EXTRACTION OF
ZINC FROM ZINC BLENDE:
Corrosion-resistant
zinc plating of iron (hot-dip galvanizing) is the major application for zinc.
Other applications are in batteries, small non-structural castings, and alloys,
such as brass. The metal is hard and brittle at most temperatures but becomes
malleable between 100 and 150 °C. Above 210 °C, the metal becomes brittle again
and can be pulverized* by beating. Zinc is a fair conductor of electricity. For
a metal, zinc has relatively low melting and boiling points (907 °C). Its
melting point is the lowest of all the transition metals aside from mercury and
cadmium. The element is normally found in association with other base metals
such as copper and lead in ores. Other minerals from which zinc is extracted
include zinc carbonate, zinc silicate, another zinc sulfide, and sometimes basic
zinc carbonate.
Methods of
Extraction:
1.
Roasting
Roasting
converts the zinc sulfide concentrate produced during processing to zinc oxide.
The carbon monoxide acts as the reducing agent i.e. it removes the oxygen from
the oxide. The impure zinc is then fractionally distilled from the mixture of
slag and other metals like lead and cadmium out of the top of the furnace in an
atmosphere rich in carbon monoxide which stops any zinc from being oxidised
back to zinc oxide. The slag and lead (with other metals like cadmium) form two
layers which can be tapped off at the base of the furnace. The zinc can be
further purified by a 2nd fractional distillation or more likely by dissolving
it in dilute sulphuric acid and purified electrotically.
1. 2. Reduction
(Smelting)
REDOX
definition reminders – reduction is a process of oxygen loss (or electron gain)
and oxidation is a process of oxygen gain (or electron loss).
ZnO obtained
during roasting is mixed with coke and heated strongly where ZnO is reduced to
Zn by carbon.
ZnO + C → Zn + CO
The reduction
is done in vertical fitting. In this process roasted are mixed with coke in the
ration of 2:1 and small briquettes are made. These briquettes are fed into
vertical report furnace, from the charging door. The report is heated
externally by burning produce gas to about 14000c. The vapour of zinc is
transferred to condenser where it condenses to give molten zinc called spelter
zinc.
1. 3. Purification
a. By
fractional distillation:
The boiling
point of Pb, Fb are higher than that of zinc while that of cadmium, arsenic are
lower than that of zinc. When distillation is carried out around 1000°c, zinc,
Cd, As, etc. distil off leaving Pb and Fe. The distillate is then heated to
800°c where Cd and As distil off leaving pure zinc. This sample of Zn is about
99% pure.
b. By electrolysis:
Zinc of
higher purity can be obtained by electrolysis. Pure zinc rod is used as cathode
while a block of impure zinc is used as anode. A mixture of Zinc Sulfate and
dill sulphuric acid is used as electrolyte. On passing current impure zinc
dissolves and equivalent amount of pure zinc is deposited at cathode.
IMPACT ON ENVIRONMENT:
The production for sulfidic zinc ores produces large amounts of sulfur dioxide and cadmium vapor. Smelter slag and other residues of process also contain significant amounts of heavy metals. About 1.1 million tonnes of metallic zinc and 130 thousand tonnes of lead were mined and smelted in the Belgian towns between 1806 and 1882. The dumps of the past mining operations leach significant amounts of zinc and cadmium, and, as a result, the sediments of the Geul River contain significant amounts of heavy metals. About two thousand years ago emissions of zinc from mining and smelting totaled 10 thousand tonnes a year. After increasing 10-fold from 1850, zinc emissions peaked at 3.4 million tonnes per year in the 1980s and declined to 2.7 million tonnes in the 1990s, although a 2005 study of the Arctic troposphere found that the concentrations there did not reflect the decline. Anthropogenic and natural emissions occur at a ratio of 20:1. Levels of zinc in rivers flowing through industrial or mining areas can be as high as 20 ppm. Effective sewage treatment greatly reduces this treatment along the Rhine, for example, has decreased zinc levels to 50 ppm. Concentrations of zinc as low as 2 ppm adversely affects the amount of oxygen that fish can carry in their blood. In some areas, this pollution is hazardous to human
health. Also, Soils contaminated with zinc through the mining of
zinc-containing ores, refining, or where zinc-containing sludge is used as
fertilizer, can contain several grams of zinc per kilogram of dry soil. 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. Zinc
levels of 2000 ppm to 180,000 ppm (18%) have been recorded in some
soil samples. Lead (Pb) and cadmium (Cd) are the main pollutants and are
associated with human health effects such as high lead blood levels in
children, arthralgia, osteomalacia, and excessive cadmium in urine.
Other acknowledgements:
Student contributions:
Cai XinTong
Nicole Ching
Phoebe Chan
Nicole Ching
Phoebe Chan
