Geology Reference
In-Depth Information
8.11.1 Chromium
Chromium's ore is chromite, a spinel of the type FeOCr
2
O
3
with variable amounts
of Mg substituting Fe and Al and Fe
3+
substituting Cr
3+
. Therefore the range of
Cr
2
O
3
in chromite usually varies from 25 to 60% with metal chromium correspon-
ding to 17 to 41%.
Chromium is used for alloying iron as a FeCr additive, for corrosion resistant
electroplating and for pigments in paints and inks. When producing ferrochromium,
the smelting process of chromite consists of a reduction that does not eliminate iron.
It is done in an electric arc furnace (see Fig. 8.10) or via metallo-thermic smelting
using aluminium, silicon or carbon as reducing agents.
Fig. 8.10 High-carbon ferro-chrome production by using a closed furnace (IPPC, 2009)
For producing pure chromium metal, iron must be separated by roasting
chromite with calcium and sodium carbonate. This reaction produces molten
sodium chromate, Na
2
Cr
2
O
4
and Fe
2
O
3
that precipitates in the high tempera-
ture melt producing CO
2
gas. A subsequent leaching process with sulphuric acid
converts the sodium chromate into sodium dichromate, Na
2
Cr
2
O
7
. The second
step is to reduce dichromate to chromium (III) and finally to chromium metal.
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