The last innovation in the salt manufacture here to be described is that of Otto Pohl, salt manu facturer and merchant, of Liverpool. This invention has, perhaps, not met with all the attention it deserves on the part of the salt manufacturers. The arrangement consists of two superimposed pans, nt one cud of which the fires arc placed ; the heated gases, passing between them to the chimney nt the other end, heat the upper pan from below in the ordinary way, while they sweep the surface of the brine in the lower pan, which thus constitutes the bed of this portion of the flue. Figs. 1219,1220,1221, 1222,1223, and 1224 show this arrangement in ground plan, long,i tudinal and transverse sections, and in side and end elevations. Milner, of Marston, near Northwieh, has a pan mounted on this same principle, which Pohl states to be an adaptation of the principle of the salting-down pans of the alkali-makers. His arrangement, however. , differs from that of Pohl in that the upper pan is diapensed with, being replaced by an arch of brickw.ork. Acoording to Pohl'a system of construction, the lower pan is 5 ft. deep. It may be made of boiler plate or of cast-iron, or, for that matter, the bottom and lower parts of ita sidea might very well be made of elm or pitch-pine, with caat iron ende and framing. Pohl tried brickwork for the conatruction of this lower pan, but abandoned it on account of leakage. In the pan figured, however, he has formed the bottom of tilea embedded in clay. Pillara of cast-iron rising from the bottom of this lower pan support the upper pan, which is of the ordi nary make, and demands no special descrip tion. The interval between the two need not, according to Pohl, be more than § in. In practice, however, 5-6 in. is not too much from the bottom of the upper pan to the surface of the brine in the lower one when con3pletely filled. The length of these pane is about 60 ft.; breadth of the upper one, about 20 ft., and of the lower one, 22 ft., the space between the two being filled all around with brickwork. Milner has made the lower pan in his arrange ment much wider than thia, or rather it may be said a lip or opening running all along each aide of the lower pan permits of the aalt as it collects being drawn to the sides by rakea, and lifted out by perforated scoops as it accumulates. According to Pohl'a armnge meat, thia might easily be managed by con tinuing the sidea of hie upper pan downwards for Bay 8-9 in., the pan being placed at such a height above the lower pan that these sides may dip 2-3 in. below the surface of the brine in the lower pan, and thus conatitute a flue f 4-5 in. deep, through which the furnace gases might peas. The lower pan might then be made eay 3 ft. wider than the upper one, ao as to leave a trough on each side about 18 in. wide, through which the aalt might be drawn. As it is, when the pan has to be drawn, which, of course, must be done as aoon aa it becomes full of Balt, the fires have to be let out, the brine run off, and the salt drawn by the door or manhole, k, Fig. 1223.
The furnacea in Otto Pohl's arrangement are four in number ; they are made about 4 ft. wide internally, and 41 ft. or even up to 61 ft. between the top of the arch and the grate bars ; a distance of 3 ft. or so is also left at the back between the end of the grate and the lower pan, the angle being filled up with a, curve of masonry as shown at e, Fig. 1219. Thia form of construction is intended to allow space for more perfect combustion, before the heated gases enter between the pans, where they tend to become rapidly cooled, with proportionate liability to depoait aoot. Fig. 1224 shows
the front elevation and the arrangement of the eliding doom b. Pohl at first carried his upper pan right over the firea. He now stops abort behind them, covering them in with arches of masaive brickwork, so as to avoid ars far aa may be loss of heat by conduction in this quarter. He also pro posed to make a sort of short circuitous flue, through which the products of combustion might be made to pass on their road to the space between the pans, by building three arches over the fires, constructed so as to reach alternately to the back aud to the front of the fire-place, like the shelves of pyrites-dust kilns (sce p. 84). These arches becoming strongly heated would aid in promoting the combustion of the smoke, while they served to catch the dust and aehea carried over from the fires. This plan, however, he appears to have abandoned. A further proviaion was made for getting rid of soot by keeping the lower pan always filled to the brim, making the end of it furthest removed from the firea a trifle lower than the fire end and Bides, and keeping it full to the brim at that end. Much of the soot falling on the surface of the brine in light flocks, would float thereon, and be carried off over the end of the pan by the draught towards the chimney.
Between that end of the pan and the entrance to the chimney, is a soot-box or closet h, with a door for cleaning it out. Notwithetanding all these precautions, large quantities of soot are liable to become oondensed, either upon the bottom of the upper pan, or between the two pans, and, falling on the surface of the brine, get carried down and mixed with the salt, rundering it black and totally unfit for food. This quality of salt, however, has been found specially suitable for the Hargreaves' salt-cake manufacture (see p. 287), so that the small quantities now pro duced flnd a ready enough sale, as the soot does not sigaify. The method shows au important economy of coal, and, according to Pohl, gives 3 tons of butter-salt with the same amount of fuel and labour aa is requisite for producing 2 tons by the old methods. The 17180 of gas from a Siemens' producer would obviate the soot completely, while it is probably preferable (according to Milner's plan) to do away altogether with the upper pan, employing merely a brick or tile covering as a reverberatory and radiating surface to throw the heat down into the lower pan, and so get rid of leakages, salt cats, and much cobbling and repairs involved in working by bottom heat. According to some experiments by Pohl, while the temperature of the upper pan remained suitable for making e,ommon salt, or ordinary fishery-salt, that of the surface of the brine in the lower pan wae maintained at full boiling, and the produce, so far as grain was concerned, was very fine butter-salt, while no scale worth mentioning forms in the lower pan. He gives as a result of 16 days' boiling with brine containing 25.27 per cent. salt, for 57 tons of slack (from Little Houlton Colliery, Lancashire) burrit,-82 tons of flne butter-salt, and 49 of common salt ; while on the old system, the 82 tone butter-salt would have taken 54 tons 13 cwt., and the 49 tons of common salt, 26i tons, or a total of 81 tone 3 cwt., showing an economy of 24 tons 3 cwt. Instead of the gases escaping into the chimney at a temperature of 315° (600° F.), as during the manufacture of salt with the ordinary common salt pans, or at a temperature of 425°-538° (800°-1000° F.), as when making butter-ealt, they never rose, even with the strongeet firing, above 142° (288° F.).