Gold
CommoditiesRare Earth Elements
Gold |
Gold is a chemical element with the symbol Au (Latin: aurum, "shining dawn") and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is dense, soft, shiny and the most malleable and ductile pure metal known. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Gold is one of the coinage metals and has served as a symbol of wealth and a store of value throughout history. Gold standards have provided a basis for monetary policies. It also has been linked to a variety of symbolisms and ideologies.
A total of 161,000 tonnes of gold have been mined in human history, as of 2009. This is roughly equivalent to 5.175 billion troy ounces or, in terms of volume, about 8,333 cubic metres, and could be valued at just under six trillion United States dollars at early March 2010 prices.
Although primarily used as a store of value, gold has many modern industrial uses including dentistry and electronics. Gold has traditionally found use because of its good resistance to oxidative corrosion and excellent quality as a conductor of electricity.
Chemically, gold is a transition metal and can form trivalent and univalent cations in solutions. Compared with other metals, pure gold is chemically least reactive, but it is attacked by aqua regia (a mixture of acids), forming chloroauric acid, but not by the individual acids, and by alkaline solutions of cyanide. Gold dissolves in mercury, forming amalgam alloys, but does not react with it. Gold is insoluble in nitric acid, which dissolves silver and base metals. This property is exploited in the gold refining technique known as "inquartation and parting". Nitric acid has long been used to confirm the presence of gold in items, and this is the origin of the colloquial term "acid test", referring to a gold standard test for genuine value.
Production
There are various ways to separate and recover gold, depending upon the nature of the ore. When gold occurs in a relatively coarse, free state, it can be recovered by mechanical means such as gravity traps and shaking tables, where the gold separates out because of its high specific gravity. Cyanidation is a chemical process used to recover gold that is very finely distributed in the ore. The process is complex; it involves adding a cyanide solution to finely ground ore and then agitating it in the presence of air to dissolve the gold. After a variety of treatment steps, the solution containing the gold is clarified and the gold is precipitated by the addition of zinc dust to the solution. The carbon-in-pulp stage of this process uses activated carbon to collect the gold without having to filter the ground ore slurry. The activated carbon is stripped of its gold in an acid bath and then recycled.
Once the gold is separated from the ore and is in the form of an impure precipitate, it is placed, along with a fluxing agent, into a high-temperature furnace. A chemical reaction takes place, in which the flux and the impurities combine to form a slag and the molten gold sinks to the bottom of the furnace where it is drawn off and poured into moulds to form doré bars, which contain the gold and any silver that may have been present in the original ore.
World industrial demand for gold includes jewellery (86%), electronics (6%) and official coins (2.5%). Gold has been a symbol of wealth from the earliest civilizations to the present. Even though it is too soft for weapons or tools, people have treasured gold for its decorative and monetary value. Since it is chemically inert, it has long been popular in dentistry for crowns and caps. Resistance to corrosion, along with electrical conductivity, makes it useful in precision electronic equipment. When a thin film of gold is applied to window glass, a building's thermal properties are improved dramatically, reducing the amount of heat gain in the summer and heat loss in the winter.
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