MANUFACTURE OF CARBONATE. SOdillIll carbonate is manufactured commercially by sev eral processes, of which only two are of impor tance--the Leblanc process and the Solvay pro cess. each named from its respective inventor. The Leblanc process consists of three stages: (1) The conversion of common salt (sodium chloride) into sodium sulphate by the action of sulphuric acid. accomplished by the aid of heat in a rever beratory furnace. This stage is called the salt cake process, 'salt cake' being the technical name applied to the sodium sulphate product. Two chemical reactions are involved in this stage, viz.: NaCI+NaIlSO4=Na,S0,-1-1IC1 (2) The decomposition of the sodium sulphate, by means of calcium carbonate and coal, at a high temperature in a furnace, the result tieing a crude product known as 'black ash,' which con sists of sodium carbonate, calcium sulphide, calcium oxide, calcium carbonate, and small quan tities of other substances. The principal reac tions taking place in this stage of the process may be expressed by the following chemical equa tions: I OC=5Na,S+ I OCO, (3) The extraction of the sodium carbonate by treating the black ash with water to dissolve the sodium salt, which yields a solution vatted `tank liquor.' containing also sodium hydrate. The crystals of sodium carbonate are obtained ulti mately by evaporation, and, when calcined, yield the dry sodium carbonate of commerce, technical ly known as soda ash. The calcium sulphide re maining undissolved in the residues is treated for its sulphur content. and the hydrochloric acid produced in the first stage of the Leblanc process is saved for use partly as such, partly for making, bleaching powder. 11: this manner from first to last there is practically nothing wasted except the calcium.
The transformation of the salt cake into black ash is generally carried out in a reverberatory furnace (Fig. 1), called a 'black-ash' or `balling furnace.' Usually 100 pounds of salt cake, 100 pounds of calcium carbonate, and 50 pounds of coal dust form a charge. The hand-worked fur nace is a long reverberatory with a hopper in the roof through which the charge is dropped into the first hearth near the flue where the heat is not very high; after thorough drying and beating, the materials are raked down onto the second or `balling hearth,' where the temperature is usu ally about 1000° C., and thoroughly rabbled until it becomes a thick, pasty mass from which car bonic acid gas escapes freely. As soon as the salt cake is decomposed, the charge begins to stiffen and carbonic acid gas (00) is evolved, as is shown by jets of blue flame (the carbonic oxide is produced by the action of coal on the excess of -calcium carbonate present). The charge is then raked into a ball and removed from the furnace to an iron truck, the escaping bubbles of gas causing the pasty mass to become porous. The shallow iron pan between the furnace hearth and the flue to the chimney is used for the evaporation of the tank liquor obtained by the lixiviation of the black ash in the third stage of the process.
The furnace operation is quite difficult, and al though the heavy tools are suspended by chains, the temperature is so intense that the quantity a man can handle at one time is limited to 300 pounds. In order to eliminate expensive hand labor and to work larger charges. revolving cy lindrical black-ash furnaces are used ; the com mon size, 16 feet long and 10 feet diameter. can treat as much as two tons of salt cake in a single charge. The lixiviation of the black ash is ac complished in a series of terraced tanks each with a false bottom perforated with small holes. The uppermost tank is charged with black ash, and water added to cover the charge; the solu tion of sodium carbonate formed, being heavier than water, sinks to the bottom of the tank and is passed through the perforations. and is with drawn by means of a pipe which delivers it to the second tank in the series, through which it passes to the third tank, etc. The operation is continuous, fresh water being added to the nearly exhausted ash in the uppermost tank to yield an unbroken flow of strong liquor. Good tank liquor contains approximately 23.5 per cent. of sodium carbonate and sodium hydrate.
The French Academy of Sciences in 1775 offered a prize for a method of making sodium carbonate from salt. Among the processes submitted was that of Nicolas Leblanc. which was of promising merit, and, being granted a pat ent in 1791, he began the manufacture on a com mercial scale. The Leblanc process is regarded as the most important discovery in the entire range of chemical manufactures, and has fur nished about one-half of the world's supply of soda. The fact that it produces both hydro chloric acid and bleaching powder as by-products has enabled it to survive competition. but the re cent introduction of electrolytic processes, which also yield bleaching powder as a by-product, is a serious menace to its future.
The Solvay process, or 'ammonia-soda process,' is based on the fact that hydrogen-ammoniutu carbonate, (NIIOHCO,, is decomposed by a strong solution of common salt, yielding sodium bicar bonate and ammonium chloride, as shown by the equation: ( NII, ) The brine is first saturated with ammonia gas, and the cooled ammoniacal liquor is subsequently charged in carbonating towers with carbonic acid gas under moderate pressure; the sodium bi carbonate. being much less soluble, separates out and leaves the more soluble ammonium chloride in solution, from which the ammonia is subse quently recovered by treatment with lime. The sodium bicarbonate is converted into sodium car bonate by calcination, and the carbonic acid gas evolved is again utilized to carbonate a second quantity of ammoniacal brine. In this cycle of operations no sulphuric acid is required and no hydrochloric acid is evolved.
The reactions involved in the ammonia-soda process were discovered by H. C. Dyar and T. 'lemming about 1838. but the process was not per fected until 1S73. In 1863 Ernest Silvay, a Del gian, constructed the first. successful plant. which has led to an enormous development of the indus try,