Haul trucks transport the ore from open pits or underground operations to processing operations.
Some ores may be stockpiled for later processing. Rock that is not economical to mine is stored in waste rock storage areas.
The grade and type of ore determine the processing method used. Additionally, the geochemical makeup of the ore, including its hardness, sulfur content, carbon content and other minerals found within, impact the cost and methods used to extract gold.
Depending on the ore, we process it using the following methods:
We feed ore into a series of crushers and grinding mills to reduce the size of the ore particles and expose the mineral. Water is also added which turns the ore into a slurry.
We send this slurry to leaching tanks, where we add a weak cyanide solution to the slurry, which leaches gold and silver into the solution. This process removes up to 93 percent of the gold and 70 percent of the silver from the ore. Carbon granules are then added to the solution. The gold is pulled from the solution and attaches to the carbon.
We then "strip" the gold from the carbon by washing it with a caustic cyanide solution. The carbon is later recycled.
Next, we pump the gold-bearing solution through electro-winning cells, which extract metals from the solution using an electrical current.
After gold has been processed, the leftover waste material is called tailings. Tailings contain small amounts of cyanide and other hazardous chemicals, so they must be disposed of in an environmentally safe way. The tailings are stored in tailings dams, which are lined with impermeable layers. While the cyanide levels in the dam are safe, steps are taken to keep wildlife away from the dams. Over time, the chemicals break down and the solids settle to the bottom so that the water can be returned to the plant to be used in processing.
We then smelt the gold, which melts it in a furnace at about 2,100°F.
From there, the liquid gold is poured into molds, creating doré bars. Doré bars are unrefined gold bullion bars containing anywhere from 60 to 95 percent gold.
We finally send the bars to a refinery for further processing into pure gold.