Scotch Soda Furnace.
Prep. 1. (From common salt or sulphate of sodium.) The latter is generally obtained by decomposing the former with sulphuric acid, the evolved gas being passed into water, or through flues filled with coke, over which a very small stream of cold water constantly flows, by which it is condensed, and forms ‘LIQUID HYDROCHLORIC ACID,’ a substance afterwards consumed, in large quantities, in the manufacture of chloride of lime, and for other purposes. The sulphate of sodium, obtained from this or any other source, is well mixed with an equal weight of chalk or limestone, and about half its weight of small coal, each being previously ground to powder, and the mixture is exposed to a strong heat in a ‘reverbatory furnace’ (see engr.) until the decomposition of the sulphate is complete, the mass during the calcination being frequently stirred about with a long iron rod; the semi-liquid is now raked into an iron trough, where it is allowed to cool, whilst the furnace is recharged with fresh materials. The crude dark-grey product, thus obtained, is known as ‘ball alkali,’ or ‘British barilla,’ and usually contains about 22 or 23% of pure hydrate of
sodium. This is now lixiviated with tepid water, and the solution, after defecation, evaporated to dryness; the residuum is mixed with a certain quantity of sawdust, coal-dust, or charcoal, and roasted in a reverberatory furnace, at a heat not exceeding 700° Fahr., until all the sulphur is expelled. The product is the ‘soda-ash,’ ‘soda salt,’ or ‘British alkali,’ of commerce, and contains about 50% of pure sodium, partly in the state of carbonate, and partly as hydrate, the remainder being chiefly sulphate of sodium and common salt. When this is purified by solution in water, defecation, evaporation, and crystallisation, it furnishes commercial crystallised carbonate of soda. When this last is redissolved, and the filtered solution is carefully crystallised, it constitutes the ordinary carbonate of sodium used in pharmacy and medicine.
Another process for the preparation of commercial carbonate of sodium, known as the ‘ammonia process,’ has of late years met with considerable adoption. The history of this process, together with the process itself, are thus described by Dr R. Wagner:[175] “Six years ago (he was writing in 1873), when the international jury at the Paris Exhibition expressed their opinion upon the state of the soda industry at that time, all the judges, whether practical or theoretical men, believed that Leblanc’s process (that previously described) would hold the field for a long time yet. This seemed still more probable, since a process had just been introduced for recovering the sulphur from the soda residues. At that time all the soda in use was prepared by this process, excepting a comparatively small amount obtained from Chili saltpetre and cryolite, although there were already tangible indications that soda could be made on a larger scale by another method, which would be cheaper than Leblanc’s process.
[175] ‘Journal of Applied Chemistry.’
“The chemical section of the international jury at the Vienna Exposition, under the presidency of Professor A. W. Hofmann, constituted a congress of chemical technology. By its labours during the course of the summer this congress of scientific men was able to authenticate the very important fact that although Leblanc’s process might in the future possess some importance for certain branches of the industry, yet in most places another soda process would be introduced in the immediate future, and entirely supersede that of Leblanc. Since the time of the Paris Exhibition this new process has grown from a small germ to a strong tree.
“The process in question, and which is called by Professor Hofmann ‘the ammonia process,’ is not new, from either a chemical or scientific point of view. It belongs to the same methods as those in which oxide of lead, bicarbonate of magnesia, quicklime, alumina, silicate of alumina, oxide of chromium, or fluosilicic acid are employed to decompose chloride of sodium,
and convert it directly into soda or its carbonate. None of these attempts met with a success deserving of notice; although for a century past efforts have been made to render them practically operative. The new process is founded upon a reaction noticed over thirty years ago—that of bicarbonate of ammonia upon a strong solution of common salt. The greater part of the sodium is precipitated as bicarbonate, while chloride of ammonium remains in solution, from which the ammonia for a second operation is expelled by quicklime. The carbonic acid necessary to convert the ammonia into bicarbonate of ammonia, and thus make the process a continuous one, is obtained by heating the bicarbonate of soda to convert it into the simple carbonate.
“The sensation which the ammonia process has created in industrial circles will render a brief history of its development not uninteresting.