is suggested by Peters as a probable reaction occurring to this
material under roasting conditions.
_Lead Sulphide_ is also occasionally present with copper ores. It melts
readily, and is not entirely decomposed at the temperatures employed
for the roasting of copper ores. The reactions on oxidation are largely
analogous to those for other sulphides.
┌─
│ PbS + O_{2} ➡ Pb + SO_{2}
─┤
│ Pb + O ➡ PbO
└─
or, PbS + 3.O ➡ PbO + SO_{2}.
Also, 2PbO + SO_{3} ➡ PbSO_{4}.PbO (basic sulphate).
_Arsenides_ are partly left as the corresponding oxides, whilst some
As_{4}O_{6} is evolved, and some basic arsenate generally remains.
Roasting Practice.
_Favourable Conditions for Successful Roasting._
(_a_) The sulphide should be in a finely divided form, so as to ensure
good contact with the air.
(_b_) The air should be supplied in a gentle current, so as to
continually provide fresh oxygen, and sweep away the inert gases which
are produced.
(_c_) The ore should be heated to a dull red heat, which is a condition
favourable for commencing the ignition and reactions. The temperature
should, of course, be well below a melting heat (_Peters_).
_The Apparatus for Roasting_ depends to some extent on the class of
material to be dealt with, which may be in the form of either (_a_)
lump ores, or (_b_) fine ores.
(=a=) =Roasting of Lump Ores.=—In modern copper-smelting work, the
practice of roasting lump ores is practically obsolete. The conditions
under which its use might still be justified are those associated with
newer mining districts, where rapid concentration of heavy sulphide
ore into matte is required, before the time is ripe for smelting the
material pyritically, and where further, it is desired to employ the
blast furnace for the smelting operations under these circumstances.