As the silica content increases, however, and the iron and sulphur contents diminish, there is a consequent decrease in the natural fuel values of the material, and as a result, the roasting is neither so efficient nor so cheaply operated, owing to the need of external fuel for giving the required roasting temperatures. On the other hand, it appears to be just this class of material which is best suited for blast-roasting.
It is found in actual working practice that material which does not contain a certain proportion of silica does not work well in the blast-roasting or sintering processes, the resulting product being found to be more irregular in composition and more difficult to operate in the sintering plant. It would therefore appear that a certain class of fine concentrate higher in silica and lower in iron and sulphur contents, which is not quite so suitable for ordinary roasting (owing to the necessity for external heating, due to lower fuel values) is eminently suited for blast roasting or sintering processes, yielding lump products very suitable for subsequent blast-furnace treatment.
The reverberatory furnace thus deals most successfully with fine table concentrates high in iron and sulphur, moderately low in silica; roasted, with its required flux, to the necessary extent, and then charged whilst still red hot into the furnaces. To relieve the reverberatories of the greater bulk of the blast-furnace flue-dust, which it treats with more difficulty, fine concentrates, as such, require to be kept out of the blast-furnace charge, either by subjecting the more siliceous material to a preparatory sintering process, or by reserving the highly pyritic variety for roasting and subsequent reverberatory treatment.
References.
Peters, E. D., “Principles of Copper Smelting.”
Offerhaus, C., “Modern Reverberatory Smelting of Copper Ores.” Eng. and Min. Journ., June 13, 1908, pp. 1189–1193; June 20, 1908, pp. 1234–1236.
Ricketts, L. D., “Experiments in Reverberatory Practice at Cananea, Mexico,” and discussion, Trans. Inst. Min. and Met., vol. xix., 1909–10, pp. 147–185.
Ricketts, L. D., “Developments of Cananea Practice.” Engineering and Mining Journal, Oct. 7th, 1911, p. 693.