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Gallery of specimens of crystalline native gold· Native gold nuggets
· "Rope gold" from Lena River,Sakha Republic, Russia. Size: 2.5×1.2×0.7 cm.
· Crystalline gold from Mina Zapata,Santa Elena de Uairen, Venezuela. Size: 3.7×1.1×0.4 cm.
· Gold leaf from Harvard Mine,Jamestown, California, USA. Size 9.3×3.2× >0.1 cm. Production Main article: List of countries by gold production
Global gold output in 2005.
The entrance to an underground gold mine in Victoria, Australia
Pure gold precipitate produced by theaqua regia refining process At the end of 2009, it was estimated that all the gold ever mined totaled 165,000 tonnes.[2] This can be represented by a cube with an edge length of about 20.28 meters. At $1,600 per ounce, 165,000 metric tonnes of gold would have a value of $8.8 trillion. World production in for 2011 was at 2,700 tonnes, compared to 2,260 tonnes for 2008. Since the 1880s, South Africa has been the source for a large proportion of the world's gold supply, with about 50% of all gold ever produced having come from South Africa. Production in 1970 accounted for 79% of the world supply, producing about 1,480 tonnes. In 2007 China (with 276 tonnes) overtook South Africa as the world's largest gold producer, the first time since 1905 that South Africa has not been the largest.[78] Mining Main article: Gold mining The city of Johannesburg located in South Africa was founded as a result of the Witwatersrand Gold Rush which resulted in the discovery of some of the largest gold deposits the world has ever seen. Gold fields located within the basin in the Free State andGauteng provinces are extensive in strike and dip requiring some of the world's deepest mines, with the Savuka and TauTona mines being currently the world's deepest gold mine at 3,777 m. The Second Boer War of 1899–1901 between the British Empire and theAfrikaner Boers was at least partly over the rights of miners and possession of the gold wealth in South Africa. Other major producers are the United States, Australia, Russia and Peru. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States. In South America, the controversial project Pascua Lama aims at exploitation of rich fields in the high mountains of Atacama Desert, at the border between Chile and Argentina. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining.[79] Prospecting Main article: Gold prospecting During the 19th century, gold rushes occurred whenever large gold deposits were discovered. The first documented discovery of gold in the United States was at the Reed Gold Mine near Georgeville, North Carolina in 1803.[80] The first major gold strike in the United States occurred in a small north Georgia town called Dahlonega.[81] Further gold rushes occurred in California, Colorado, the Black Hills, Otagoin New Zealand, Australia, Witwatersrand in South Africa, and the Klondike in Canada. Due to the current high price of gold, its is now being recovered from used computer and cell phone components.[citation needed] Extraction Main article: Gold extraction Gold extraction is most economical in large, easily mined deposits. Ore grades as little as 0.5 mg/kg (0.5 parts per million, ppm) can be economical. Typical ore grades in open-pit mines are 1–5 mg/kg (1–5 ppm); ore grades in underground or hard rock mines are usually at least 3 mg/kg (3 ppm). Because ore grades of 30 mg/kg (30 ppm) are usually needed before gold is visible to the naked eye, in most gold mines the gold is invisible. The average gold mining and extraction costs were about US$317/oz in 2007, but these can vary widely depending on mining type and ore quality; global mine production amounted to 2,471.1 tonnes.[82] Refining After initial production, gold is often subsequently refined industrially by the Wohlwill process which is based on electrolysis or by the Miller process, that is chlorination in the melt. The Wohlwill process results in higher purity, but is more complex and is only applied in small-scale installations.[83][84] Other methods of assaying and purifying smaller amounts of gold include parting and inquartation as well as cupellation, or refining methods based on the dissolution of gold in aqua regia.[85] Consumption The consumption of gold produced in the world is about 50% in jewelry, 40% in investments, and 10% in industry. India is the world's largest single consumer of gold, as Indians buy about 25% of the world's gold,[86] purchasing approximately 800 tonnes of gold every year, mostly for jewelry. India is also the largest importer of gold; in 2008, India imported around 400 tonnes of gold.[87] Indian households hold 18,000 tonnes of gold which represents 11% of the global stock and worth more than $950 billion.[88]
Pollution Gold production is associated with contribution to hazardous pollution.[91] The ore generally containing less than one ppm gold metal, is ground and mixed with sodium cyanide or mercury to react with gold in the ore for gold separation. Cyanide is highly poisonous chemical, which can kill living creatures when exposed in minute quantities. Many cyanide spills[92] from gold mines have taken place worldwide both in developed and poor countries which killed the marine life in long stretches of affected rivers. Environmentalists consider these pollution disasters as major environmental disasters.[93][94] When mercury is used in gold production, minute quantity of mercury compounds reach water bodies, causing heavy metal contamination of water. Mercury enters into the human food chain in the form of methyl mercury through fish, etc. Mercury poisoning in humans causes incurable brain function damage and severe retardation. Thirty tonnes of used ore is dumped as waste for producing one finger ring of gold.[95] Gold ore dumps are the source of many heavy elements such as cadmium, lead, zinc, copper, arsenic, selenium and mercury. Water is unsuitable for human consumption if these heavy metals are found in more than one ppm concentration. When sulfide bearing minerals in these ore dumps are exposed to air and water, the sulfide transforms in to sulfuric acid which in turn dissolves the heavy metals facilitating their passage in to surface water and ground water. This process is called acid mine drainage. The gold ore dumps are considered as long term man made hazardous waste next only to nuclear wastedumps.[95] Billions of dollars need to be spent to mitigate the heavy metals pollution from worldwide gold ore dumps which are increasing every year. Gold extraction is also highly energy intensive industry (25 kW·h of electricity per one gram of gold production) to extract ore from deep mines and to grind the large quantity of ore for further chemical extraction.[96] Chemistry
Gold (III) chloride solution in water Although gold is the most noble of the noble metals,[97][98] it still forms many diverse compounds. The oxidation state of gold in its compounds ranges from −1 to +5, but Au(I) and Au(III) dominate its chemistry. Au(I), referred to as the aurous ion, is the most common oxidation state with soft ligands such as thioethers, thiolates, and tertiary phosphines. Au(I) compounds are typically linear. A good example is Au(CN)2−, which is the soluble form of gold encountered in mining. Curiously, aurous complexes of water are rare. The binarygold halides, such as AuCl, form zigzag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.[99] Au(III) (auric) is a common oxidation state, and is illustrated by gold(III) chloride, Au2Cl6. The gold atom centers in Au(III) complexes, like other d8 compounds, are typically square planar, with chemical bonds that have both covalent and ionic character. Aqua regia, a 1:3 mixture of nitric acid and hydrochloric acid, dissolves gold. Nitric acid oxidizes the metal to +3 ions, but only in minute amounts, typically undetectable in the pure acid because of the chemical equilibrium of the reaction. However, the ions are removed from the equilibrium by hydrochloric acid, forming AuCl4− ions, or chloroauric acid, thereby enabling further oxidation. Some free halogens react with gold.[100] Gold also reacts in alkaline solutions of potassium cyanide. With mercury, it forms anamalgam. Date: 2016-01-14; view: 675
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