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.
