James Richard Fromm
Copper is occasionally found as the free metal in nature and is then known as native copper. Artifacts of native copper are found in primitive areas, such as among the Indians of the Pacific Northwest, and are known from antiquity, dating back to about 5000 BC. In Asia and Asia Minor, such native copper was annealed (heated) to obtain greater workability since about 4200 BC. Native copper is often found somewhat dispersed in rock. Upon heating, the metal melts and flows away from the rock, an early smelting method known as liquation. Native copper is available only in small quantity and was more of decorative than of practical use for that reason.
About 3000 BC, the oxide ores of copper were first reduced with carbon in the form of charcoal, initiating the significant production of metallic copper and the Bronze Age. The Bronze Age is normally considered to run from about 3000 BC, when the stone tools and weapons which characterize the Stone Age begin to be replaced by those of copper and its alloys. Bronze, a rather durable brown alloy consisting essentially of copper and tin, was by far the most important form of copper used.
The other important alloy of copper is the yellowish alloy brass, which consists of copper and zinc. Zinc itself was not known in the ancient world, and was produced as zinc metal in the Western world only after about AD 1600. However, brass objects are known which date from as early as 800 BC. This ancient brass was produced by melting copper together with a zinc carbonate ore and charcoal. Effectively, the copper metal served as the reducing agent for the zinc and was in turn reduced to copper again by the charcoal. In modern zinc production, zinc sulfide ores are roasted in air to ZnO which is then reduced using carbon monoxide (from carbon combustion) to give Zn(g). The metallic zinc condenses to the solid metal as it leaves the furnace.
The Bronze Age is generally considered to end about 1200 BC, as its bronze implements were replaced by the iron tools and weapons characteristic of the Iron Age which followed it. The two oxides of copper Cu2O (cuprite) and CuO (tenorite) are both reducible to the metal by use of carbon or charcoal. The reactions are
2CuO + C CO2(g) + 2Cu,
for which DG0 is -134.9 kJ/mole reaction, and
2Cu2O + C CO2(g) + 4Cu,
for which DG0 is -102.4 kJ/mole reaction.
Another ore of copper, malachite, has the chemical formula CuCO3.Cu(OH)2. It too can be smelted using charcoal, the slightly unfavorable free energy of reaction being overcome by the favorable free energy of reaction of the following reduction step,
CuCO3.Cu(OH)2 CO2(g) + H2O(g) + 2CuO,
for which DG0 = +134.3 kJ/mole.
Toward the end of the Bronze Age, about 1500 BC, roasting was added to convert the more abundant sulfide ores of copper to the oxides, considerably increasing the copper supply. The most abundant ore is chalcopyrite (CuFeS2), but copper sulfides themselves are also found. The roasting process of the sulfide ore takes place after its concentration and melting to form matte copper, which is either Cu2S (chalcocite) or CuS (covellite). The roasting is exothermic as is the roasting of silver and lead sulfides. The reaction is
2CuS + 3O2 2SO2 + 2CuO.
The standard free energy change for this reaction is -752.6 kJ/mole reaction.
In the sulfide ores of copper one of the major impurities is iron(II) sulfide, FeS. In the roasting process the iron sulfide is converted to oxide and then by reaction with silicates present to a slag which separates from the copper oxide in the reduction step. These reactions are
2FeS + 3O2 2SO2(g) + 2FeO,
FeO + SiO2(l) FeSiO3(l).
After the reduction or smelting step, copper can be refined by blowing air through the molten metal. This oxidizes any other metals which are present as impurities; the oxides rise to the surface and can be skimmed off. If the refining is carried out at too high a temperature or the airblast is too strong, some of the copper is oxidized as well and the metal is made brittle by the oxide present. The excess oxide can be reduced by stirring the molten metal with poles of green wood (poling). This process, devised in the ancient world, is still used today and produces so-called "blister" copper, about 99% pure.
The production of copper from its ores today is still virtually the same as in the ancient world. The significant improvements have been in later purification steps and the ability to process economically larger tonnages of lower-grade ore by concentration procedures such as froth flotation, to the point where ores containing 1% copper can be economically mined. Modern copper refineries use an electrolytic step to effect further purification of the metal; ancient refineries did not. We will return to this electrolytic step in a later section.
Tin ore, at least in the ancient world, was normally cassiterite, or stream-tin. Cassiterite is tin(IV) oxide, SnO2, a dense black material. It is normally found in the beds of streams in association with alluvial gold and hence was well known in ancient times. Although the metal could be, and was sometimes in the ancient world, produced from the oxide, it was more usual to add the stream-tin directly to copper or copper oxide and reduce the tin there either with copper or with the excess of charcoal which would be present, thus giving the desired alloy, bronze.
Reduction of tin with carbon is not an easy process since the free energies of reduction are unfavorable. For the reaction
SnO2 + C Sn + CO2, DG0 = +125.2 kJ/mole,
and for the reaction
SnO2 + C 2SnO + CO2, DG0 = +131.0 kJ/mole.
However, reduction with carbon monoxide is considerably less unfavorable.
For the reaction
SnO2 + CO SnO + CO2, DG0 = +5.5 kJ/mole,
and for the reaction
SnO + CO Sn + CO2, DG0 = -0.3 kJ/mole.
Reduction probably occurs by the carbon monoxide route; it is facilitated by use of excess carbon whose free energy of combustion carries the unfavorable tin reduction reaction over. Modern tin reduction plants employ a reverberating furnace with a blast of air and combustion of large quantities of excess carbon to carry the reaction.