Gold mining
Gold mining consists of the processes and techniques employed in the removal of gold from the ground. There are several techniques by which gold may be extracted from earth and rock:
Gold panning
Gold panning is a manual technique of sorting gold. Wide, shallow pans are filled with sand and gravel that may contain gold. Water is added and the pans are shaken, sorting the gold from the rock and other material. The gravel is usually removed from streambeds, often at a bend in the stream, where the weight of gold causes it to settle out of the water flow. This type of gold found in streams or dry streams are called placer deposits.
Hydraulic mining
Hydraulic mining is a type of placer mining used in areas where large amounts of loose gravel and sand or soil are poorly packed and may be washed away with a heavy stream of water. Fire hoses (Water cannons) are sometimes used to strip away entire hills of loose gravel, which are then run through a sluice (a wooden trough with ripples). Gold, being heavier, does not move as easily as other material in the sluice. This technique can damage the environment, causing mud in streams below the mining site and erosion damage at the site itself.
Hard rock mining
Hard rock mining is removing rock from the ground, in which miners tunnel and blast into rock, seeking deposits of gold. Veins of gold ore are often found several inches or feet wide in certain rock formations in a volcanic deposit and in certain bed layers in a sedimentary deposit, hence the minerals may be removed, collected, and treated to process the gold and other valuable metals (such as silver) from them. This technique uses the most energy and is typically viable only where exceptional gold grades warrant the associated expense. Deposits where this is the case may have significantly higher grades than those needed to offset the costs and so underground mining can be quite profitable. Hard rock mining produces much of the world's gold.
Typical hard rock mining involves a cycle wherein holes are drilled in the rock to be broken, explosives are placed in the holes then detonated. Rock broken by the explosion is then removed from the area, typically with mechanized equipment. Where ground conditions warrant, rock bolts and other means of ground support may then be installed and the cycle can be begun again with drilling. By this means an advance of some few meters may be achieved.
Cyanide process
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 and/or silver, and is then separated from the ground rock as gold cyanide and/or silver cyanide solution. Zinc is added to the solution, precipitating out residual zinc, as well as the desirable silver and gold metals. The zinc is removed with sulphuric acid, leaving a silver and/or gold sludge that is generally smelted into a doré that is shipped to a metals refinery for final processing into 99.9999% pure metals.
The cyanide technique is very simple and straightforward to apply and a popular method for low-grade gold and silver ore processing. Like most industrial chemical processes, there are potential environmental hazards presented with this extraction method in addition to the high toxicity presented by the cyanide itself. This was seen in the environmental disaster in Central-Eastern Europe in year 2000, when during the night of 30 January, a dam at a goldmine reprocessing facility in Romania released approximately 100,000m³ of wastewater contaminated with heavy metal sludge and up to 120 tonnes of cyanide into the rivers of Tisza and Danube.
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