Gold
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Gold Introduction Produced jointly by the Minerals Council of Australia and AGSO Gold, the only yellow metal, has the chemical symbol Au, which is derived from the Latin word for gold — aurum. It has a density nearly twice that of lead, is a good conductor of electricity and heat, and is so malleable that it can be rolled thin enough to allow light to pass through. Common acids will not dissolve gold but 'aqua regia' (a mixture of nitric and hydrochloric acids) will, as will alkaline cyanide solutions. Gold had a significant historical role in Australia, which had its first gold rush in 1851 after the mineral was found near Bathurst (NSW). The Bathurst gold rush was followed by discoveries in Victoria. Gold fever drew tens of thousands of immigrants from many parts of the world to the Australian colonies. Ballarat and Bendigo in Victoria became sites of major rushes. Later, in the early 1890s, great finds were at Coolgardie and Kalgoorlie in Western Australia. Within ten years of the rushes to Bathurst, Ballarat and Bendigo, Australia's population trebled to more than one million people. Gold discoveries spurred the development of inland towns, communications, transport and foreign trade. Although gold boosted Australia's development, it's importance declined during most of the 20th century as other minerals became of greater economic significance. It underwent a resurgence in the 1980s and 1990s when the application of new technology allowed lower grade ores to be processed economically. Occurrence Gold usually occurs in its metallic state, commonly associated with sulphide minerals such as pyrite, but it does not form a separate sulphide mineral itself. The only economically important occurrence of gold in chemical combination is with tellurium as telluride minerals. Most gold mined in Australia today cannot be seen in the rock. It is very fine grained and mostly has a concentration of less than 5 grams in every tonne of rock mined. Primary gold deposits are formed from gold-bearing fluids at sites where the chemistry and physical characteristics permit gold deposition. Primary deposits are often modified by weathering, but secondary deposits are formed only after the complete breakdown of the host rock has occurred. Liberated gold is concentrated in alluvial (placer) deposits. Examples of primary deposits include those mined at Kalgoorlie in the Super Pit, Telfer, St Ives, Norseman and Mount Magnet (WA), Kidston and Mount Leyshon (Qld), The Granites (NT), Stawell (Vic), Peak in the Cobar area (NSW), Henty (Tas) and Olympic Dam (SA) where gold occurs with copper and uranium. Secondary (alluvial) deposits are no longer major sources in Australia but alluvial gold sparked the rushes of the 1850s. Gold is also found as a minor component in many base metal deposits and is recovered as a by-product at some operations such as Rosebery and Hellyer (Tas). Australian resources and deposits Australia's economic gold resource is just over 4400 tonnes. About two-thirds of Australia's resources occur in Western Australia, with the remainder in all other States and the Northern Territory. Virtually all resources occur in primary deposits, many of which have undergone some degree of weathering. Weathered primary deposits are important to the gold industry because they are usually easier and cheaper to mine and the gold is easier to recover. Australia in the world Australia has about 10 per cent of world economic gold resources and is ranked third after South Africa and USA. It is the world's third largest producer, after South Africa and USA and accounts for about 12 per cent of world output. Mining Most of Australia's gold production comes from open-cut mines. Large capacity earth-moving equipment is used to remove waste rock from above the ore body and then to mine the ore. Waste and ore are blasted to break them into sizes suitable for handling and transport to waste dumps or, for ore, to the crusher. Underground mining is used where the depth of ore below the surface makes open-cut mining uneconomic. Vertical shafts and declines (spiral tunnels) are used to move people and equipment into and out of the mine, to provide ventilation and for hauling the waste rock and ore to the surface. Processing Coarse gold may be removed by gravity concentration. The processing required to recover fine gold from crushed ore is determined by the free-milling or refractory nature of the ore. Free-milling ore is ore from which gold can be recovered by crushing, grinding and cyanidation (treatment with a dilute cyanide solution) without additional processing. Free-milling oxide ores are suitable for direct cyanidation of the crushed and ground ore. Refractory ore is ore where gold is locked in the sulphide minerals so that additional processing, such as roasting or biological leaching, is required before cyanidation to achieve satisfactory levels of gold recovery. Sulphide minerals in refractory ores are converted to oxides by either roasting or biological leaching to release the gold. In biological leaching the oxidation is caused by the action of certain bacteria on the ore. The tonnage of refractory ore to be roasted or leached is greatly reduced by first producing a finely ground concentrate. Ground ore or treated concentrate is placed in a weak solution of sodium cyanide, which dissolves gold and forms a slurry of gold-bearing solution and barren solids. Some ores may be treated by heap-leaching. This involves sprinkling a weak cyanide solution over an open pile of ore stacked on an impervious base. The solution percolates through the ore, leaching gold as it goes, and is drawn off at the base and then treated to recover the gold. In both cases, the gold is recovered from the gold-bearing solution in the carbon process in which pellets of activated carbon, made from charred coconut husks, are added to the slurry and the gold-bearing ions are adsorbed onto the pellet surface. The pellet load is moved through a number of linked tanks containing slurry in a direction opposite to the slurry movement. Pellets, loaded with gold, are removed and gold is stripped from them by washing in a solution of hot cyanide. Carbon used in the process is recycled. An electric current is passed through the new solution and the gold is deposited on a steel wool cathode. The gold laden cathode is treated with hydrochloric acid to dissolve any residual steel. The gold sludge is filtered and dried and is then ready for smelting. At this stage the gold-bearing material may still contain silver and base metals. Gold is smelted in a crucible furnace to produce unrefined bullion. In smelting, base metal impurities are oxidised and absorbed, leaving the precious metals to be poured into ingot moulds. Smelted gold is then refined. Refining Several refining processes are used in Australia — chlorination, electrolytic and aqua regia. In the chlorination process (Miller process), chlorine is introduced to melted bullion in a crucible furnace. The gas reacts with silver and any remaining base metals to form chlorides, bubbles of which rise to the surface of the molten bullion and are removed. The molten, refined gold is cast into bars. The electrolytic process (Wohlwill process) involves dissolving gold from the bullion (anode), in a chloride solution and redepositing the gold on a pure gold or titanium cathode. Silver remains on the anode. The cathodes are melted and cast. In the third process the unrefined bullion is dissolved in aqua regia and silver is precipitated as silver chloride. Sulphur dioxide gas is passed through the remaining solution and gold is precipitated as a fine metallic powder. The gold is then melted and cast. Uses Gold has long been a medium of exchange and investment. It is widely used to produce coins such as the Australian Nugget. Although Governments of most countries include holdings of gold as part of their monetary reserves many are starting to reduce their holdings. Financial institutions and individuals also use gold as a store of wealth. The main uses of gold are jewellery, dentistry, the adornment of buildings and for artistic purposes. In jewellery, gold is often mixed with other metals to produce alloys of different colours. White gold is an alloy of gold with silver, palladium, nickel and copper. Yellow, green and red golds are produced by alloying gold with copper and silver in different proportions. The gold content of jewellery is expressed as a number of carats — pure gold is said to be 24 carat gold; 18 carat gold contains 18/24 or 75 per cent pure gold. High electrical conductivity, malleability and ductility favour the use of gold in electronic and computer circuitry, radar equipment and satellites. Because of gold's heat reflecting properties it was used as a film coating on the Apollo 14 lunar module, the vehicle which first landed man on the moon. An important and growing use is in the mechanism and circuitry of safety air bags in motor vehicles.
Blainey, G.L. 1993 The Rush that never ended: A history of Australian Mining, 4th edition, Melbourne University Press, Melbourne. Australian Geological Survey Organisation 1999 Australia's identified mineral resources, Bureau of Resource Sciences, Canberra. Geological Survey of New South Wales 1986 Gold in New South Wales, Department of Mineral Resources, Sydney. Hughes, F.E. (Ed) 1990 Geology of the Minerals Deposits of Australia and Papua New Guinea, Volumes 1 and 2. The Australasian Institute of Mining and Metallurgy, Melbourne. Raymond, R. 1974 Out of the fiery furnace, The impact of metals on the history of mankind, Macmillan, Melbourne. Woodcock, J.T. and Hamilton, J.K. (Eds) 1993 Australasian Mining and Metallurgy: The Sir Maurice Mawby Memorial Volume, Volume 2. The Australasian Institute of Mining and Metallurgy, Melbourne. |
