A sodium cyanide solution is commonly used to leach gold from the mineral. Cyanide dissolves gold from the mineral in the solution as it seeps through the heap. Gold mining refers to the processes needed to extract gold from its minerals. The vast majority of gold is extracted from dilute minerals through a combination of chemical processes.
Every year about 2000 tons are obtained from the earth, plus another 300 tons from recycling. The mill operators then thicken the suspension with water to form pulp and pass it through a series of leaching tanks. Leaching dissolves gold from the mineral using a chemical solvent. The most common solvent is cyanide, which must be combined with oxygen in a process known as carbon in the pulp.
As cyanide and oxygen react chemically, the gold in the pulp dissolves. When workers insert tiny grains of carbon into the tank, the gold adheres to the carbon. By filtering the pulp through screens, the carbon containing gold is separated. Then, several processes can be used to separate the gold from the ore.
The most common techniques in the United States use cyanide in a variety of ways. In one, the ground mineral is placed in a tank containing a weak cyanide solution and zinc is added. Zinc causes a chemical reaction that separates gold from the mineral. The gold is then removed from the solution with a filter press.
Although each ounce of gold recovered requires the processing of about 100,000 ounces of ore, there is so much gold deposited in rock veins that this method accounts for more than half of the world's total gold production today. Gold in veins can be the size of microscopic particles, in nuggets or sheets, or in gold compounds. Regardless of how it is found, the mineral requires extensive extraction and refining. A third of all gold is produced as a by-product of copper, lead and zinc production.
Copper, for example, must be electrolytically refined to increase its purity from 99% to more than 99.99%, as required for many industrial purposes. In the refining process, an impure copper anode is electrolyzed in a bath in which the cathode is a very thin sheet of highly refined copper. As the process continues, copper ions leave the impure anode and settle as atoms on the cathode. Since the impurities are not transported through the bath, as the anode is consumed, the impurities fall to the bottom in the form of mud.
This anodic sludge contains gold in sufficient quantities for the recovery to be cost-effective. A third of all gold is obtained from these by-products. Silver and platinum are also recovered from copper anode sludge in quantities large enough to more than pay for the entire refining process. To obtain gold from vein ore, the ore is first crushed in rod or ball mills.
This process reduces the mineral to a powdered substance from which gold can be extracted by amalgamation with mercury or by pleasure procedures. At this point, around 70% recovers. The rest are dissolved in dilute solutions of sodium cyanide or calcium cyanide. The addition of metallic zinc to these solutions causes metallic gold to precipitate.
This precipitate is refined by casting. The purification is completed by electrolysis and the sludge produced will contain commercial quantities of silver, platinum, osmium and other rare earth metals. During these ancient times, gold was extracted from alluvial deposits, that is, particles of elemental gold found in river sand. The purity of alloyed gold is expressed by the karat system, where the percentage of gold by weight is expressed as a fraction of 24.The distribution of gold seems to validate the theory that gold was dragged to the Earth's surface from great depths by geological activity, perhaps with other metals as a solid solution within molten rock.
Elemental gold is an excellent conductor of electricity and heat, second only to the other members of group 1B, copper and silver. Finally, the gold-containing solution is ready for electroextraction, which recovers gold from leaching chemicals. After the discovery (184%) of gold in California, North America became the world's leading supplier of the metal. In 1887, John Stewart MacArthur, working in collaboration with brothers Robert and William Forrest for the Tennant Company in Glasgow (Scotland), developed the MacArthur-Forrest process for the extraction of gold ores.
The carbon is moved to an extraction vessel where a hot caustic solution separates gold from carbon. Since the 1970s, gold has been bought and sold on the market, with highly fluctuating prices, and gold reserves have only a very indirect relationship with currency values. According to de Lecerda and Salomons (199), mercury was used for the first time for extraction around 1000 BC. C., according to Meech et al.
(199), mercury was used to obtain gold until the last period of the first few millennia. Of the world's known gold ore mineral reserves, 50 percent are in South Africa and most of the rest are divided between Russia, Canada, Australia, Brazil and the United States. Miller's process of refining impure gold with chlorine gas (patented in Great Britain in 1886) and Emil Wohlwill's electrorefining process (introduced in Hamburg, Germany). Endogenetic gold minerals include deposits of elemental gold in veins and veins in quartzite or mixtures of quartzite and several iron sulfide minerals, in particular pyrite (FeS) and pyrrhotite (Fe1-XS).
Previously, the extraction of gold from alluvium minerals was carried out using techniques associated with pleasure mining, such as the simple extraction and isolation of gold, allowing the direct recovery of small nuggets and flakes of gold. Exogenetic gold can also exist in the form of oxidized minerals that have been formed through a process called secondary enrichment, in which other metallic elements and sulfides are gradually eliminated, leaving behind gold and insoluble oxide minerals as deposits on the surface. A similar arrangement traps metal over wool and may have been the origin of the legend of Jason's quest for the golden fleece. .