Thermodynamics of Mineral Resources - Thanatia: The Destiny of the Earth's Mineral Resources - page 285

not subject to externalities such as market price and arguably gives a more rational
and objective appraisal of the effort that both Man and Nature combined spent
in obtaining the good(s). Any improvement to the process e ciency immediately
decreases the cost of production with the sensitivity of the e ciency increasing with
the rarity of resources. Hence the importance of optimising the use of those inputs
with the greatest exergy replacement costs, as they are the most important in terms
of conservation.
9.7.2 Allocation
The calculation of exergy costs is truly a complex one, especially in the case of
minerals. As was seen in Chap. 8, no one single commodity deposit exists. Con-
trarily, mineral ores are composed by a group of substances that may or may not be
commercially exploitable (see Fig. 8.1). Thus, with each ore, products, byproducts
and wastes appear. An example of this is the simultaneous mining of copper and
gold ore whereby the high prices of the latter may drive the exploitation of both.
If one wants to account for the exergy costs of a certain non-fuel commodity,
the different energy expenditures (and eventually environmental impacts) of the
production process need to be allocated according to each obtained product, be it
a main or a byproduct. Such a task is by no means trivial and different procedures
have been proposed and used:
(1) As a function of market price: If one takes the example of the gold-copper
ore, much of the environmental burden (including energy consumption) has to
be ascribed to gold production, seeing as it is this metal that pays the highest
premiums. However, the problem with this is that burdens do not change should
the gold price fluctuate, which it does frequently. Clearly resource consumption
and greenhouse gas emissions are related to physical expenditures, not prices.
(2) Through tonnage: Using the gold-copper example again, the majority of envi-
ronmental impacts would be ascribed to copper, as in tonnage terms it is the
most significant. The issue with this is, although based on physical phenomena,
such a procedure does not reflect the physical value of minerals.
(3) Through exergy replacement costs: In the authors' opinion, the most rational
way to allocate energy through the different mineral substances contained in an
ore is by considering exergy replacement costs 13 . The greater a commodity's
replacement cost, the more energy (or environmental burden) is allocated to it
and subsequently the strengths of price and tonnage as indicators are combined,
with the highest importance placed on those minerals that are more “socially
valued”. It is thus a physical measure independent of monetary arbitrariness
supported by the rigorous theory of Thermoeconomics.
13 Exergy should be calculated in absolute energy units, i.e. GJ or ktoe. This means that specific
exergy replacement costs (GJ/t) (see Table 12.2) need to be multiplied by the tonnage of the given
commodity.