Geology Reference
In-Depth Information
8.8.4 Cadmium
Cadmium has no specific ore as su
cient quantities are produced as a byprod-
uct from the smelting of either zinc or lead from their ore, sphalerite (ZnS), in
which greenockite CdS is a significant impurity constituting as much as 3%. The
purification process of zinc solutions with zinc powder can selectively produce cad-
mium cementate by a careful control of temperature, reaction time, pH, surfactants
and metallic zinc excess. Cadmium can be recovered from this compound via the
hydrometallurgical route. This consists of leaching cementate in sulphuric acid,
followed by purification with caustic soda and sodium nitrate to remove the re-
maining zinc and finally, electro-winning. Cadmium can also be recovered from
cadmium chloride liquor via an ion exchange process in a trommel with a zinc strip.
In this case the metallic zinc becomes converted into zinc chloride and cadmium
is deposited as a metallic sponge. The refining process of cadmium predominately
consists of removing its residual zinc via high temperature distillation with the sub-
sequent melting together with caustic soda and sodium nitrate (Gouvea and Morais,
2007).
According to Morral et al. (1985), 6.48 GJ/t of metal, which could be considered
as the energy used in the process of metal refining, is required to obtain a kilogram
of cadmium from an aqueous solution. On the other hand, if one is to consider
the process of metal concentrate as a form of sulphuric acid leaching, the energy
requirement for a transaction of this type is (mean value) 103.5 GJ/t of cadmium,
should the initial concentration be 0.4%, which equates to the typical zinc ore grade
(Kihlstedt, 1975). A more recent publication (IPPC, 2002) reports that the content
of Cd in zinc sludge is around 45%. In turn, 339 kg of sludge is contained within
1,000 kg of Zn. So, if the energy consumption of the smelting stage is 40.4 GJ/t
plus 1,250 kWh/t of zinc, this gives an approximate energy consumption of cadmium
equal to 279 GJ/t.
8.8.5 Indium, Germanium and other co-products
Lead and zinc ores contain other metals, not just cadmium, in varying quantities
that are beginning to receive huge commercial interest - mainly from those in the
electronics and energy storage sectors. These are indium, gallium, germanium,
arsenic, tellurium, antimony, bismuth and selenium. As described previously, all
appear concentrated in the residual slag, dross, flue dusts or precipitates. Their
recovery is done on a metal by metal basis and can include various combinations
of leaching, cementation, solvent extraction, chlorination, vacuum distillation and
electro-winning techniques. In addition, most of the refining processes use fine
chemistry and many of these techniques are confidential (Ally et al., 2001).
Indium is obtained from the zinc oxide residue generated in the leaching process
with dilute sulphuric acid. This residue is first treated with dilute hydrochloric acid
to remove the lead before leaving a filtrate constituted by As, Sb, Sn and less than
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