Recycling Solutions - Thanatia: The Destiny of the Earth's Mineral Resources - page 441

and Ag (Hagelüken and Meskers, 2010). Hence, each element that has yet to be
recuperated becomes an impurity 6 contained within the principal metal that later
must be refined or treated in order to eliminate it.
The production of minor metals needs secondary metallurgical processes which
are undertaken in addition to the mining of the principally extracted metal. Ge-
nerally speaking, metallurgical processes require the use of very specific treatment
technologies with, as the reader has seen in Chap. 8, each metal entailing individual
separation and refining technologies. Given that the principal process doesn't typi-
cally involve the minor metal, the separation of the latter is far from e cient. This
then leads to high embodied energy values and consequently large losses in metal
yield. Minor metal recovery processes whilst highly sophisticated are also capital
intensive and more closely linked to fine chemistry than the bulky technologies asso-
ciated with conventional metallurgy. Such investments can have uncertain payback
periods, given that they are a function of market instabilities and price. Two good
examples of this are the PGM and the rare earth metal groups.
Should the mining-metallurgical technologies be inappropriate or more impor-
tantly the concentration of the minor metal be too low, almost all of it will remain
in waste rock, tailings or slags. There will always be gangue left for re-exploitation
some point into the future. But rock removal is far from a precise process and for
the minor metals neither is it selective. The threshold of whether a metal should be
fully extracted from resulting waste streams is simply a question of economics. For
precious metals, it pays to recover any minute quantities when for others it simply
does not. And, no one really knows the amount of resources that remain in waste
rock from those mines that are currently in operation or have been closed. Only
those veins with an acceptable cut-off grade are exploited, but even so they exist
in variable, rather than discrete compositions. Furthermore, it is improbable that
this mineral exploitation involves off-premise accountability of the ore grades con-
tained in the waste rock, leaving a notable quantity of resources which are neither
exploited nor recognised in mining statistics.
Opening a new mine implies amongst others, enormous investment in machinery
and infrastructures along with significant social and environmental impacts, some
of which may be indefinite. Yet in contrast, the extraction of minerals of a low
ore grade in working mines presents a great opportunity for the recuperation of the
minor metals. To that effect, as energy and water consumption are strongly depen-
dent on ore grade, improving such costs automatically favours low-grade mineral
extraction. This is especially the case for grinding operations 7 .
6 When the impurities are traces of heavy metals, this translates to an environmental problem on
the release of wastewater, dusts or flue gases.
7 Norgate (2010) proposes the use of comminution systems such as stirred mills and high pressure
grinding rolls (HPGR) in order to obtain energy savings of around 15-30% compared to traditional
dry circuits. Here any water is re-used as the process demands.