SODA.—Sulphate of. Mechanical Roaster.
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works, which is usually put into the pan along withthe salt), anil rises up to 14 tons. From blindfurnaces it rarely exceeds 8 or 9 tons.
The decomposition of the salt takes place mostlyin the pan. About two-thirds of the hydrochloricacid is evolved in the pan, and that principallyduring the first stage (the “dissolving”),and only one-third or thereabouts inthe furnace. In the pan a great deal ofsodium bisulphate (hydro-sodicsulphate,
NatIS0 4 ) is formed, which in thenaco reacts on the unde-composed sodium chlo-ride, thus—-
+ HC1.
The decomposition isnever quite complete;even weak sulphate con-tains a little bisulphate (com-monly styled “freeacid”), alongwith undecomposcd sodiumchloride, and in strong sulphatethe proportion of it may amountto 1 or 1-1 per cent, of S0 8 inthe state of FfaIIS0 4 on thesulphate. Salt-cake containingmore than this is consideredfaulty. Strong sulphate oughtnot to contain more than £ tof per cent, of sodium chloride.
Jones' mechanical furnace isshown in Figs. 6, 7, and 8. Fig.
C is a section on line A-B Fig. 7;
Fig. 7, a plan on line C-D Fig. 6;and Fig. 8, section on line e-fFigs. 6 and 7. This furnacecombines the w r ork hithertodone in the decomposing panand the drying or roasting fur-nace, and works with a largosaving of labour and fuel. Itconsists essentially of a circularflat pan, 14 to 16 feet in diameter, heated by a cokefire from above. In this pan a vertical shaft is setin motion by overhead gear, from which four armsproceed horizontally; and to the latter a kind ofknives or ploughs are attached, at varying distancesfrom the centre, so that during the revolution ofthe shaft every portion of the contents of the panis kept stirred up. The action is continued tillpractically all the hydrochloric acid is driven off,and the contents of the pan are then discharged inthe state of excellent finished sulphate of soda. Agreat advantage of this process is this—that if thecharge be either too strong or too weak, it can bemade quite right at any moment by adding moreacid or more salt, as the case may be. In practice,generally an excess of acid is run in at first; andwhen it has been thoroughly mixed, it is determinedby analysis how much salt is required to make thesulphate just right. At present only three chargesVOL. II.
are worked in twenty-four hours, of about 4 tonsof salt each for a 14-feet pan; but it is intended to
Fig. 6.
0 FEE
Fig. 8.
fill and discharge the pan automatically in future,which will make it possible to work a charge everysix hours, of 5 tons for a 16-feet pan, or 20 tons100