MEND Report 2.25.3
September 1997

EXECUTIVE SUMMARY

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 (2.0x10^-5 cm/s), desulphurized tailings (3.5x10^-5 cm/s) and LSSS (3x10^-3 cm/s). However, the hydraulic conductivity was decreased by one order of magnitude at the surface of the compost cover (from 2.1x10^-1 to 2.6x10^-2 cm/s) and the peat cover (from 6.8x10^-2 to 4.8x10^-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.

Last Modified: 2003-11-26		Important Notices

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