Richard Kelham asks if there was any commercial exploitation of
malachite, a copper carbonate/hydroxide.
Yes, very much so. Being a nicely coloured stone, it may well have
been the origin of the Bronze Age, when early people built their
hearths of the nice green stone and discovered smelting. The black
oxide, when heated with charcoal, yields metallic copper.
One of the earliest "commercial" examples was the use of malachite
by the Zimbabwe Empire, which produced copper currency from
malachite deposits at Kansanshi and Bwana Mkubwa.
Typically, malachite forms near the surface in the zone penetrated
by air and groundwater, so converting the original sulphide minerals
in an "oxide capping". At Malanjkand in India, for example, malachite
was the dominant copper mineral near the surface, and at Chambishi,
Zambia, there was malachite to a depth of 50 metres.
The "problem" of malachite for modern technology is that the flotation
process works best on sulphide minerals, so malachite is lost to tailings.
Some reagents will collect it, but generally the upper zones of ore which
contained it were known generally as "refractory" ores. In modern times
such ores were worked at the Aljoujt mine in Mauritania, between 1970
and 1983, using a furnace system known as TORCO ( Treatment Of
Refractory Copper Ores). Once the open-pit had worked its way through
the upper zone of ore, which graded 2.7% copper, the furnace was
closed down.
Tony Brewis
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