The commercial product nearly always contains a certain percentage of sesquicarbonate. It is used largely in the manufacture of baking powders, seidlitz powders, and effervescing drinks ; also to some small extent in dyeing and printing as a mild alkali, where a more energetic carbonate might be hurtful, The total manufacture of the country reaches 20,000 tons per annum. At the present time, belling and cost price are about Si. 15s. per ton. Roughly speaking, one ton of " bicarb." requires two tons of soda crystals.
Hydrate of Sodium. Caustic Soda. NaHO.—This substance is a white, opaque, hard solid, possessing a fibrous texture. Its specific gravity is 2.00. It is fusible below a red heat, and is less volatile than the corresponding potassium hydrate. It is very soluble in water, attracting moisture from the air. TJpon drying, carbon dioxide is absorbed, and the hydrate becomes converted into the carbonate.
The saturated solution has a specific gravity of 1.5 at ordinary temperatures. At 18°, 100 parts of water, dissolve 60.53 parts of hydrate ; at 70°, 116.75 parts; at 80°, 127.02 parts. The following table (Tiinnermaun) gives the amount of soda (Na,0) in solutions of varying densities :— The solution dissolves hair, wool, and most animal substances, also sulphur and the metallic sulphides, silica and alumina. It acts as a strong caustic, and is a powerful alkali.
It has already been stated that caustic soda is produced when water is added to the monoxide of sodium. Upon a large scale, however, it is manufactured by depriving the carbonate of its carbonic acid by the action of hydrate of calcium.
The credit of first preparing caustic soda upon a, large scale is probably due to Weisenfeldt, who introduced at the St. Rollox Works, Glasgow, in 1844, a process of fusing the red liquors from the black salts with nitre. The caustic produced in this way was of good quality and white colour. Since that time the manufacture has been gradually improved, the most notable alterations being those of Gossage, in 1853, who proposed to utilize the tank liquors, fishing out the carbonate and other sodium salts during evaporation, and preparing caustic soda from the mother liquors; Stott, in 1855, who patented a process for removing the sulphides from tank liquor by means of oxide of iron, zinc, or manganese; Bakewell, in 1857, who first adopted the sheet-iron drum now in ordinary use ; Thomas, in 1858, who patented the use of calcium hydrate and the oxidation of the sulphides by air ; Ralston, in 1860, who first produced what is usually termed " white" caustic, by continuing the evaporation of the caustic liquors, until the iron separated out as oxide and precipitated to the bottom of the vessel, leaving a clear supernatant caustic solution. This introduction of a pure
70 per cent. white caustic was followed up by the publication, in 1872, of Pauli's process now in general use. He proposed to fuse the ordinary " cre,am " caustic, and keep up the heat until the oxide of iron and silicate of alumina separate out.
Soda liquor from three sources is employed in the production of caustic soda : (1) from the finished soda ash by dissolving it in hot water; (2) direct from the lixiviation of the balls; (3) from the red liquors. The first of these processes is a very roundabout and expensive plan ; the dissolving of the ash L9 conducted in a similar manner to that described when treating of refined alkali, and the solution then simply takes the place of a pure tank liquor. Red liquors are chiefly employed in the production of cream caustic, on account of the impurities contained in them. The second method, the eausticizing of tank liquors, is the most important process for the production of caustic soda, and to it the attention of the reader is directed.
The ball mixture is first adjusted to the process. A large excess of limestone or chalk is added, and the lime mud frorn the causticizer is usually worked up in the ball furnace. So the mixture may assume either of the following proportions :— The admixture of lime mud of course varies, and with it tbe amount of limestone or chalk.
The tank liquors after settling are pumped into the " causticizer." If white caustic is to be made this settling part of the operation must be thorough. The causticizers are extremely various in sizes and shapes. Often old boilers, cut in half crosswise, are used. The best apparatus, in which the liquors are both causticized and oxidized, and at the same time thoroughly agitated, is shown in Figs. 271 and 272. But little explanation is necessary. The air oxidizes the sulphides and performs the necessary agitation of the contents of the vessel, and steam helps in the agitation and heats the liquors. Steam and air are admitted below a perforated false bottom, the plan of which is given in Fig. 272. Sometimes a previous oxidation by a special blower is resorted to before the liquors are introduced into the oauaticizer, and mechanical agitation, by an engine fixed to the aide of the vessel, adopted. A sludge valve serves to run off the residue, or "lime mud," and the clear caustic) liquors are decanted by any convenient form of siphon.