The chemical element copper is a reddish metal at the head of group IB in the periodic table.  Its symbol is Cu;  atomic number, 29;  and atomic weight, 63.546.  Copper follows the first transitional series of elements.

Copper was the first metal used by humans and is second only to iron in its utility through the ages.  The name is derived from the Latin cuprum, "copper," from the earlier Latin Cyprium, "Cyprian metal." The discovery of the metal dates from prehistoric times, and it is estimated that copper was first used about 5000 BC or even earlier.

Natural Occurrence and Extraction

In Roman times much of the copper was obtained from the island of Cyprus, as the name implies.  Copper today is mined in many parts of the world, the largest producers at present being Chile, Peru, Poland, the United States, Zaire, and Zambia. More than 160 minerals containing copper are known.  Copper constitutes 70 parts per million of the Earth's crust and is present to the extent of 0.020-0.001 parts per million in seawater.

Copper in its native state--such as that found in the Lake Superior region of North America--is often so pure that it requires only melting with a flux to produce "lake copper," which for many years was the world standard for pure copper. About 80% of all copper mined today, however, is derived from low-grade ores containing 2% or less of the element.

Half of the world's copper deposits are in the form of chalcopyrite ore.  All important copper-bearing ores fall into two main classes:  oxidized ores and sulfide ores.  Sulfide ores are more important commercially.  Ores are removed either by open-pit or by underground mining.  Ores containing as little as 0.4% copper can be mined profitably in open-pit mining, but underground mining is profitable only if an ore contains 0.7%-6% copper.

The oxidized ores, such as cuprite and tenorite, can be reduced directly to metallic copper by heating with carbon in a furnace, but the sulfide ores, such as chalcopyrite and chalcocite, require a more complex treatment in which low-grade ores have to be enriched before smelting begins.  This involves the ore-flotation process, in which the ore is crushed and powdered before it is agitated with water containing a foaming agent and an agent to make the copper-bearing particles water-repellent.  These particles accumulate in the froth on the surface of the flotation tank, and this froth is skimmed off and heated to about 800 deg C to remove some of the water as well as antimony, arsenic, and sulfur, which are also present.  The residue is then mixed with silica and melted in a furnace at 1,400 deg-1,500 deg C.  This produces two liquid layers:  a lower layer of copper matte (cuprous sulfide mixed with iron sulfide and oxides), and an upper layer of silicate slag, which is drawn off.  Silica or siliceous copper ore is added to the liquid matte in a converter, and air under pressure is blown through the liquid.

Upon removal of the iron slag, the copper(I) sulfide that remains is reduced to copper by heating in a controlled amount of air.  The remaining molten copper, which is 98%-99% pure, is either cast into blocks of blister copper or into anodes.

The final stage of purification is mainly by electrolytic refining, which yields copper of 99.95%-99.97% purity.  The impure copper is made the anode of an electrolytic cell that contains pure strips of copper as the cathode and an electrolyte of aqueous copper(II) sulfate.  During electrolysis, copper is transferred from the anode to the cathode.  An anode sludge containing silver and gold is produced during this process, and this increases its economic feasibility.

Physical and Chemical Properties

Eleven isotopes of copper are known, two of which are not radioactive and occur with a natural abundance of 69.09% and 30.91%, respectively.  Copper melts at 1,083.4 deg plus or minus 0.2 deg C (in a vacuum), boils at 2,567 deg C, and has a density of 8.96 at 20 deg C.  The element has a hardness of 3, takes on a bright metallic luster, has a cubic crystal structure, and is malleable, ductile, and a good conductor of heat and electricity, second only to silver in electrical conductivity.

Copper exhibits oxidation states of +2 (the most common, forming Cu(II) compounds), and +1 (Cu(I), stable only in aqueous solution if part of a stable complex ion);  a few compounds of copper(III) are also known.  Although the electronic configuration of copper is formally similar to that of the alkali metals (Group IA) in general and potassium in particular, the behavior of copper is considerably different from that of the alkali metals.  The shielding of the outer electron from the attraction of the nucleus is stronger than in copper.  Thus the outer electron in copper is more tightly bound, resulting in a comparatively high first ionization potential and a relatively small ionic radius for copper.

The outstanding feature of copper and the other metals of Group IB (gold and silver) is their resistance to chemical attack. Copper is slowly attacked by moist air, and its surface gradually becomes covered with the characteristic green patina that consists of basic sulfate.  At about 300 deg C copper is attacked by air or oxygen, and a black coating of copper(II) oxide forms at the surface;  at a temperature of 1,000 deg C copper(I) oxide is formed instead.  The metal is attacked by sulfur vapor, with the formation of copper(I) sulfide;  and by the halogens, which form copper(II) halides, except iodine, which forms copper(I) iodide.  Copper is not attacked by water or steam, and dilute nonoxidizing acids, such as dilute hydrochloric and dilute sulfuric acids, have no effect in the absence of an oxidizing agent.  The metal is attacked by boiling concentrated hydrochloric acid with the evolution of hydrogen, by hot concentrated sulfuric acid, and by dilute or concentrated nitric acid.

Alloys of Copper

Copper mixes well with many elements, and more than 1,000 different alloys have been formed, several of which are technologically significant.  The presence of the other element or elements can modify the hot or cold machining properties, tensile strength, corrosion fatigue, and wear resistance of the copper;  it is also possible to create alloys of pleasing colors.

The best-known alloy of copper is \Tbrass\t, which consists of copper containing between 5% and 40% zinc.  It possesses a high tensile strength, hardness, and wear-resistance.  The addition of 0.5%-3% lead to a brass alloy (leaded brass) improves the machinability of brass, and brass containing 30%-40% zinc and 1% tin (tin brass) has a high corrosion-resistance.

Another useful alloy of copper is nickel silver, which consists of copper (55%-65%), nickel (10%-18%), and zinc (17%-27%).  It is used as a base for silver-plating items such as costume jewelry and tableware.

Phosphor bronze is formed by the addition of up to 0.35% of phosphorus to copper-tin alloys containing up to 10% tin.  This alloy has great resiliency, fatigue endurance, hardness, and corrosion resistance;  these properties make it suitable for use in springs and diaphragms.

Silicon bronze, consisting of 1%-3% silicon, 95%-96% copper, and small amounts of other metals--for example lead, tin, zinc, manganese, iron, or nickel--is as strong as mild steel and has a high resistance to corrosion.  It is used in the production of equipment for chemical plants in which corrosive liquids are handled.

Aluminum-copper bronzes contain aluminum (5%-12%) and sometimes zinc and silicon;  they are also corrosion resistant, and have good strength, hardness, and wear resistance.  They are used for carrying corrosive liquids such as hot brine in salt refineries.

Beryllium-copper alloys, containing 2% beryllium, have a high corrosion resistance and high tensile strength, with considerable fatigue and wear resistance.  They find wide application where high strength is required and for making non-spark-forming tools.

Relationship to Life Sciences

Copper is a trace element essential to the healthy life of many plants and animals, in which it usually occurs as part of the oxidizing enzymes such as ascorbic acid oxidase, tyrosinase, lactase, and monoamine oxidase.  These enzymes, which are high-molecular-weight proteins containing 0.05%-0.35% of Cu, play an important part in living oxidation and reduction reactions, in which the copper undergoes cyclic changes between Cu(I) and Cu(II) oxidation states.  The metal is tightly bound to ligand sites, containing oxygen, sulfur, or nitrogen atoms on the protein.

The normal diet of humans includes between 2 and 5 mg of copper per day, exceeding the body maintenance requirements of about 2 mg per day.  The hereditary deficiency of the protein ceruloplasmin, known as Wilson's disease, is associated with a pathological increase in the copper content of almost all tissues, particularly the brain and liver.  Albino mammals lack the normal form of the copper-containing enzyme tyrosinase, which participates in the synthesis of the pigment melanin. Copper can be toxic in large quantities, especially to lower organisms such as bacilli, fungi, and algae.

Compounds of Copper

The most important simple salts of copper are copper(II) oxide, CuO, and copper(II) sulfate, also known as blue vitriol.

Copper(II) oxide may be obtained as a black solid by heating either copper(II) carbonate or copper(II) nitrate.  On heating copper(II) oxide to about 800 deg C it decomposes to copper(I) oxide and oxygen.  Copper(II) oxide reacts readily with warm mineral acids to form copper(II) salts.  Copper(II) sulfate is prepared by reacting either copper(II) oxide or copper(II) carbonate with dilute sulfuric acid.  The mixture is evaporated by heat to obtain a saturated solution, and the blue solid pentahydrate crystals separate on cooling.  On an industrial scale, copper(II) sulfate is obtained by forcing air through a hot mixture of copper and sulfuric acid.

Copper resembles the transition metals in forming a large number of organometallic compounds or complexes;  in recent years this has led to renewed interest in the study of inorganic chemistry.

Applications of Copper and Its Compounds

The electrical industry is a major consumer of copper.  The metal is used for the windings of generators and for conveying electrical power.  Its resistance to chemical attack and its high thermal conductivity make copper a useful metal for condensers in chemical plants and for car radiators.  Copper tubing is widely employed in plumbing, and finely divided copper is used as an industrial catalyst in the oxidation of methanol to formaldehyde.  Copper compounds, such as Fehling solution, are used in analytical tests for sugars.

Copper(II) sulfate has many industrial applications, including the preparation of Bordeaux mixture (a fungicide) and the manufacture of other copper compounds.  It is also used in electroplating solutions, in textile dyeing, and as a timber preservative.  Probably the earliest use of copper as a fungicide was in the form of copper sulfate solution employed as a seed dressing to destroy cereal disease, such as bunt. J. ALISTAIR \TKerr\t

The Copper Industry Today

The world's largest producer of copper currently is Chile.  The United States, Canada, and the \TUSSR\t are other major producers, followed by Zaire, Zambia, and Peru.  Annual world mine production in the late-1980s totaled about 8 million metric tons.

Consumption of copper has declined sharply in recent years.  In the United States alone, total consumption fell from a 1973 high of 2.2 million metric tons to 1.6 million in 1982;  of this total, almost half was in the form of copper recovered from scrap.  The worldwide recession of the early 1980s--when such heavy copper consumers as the housing and automobile industries experienced sharply lower sales--is partly responsible for the decline in copper use.  In addition, new materials are increasingly replacing copper:  plastic for pipe and tubing, lightweight aluminum for automobile parts.

As a result of the sharp decline in copper demand, many U.S. and Canadian producers have either closed their facilities or curtailed production.  One of the largest open-pit mines in the world, the Berkeley Pit in Butte, Mont., was closed in 1982. The state of Arizona, the leading producer, was particularly hard hit;  by 1986 employment in the copper industry there had dropped to less than 7,000 from a postwar high of 25,000.  The closures, cutbacks, and wage reductions occasioned bitter, sometimes violent labor disputes.

In contrast, government-subsidized copper industries in such countries as Chile and Zaire continued to produce copper even when its market price was lower than the costs of mining it. Chile operates its mines at 90% of capacity and Zaire at 95%. Some 40% of total world copper production is now controlled by governments, most of them in the Third World.  Many of these governments receive financial aid from the United States, or from such entities as the World Bank, which the United States supports.  This was a sore point with U.S.  producers.

The recession of the early 1980s was followed by an upswing in world economies, beginning in 1984.  By 1988 copper consumption in the United States had soared to its highest point in a decade.  U.S.  mines reopened and miners were reemployed.

Copper wire for electric and electronic uses was the most important area of refined copper sales.  Brass products, industrial machinery, automobiles and other forms of transport, and consumer products were all large copper consumers.  In the fields where copper traditionally has been strong, only housing failed to revive.

Solvent extraction, a new copper refining technique, is proving highly successful.  In a process resembling that recently developed for treating gold ores, low-grade copper ores--once considered valueless--are treated with a weak acid solution, and after being combined with other chemicals, the leachate is subjected to electrolysis.  Copper derived from leachates is called "electrowon," and costs for electrowon copper are far below those for copper derived from smelting.

Bibliography:  Barrett, E.  M., The Mexican Colonial Copper Industry (1987);  Bowen, Robert, and Gunatilaka, Anada, Copper: Its Geology and Economics (1977);  Cunningham, Simon, The Copper Industry in Zambia (1981);  Herbert, E., Red Gold of Africa (1984);  Mezger, Dorothea, Copper in the World Economy (1980);  Moran, Theodore H., Multinational Corporations and the Politics of Dependence;  Copper in Chile (1975);  West, E.  G., Copper and its Alloys (1982).