Geology Reference
In-Depth Information
feed material of 7-25% Ni. The raw materials for PGMs extraction are concentrates
produced from ores, mattes and slimes from nickel and copper operations. PGMs
can also be present in the anode slimes once gold and silver have been obtained
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.
The PGMs are extracted by a variety of hydrometallurgical processes which es-
sentially consist of separation and purification processes. The low grade feedstock
is crushed and melted to form a homogeneous product. As in the case of gold, the
classical procedure consists of leaching the product in aqua regia and then allow-
ing the metals to separate by selective precipitation. Firstly, aqua regia dissolves
Pt and Pd leaving the other metals in the solid leach residue. Then platinum is
precipitated with ammonium chloride and the salt is further heated to liberate the
metal. Afterwards palladium is precipitated from the solution with ammonia. The
solid leach residue contains Rh;Ir;Ru and Os. Rhodium is separated as a sulphate
by treating this leach with molten sodium bisulphate. It dissolves in water and is
easily separated by reduction with zinc powder. Ruthenium is then separated as
sodium ruthenate by fusing the residue with sodium nitrate salts. It is subsequently
converted into RuO
4
with chlorine gas. As RuO
4
is a volatile compound, it can be
separated by distillation and later reduced to the metallic state. Osmium is recov-
ered in a similar way but using a distillation in acidic solutions. Iridium is obtained
from treating the residue with sodium peroxide that solubilises in hydrochloric acid.
The metal is obtained by calcination of the precipitate obtained with ammonium
chloride (IPPC, 2009).
An alternate method to obtain PGMs is through organometallic processes
whereby all are simultaneously dissolved in hydrochloric acid/chlorine solution and
then each metal is selectively extracted. The solvents used are organic molecules
that form stable organometallic complexes which can be subsequently treated to
obtain a pure metal sponge. Separation and purification treatments may consist of
a variety of chemical precipitation and dissolution, liquid extraction, distillation of
tetroxides, ion exchange, pyrolysis, reduction and electrolytic processes.
8.9.4 Energy and environmental issues of precious metals
According to IPPC (2009), precious metals have a significant impact on the envi-
ronment. Air emissions contain heavy metals, SO
2
, CO
2
, NO
x
, chloride, VOCs,
dioxins, furans and dust. In addition to being energy intensive, the specific isolation
processes use strong reagents such as hydrochloric acid/chlorine, aqua regia, caus-
tic soda, a variety of organic and inorganic solvents, including cyanide, reducing
and oxidising agents. The allocation of emissions is complex since their production
processes are subsidiary of other base metal production.
Sulphur dioxide emissions, which need to be collected in order to be converted
in the sulphuric acid plant come from sulphide roasting, the acid digestion stages
and fuel combustion. Chlorine and hydrochloric acid gases meanwhile are other
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See http : ==www:goldandsilvermines:com=platinum:htm. Accessed Aug. 2011.
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