Geology Reference
In-Depth Information
This is done by first reducing the dichromate to chromite with coal and then reduc-
ing the chromite to chrome metal using aluminium. An alternative way to obtain
chromium is by the electrolysis of chromic acid, which is obtained from the dichro-
mate produced in the leaching process.
Alternatively, chromium metal can be produced from ferrochromium by elec-
trolysis as follows. First FeCr is leached in sulphuric acid at 200
o
C where ferrous
sulphate precipitates. Then chrome alum (chromium (III) potassium sulphate) is
formed upon the addition of potassium sulphate. After several crystallisation, fil-
tration and ageing process steps, alum is ready to be used as an electrolyte for a
diaphragm cell (IPPC, 2009).
According to Chapman and Roberts (1983) the energy consumption in the pro-
cess of mining and ore concentration is 1.5 GJ/t of FeCr and the energy required
to obtain high-carbon ferrochrome from ore concentrate is 62.6 GJ/t of FeCr. Al-
ternatively, Classen et al. (2007) (Ecoinvent) report that the energy required in the
mining and concentration stages of this ore is 0.081 GJ/t of chromite, equivalent
to 0.084 GJ/t of ferrochrome, at a refining grade of 81%. In the smelting pro-
cess, the same authors report an energy consumption of approximately 36.3 GJ/t
of ferrochrome produced whereas IPPC (2009) gives a figure of between 41.5-43.6
GJ/t.
With regards to health impacts, whereas chromium (III) is very stable and easily
excreted from the body, chromium (VI) is quite aggressive if ingested because it is a
very active oxidant. An additional concern is chromate dust due to its carcinogenic
properties and ability to trigger allergic reactions (EPA, 1984; WHO, 2009).
8.11.2 Manganese
Pyrolusite (MnO
2
) is the most important manganese ore, which is usually found
either hydrated or in its anhydrous form with variable amounts of iron oxides and
a Mn content as high as 35%. There are also vast quantities of pure manganese
nodules on the ocean bed but they are as yet not economically exploitable.
There are two commercial routes to produce manganese metal from its ore.
One is electrolysis of aqueous manganese salts and the second is electrothermal
decomposition of the ore.
The former consists of first reducing the ore in a rotary kiln at 850-1000
o
C with
coal as a reducing agent. In this method, once the calcine produced is cooled, it is
leached in the sulphuric acid taken from the electrolytic bath. In the purification
process of the leach, iron is removed by oxidative precipitation whilst nickel and
cobalt are removed by sulphide precipitation and then filtered. The electro-winning
of manganese sulphate has a poor e
ciency due to its low reduction potential com-
pared to hydrogen. To avoid this problem, sulphur dioxide is added to the electrolyte
for electro-winning. Manganese metal is then plated on titanium cathodes, stripped
off, washed, dried and degassed. The sulphuric acid from the anodes is subsequently
recycled for re-entry into the leaching process.
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