Cold temperature column leaching tests at 2 °C and 10 °C were conducted for Cullaton Lake B and Shear (S) – Zones tailings to evaluate their oxidation and leaching characteristics at low ambient temperatures. The tests were an extension of an earlier room temperature study at 25 °C (Phase 1) in which both B and S – Zones tailings produced acidic drainage under laboratory simulation conditions (Davé, 1992).

The low temperature leaching studies were designed to simulate conditions similar to those expected within the tailings at the Cullaton Lake site after implementation of a decommissioning plan of covering the tailings with approximately 1.5 m of wasterock and overburden for promoting freezing and permafrost conditions within
the tailings. It was anticipated that during the frost free period some of the tailings might thaw to temperatures ranging between 0 °C and 10 °C.

These tests were conducted in clear acrylic columns, 12.5 cm in diameter and 80 cm high. Duplicate columns were filled with well mixed tailings from each zone, inoculated with a solution containing Thiobacillus ferrooxidans culture and placed in a walk-in refrigerated experimental cold chamber. The tailings were allowed to weather under unsaturated conditions at the desired cold temperatures of 2 °C and 10 °C, and leached by rinsing with well aerated natural lake water chilled to the same experimental temperature.

The leaching scheme consisted of batch additions of 1 l (litre) natural lake water, once every one or two weeks, to each experimental column undergoing weathering. The columns were allowed to drain freely during the intermediate period. Effluent was collected and analyzed for pH, redox potential (Eh), electrical conductance (Ec), total acidity, total alkalinity, and dissolved concentrations of SO42-, total Fe, Ca, Mg, Al, Mn, Sb, As, Cu, Ni, Zn, Pb, Hg, Si, and CN1-. The tailings were first weathered and leached at 2 °C for two years, and then at 10 °C
for another year. For starting the leaching experiment at 10 °C, the experimental chamber and tailings were warmed to the desired temperature, individual tailings homogenized and the columns recharged. The column leaching results are as follows:

Oxidation and acid generation occurred in both B and S – Zones tailings at low (2 °C) and intermediate (10 °C) temperatures. The rate of acid generation was low and the occurrence of acidic drainage was delayed at these temperatures compared to those observed at 25 °C in the previous study by Davé (1992).

The B – Zone tailings contained relatively high sulphide (2.31% as S) and high total available alkalinity (45.36 kg CaCO3/tonne tailings), and had a large negative net neutralization potential (NNP) of -26.84 kg CaCO3/tonne tailings. The acid generation rate in the tailings at 2°C was low such that a reasonable degree of acid neutralization was achieved and acidic drainage prevented during most of the leaching period at this temperature.

Acidic drainage occurred near the end of the leaching period at 2 °C but the overall impact in terms of total acidity and effluent metal loading was low.

Near the end of the two year leaching at 2 °C, the drainage effluent was characterized by a pH of ~ 6.6, acidity ~ 300 mg CaCO3 /l, and concentrations of SO42- at ~ 2,000 mg/l, Fe ~ 150 mg/l, Ca ~ 500 mg/l, Mg ~ 100 mg/l, Al ~ 0.15 mg/l, and Mn ~ 20 mg/l. Trace amounts of As ~ 0.1 mg/l, Ni ~ 0.1 mg/l, and Pb ~ 0.5 mg/l were also
observed. Cu, Sb, Hg, and Zn levels were below detection limits.

A cumulative total of 21% of the total sulphur contained in the B – Zone tailings was mobilized and removed as total sulphate in the effluent during the two year leaching period at 2 °C. Approximately 10% of this total sulphur was present in the soluble form which leached rapidly, within two months, at the beginning of the experiment. The remaining ~ 11% of the total sulphur was removed as a result of acid generation and neutralization at 2 °C.

The magnitude of acidic drainage initially increased for B – Zone tailings during leaching at 10°C. Concentration peaks for acidity, iron and some metals were observed after approximately two months of leaching. The effluent was characterized by a pH of ~ 5.0, moderate acidity ~ 850 mg CaCO3/l, and concentrations of SO42- at ~ 3,500 mg/l, Fe ~ 450 mg/l, Ca ~ 500 mg/l, Mg ~ 100 mg/l, Al ~ 0.2 mg/l, and Mn ~ 22 mg/l. Low concentrations of As ~ 0.2 mg/l, Ni ~ 0.15 mg/l, Pb ~ 0.5 mg/l, and Sb ~ 0.05 mg/l, were also observed, but Cu, Hg, and Zn levels were below detection. After peaking, the magnitude of the acidic drainage gradually decreased with time to a low level in the last six months of leaching.

A cumulative total of 5.3% of the total sulphur contained in the B – Zone tailings was further mobilized and released during one year of leaching at 10 °C.

In contrast to the room temperature (25°C) leaching of the B – Zone tailings (Davé, 1992), the above results indicated very slow oxidation at 2°C and an initial slight to moderate oxidation during leaching at 10°C. The overall impact of acidic drainage from B – Zone tailings was low, in the short term, at colder temperatures due to complete acid neutralization. The tailings retained appreciable amounts of moisture (85-100% pore volume saturation) during laboratory leaching, which further limited oxidation and acidification, thereby controlling acidic drainage and reducing its impact.

The S – Zone tailings contained relatively low sulphide (0.4% as S) and low total available alkalinity (2.0 kg CaCO3/tonne tailings), and had a moderate negative net neutralization potential (NNP) of -10.5 kg CaCO3/tonne tailings. The acid generation rate in these tailings was also low at 2 °C, but because of low available alkalinity there was insufficient acid neutralization and acidic drainage occurred early in the 2 °C leaching period.

During the middle of the two year leaching period at 2 °C, acid drainage peaked where effluent was characterized with a pH of ~ 3.0, moderate acidity ~ 600 mg CaCO3/l, and dissolved concentrations of SO42- at ~ 700 mg/l, Fe ~ 175 mg/l, Al ~ 25 mg/l, Mn ~ 30 mg/l, As ~ 0.4 mg/l, Cu ~1.2 mg/l, Ni ~ 1.0 mg/l, Zn ~0.4 mg/l, Pb ~ 2.0 mg/l,
and Si ~ 50 mg/l. Effluent concentrations of Ca and Mg were low at 30 and 10 mg/l respectively. Sb was present at trace levels, ~ 0.03 mg/l, and Hg levels were below detection.

A cumulative total of 42% of the total sulphur contained in the S – Zone tailings was mobilized and released during the two year leaching at 2 °C. The tailings contained approximately 25% of the total sulphur in the soluble form which was quickly removed within first two months of leaching at 2 °C. Further leaching at 2 °C resulted in an additional mobilization and removal of sulphate in the amount equivalent to 17% of the total sulphur contained in the tailings.

With the raising of the leaching temperature to 10 °C, the acidic drainage continued and increased in magnitude for S – Zone tailings. Similar to B – Zone tailings, effluent concentration peaks for acidity, iron and metal loading were also observed initially during leaching at 10°C. The effluent was characterized by a pH of ~ 3.0, moderate
acidity of ~ 650 mg CaCO3/l, and dissolved concentrations of SO4 2- at ~ 1,500 mg/l, Fe ~ 225 mg/l, Ca ~ 500 mg/l, Mg ~ 100 mg/l, Al ~ 25 mg/l, Mn ~ 30 mg/l, As ~ 0.2 mg/l, Cu ~ 1.2 mg/l, Ni ~ 1.5 mg/l, Zn ~ 1.0 mg/l, Pb ~ 2.0 mg/l, and Si at 50 mg/l. As was the case for 2°C leaching, the effluent concentrations of Sb were at trace levels, ~ 0.06 mg/l, and Hg concentrations were below detection. The magnitude of the acid drainage, after peaking, also decreased slowly with time to a low level in the last six months of leaching.

A cumulative total of 13% of the total sulphur contained in the S-Zone tailings was further mobilized and released during one year of leaching at 10 °C.

The S – Zone tailings were also characterized by a high moisture retention and poor drainage that limited the oxidation, acidification and overall impact to the well drained upper layer.

In contrast to what was seen with B – Zone tailings, no significant reduction in acidic drainage from S – Zone tailings was observed during leaching at lower temperatures from that of room temperature (25°C) in the earlier study, reflecting the low total available alkalinity and inadequate neutralization in S – Zone tailings.

It is recommended that the data should be further examined and analyzed to obtain acid generation and metal loading rates at the three study temperatures (25°C, 10°C, and 2°C). Quantitative microbiological studies should also be undertaken to distinguish between (and measure) chemical and biotic oxidation at low temperatures.

The tailings facility at the Cullaton Lake site should also be assessed for its current physical, chemical, mineralogical and biological status by undertaking suitable field studies.