How gold is made

In most gold mines the gold is invisible, and a ton of ore may hold only a few grams of it.

stations: 6
things upstream: 11
raw sources: 7
steps from the ground: 3

The short version

Gold is dug from rock that holds only a few parts per million of it. The ore is ground to a slurry and mixed with a dilute cyanide solution, which dissolves the gold. Activated carbon or zinc dust pulls the gold back out of the liquid, and furnaces melt it into bars that are refined to high purity.

Dig the ore

Open pits typically yield 1 to 5 parts per million of gold, and underground mines at least 3. Ore as low as 0.5 parts per million can pay. Gold becomes visible to the naked eye only at around 30 parts per million.

Goes in
Gold ore
Grind it to a slurry

Grinding mills reduce the rock to fine particles, and water is added to turn it into a pulp. Some ores are first concentrated by froth flotation or by gravity.

Goes in
Water
Dissolve the gold in cyanide

Sodium cyanide solution is mixed into the pulp, and with oxygen present the gold dissolves as a soluble complex. Lime holds the slurry strongly alkaline, at a pH above 10.5, so that the cyanide stays in solution rather than escaping as gas.

Goes in
Quicklime
Capture it on carbon or zinc

Gold is pulled out of the liquid by adsorbing it onto activated carbon. In the older Merrill-Crowe process, zinc dust and a vacuum precipitate it instead. What is left of the cyanide in the waste stream is treated to break it down.

Goes in
Zinc
Melt it into bars

The recovered gold is smelted in a furnace and poured into rough bars, which still hold some other metals.
Refine it to purity

Refiners use the Miller process, where chlorine gas is bubbled through the melt, or the Wohlwill process, which uses electrolysis and gives higher purity. Bullion coins can be 99.99 percent gold.

Goes in
Chlorine

End of the lineGold Next it goes into microchip, led, sewing needle and 6 more.

Worth repeating

A single gram of gold can be beaten into a sheet of 1 square meter.
Around 201,296 tonnes of gold exist above ground, which would fit into a cube 21.7 meters on a side.
Recycling a kilogram of gold produces 53 kilograms of CO2 equivalent, while mining a kilogram produces 16 tonnes.