Geology Reference
In-Depth Information
rare gold element in a concentrated state; a nickel sulphide deposit is preferred to a
nickel oxide one (laterites) since the former involves considerably lower energy costs
in the smelting process; a hard rock has a high crushing and grinding energy cost
and this is why the cement industry prefers to buy comminuted limestone. Concen-
tration, composition and cohesion are mineral properties that determine whether
a mine is commercially exploitable or not. These properties, together with others
such as proximity to customers, reduced environmental impacts or water availabil-
ity encourages mining exploitation because of the savings in energy and operation
costs.
Mineral rarity requires a definition. When one thinks of rarity, there is a ten-
dency to express it in terms of quantity (i.e. whether there is a little or a lot).
However, the authors believe that this concept can be better described in energy
terms. Therefore, mineral rarity could arguably be defined as “the amount of exergy
resources needed to obtain a mineral commodity from bare rock, using the best avai-
lable technology”.
The authors in their mineral resource assessment have chosen the common bare
rock (Thanatia) as the reference baseline. Accordingly, thermodynamic rarity of
minerals is precisely defined as “the actual amount of exergy resources needed to
obtain a mineral commodity from Thanatia to the market conditions using the cur-
rent best available technologies”. Consequently a mineral's “thermodynamic rarity”
equates to its natural bonus (measured in terms of exergy replacement costs) plus
its mining, beneficiation
7
, smelting and refining exergy cost. Rarity becomes thus
a quantifiable thermodynamic property measured in kJ. Moreover, as it is addi-
tive, indirect exergy costs related to water availability, environmental impact and
transport from mine to customer can be incorporated into the definition.
Thermodynamic rarity varies from mineral to mineral as a function of its ab-
solute scarcity in Nature and the state of technological development. Generally
speaking, if technology does not change, the thermodynamic rarity of a given mi-
neral will remain more or less constant
8
since it depends on fixed initial and final
states, i.e, on Thanatia and on the commodity's quality following refining, which is
usually commercially imposed (see Fig. 4.3). That said, as minerals are extracted,
ore grades decline and hence, mining, beneficiation and refining costs increase. Yet
the natural bonus simultaneously decreases and it becomes “easier” to replace low
quality resources (see Fig. 4.4). Hence why at constant technological conditions,
thermodynamic rarity will also remain constant with the hidden avoided costs trans-
formed into real ones as mining operations become more energy intensive. If, by
way of contrast, technological improvements appear, thermodynamic rarity will de-
crease due to the reduction of both, exergy replacement costs and “OTR” costs, i.e.
mining, beneficiation, smelting and refining costs (see Fig. 4.5).
7
Beneficiation refers to the processes required to concentrate the ore prior to the smelting and
refining stages. See Chap. 7 for details.
8
In reality, it won't strictly remain constant because as will be seen in Chap. 12, exergy replace-
ment costs do not vary lineally with ore grade.
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