It has been complained of this kiln that a great amount of dust is produced in it, which is continually choking the flues, and that the bars soon wear out. From the latter circumstance it is necessary to fire the furnace three weeks before charging, when starting the kiln, so that the temperature may rise gradually to a white heat. Probably these difficulties account for the fact that when using pyrites simply for making sulphuric acid, manufacturers prefer Olivier and Perrot's kiln, which we now proceed to describe.
Fig. 74 shows a vertical longitudinal section, and Fig. 75 a vertical cross-section of this kiln, as arranged four in a group, thus forming an oblong quadrangle. The lump ore is burnt below in the space A, while the dust is roasted above ou the seven fire-clay plates, about 4 in. thick, by which the kiln is divided. The uuder part A is furnished with a fireplace b, which is made of four-sided bars that can be turned round, and through which the roasted ore falls into the ashpit C below, as already described. From time to time the ashes are drawn out at the opening c, furnished with a tightly shutting iron door. In this door, or better nearer under the fireplace, holes are made iu the wall, through which the necessary air finds its way into the kilo, and which may be shut as required by clay plugs. The openings d, provided with iron doors, are used when charging the kiln. The dust is spread on the clay slabs,a about 2 in. thick, and is burnt by the heated gases arising from the combustion of the lump ore in the fireplace of A. These gases ascend through the spaces a left in the walls, pass over the surface of the clay plates as shown by the arrows in Fig. 75, and then through the flue f to the chambers. The little holes stopped with clay plugs, allow the roasting of the dust on the slabs to be watched. The slabs are charged through the openings g, which can be closed with iron doors, and after perfect roasting the dust is raked into the channel D which lies at the end of the slabs, and extends througliout the whole length of the kiln. This channel D must remain filled with spent ore during the process. When a new charge is to be inserted ou the slabs, the channel D is first emptied through the doorway h, this is then shut, and the spent ore from the lowest slab is raked into the channel D, whose dimensions are so regulated that it then becomes filled exactly up to the level of the slab. The lowest Blab is now supplied with fresh ore, the corresponding door g is re-shnt, and the spent dust from the second slab is raked into the channel D, which ie thus filled just up to the level of the second slab. This is continued till all the slabs are supplied. In this kiln ores may be burnt in
the proportion of thirty-five parts lump ore and sixty-five parts dust.
As the kiln gases have such a high temperature as to require cooling before entering the chambers, the heat thus evolved is sometimes utilized for concentrating acid in leaden pans as seen in the figures, or for heating the boiler which supplk s steam to the chambers.
Regtilat,ivi the Openttion.—As the percentage of sulphur contained in tho material used has very much to do with the conduct of the operation and fern's the basis of all calculations, its varying proportions must be ascertained. This may be done in the following manner: Polouze mixes 1 grm. of powdered ore with 5 grin. salt, 7 grm. chlorate of potash, and 5 grin. anhydrous carbonate of soda, fuses the mixture, and keeps it red hot for ten minutes. The sulphur will thus be reduced by the chlorate of potash to sulphuric acid, which displaces a certain part of the earbonie aeid and unites with the sodium from which the carbonie acid has been evolved. The fused mass is dissolved in boiling water, and the proportion of carbonate of soda remaining is alkalimetrically measured. From the difference between the amount of carbonate of soda existing before the fusion and that formed afterwards, a simple calculation gives the equivalent of sulphur which was in the ore. When the pyrites contains quartz or silica, the solution in boiling water is the more necessary, because in this case the alkaline silicates are not fully dissolved in cold water, and will also interfere with the alkalimetric estimation. This process, moreover, can not be used for pyrites containing sulphates of alkaline earths, because these will be decomposed by the carbonate of soda.
Another test consists in mixing 1 grm. of the finely powdered pyrites with 3 grm. anhydrous carbonate of soda and the same quantity of saltpetre. This is fused together in a muffle furnace at a red heat, dissolved in het water and the solution filtered into a beaker. The acid liquid is then boiled for a short time, and the sulphuric acid estimated, whence the equivalent of sulphur can be ealeulated. This estimation is performed by a solution of chloride of barium, so regulated that 1 co. of this is equal to 2 per eent. of sulphur.