The soda-salts (chiefly carbonate of soda), thus obtained by evaporation of the lye, contain caustic soda, which requires to be carbonated, and a little sulphide of sodium, which it is necessary to get rid of. They are accordingly transferred to a reverberatory furnace, and calcined, at a moderate heat, along with sawdust, or sometimes with small coal, the mixture being stirred with iron paddles. By this treatment, the caustic soda is converted into carbonate of soda, the sulphur is mostly expelled, and we now obtain the or alkali of commerce, which generally contains about 50 per cent of real soda, NaO; the other ingredients, besides the carbonic acid with which it is combined, being chiefly water, sulphur, and common salt. Sometimes it is further purified, and it is then known as white alkali.
Soda crystals, or what is commonly called "washing-soda," are obtained by dissolv ing the soda-ash in hot water, then filtering the solution and boiling it till the specific gravity reaches 1.3, when it is transfered to the crystallizing coolers: Bars of wood or iron are laid across these vessels to sustain the mass of crystals which form, and in ten days at most the crystallization is complete. Crystals of soda are purer than soda-ash, but they are of much less value, weight for weight, because of the large quantity of water which enters into their constitution, amounting to 62f per cent.
The manufacture of caustic-soda is now an important branch of the alkali trade. For soap-making, bleaching, and several other purposes, carbonate of soda requires to be rendered caustic by quicklime. 3lanufacturers have, accordingly, taken to the plan of treating the black-ash liquor with hydrate of lime, and so obtain caustic-soda at this stage, instead of sending it into the market as a purified carbonate of soda, for purposes it requires to be decarbonated again. Another plan consists in mixiug a small
quantity of chloride of lime, or nitrate of soda, with the soda-lye from the blaek-ush. It is then concentrated into a strong solution, and finally brought to the state of a fused mass in round iron pots heated to redness. Great attention is now given to the recovery of sulphur from the enormous " waste heaps" which accumulate at all large alkali works, since there is fully 'a ton of this "waste" produced for every ton of common salt used. It consists largely of noxious sulphides of calcium, and is a source both of river and, in warm weather especially, of atmospheric pollution. Mond's process, or some modification of it, is usually adopted. Some chemical changes are first produced in the waste by the action of common air, and the sulphur afterward precipitated by hydrochloric acid.
Various processes have been at different times proposed, and many have been pat ented, for making carbonate of soda by other methods than that of Leblanc; but only one of these has had any measure of success. We allude to what is called the " ammo nia process," patented by Dyer & Hemming in 1838. Several improvements—some of them quite recent—have been made on time original method, nod time process is now worked on the large scale at Northwich in Cheshire, and at several places on the conti nent. It consists in a solution of common salt with bicarbonate of ammonia, by which bicarbonate of soda is produced, and the chloride of ammonium remains in solution.
Sulphate of soda is made at 'Widnes, in Lancashire, by Mr. Hargreaves's process of decomposing bricks of common salt with sulphurous acid and steam. This method, by avoiding the direct use of sulphuric acid, saves the expense of nitrate of soda; and there is also a saving in the wear and tear of apparatus, owing to the lower temperature required.