Note 14 — Chapter XII · SODIUM
George Kamensky translation
The whole of the sulphur used in the production of the sulphuric acid employed in decomposing the common salt is contained in this residue. This is the great burden and expense of the soda works which use Leblanc's method. As an instructive example from a chemical point of view, it is worth while mentioning here two of the various methods of recovering the sulphur from the soda waste. Chance's process is treated in Chapter XX., Note 6.
Kynaston (1885) treats the soda waste with a solution (sp. gr. l·21) of magnesium chloride, which disengages sulphuretted hydrogen: CaS + MgCl2 + 2H2O = CaCl2 + Mg(OH)2 + H2S. Sulphurous anhydride is passed through the residue in order to form the insoluble calcium sulphite: CaCl2 + Mg(OH)2 + SO2 = CaSO3 + MgCl2 + H2O. The solution of magnesium chloride obtained is again used, and the washed calcium sulphite is brought into contact at a low temperature with hydrochloric acid (a weak aqueous solution) and hydrogen sulphide, the whole of the sulphur then separating:
CaSO3 + 2H2S + 2HCl = CaCl2 + 3H2O + 3S.
But most efforts have been directed towards avoiding the formation of soda waste.