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Copper in the USA: Bright Future Glorious Past
At the beginning of copper’s flow through the economy are the mining companies, which process vast quantities of low-grade ore, mostly from open-pit mines in order to produce copper. Approximately two tons of overburden must be removed along with each ton of copper ore. The ratio of overburden to ore is sometimes as high as 5 to 1. The ore itself averages less than 0.7% copper in US mines.
Copper ore normally is crushed, ground, and concentrated, usually by flotation, to produce a beneficiated ore containing about 25% copper. The ore concentrates are reduced to the metallic state, most often by a pyrometallurgical process. Traditionally, the concentrated ore is processed in a primary smelting reactor, such as a reverberatory furnace ², to produce a copper sulfide-iron sulfide matte, up to 60 percent copper. Today reverberatory technology is rapidly being replaced by oxygen/flash smelting, which greatly reduces the volume of off-gases. Sulfuric acid is manufactured from the sulfur dioxide contained in these off-gases, reducing air pollution by 95% or more and providing an important co-product of copper smelting. The matte is oxidized in a converter to convert the iron sulfides to iron oxides, which separate out in a slag, and to reduce the copper sulfide to blister copper, which contains at least 98.5 percent copper. Current technology combines the converting step with the preceding smelting step. Fire refining of blister copper then removes most of the oxygen and other impurities, leaving a product at least 99.5 percent pure, which is cast into anodes. Finally, most anode copper is electrolytically refined ³, usually to a purity of at least 99.95 percent.
The resulting cathodes are the normal end product of the producer companies and are a common item of commerce. In recent years, many producers have installed continuous-cast rod mills to directly convert cathode copper to wire rod (typically 5/16 inch in diameter), the feed material for the wire and cable mills. Primary producers may also convert the cathode to cakes or billets of copper for sale to brass mills. The consumption of refined copper (mostly cathodes) in the United States was about 2.5 million short tons (2.3 million metric tons) in 1989, about 27% of the total free world usage of 9.2 million short tons (8.3 million metric tons).
Hydrometallurgical processing is an increasingly important alternative to pyrometallurgy, particularly for nonsulfide ores, such as oxides, silicates, and carbonates. Weak acid is percolated through ore or waste dumps of rejected materials. Copper is leached out of the ore by the acid solution extraction, to produce an electrolyte suitable for electrowinning, wherein copper is extracted electrolytically much as anode copper is electrorefined. Electro-won copper is equal in quality to that produced by electrolytic refining.
In recent years well over half the copper consumed in the United States has been derived from recycled scrap, and this percentage has grown somewhat over the last two decades. About 55% of this scrap in recent years has been “new” scrap, such as turnings from screw-machined rod, and 45% has been “old” scrap, such as used electrical cable or auto radiators. Scrap recycled within a particular plant or company-runaround scrap-is not included in these statistics. About one-third of the scrap recycled in the United States is fed into the smelting or refining stream and quickly loses its identity. The remainder is consumed directly by brass mills; by ingot makers, whose main function is to process scrap into alloy ingot for use by foundries; by foundries themselves; by powder plants; and by others such as the chemical, aluminum, and steel industries.