Two advantages are at once apparent :—no nitrate of sodium is necessary to effect the conversion of the sulphurous into sulphuric acid, and an inferior descript;on of salt can be used. The form of apparatus employed is given in Figs. 240, 241, and 242. The salt is first prepared by being moistened with steam. It is then spread upon a suitable floor boated with underneath flues, dried, and broken up into small lumps, the fine being returned to the steaming-box. A better method of preparing these lumps bas lately been introduced. It consists in spreading the steamed salt upon a series of iron plates set olose together, and travelling upon endless chains over a flue inclined at an angle of 35° or so, Aa each plate, or flap, reaches the upper end it falls over, and the mass of naked salt falls upon a prepared floor, and shivers into fragments. In this way, comparatively little " arnalla " is made. When prepared, the salt is charged into a series of huge iron cylinders, about 12 ft. high and 15 ft. :in diameter—see Al A' A', &c., in Fig. 240. In these cylinders it lies upon a aeries of movable grids or bars, as H, Fig. 241. It is usual to have eight such cylinders, arranged in two rows, with a apace between for flues and working the charges. The cylinders are connected with huge iron arms or siphons D, working upon pulleys fixed in the roof, with a flue C, conveying the sulphurous acid, &c., from the pyrites burners, and with each other by the circu lating pipes E. The sulphate when finished is withdrawn by the door B, the grids H being knocked away from the tripods upon which they stand. The gas leaves the cylinders by an opening in the bottom of the drawing doorway, and passes into the flue G, from whence it is withdrawn by a Roots blower, or other suitable exhausting apparatus. Fig. 242 shows the brickwork foundation upon which the cylinders stand. The flues are covered over upon the top by a layer of ashes to keep in the beat, the burners being also constructed to prevent, as far as possible, loss of heat by radiation. The oylindera are heated, as may be required by fireplaces at the foot of each, the products of com bustion passing up the small flues which will be noticed at the side of the cylinders, Fig. 240. These flues are only just of sufficient size to prevent choking. The heat is also sometimes admitted into a space some 10 in. high, left between the cylinder covers and a second covering plate, passing down the opposite side of the cylinder, and finally issuing to the chimney. The cylinders are so arranged that each one can become in turn the first, the last, and an intermediate one. The gases are admitted first into the cylinder whose contents are most nearly converted. into sulphate, and pass from that through the intermediate cylinders, until they act upon the cylinder just filled with freshly prepared salt. When a cylinder is finished it is detached from the circle, emptied, recharged, and then becomes the last of the series.
Although this process is beautifully self-acting, it requires most careful construction and watching. The heat must be well got up before the gas from the burners is admitted, and never allowed to fall below 750°. The contraotion and expansion of the cylinders must be carefully guarded against, and the whole plant erected in as faultless and solid a manner as possible. Occasionally, the salt gets choked in the cylinder, and has to be entirely emptied out and worked.
over again at a great expense, or sold as half-finished sulphate. The management of draught by efficient aspirating apparatus is not so difficult a matter as might at first appear, and the sulphate produced when the process is carefully worked, is of most excellent quality, free from iron, and testing 98 per cent. of sulphate. The production and condensation of hydrochloric aoid is equal to, and as easily managed as, the product of the hand furnaces, and the amount of fuel used is only about 8 cwt. per ton of sulphate. Upon the score of plant expense, there is nothing to choose between the " Hargreaves " and the old process, but the sulphate from the former process can undoubtedly be made at a reduced cost of 5s. per ton-35s. against 40s.
The process of Messrs. Cammack and Walker, a revolving cylinder and continuous supply of salt and sulphuric acid, has already been noticed. Together with several other new devices, such as those of Measo (revolving pan), and Black, and Hill, it has not yet been placed upon a really working footing.
The second operation of the Lo Blano process consists of calcining the sulphate of sodium with chalk and small coal, producing the impure carbo nate of soda known as "black ash," or "ball soda." It has already been shown what constitutes a good sulphate — freedom from uncombined salt and acid, and a good canary colour. The presence of reddish lumps, while showing, indeed, that the sulphate has been well fired, indicates also the presence of a considerable amount of free salt. The salt cake is then, technically, " weak." Two varieties of carbonate of lime are used, chalk and limestone. The former is the material chiefly used in the Tyne district, for the hand ball furnaces ; the latter in Lancashire. The best chalk comes from the neighbourhood of Londou, Northfleet, Greenhithe, &c., and costs about 2s. 6d. per ton delivered to the works as block chalk. "Smalls" cost about Is. 6d. per ton. The small cost results from the custom for small coasting vessels to take it in as ballast upon their return journey. Usually containing some 12 to 15 per cent, of moisture, a portion of the chalk is dried in kilns aud mixed with the fresh, damp material, iu quantity just enough to make the whole run well in the mill. It is then crushed between fluted rollers, or edge stones, and is ready for the furnace. If used in lumps, or wet, tho sulphate in the furnace is fluxed and burned before the chalk is acted upon, and the " ball " spoiled. Moreover, it is necessary, when lumpy chalk has to be used, to put in a considerable excess, which iu the tanks gives rise to oaustic soda. Good limestone, Buxton, Irish, or Welsh, as used in Lancashire and other districts, has about the following composition :— In revolving ball furnaces the use of limestone is universal.
The selection of a good " mixing" coal, as it is called, is an important matter, and the quality must be kept as uniform as possible. It should leave when ignited as small a quantity of ash as possible, and must be free from slaty and siliceous matter. A bituminous gas coal mixes well. The ash should not exceed 5 per cent., or the sulphur 0'75 per cent.
These three materials, in the proportion of 3 cwt. of sulphate to 3/ or 3 cwt. of chalk aud about 1/ cwt. of small coal, are introduced into the ball furnace by means of a hopper, or thrown by hand upon the charging bed. The mixture varies with the judgment of the individual manufacturer or with the state and quality of the materials, but the above proportions represent the usual charge. The furnace is reverberatory ; elevation, section, and plan are given in Figs. 243, 244, and 245. The fire is placed at the end, and is about 4 ft. by 6 ft. The two bars that will be noticed below the grates afford leverage to the poker which is used to break up the scars or " clinkers." When these scars are removed and fall into the " cave " or firehole, they must be cooled with water to prevent damsge to the iron. It is usual to place a water-tap in the firehole for the purpose. Coal is introduced through the firehole at the end of the furnace, which is covered with a hanging door of cast iron lined with " half thicks." Between the fire and the first bed of the furnace a long narrow slit will be observed. This allows a current of cool air to pass continually under the bridge. Ono side of it is formed by what is called a " bridge plate "—a long cast-iron slab of peculiar construction, flanged at each end and bolted into the side plates of the furnace.