Geology Reference
In-Depth Information
The state of technology also plays an important role. This fact is highlighted
by aluminium. Even if its ore grade is similar to that of chromium or manganese,
the elevated value of k(x = x
m
) is indicative of the significant irreversibility of the
production process. Something similar occurs with tantalum, which has a high unit
exergy cost value compared to other minerals with similar ore grades such as tin or
tungsten (wolfram). This can be attributed to the elevated energy intensity in the
mining and concentration steps of tantalum.
A particular case is that of nickel and its ores. Historically, the metal was likely
to be obtained from sulphide ores due to the major energy requirement of laterites
in the refining process (Sec. 8.7). Nevertheless, more Ni resources are in the form of
laterites (60%) than sulphides (40%). Yet focusing only in the concentration energy,
sulphide ores have larger concentration requirements than lateritics, as revealed by
the larger unit exergy costs.
Of special interest is the value of k(x = x
c
). When multiplied by the minimum
exergy required to concentrate the mineral from x
c
to x
m
, it represents the amount
of energy used to mine and concentrate a substance from the bedrock (Thanatia)
to the current conditions in mineral deposits. It provides a measure of the mineral
exergy bonus on Earth. The value of the crepuscular unit exergy cost k(x = x
c
)
is always greater than that of the current mineral deposits k(x = x
m
). And this
difference increases with the separation between the crepuscular grade x
c
and the
average ore grade in mines x
m
. For instance, the crepuscular k-value of silicon, lime
or titanium is in the same order of magnitude as the k-value of the mine because
their average ore grades are close to those of Thanatia. The opposite happens with
antimony, bismuth or tantalum, which have a very low crepuscular grade compared
to the current average values, therefore the crepuscular k-value is considerably larger
compared to that of the mine.
Considering all such facts, the exergy replacement costs provide hints as to which
minerals would be the most di
cult to replace after complete dispersion. Extracting
and dispersing a mineral with large exergy replacement costs implies an irreversible
loss of natural capital that mankind could not realistically replace, given that to do
so, would require huge amounts of energy, labour and effort. The minerals with the
highest exergy replacement costs according to the authors' calculations are gold,
tantalum, mercury, silver, cobalt, cadmium and tungsten.
It should be stated that the values obtained are preliminary assessments. Im-
portant assumptions have been made, such as only one ore is assigned for each
substance or that the same technology is applied across the whole range of grades
analysed (including the crepuscular ore grade). One of the major limitations found
is the lack of real data over time. So estimation of future trends without real and
reliable information becomes subjective. Therefore, the results and data provided
are an attempt to create indicators based on physical facts rather than on ever
changing market policies.
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