Geology Reference
In-Depth Information
geologically scarce but dispersed and chemically challenging to isolate. So their
depletion per se is not a problem since there are more REE in the crust than
say copper. The issue comes in their thermodynamic rarity which is itself derived
from several combining factors consisting of concentration, separation and chemical
composition. Firstly, exploitable concentrations of REE-containing minerals such
as bastnaetsite, monazite or clays are di
cult to find as they are highly dispersed.
Secondly, REE occur all together. Some of them, like cerium and lanthanum are
relatively abundant in comparison to the remaining fifteen. And not all are equally
demanded, for instance dysprosium is mass used as a critical component in per-
manent magnets. Thirdly, given that the chemical properties of all REE are very
similar, their separation, mainly by ion exchange techniques, is complex and ine
-
cient as in the case of gadolinium (see Chap. 8). And finally, as with alkaline and
alkaline-earth metals, REE need great amounts of energy in the reduction to their
metallic state. Therefore in a number of applications, to save energy, reactants and
effort, mischmetal, rare earth oxide, REO, or even the naturally occurring total
rare earth oxide, TREO is commonly used instead of the pure metal. All of the
above lead to REE having a very high exergy replacement cost and beneficiation
cost, i.e. thermodynamic rarity, when compared to other metals. In short, rare
earths are truly rare from a thermodynamic (exergy) perspective.
Finally, this property could be used to properly value minerals in a way that
protects the geological heritage and the mineral wealth of future generations. No
matter how exhausted a mineral is, its value should consider: the actual amount
of resources needed to convert a mineral into a commodity and its replacement
cost. As ore grades continue to decline over time, it is important that the current
replacement cost is converted into money that can then be used to offset the higher
extraction costs sure to be experienced into the future. And, as thermodynamic
rarity can be considered almost constant throughout time (provided that technology
does not dramatically change), the cumulated replacement costs converted into
monetary values should represent the price that former generations would have
to pay those of the future. Obviously, this also means that the current market
prices of mineral commodities should increase so as to include replacement costs
and reach a stabilised value no matter their current ore grade. Such thinking is a
step towards what constitutes a thermodynamic solution to the very much sought
after intergenerational justice.
4.6 Summary of the chapter
The extraction of materials from the Earth's crust implies a net reduction of Na-
ture's exergy stock. This is because one is currently unable to recycle all materials
that are mined. Consequently, the Earth is approaching, albeit gradually, a de-
graded planet of minimum exergy, with an absence of fuel and non-fuel mineral
deposits, simply as a consequence of the Second Law.
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