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Chromium ore, or chromite, occurs exclusively in rocks
formed by the intrusion and solidification of molten lava or
magma which is very rich in the heavy, iron containing minerals
such as pyroxenes and olivines. Within these rocks, often referred
to as ultramafic igneous rocks, chromium occurs as a chromium
spinel, a highly complex mineral made up, in its basic form,
of magnesium as MgO and aluminium as Al2O3.
However, the magnesium can be substituted in varying proportions
by divalent iron, and the aluminium can be substituted, also
in varying proportions, by trivalent chromium and trivalent
iron. Thus the chromium spinel may be represented as:
Large variations in the total and relative amounts of Cr and
Fe in the lattice occur in different deposits. These affect
the ore grade not only in terms of the Cr2O3
content but also in the Cr:Fe ratio which determines the chromium
content of the ferrochromium produced. The variations also affect
the reducibility (relative ease of reduction) of the ore. For
example, increasing amounts of magnesium compared with iron
in the divalent site will make the spinel more difficult to
reduce. Conversely, increasing amounts of iron in the trivalent
site, replacing aluminium, will increase the reducibility of
the spinel. Overall, the chromite ore can be given a refractory
index (relative resistance to reduction) as follows:
| Refractory index= wt.% |
Cr2O3+MgO+Al2O3
|
| |
|
| |
(total Fe as FeO) +SiO2 |
The greater the index, the more refractory, or less reducible,
the ore.
Chromium is the most abundant of the Group V1A family of elements
and at an average concentration of nearly 400ppm in the earth's
crust it is the 13th most common element. However, as with all
minerals or elements, economic deposits occur only where it has
been concentrated in nature. The chromium spinel is a heavy mineral
and it concentrates through gravity separation from most of the
other molten material in the magma during crystallisation from
the cooling magma. Commercial chromite deposits are found mainly
in two forms: stratiform seams in basin-like intrusions, often
multiple seams through repeated igneous injections, and the more
irregular podiform or lenticular deposits.
The best known example of a stratiform deposit is the Bushveld
Igneous Complex of South Africa. This complex contains most of
the world's chromite reserves. The Great Dyke of Zimbabwe, traversing
nearly the length of the country, is very similar and has been
linked to the Bushveld in geological history. These two features
are well-known also for their important and very large commercial
deposits of the platinum-group metals.
Other stratiform deposits occur in Madagascar and in the Orissa
district of India.
Stratiform deposits are generally very large complexes. They can
be more than 5,000 metres thick and cover thousands of square
kilometres. For example, the largest, the Bushveld, covers an
area of 12,000 square kilometres.
The podiform deposits are relatively small in comparison and may
be shaped as pods, lenses, slabs or other irregular shapes. Many
have been extensively altered to serpentine and they are often
faulted. They are generally richer in chromium than the stratiform
deposits and have higher Cr:Fe ratios. Ore reserves in Kazakhstan
are of the podiform type. Podiform ores were originally highly
sought after, especially those from the deposits in Zimbabwe,
as the best source of metallurgical grade chromite for high-carbon
ferrochromium. These ores also tend to be massive (hard lumpy)
ores, as opposed to the softer, more friable ores from the stratiform
deposits, and this makes for better electric smelting operation.
There is a third type of chromite deposit but of very limited
commercial significance. These are the eluvial deposits that have
been formed by weathering of chromite-bearing rock and release
of the chromite spinels with subsequent gravity concentration
by flowing water.
Chromium may also be concentrated in high-iron lateritic deposits
containing nickel and there have been attempts to smelt these
to produce a chromium-nickel pig iron for subsequent use in the
stainless steel industry.
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