What is Copper?

Copper is a chemical element with the symbol Cu (from Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish. It is used as a conductor of heat and electricity, a building material, and a constituent of various metal alloys.
The metal and its alloys have been used for thousands of years. In the Roman era, copper was principally mined on Cyprus, hence the origin of the name of the metal as сyprium (metal of Cyprus), later shortened to сuprum. Its compounds are commonly encountered as copper(II) salts, which often impart blue or green colors to minerals such as turquoise and have been widely used historically as pigments. Architectural structures built with copper corrode to give green verdigris (or patina). Decorative art prominently features copper, both by itself and as part of pigments.
Copper(II) ions are water-soluble, where they function at low concentration as bacteriostatic substances, fungicides, and wood preservatives. In sufficient amounts, they are poisonous to higher organisms; at lower concentrations it is an essential trace nutrient to all higher plant and animal life. The main areas where copper is found in animals are tissues, liver, muscle and bone.

Characteristics

Physical

 
Copper, silver and gold are in group 11 of the periodic table, and they share certain attributes: they have one s-orbital electron on top of a filled d-electron shell and are characterized by high ductility and electrical conductivity. The filled d-shells in these elements do not contribute much to the interatomic interactions, which are dominated by the s-electrons through metallic bonds. Contrary to metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are relatively weak. This explains the low hardness and high ductility of single crystals of copper.[1] At the macroscopic scale, introduction of extended defects to the crystal lattice, such as grain boundaries, hinders flow of the material under applied stress thereby increasing its hardness. For his reason, copper is usually supplied in a fine-grained polycrystalline form, which has greater strength than monocrystalline forms.

The low hardness of copper partly explains its high electrical (59.6×106 S/m) and thus also high thermal conductivity, which are the second highest among pure metals at room temperature.[3] This is because the resistivity to electron transport in metals at room temperature mostly originates from scattering of electrons on thermal vibrations of the lattice, which are relatively weak for a soft metal.[1] The maximum permissible current density of copper in open air is approximately 3.1×106 A/m2 of cross-sectional area, above which it begins to heat excessively.[4] As with other metals, if copper is placed against another metal, galvanic corrosion will occur.[5]
Together with osmium (bluish), caesium (yellow) and gold (yellow), copper is one of only four elemental metals with a natural color other than gray or silver.[6] Pure copper is orange-red and acquires a reddish tarnish when exposed to air. The characteristic color of copper results from the electronic transitions between the filled 3d and half-empty 4s atomic shells – the energy difference between these shells is such that it corresponds to orange light. The same mechanism accounts for the yellow color of gold.[1]

Chemical

Copper forms a rich variety of compounds with oxidation states +1 and +2, which are often called cuprous and cupric, respectively.[7] It does not react with water, but it slowly reacts with atmospheric oxygen forming a layer of brown-black copper oxide. In contrast to the oxidation of iron by wet air, this oxide layer stops the further, bulk corrosion. A green layer of verdigris (copper carbonate) can often be seen on old copper constructions, such as the Statue of Liberty, the largest copper statue in the world build using repoussé and chasing.[8] Hydrogen sulfides and sulfides react with copper to form various copper sulfides on the surface. In the latter case, the copper corrodes, as is seen when copper is exposed to air containing sulfur compounds.[9] Oxygen-containing ammonia solutions give water-soluble complexes with copper, as do oxygen and hydrochloric acid to form copper chlorides and acidified hydrogen peroxide to form copper(II) salts. Copper(II) chloride and copper comproportionate to form copper(I) chloride.[10]

Occurrence

Copper can be found as either native copper or as part of minerals. Native copper is a polycrystal, with the largest single crystals found to date measuring 4.4×3.2×3.2 cm.[14] The largest mass of elemental copper weighed 420 tonnes and was found in 1857 on the Keweenaw Peninsula in Michigan, US.[15] There are many examples of copper-containing minerals: chalcopyrite and chalcocite are copper sulfides, azurite and malachite are copper carbonates and cuprite is a copper oxide.[3] Copper is present in the Earth's crust at a concentration of about 50 parts per million (ppm),[15] and is also synthesised in massive stars.[16]

Production

Most copper is mined or extracted as copper sulfides from large open pit mines in porphyry copper deposits that contain 0.4 to 1.0% copper. Examples include Chuquicamata in Chile, Bingham Canyon Mine in Utah, United States and El Chino Mine in New Mexico, United States. According to the British Geological Survey, in 2005, Chile was the top mine producer of copper with at least one-third world share followed by the United States, Indonesia and Peru.[3] The amount of copper in use is increasing and the quantity available is barely sufficient to allow all countries to reach developed world levels of usage.[17]

Reserves

Copper has been in use at least 10,000 years, but more than 95% of all copper ever mined and smelted has been extracted since 1900. As with many natural resources, the total amount of copper on Earth is vast (around 1014 tons just in the top kilometer of Earth's crust, or about 5 million years worth at the current rate of extraction). However, only a tiny fraction of these reserves is economically viable, given present-day prices and technologies. Various estimates of existing copper reserves available for mining vary from 25 years to 60 years, depending on core assumptions such as the growth rate.[18] Recycling is a major source of copper in the modern world.[19] Because of these and other factors, the future of copper production and supply is the subject of much debate, including the concept of Peak copper, analogue to Peak Oil.
The price of copper has historically been unstable,[20] and it quintupled from the 60-year low of US$0.60/lb (US$1.32/kg) in June 1999 to US$3.75 per pound (US$8.27/kg) in May 2006. It dropped to US$2.40/lb (US$5.29/kg) in February 2007, then rebounded to US$3.50/lb (US$7.71/kg) in April 2007.[21] In February 2009, weakening global demand and a steep fall in commodity prices since the previous year's highs left copper prices at US$1.51/lb.[22]
Methods
The concentration of copper in ores averages only 0.6%, and most commercial ores are sulfides, especially chalcopyrite (CuFeS2) and to a lesser extent chalcocite (Cu2S).[23] These minerals are concentrated from crushed ores to the level of 10–15% copper by froth flotation or bioleaching.[24] Heating this material with silica in flash smelting removes much of the iron as slag. The process exploits the greater ease of converting iron sulfides into its oxides, which in turn react with the silica to form the silicate slag, which floats on top of the heated mass. The resulting copper matte consisting of Cu2S is then roasted to convert all sulfides into oxides:[23]
2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2
The cuprous oxide is converted to blister copper upon heating:
2 Cu2O → 4 Cu + O2
This step exploits the relatively easy reduction of copper oxides to copper metal. Natural gas is blown across the blister to remove most of the remaining oxygen and electrorefining is performed on the resulting material to produce pure copper:[25]

Cu2+ + 2 e– → Cu

Recycling
Copper, like aluminium, is 100% recyclable without any loss of quality whether in a raw state or contained in a manufactured product. In volume, copper is the third most recycled metal after iron and aluminium. It is estimated that 80% of the copper ever mined is still in use today.[26]
The process of recycling copper follows roughly the same steps as is used to extract copper, but requires fewer steps. High purity scrap copper is melted in a furnace and then reduced and cast into billets and ingots; lower purity scrap is refined by electroplating in a bath of sulfuric acid.[27]

Applications

The major applications of copper are in electrical wires (60%), roofing and plumbing (20%) and industrial machinery (15%). Copper is mostly used as a metal, but when a higher hardness is required it is combined with other elements to make an alloy (5% of total use) such as brass and bronze.[15] A small part of copper supply is used in production of compounds for nutritional supplements and fungicides in agriculture.[28][60] Machining of copper is possible, although it is usually necessary to use an alloy for intricate parts to get good machinability characteristics.

Electronics and related devices

The electrical properties of copper are exploited in copper wires and devices such as electromagnets. Integrated circuits and printed circuit boards increasingly feature copper in place of aluminium because of its superior electrical conductivity; heat sinks and heat exchangers use copper as a result of its superior heat dissipation capacity to aluminium. Vacuum tubes, cathode ray tubes, and the magnetrons in microwave ovens use copper, as do wave guides for microwave radiation.[61]

Architecture and industry

Because of the waterproof nature of copper, it has been used as the roofing material of many buildings since ancient times. The green colour on these buildings is due to a long-term chemical reaction: copper is first oxidised to copper(II) oxide, then to cuprous and cupric sulfide and finally to copper(II) carbonate, also called verdigris, which is highly corrosion-resistant.[62] The copper used in this application is phosphorus deoxidized copper (Cu-DHP).[63] Lightning rods use copper as a means to divert electric current throughout the ground instead of destroying the main structure.[64] Copper has excellent brazing and soldering properties and can be welded; the best results are obtained with gas metal arc welding.[65]

Copper in alloys

See also: List of copper alloys

Numerous copper alloys exist, many with important uses. Brass is an alloy of copper and zinc and bronze usually refers to copper-tin alloys, but can refer to any alloy of copper such as aluminium bronze. Copper is one of the most important constituents of carat silver and gold alloys and carat solders used in the jewelry industry, modifying the color, hardness and melting point of the resulting alloys.[66]

The alloy of copper and nickel, called cupronickel, is used in low-denomination statuary coins, often for the outer cladding. The US 5-cent coin called nickel consists of 75% copper and 25% nickel and has a homogeneous composition.