(3) The roaster gases may be utilised for making acid.
In modern practice the work of the reverberatory plant is controlled at the roasters. The reverberatory foreman smelts whatever mixture is sent from the roasting plant, and if the grade of the resulting matte is not satisfactory, it is in the roasting operations that the required change is made for the correct adjustment of the sulphur and for controlling the consequent tenor of the matte.
The Reactions of Roasting.—The operation of roasting is the exposing of a substance to the effects of heat and air, in order to oxidise it, and to render it more suitable for subsequent smelting operations.[7]
In the case of the ordinary sulphide copper ores, roasting not only (a) reduces sulphur, and so ensures good concentration on smelting, but (b) by oxidising the iron, provides a ready flux for siliceous gangues. The more important reactions occurring to the usual constituents of the copper ores which are roasted, may be summarised as follows:—
Iron Pyrites.—First loses free sulphur at a low temperature: it is generally assumed that FeS is left, but the residual sulphide rarely attains this composition—
FeS2 ➡ FeS + S.
Iron Sulphide.—Sulphur has a great affinity for oxygen, to form SO2 and it may be assumed that this reaction first takes place thus—
FeS + O2 ➡ (Fe) + SO2 (i.)
The iron is however instantly oxidised by the excess oxygen always present—
(Fe) + O ➡ FeO (?) (ii.)