|A COPPER BEARING MINERAL WITH AS MUCH
AS 50% COPPER, WHICH IS WHERE IT GETS ITS GREEN COLOR. The distinctive
bright-green hydrous CARBONATE MINERAL malachite is a common but minor
ore of copper. It is usually found in copper deposits associated with
LIMESTONE, occurring with AZURITE as the weathering product of other
copper ore minerals. Hardness is 3 1/2 to 4, streak is pale green,
specific gravity is 3.9 to 4.1, and luster is adamantine to silky.
Malachite forms needlelike prismatic crystals (monoclinic system) that
are rarely distinct; it is usually found in granular, earthy, or
fibrous masses and rounded, banded crusts. Malachite is used as a
decorative stone when cut and polished, a semiprecious gem, and a green
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
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
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
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
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 BRASS, 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.
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.
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.
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.