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 SO42- 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.