The burning of the lump ore of iron and copper pyrites may be performed in small shaft kilns, first invented in this country. Their shape and size vary very much according to the nature of the ore they are intended to roast. Figs. 59, 60, and 61 show a kiln without a fire-grating, about 10 ft. high and 3 ft. in diameter. In the vertical view to the ]eft of the line A B is seen the outside view of the kiln, and on the right of the same line a vertical section of the kiln on the line C D of the plan. Fig. 61 gives a vertical section of the lower part of the kiln on the line E F of the plan. The moulded and lump ores, which latter are previously reduced to about 1/ in. in diameter, are inserted through the opening a fitted with an iron cover, and after being completely burnt they are drawn out at the bottom at b b' with iron rakes.
In order to lighten the labour, the sole of the kiln is formed as a cone rising in the shape c c, and down which the ore easily runs. The air necessary for the combustion enters partly at b b', and partly at the holes d in the side wall of the kiln, which are closed according to need by pieces of brick. These openings also serve for the introduction of iron pokers when the mass needs stirring or breaking up. This happens especially with copper ores which very easily sinter together. The height to which the kiln is filled depends upon the quality of the ore to be burnt. The more difficult of burning, the greater the quantity of ore necessary at a time. It is very important to be sure that sufficient air is admitted to fully burn off the sulphur of the upper layers. The gases pass through e into the leaden chambers. In many works they are taken first through a brick chamber where much of the dust mechanically carried in the gas is deposited. When the nitric acid is derived from nitre and sulphuric acid, the pots containing the mixture are put through a close-shutting door into the channel e, where the temperature is high enough to produce decom position.
When starting the kiln it must first be made red hot by means of an ordinary fire. Coal or coke may be used for the purpose, and these aro introduced like the ore at a. During the combustion of the coal, the hole a remains uncovered to give a draught to the fire, while the channel e is shut by the damper f to prevent the coal smoke, &e., from entering the chambers. When the kiln is suffi ciently hot, it is cleaned out, put again into communication with the chambers, and then fed gradually with ore in such a manner that each successive charge becomes thoroughly hot before the next is admitted. When the kiln has in this way been filled to the proper height with glowing ore, the work oontinuee regularly, and the new charge is introduced every 12, 6, or 4 hours.
The amount of ore which can be properly burnt in such a kiln depends upon the physical and chemical properties of the ore. To feed such a chamber system as we have described with sul phurous acid, from two to six such kilns will be necessary, according to the amount of sulphur in the ore used. They are then built all together, so that one channel serves to conduct all their
gases into a vertical shaft leading to the chambers.
Ore which crumbles readily settles into such a compact mass in perpendicular-walled kilns that the draught becomes choked. In order to check this evil as much as possible, kilns for burning such ores are built with two walls eloping together towards the bottom, these walls being much longer than the other two. The great height of these kilns affords considerable advantages in burning ores of slow combustion ; but they always possess the fault that the great mass at the bottom makes the draught very difficult to regulate and to supply in sufficiency. To overcome this defect, they have recently been provided with gratings under which is an empty space or ashpit, into which the necessary air is admitted through a tight-fitting iron door furnished with holes.
Fig. G2 shows a view as well as a vertical section (through the line c d, d e of the plan, Fig. 63) of such a kiln, and Fig. 64 is a vertical section of the same through the line a b of the plan. The shaft B of this kiln is of much less height than that of the kiln shown in Figs. 59, 60, and 61, and is much larger above than below. This kiln is well suited for ores which burn rapidly. The mineral is introduced from above at the opening f, which is fitted with a close-shutting cover. The with drawal of the burnt ore through the door A is facilitated by having the grate g which carries the ore, arranged to be in a sloping position. Small fragments fall through the grating into the ashpit, and are removed from time to time through the door i, which is furnished with ventilation holes. About 10 in. above the grating the front wall of the kiln is provided with a row of holes k, in which are boxes fitted with round iron bars 1. These can be used in breaking up the mass, and also serve to prevent the falling of the overlying mass while the spent ore is being withdrawn. The opening m provided with a door is useful for watching the process and for inserting an iron bar to stir up the mineral when that becomes necessary. The larger opening a (also shut with a door) can be used for the same purpose, and also for introducing the nitre pot into the channel 0, when the nitrogen compounds are derived in that manner. The gases next pass from the channel C into the wide channel D, which in some works is made 300 ft. long, and in which the dust as well as a great portion of the arsenic contained in the pyrites will settle. Auy re quired number of kilns may be put into com munication with this flue, and their collective gases thus be taken to the chambers. The gutter E lying below the sole of the furnace is for protecting the kiln from wet.