Haul trucks transport the ore from open pits or underground to processing operations. Some ore may be stockpiled for later processing. Rock that is not economical to process is stored in overburden rock storage areas.
Newmont uses two ore processing techniques to extract gold: mill processing and heap leaching. 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 it, impacts the cost and methods used to extract gold.
Heap leaching is used when the ore contains a lower grade of mineral content. The basic process is as follows:
- Low grade ore can be dumped directly on a leach pad (this method is called run of mine) or can be crushed and stacked on top of slightly sloped ground that has been lined with an impermeable plastic.
- A leaching solvent, commonly a weak cyanide solution, is then applied to the surface of the ore heap using drip irrigation.
- As the solvent percolates through the ore heap, the precious metals dissolve into the solution and travel to storage ponds at the base of the leach pad – a process that can take upwards of two months.
- Once collected, this gold-bearing solution is pumped to process facilities where the gold is extracted using a process known as carbon stripping or collection on carbon.
- Cyanide levels are readjusted in the leftover or barren solution so it can be recycled back into the leach cycle.
Mill processing is used when the ore contains a higher grade of mineral content. The basic process is as follows:
- 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 recovers 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 attaches to the carbon and is pulled from the solution.
- 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 electrowinning cells, in a process that uses an electric current to recover metals from the solution.
- After the ore has been processed and gold extracted, the leftover waste material, called tailings, contains small amounts of cyanide and other chemicals, so must be disposed of in an environmentally safe way. The tailings are stored in tailings dams, which are lined with impermeable layers. Although the cyanide levels in the dams are safe, steps are taken to keep wildlife away from the dams. Over time, the chemicals break down, the solids settle to the bottom and the water can be returned to the plant to be reused in processing.
- We then smelt the gold, melting it in a furnace at about 1,200 degrees Fahrenheit.
- 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.
- Finally, we send the bars to a refinery for further processing into pure gold.