The Exergy Replacement Costs of Mineral Wealth - Thanatia: The Destiny of the Earth's Mineral Resources - page 370

The drawback of using exergy costs is that as opposed to exergy, it depends
on the state of technology and hence, varies with time. Nevertheless, it provides
a mineral appreciation which is closer to common accounting principles. Again,
the authors reiterate that these numbers should be taken with caution, due to the
existence of many assumptions. They do serve however as an illustration of the
order of magnitude as to the physical value of mineral wealth.

12.8 Summary of the chapter

The aim of this chapter has been the exergy cost assessment of the mineral en-
dowment on Earth. To that effect, the calculation procedures for obtaining exergy
replacement costs initially introduced in Chap. 9 were shown.
Unit exergy replacement costs k, i.e. the ratio between the real energy and the
minimum exergy required to undertake a certain process varies with time and with
ore grade. This is because as ore grades decrease, the energy required to extract a
certain mineral from the mine increases, whereas with su cient time, technological
progress improves the e ciency of processes and thereby reduces energy consump-
tion.
Therefore, ideally, unit exergy replacement costs should be obtained empirically
from real data of energy vs. ore grade trends. However, such information is limited
to gold, copper, nickel, cobalt and uranium. The results obtained suggested an expo-
nential relationship of energy vs. grade varying from x 0:2

m to x 0: m . Accordingly,
for the minerals where no empirical data was available, the general relationship:
E(x m ) = Ax m 0:5 was assumed.
With energy values obtained in Table 8.3 and average ore grades presented in
Table 6.10, the exergy replacement costs of 37 mineral commodities were calculated.
As a case study, the exergy replacement costs associated with 2008's global
mineral production were ascertained. The results obtained quantify that the useful
energy that Man saves, thanks to the existence of mineral deposits, is at least in
the same order of magnitude as the annual depletion of coal, oil or natural gas.
The analysed substances represent however only a part of the globally used
mineral resources, so the complete saving if one includes all minerals is even larger.
This analysis thus enhances and puts numbers to the importance and necessity of
material reuse and recycling.
But how will the mineral endowment on Earth vary in the future? Will there be
enough resources to support the increasing demand for minerals? All such questions
are explored in the following chapter.