How gold is extracted
Did you know that a deadly poison can also help extract precious gold from mines?
It had long been known that cyanide ions (CN-) stop the body from absorbing oxygen resulting in a very quick death and that gold (Au2+) also dissolves in cyanide, but in the 19th century people in the mining business thought to use this basic chemistry to help extract the small amounts of gold in low-grade ores.
The quartz from the mine containing the small amounts of gold is ground up and the cyanide solution is added. Two cyanide atoms attach themselves to one gold atom and the newly formed molecules float in the ‘pregnant’ mix.
The final step is to precipitate out the gold which is done by running an electrical current in the solution-one side of the tank is positively charged, the other negative. Because the gold ions are positively charged, they move towards the negative charge. The gold is then melted off and poured into a mould. Often silver (Ag2+) lingers in the solution as it is sometimes present in the mines, resulting in a lower grade bouillon.
In 1899 Karangahake, near Waihi, became the first place in the world to trial this process and it is still being used today. It is important that mines carefully monitor the gold cyanidation process because the many toxic chemicals used in the process could cause serious harm to the environment in the unfortunate event of an accident.