Gold ore processin
Cyanide extraction of gold may be used in areas where fine gold-bearing rocks are found. Sodium cyanide solution is mixed with finely ground rock that is proven to contain gold or silver, and is then separated from the ground rock as gold cyanide or silver cyanide solution.Zinc is added to precipitate out residual zinc as well as the silver and gold metals. The zinc is removed with sulfuric acid, leaving a silver or gold sludge that is generally smelted into an ingot then shipped to a metals refinery for final processing into 99.9999% pure metals.
Advancements in the 1970s have seen activated carbon used in extracting gold from the leach solution. The gold is absorbed into the porous matrix of the carbon. Activated carbon has so much internal surface area, that fifteen grams (half an ounce) has the equivalent surface area of the Melbourne Cricket Ground (18,100 square metres (195,000 sq ft)). The gold can be removed from the carbon by using a strong solution of caustic soda and cyanide, a process known as elution. Gold is then plated out onto steel wool throughelectrowinning. Gold specific resins can also be used in place of activated carbon, or where selective separation of gold from copper or other dissolved metals is required.
The cyanide technique is crude and is commonly used in low grade gold and silver ore processing. There are many environmental hazards associated with this extraction method in addition to the high toxicity of the cyanide itself. An acute example was demonstrated in the 2000 Baia Mare cyanide spill environmental disaster when a break in a waste pond dam gold mine reprocessing facility near Baia Mare in northern Romania released approximately 100,000 cubic metres (3,500,000 cu ft) of waste water contaminated with heavy metal sludge and up to 120 long tons (122 t) of cyanide into the Tisza River.]