Geology Reference
In-Depth Information
9.5.2 The exergy of non-fuel mineral resources
When the system under analysis is a mineral resource, its temperature, pressure,
velocity or height with respect to the reference environment does not inform about
its inherent physical value. In fact, if only such properties were taken into account,
a mineral would have zero exergy. In contrast, its physical value is rather associated
with its specific composition, concentration and cohesion size, what makes it unique
with respect to the surrounding environment and useful for society.
Accordingly, total mineral exergy can be calculated as the sum of the following
components:
Chemical exergy, the minimum theoretical work that should be spent in order
to provide minerals with a specific composition from the reference environment
(Eq. (9.29)).
Concentration exergy, the minimum theoretical work that should be spent in or-
der to provide minerals at a specific concentration with respect to the dispersed
state of Thanatia (Eq. (9.30)).
Comminution exergy, the minimum theoretical work to be spent in bringing the
cohesion energy of the constituents in Thanatia to the mineral cohesion energy
found in the mine (Eq. (9.31)).
With knowledge of the above, the total exergy of mineral resources B
ti
can be
then calculated using Eq. (9.28), where unit exergies are converted into absolutes,
by multiplying them by the moles of the substance under consideration (n).
B
ti
= n
i
b
chi
+ n
i
b
ci
+ n
i
b
comi
(9.28)
In the next subsections, each of the three components of mineral exergy are
described.
9.5.2.1 Chemical exergy and the reference environment
The energy involved in the formation process of a given mineral comprises of the
creation of the compound from its elements and the cohesion of the molecules to
form the mineral's crystal structure (stages I to III explained in Sec. 9.2). Such
a process can be assessed with the mineral's standard chemical exergy, which is
equivalent to the minimum theoretical work needed to obtain the resource at a
specific composition from the reference environment.
Technically, chemical exergy expresses the exergy of a substance at ambient tem-
perature and pressure. It is defined as the maximum work that can be obtained
when the substance considered is brought in a reversible way to the state of re-
ference substances (R.S.) in the reference environment. By definition this process
must only use the heat and reference substances already contained in the R.E. Che-
mical exergy, (b
chi
), results from a conventional assumption of a standard ambient
temperature and pressure and a standard concentration of reference substances in
the natural environment. It can be calculated by means of the exergy balance of a
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