August 2011

The evolution of an operation

Tracing the history of uranium milling at Rabbit Lake

By Chuck Edwards

Rabbit Lake, owned by Cameco Corporation, is the longest producing uranium operation in Saskatchewan | Photo courtesy of Cameco Corporation

The Athabasca Basin in northern Saskatchewan has been one of the world’s premier uranium camps, yielding more than 700 million pounds of uranium oxide since milling operations first began in the area in 1975. It occupies nearly one-third of the surface area of the Saskatchewan Shield and famously hosts many high-grade uranium deposits.

The first uranium mill was Rabbit Lake, which came into operation on June 10, 1975. The mill was operated by Gulf Minerals (Canada), which made several modifications during the first years of operation to solve problems and to increase production. Further changes were made by Eldorado Nuclear, which acquired Gulf Minerals (Canada) in 1982. 

Initial refinements

The autogenous (AG) mill in which the ore was crushed and ground was quickly converted to semi-autogenous grinding (SAG). As well, soon after startup, the urethane liners failed throughout the acid section of the mill, including the tailings line. Linings in the counter current decantation (CCD) thickeners, solvent extraction (SX) mixer-settlers and associated tanks were replaced with fiberglass, while the tailings line was replaced with high-density polyethylene piping.

The CCD underflow diaphragm pumps were a continual problem. The pumping system was first recommissioned with hydraulic drives, but eventually the diaphragm pumps were replaced with centrifugal pumps. There were also chronic difficulties with ore crushing. Eventually, crushing was abandoned altogether and the ore was instead broken with a hydraulic breaker and passed through a grizzly feeder.

Modifications were made in 1977 to control the radium in the effluent, through barium chloride precipitation and pond settling of the radium/barium sulphate precipitates. Pressure sand filters were later installed when pond settling failed to produce a solid-free effluent.

By the early 1980s, the Rabbit Lake mill was a successful, smooth-running operation. The startup mill feed rate was 60 tph (1,600 tpd), but by 1981, the average milling rate had increased approximately 28 per cent to 77 tph (2,040 tpd).

Adjusting to a new ore

The Rabbit Lake ore body was scheduled to be mined out by May 1984. Stockpiled ore would continue to provide mill feed until late 1985, at which point the mill feed would switch to the much “dirtier” (higher arsenic content) B-zone ore. The initial ore was “clean” (lower arsenic content) in comparison; however, even with this clean ore there was considerable crud build-up in the mill’s SX mixer-settlers.

Rabbit Lake used ammonia for SX stripping. It ran tests with the B-zone ore using the same process of ammonia stripping, and found that crud formation was much higher. In addition, Rabbit Lake was now required by regulators to reduce the ammonia in its effluent by 97 per cent, from approximately 500 mg/L to a maximum of 15 mg/L. Either a process to remove ammonia from the effluent would have to be installed, or they would have to change to a completely ammonia-free process.

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