The Exergy Replacement Costs of Mineral Wealth - Thanatia: The Destiny of the Earth's Mineral Resources - page 352

required to form the mineral from the concentration in Thanatia to that found in
mineral deposits (from R#0 to R#1 as shown in Fig. 4.2 on p. 93.).
b c (x c ! x m ) = b c (x = x c ) b c (x = x m ) (12.2)

Hence, even if the term b ci is used in this topic for simplification purposes, it is
rather b c (x c ! x m ) that presents the correct notation.
On the other hand, the exergy replacement cost is defined as the total exergy
required to mine and concentrate the mineral resources from Thanatia, using cur-
rently available technologies. Such values are therefore not absolute and universal,
as opposed to the exergy property. Exergy costs are a function of the type of mineral
analysed and its ore grade, extraction and separation technologies and associated
energy consumption, which in turn vary with time (i.e. according to the learning
curve).
The calculation of a resource's exergy cost b t constitutes a chemical cost (k ch
b chi ), accounting for the chemical production processes of the substance, and a
concentration cost (k c b ci ), relating to the concentration processes.
b ti = k ch b chi + k c b ci = b ch + b c (12.3)

The variable and dimensionless k represents the unit exergy cost of a mineral. It
is defined as the ratio between the energy invested in the real process for mining and
concentrating the mineral from the ore x m to the pre-smelting and refining grade
conditions x r (E x m !x r ), and the minimum theoretical energy (exergy) required to
undertake the same process (b x m !x r ).
k = E(x m ! x r )
b x m !x r

(12.4)

The chemical exergy cost b ch of a resource comes into play when the chosen
reference environment does not contain the substance under consideration. Since the
Crepuscular Earth Model contains, in principle, most of the minerals found in the
crust, the chemical exergy will not appear. The authors thus focus predominately
on the concentration exergy replacement cost b c and the unit concentration costs
k c .

Since the energy required for mining is a function of the mine's ore grade and the
technology used, so is the unit exergy cost (Eq. (12.5)). As Ruth (1995) states, both
variables have an opposite effect on the energy used. The lower the ore grade, the
more energy is required for mining it. On the contrary, technological development
usually improves the e ciency of mining processes and hence, decreases energy
consumption. This will be discussed in more detail in Sec. 12.3.
k = k(x;t) (12.5)

It is therefore di cult to extrapolate k into the future for the practical impos-
sibility of predicting changes in scientific and technological knowledge.
Another problem with k is that it is a discrete function, as the technology applied
can vary with the concentration ranges of a particular deposit. Also in turn, each