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those substances in Thanatia into the mineral ores currently found in natural
deposits.
Common sense will regard route (a) as the most logical, since one can always
compare the exergy needed to extract a material from a mine with that of taking it
from bare rock. However the thermodynamic procedure in (b) does not conceptually
differ from the indirect route and the same formulas apply. Therefore the avoided
exergy cost of route (a) can be approximated to the mineral exergy bonus of a given
deposit as obtained in route (b). It should be noted that the final result will depend
on the technologies applied. Nevertheless, the order of magnitude (which is what is
really being looked for) should not differ significantly.
It is the direct route that will provide the basis for the subsequent calculations
presented in the third part of this topic and explained in detail in Chap. 12.
It should be stated that the exergy replacement cost of minerals cannot be con-
sidered as pure Physics. As opposed to pure exergy, it is not a thermodynamic pro-
perty but rather an indicator which relies on the level of technological e
ciency in
the mining and refining of minerals. In this respect, Steen and Borg (2002) screened
many current technologies available for obtaining metals from the bedrock and con-
cluded that mining, crushing and grinding are some of the basic determinants in
cost estimates. Also having observed technological developments throughout the
last decades, it quickly becomes apparent that no major improvements have been
introduced into mining, crushing and/or grinding. Massive changes in the near fu-
ture are unlikely. Therefore and in terms of orders of magnitude, the question of
technological dependence is closer to a theoretical conjecture than a real problem.
So exergy replacement costs provide reasonable values and remain fairly stable with
technological change. Furthermore, the conceptual drawback of being an indicator
rather than a thermodynamic property, could be advantageous: any technological
upgrade will be noted down as a success story, providing knowledge and not debt,
to future generations.
Hence, this topic uses both indicators: exergy and exergy costs with a common
reference baseline (Thanatia). One first defines the exergy for the reservoirs in
Fig. 4.2 and then goes on to evaluate their exergy costs. Presented over time, they
can give an indication as to the speed at which mineral degradation is occurring.
In summary, exergy and exergy replacement cost constitute useful tools for re-
source classification according to their respective depletion states, since they give an
idea as to the degree of resource quality, or what the authors name “thermodynamic
rarity” as explained in the next section.
4.5 Thermodynamic rarity
A mine is a rarity in the Earth's crust and is more valuable the rarer the mine-
ral it provides. High concentration, suitable composition and weak cohesion are
characteristics scarcely found in common bedrock: a gold mine provides the very
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