SODA.— White-ash.

829

can be either used for drying chalk, or for a preli-minary heating of tack liquor, or, of course, for anyother purpose.

The carbonating operation is carried on as fol-lows :—The black salt is first charged upon the backbed, where it is not disturbed, and where the mois-

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Fig. 28.

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ture is expelled to a great extent. The work onlybegins properly after it has been shifted to the work-ing bed. Each charge weighs about 16 cwts. afterfurnacing. The second bed is charged when thefirst charge is about half worked, and the damper isthen raised, because at that moment more fire isneeded, and the salt on the back bed impedes thedraught; but when the latter, after drawing thecharge from the front bed, has been shifted to it,the damper is let down again, and the first stage ofthe work, “ the drying,” begins. The heat at thisstage does not very much exceed the melting pointof lead; the fire is kept “ green,” that is, light andsmoky, and the stuff is turned over now and then.The moisture is seen to escape in copious fumes; ifthere were too much heat in this stage the salts wouldbe locally dried and fluxed, and would be found tobe full of sulphide and caustic. This stage takesabout two hours. When all steam is off and thestuff appears loose and pure a little more heat isapplied, and the stuff is turned over very assidu-ously, as this is the proper “carbonating” stage;that is, now the sulphide is partly oxidised to sul-phate, partly converted into carbonate, the saw-dust is burnt, and the resulting carbonic acid,along with that of the fire, converts the caustic sodainto carbonate. At this stage the furnace must befilled as much as possible with carbonic acid, and agood result can only be attained by great care andindustry on the part of the furnace-man. Gradu-ally the fire is increased, and the last stage is that of“ firing” the mass, when it has to be brought to agood red heat, constantly turning it over and com-pletely burning out the sawdust. This succeeds better

with sawdust from pine wood than with that fromhardwood. The furnace-man shifts the stuff por-tion by portion to the fire-bridge, and allows it toget just hot enough to burn out the sawdust, butremoves it promptly before it can even begin to flux,and shifts another portion into the same place; heopens his working door more or less, or lets it downaltogether for a while, just as the state of his furnacerequires it. The less caustic there is in the stuff themore it will “ stand the fire ; ” if it is too caustic itis “soft ” and liable to fluxing, and cannot be heatedas it ought, viz., ultimately with a clear fire, comingout red hot right through. The whole operationshould not be done in less than five hours, to do itproperly. It can hardly be described in words asit has to be done in practice, very careful manipula-tion being necessary throughout. The same blacksalt will, in the hands of a good workman, yield anash of 52 per cent, soda and only 1 per cent, caustic,which in the hands of another only yields 51 or even50 per cent., and 2 to 2| per cent, caustic. In anywell-carbonated ash there ought to be no more than1 per cent, sulphite, and no sulphide whatever, oronly the merest trace.

The action in the carbonating furnace has beenstudied by Moorhouse, who unfortunately did sowhen he had some abnormally sulphuretted salt todeal with, yielding abnormally caustic ash ; never-theless, his results are very interesting. The fur-nace was charged at 5’30 p.m., and samples weretaken from it at seven, eight, and nine o’clock, andultimately of the ash drawn out at 10T5 p.m. Theresults of the testings are embodied in the followingtable:—