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Mining News: Fort Knox heap leach yields first gold Kinross Gold Corp. loaded tons of lower-grade ore onto a 70-acre pad in August; poured first bars of yellow metal in November Shane Lasley Mining News
About 400 ounces, or 11.4 kilograms, of molten gold poured into bars at Kinross Gold Corp.’s Fort Knox Mine on Nov. 11 marked the first of the precious metal produced from the Walter Creek Valley Heap Leach Facility at the Interior Alaska gold mine.
Startup of the heap-leach facility is expected to increase Fort Knox annual production to average 370,000 gold ounces, or about 10,489 kilograms.
Fairbanks Gold Mining Inc., the Kinross affiliate that operates Fort Knox, has been working on the heap-leach processing since early in 2008. While construction of the heap-leach pad is an on-going process, crews completed construction to a stage that allowed the company to begin loading low-grade ore onto the pad in early August.
Ore stacked on the pad consists of run-of-mine rock from the Fort Knox Pit as well as ore from the Barnes Creek and Fish Creek stockpiles, which currently contain 29 million short tons of lower grade ore ready to be processed.
When completed, the heap-leach pad will cover 310 acres of the Walter Creek Valley and have the capacity to hold 161.2 million short tons of ore. The construction is being carried out in five stages, each stage increasing the size and capacity of the pad.
With the first stage complete, the pad area covers 70 acres and can hold 13.7 million short tons of material.
Milestones reached Johnny Blizzard, technical services manager for Kinross, told an audience at the Alaska Miners Association 2009 Annual Convention Nov. 6 that the mine reached a major milestone Oct. 27 when ore was loaded to the 1,640-foot level.
“The 1,640 is important to us, because once we get it done, there is no reason why we cannot flood it to the ultimate flood elevation of the ‘1,635,’ ” Blizzard explained.
To reach the 1,640 level, three 30-foot lifts were stacked on the heap-leach pad. The top of the pile will reach the 1,750-foot elevation when all 13.7 million short tons of stage-one ore is placed on the pad.
As the company closed in on having ore stacked to the 1640 elevation, it began flooding the pad area with cyanide solution.
Blizzard explained that a small amount of cyanide is added to water. As this solution runs through the low-grade ore, it dissolves the gold. The solution is then pumped through a recovery circuit where the dissolved gold attaches itself to carbon. Once loaded with gold, the carbon is run through the mill for further processing.
The gold-dissolving solution began being dripped over the low-grade ore on Oct. 14, and on Oct. 30 the pumps were turned on to carry the gold-bearing liquid to the carbon-in-columns recovery circuit.
Blizzard details construction Construction of the heap-leach facility began in the spring of 2008, but due to an uncommonly wet summer, construction crews were unable to advance the project as far as planned. Though the crews at Fort Knox are not completely opposed to working in the rain, maintaining optimum moisture content to meet soil compaction requirements during the wet summer proved to be difficult.
Blizzard described an enormous earthmoving endeavor to the miners.
He said the entire pad area is double-lined with multiple engineered layers that allow the leaching process to function properly, even in the extreme cold of Interior Alaska, while ensuring the cyanide solution remains contained.
Blizzard said that a 16-inch-layer of screened and compacted material is laid down under the entire pad area and covered by an impermeable geomembrane liner, providing a secondary containment system.
A piping system and three feet of clean gravel is placed on top of the secondary-liner to allow the solution to quickly flow to a sump in the event that the primary-liner was compromised. If solution should be detected in the secondary area, it would be pumped from the sump to the top of the pad.
He said a layer of low-permeability, compacted material is placed on top of the secondary drain gravel to provide a cushion for the primary-liner, which consists of an impermeable geomembrane covered with a woven geotextile material to provide additional protection.
To capture the gold-bearing solution, a piping system and three feet of mill-reject material (two-inch-minus material from the mill) is laid on top of the primary-liner. This allows rapid flow of the (gold-bearing) solution to a sump area where it is pumped to a carbon-in-columns recovery plant where the gold is captured.
When asked about the heap-leach facility’s ability to operate without freezing during the brutal cold of Interior Alaska winters, Blizzard quipped, “That’s the million-dollar question. We are confident this thing is going to run. We know how to make it run. The details around that though, are proprietary. You gotta pay to play.”
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