How is zinc extracted from its ore?
Over 95% of the world’s zinc is produced from zinc blende (ZnS). The
low concentrations of the zinc blende mineral in most zinc ores mean that the
ore must first be concentrated through froth flotation before extracting the
metal as zinc ores contain 5-15% zinc. Froth flotation is where a strong
current of air is blown through the suspension, producing a heavy froth or foam
on the surface. The metal sulphide is wetted by the oil but the gangues is not
and the sulphide-oil mixture is carried to the surface by films of oil The
froth is skimmed off, the gangue settles down on the bottom or remains
underneath the froth. By this flotation method it is possible to
obtain the concentrate containing at least 50% zinc.
Next, the concentrate goes through a process called
sintering. The
concentrate is brought to a temperature of more than 900°C where zinc sulphide converts
into the more active zinc oxide. At the same time sulphur reacts with oxygen
giving out sulphur dioxide which subsequently is converted to sulphuric acid, an
important commercial by-product.
2ZnS + 3O2 --> 2ZnO +2SO2
2ZnS + 3O2 --> 2ZnO +2SO2
Then, metallic zinc can be recovered, by using either electrolytic
refining (hydrometallurgical)or by thermal smelting (pyrometallurgical).
In thermal smelting, The crude zinc oxide is heated
with coke (carbon) at 1,000ºC. The zinc metal formed.
ZnO +
C --> Zn(vapour) + CO(gas)
Zinc is
found in vapour form as, unusually for a metal, zinc boils at the relatively
low temperature of 907ºC. The zinc vapour is condensed in the absence of air,
so that the zinc does not oxidize with oxygen in the air again to form the
oxide. Zinc produced by this process is about 98.5% pure and can be further refined
if required. Around 15% of the world’s zinc is produced this way.
In
electrolytic refining, Sulfuric acid is added to zinc oxide.
ZnO +
H2SO4 --> ZnSO4(aq) + H2O(l)
However, the dissolved solution contains
some impurities which need to be eliminated in order to obtain a high-purity
zinc product at the end of the production process. Purification is mainly done by adding zinc
dust to the solution. As all the elements to be removed lie below zinc in the reactivity
series they can be precipitated by cementation. Cementation is a specialized precipitation process in which a more
reactive metal is used to remove a more noble metal from solution. The thus obtained purified solution passes
an electrolytic process where the purified solution is electrolyzed between
lead alloy anodes and aluminium cathodes. An electrical current is circulated
through the electrolyte by applying an electrical difference of 3.3-3.5 volts
between the anode and cathode causing the zinc to deposit on the aluminium
cathodes in high purity. The deposited zinc is stripped off, dried, melted and
cast into ingots. The zinc ingots may have different grades: High Grade (HG)
99.95% and Special High Grade (SHG) 99.99% of zinc. About 80% of the world's zinc is
produced using this process.
Impacts
- Drinking water contains certain amounts of zinc, which may be higher when it is stored in metal tanks. Industrial sources or toxic waste sites may cause the zinc amounts in drinking water to reach levels that can cause health problems.
- Levels of zinc in excess of 500 ppm in soil interfere with the ability of plants to absorb other essential metals, such as iron and manganese.
- Concentrations of zinc as low as 2 ppm also adversely affects the amount of oxygen that fish can carry in their blood and thus might lead to the death of the fishes.
- Extraction of zinc also produces greenhouse gases such as sulphur dioxide and carbon monoxide. This in turn would cause an enhanced greenhouse effect on the environment which would lead to global warming.
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