The power required to drive the Thwaite-Denny furnace is four indicated horse-power.
The Molesworth Furnace also is a revolving cylindrical appliance, which, to say the least of it, is in many respects novel and ingenious. It consists of a slightly cone-shaped, cast-iron cylinder about fourteen feet long, the outlet end being the larger to allow for the expansion of the gases. Internal studs are so arranged as to keep the ore agitated; and spiral flanges convey it to the outlet end continually, shooting it across the cylinder. The cylinder is encased in a brick furnace. The firing is provided from outside, the inventor maintaining that the products of combustion are inimical to rapid oxidisation, to specially promote which he introduces an excess of oxygen produced in a small retort set in the roof of the furnace and fed from time to time with small quantities of nitrate of soda and sulphuric acid. Ores containing much sulphur virtually calcine themselves. I have seen this appliance doing good work. The difficulties appeared to be principally mechanical.
There are other furnaces which work with outside heat, but I have not seen them in action.
“C.”—Shaft Type of Furnace.
In one form of this furnace, instead of allowing the ore to descend in a direct clear fall the descent is impeded by inclined planes placed at different levels in the height of the shaft, the ore descending from one plane to the other.
The Stetefeldt Shaft Furnace.—Although very expensive in first cost, has many advantages. No motive power is required and the structure of the furnace is of a durable character. Its disadvantages are:—Want of control, and the occasionally imperfect character of the roasting originating therefrom.
Three sizes of Stetefeldt’s furnaces are constructed:
| The largest will roast from | 40 to 80 | tons per diem. |
| The intermediate ” ” | 20 ” 40 | ” ” |
| The smallest ” ” | 10 ” 20 | ” ” |
A good furnace should bring down the sulphur contents even of concentrates so as to be innocuous to mercuric amalgamation. The sulphur left in the ore should never be allowed to exceed two per cent.
A forty per cent. pyritous or other sulphide ore should be roasted in a revolving furnace in thirty to forty minutes, and without any auxiliary fuel.