Geology Reference
In-Depth Information
electrolytic copper slimes by cementation with copper as occurs in the Outokumpu
process. Copper telluride is formed and then leached with caustic soda and air to
produce the sodium telluride solution. A controlled oxidative process follows which
can produce either commercial grade tellurium metal or its oxide TeO
2
(Hoffmann
and Reimers, 2000; Langner, 2000; Fthenakis et al., 2007).
8.6 Tin
8.6.1 Process
About 80% of the world's tin resources are found in alluvial deposits containing
around 0.2 kg=m
3
(0.01% Sn). The principal ore is cassiterite (SnO
2
), which can
be found mixed with other minerals such as stannites (Cu
2
S, FeS, SnS
2
). These
deposits have grades varying between 1% and 0.1%, with the percentage required
for commercial exploitation currently standing at 0.35%.
Whilst tin rich ores containing large amounts of iron, sulphur, bismuth and
tungsten require no pretreatment, the key problem in obtaining tin in the poorer
quality deposits is the need to prepare a high tin concentration (35 to 70% Sn).
The vein ores must be crushed and ground before undergoing a hydro-selection or a
flotation process in order to remove much of the gangue. Magnetic separation can
then remove the accompanying wolframite.
Once it is concentrated, the extraction of tin from its ores is undertaken via
a fusion reduction, in which the metal is melted in a reverberatory furnace and
then separated from a viscous slag containing little in the way of tin content. The
melting reverberatory furnace comprises three distinct stages: primary melting of
tin concentrates with suitable addition of fluxes and fuel, a recovery process of tin
from the slag and a final elimination of metal impurities by the way of a reduction
process and tin fusion. An electric furnace treatment follows in a two-step process.
In the former step, cassiterite melts alongside a tin-iron alloy obtained in the second
stage fusion. This fusion produces a relatively pure tin, plus a tin-rich slag. This
granulated slag is melted with coal and limestone flux to produce yet more tin (an
iron alloy that is recirculated and a runoff containing relatively little tin). The final
refining of tin may include liquefaction and boiling, or can be done via electrolytic
refining (Botero, 2000).
8.6.2 Energy and environmental issues
According to the Bureau of International Recycling (Grimes et al., 2008), the energy
requirement for the complete life cycle of tin production is 200 GJ/t (including
mining and metallurgical processes). They also report that the energy require-
ment for the primary production of one tonne of tin is 18.2 GJ (smelting process
only), whereas for its secondary production this figure drops to only 0.2 GJ/t Sn.
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