Besides merely evaporating the liquors, the action io the boiling-down pan is to convert a large quantity of the sodium hydrate into carbonate, and oxidize the sulphides to some extent. All the sulphur is carried through the process, none being volatilized as is sometimes stated. A certain amount of finely divided sulphate of soda is carried over from the furnace, by the current of gases, and settles in the liquors.
In France, the soda liquor is boiled down and calcined in the same furnace. This is a very primitive method, turning out a good carbonate, but expensive and only producing a very limited output.
Tu keep the salts from being contaminated by the products of combustion in the ball furnace and the carrying over of sulphate and insoluble matter, boiling down by bottom heat is very often resorted to. The various descriptions of pan and setting are shown in Figs. 252 to 257. The apparatus is usually termed a " boat " pan from its shape. It will be noticed that the pan is so set in brickwork that the fire only plays upon the sides about half-way up. Consequently the salt, as it crystallizes out, acoumulatcs at the bottom of the pan and is then "fished " out up the sloping sides, being protected by the solid brickwork from being burned. "Fished " salts yield a very fine car bonate, 52 to 57 per cent. ; the remainder, containing the caustic soda and certain other salts, forms a " caustio " ash, containing up to 10 per cent. of sodium hydrate, and on that account esteemed by paper makers and soap manufacturers, who, in any case, havo to causticize their lye.
An ingenious form of pan has been occasionally tried. It consists of two compartments, the ono heated and the other kept cool, connected by a large tube. Tho liquors are kept in conetant circu lation between the two compartments, crystallizing out in the cold one, and thy mother liquors being pumped back. It has also been proposed to fish salts of different value from tho boiling down pan at different stages of concentration, leaving the mother liquors to be finally worked up into a caustio ash. Upon the whole the method of boiling down by the waste heat passing over the surface of the liquors is the most economical, proper care in the subsequent finishing process rendering it perfectly easy to produce a satisfactory carbonate.
It now remains to carbouate, or finish, the black salts. Wheeled from the depot, or trainer, they are thrown into a reverberatory furnace, very closely resembling the sulphate of soda "roaster." The chief difference is that the bridge of the "carbonator" is carried higher so as to keep the flame from too immediate contact with the salt. Sometimes a single-bedded, sometimes a double-bedded carbonator is used, the latter undoubtedly doing the better work. In the case of a single-bedded furnace the charge of salt, weighing about 15 cwt., is thrown upon the bed and the fire kept well damped until the mass is thoroughly dried. As soon as this stage is reached, the damper is drawn up, the fire "cleared," and the charge carefully and almost continuously worked with a heavy paddle. If the fire is allowed to burn with a clear flame before the salt is dry, certain portions of it, especially in the neighbourhood of the bridge, will flux and be spoiled. The working of a batch of salt requires considerable skill. It must be sliced in thin portions and worked first towards the fire and then back again, so that every portion may be exposed to the heat, hut nooe long enough to be fluxed. Tho work accomplished in the carhonator is to oxidize the sulphide of sodium into eulphate and carbonate the caustic soda. The latter is brought about both by the presence of carbon dioxide in the furnace and by the carbonizing of the sawdust which was thrown into the salt in the pan. Chiefly the former ; the sawdust tends mainly to keep the salt open so that it is readily accessible to the influence of the heat of the carbonator. When the charge is cleared—whou all appearance of sulphur has vanished from the manipulation of the paddle, the fire is urged to a strong red heat and the door shut down for a few minutes. The charge is then raked out into iron harrows.
When a double-bedded carbonator is used, and it is by far the more preferable furnace of the two, a charge of black salt dries upon the bed furthest from the fire, while another is being worked upon the first bed. Considerable economy of fuel and an increased output are thereby secured. A double-bedded carbonator closely resembles a double-bedded roaster in every particular.