Geology Reference
In-Depth Information
Secondary silver sources typically include scrap jewellery, circuit boards and pho-
tographic materials whilst the primary metallurgical source appears in anode slimes,
sludges, ashes, sweeps and other forms of waste residue. The pyro-metallurgical
route is used to treat such residues which may also contain copper, lead and other
precious metals (IPPC, 2009). All of these metals are blended and smelted in a
variety of conventional furnaces in a reducing atmosphere with fluxes to collect the
non-metallic components such as slag. The molten lead or copper also contains dis-
solved precious metals, which is subsequently oxidised in a cupellation furnace that
oxides lead to litharge (PbO) and/or copper to Cu
2
O, liberating the doré (silver
and gold), gold and PGMs for subsequent refining since precious metals do not rust.
In the hydrometallurgical production of zinc, the enriched Pb=Ag leach residue is
further processed in a lead smelter, in a process which has already been described
in Sec. 8.8.1. The zinc crust generated, containing the Au and Ag (Parkes crust
feed) is then skimmed off. It typically contains 1 kg of Ag per 5.77 kg (Ecoinvent -
Classen et al. (2007)).
The anode slimes produced in the electrolytic refining of copper also generally
contain significant quantities of precious metals. They are thus treated for their
gold and platinum group metals content. Acid leaching is used for the removal
of Cu, Ni, Se and Te. Subsequently, a smelting process employing silica and
sodium carbonate as fluxes, produces a ready to be electrolytically refined bullion
which is approximately 60% rich in precious metals. The electro-refining takes
place in a Moebius cell using titanium cathodes and an acidified silver nitrate as
the electrolyte. The silver ions dissolve at the anode whilst silver crystals deposit
on the cathodes. The silver gained is then melted in a crucible furnace ready for
casting (IPPC, 2009). The slimes are further treated for gold and PGM recovery.
8.9.2 Gold process
Native gold comes from alluvial deposits and small nuggets which have been im-
memorially extracted in placer mines using water panning and sluicing techniques.
Gold can be also found in the form of tellurides but the latter do not constitute com-
mercial ores. It is usually obtained as a co-product of sulphide mining, especially in
copper ores. It can also appear alloyed with small amounts of silver or amalgamated
with mercury and in fact the latter can be used for its extraction since mercury eas-
ily amalgamates gold and silver. This action however entails important detrimental
environmental consequences as will be seen later. Subsequently, mercury should be
recovered in a distillation retort.
Currently, low grade gold ores necessitate specific and elaborated mining, hydro
and pyro-metallurgical techniques. Cyanide heap leaching of the ground ore is
a key operation. Gold bearing sulphide ores are refractory to cyanidation and
require ultra-fine grinding and roasting or any other sulphide oxidation pretreatment
processes. Iron and copper also need to be removed and are done so as explained
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