II. The Ammonia-Soda Process
In spite of the great improvements effected during recent times the Leblanc process cannot economically compete with the ammonia-soda process, principally for two reasons. The sodium in the latter costs next to nothing, being obtained from natural or artificial brine in which the sodium chloride possesses an extremely slight value. The fuel required is less than half the amount used in the Leblanc process. Moreover, the ammonia process has been gradually elaborated into a very complicated but perfectly regularly working scheme, in which the cost of labour and the loss of ammonia are reduced to a minimum. The only way in which the Leblanc process could still hold its own was by being turned in the direction of making caustic soda, to which it lends itself more easily than the ammonia-soda process; but the latter has invaded even this field. One advantage, however, still remained to the Leblanc process. All endeavours to obtain either hydrochloric acid or free chlorine in the ammonia-soda process have proved commercial failures, all the chlorine of the sodium chloride being ultimately lost in the shape of worthless calcium chloride. The Leblanc process thus remained the sole purveyor of chlorine in its active forms, and in this way the fact is accounted for that, at least in Great Britain, the Leblanc process still furnishes nearly half of all the alkali made, though in other countries its proportional share is very much less. The profit made upon the chlorine produced has to make up for the loss on the alkali.
From Thorpe’s Dictionary of Applied Chemistry, by permission of Longmans, Green & Co.
AA, Tower; B, ammoniacal brine main; E, gas-inlet; Z, vacuum filter; V, pipe to air-pump.
The ammonia-soda process was first patented in 1838 by H. G. Dyar and J. Hemming, who carried it out on an experimental scale in Whitechapel. Many attempts were soon after made in the same direction, both in England and on the continent of Europe, the most remarkable of which was the ingenious combination of apparatus devised by J. J. T. Schloesing and E. Rolland. But a really economical solution of the problem was first definitely found in 1872 by Ernest Solvay, as the result of investigations begun about ten years previously. The greater portion of all the soda-ash of commerce is now made by Solvay’s apparatus, which alone we shall describe in this place, although it should be borne in mind that the principles laid down by Dyar and Hemming have been and are still successfully carried out in a number of factories by an entirely different kind of apparatus.
The leading reaction of this process is the mutual decomposition of ammonium bicarbonate and sodium chloride: NaCl+NH4HCO3＝NaHCO3+NH4Cl. It begins, however, not with ready-made ammonium bicarbonate, but with the substances from which it is formed—ammonia, water and carbon dioxide—which are made to act on sodium chloride. In practice the process is carried out as follows. A nearly saturated solution of sodium chloride is obtained by purifying natural or artificial brine, i.e. an impure solution of common salt, especially removing the alkaline earths and so forth by addition of sodium or ammonium carbonate and settling out the precipitate formed. This solution is saturated with ammonia, produced in the recovery plant (see below), in vessels provided with mechanical agitators and strongly cooled by coils of pipes through which cold water is made to flow. These vessels, as well as all others which are used in the process, are not open to the air, but communicate with it through washers in which fresh salt solution is employed for retaining any escaping vapours of ammonia. The ammoniacal salt solution is now saturated with carbon dioxide. This is employed in the shape of lime-kiln gases, obtained in a comparatively pure and strong form (up to 33% CO2), in very large kilns, charged with limestone and coke. The kilns are closed at the top, and the gases are drawn out by powerful air-pumps, washers being interposed between the kilns and the pumps for the purpose of purifying and cooling the gas. The heat evolved by the compression in the air-pumps (which rises to four atmospheres or upwards) is again removed by cooling, and the gas is now passed upwards in the “Solvay tower” (fig. 10). This is a tall iron erection, built up from superposed cylinders, which are separated from one another by perforated horizontal diaphragms, constructed in such a way that the gases are over and over again subdivided into many smaller streams and are thus thoroughly brought into contact with the ammoniacal salt solution with which the tower is about two-thirds filled. There the reaction mentioned above takes place, and owing to the concentration of the liquid the sodium bicarbonate formed is to a great extent precipitated in the shape of small crystals, forming with the mother-liquor a thin magma. This takes place with considerable evolution of heat which is removed by internal and external cooling with water. The temperature must not be allowed to rise beyond a certain point, for the reaction NaCl+NH4HCO3＝NaHCO3+NH4Cl is reversible, and at a temperature of about 60° or 70° C. it is in fact practically going the wrong way, viz. from right to left. On the other hand the cooling must not be carried too far, for in this case the crystals of sodium bicarbonate become so fine that the muddy mass is very difficult to filter. The best temperature seems to be about 30° C.
Either at certain intervals, or continuously, a portion of the contents of the tower is withdrawn and fresh ammoniacal salt solution is introduced higher up. The muddy liquid running out is passed on to the vacuum filters (Z, fig. 10). Here a separation takes place between the crystals of sodium bicarbonate and the mother-liquor. The former are washed with water until the chlorides are nearly removed, and are then carried into the drying apparatus.
This must be constructed in such a manner that the bicarbonate, which always contains some ammonium salts, is first freed from these by moderate heating (of course taking care that the ammonia is completely recovered), and later on, by raising the temperature, it is decomposed into solid sodium carbonate and gaseous carbon dioxide. The former needs only grinding to constitute the final product, ammonia-soda ash; the latter is again employed in the process of treating the ammoniacal salt solution with carbon dioxide. Various forms of apparatus are employed for this treatment of the crude bicarbonate—sometimes semi-circular troughs with mechanical agitators on the principle of the Thelen pan (see above)—all acting on the principle that the escaping ammonia and carbon dioxide must be fully utilized over again. The soda-ash obtained in the end is of a high degree of purity, testing from 98 to 99% Na2CO3, the remaining 1 or 2% consisting principally of NaCl.
A very important part of the process has still to be described, viz. the recovery of the ammonia from the mother-liquor coming from the vacuum filters and various washing liquors. Unless
