How Gold is Mined

 

Exploration

The gold mining process begins with geologists discovering a deposit.

 

           
Exploration Drilling

After finding indications of gold, drill rigs are used to obtain samples from below the surface. These samples are analyzed for their gold content. If there is enough gold in the deposit to be produced economically, a mine is permitted and developed.

 

           
Open Pit Mining

The ore is mined in four steps: drilling, blasting, loading and hauling.

Blasthole Drilling

In the case of a surface mine, a pattern of holes is drilled in the pit and filled with explosives.

 

           
Blasting

The explosives are detonated in order to break up the ground
so it can be loaded by large shovels or front-end loaders into haul trucks.

 

           
Underground Mining

Some orebodies are more economically mined using underground methods. In this case, a tunnel called an adit or a shaft, is dug into the earth. Sort tunnels leading from the adit or shaft, called stopes, are dug to access the ore. The surface containing the ore, called a face, is drilled and loaded with explosives. Following blasting, the broken ore is loaded into trucks and taken to the surface. Once mining is completed in a particular stope, it is backfilled with a cement compound.

 

           
Ore and Waste Haulage

The haul trucks transport the ore to various areas for processing. The grade and type of ore determine the processing method used. Higher-grade ores are taken to a mill. Lower grade ores are taken to leach pads. Some ores may be stockpiled for later processing.

 

           
Ore Processing

Heap leaching

The ore is crushed or placed directly on lined leach pads where a weak cyanide solution is applied to the surface of the heap. The solution percolates down through the ore, where it leaches the gold and flows to a central collection location. All of the solution is recovered in this closed system, preventing any infiltration into the ground below.

 

           
Milling

The ore is fed into a series of grinding mills where steel balls grind the ore to a fine slurry or powder.

 

           
Oxidization

Some types of ore require further processing before gold is recovered. In this case, the slurry is pressure-oxidized in an autoclave (shown on right) before going to the leaching tanks or a dry powder is fed through a roaster in which it is oxidized using heat before being sent to the leaching tanks as a slurry.

 

           
Leaching

The slurry is thickened and run through a series of leaching tanks. The gold in the slurry adheres to carbon in the tanks.

 

           
Stripping

The carbon is then moved into a stripping vessel where the gold is removed from the carbon by pumping a hot caustic solution through the carbon. The carbon is later recycled.

 

           
Electro-winning

The gold-bearing solution is pumped through electro-winning cells or through a zinc precipitation circuit where the gold is recovered from the solution.

 

           
Smelting

The gold is then melted in a furnace at about 2,100°F and poured into molds, creating doré bars.

 

           
Gold Bullion

Doré bars are unrefined gold bullion bars containing anywhere from 60% to 95% gold.

 

           

Refining

The bars are sent to a refinery for further processing into pure gold.

 

           
Reclamation

To restore the landscape for future uses such as ranching, recreation or wildlife habitat protection, reclamation, including revegetation, is implemented concurrently during mining.