Geology Reference
In-Depth Information
The mining and refining exergy costs (in MJ) can be defined as the actual
exergy expenditure needed to extract, concentrate and refine a given mineral to
produce a given material. Since these values are real and belong to the “over the
rainbow” path, calculating exergy costs is equivalent to obtaining embodied exergies.
Assessing these costs permits an identification of the generic process R#1 to R#2
shown in Fig. 4.2.
In the “down the rainbow” path, the aim is to obtain the exergy replacement
costs of minerals from a hypothetical grave to the cradle. Since such processes do
not exist in reality, exergy replacement costs cannot be seen as embodied exergies,
as it happened in the previous case. Such costs are thus more complex to assess
and two possible routes can be considered: an indirect and a direct one (Valero and
Valero D., 2010).
(a) Indirect route (R#0 ! R#2 ! R#1). Let's first suppose one wants to obtain a
given material from the bare Earth's crust using current technology. That is to
say that one wants to assess the exergy cost needed to extract the material from
the “common bedrock”. Steen and Borg (2002) provide some examples of indus-
trial processes that could be used for assessing the energy (exergy) expenditure
to reach R#2 from R#0 for a given material. Fortunately, current industry
knowledge relating to real chemical reactions and concentration methods can
provide approximate values of this exergy expenditure. So most mineral de-
posits are in the form of sulphides, oxides and other chemical substances which
are quite familiar to those in the chemical industry. Data on how to obtain
these substances from the corresponding silicates or other reference substances
are reasonably easy to find in most cases. Once the exergy cost of the mate-
rial in R#2 has been calculated, the exergy replacement cost of the mine in
R#1 can be easily obtained as the difference between the exergy in R#2 minus
the real exergy costs of extraction, concentration and refining (the process be-
tween R#1 ! R#2). Thus the exergy replacement cost of the mine can now
be interpreted as an “avoided cost” (MJ) that Nature provides without charge
in the form of a naturally concentrated material. This can be mathematically
expressed as in Eq. (4.3):
B (R#0!R#1) = B (R#0!R#2) B (R#1!R#2)
(4.3)
(b) Direct route (R#0 ! R#1). This second route consists of directly assessing the
“exergy bonus” (that which Nature provides freely) found in naturally occurring
deposits. For this reason, the exergy replacement cost of a mineral deposit
(in MJ) is defined as the exergy that would be expended in recovering the
deposit from Thanatia (steps R#0 ! R#1 in Fig. 4.2) with currently available
technology. Therefore, the calculation of these costs requires a definition of
the hypothetical processes R#0 ! R#1 for the mining and concentration of
 
Search WWH ::




Custom Search