The carbonating process is one requiring great care and judgment. If the "hard" and "soft" salts are not properly mixed—the salts from the furnace end of the pan, and the end nearest the flue—the charge either fluxes before being thoroughly finished or cannot be " cleared " at all. When the salt has been fished it requires only drying, the beat being ke sufficiently low not to melt the charge.
The carbonate when drawn, now called "alkali " or " soda ash," is weighed and tipped up in the "alkali house." The quality may be judged pretty accurately by its appearance. It should come out of the carhonator red hot, showing the heat well when tipped out of the barrow. When cool, the ash should have:a bluish shade over the white, proceeding probably from some compound of sodium and manganese, and should not assume a dull, clayey appearance. Above all, it should not be grey. If the heat in the carbonating process has been set away too soon, the ash will assume a reddish " foxy " appearance, hut will sometimes, even then, grind a very fair colour. A usual amount of work for a carbonating furnace is 30 cwt. of finished ash per shift, divided into two charges taking about five hours each to work off.
Every hatch of soda ash should be carefully tested for carbonate and sulphide of sodium, and once a week or so, a complete analysis should be made of a mixed sample of all the week's work. The amount of sulphide of sodium should not exceed a trace--i. e. the merest brown coloration should be given when a few drops of acetate of lead are added to a solution of the ash. The sulphite should not exceed 0.10 per cent. ; the insoluble residue 1 per cent.; the chloride of sodium 1.25 per cent. ; the sodium sulphate 7 per cent. ; the sodium hydrate 1 per cent. ; except in the case of a caustic ash. The points chiefly to he attended to aro the amounts of sodium sulphide and sodium sulphate. The former ought at any rate not to exceed • 01 per cent. If it does, the salts have been imperfectly'worked, for however "dirty " they may be, it is perfectly possible to clear even the worst samples. If the sulphate of soda exceeds 7 or 8 per cent. the balls have been badly worked, the undecomposed sulphate coming through the process, or there has been an excess of sulphide of sodium in the tank liquors.
When the carbonating process is completed, a manufacturer can tell the result of the whole operation of soda making, not only as regards quality, but loss during the intricacies of the process A considerable amount of loss is inevitable. The plant leaks in various directions, however well it
may be cared for and looked after ; a certain amouut of soda goes away with the weak tank liquors, in the volatilization of sodium salts, in the formation of salts—e. g. sulphide and sulphate—which do not reckon as available soda, in incomplete work in the sulphate of soda process, &c. As a rule, the average production of carbonate from sulphate is not above 69 per cent.—perhaps hardly so much. In a carefully conducted works,•71 parts of ash of "natural strength "—i. e. 52 to 53 per cent.—should be obtained from 100 parts of sulphate. Theoretically, 75 parts should be obtained.
A very great improvement upon the ordinary carbonating furnace has recently been introduced by Mactear, of the St. Rollox Works, in the shape of a mechanical furnace. The apparatus is shown in Figs. 258,259. It consists of a revolving circular bed, about 20 ft. in diameter, made of boiler plate, and supported upon cast-iron arms which radiate from a hollow central casting. The bed also has an opening in it corresponding to this centre piece. The arms, and consequently the bed, revolve with bevelled wheels upon a rail or race laid upon any substantial foundation, and are driven by suitable engine, allowing of perfect regulation of speed. Through the centre of the foundation a roadway is loft, upon which travels a set of waggons to receive the finished carbonate. The arch of the furnace is supported from an angle iron carried by iron columns set in the founda tions. The keystone of the arch is a cast-iron ring, corresponding to the centre of the bed, and forming an opening through which rises and falls, as may be required, a closing piece held in its position by a groove at the bottom and iron hoop at the top. By the raising of this centre piece, the opening in the sole of the furnace is uncovered, and the carbonate discharged into the waggons. The pan itself is lined with bricks, and round the outside a lute is formed, into which dips a cast-iron flange depending from the arch. As this lute gets immediately filled with portions of the charge, all iogress of cold air is prevented.