Environmental Engineering Reference
In-Depth Information
In conventional wet tailings disposal schemes, washed solids (termed red mud or bauxite
residues) are pumped to tailings ponds for final disposal (see Chapter Eighteen for a discus-
sion of tailings management practices). In recent years dry tailings disposal schemes emerged
as the preferred tailings management practices in which washed solids are dewatered using
filter units to produce bauxite residue cake. Red mud consists of iron oxide, sodium alumin-
ium silicate, titanium oxide, and some other metals, and is alkaline with a pH of about 13.
Red mud consists of iron oxide,
sodium aluminium silicate,
titanium oxide, and some other
metals, and is alkali with a pH of
about 13.
Precipitation and Filtration
After separation of solids, the remaining liquor (or pregnant liquor) is passed to precipita-
tion via heat exchangers where the pregnant liquor is cooled to 60 to 70ºC. Recovered heat
is transferred to the spent liquor (liquor from which aluminium ions have been separated)
that is returned to the digestion process.
The cooled pregnant liquor is seeded with hydrate crystals, which act as a nucleus
(or seed crystal) for precipitation. Seed crystals grow as they settle through the liquor by
attracting dissolved aluminium ions to them. The chemical reaction during precipitation
that removes dissolved aluminium ions from the pregnant liquor is as follows:
Al
2
O
3
(6.18)
2NaAlO
2
4H
2
O
→
3H
2
O
2NaOH
The reaction is slow with a large fraction of soluble alumina remaining in the spent liquor.
While the retention time in precipitation tanks is about 35 to 40 hours, most of the yield
and particle growth is obtained in the first few hours.
Calcination
Calcination transforms precipitated hydrates into alumina. After filtration, alumina hydrates
are fed into a rotating, cylindrical kiln that is tilted to allow gravity to move the material
through it, heating it to a temperature of about 1,100ºC to 1,300ºC. After leaving the kiln,
the crystals pass through a cooler. The final product is in the form of white alumina powder.
6.4 PYROMETALLURGY AND RELATED
ENVIRONMENTAL CONCERNS
The main environmental challenges in pyrometallurgical processes are the production of
large quantities of gaseous emissions (e.g. sulphur dioxide, vaporized metals, and dust),
solid emissions (e.g. slag, gypsum, and wastewater treatment sludge), and liquid emissions
(e.g. effluents and cooling water).
Gaseous Emissions
Control of gaseous emissions is a continuing challenge to the pyrometallurgical industry.
Emission controls focus mainly on stack emissions, but fugitive emissions also contribute
to the overall emissions of a smelter. Fugitive emissions are emissions that are not released
through a vent or stack. Examples of fugitive emissions include volatilization of vapour
from vats or open vessels during material movement or dust from stockpiles and conveyors.
Control of gaseous emissions
is a continuing challenge to the
pyrometallurgical industry.
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