Developments of Roasting Practice.—The main objects sought in roasting practice have been—
- (1) To have as large a surface of material exposed to heat and air as possible.
- (a) By elongating and multiplying the beds of the furnace.
- (b) By furrowing and rabbling the charge.
- (2) Continually to expose fresh surfaces of ore to oxidation.
- (a) First by hand-rabbling.
- (b) Later by movable furnace hearths.
- (c) By mechanical rabbling.
- (3) To obtain a continuous output—
- (a) By mechanical charging, rabbling, and discharging.
The Development of the Roasting Furnace.
A. Fixed Hearth.—In Great Britain from 1583 onward, roasting in small reverberatory furnaces seems to have been the usual method, and up to 1850 the furnaces appear to have been only of moderate dimensions, with a single hearth, 16 feet × 13 feet 6 inches, constructed of firebricks set on end, and with a fire-box 7 feet × 2 feet 3 inches × 18 inches. Rabbling was done by a long rake, the material being charged and worked through one door. This method of working wasted time, made the process intermittent, and caused continual cooling down of the furnace, involving large fuel costs and much labour. The first improvements were to lengthen the hearth, to add more working doors, and to put the charge into the furnace by a hopper passing through the roof. It was next found best to elongate the hearth still further, and to drop the level of the bed in stages by about 2 inches at a time, thus ensuring better control of working. By this means the best type of hand-calciner was arrived at, consisting of four beds, each 16 feet × 16 feet, the whole charge being moved forward from one bed to the next at each stage of the process.
In roasting, the ore is first placed in the coolest part of the furnace, and is worked towards the fire, so that the charge travels in one direction, and the flame and furnace gases in the opposite direction to meet it.
The advantages of this system are that—
- (a) The clotting of the sulphides is prevented, since the first part of the roasting proceeds at a comparatively very low temperature.
- (b) The sulphur in the ore often provides sufficient heat to maintain the roasting in progress during the early stages.
- (c) The hottest parts of the furnace are where the roasted infusible oxides arrive, so but little clotting or sintering occurs here.
The capacity of the four-bedded hand-roaster is 7 to 15 tons per twenty-four hours, depending on the sulphur proportion in the charge and in the roasted product.
It is a very useful form of furnace when labour is cheap. The furnace works very efficiently, but in the New World, where manual labour was dear, labour costs became prohibitive, and in order to economise in this direction, mechanical rabbling was introduced.
The O’Harra Calciner (1885) was essentially the old type of furnace, double hearthed and mechanically rabbled. It consisted of long straight furnace hearths. The rabbles were ploughs dragged through the furnace by means of endless chains which were carried over grooved pulleys, situated outside the furnace, at the ends. This was an important invention, giving a continuous feed and discharge, a much larger output, and efficient and regular stirring without much hand labour. The rabbles became cooled on issuing from the hearth. The capacity was 50 tons per day from furnaces of 90 feet × 9 feet hearths, giving a roasting capacity of 61 lbs. of ore per square foot of hearth area, compared with about 33 lbs. per square foot with the old hand calciner. In working the early forms of this furnace there were many mechanical troubles and breakdowns, and the subsequent modifications of this form consisted largely of devices for the purpose of overcoming such difficulties.