The salt most highly esteemed for the process is that obtained from the Cheshire or Worcester shire brine. It is both pure and cheap, containing about 95 per cent. of ohloride, and very slight impurities except water—which averages about 4 per cent. The brine salt, moreover, is in fairly sized and regular crystals, a formation which renders it peculiarly adapted to the decomposing process. Other kinds of salt have been often tried, notably the German rock, which can be imported at about 2s. per ton less cost than the Cheshire article. The grinding of the rock salt, however, causes considerable admixture of flue material, which is apt to cake on the bottom of the pan, and cause it to become overheated at that particular spot, and crack. Moreover, there is always an appreciable amount of calcium sulphato in rock salt, which goes through into the sodium sulphate and deteriorates the quality. Too much stress cannot bo laid upon tho importance of caring for the " life ' of a decomposing pan, as the operation of replacing a broken one is both a great nuisance and a great expense. The sulphuric acid employed is ordinary " chamber " acid, preferably at about 133° Tw., and as hot as possible. Formerly it was necessary to heat and concentrate tho acid in what was called an " evaporating pan," or "concentrating pan," but thanks to the introduction of Glover's towers, a suffioient supply of strong and hot acid is always available without any special means of preparation.
The process of decomposition has been sufficiently set forth in the article upon hydrochloric acid. The salt and acid are mixed in the " pan," thoroughly stirred up, and boiled for about forty minutes. The mixture, of a pasty consistency, and composed partly of sulphate and partly of bi sulphate of soda, is then pushed over into the " drier " or "roaster," and subjected to further careful manipulation and furnacing, whereby the whole of the soda is obtained in the form of sulphate, and the remaining portions of hydrochloric acid gas driven off.
Details of apparatus in common use, and different from those already described under Hydro ohloric Acid, are given in Figs. 214 to 233. It may be premised that the original sulphate furnace was built entirely of brickwork, and consisted of only one bed. When it was found that no brickwork would resist the wear and tear of the process and the ravages of acid substances, a lead lining was introduced, and this was employed until about thirty years ago, when metal pans and separate furnaces were introduced. In all probability, the pan now in common use will have, in its turn, to give way to some such mechanical contrivance as that of Messrs. Jones and Walsh. In Figs. 214 to 218 are shown five different pan settings. In Fig. 214 the edge of the pan is flanged and brought outside the brickwork arch, that any boiling over may be at once seen and rendered harmless. This method of setting originated with the alkali inspectors, who found considerable escapes of gas from the irruption of the contents of the pan into the underneath flues. It is also safer for the pan; but the small brick arch rendered necessary is difficult to keep in repair, and the necessity for its removal makes the replacing of the pan needlessly expensive and tedious. Figs. 215 and 216 show two methods of seating the pan in the brickwork. Figs. 217 and 218 show two forms of irun
adapted to the exigencies of pans set with the rim visible, and intended to obviate.he evils of this style of setting, already pointed out. The expense entailed by the wear and tear of these iron covers, how ever, need not be enlarged upon. Fig. 219 gives the elevation of a single-bedded furnace, an old style of finishing apparatus, but capable of turning out thoroughly good sulphate, if the batch be not too large. In Figs. 220, 221, and 222, the elevation, longitudinal section, and plan of an improved single-bedded arrangement are given. The pan has a furnace upon each side of it, the charge being pushed alternately into each. In this way, every batch of sulphate has about two hours allotted to its finishing, and with a moderate expenditure of fuel can be " brought down " exceedingly fine, en operation that evory manufacturer and intelligent consumer of sulphate knows the import anco of. In Figs. 223 to 226 are shown different methods of setting a pan with two roasters. In Figs. 223 and 224, the rim is bedded solid in the brickwork; in Figs. 225 and 226, it is brought in sight all round.
The furnaces sot forth hitherto have been all "open"; i.e., tbo flame acts directly upon the etch, an. .e pro. nets o com•ustion go with the ly. roc one acid gas into the condenser. To lessen this evil, coke may be used instead of coal, but is of course more expensive. In Figs. 227 and 228, are given the sectional elevation and plan of the " close " roaster commonly used in Lancashire and most other centres of the trade, except the Tyne. It will he noted that here the heat and products of combustion pass over and under the furnace, never coming into actual contact with the materials. Two great evils attend this construction of furnace :—(1), the greater draught being in the smoke flue, there is constant liability to lose gas from leakage ; (2), the sulphate, as a rule, is only imperfectly fired. From the first of these evils arises not only loss of gas, but damage to the surrounding vegetation, &c. To obviate this tendency to leakage, the late Mr. Deacon proposed the plan set forth in Figs. 229 to 232. The fireplace is built, as will be noticed, several feet below the sole of the furnace, and from this difference of level there is always a heated column of air and gases over the fire-bars ; in fact, a chimney is interposed between the flues around the furnace and the fire. By this means the necessity for a great draught is avoided, the gases and products of combustion being actually checked while passing round the furnace, and, if escaping at all, finding their way into the interior of the muffle. At any rate all tendency on the part of the hydrochloric acid gas to leak into the flues is effectually prevented. A bye-flue leads the products of combustion direct to the chimney, instead of underneath the pan, whenever required, when the latter is empty or too hot. The draught of the chimney should be carefully regulated by a small damper, but the connections should be of large aize, and the final stalk should have as regular a dranght as possiblo. This regulation of draught, so as to give sufficient power over the materials in the furnace, and turn out thoroughly ftred sulphate, is the diffioulty met with in working an exceedingly ingenious furnace.