Geology Reference
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0.2% of In. The latter is treated with soda whose precipitate contains most of the
In. Successive leaching with sodium hydroxide and hydrochloric acid then allows
for the relative isolation of In in solution. The solution is purified by cementation
of Cu and As with iron, then Sn with lead and finally In with aluminium (Felix,
1996).
Since indium is obtained as a byproduct of the zinc production process, it can
be assumed that no extra energy is required in the mining and concentration of this
metal. For the refining from indium sponge, it is estimated that the sulphuric acid
leaching of the zinc byproduct obtained, would need between 750 and 5,000 GJ/t
of indium (2,875 GJ/t on average), for an initial concentration of 100 ppm (Botero,
2000).
Germanium is extracted in the Zn recovery processes with some additional quan-
tities coming from the electrolytic refining processes of copper and lead. The con-
centration process is the same as indium's. Here the key is producing germanium
tetrachloride which can be separated from other metal tetrachlorides using fractional
distillation. Once separated, the germanium tetrachloride is hydrolysed to the dio-
xide and then conventionally reduced with hydrogen or carbon to the metal element
(Fthenakis et al., 2007). Kellogg (1977) indicates that the energy requirement for
germanium production is 2,215 GJ/t of metal. Since germanium is a byproduct of
zinc production, it is assumed that this quantity of energy corresponds fully to the
process of refining the metal.
The impurities removed via the sulphuric leaching of zinc ore also contain gal-
lium together with aluminum and iron. Essentially the process of obtaining gallium
consists of treating these impurities with hydrochloric acid to dissolve Ga and Al.
Gallium is then separated from aluminium by a solvent extraction with ether and
is subsequently recovered by electrolysis. According to Kellogg (1977) gallium pro-
duction requires 12,660 GJ/t, which means that it has the third greatest energy
requirement after industrial diamonds and gold. Since gallium is obtained as a
byproduct of bauxite mining and given the scarcity of information pertaining to the
metallurgical processes for obtaining it, it is assumed that its energy requirement
corresponds entirely to the metal refining process.
8.9 Precious metals
Primary silver, gold and platinum group metals (PGMs) can be obtained as byprod-
ucts from the processing of anode slimes from copper production, leach residues
and crude metal from zinc and lead production. Silver and gold typically appear
more frequently than PGMs. Conventional pyro-metallurgical or hydrometallurgi-
cal routes and solvent extraction stages are used. The exact process details however
remain commercially confidential. A general flowsheet for precious metal production
is shown in Fig. 8.9.
 
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