Geology Reference
In-Depth Information
The next step, the purification process, removes impurities from the leach solu-
tion that could affect the quality of electrolysis. This removal is achieved by adding
zinc powder to the solution which serves to chemically replace the copper, cadmium,
arsenic, antimony, germanium, tellurium and thallium - all of which precipitate out
of the solution as sludge. A similar process is undertaken via arsenic and antimony
oxide catalysts, in order to replace nickel and cobalt in an operation called cemen-
tation. Once all of impurities have precipitated out, the solution is ready for the
final step of electro-winning, whereby the cell house employs lead anodes, where
the oxygen is formed and aluminium cathodes, where the zinc metal deposits. Any
spent electrolyte is recycled back to the leaching stage. An electrical induction fur-
nace melts and casts the zinc cathodes into ingots. The electrolysis phase is the
greater energy consumer of zinc smelting.
8.8.3 Energy and environmental issues of lead and zinc production
The lead and zinc industry cause substantial environmental impacts to air and wa-
ter via the production of hazardous wastes. Acid rain, poisonous heavy metals and
greenhouse gases are of the greatest concern. Emissions to air typically include sul-
phur dioxide and acid mists, nitrogen oxides, zinc sulphate, metals, VOCs, dioxins,
furans and dusts. They can be formed either as controlled emissions from known
sources or escape diffusely, meaning that they can appear from almost anywhere on
site.
There are many emission sources to be controlled and treated: from the roast-
ing, the sulphuric acid plant, smelting and refining operations, electrolysis, casting
or transport and handling. The oxidation stages of sulphides produce sulphur dio-
xide. Sulphur dioxide emissions can reach up 3,000 g per tonne of metal produced
albeit that there are plants that emit less than 1,200 g/t metal. Furthermore, even
though sulphur dioxide is oxidised and converted into sulphuric acid, typical SO
2
concentrations in the off-gas range from anything between 200 to 2,300 mg=Nm
3
.
Diffuse emissions can of course be avoided through the correct sealing of furnaces
and extraction systems.
Roasting, smelting and furnaces with oxygen stages provide sources of NO
x
with
the range for the complete process between 20 to 400 mg NO
x
=Nm
3
off-gases. Gas
cleaning processes and the sulphuric acid plant avoid larger emissions. Other gases
of concern are the highly toxic stibine and arsine (which is also pyrophoric). The
former is produced in the wet treatment of residues derived from the recovery of
lead batteries whilst the latter is emitted in the zinc purification process together
with cadmium in the cadmium plants, especially in the distillation stages.
Roasting, smelting processes and slag treatment can also emit direct and dif-
fuse emissions of various dusts and metals. Here the dust removal is critical since
Zn;Cd;Pb;Hg;Se;Cu;Ni;As;Sb;Co and Cr can be eventually recovered thereby
avoiding ecosystems damage. Dusts can also be produced in the melting, alloying
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