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.