An Introduction to Mining and Metallurgy - Thanatia: The Destiny of the Earth's Mineral Resources - page 172

evaporation, organic solvent extraction, membrane separation, ion exchange, ad-
sorption and other physical separation techniques. These are combined with re-
duction processes. Electrolysis reduction in a weakly acidic aqueous solution is an
important technique in particular, as is electrorefining (electrowinning) for the high
purification performed in obtaining most non-ferrous metals.
Biohydrometallurgy 7 is an intermediary field between biotechnology and hy-
drometallurgy that uses microbes for the bioleaching and bio-oxidation of metal
ores (see for instance Rossi (1990)). In theory, almost any chemical process can
be undertaken with bacteria and this opens up a large field for development. Such
processes are now used, for example, in the treatment of heavy metals and wastew-
ater as well as in the recovery of high value metals such as copper, nickel, lead,
zinc and gold. In fact, at present, over 15% of the total U.S. copper production
comes from bacterial leaching. Much of this is dump leaching on mine tailings but
there is a growing sophistication in in situ leaching, in which bacteria-laden wa-
ters are pumped into ore deposits and retrieved through separate pumping wells.
Bacterial mineral leaching processes require relatively small energy inputs and offer
accessibility benefits where ores are di cult to find. However, for the bulk of the
metal processing industry, this technology has limited commercial application since
passive bioprocesses are slow and bacteria-metal resistant limits still need to be
determined (Sarveswara Rao and Acharya, 2008).
Biological options aside, a large part of metallurgical processes of metal refining
take place with pyrometallurgical techniques, however the hydrometallurgical ones
are increasing in importance due to their potentially smaller environmental impact.
Lakshmanan et al. (2003) for example, states that hydrometallurgical processing
has shown itself to be highly innovative and economic with environmental prospects.
King (2007) also comments that “many positive things come from the forced changes
in the non-ferrous metals industry and perhaps the best example is the closure of
the sulphur loop. It is now accepted that in pyrometallurgical operations, SO 2 will
be captured and sulphuric acid will be produced. In turn, this acid can be re-used
by industry in applications such as the leaching of oxide ores or the leaching of
sulphide concentrates”.
That said, none of the methods differ greatly in GHG emissions for the very
technologically optimised case of copper. Mudd (2010b), based on Norgate and
Rankin (2002), estimated that Australia's average 2008 ore grade of 0.95% Cu,
contributed to 8.7 and 8.9 t CO 2 e=t Cu for pyrometallurgical and hydrometal-
lurgical processing respectively. Unfortunately and as Marsden (2008) points out
for the well-known case of copper, hydrometallurgical processes are strongly depen-
dent “on ore mineralogy, ore grade, by-products, the metallurgical response of the
ore to the process options, the presence of deleterious elements and other factors
including throughput rate and those environmental, geographical, etc in nature.
7 A crude form of leaching metals from low-grade ores using living organisms which can be traced
back to ancient societies.