256

AUSEXIC.— Aiisenious Acid.

flame and air that pass through. In each turn itcomes a little lower down the tube, and ultimatelyreaches the lower end, whence it falls into achamber, c, in a calcined state, the sulphurand arsenic having been driven off to the fluechambers.

In this calciner as much as 6 or 7 tons of ore canbe treated in twenty-four hours. For such a size aswe have spoken of the cylinder is not uncommonlymade of an old tube of a Cornish boiler; larger ones,however, have been tried, and have done well. Theboiler-shell itself may be used for the purpose, thediameter of which would be 5 feet C inches or so;in this case the fire-brick lining could be 9 inchesthick, so as to leave 4 feet clear inside. It lias beenfound that it is not necessary (at all events with thelarge ones) to give any inclination to the cylinder;the slope which the ore acquires on being fed fromthe top is sufficient to carry it down the length ofthe tube in its revolutions.

(d). Calcining in Kilns. —For certain kinds of orethis is probably the cheapest method of any. If theore can be mixed so as to contain something like 20

per cent, ofl'is 4 - pyrites, the

only expendi-ture of fuelwill be at firstlighting. Thekilns are of thesimplest con-struction. Fig.4 shows threeside by side,of which themiddle one isseen in section.The diameteris 3 feet, the height 10 feet. The top is coveredwith an iron plate, which is removed when theore is to be introduced, while the flues (one ofwhich is represented by dotted lines) lead awayfrom just beneath it and then unite. At firststarting the kiln is filled nearly to the topwith rubbish; above is put ore with a littlefuel; as this gets hot and begins to burn ofitself, some of the rubbish is raked out frombelow, while more ore (without fuel) is addedabove. In the course of time all the rubbishwill have been got rid of, and the kiln will be filledwith a burning mass of ore. Some is then rakedfrom the bottom five or six times in twenty-fourhours, and a corresponding amount of ore is put inat top; in fliis way from 1 ton to 1 \ ton is done perday per kiln, the work being continuously kept up.There will be observed in the kilns three holes(closed up by bricks) at different levels; these areto give access to the ore in case poking should berequired to make it descend. With the poorer oresa proportion of fuel may be mixed to produce thenecessary heat.

For this method of roasting the ore may be crushedso as to pass through lf-inch meshes. We would

point out that, for ores contaiiring tin and a properproportion of pyrites, it might bo advisable to roastthem thus and afterwards send them to the stamps,for which they would have been well prepared bythe calcination.

(<;). Roasting in a Muffle Furnace. —This is prac-tised at Altenberg and at Keichenstein in Silesia.Figs. 5 and 6 are explanatory of the furnace; thefirst shows a longitudinal and the second a transversesection; the same characters in each indicate thesame parts. The muffle of the furnace, where theore is deposited for roasting, is indicated by a ; b isa door in the front of the muffle, through which theworkman introduces his slicer to work the chargeand draw out the exhausted material; c c c are theflues round the muffle, where the smoke and flamefrom the fire are carried forward to the main flue, e,leading to the chimney. From the muffle two chan-nels, //, lead to the first chamber, B, appropriated

Fig- 5.

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for the condensation of the acid, the further courseof which will be spoken of below. For this processthe ore is reduced to a moderate size by crushing,in which state it is called schlicch. About 10 cwts. ofthe schliech forms the charge, which is introducedthrough the opening, d, and spread upon the inclinedbed of the furnace to thedepth of from 2 to 4 F ‘g e -

inches; this is raised toa gentle red heat, whichis afterwards slightlylowered, and then main-tained equably, the orebeing at the same timefrequently stirred. Thedoor, b, is kept open,that the air entering may

oxidize the sulphur and arsenic. In about twelvehours this oxidation is effected; the exhausted oreis then raked out and another charge introduced asbefore.

The Flues or Chambers. —By whichever of theseprocesses (except it be the last mentioned) theore may be roasted, the arsenious acid that maybe volatilized is intercepted in chambers con-structed in the course of the flue, through whichall the products of combustion pass betweenthe furnace and the stack. The best arrange-ment of flue-chambers is that shown in the accom-panying diagrams (Figs. 7 and 8). There is along line of chambers so arranged that the smokeshall zig-zag through them ; each chamber is 3 feetwide, about 15 feet long, and 6 feet 6 inches highto the top of the arch. To every two chambers is alow- doorway from outside, which is closed by an

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