We now come to kilns for burning dust pyrites. As the dust prevtiats the passage of air when disposed in thick layers, it must be spread in a thin coating over which the draught can pass. The heat generated is so much lessened under the circumstances, however, that the combustion can only be maintained by the assistance of additional heat from without. With this object the dust has been, till recently, burnt in so-called muffle furnaces, having an enclosed roasting space, heated by flre on tho outside. Such wero also formerly used for renAine- copper pyrites, And are still employed for zinc-blende ; but they are no longer in use for treating iron pyrites dust for sulphuric acid making, as other and better plans have been devised. Muffle furnaces therefore ore no longer to be found in works where sulphuric acid is the chief product, but only where the sulphurous acid from the smelting of ores is applied to the manufacture of sulphuric acid as a bye-product.
A plan common in Germany consists in dividing the kiln into two parts by a low wall, in each of which are placed four fire-clay plates 6 ft. long, 1 ft. 7 in. wide, and 4 in. thick, ranged at 8 in. distance one above another. Each of the eight portions of roasting surface is provided with a door 12 in. wide and 6 in. high, fitted with holes for the admission of air, by which the charge is introduced, and at the back is an opening, 4 in. square, through which the liberated sulphurous acid escapes into a vertical flue, 3 ft. long and 8 in. wide, in connection with the chambers. The nitre pots are inserted in this flue. The whole roasting surface is made red hot by a coal fire ; the flame branches into all the flues, but re-unites at one, and escapes by a shaft.
In many Belgian works the dust pyrites is roasted iu muffle furnaces having a single fire-bed 30 ft. long x 8 ft. broad. This is made of fire-clay slabs, 3 in. thick, supported by the side walls of the kiln and by several subsidiary walls, 4i in. thick, and heated from below. Ia one of the long sides of the kiln three fireplaces are made, the heat from which is divided and spread under the whole of the bed-plate. An arch is turned over the bed-plate so that the roasting space is 16 iu. high at the walls and 2 ft. 4 in. in the centre. At one end of the kiln is a pipe which conducts the gases to the chambers. Near this is an opening, that may be closed at pleasure, through which the kiln is charged, and at the opposite end, in the sole of the furnace, is a second opening, 8 in. square, through which the burnt pyrites is withdrawn from the ashpit into which it has fallen. This is shut by an iron door, perforated with holes for the admission of air. In one
of the long sides of the roasting chamber four holes are made, at which the workman inserts the tool for raking out the spent dust before inserting each new charge. About five tons of dust can be burnt in this kiln per twenty-four hours, divided into six or eight charges daily, and spread about 3 to 4 in. thick over the surface of the bed-plate. The with drawal of the burnt ore and the introduction and spreading of the fresh charge occupy about one hour. Besides this the dust must he turned over about every half hour, which occupies ten minutes. It re quires two workmen.
In this country Spence's kiln, as shown in Fig. 68, is chiefly used. The fire-lump bed c, 30 to 45 ft. long, is heated from below by a fire in the furnace-grate a, whilst the pyrites is spread out in a layer about 2 to 3 in. thick on the sole of the chamber d. The necessary air enters at the passage f (by which also the spent dust is withdrawn) into the roasting chamber, and the gas formed passes at g into the leaden chamber. The charge is introduced at the lateral hole a farthest from the fire, as the charge immediately before is turned over towards f, and thus leaves a space. The tools used for this purpose are inserted at the holes e, e', e". By this process the ore is pushed, as it gets poorer in sulphur, always into a hotter part of the kiln, where also the air is richer in oxygen. Thus the burning of the last portions of the sulphur, which is generally so difficult, is greatly assisted, and with due care very little sulphur is left in the ore.
.At Swansea these kilns are generally about 30 ft. long, and are charged every two hours with about 10 cwt. of copper pyrites dust, which remains twelve hours in the kiln, thus daily 6 tons of ore are roasted.
Among the newest forms of kiln for roasting dust ores is Gerstenhoefer's, consisting of a tall shaft, while Olivier and Perret's furnace is intended for treating lump and dust ores at the same time.
The former kiln is shown in Figs. 69, 70, 71, and 72. Fig. 69 is partly in elevation and partly in section along the line V V of Fig. 70, the latter being a part plan and a part horizontal section of the former along the line Y Y. Fig. 71 is a vertical section along the line X X of Figs. 70 and 72, the latter being a horizontal section along the line Z Z of Figs. 69 and 71. The main shaft of the kiln, formed of fire-bricks, is about 17 ft. high, 4 ft. 3 in. long, and 2 ft. 9 in. wide.