Geology Reference
In-Depth Information
Chapter12
The Exergy Replacement Costs of Mineral
Wealth
12.1 Introduction
In the previous chapters the authors stated that the exergy property provides a
reference value for the evaluation of minerals, equivalent to the thermodynamic
minimum to restore them from Thanatia. Unfortunately the results obtained using
solely exergy as an accounting property are very far removed from a more realistic
appreciation as to the value of minerals.
This is why exergy replacement costs are used in this chapter for the assessment
of mineral depletion. The exergy replacement costs provide a measure of the amount
of energy one should invest with conventional technologies in recovering the mineral
deposits to their initial conditions of composition and concentration from Thanatia.
12.2 Formulas for assessing the exergy replacement costs of
minerals
As explained in Chap. 9, the exergy of a mineral resource has at least two compo-
nents (neglecting the cohesion exergy): one associated with its chemical composition
and the other with its concentration. Hence, the total exergy (b
t
), representing the
minimum exergy required for obtaining the resource from Thanatia to the initial
conditions in the mineral deposit, is calculated as the sum of the two exergy com-
ponents: chemical b
chi
(Eq. (9.29)) and concentration b
ci
(Eq. (9.30)) as explained
in Sec. 9.2.
b
ti
= b
chi
+ b
ci
(12.1)
It should be remembered, that chemical exergy is obtained from a defined re-
ference environment (in this case, the one described in Sec. 9.5.2.1). For the cal-
culation of concentration exergy meanwhile one has to identify the ore grade (the
average mineral concentration in a mine x
m
) and the average concentration in the
Earth's crust x
c
(as shown in the Crepuscular Earth Model). The value of x in
Eq. (9.30) is then replaced by x
c
or x
m
to obtain their respective exergies, whilst
the difference between them b
c
(x
c
! x
m
) represents the minimum energy (exergy)
351
Search WWH ::
Custom Search