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SODA.— Sulphate Roasters.

This arrangement entails more labour in discharginga furnace, as each batch has to be lifted twice ; butit prevents to a certain extent the nuisance of thegas arising from freshly drawn sulphate.

The work in the pan or furnace is as follows:—■Supposing the pan has just been heated up, or elsethe previous charge has just been “ shoved,” the saltbelonging to the new charge is shovelled in as quicklyas possible through the charging door; and whenabout half of it is in, the acid syphon is let down sothat the sulphuric acid begins to run in as well.The quantity of salt for one charge varies verymuch. In working with open driers it is almostuniversally the custom to work one batch each hour—these batches varying from 6 cwts. of salt up to 10cwts. With close roasters the charges are mostlylarger, beingfrom 12 to 15 or even 18 cwts. of salt; butthen only from 8 to 12 batches are made in twenty-four hours. In any case the quantity of sulphuricacid must be exactly measured, though the bulk ofacid for a certain quantity of salt will, of course, varyvery much according to the strength of the acid andto its temperature. As far as the strength is con-cerned, the stronger the acid is, the better, up to144° Tw.—a strength which is frequently reachedat the present time when Glover’s towers allow ofthe concentration of all the acid made in most alkaliworks. On the other hand, where there is no con-venience for concentrating the acid, it is sometimesused as low as 120° Tw.; but the weaker the acid,the more time and coal does it take to do the work,the more difficult is the condensation, owing to thelarge amount of steam, and the more quickly are thepans worn out. As, however, in no case does theacid run with exactly equal strength, its specificgravity must constantly be tested with the hydro-meter, and more or less run into the pan accordingto its indications.

If the acid comes hot enough from the towersor pans, it is not necessary to heat it in the gaugecistern; but if cold, the latter ought to be heatedso that the temperature of the acid may at leastreach 120° Fahr., otherwise it cools the pan (whichis already greatly cooled by the salt) far too much,and may even cause it to crack. The absolutequantity of the acid to be used depends very muchupon the quality of the sulphate desired; if strongsulphate has to be made, as is usual in open driers,' theproportion of acid of 144° Tw. to Cheshire salt ofaverage moisture will be about as 95 : 100. Whenthe salt is either drier or wetter than usual, a cor-responding allowance of acid must be made. Theresult of the testing of the fumaced sulphate of sodafurnishes, however, a daily control whether the pro-per quantity of sulphuric acid has been used or not.

When all the acid has been run in, and all thesalt is charged, the whole is well mixed by means ofa light iron rake. At this stage the mass is very liableto froth and even to boil over, which very soondestroys the pan arch; and it is therefore usual tothrow a few ounces of tallow or other grease intothe pan, in order to keep down the froth. The dooris then put on and made tight with salt, but the

rake is left in and occasionally worked about; thesalt serving as a kind of stuffing box for the gas.After about half an hour the salt will be dissolvedin the acid (for large batches it takes more time);the open coarsely-crystallized form of the Cheshire“common salt” greatly facilitates the dissolving,whilst the finely - crystallized “ butter salt ” ofCheshire, or finely-ground rock-salt, are much moredifficult to treat, because they do not allow the acidto penetrate very easily through their whole mass.Rock - salt must be somewhat coarsely ground.After the salt is all dissolved, the fire is urgedmore strongly, in order to “boil down” the batch,and this is continued until the mass becomes sostiff that the workman feels considerable resist-ance to the working of the rake in it. He thenslackens the fire, pulls up the damper, and removeshis charge into the furnace, which by that timemust have been emptied of its own charge. Theremoval is effected, in case of “shoving,” by meansof an iron rake whose head is tent in a slight curve,and follows the sweep of the pan; in case of “ cast-ing,” by means of a large, slightly hollowed shovelwith iron handle. The gangway is a little higherthan the furnace beds, which facilitates the pushingover. The pan is then at once charged as before.In one work a second charge is put in the pan beforethe first is removed, and only half is shoved over;this is said to save the pans greatly from breakage,as they never go empty.

In the furnace the batch is at once spread out inan even, thin layer, and exposed to the heat of thefire. During the operation it is several times turnedover by means of a “paddle,” similar to the ballfurnace paddles to be described afterwards; thelumps are crushed down, and the whole is nowand then raked through with a tooth-rake. Greatcare is requisite to mix up all parts of the batch,otherwise there will be portions of it very poor,that is, containing undecomposed sodium chloride;and others over strong, containing sodium bisul-phate. It is evident that this careful working isvery much easier in a reverberatory furnace, bothbecause it is lighted up by the fire, and because theheat is so much greater than in a muffle furnace,which is at first quite dark, and only towards theend of the operation becomes a little lighter as thegas passes away and the furnace gets hotter. Thusnot only does the operation in the blind furnace lastvery much longer (say, twice as long), but it doesnot on an average yield as thoroughly decomposedsulphate of soda as that in open driers. In the lat-ter the operation is only finished when the mass hasbecome red hot all through, and when very little gascomes off it. The salt cake must not be allowedto flux, since this would spoil the quality of thesulphate for ball making.

The yield of furnaced salt-cake from salt ofaverage moisture (say 5 per cent.) is generallyassumed at 112 parts for 100 parts of salt. Theoutput of each pan and furnace, with open driers,is rarely below 11 tons per twenty-four hours(counting in the nitre cake from the sulphuric acid