Most modern coins are mode from the cheaper, more plentiful metals such as copper and tin – the ‘base metals’. Among these, pure copper is occasionally found inside rocks and is know as ‘native’ copper. Most commonly, it is found in ores in the form of copper sulphide (copper combined with sulphur) or, less often, copper oxide (copper combined with oxygen).
Rock containing the copper ore is obtained from opencast mines, which are like huge quarries, or dug out of mines. The ore-bearing rock is crushed in a ball mill by heavy rollers, each 6 metres in diameter, 7 metres wide and driven by a 5,000 horse power electrical motor. After crushing, the copper ore is separated from the rest of the rock, at which point it is known as a concentrate.
Copper sulphide ore is simply melted in a furnace to separate the sulphur from the copper. Oxide ore is dissolved in a tank of sulphuric acid, then the copper is extracted by electrolysis. Two copper rods 9 called electrodes) are lowered into the acid and an electric current is passed through the acid from the one electrode to the other. Pure copper builds up on the negative electrode. Any impurities end up as a slimy sludge on the bottom of the tank. When the process is finished, the electrodes are lifted out of the acid, melted and then cast into ingots. Copper extracted in this way is knows as electrolytic copper and is very pure.
The most important zinc ore is zinc sulphide. Crushed zinc ore is heated in a furnace to convert the zinc sulphide into zinc oxide. Then the oxide is heated with carbon to remove the oxygen and produce pure zinc. The oxygen and carbon combine to form carbon dioxide gas, which floats away, leaving the pure metal.
The main nickel ore is nickel sulphide. As with zinc production, the first stage in producing nickel from the mined ore is to heat it in a furnace to convert the nickel sulphide into nickel oxide. A mixture of hydrogen and carbon monoxide gas, known as producer gas, is then pumped into the furnace. This combines with the oxygen, leaving an impure nickel.
In North America, the nickel is usually refined by electrolysis, elsewhere the Mond process is generally used. The Mond process consists of a two-stage operation. First, the nickel is heated with producer gas, but this time to only 5 degrees, which is a much lower temperature than before. This produces a compound called nickel carbonyl, which consists of nickel, carbon and oxygen. The nickel carbonyl is then heated to about 180 degrees. At that temperature it gives off carbon monoxide, leaving little pellets of pure nickel.