It will be noticed that the roaster is sometimes fired from the end and sometimes from the side. The former is the better plan of the two, as giving a more regular heat over the sole, and admitting of better manipulation. A thoroughly good armngement is shown in Fig. 233, two small fireplaces being arranged at the end of the furnace. Fig. 234 gives the end plates in section, with flange,d joint and binder " runners." A sectional elevation of pan, roaster, and condensing apparatus is given in Fig. 235. The connection between the furnace and its condenser should be of course much longer than is repre sented in the drawing, to allow the gases to cool before they enter the tower.
The weight of salt constituting a batch varies very much, depending chiefly upon the size and construction of the roaster. A good charge for a single-bedded furnace is 6i cwt. per hour ; when a large double-bedded roaster is used, the weight of salt may go to 10 or even 12 cwt. per hour. A batch of the latter size is however likely to be badly decomposed and worked. The manipulation of the pan is one of the most delicate operations in an alkali works, requiring not only great strength but judgment and long experience. The men at the furnace, on the other hand, can be readily trained to their work.
Good sulphate should come out of the furnace red hot, and present when cool a bright canary colour, with no shade of green upon it. The lumps when broken should show no centres of unde composed salt. Upon an average, about 112 parts of sulphate are obtained from 100 parts of salt, the rate of charging being usually one batch per hour. The work is continued, of course, day and night, the fires only being drawn at the week's end. The following is about the composition of an average English salt cake :— A careful manufacturer, however, will keep the amounts of both sodium chloride and free acid below those set down. The best hand-made salt cake should test 97 per cent. of sulphate of soda. and not more than 0.5 per cent. of salt and the same amount of free sulphuric acid. The batches are usually tested every three or four hours during the day, but only for free salt and acid. The sample is first tested for the acid by a standard alkaline solution, and then with nitrate of silver for chloride, after adding a little potassium chromate.
The two most noteworthy changes that have recently been made in the facture of sulphate of soda are the processes of Jones and Walsh, and Hargreaves. The former simply substitutes mechanical for hand labour, and completes the whole process of decomposing and " drying " in one furnace. Plans and sections
are given in Figs. 236, 237, 238, and 239, which will readily explain themselves. A large shallow pan, formed of cast-iron plates in sections, firmly bolted together, is set upon a suitable bed of brickwork or masonry. The fire passes over the pan, acting directly upon the contents, and a vertical shaft, fitted with four arms, with ploughs attached, and driven by overhead gear, keeps every portion of the charge constantly stirred up. The salt is first introduced, and then the acid run upon it, to avoid possible breakage. When the operation is finished, the contents are raked out, or occasionally discharged by mechanical contrivance, and a fresh batch is introduced. Owing to its size, a Jones pan only takes a charge about every six hours. Five tons of salt can be worked at once. There are undoubtedly many advantages ing this system. An enormous amount of work, compared with the hand furnace, is got through, and the sulphate is thoroughly well fired. The consumption of usually coke—is brought down to about 1 cwt. per hour, and the attendance of only one man, in place of the " pan-man " and two " driers " of the older system, is required. But as at present arranged there are serious drawbacks. Chiefly the amount of draught with which it is necessary to work renders the condensation of the hydrochloric acid difficult, and, owing to the effects of contraction and expansion of both metal and brickwork, the pan is difficult to keep tight and in good repair. Still, the Jones furnace is the best mecha nical contrivance that has yet appeared, and probably, with some modifications, is destined to supersede the old hand furnaces ; unless, indeed, some such new process as the " Hargreaves ' revolutionizes the whole operation of sulphate manufacture.
The process of Messrs. Hargreaves and Robinson aima at no mere modification of the preson plan, but proposes to produce aulphate of soda by the direct action of sulphurous acid, oxygen (from the air), and steam, upon ohloride of sodium. The idea is not altogether a new one, Longmaid having proposed, twenty-five years ago, to roast salt and pyrites in a reverberatory furnace, and obtain sulphate of sodium, oxide of iron, and chlorine, 4FeS, 16NaC1 + 190, = + 8C1,, but his process was never successfully carried out. The reaction in the Hargreavea process is a follows, requiring a temperature of from 370° to 480° (700° to 900 F.) ;— 2NaC1 + SO, 0 + = 2HC1.