New equipment to cut mine's cyanide use

Posted: Monday, August 12, 2002

FAIRBANKS - New equipment at the Fort Knox gold mine is expected to cut cyanide use by 25 percent and enhance gold recovery.

The $5.5 million thickener tank is expected to pay for itself in two years from gold production and savings on chemicals used in its gold recovery processes.

Fort Knox uses 1,600 tons of cyanide a year. The addition would reduce that amount to 1,200 tons.

Cyanide is used to leach gold from crushed precious-metal-bearing rock.

The idea for the thickener tank came from a Fairbanks Gold Mining Inc. engineer who wondered if temperature made a difference in gold recovery. Fairbanks Gold, a subsidiary of Kinross Gold Corp., operates Fort Knox.

After studying the company's operation for two years, it turned out that temperature did matter, said Clyde Gillespie, Fairbanks Gold senior environmental engineer.

The tank will allow reinjection of cyanide-laced hot water into the gold-refining system.

Here's how it works:

Fairbanks Gold mines hard rock containing microscopic bits of gold from its Fort Knox and True North deposits, both located about 30 miles north of Fairbanks. Currently, that rock is sent through a crusher and then a grinding mill until it is a fine sand.

Water is added during the process and the resulting slurry is sent to a thickener tank where the water is reduced. The sandy ore mixture is sent through a series of huge tanks where cyanide is added to leach gold from the grains of sand and then carbon is added to strip out the gold particles.

Once the pale-yellow gold is recovered - about 1,200 ounces a day - the leftover mixture is treated to "kill" the cyanide. The water is siphoned into a holding pond and the sand is trucked to a disposal area.

The water in the holding pond, no longer hot and containing trace amounts of cyanide, is reused in the refining process.

The new thickening tank will change the operation slightly, but significantly, Gillespie said.

Instead of sending the gold-stripped slurry through the cyanide "killing" process, the mixture will be sent to the new thickening tank.

There the water, hot and cyanide-laced, will be drained off the slurry and put directly back into the gold-recovery system. The leftover slurry will be treated to "kill" the cyanide. Holding pond water will still be available as needed to make up the difference in the process.

This will help in two ways. The recycled water will not require as much energy to reheat and the hot water is more effective in the gold recovery process.

"For every 10 percent increase of temperature, there is a 1 percent increase in gold recovery," Gillespie told the Fairbanks Daily News-Miner.

The 1 million gallon tank will be enclosed in a steel building and will require one operator at the controls.

Construction is expected to be completed by October.

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