This mechanical carbonator has proved a great success, and is being rapidly adopted by the best alkali makers. A furnace of the dimensions stated can turn out 150 tons of finished ash per week, the cost of labour being 11d. per ton, and the amount of fuel on an average, 8 cwt. per on of carbonate. The quality of the ash, too, is more certain than in a hand furnace, as the mechanical working can be absolutely relied upon. In decreasing the amount of caustic °oda the superior working is especially shovrn.
Until about twenty years ago this first carbonate, or "soda ash," was looked upon merely as a erthle produet--a prelude to further processes. With increased knowledge, and skill, and more perfect apparatus, tho quality has been so much improved that soda ash forms the great bulk of commercial carbonate of soda, having superseded refined alkali, to a great extent, in the soap, glass, and paper trades. It is rarely sold in the rough state in which it comes from the furnace, but is usually ground to a fine powder. For this ptupese it is put through horizontal stones, similar to those of a flour mill, from which it rune into casks placed beneath the spout. Occasionally vertical, " eilgo," stones are used, but the ash requires then to be sifted before being packed. The best stones aro blue lava from Italy and the Lower Rhino. These resist the heat of the ash better than the French " burrs " occasionally nsed. The carbonate, after being spread upon the floor of the packing house to cool, should bo fed into the mill by a set of elevators, aud the cask into which the finely ground produet runs should be kept constantly rocked or shaken by any suitable con trivance so that it may be paeked as tightly as possible. The cost of grinding and packing is about Is. per ton, and the weight of cask when filled varies from 1 to 15 ewt., depending upon the requirements of the consumer. If a lower strength of ash be required than that produced from the furnace—say 48 per cent. instead of 52 per eent.—a certain qnantity of dried chloride of sodium, or "kelp salt," is mixed with it. To keep this mixture, and the strength, right, it is necessary to test every cask.
Soda ash is sold by the percentage of sodium carbonate it contains, and at so much "per degree!' The testing is of the simplest description, by neutralization of a solution in hot water of a known weight of soda ash by a standard solution of pure sulphuric acid. It may be noted that all available soda—the hydrate, silicate, and aluminate—tests as carbonate. The equivalent of sodium is usually taken as 24 instead of the correct fig,ure 23, and of the carbonate 108 instead of 106. Ilenee the correction to " English degrees " in the table annexed—the degrees upon which the carbonate is usually reckoned. The Decroizilles' degrees represent the Frenchitandard and show the number of parts of oil of vitriol neutralized by 100 parts of the sample. This alkali metrical table has been drawn up by John Pattinson, of Newcastle-upon-Tyne.
Refined Alkali.—For the finer sorts of glass, and for various other purposes, a purer article than soda ash is required, and this is readily obtained by dissolving the ash in hot water, settling, boiling down, and re-furnacing. All the insoluble impurities of the ash are thereby removed, the iron and
the lower sulphur compounds thoroughly oxidized, while the sodium hydrate is converted into car bonate. The process of refining is as follows :—The soda ash in its rough state, as it comes from the carbonator, is thrown into some such dissolver as that shown in Figs. 260 and 261, and hot water run in. Any waste heat is utilized for warming the water, but it is also necessary to have a small steam pipe in the dissolver itself. The agitators shown greatly facilitate solution. The liquor, as nearly saturated as possible, is taken off by a pipe set about two-thirds up the side of the dissolver, a second outlet at the bottom serving to empty the vessel completely when requisite. If much sulphide be present in the ash, a few pounds of bleaching powder may be added in the dissolver, but inasmuch as this destroys also a certain amount of carbonate, its use is not recom mended. The soda solution is drawn off as it forms and run through a series of shallow settlers, preferably half boilers, into the pumping well. From here it is pumped up into any convenient arrangement of. tanks and left for about five hours to settle. The solution being run very hot from the dissolver, does not cool down to crystallizing point in the settlers, unless left too long, and therefore no loss need be feared upon this head. After thoroughly settling, the clear liquor, to within about 12 in. of the bottom of the tank, is run off into a pan almost exactly similar in con struction to the boiling-down pan of a hand ball furnace The arch. however. is kept higher, the pan has two fireplaces, one at each end, and the products of combustion are taken out from tho centre. This boiling-down operation results in a great waste of heat, as the flames must be kept from too close contact with the liquor, to prevent contamination with carbonaceous matte r, and a rapid draught must be used. Sometimes a pan has only one fireplace, but this arrangement does not work well ; nor can the boiling down be performed by any waste heat. When the salts assume a pasty con sistency, with large lumps of nearly dried salt mixed through, the doors are removed and the whole mass is raked out into a drainer placed in front. The doors are then replaced and a fresh charge of lye is run in. The drained salt is treated precisely like the black salts— dried and furuaced in a double-bedded reverberatory furnace with a high arch. The finishing process re quires great care to prevent fluxing, a clear flame being only allowed to play upon the charge when it is thoroughly dried. It is altogether more akin to the drying of fish salt in the carbonator. Nearly all the sodium hydrate is now converted into carbonate and the sulphite into sulphate. A loss of about 5 per cent. of material is incurred by the refining process, and a de crease of about per cent. in strength, 53 per cent.