Hargreaves’ process produces sodium sulphate (without previous conversion of sulphur dioxide into sulphuric acid) directly by the passage of gases from the pyrites burners, air and steam, through salt blocks placed in vertical cast-iron retorts, a number of which are connected in series. A fan draws the gases through the system and leads the hydrochloric acid fumes to the condenser.
Sodium sulphate is used in the manufacture of glass, ultramarine, &c. Further, the sulphate is converted into Glauber’s salts by dissolving the anhydrous sulphate obtained in the muffle furnace, purifying with lime, and allowing the clear salt solution to crystallise out in pans.
A further use of the sulphate is the preparation of sodium sulphide, which is effected (as in the first part of the Leblanc soda process) by melting together sulphate and coal in a reverberatory furnace. If the acid sulphate (bisulphate) or sulphate containing bisulphate is used much sulphur dioxide gas comes off.
The mass is then lixiviated in the usual soda liquor vats and the lye either treated so as to obtain crystals or evaporated to strong sodium sulphide which is poured like caustic soda into metal drums where it solidifies.
In Solvay’s ammonia soda process ammonia recovered from the waste produced in the industry is led into a solution of salt until saturation is complete. This is effected generally in column apparatus such as is used in distillation of spirit. The solution is then driven automatically by compressed air to the carbonising apparatus in which the solution is saturated with carbonic acid; this apparatus is a cylindrical tower somewhat similar to the series of vessels used for saturating purposes in sugar factories through which carbonic acid gas passes. In this process crystalline bi-carbonate of soda is first formed, which is separated from the ammoniacal mother liquor by filtration, centrifugalisation, and washing. The carbonate is then obtained by heating (calcining in pans), during which carbonic acid gas escapes, and this, together with the carbonic acid produced in the lime kilns, is utilised for further carbonisation again. The lime formed during the production of carbonic acid in the lime kilns serves to drive the ammonia out of the ammoniacal mother liquor, so that the ammonia necessary for the process is recovered and used over and over again. The waste which results from the action of the lime on the ammonium chloride liquor is harmless—calcium chloride liquor.
The electrolytic manufacture of soda from salt requires mention, in which chlorine (at the anode) and caustic soda (at the cathode) are formed; the latter is treated with carbonic acid to make soda.
Effects on Health.—Leymann’s observations show that in the department concerned with the Leblanc soda process and production of sodium sulphide, relatively more sickness is noted than, for example, in the manufacture of sulphuric and nitric acids.
In the preparation of the sulphate, possibility of injury to health or poisoning arises from the fumes containing hydrochloric or sulphuric acid in operations at the muffle furnace; in Hargreaves’ process there may be exposure to the effect of sulphur dioxide. Hydrochloric and sulphuric acid vapours can escape from the muffle furnace when charging, from leakages in it, and especially when withdrawing the still hot sulphate. Large quantities of acid vapours escape from the glowing mass, especially if coal is not added freely and if it is not strongly calcined. Persons employed at the saltcake furnaces suffer, according to Jurisch, apart from injury to the lungs, from defective teeth. The teeth of English workers especially, it is said, from the practice of holding flannel in their mouths with the idea of protecting themselves from the effect of the vapours, are almost entirely eroded by the action of the hydrochloric acid absorbed by the saliva. Hydrochloric acid vapour, further, can escape from the absorbing apparatus if this is not kept entirely sealed, and the hydrochloric acid altogether absorbed—a difficult matter. Nevertheless, definite acute industrial poisoning from gaseous hydrochloric acid is rare, no doubt because the workers do not inhale it in concentrated form.
Injury to the skin from the acid absorbed in water may occur in filling, unloading, and transport, especially when in carboys, but the burns, if immediately washed, are very slight in comparison with those from sulphuric or nitric acids. Injury to health or inconvenience from sulphuretted hydrogen is at all events possible in the de-arsenicating process by means of sulphuretted hydrogen gas. At the saltcake furnace when worked by hand the fumes containing carbonic oxide gas may be troublesome. In the production of caustic soda severe corrosive action on the skin is frequent. Leymann found that 13·8 per cent. of the persons employed in the caustic soda department were reported as suffering from burns, and calls attention to the fact that on introducing the lime into the hot soda lye the contents of the vessel may easily froth over. Heinzerling refers to the not infrequent occurrence of eye injuries in the preparation of caustic soda, due to the spurting of lye or of solid particles of caustic soda.