Science Fair Project Encyclopedia
Gold panning is a manual gravimetric technique of sorting. Wide, shallow pans are filled with sand and fine gravel that is thought to contain gold. Water is added and the pans are shaken, sorting the gold, by density, from the rock and other undesirable material. The gravel is typically removed from streambeds, often at a bend in the streambed, where the weight of gold causes it to settle out of the streamflow. These concentrations of gold found in streams or dry streambeds are called placer deposits.
Hydraulic mining is a form of placer mining employed in areas where large amounts of loose gravel and sand or soil are poorly consolidated and may be washed away with a heavy stream of water. Water cannons are sometimes used to strip away entire hillsides of loose material, which are then run through a sluice. Gold, being denser, does not move as readily as other sediment and concentrates in the sluice, from which it is collected. This technique can damage the environment, causing siltification in streams below the mining site and erosion damage at the site itself.
Hard rock mining
Hard rock mining is most consistent with the popular conception of removing rock from the ground, in which miners tunnel and blast into rock, seeking hydrothermal deposits of gold. Veins of gold ore are often found several inches or feet wide in certain rock formations, whence the minerals may be removed, collected, and treated to process the gold and other valuable metals (such as silver) from them. This technique is possibly the most energy-intensive, but the most profitable way to harvest significant quantities of the metal.
Cyanide process extraction of gold may be used in areas where gold-bearing rocks or finer materials are found at the surface. Sodium cyanide solution is allowed to leach through a pile of finely-ground rock that contains gold and silver, and is then collected as gold cyanide and silver cyanide solution. Zinc is added to the solution, precipitating out zinc, silver, and gold. The zinc is removed with sulphuric acid, leaving a silver and gold amalgam that may be further processed into the individual metals.
The cyanide technique is simple and straightforward to apply. It is popular in areas where mine tailings may contain large quantities of valuable metals despite previous processing, even previous treatments with cyanide. This technique can be dangerous and can cause significant environmental damage (see also: Summitville mine) due to the toxicity of cyanide.
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details