth of water required cannot presently be confidently determined. The present prediction tools do not consider other important tailings properties such as thixotropy and cohesion.

We recommend that additional research and field work be initiated to address the uncertainties and issues raised in this review. The key areas for such work may include verification of resuspension predictions at a few selected sites, fundamental understanding of tailings properties and behaviour, the nature and rate of accumulation of the organic matter-iron hydroxide sediment at the tailings-water interface, and the contribution of resuspension to tailings oxidation and long-term water quality.

Methods such as subaqueous disposal are available to control and reduce acidic drainage from tailings and can be applied to many, but not all tailings management facilities. These available methods have various drawbacks and in particular high cost. There is a need for alternate methods which can reduce the cost of closure and perhaps eliminate some of the drawbacks associated with the available methods. This report reviews one such alternate method that is increasingly being considered for the closure of sulphide tailings impoundments – the application of elevated water table concepts.

The concept of using an elevated water table within tailings is of great interest as available data suggests that the rate of oxidation of sulphide tailings, in conditions where the water table approaches the surface of the tailings, is very low and similar to the rate of oxidation of sulphide tailings maintained under water cover. There are three basic approaches that can be applied to raise the elevation of the water table and the associated capillary zone within tailings:

1. Modifying the water balance of the tailings;
2. Enhancing the water retention ability of the tailings; and
3. Constructing groundwater flow barriers within the tailings.

Modification of the water balance involves increasing water input into, or decreasing water losses from, the tailings. Water retention can be improved by enhancing the physical characteristics of the tailings prior to placement – thickened tailings which provide a very high level of saturation and considerable capillary zone height represent one means of achieving this objective. The concept of a groundwater flow barrier involves the installation of a barrier within tailings to reduce the horizontal and often preferential downgradient flow of pore water within the tailings. The suitability of these approaches is dependent upon site specific conditions.

Not all tailings management facilities are well suited for the application of elevated water tables. Tailings stacks were historically designed to minimize the water level within the tailings and maintain as small a surface pond as possible. The application of elevated water table concepts to raised stacks presents a considerable challenge. The application of the concept to other tailings impoundments (e.g. valley dam impoundments) is possible and may be economically beneficial in comparison to other closure options.

Elevated water tables have been an intrinsic component of closure strategies for tailings management facilities but have only recently become proposed or applied as a principal basis of closure plans. As such, experience in the application and performance assessment of elevated water tables is recent and being accumulated. The developing state of knowledge is evidenced by the relatively limited data available regarding the application of elevated water tables to sulphide tailings impoundments. Nine sites, however, provide relevant but preliminary data:

* Five sites (the Elura mine in Australia, the Greens Creek mine in Alaska, the Cluff Lake mine in Saskatchewan, Les Mines Selbaie in Québec, and the Kidd Creek metallurgical site in Ontario) involve the ongoing or discontinued disposal of thickened tailings. These sites represent a broad range of site conditions, and tailings disposal and closure strategies.
* Three sites (the Falconbridge New Tailings site, the Dona Lake mine site, and the Stanrock mine site – all in Ontario) involve the modification of the water balance of the tailings. At the Falconbridge New Tailings site, research has been carried out to investigate the important relationships between oxygen consumption rates within tailings, the degree of saturation, and depth to the tailings water table. At the Dona Lake site, a closure plan is being implemented which relies in part on the elevation of the water table within the coarse tailings beach area. The proposed approach for the historic Stanrock tailings stack is based on replacing spigotted tailings dams with engineered water retaining structures – this would ultimately allow the water table within the tailings to rise and inhibit acid production from much of the tailings. Chemical treatment of acidic drainage from the Stanrock tailings would continue for an interim period following the construction of the new containment structures.

* The Sturgeon Lake site in Ontario involves a combination of a modified water balance and a groundwater flow barrier.

Chemical treatment of acidic drainage may be required, in some instances, after the application of an elevated water table concept. Key sources of acid production could include sulphide solids in the near surface and not sufficiently saturated zone, and fluctuations in the water table elevation. The effect of water table fluctuation could be mitigated in some cases by the use of a cover to reduce evaporative losses, increase infiltration, or to elevate the water table to the cover. The additional and interim use of chemical treatment may be cost beneficial when compared to other options.

Numerical modelling techniques are being used to predict the performance of elevated water table concepts in controlling or reducing acidic drainage. These techniques draw upon soil science and civil engineering methods of predicting water movement in soil, and can be complemented by numerical models for the prediction of acidic production in tailings. These models are important as they provide a means of preliminarily assessing the performance of a closure option, and of addressing the effect of changes in ambient conditions (e.g. drought conditions).

Preliminary and conceptual cost comparisons were carried out based on the closure of three types of tailings impoundments: a tailings stack; a valley impoundment underlain by a pervious zone; and a valley impoundment underlain by an impervious zone. Closure options included the perpetual collection and treatment of acidic drainage, the use of an engineered cover, and applications of elevated water table concepts. The first order estimates of closure costs indicate that elevated water table concepts, when suitable, can provide significant closure cost saving in comparison to collection and treatment, and the use of an engineered cap.

The Design Guide is directed at a wide range of audiences including regulatory agencies in the developed and developing world, small, intermediate and large mining companies, environmental consultants, non government organizations (NGO’s) and the general public. As some of these groups are not highly experienced in the technical issues relating to tailings systems, very broad and general coverage has been included in areas such as the geotechnical aspects, hydraulic design and risk assessment. This information is included in order to illustrate how the detailed geochemical design information can be integrated to yield a complete design package. The logic flow charts described are intended as a guide to those seeking to make a preliminary evaluation of the applicability of the subaqueous disposal system to their particular project. These charts can also be used to identify variables which can be optimized in establishing the basic configuration of the impoundment. After this prescreening, the project proponent may then retain specialized design consultants to assist in the development of the final detailed design. As a further guide, references are provided to documents that contain information on detailed design procedures and standards for such factors as risk assessments and geotechnical and hydraulic designs which are described in general terms in this Guide. Readers are encouraged to consult the references to obtain a more complete understanding of the complexities of impoundment design and construction. Where needed, appropriate consulting expertise should be retained to develop a proper and complete tailings system design.

The influence of paste backfill on operational and long-term mine and ground water quality has been identified as one of the priorities of the MEND Program. This report provides a brief summary pertaining to current practices in the geochemical characterization of both cemented and uncemented paste backfill, and methods used to predict environmental impacts to surface and ground water quality associated with the application of paste backfill in underground applications. Data was collected via a literature review, and a survey of mines known to use paste backfill.

The findings indicate that the amount of available information and research on the influence of underground paste backfill on mine water quality is typical of a relatively new field. To date, research by the community at large has focused on the structural characteristics of paste in terms of meeting the required backfill strength using the most economic amount and mix of binder materials. In light of the belief that the chemical reactivity of tailings and the volume of leachate generated are reduced by thickening, and by the addition of alkaline additives such as cement, little information on the influence of paste backfill on mine water quality appears to have been developed.

Exceptions have been where:
• the mineralogy and reactivity are extreme, with potential effects on paste strength;
• a portion of the paste is being deposited on surface (with potential surface water impacts); and,
• concern regarding potential ground water contamination from underground waste disposal in the United States led to initiation of the Underground Injection Control (UIC) Program that incidentally includes placement of mine waste backfill in underground mines under its legislation (Levens et al., 1996).

Recognition of the fact that any backfill has the potential to generate contaminant plumes in the long term, and potentially influence ground and/or surface water appears to have increased the site-specific evaluation of paste characteristics of newly proposed mines in recent years.

Despite the lack of extensive detailed study, the use of paste backfill in underground environments has been generally considered beneficial to reduce overall environmental impacts associated with mining, due to:

1. Reduction in the volume of tailings requiring surface disposal, thereby reducing surface impacts through footprint reduction;
2. Use of the full tailings stream in the backfill, rather than the coarse fraction used in more conventional sand fill, thereby reducing the need to handle and dispose of a separate slimes stream;
3. Reduction in the potential for tailings to oxidize or leach due to the nature of thickened tailings placed as underground backfill because of:
• Less free water, which reduces leachate generation;
• Less available oxygen as a result of the higher degree of saturation;
• Preferential flow of ground water around backfill, rather than through it due to the lower hydraulic conductivity of the paste backfill;
• The addition of cement that provides extra neutralization potential (NP) and decreases effective porosity; and,
• The potential for flooding at closure which reduces sulphide oxidation in long-term.

The general theories associated with paste backfill characteristics and geochemical reactivity appear sound, but there does not appear to be much field validation on the actual influence of key parameters. Lack of controlled conditions in active mine environments appears to significantly limit the ability to separately assess potential scale up issues. The field would benefit from research targeted at the specific components of paste theory (such as the separation of the influence of thickening and binder addition), examination of scale-up issues (preferably in the controlled environment of an isolated well characterized and instrumented backfilled stope), collection of detailed case studies, and additional monitoring of mine waters to assess the influence of paste backfill on mine water quality over time. The lack of detailed information currently available is of concern, and highlights the need to compile detailed site data and monitoring data for future assessment and validation of predictions currently being initiated. And as with any new field, establishing a standard base of terminology would be useful.

In the bigger picture, there may be a need to better define the potential importance of this issue, such that priorities for studying this matter can be assessed. For example, are existing backfilled mines producing significant ground water contaminant plumes? Certainly sidehill mines that continue to drain from portals or other openings are known to be potential closure problems when not suitably mitigated (i.e. Britannia Mine in B.C., Canada; Summitville Mine in Colorado, U.S.). And there appears to be sufficient information to suggest that there might be potential impacts from backfilled mines where the wall rock and backfill are particularly reactive (i.e. Bernier and Li, 2003). However, a general survey of existing underground mines might put the significance of the issue in perspective.

In 1987, City Resources (Canada) Limited initiated a program of acid generation testing as part of feasibility studies for the Cinola Gold Project located on the Queen Charlotte Islands, off the north coast of British Columbia. The program was designed to quantify acid generation potential and evaluate waste rock management options for the proposed open pit gold mine. In 1990, the Mine Environment Neutral Drainage (MEND) Program and the BC Acid Mine Drainage Task Force began funding further monitoring of tests to evaluate long term weathering conditions in five humidity cells, five leach columns and four on-site waste rock pads. The study was concluded in 1993.

The project area experiences mild damp maritime conditions. Most precipitation occurs in the winter, primarily as rain. Bedrock in the area is mantled by thin till deposits. The Cinola deposit was formed by localization of relatively low temperature hydrothermal fluids along a fault system. During mineralization, the surrounding conglomerates, sandstones, siltstones and mudstones were altered to clay minerals (kaolinite and illite, a low temperature form of muscovite) and pyrite was disseminated throughout the rock mass at concentrations typically in the range of 1 to 2%. Calcite occurs at similar or lower concentrations. Several rock types were classified as potentially acid generating based on conventional acid-base accounting.

The study yielded important data on long term weathering trends for tests conducted at different scales. Five humidity cells (four of which contained acid generating waste rock) were continued for 140 weeks and showed three distinctive stages in pH and sulphate production rate. In Stage 1, leachate pH was near 7.0 and sulphate production was slow (less than 20 mg SO4/kg/week). As readily available neutralizing minerals were removed, leachate pH decreased to less than 3 and sulphate production increased rapidly (to greater than 1000 mg SO4/kg/week). In Stage 3, sulphate production slowly decreased and leachate pH slowly increased to between 3 and 4. After more than 100 weeks of leaching, sulphate production decreased by one or two orders of magnitude and leachate continued to be acidic (leachate pH<4). Trends in waste rock pad leachate were less apparent due to annual flushing events related to late summer and fall rainfall. However, a qualitatively similar long term decrease in sulphate production and increase in leachate pH was observed. After five years, the waste pad leachate continued to be strongly acidic for three test piles and concentrations of iron, copper, zinc and arsenic were indicative of acidic weathering conditions.

The leach column study evaluated the delay in release of heavy metals and acid drainage achieved by addition of varying concentrations of crushed limestone to waste rock. Long term trends were very similar to humidity cells. Two simple relationships were defined relating limestone content (ie. neutralization potential) to (1) time elapsed until the first indications of net acid generation were observed; and (2) time elapsed until peak acid generation was observed. Release of zinc followed the first type of relationship whereas release of iron and copper followed the second relationship. The study showed that although limestone addition delayed full acid generation and release of copper and iron according to an exponential relationship, significant zinc loads may be released in pH-neutral drainage several years before acid drainage is produced.

When comparing different types of weathering tests, the study demonstrated that the small- and large-scale tests produced similar results for qualitative long term trends, sulphide oxidation reaction kinetics, and average sulphate release rates expressed as estimates of exposed particle surface areas. The test results were dissimilar for metal release rates probably due to complex mineral precipitation controls operating under widely varying solid-to-liquid ratios. These conclusions apply to the Cinola epithermal deposit where sulphide mineralogy is simple relative to complex deposits containing two or more different types of iron sulphide in a variety of forms.

It was concluded that further research is needed to address a number of issues. These include: evaluation of solid weathering products; long term kinetics of weathering in mineralogically complex materials; long term kinetics of weathering in marginally-acid generating materials; long term leachate quality for sulphide-depleted weathered waste rock; application of small-scale test results to large scale waste rock piles; and evaluation of the effect of heterogeneities in mixing of calcareous and potentially acid generating waste rock.

This review of existing information on acid-generating waste rock was based on searches of numerous computerized databases, several visual searches of libraries, and direct contacts with researchers, technical organizations, and mining companies. Relevant documents were examined for detailed information on theory, laboratory experiments, modelling, and field studies. Documents were also critically assessed to determine their validity, applicability, and weaknesses.

In order to organize and present the findings of the critical review, waste-rock piles were conceptually viewed as natural systems with transport of water, solids, and gases through the piles as well as reactions operating within the piles. The internal reactions included acid generation, bacterial activity, acid neutralization, and metal leaching. The effects of pile construction, phase transport and acid-control technologies on the system were also addressed. Finally, recommendations were offered for further studies and for decommissioning of waste-rock piles.

This review demonstrated that groundwater movement through, and beneath, waste-rock piles represents the primary pathway for contaminant migration. Consequently, detailed hydrogeologic studies of piles and their surrounding environments are primary requirements in understanding and predicting the migration of acid drainage. Furthermore, the process of acid generation was found to be highly complex and most published assessments of acid generation failed to consider this complexity.