Geology Reference
In-Depth Information
In modern plants most of the sulphur dioxide produced gases are collected and
converted into sulphuric acid. The sulphuric acid produced can absorb a large part
of the NO
x
8
but its quality may be affected. Chlorine is also formed during the
electrolysis of chloride solutions. It is collected and returned to the leaching stage.
VOCs are emitted in the solvent extraction stages. They can be removed using
condensers, by cooling the air flowing in the ventilation system thereby recovering
the solvent. Dust and heavy metals are typical emission problems stemming from
the overall smelting process and can be easily solved using fabric filters and scrub-
bers, returning the off-gases to the leaching process. Such techniques can still leave
significant diffuse emissions to enter the atmosphere, should there be a deficiency
in the sealing of installations. Carbon monoxide, for its part, is used in the nickel
carbonyl refining route where hydrogen is used as a reducing agent. These gases are
explosive and/or very toxic and therefore their recovery and gas exhaust systems
need to be robust and very well sealed.
Emissions to water are the cations such as Cu;Ni;Co and Cr. Additionally,
anions such as the fluorides, chlorides, arsenides and sulphates originating in the
hydrometallurgical processes, matte granulation and wastewater from wet scrubbers
and wet electrostatic precipitators are common. The treatment of liquid e
uents
generates gypsum and metal hydroxides. Depending on their monetary value, they
can be returned to the metallurgical process. If not recycled, the disposal of these
residues is done so in lined storage ponds, which hold the possibility of percolating
into groundwaters. Other important residues are the solids, typically slags from
smelting processes. This slag can be as much as 4 to 10 times the weight of the
metal produced.
Marcuson et al. (2009) consider that environmental constraints in the Ni in-
dustry will intensify in the coming decades: future technology must not only solve
metallurgical problems but do so at substantively lower environmental costs (a pat-
tern which has generally not prevailed in the past).
Hence, the overall environmental impact of nickel and cobalt smelting is con-
siderable and although current technology may reduce it, there remains a worrying
historical legacy. For example, the cumulative impacts of the smelting and refin-
ing processes of the 1960s over a wide area surrounding Sudbury, Canada, led to
heavy metal soil contamination, acid rain linked to SO
2
emissions (1.5 to 2.7 Mt
SO
2
/year), acidified wetlands, important declines in biodiversity (especially fish),
vegetation dieback and heavy soil erosion (Mudd, 2010a). A similar situation in
Russia at Noril'sk has meant that soil pollution is so severe that it has become
economically feasible to extract Pt and Pd from its contents.
As for energy consumption, according to IPPC (2009), the energy used for the
production of matte from sulphidic concentrates is reported to be in the range 25
to 65 GJ per tonne of nickel for ores that contain between 4 to 15% Ni. The energy
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The processes occurring at high temperatures in the presence of air are prone to form nitrogen
oxides.
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