Geology Reference
In-Depth Information
costs predominate over the rest in the overall industrial chain (from cradle to mar-
ket). Two generic ways can be used: pyrometallurgical and hydrometallurgical.
Sometimes, as in the case of nickel, a combination of both pyrometallurgy and hy-
drometallurgy takes place. The former is generically referred to as “smelting and
refining” and is the most commonly used technique. It melts and reduces the ore us-
ing thermal energy. The latter is performed in aqueous solutions, taking advantage
of differential solubilities and electrochemical properties. It thus depends on the
particular solubility of an ore in strong basic or more frequently acidic water. Solu-
bility in water requires less energy than smelting but does need aggressive chemicals
and a finer grinding, which is a very energy intensive operation in its own right.
7.5.1 Pyrometallurgy
The Encyclopedia Britannica defines smelting as the “process by which a metal is
obtained, either as the element or as a simple compound, from its ore by heating
beyond the melting point, ordinarily in the presence of oxidising agents, such as
air, or reducing agents, such as coke”. Refining typically refers to electrorefining
processes, from which the output is a metal pure enough to satisfy market demand.
Prior to smelting, carbonates and sulphides require a roasting or a calcination
process that converts them into the corresponding metal oxide. For instance, in
the case of malachite (CuCO
3
), limestone (CaCO
3
) and magnesite (MgCO
3
) all
undergo a simple thermal decomposition leaving the metal oxide free and liberating
CO
2
into the atmosphere. In the case of galena (PbS), it is first converted into
sulphate and then thermally decomposed into the metal oxide, PbO and SO
2
gases.
These gases are then further oxidised into sulphuric acid to treat the galena ore. In
the case of copper sulphide (CuS), the exothermic reaction with air to form SO
2
acts as a fuel and almost only initial activation energy is needed to bring the system
to incandescence. Sulphate roasting is also used in the production of water-soluble
cobalt and zinc oxides for further hydrometallurgical processing.
Each smelting process varies according to the nature of the ore, the metal in-
volved and the purity of the final product. All however require a high temperature
furnace with the addition of suitable reducing agents such as coke. The furnace
produces impure molten bullion and waste materials that become easily separated
by differences in density. In the case of sulphides, important by-products like gold
and silver may also be obtained. Metals and alloys like lead, magnesium and espe-
cially iron and ferrous compounds such as ferrochromium and ferromanganese can
be produced either in a blast furnace or in the commonly used electric furnace at
the expense of carbon (coal, coke or charcoal) which combusts into CO
2
gas. The
reduction
4
of alumina ore occurs via electrolysis using as the electrolyte a molten
cryolite (Na
3
AlF
6
) capable of dis
solving alumina (see Sec. B.2.1).
4
As opposed to roasting, which heats raw material to a very high temperature in contact with
air or oxygen-enriched air, reduction processes must be undertaken at high temperatures and in
the absence of air.
Search WWH ::
Custom Search