Before charging the kiln it must be made white hot. For this purpose fire-bars must be put into the furnace a, Figs. 69 and 71, and the hole 1, which will have been opened for this purpose, is re-closed, and a strong fire is made in the grate. The necessary fuel is inserted at the openings c, which may be closed with cast-iron doors. The lowest door b remains open to admit air. The con neotion of the kiln with the chambers is shut while the kiln is being heated, and on the other hand a side flue is opened through which the combustion gases escape. When the kilo is white hot the charging is slowly commenced. To keep the burning regular, the ore must be supplied as a dry powder of constant grade, and for this reason it is sifted before use. It is poured into the cast-iron box A, fitted with a wooden hopper, and which contains tho apparatus for regulating the admission.
This consists of two corrugated rollers, with ribs about in. broad, 2 in. in diameter hollows, and 3d in.
in the ribs, made to revolve regularly by the worm d on the pulley shaft e. By the pace of these rollers, which at first is only one revolution in five minutes (thus taking seven hours to feed the kiln), the supply is regulated. The cover f, above the rollers, can be shut or opened at pleasure, and serves to protect them from the weight of the superincumbent mass of mineral. The ore taken in by the rollers is dropped into the slit g, which is shut by the ore lying in the half-oylinder h above the rollers, so that no kiln gas can escape. From the slit g the ore falls on the fire-clay prism i, and thence on both sides to a row of four prismatic fire-clay bars k, whence it falls again on to seven such bars lying immediately below. Beneath are fifteen such rows, alternately six and seven bars, and so arranged that the bars above always correspond to spaces helow, and the spaces above to bars below. The ore thus falls gradually to the bottom of the kiln B, which serves as a collecting space. As soon as the ore begins to fall upon the fourth row of bars counting from the bottom, the fire is suspended iu the furnace a. Theu the fire-bars are drawn out one by one, the holes left are walled up, the ashpit is cleared out, and the kiln completely closed, save the openings necessary for supplying air. The gases are still allowed to escape for a short time, then the connection between the kiln and the escape flue is altogether closed, and that leading to the chambers re-opened. Formerly hot air was forced into the kiln, but now cold air only is used in the roasting of iron and copper pyrites. For sulphides which burn with great
difficulty, as zine-blende, it is preferable to have hot air. The falling ore comes into contact with ascending air in such a way that, in consequence of the oxidization and desulphurization, it constantly finds air richer iu oxygen, by which its complete roasting is much aszisted.
The sulphurous acid formed and the excess of oxygen and nitrogen from the air admitted leave the kilo at the top through the flues m (which are furnthed with closing doors n, for convenience in cleaning) into the main flue C, and thence through the dust chamber D into the leaden chamber. The openings o and p, shut with folding iron doors, are used when cleaning the main flue and the dust chamber. This last-named is roofed with iron plates, on which the ore is dried. In the front wall of the kiln are openings q corresponding with the spaces between the bars. These are fitted with iron boxes r, pierced by round holes furnished with fire-clay plugs. These serve for watching the progress of the roasting, and for the introduction of an iron scraper, should the spaces between the bars become clogged. It is well to see to this every three hours. Also occasionally the dust must be removed from the uppermost part of the kiln, where it accu mulates. This is effected through the holes s (Fig. 69). The form of the boxes and bars needs no further explanation than the view shown in Fig. 73 From two to five tons can be burnt daily in this kiln. At this rate the pit B must be emptied every six hours. In order to reduce as far as possible the amount of air thus perforce admitted, the scraper or rake is introduced through a little opening formed in the door b. In working the kiln four men are necessary, but they can manage more than one kiln at a time. The result of the working depends greatly upon the care bestowed by the workmen in cleaning the spaces between the bars. In the proper conduct of the working, the greatest heat prevails at the upper part of the shaft, lower down it decreases to low red, whilst the lowest bars cease to glow at all. When air is admitted in too great quantity the heat spreads downwards, and when deficient it retreats upwards; in the former case the kiln becomes too hot, in the latter too cold. When the heat is too great the draught must be diminished, or the supply of ore increased, by augmenting the pace of the rollers. Should fritting of the ore ensue from too rapid combustion, the fault must be checked by substituting spent ore for raw in feeding the kiln.