ash making only a 52.5 per cont. refined alkali. Some times the " white salt" in the drainer is washed with water to obtain a purer carbonate, but such a process is entirely superfluous. The mud from the settlers and dissolver is washed with hot water, and the washings aro ran into the dissolver or the boiling-down pan.
More usually, however, the whole of the residue is put upon the balls in the lixiviating tanks.
Refined alkali is ground and packed in manner pre cisely resembling soda ash. Formerly, nearly the whole of the alkali of commerce consisted of this refined article, but its use has given way to a great extent to that of a better prepared soda ash. The cost of refining is about 25s. per ton, the product being of a beautifully white colour, and containing from 48 to 53 per cent, of carbonate, depending upon the strength of the ash from which it is made.
Soda crystals.—This well-known product of the alkali trade is a crystalline compound of carbonate of soda with ten atoms of For most household and cleansing purposes, also in bleaching, tanning, and dyeing, soda crystals still hold their own, though probably, as purer forms of carbonate, such, for instance, as the product of the ammonia process, to which further reference will be made, are introduced, the use of crystals will be to a great extent superseded. A manifest disadvantage is the bulk of water of crystallization contained. Roughly speaking, 1 ton of soda ash makes 2 tone of crystals, so that all the expenses of freight, carriage, and manipulation are doubled. The greatest part of the trade in crystals is done with foreign countries where heavy import duties upon soda ash give the manufactured product the advantage. Also for all household purposes, crystals are esteemed, as being cleaner and more readily handled.
The manufacture of crystals is as follows :—Good, thoroughly carbonated soda ash, of as high strength as possible, is thrown, in the rough state, into a dissolver and treated in a manner similar to that set forth when speaking of refined alkali. The ash should be free from caustic soda because the presence of this salt spoils the appearance of the crystals, and increases the volume of mother liquor. If the ash contain any appreciable amount of sulphide of sodium, it must be cleared by introducing a small quantity of bleaching powder into the dissolver. The solution, having a specific gravity of about 1.225, is run from the dissolver into a series of settlers—large wrought-iron tanks about 10 ft. square by 7 ft. deep, well stayed with cross rods of iron. After thoroughly settling, the deer liquors are run off by any convenient siphon arrangement into a set of coolers or "cones," where they remain for a fortnight to three weeks, depending upon the state of the weather, until a pretty nearly solid mnss of crystals forms. These crystallizing cones are of various shapes
and description. The best form, upon the whole, is shown in Figs. 262, 263, and consists of a circular east-iron pan, 2 ft. deep and about 9 ft. in diameter, by 1 in. thick. A hole in the centre serves to remove the mother liquor when the plug shown in Fig. 263 is withdrawn. The crystals form in the manner set forth. That which sets round the sides of the cone is called " block," and in cool weather is a solid mass 9 in. or so in thickness. Finer, larger crystals form in the centre and upon the surface, called "points," and are more esteemed. To assist these points to form, it is usual to place strips of iron or wood across the surface of the liquor. Crystals of a very large size may he " grown " by removing a few points and suspending them in fresh liquor, repeating the operation almost to an indefinite extent. When it is judged that all the available soda has crystal lized out, the plug is withdrawn from the centre of the cone, the mother liquors are drained away, along a spout placed beneath, into a well or any convenient receptacle, and the crystals removed to the drainer. This is usually a long sloping bench or hopper, set with its top on a level with the bottom of the cones. At the bottom of the slope, are a series of small doors, and when these are raised, the crystals, after they have drained, are run into casks placed beneath, and packed ready for sale. The manufacture of crystals requires considerable experience and great care. The liquors must be allowed to settle thoroughly, the cones must be kept well filled, so as not to rust, and they must be emptied before the crystals "salt," or become opaque. The roof over the cones must be kept in perfect repair to prevent any disturbances of the liquor while crystallizing. Finally, the whole plant must be arranged so that a free current of cool air plays upon all sides of the cones. Neglect of this last-named precaution is a potent source both of bad crystals and bad yields. The mother liquors are pumped into a boiling-down pan, and treated exactly like refined alkali. The product—" weak alkali," as it is called—varies in strength with the amount of mother liquor. In hot weather, when less soda crystallizes out, the weak alkali will test up to 41 per cent. of carbonate. In winter, when the "crops" of crystals are large, and the mother liquors proportionately weak, the strength of the weak alkali will not run beyond 36 or 37 per cent. This product is highly esteemed by glass manu facturers, as they only pay for a small amount of carbonate of soda, and get a large amount of sul phate of soda—which of course remains in the mother liquor, and lowers the test—given to them. The mud from the settlers is washed or put upon the balls in the lixiviating tanks.