FeS + 3,O ➡ FeO + SO_{2}.
This sulphur oxidation is an important source of heat, and in the early
stages of roasting, sulphur is seen burning with the familiar blue
flame, and the mass becomes red hot; stirring being required to prevent
the material from sintering by the heat generated within itself.
The oxidation of the iron generally proceeds further, yielding higher
and more stable oxides—
┌─
│ 2FeO + O ➡ Fe_{2}O_{3}.
─┤
│ 3FeO + O ➡ Fe_{3}O_{4}.
└─
The SO_{2} in the presence of oxygen and in contact with strongly
heated material further tends to form SO_{3}, which is a powerful
oxidising agent, and plays a considerable part in the various oxidising
reactions which occur.
_Pyrrhottite_ behaves in much the same way; it may be regarded as
consisting of _x_FeS + a little extra sulphur. It does not roast quite
so easily as pyrites, partly on account of physical characteristics,
and partly because, in the case of pyrites, the greater amount of
excess sulphur which is first driven off, tends to leave the mass more
porous and so assists oxidation.
_Copper Sulphide._—Its characteristics on oxidation have already been
indicated in Lecture III., p. 36. It melts easily, often at roasting
temperatures, hence careful heating and attention are required when
much is present.
The reactions are probably analogous to those of FeS oxidation, in the
primary oxidation of the sulphur and the instantaneous oxidation of the
nascent copper—
┌─
│ Cu_{2}S + O_{2} ➡ (2Cu) + SO_{2}
─┤
│ (2Cu) + O ➡ Cu_{2}O,
└─
thus Cu_{2}S + 3.O ➡ Cu_{2}O + SO_{2};
this being accompanied by simultaneous action of the following nature:—
Cu_{2}O + SO_{2} + 2,O ➡ 2CuO + SO_{3}
CuO + SO_{3} ➡ CuSO_{4}
CuSO_{4} + Cu_{2}O ➡ 3CuO + SO_{2}.
In addition to the tendency to melt, copper sulphide roasts less
perfectly than the FeS, usually yielding oxides which are accompanied
by small quantities of sulphate.
_Chalcopyrite_ is the commonest copper ore, and the material most
frequently subjected to roasting in copper smelting practice.
Consisting of Cu_{2}S. Fe_{2}S_{3}, and accompanied usually by a
large excess of FeS_{2}, it behaves very much like a mixture of these
sulphides when treated in the roaster furnace, hence the reactions on
roasting follow on the lines just indicated.
In practice the roasting is never carried to such a degree that all the
sulphur is eliminated, since it is essential to retain some sulphur
in order to collect the copper in the form of matte, and also because
the time, and the cost of the fuel required to roast all of it off,
would be prohibitive. Consequently, the products from the roasting of
chalcopyrite consist principally of oxides of iron and copper, together
with a certain amount of copper sulphate, very little iron sulphate,
and some undecomposed sulphides.
The actual form in which the sulphur is present at the end of the
roasting operation is not usually of very special importance in
practice, especially where the previous experience with the roasted
material determines the extent to which the roasting is conducted,
since the greater part of the sulphur eventually produces the sulphide
and constitutes the matte, on smelting the roasted charge; although
some is also eliminated as SO_{2} by interaction with oxides. In
modern roasting practice, therefore, all that is usually required is
to roast the ore down to, say, 5 per cent., 6 per cent., 8 per cent.,
or whatever proportion of sulphur is necessary to yield the required
grade of converter-matte in the reverberatories, as judged by previous
experience of the furnace plant and working. Much SO_{2} is evolved
during the roasting, though it is usually largely diluted with nitrogen
from the air used up.
_Other Foreign Constituents of Copper Ores—Zinc Sulphide._—ZnS is
sometimes present. Some remains unchanged on roasting, as the heat in
ordinary practice is not great enough to thoroughly decompose it. Some
oxide and some sulphate are also produced.