the solubility of sodium carbonate (and sulphate) inwater does not increase regularly with the tempera-ture, but is highest at 38° C., so that no crystalliza-tion can take place whilst the boiling hot solutioncools down to this temperature. But it is alwaysbest to surround the settlers with a bad conductorfor heat (bricks, wood, &c.) The completely settledand clear solution is now evaporated in an open pan,covered by an arch, by surface heat; the constructionof the pan itself and of its drainer is exactly likethat of the common open black-salt pan as shown onthe Plates, except that the white-salt pans are generallyonger (up to 30 feet), and provided with a fire-placeat each end, the flame escaping through a flue builtin the arch in the centre of the length of the pan.The reason for this is to avoid the formation ofsmoke as much as possible, which, experience shows,cannot be avoided with a single fire at one end evenwhen only coke is used as fuel, as is generally the case.The liquor is boiled down to a paste, the pansemptied into the drainer, and the drained white saltfurnaced in a furnace exactly like a carbonatingfurnace, but of course with much greater ease thanin the ordinary carbonating operation. It is notusual (as stated in text-books) to employ the wasteheat of the finishing furnace (white ash furnace) forevaporating the liquor; it is nothing like sufficientfor this purpose, and separate fires must be pro-vided, which moreover allows the use of coals inthe furnace, whilst coke is used for boiling down.The refining operation causes a loss of about 5 percent, of the weight of soda ash originally employed.The refined alkali is ground and packed exactly likeordinary ash. The mud remaining in the settlers iswashed with hot water; the washings are used fordissolving fresh soda ash, and the residue is thrownaway. This operation is rather troublesome, as themud is of a slimy character and difficult to wash ; itis, therefore, sometimes put back among the ballsinto the lixiviating tanks. In large works it mightpuy to put it on a bed of porous bricks, and exhaustthe air underneath to quicken the filtration, similarto the operation in caustic soda making.
Soda Crystals. —For some purposes even purersodium carbonate is required than furnished by therefined alkali of trade, and formerly (before theperfection of fishing and the introduction of theammonia process) the only available method forobtaining this on a large scale was by the manufac-ture of crystallized soda. This compound (Na 2 C0 3+ 10II 2 O) contains 63 per cent, water, all of whichhas to pay the same freight as the 37 per cent,sodium carbonate, and indeed much more, inasmuchas soda crystals are much bulkier than soda ash,and the freight for them is consequently higher,weight for weight, than that of soda ash. It wouldtherefore seem to be quite out of the question tomanufacture large quantities of this article, as itsgreater purity in comparison to that of soda ash orrefined alkali is only very rarely a sufficient con-sideration for the extra cost and freight. And yet,for all that, soda crystals are manufactured on acolossal scale both in this country and abroad; in-
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deed, many factories abroad do nothing but buyEnglish soda ash and make it into crystals. The ex-planation of this apparently strange fact is this—that the large proportion of water which is adrawback for most commercial applications, is ofgreat value for the domestic use of soda in washingand other household operations. The large bulk ofthe enormous quantities of crystals manufactured isultimately used for such purposes. The crystals aremore easily divided in small portions without soilingcupboards, &c., and they do not act so strongly uponthe hands of the washerwomen and cooks who usethem as the ordinary calcined alkali would do.
The manufacture of soda crystals seems easy; butit is nevertheless an operation requiring some care,as it is out of the question to crystallize more thanonce, and the crystals must be both of good colourand shape from the first. To begin with, the sodaash used for this purpose must be as well carbonatedas possible. It does not matter in the least howyellow (from ferric oxide) it is, so long as it is freefrom sulphides and moderately so from caustic.Whilst bleachers, papermakers, soapmakers, &c.,rather like some percentage of caustic in the ash,crystalmakers insist upon getting no more than 1 to1J per cent, of caustic in their ash. Any largerproportion both spoils the colour of the crystals (byits action on ulmates and other organic com-pounds) and causes a loss in crystallizing, sincecaustic soda, of course, does not crystallize, butremains in the mother liquors. The operations ofdissolving and settling are exactly like those inmaking refined alkali, which have been describedabove. The solution, after having been bleached(if necessary) and completely settled, is run intocoolers, generally made of cast iron, and known inthe factories as “cones” (from their original, shape),and allowed to stand for a week or ten days, accord-ing to the season, in order to crystallize. The conesare very frequently made in the shape of circularpans, about 9 feet in diameter and 3 feet deep ; buta more convenient shape is shown in the woodcut,viz., a rectangular pan with rounded corners (Fig. 31),
Fig. 31.
with the bottom sloping to one side, and with a plug-hole and plug at this low end. Whilst in the ordi-nary circular cones the mother liquor has to bepartly siphoned and partly baled off, it runs awayby itself from the cones just described after the plughas been loosened, and the crystals drain more105