The two methods employed are—(A) open-air roasting, (B) roasting in kilns.
A. Open-air Roasting of Lump Ores.—This method is conducted in heaps or stalls, and the features just considered apply particularly to this branch of roasting practice. The modern tendency is to avoid heap-roasting altogether, and it is only conducted when the conditions are exceptional.
Amongst the many grave objections to open-air roasting are—
(a) It is very slow, since a long period of time is required for the oxidising effect to penetrate through massive lumps of ore.
(b) A large amount of capital is tied up in the material at the roast-yards.
(c) The losses occasioned by wind and rain are very considerable.
(d) It is difficult to use up a large quantity of fines in the roast-heaps.
(e) Difficulties arise owing to damage by the fume, and from interference by litigation.
There is one special instance of a modern smelter making a great success of heap-roasting—namely, at Rio Tinto—but the circumstances are peculiar, as the roasting is followed by leaching operations of the immense ore heaps in situ.
This branch of roasting need not be considered at length, and the older standard text-books give full descriptions of the various methods employed. The following particulars are important, however, when under exceptional circumstances such work has to be undertaken:—