Thermodynamics of Mineral Resources - Thanatia: The Destiny of the Earth's Mineral Resources - page 286

Table 9.2 shows an example of how costs are allocated as a function of ton-
nage, price (for 2000 and 2011) and exergy replacement costs of a mean porphry
CuAu deposit containing copper, gold and silver (Cox and Singer, 1992). The
exergy replacement costs used for the aforementioned minerals are those provided
in Table 12.2 and their prices are those published in USGS (2011b). According to
the different allocation methods used, if the tonnage indicator is selected, none of
the burden is ascribed to gold or silver due to their much lower grade compared
to copper. If exergy replacement cost is applied meanwhile, the results are similar
to those obtained via the price indicator. This supports the idea that the exergy
replacement cost indicator is very close to the value society places on minerals.
However, contrarily to prices, exergy replacement costs do not fluctuate with ex-
ternal factors linked to market mechanisms but remains constant as can be seen in
Fig. 9.6. That said, they are a function of the technology used and can be obtained
from energy data published in different sources such as that provided by the Ecoin-
vent database. Hence, the values are only as reliable as the sources used and are
subject to improvements.

Table 9.2 Cost allocation of porphry CuAu deposits as a function of tonnage, price and
exergy replacement costs

$ 2011 alloc. B alloc. Grade Tonnage

alloc.

$ 2000 alloc.

Gold 3.80E-07 0% 26% 29% 28%

Silver 1.59E-06 0% 2% 3% 1%

Copper 5.00E-03 100% 72% 68% 70%

9.7.3 The importance of disaggregation

As with any industrial process, the analysis of a given mining or metallurgical one
must be highly disaggregated in order to identify, in each subsystem, the purpose
of production. One is then able to identify at any step along the value producing
chain the actual cost of any intermediary product. It is therefore possible to ob-
jectively decide at which point to sell it or whether in fact it is more beneficial to
continue with the refining process. Contrarily, if the process is highly aggregated it
becomes almost impossible to allocate exergy expenditures in proportion to exergy
(or exergy replacement costs). In such cases one only knows that a given product is
economically “more important” than another and so tends to allocate resource costs
and GHG emissions in proportion to market prices. Yet when one disaggregates the
overall system into sub-processes, prices become irrelevant and it becomes possible
to see how energy is converted, i.e. where and how many exergy expenditures were
used to produce the range of products. It also becomes easier to see the e ciency
of each sub-process and allocate costs in a more consistent way. Accordingly, a
product is subject to two distinct phenomena: an internal process of cost formation