Copper is a metal and chemical element with the symbol Cu. Its name derives from the Latin cuprum. It is a soft ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a reddish/orange colour. Copper can occur in nature in its pure form - called native copper - but usually it is found as a mineral ore. It is perhaps the most useful metal to man after iron.
This article offers a non-technical background to copper, to accompany the section on Río Tinto mines.
Most copper is mined as mineral ores of sulphides (chalopcopy and chalcocite) but it can also be carbonates (azurite and malachite) or oxides (cuprite)
After gold, copper was the first metal to be used by humans, around 8,000 BC and probably in the Middle East from that era. Exhibited in the British Museum is a heavy copper pin that was discovered in an Egyptian tomb.
By 3500 BC, it was the first metal to be smelted from the ore and cast into a mould. It was also the first metal to be purposefully alloyed with another metal, tin, to create bronze - around 3500 BC in India, Mesopotamia and Greece.
This established the Bronze Age, which ended with the Iron Age in 600 BC in northern Europe. The Bronze Age gave way to the Iron Age, as iron was easier to find and easier than bronze to process into a usable metal.
Copper compounds are known as the copper salts, azurite, malachite and turquoise, and have historically been used as pigments.
Copper does not react with water, but it does react slowly with oxygen to form a layer of brown-black copper oxide which, unlike rust on iron, protects the underlying copper from more extensive corrosion. If kept well polished it is highly ornamental. When rolled, hammered or drawn, copper becomes hard, almost as strong as mild steel. However, it loses its strength if "annealed" by being heated to a low red heat and quenched in cold water.
Copper architectural structures weather to give a green verdigris (or patina) coating which is copper carbonate - or, if near the sea, copper chloride. The Statue of Liberty in New York is an example of this.
The most famous copper mine in Britain was the Parys mine in Anglesey, near Amlwch, peaking at 5,000 tonnes a year. The chief copper district in the USA is the mineral range near the south shore of Lake Superior. Projecting into Lake Michigan is the Keweenaw Peninsula containing the Calumet, Hecla and Tamarack mines with shafts one mile (1.6 kilometres) deep. Chile is now the second leading copper-producing country, with perhaps the largest deposits in the world, closely followed by deposits in northern Zimbabwe and at Katanga, in the Democratic Republic of the Congo. Japan also has extensive copper deposits, while in Europe the largest mine is Río Tinto.
EXTRACTION FROM ORE
The concentration of copper in mined ores averages only 0.6%. The extraction process is extremely complicated and this was one of the main problems experienced during the early development of the Río Tinto mines.
In some mines a picking process is carried out. As the ore passes along a conveyor belt, trained operatives pick out the ore that is too poor to warrant further treatment. The ore is broken up coarsely by jaw breakers, and then more finely by rollers. The fine ore is placed in revolving chambers, with heavy iron balls rolling around to crush it further.
The crushed ore is sorted into sizes by screens and in various water flowing tanks with riffles. In principle, particles containing the metal, being heavier, separate from those which do not. Treating the finest ore dust is done by mixing the slime with an oily froth that sticks to the metallic particles. This is called froth flotation or bioleaching. The copper content has now reached 15%.
The concentrates are roasted in a furnace, which drives off a great deal of the sulphur. The actual smelting is done in a blast furnace or reverberatory furnace. Heating this material with silica in flash smelting removes much of the iron (as slag) that floats on the top of the heated mass. The material is now 30% to 40% pure.
Most large smelting works now use a Bessemer converter. Air is blown through the molten matte, until the colour of the flame indicates that the process has been finished. Here the cuprous is converted into blister copper upon heating. Natural gas is blown across the blister to remove most of the remaining oxygen and it is formed into plates.
Electro refining is performed on the plates by immersing them in a tank of copper solution, and a small amount of sulphuric acid impurities, including gold and silver, fall to the bottom of the tanks as the anodes are eaten away. This process produces "electrolytic" copper, which is only five parts per 10,000 impure. This is the purest copper produced today.
Modern-day applications of copper are electrical wires and electrical fittings, roofing and plumbing, and industrial machinery.
Copper as an alloy with 11% tin makes bronze, which is harder and more durable than copper. Bronze was first used for tools, weapons and armour.
Copper as an alloy with zinc in various proportions produces brass, which is used for decoration due to its bright gold-like properties, or where low friction is required such as locks, gears, bearings, doorknobs and ammunition casings (where it is important to avoid striking a spark). It is more malleable with a lower melting point (920 C) and is used for hand-worked items such as musical instruments and monumental plaques and ornate bed-heads.
Principal research sources:
Wikipedia: Copper, Bronze and Zinc
Page extracts from Newnes Pictorial Knowledge (A Young Person's Encyclopaedia)