How To Extract Gold

(via The National Geographic Magazine)

Some 2½ tons of rock must be processed to produce an ounce of gold and a sliver of silver.

1. Like a mighty nutcracker, steel jaws shatter ore into softball-size fragments. Hand sorters discard pieces lacking gold.

2. After further crushing, the ore mixes with water and enters a revolving cylinder, to be pulverized by tumbling steel balls or bars.

3. Air jets and mechanical arms in agitator tanks mix cyanide into powdered ore and water, called slime. This releases gold from rock.

4. The gold-cyanide solution and slime funnel into vast tanks where the rock particles slowly sink. The clarified solution is fed into filtrations units.

5. Gold-cyanide solution is filtered to strain out any remaining rock particles, and then is deaerated.

6. Zinc dust added to the solution separates the cyanide from the gold, which emerges as an impure powder.

7. The gold is melted with fluxes such as borax. As the metal cools in the bottom of a conical mold, the fluxes combine with impurities and float as slag.

8. Final product: a shining ‘button’ 90 percent gold, the rest silver. Further processing at a central refinery yields the 99.6 percent pure gold.