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.
The advantages possessed by the method are—
(1) No preliminary crushing is required.
(2) The product is largely in the form of lumps, and hence immediately suitable for blast-furnace work.
(3) The plant and appliances required are simple.