Very few metals emerge glittering and perfect from the earth. Nuggets of gold are sometimes found; in fact in 1869 a nugget of pure gold weighing 154lb (69.85kg) was found in Victoria, Australia. And in 1856 a 500 ton lump of pure copper was dug up from a mine in Michigan, USA.
Some other metals, however, appear in drab disguises, combined with oxygen, sulphur, carbon and other elements to form ores that look little different from rocks or earth.
The first step towards obtaining the pure metal is to separate the ore from the dirt and stones dug up with it. Different methods are needed for different metals.
One way is said to have been discovered by the wife of a lead miner who found that particles of lead stuck to the froth when she washed his dirty clothes.
Lead and copper-mining companies now add ore to aerated, frothing liquid containing a chemical called a collector that enables the mineral particles to cling to the surface of air bubbles, while the waste is wetted and sinks. The valuable material is carried away on the froth to be skimmed off and dried.
Heat is often used to extract the pure metal from the ore, in a process called smelting. Early man discovered that when ores were heated in a fire with charcoal, a spongy mass of metal was left which could be beaten into weapons, tools or ornaments.
Copper was smelted in this way in ancient Egypt, and later the same method was used to produce an even more useful metal, iron. In medieval England it was found that the use of furnaces, which bellows to produce a forced draught of air, would increase the temperature of the fire and produce not a lump of metal, but a stream of liquid iron that could be cast in moulds.
Iron ore is known chemically as iron oxide because the metal is combined with oxygen in its natural state. In the smelting process, the iron oxide reacts with charcoal, made by converting wood into carbon. The oxygen atoms are detached from the iron, and attach themselves to the carbon, forming carbon oxide gas. This escapes leaving behind the iron.
The modern version of the same process uses coke as a source of carbon rather than charcoal, and takes place in huge blast furnaces capable of producing thousands of tons of iron a day.
The iron produced is called pig iron. This contains too much carbon to be useful, so must be converted into steel, by removing the carbon, or into cast iron by blending. Steel is the most important form of iron.
Aluminium occurs in combination with oxygen in bauxite ore. Though it is the most plentiful metal of all, making up 8 per cent of the Earth’s crust, it was not produced in any quantity until the end of the 19th century, because it requires a large amount of energy to separate it from oxygen.
The method used in electrolysis. An electric current is passed through a molten bath of aluminium oxide, which removes the oxygen, leaving behind liquid aluminium. The major difficulty is the very high melting point of aluminium oxide – over 3600ºF (2000ºC), compared to about 2900ºF (1600ºC) for iron.
The problem is solved by mixing the aluminium oxide with a mineral called cryolite (sodium aluminium fluoride) which lowers the melting point to a more manageable and cheaper 1800ºF (1000ºC).
Gold is one of the metals produced by chemical means. It often occurs as fine grains in the beds of streams. The problem is to separate the very small amounts of gold from the mass of useless material.
In ancient times the fleece of a sheep, immersed in a stream, was used to collect grains of gold – perhaps the origin of the Golden Fleece sought by the Argonauts. And prospectors ‘panned’ for gold – swirling the dirt from a stream in a pan of water until the lighter gravel was washed away, leaving the denser gold in the pan.
Today a chemical is used. The crushed ore is mixed with a solution of potassium cyanide, which dissolves the gold. The solution containing the gold is then filtered, to remove undissolved impurities, and the gold is finally precipitated out. A ton of ore will produce just over one-third of an ounce (10 grams) of gold.
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