The term gold mining covers geological assessment, exploration methods, drilling, development, processing, refinement, financing, and delivery of gold products. Gold is a rare substance, and most deposits are found in low concentration in the rocks. The average concentration of gold in the Earths crust stands at 0.005 parts per one million (5 grams per one ton). The gold ore should contain a geological concentration of at least 1000 times above the pointed average in order to be found suitable for mining purposes. In brief, the extraction processes are expensive because miners have to process large volumes of ore to obtain small quantities of gold.
Gold is usually found in alloys with silver and mercury. In rare cases, gold can be found in combination with other elements such as calaverite, krennerite, and nagyagite, among others. Gold is concentrated above and below the earth surface. Pacer deposits are found at points where rivers cut through rocks which contain concentrated gold. Underground gold deposits are formed in combination with other minerals such as sulphides and pyrites.
Gold has symbolized power and wealth for many millennia. By the 3rd century AD, the Roman Empire attracted skilled gold craftsmen who created refined pieces of jewelry. The Romans extracted gold from river deposits, dug short tunnels, and worked below the ground in vertical tunnels.
Several methods serve the purpose of gold extraction. Gold panning employs shallow pan sieves that are filled with gravel and held in flowing water. Gold settles at the bottom of the sieve. Small scale mining has employed sluicing techniques. The sluice box is placed in streams to catch the gold bearing substances. Metal detecting is another manual method that locates gold deposits beneath the earth surface. If present, the gold will change the electromagnetic field of the metal detector. Dredging is another small scale mining method. This piece of equipment represents human-operated suction dredge which comprises of sluice box and suction hose. Hard rock mining is employed when the gold material is contained within rocks. This method uses open-pit and underground mining via shafts. Most gold mining companies use this method to extract gold.
Sometimes, the gold is a secondary substance in the ore. Copper mines as well as sand and gravel pits may contain gold material. In this case, gold represents a byproduct from the extraction of another principle product. Another processing method uses sodium cyanide. The solution of sodium cyanide is mixed with gold bearing rock and then separated in the form of gold cyanide. Secondly, zinc is mixed with the solution and precipitates the gold metal. Gold is produced by removing the zinc with sulfuric acid. Refractory gold ores are resistant to standard extraction via cyanidation. The refractory ore may contain sulfides or organic carbon. For instance, the organic carbon will absorb the gold cyanide. Then, other treatment procedures are followed to extract gold. Pre-treatment includes bio-oxidation and pressure oxidation, roasting, and ultra-fine grinding.