Geology Reference
In-Depth Information
emissions that can escape during the Miller process, digestion, electrolytic and
purification operations. The chlorine is recovered wherever possible and scrubbers
can be used to remove it from off-gases. Dust containing heavy metals such as Cu,
Pb, As and Cd forms in the incinerators, furnaces and cupels and their emissions can
be collected by filters. Nitrogen oxides that are produced in the high temperature
combustion processes and in those which use nitric acid can be further emissions
sources. Once collected in scrubbers, NO
x
is oxidised to nitric acid and recovered.
VOCs, which can be destroyed with catalytic oxidation processes stem from solvent
extraction. Dioxins and furans meanwhile come from the poor combustion of oil
and plastic in the secondary feed material.
Water e
uent, both quantity and quality, is also a key issue as a significant
amount of it is used in the beneficiation processes with copious amounts also applied
in the pyrometallurgical and hydrometallurgical processes for cooling, leaching and
solvent extraction. Wastewater quality is affected with the quantity released often
containing a mixture of suspended solids, metal compounds and oils as well as
anions such as cyanide or nitrite. Metals frequently detected in wastewater streams
are As, Pb, Hg, Zn, Cd, Cu, Ni, Fe, Se and Ag which are treated along with
solids using precipitation and/or ion exchange techniques (IPPC, 2009).
Noril'sk Nickel Company in Russia and the Bushveld complex in South Africa
are responsible for 90% of the primary world production of PGMs. Based on data
of the aforementioned companies and from North American mines, Saurat (2006)
proposes an emission per kg of PGMs, of 1.79 t of SO
2
; 23.45 t of CO
2
e and a
total materials requirement of 377.2 t, from which nearly 99.5% is mined rock. A
kilogram of PGM is equivalent to 0.753 kg of Pt, 0.32 kg of Pd and, 0.0377 kg of
Rh. Per kg of PGMs, 164 kg of Ni and 115 kg of Cu are also produced.
Mudd (2007b) and Glaister and Mudd (2010) analyse the water use, energy unit
costs and GHG emissions in global Au and PGMs productions from data supplied
by major companies. These authors present water use figures for a production-
weighted average of 1 kg of gold and 1 kg of PGMs. These are 691 m
3
=kg Au
(ranging from 224 to 1783) and 391.5 m
3
=kg PGM (ranging from 214-1612). These
values are within the range of values given by Saurat (2006) and IPPC (2009).
Moreover, they indicate that gold's water demand stems from the need to mine low
ore grades, whereas PGMs use more water in their refining processes.
Mudd (2007b) and Glaister and Mudd (2010) also propose a production-weighted
average unit energy cost of 143 GJ/kg of Au (ranging from 120 to 213) and 175
GJ/kg of PGM (ranging from 100 to 255). Similarly, an average of 11.5 t CO
2
e=kg of Au (ranging from 224 to 1,783) and, 39.4 t CO
2
e=kg of PGM (ranging
from 214 to 1,612) are proposed. For silver, unit energy costs and GHG emissions
are expected to be proportional and slightly lower than those of gold, since both
commodities are produced parallely. This is because silver ore concentration is
higher than that of gold and once doré has been obtained, the refining process
consumes similar amounts of energy. Specifically, Kellogg (1977) reported an energy
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