_Iron Sulphide._—Sulphur has a great affinity for oxygen, to form SO_{2} and it may be assumed that this reaction first takes place thus— FeS + O_{2} ➡ (Fe) + SO_{2} (i.) The iron is however instantly oxidised by the excess oxygen always present— (Fe) + O ➡ FeO (?) (ii.) Or, combining (i.) and (ii.)— 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.