Geology Reference
In-Depth Information
in their corresponding sections (Sec. B.2.31 and Sec. B.2.17). When gold appears
as a co-product of copper or other metals in their metallurgical extraction, the
primary gold source comes from the anode slimes of the electro-refining processes
10
.
According to the Ecoinvent database (Classen et al., 2007), some 2,159 kg of blister
Cu feed produces 143 kg of precious metal slime feed with slime refining reducing it
down to 36 kg. This quantity in turn is composed of 1 kg of gold, 32.9 kg of silver
and 1.82 kg of palladium.
The separation process of silver and other impurities such as lead from gold
is named gold parting. It is done in the Miller process which consists of melting
impure gold in an induction furnace at 1000
o
C while blowing chlorine gas through
the melt. Borax is added as a flux for a better skimming. Nearly all elements, except
gold, form chlorides that float on top of the melt, as insoluble salts or volatiles. The
gases meanwhile, especially zinc chloride, are captured in the scrubbing system.
The resulting gold purity can reach 98-99.5%. If a higher purity is needed, then the
gold is cast into the anodes for further electro-refining in the Wohlwill electrolytic
process. This occurs in an electrolyte bath of acidic gold chloride at 70
o
C where
the anode dissolves and pure gold deposits at the titanium cathodes. Platinum and
palladium meanwhile remain in the electrolyte solution as slimes and are recovered
later (IPPC, 2009).
Gold easily dissolves in aqua regia (a mixture of nitric and hydrochloric acids in a
ratio of 1:3 parts) or in hydrochloric acid/chlorine solution, forming a gold chloride
which dissolves in water and later precipitates as a metal by means of different
reagents. This technique may be used to treat gold containing anode slimes from
silver electrolysis, as has already been described. Gold can also be removed from
cyanide by electrolysis in enclosed installations. The hydrogen cyanide produced is
immediately oxidised with a very strong oxidant such as hydrogen peroxide.
8.9.3 Platinum Group Metals process
This group is composed of Pt, Pd, Rh, Ru, Ir and Os. Their ores are alluvial
deposits coming from ultrabasic magmas from the Earth's interior. Their present
economic limit of exploitation is with ores containing at least 0.05 ppm. All appear
in minerals such as nickel-pyrites and chromium-pyrites carrying arsenides, stibines
and sulphides of PGMs. According to Hochfeld (1997), a typical ore contains metals
in the following ranges: Ni (0.1-0.3%), Cu (0.1-0.17%), Pt (3.2-6.0 ppm), Pd (2-20
ppm), Rh (0.21-054 ppm), Ir (0.06-0.21 ppm), Os (0.04-0.07 ppm) and Au (0.07-
0.26 ppm).
Since most of PGMs are obtained from nickel sulphides, the beneficiation is the
same as that of the originating ore. Thereupon, the ore's iron sulphide content is
decreased via floatation pretreatment, concentrating the ore some 20-30 fold to get a
10
See http : ==geology:com=publications=getting gold=goldextraction:shtml. Accessed Aug.
2011.
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