Process and recovery The Key Lake mill uses a seven-step process:
- blend McArthur River ore with low grade mineralized material to lower the grade;
- dissolve the uranium using a leaching circuit
- clarify the uranium in solution using a counter current decantation circuit;
- concentrate it using a solvent extraction circuit;
- precipitate it with ammonia;
- thicken, dewater and dry/roast it in a calcining kiln;
- package it as 98% U3O8 (yellowcake).

High grade McArthur River ore slurry arriving at the Key Lake receiving plant is unloaded from the truck mounted containers by a vacuum aspiration system and pumped to one of four large air agitated slurry storage pachuca tanks. High grade slurry is pumped from a pachuca to the blending tank located in the crushing and grinding plant. There it is mixed with low grade slurry prepared by grinding gravity concentrated mineralized waste hauled from McArthur River or left over from the original Key Lake mining operations. The resulting slurry, blended to a target of 4% U3O8, is pumped to one of three storage pachuca tanks located in the leaching plant. Blending is necessary because the original Key Lake processing facilities were not designed, from a radiation protection perspective, to accommodate the high ore grades found at McArthur River. In addition to reducing the radiation exposure in the mill, the dilution of the high grade ore serves two other purposes: recovery of uranium from the low grade material; and final disposal of the low grade waste.

Sulphuric acid produced on site by burning/converting sulphur is used to leach uranium, along with various impurities, from the host ore in a single stage atmospheric leach circuit. At the time of this report, the existing high pressure (540 kPa) autoclave secondary leaching circuit is on stand-by as the current ore being milled is amenable to leaching at atmospheric pressure. The slurry is heated to a target temperature of 60° C and nearly pure oxygen, produced on site, is injected into the leach vessels to oxidize the uranium minerals and thereby permit uranium dissolution. Approximately 99.5% of the uranium and varying percentages of the impurities contained in the ore enter the leach solution during leaching.

Counter current decantation (CCD) consists of eight thickeners and a clarifier located outdoors on the mill terrace beside the leach plant. Acidic water is introduced at the tail end of the circuit and advanced from thickener to thickener in the opposite direction to the leached solids flow. The result is that the dissolved components are washed away from the leached solids. The washed leach residue is sent to the Deilmann TMF for neutralization and disposal. Pregnant solution containing 10 to 15 g/L dissolved uranium and varying levels of impurities is clarified to remove residual solids and pumped through sand filters to the solvent extraction plant.

In the solvent extraction plant, the filtered pregnant solution is mixed with an organic solvent consisting of isodecanol and amine dissolved in kerosene. The uranium transfers from the aqueous solution to the organic phase leaving behind most of the dissolved impurities. The waste solution (raffinate) containing the impurities is pumped to the bulk neutralization plant for treatment and disposal. The organic solvent, loaded with uranium, is contacted with ammonia in ammonium sulphate solution, causing the uranium to transfer back to a highly concentrated aqueous phase known as loaded strip solution. Special treatment circuits are available to deal with problem impurity elements such as arsenic, molybdenum and selenium that tend to follow the uranium through the solvent extraction process.

Using ammonia, uranium is precipitated from loaded strip solution in the yellowcake plant as ammonium diuranate. The precipitate is dewatered in a thickener followed by a centrifuge then calcined to U3O8 in a multi hearth furnace at 840 oC. The final calcined product is packed in 200 litre drums for shipment to refineries around the world. Excess ammonium sulphate is recovered in a crystallization circuit by evaporating the water and drying the resulting by-product, which is sold locally for use as a high purity fertilizer.