Thermodynamics of Mineral Resources - Thanatia: The Destiny of the Earth's Mineral Resources - page 271

made up of the two corresponding electrodes. The species that oxidises presents the
smaller potential and will thus be the anode, leaving the other species to reduce in
the cathode.

E cell = E red (cathode) E red (anode) (9.25)
Reaction kinetics depends on mass transfer mechanisms from the electrolyte to
the electrode and vice versa, the adsorption process of reactant particles in the elec-
trode interphase and the crystallographic structure of the electrode surface and its
properties. The reaction is in no way reversible. In fact, there are associated overpo-
tentials and resistance-related potential loss forcing an increase in both the voltage
and current density to a greater extent than the minimum dictated by the Nernst
equation. There is also the need to maintain an appropriate bath temperature as
well as other ancillary operations.
With this knowledge as a basis, one can begin to understand the electrometallur-
gical processes as described throughout Chap. 8. These take place in an electrolyte
bath where metal reduction occurs at the cathode which receives the electrons pro-
vided by the oxidation process occurring at the anode. This is activated by a power
supply that must overcome the potential difference between the two half-cell re-
actions. The electrolyte must facilitate the migration of ions and may consist of
a molten salt or of an aqueous solution. The choice between the two depends on
the reduction potential of the metal to be obtained. For example, in the case of
aluminum and magnesium, where their reduction potentials are lower than that of
water, electrolytic winning requires molten salts to avoid the production of hydro-
gen.
According to Free et al. (2012), the cell voltage for metal reduction operations
is commonly in the 2-4 Volt range for electrowinning and 0.2-0.4 Volts for electrore-
fining with the energy cost in kWh/t of metal as follows:

26800nE cell
A w

E(kWh=t) = (9.26)

where E cell is the actual cell voltage including the sum of the reversible potentials of
the anode and cathode plus all overpotentials and potential losses, n is the number
of electrons per mole of metal being reduced, A w is its atomic weight and is the
current e ciency of the bath which expresses the amount of current effectively used
in reducing the metal.
Electrowinning is a very energy intensive process. Its electricity cost (pure exergy
cost) is commonly greater than 1,500 kWh/t of reduced metal. There is therefore
considerable room for improvement especially in the research of electrodic processes.
In contrast, electrorefining used for obtaining high purity non-ferrous metals often
needs less than 20% of that required for the electrowinning of the same metal. The
reason for this is that the electrorefining process employs the same metal both in
the cathode and the anode, thus eliminating their electrode potential difference.
The anodic overvoltage also reduces. However the control of impurities may add
further complexities and costs to this process.