Geology Reference
In-Depth Information
These raw materials are then converted into useful products in the entry-gate-
to-exit-gate, becoming part of the stock in use (Reservoir #2 to #3). At a product's
end-of-life (EoL) it becomes a waste - either in the form of pollution (Reservoir #0)
or as a heterogeneous mix in landfill (Reservoir #4). Fortunately, an increasingly
important amount of such products are recycled before or after being disposed of in
landfills. In the latter case the process is referred to as urban mining (a much more
energy intensive process than new or old scrap recycling - see Sec. 14.4). In any
recycling option, the replacement costs (which are hypothetical) as well as those
relating to mining and concentration are saved, remaining manufacturing costs and
those associated to the corresponding EoL technologies (see Sec. 14.5). Yet, despite
such initiatives and even incentives, most products end up as wastes (Reservoir #4)
becoming eventually degraded and dispersed, prior to arriving at the final resting
place (to the grave #0), that is to say, Thanatia.
In this way, at least four main costs in the life cycle of a product come into
play (Fig. 4.2). The first one, representing the hypothetical effort Nature spent in
producing minerals in a concentrated state, is in fact a hidden and avoided and cost
and belongs to the DTR path. The second one is that associated with the BoL,
i.e. with mining, mineral processing, smelting and refining. The third relates to
the fabrication and manufacturing costs resulting from transforming raw materials
into useful products. The last, the EoL (recycling) costs are pretty self-explanatory
and increase as the end-of-life material approaches the landfill stage.
Even though Fig. 4.2 is a general diagram, each particular substance may have its
own one. The size and the rate of change for each reservoir depends on the quantity
available of the raw mineral (R#1), the annual rate of mineral extraction (R#1 to
R#2), the annual rate of disposal (R#3 to R#4), the annual rate of recycling (R#4
to R#2), the rate of unrecoverable mine tailings (R#1 to R#0), and the rate of
unrecoverable losses found in any process generally (R#i to R#0). Each reservoir
should have at least three inventory properties: quality, as expressed in terms of
1) concentration and 2) chemical composition and 3) quantity, expressed in the
amount of each material at a given quality. Also the relative position of reservoirs
in the y-axis depends on how the quality and quantity of the given reservoir varies
with time, due to rates of processing and technological improvement.
Following Fig. 4.2, the authors propose two possible physical indicators for each
reservoir. One is pure exergy and the second, exergy cost. As will be seen in
Chap. 11, exergy is still an insu
cient unit for the realistic quantification of re-
source depletion, meaning one should then resort additionally to exergy costs. The
authors refer to exergy cost (in MJ) of a product as the actual exergy expenditure
in its production process, once the limits of the analysis, the process itself and
the e
ciencies of each process component have been defined. This also implies a
definition as to what can be considered a feedstock, raw material, fuel, product,
byproduct or waste.
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