This report describes the results of laboratory column leaching studies that were conducted as a part
of a broad research program titled “Development of Wet Barriers on Pyritic Uranium Tailings for
Controlling Acid Generation”. The research was undertaken jointly by CANMET, Elliot Lake
Laboratory and Rio Algom Limited, under the Canadian Mine Environmental Neutral Drainage
(MEND) Program.
Purpose:
Column leaching studies were conducted for evaluating the oxidation and leaching characteristics of
pyritic uranium tailings with and without limestone amendments and crushed waste rock, under
unsaturated and submerged conditions. These experiments were designed to determine the acid
generation characteristics and subsequent metal and radionuclide releases of test samples subjected
to submersion underwater and exposed conditions.
Methods:
The leaching experiments were conducted using PVC cylindrical column lysimeters filled with total
mill tailings, coarse tailings, tailings amended with 7.5% (w/w) limestone of varying screen sizes, and
crushed waste rock samples obtained from Quirke Mine. The experiments provided unsaturated
conditions for leaching of test samples as well as a submerged condition for coarse tailings where a
shallow water cover, 0.45 m in depth, was established using distilled water. The submerged coarse
tailings were also rinsed rapidly during the initial leaching period to remove gypsum and other soluble
minerals for studying oxidation and leaching characteristics of such tailings in the absence of gypsum
controlled sulphate solubility.
All tests were conducted at laboratory room temperature (22 ºC) and light conditions, which aided in
enhancing the chemical reactivity of test materials in laboratory simulations compared to field
conditions.
Duration of the Study:
The experiments, conducted at the CANMET, Elliot Lake Laboratory, were started in April 1989 and
concluded in June 1993.
Results:
Coarse Tailings:
Unsaturated Condition:
- The unsaturated coarse tailings without limestone amendment oxidized
readily and produced highly acidic drainage (pH ~ 1.0 – 2.0 and acidity
~ 12,000 – 30,000 mg CaCO3/l); - The coarse tailings amended with coarse limestone (screen size -6.3 mm)
also produced highly acidic drainage (pH ~ 2.5 – 3.0 and acidity ~ 4,000 –
12,000 mg CaCO3/l), but its onset was delayed by approximately one
year in comparison to coarse tailings without limestone. The acid drainage
occurred with 90% of the available alkalinity still remaining in the sample; - The coarse tailings amended with pulverized wet ground limestone did not
produce any acidic drainage during the entire four year study period.
Oxidation and acid generation processes were active in these tailings as
well, but the fine grained limestone provided complete acid neutralization
and acidic drainage was prevented during the study period.
Submerged Condition:
- The submerged coarse tailings did not produce any porewater acidic
drainage for the first three years, but in the fourth year a weak acidic
drainage (pH ~ 5.5 – 7.0 and acidity ~ 10 -20 mg CaCO3/l) was observed - In comparison to unsaturated coarse tailings, submersion of tailings
underwater decreased cumulative porewater acidity, sulphate and iron
loadings by factors of 295, 200 and 950 respectively; - The submersion of coarse tailings also resulted in increased mobility and
release of Ra-226 in the porewater when tailings developed acidic
conditions at the surface after they were depleted of available alkalinity
and gypsum. There was a 10 fold increase in the cumulative loading of
Ra-226 in the porewater over that of unsaturated coarse tailings; - The increased drainage of Ra-226 in the porewater was also
accompanied by that of iron, both occurring near the end of the fourth
year, where as gypsum was completely removed from the tailings during
the first three months of leaching; - The surface water above submerged tailings contained low concentrations
of dissolved metals, when the water cover above the tailings was
continuously maintained in a well oxygenated condition by fresh water
inflow and where both surface and pore water flows were present in equal
proportions. The dissolved Ra-226 concentration in the surface water was
also low, ranging between 30 and 140 mBq/l.
Total Mill Tailings:
Unsaturated Condition:
- The total mill tailings, in all test conditions, drained poorly and retained
appreciable amounts of moisture which controlled their oxidation and acid
generation rates; - The unsaturated total mill tailings without limestone amendment also
produced acid drainage, but it was moderate and less acidic (pH ~ 2.0 –
3.0 and acidity ~ 500 – 1500 mg CaCO3/l) than for coarse tailings under
similar conditions; - The total mill tailings amended with various screen-sized limestone did not
produce acid drainages during the entire study period. The tailings were
oxidizing slowly but the added limestone was actively consumed in
providing complete acid neutralization in all amended samples.
Waste Rock:
Unsaturated Conditions:
- The unsaturated crushed waste rock having a high acid generation
potential (NNP = -17.8 kg CaCO3/tonne) also oxidized and started to
produce highly acidic drainage ( pH ~ 2.0 – 2.5 and acidity ~ 7,000 –
20,000 mg CaCO3/l) in the fourth year of the study period; - The unsaturated crushed waste rock having medium (NNP = -10.7 kg
CaCO3/tonne) and low (NNP = +13.8 kg CaCO3/tonne) acid generation
potentials did not produce acidic drainage during the entire study period.