An Introduction to Mining and Metallurgy - Thanatia: The Destiny of the Earth's Mineral Resources - page 171

The newly developed technique of flash smelting of sulphide ores, meanwhile,
takes advantage of the accompanying iron sulphides 5 . This process forms a slag
with fluxing material (limestone), an iron oxide and another stream, a matte that
contains a partially oxidised metal-iron concentrate. The energy liberated by the
oxidation of iron sulphide is recovered to reduce the sulphide metal to its elementary
state. Copper and nickel are produced in this way. A final converting furnace
oxidises the iron sulphide of the molten matte with blown air. This two-step process
also facilitates the recovery of SO 2 gases.
Electrorefining is the last step used in the purification of non-ferrous metals. It
is an electroplating process, whereby the non-purified metal constitutes the anode,
which is dissolved in an acidic electrolyte transporting media. On passing an electric
current, the metallic ions migrate to the cathode where they deposit as pure metal.
Impurities containing oxygen, sulphur and other base metals are thus eliminated.
It is very important to understand the behaviour of elements in order to re-
cover valuable metals such as Ni;Cu and Co or to eliminate detrimental impurities
such as As;Sb and Bi, especially as the processes described above work at very
high temperatures and release volatile and highly toxic gases, including those of
heavy metals (Tan and Neuschutz, 2001). This is because thermochemistry plays a
key role. It dictates the Gibbs free energies and the temperatures for the thermal
decomposition, melting, reducing and electrorefining processes. A comprehensive
overview of this behaviour is provided by the Ellingham diagram (Ellingham, 1944)
in which G of any metal reaction (mainly oxidation processes) is graphically rep-
resented over temperature (a thermodynamic overview of smelting processes and
the Ellingham diagram are explained in detail in Sec. 9.4.3.1).
7.5.2 Hydrometallurgy
Hydrometallurgical techniques can be used for the recovery of ores, concentrates,
recycled or residual materials. They offer an alternative way of energy savings by
taking advantage of aqueous chemistry. Leaching or metal lixiviation is the first
and main stage. A reagent dissolved in water extracts valuable metals producing
the pregnant liquor stream PLS, ready to be treated. The most common reagent is
sulphuric acid 6 but ammonia and alkalis may also be used. The most commonly
utilised process occurs in the open air where the material is arranged in extensive
heaps or in dumps and the leach solution is sprayed and left to percolate through
the material and emulate naturally occurring processes. Alternatively, large tanks
or vats can be used to better control pH, oxygen, pressure or temperature. The re-
covery of metals from the PLS may undergo a wide and an ever-expanding range of
versatile techniques such as concentration, purification from undesired substances,
5 From a thermodynamic viewpoint it is not reasonable to first convert sulphur ore into an oxide
for metal recovery when it is cheaper from an energy perspective to convert it directly. This is
because the chemical potential of the sulphide is higher than that of the oxide.
6 Sulphuric acid is corrosive and non-metal-selective. It thus complicates the process downstream.