Geology Reference
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plained through changes in energy, temperature and the Gibbs function variation
that occurs in redox reactions and which determines whether or not a reaction takes
place. In pyrometallurgical processes high temperatures are required. The reactions
involved can be depicted by an Ellingham diagram, whereby G is plotted against
temperature.
Meanwhile, in electrometallurgical and hydrometallurgical processes, redox re-
actions are undertaken in an electrolyte bath which consists of a molten salt or an
aqueous solution that facilitates ionic migration. Metal reduction happens at the
cathode on receiving the electrons freed by the oxidation process at the anode. This
can only occur should the power supply be su
cient enough to overcome the poten-
tial difference between the two half-cell reactions. The minimum potential required
to force a reaction is dictated by the Nernst equation, which states the relationship
between the Gibbs free energy and the concentration of reactants. A helpful tool
for understanding the nature of hydrometallurgy is the Pourbaix diagram, which
results from the plotting of all equilibrium lines corresponding to the various species
that a given element may form such as a pure metal, ion or hydroxide against pH.
Thermodynamics also permits a physical evaluation of mineral resources, which
can be done using exergy instead of entropy or Gibbs free energy to generate a more
concrete and understandable indicator. Exergy measures a system's potential for
change whilst not in thermodynamic equilibrium with a given reference environ-
ment. With respect to the latter all materials have a quantifiable exergy content
which is characterised by its thermodynamic properties. These can include: inter-
nal energy, temperature, pressure, specific volume, entropy, velocity or altitude and
lend themselves to specific exergy components, which may in turn include thermo-
mechanical, kinetic and potential exergies. Exergy has been extensively used in the
analysis of industrial processes, where the aforementioned variables are core com-
ponents. Yet, when it comes to the evaluation of global mineral wealth, additional
parameters come into play: chemical composition, concentration and comminution.
Chemical composition accounts for the formation of the mineral from the refe-
rence environment. The authors have used their update of Szargut's R.E. for the
calculation of mineral chemical exergy. Yet such a conception of a R.E. is not suf-
ficient for a complete exergy evaluation of mineral resources, as it cannot be used
as a baseline for concentration and comminution exergies. For this reason, the au-
thors constructed a model founded on geospheric resemblance depicting Thanatia.
Accordingly concentration exergy, which exhibits a negative logarithmic behaviour
with grade, expresses the minimum amount of effort Nature had to “virtually” spend
to bring the minerals from the concentration present in the dispersed state of Tha-
natia to that of a mine. Its negative logarithmic curve means that as ore grade tends
to zero, so does the deposit's exergy, whilst that required for replacing the mine
tends to infinity. Finally comminution exergy accounts for the minimum energy
required to rebind the solids in Thanatia back to their original condition in the
mine. It has been observed that this third component is very small in order of
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