Flowsheet A YELLOWCAKE RECIPE

After crushing and grinding come leaching and counter-current decantation (ccd), solvent extraction, yellowcake precipitation, ammonium- sulphate crystallization, and bulk neutralization and tailings disposal.

The design criteria for the mill were as follows: Throughput 710 tonnes/day Average ore grade 2.4% U3O8

2.3% Ni

1.3% As Leach extraction 99% Overall recovery 97% Annual U3O8 output 5.45 million kg Leaching and CCD

A 2-stage leaching circuit (one an atmospheric leach, the other achieved in autoclaves), followed by a ccd circuit running at high acidity, was selected. This prevented uranyl and ferric arsenate precipitation and ensured a 99% leach extraction. Secondary leaching is done with in 100-g-per-l sulphuric acid using as many as 10 autoclaves at 70 degrees c and 550 kpa. Oxygen is used as the oxidant in the autoclaves. This process was used because of the high sulphide content of the ore.

Eight 20-m-diameter countercurrent decantation thickeners wash the pregnant solution from the barren wastes. Solids travel to the No. 8 from the No. 1 thickener, with the underflow from the last thickener discarded after treatment. Wash water, which is contaminated mine water added at the No. 8 thickener, and uranium- bearing solution is transformed through the circuit in the opposite direction to the solids Solvent Extraction

This circuit was the first in North America to feature Krebs mixer-settler units. Incorporating the Krebs units reduced the building size by one-third. Thirteen of these units (four extraction, three acid scrub, four stripping, one ammonia and one regeneration) are installed. Pregnant solution at 6 to 8 g per l U3O8 is treated in a reactor/ clarifier and the solution is contacted with organics comprising 91% kerosene, 6% amine and 3% isodecanol. To remove arsenic, the loaded organic passes through acid scrub mixers. Finally, it is stripped to remove uranium from the organic and a loaded strip solution at 40 to 50 g per l U3O8 is pumped to yellowcake precipitation. Yellowcake Precipitation

Gaseous ammonia mixed with air is injected into one precipitation vessel. Dilute yellowcake slurry overflows to a 14-m-diameter thickener. Thickener underflow is centrifuged and washed in stage and the washed yellowcake is fed to a propane-fired, 6-hearth calciner. The calcined yellowcake is loaded into 200-l drums for shipment to refiners. Ammonium-Sulphate Crystallization

Stripping and precipitating uranium with ammonia generates ammonium sulphate. This must be continually removed. After initial study, planners decided that crystallized ammonium sulphate could be produced for sale to the Canadian fertilizer market. A 4-effect evaporator/crystallizer plant was installed for this purpose. Vapor condensate from the plant could be controlled to fewer than 300 parts per million ammonia, allowing its use after acidification as arsenic scrub solution in solvent extraction.

All waste streams are treated in the bulk neutralization/radium removal plant to meet federal and provincial effluent guidelines. Oxygen for mill use is produced in a 35-tonne-per-day cryogenic plant while an acid plant produces the mill’s sulphuric acid requirements of some 36,000 tonnes per year. Compressed air to the mill is provided by Atlas Copco compressors.

The Key Lake mill is highly computerized. A Fisher provox microprocessor-based, distributed control system is used for control panel instrumentation. Motor control functions are performed by Modicon programmable “logic controllers.” The plant is serviced by six control rooms (one each for crushing/grinding, leaching/ ccd, solvent extraction, yellowcake precipitation/ammonium sulphate crystallization, effluent treatment, and powerhouse/acid plant. Each control room contains two provox consoles linked to a common data highway. The entire plant can be operated by the two plant control stations in the leach plant control room. To boost process efficiency and reduce manpower, 2,400 loops were installed plant-wide. Tailings Treatment An environmental impact statement, prepared in October, 1979, was approved by the Saskatchewan government (after extensive publ ic hearings) in 1981. Besides being time- consuming, the environmental approval process entailed costly treatment facilities — $100 million to be precise.

Disposal into the tailings dam involves a technique called sub-aerial tailings deposition. This sub-aerial technique allows for quick solidification of the tailings. The 600×600-m pond is sealed with a 20-cm-thick bentonite layer. Bentonite, a mineral mined in southern Saskatchewan, expands on contact with water to form an impermeable seal. Above the bentonite, a drainage system sits in filter sand. The pond is sloped to provide proper drainage. Excess water returns to the mill for treatment. Any water discharged into the environment must meet regulatory standards.

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