Geology Reference
In-Depth Information
electrochemical redox reactions as was seen in Chap. 8. In any case, thermodynamic
separations are always present.
All the above point to the fact that a linear growth in purification requires an ex-
ponential energy input, as was seen in Sec. 9.3. Furthermore, the technology used in
separations is of various orders of magnitude greater than the minimum thermody-
namic demand. If one combines the energy employed in physico-chemical separation
with that required in the transport of materials and chemical transformation, the
e
ciencies experienced fall extraordinarily far from the minimum (exergy).
A particular physical separation process is grinding, which consists of reducing
particle size. This process consumes significant amounts of energy. This quantity
can be indicated by Bond's equation which relates the amount of energy needed
in comminution with the material's final particle size. Once again, the reader will
see that a linear reduction in size leads to an exponential growth in energy input
(Sec. 9.5.2.3).
In summary, the principal objective of mining and metallurgy generally, is a sep-
aration, concentration and isolation of substances extracted from the Earth, which
are frequently found mixed together in an oxidised state. Such processes are subject
to the exponential law of thermodynamic separation. Man's technology, well, “his”
imitation of Nature, is in addition so far removed from the thermodynamic mini-
mum. And, of all the energy transformation processes, those involved in separation
are, beyond doubt, the least e
cient. For this reason, the embodied exergy and
exergy values of whichever element chosen are so far apart.
This entropic vision is, in fact, based on the Second Law. It is a fundamental law
and is simply not a matter of market demand nor related with social lifestyles. Man's
technology cannot overcome it but society can decrease its velocity by decelerating
mineral extraction.
9.4.3.1 A thermodynamic overview of smelting processes
The Second Law's power of explanation does not end in entropy. In fact the evo-
lution of technology in smelting and refining processes is closely related to the way
elements behave in redox reactions and the energy and temperatures involved. This
section describes the thermodynamic behaviour of smelting and refining.
As stated in Sec. 7.5, the vast majority of processes are classified into those that
use heat (pyrometallurgical route), those which use selective solving (hydrometal-
lurgical route) or those that use electricity (electrolytic route). Few processes use
only one route and most use them in combination. Refining processes are usually
electrolytic (electro-winning) in nature, regardless of which of the routes were pre-
viously followed.
The aim of smelting is to overcome the thermodynamic and kinetic barriers
present in a given operation, involving the lowest amount of energy, economic and
environmental costs possible, according to the ore composition and concentration
and the chemical behaviour of the required metal(s) which in turn determines the
Search WWH ::
Custom Search