Geology Reference
In-Depth Information
14.7 Entropic backfire: reagents and recyclate entropy generation
As an element cannot be destroyed but instead becomes dispersed or degraded
(should it come into contact with other elements) it can theoretically be reused over
and over again with the empirical limit hinging on the energy consumed. There are
however associated side-effects in this practice that can be understood through the
Second Law.
The reader is asked to consider the case of a separation of a mixture containing
various metallic substances present in a recyclate 10 . Here, if one is to selectively
separate a metal from the mixture, a set of determined chemical and thermal pro-
cesses would have to be applied. Such processes could include, for example, selective
solvent extraction, phase changes or dissolution with acids or bases, etc. As the ob-
jective is to extract the paying metal, what happens to the waste is of secondary
importance. Yet from a Second Law perspective the overall level of entropy has
increased. But where exactly? The answer is in the waste generated from the
use of solvents, reagents or fuels and sometimes in that of the recyclate. This en-
tropy augmentation can subsequently be referred to as the “entropy generation in
reagents”.
Furthermore, the recyclate has been partially separated into the desired metal
and the “waste from recyclate”, that is to say those residues that could not be
salvaged. This waste from recyclate, in theory and with a su cient quantity of
new reactants, could be converted into new recyclate that could in turn be sepa-
rated further into a secondary new paying metal with an additional amount of new
reagents.
The question now is whether it is worthy to continue with subsequent separation
processes or if it is better to obtain the new paying metal from mines directly. If
in the first extraction, the process has been optimised for salvaging a certain pay-
ing metal, the resulting waste from recyclate may not present favourable chemical
conditions for a second paying metal. Consequently, recovering additional metals
from such waste is not only extremely complex but also expensive. The reason for
this phenomenon is that as explained previously, natural processes tend to generate
families of elements with similar chemical characteristics (see Fig. 8.1 on p. 179),
whereas in waste streams all combinations of minor metals may be possible. Such
elements do not generally present similar chemical behaviours that favour their mu-
tual separation. In short, if the recovery process is selective for a given paying metal,
the salvaging of additional minor paying metals is often blocked from an economic
point of view, since it is literally more cost effective to extract virgin material.
In thermodynamic terms this means that that the “final waste from recyclate”
contains more entropy than the initial recyclate with the difference between the
two known as “recyclate entropy generation”. This term describes how in all real
processes the specific separation of components from a recyclate creates waste which
10 See the theory behind the entropy generation in mixtures in Sec. 9.3.
 
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