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.
