Ever wondered how gold is actually retrieved from the earth? Although movies may have you believe gold is found in big nuggets on the surface or by panning in a river, this is rarely true on a commercial scale.
The process of getting gold out of the earth is a little more complicated (and costly) than that, so read on to find out exactly how gold is mined here in Australia.
Exploration
Gold exploration begins at the exploration phase. The first job is to establish if there is gold in the ground. This might be a brand-new site, or an existing resource that could be extended. A geologist will conduct in-depth investigations and analysis of the area through detailed structural mapping, sampling and geophysics to understand the target areas.
If this shows potential for gold mineralisation, exploration activities move into initial drilling. This involves drilling holes into a potential orebody, logging the rocks encountered and taking regular samples. The results of this sampling are used to determine the size and grade of a potential deposit.
If the initial drilling is successful at finding gold, much more drilling will follow until a resource can be confidently defined. This is handed over to engineers to assess what can be economically mined.
Mining
Open cut vs underground
There are two types of gold mines in Australia – open cut and underground. Open cut or open pit is a surface mining technique that extracts minerals from an open pit in the ground.
This is the most common method for mining minerals across the world as it is simpler and cheaper for mining deposits close to the surface. Most of Australia's gold comes from open-cut mines like the Super Pit in Kalgoorlie Boulder.
Giant shovels and trucks remove waste rock from above the ore body and then the ore is mined. Waste and ore are blasted to break them into sizes suitable for handling and transport to waste dumps or, in the case of the ore, to the crusher.
Underground mining is used where the depth of ore below the surface makes open-cut mining uneconomic. Vertical shafts and long, winding tunnels (Declines) are used to move people and equipment in and out of the mine, to provide ventilation and for hauling the waste rock and ore to the surface. Extensions of deposits mined by open pit methods may be mined later by underground methods beneath the old open pit.
Processing
Crushing and grinding
Now the ore is out of the ground, it needs to be processed. Once recovered from the mine, the ore is fed into a crusher at a processing plant. The huge jaws crush the rock to small pebble-sized pieces which then go to the mill. Steel balls roll around inside the mill, crushing the rock further into dust. Water is added at this point to turn the rock into slurry. The slurry is then pumped into a cyclone, which spins allowing coarser materials to fall to the bottom, before being crushed and ground again. Lighter material is pushed up and onto the next stage.
Coarser gold is recovered with a gravity circuit. Different elements settle at different rates, so a gravity circuit uses water and pulses to agitate the slurry. The gold settles faster than other minerals, so it is separated and can be collected.
Flotation and oxidisation
The flotation stage is the first opportunity to separate the gold bearing ore from waste rock. Chemicals are added to help the gold particles bond to the sulphur inside the rock. Air is then used to agitate the slurry and float the gold bonded sulphur to the top.
This mixture is then thickened and fed into an autoclave to break the bonds between the sulphur and gold – a chemical process that replicates natural oxidisation. Gold is then released as the oxygen and sulphur react under pressure.
Leaching and adsorption
The oxidised slurry then moves to the carbon leach circuit to dissolve the gold out of the rock. This is done via gravity fed tanks and chemicals like sodium cyanide. The sodium cyanide leaches the gold from the rocks turning it into a liquid. This process can take days! By the time the rock particles reach the last tank (this bit takes multiple days!) almost all of the gold is recovered.
At the same time, activated carbon (coconut husks) are pumped upstream through the tank system to catch the leached liquid gold. Once full of gold the carbon travels to a gold desorption column designed to wash the gold out of the carbon.
When this process is complete, all that is left is eluate containing hot water, gold solution, cyanide, and caustic soda. The eluate is transported to the gold room and electroplated onto cathodes wrapped in steel wool.
The steel wool is then melted in a furnace and causes the gold to sink to the bottom and remaining waste (slag) to rise to the top. As the bar is poured, the slag pours out, followed by the gold, causing the slag to overflow out of the mould, leaving behind the gold.
And finally, we have our gold bar!
A sample is taken to send to the state’s Mint to determine the quality before the gold bar is then removed and cleaned. It is weighed and numbered before being shipped to the Mint.
So, there you have it! The long and incredible process people in the gold industry use to get this precious mineral from ground to Mint.