text: The principal technology is the cyanide process, in which gold is leached from the ore by treatment with a solution of cyanide.  The first step is comminution (grinding) to increase surface area and expose the gold to the extracting solution. The extraction is conducted by dump leaching or heap leaching processes. Sodium cyanide is produced on a billion-ton/year scale mainly for this purpose.  "Black cyanide", a carbon-contaminated form of calcium cyanide (Ca(CN)) is often used because it is cheap.  The crude ore is washed with a c. 0.3% solution of cyanide in air, often repeatedly, and the aqueous extract is collected and refined further. Recovery from solution typically involves adsorption on activated carbon, the carbon in pulp process.
  Thiosulfate leaching has been proven to be effective on ores with high soluble copper values or ores which experience preg-robbing.
  Leaching through bulk leach extractable gold, or BLEG, is also a process that is used to test an area for gold concentrations where gold may not be immediately visible.