NICOLAS) .
In brief, the process consists of two parts, the salt-cake process, and the "black ash" process. The salt-cake process is described elsewhere (see HYDROCHLORIC ACID) ; the reaction of sulphuric acid with common salt produces hydrochloric acid and sodium sulphate (salt cake).
(Sulphuric acid) (Salt) (Sodium sulphate) (Hydrochloric acid) The second stage involves the conversion of the salt cake to carbonate by heating with limestone and coal; the former to provide the required carbonic acid and the latter to reduce the salt cake to a form in which it will react with the limestone: thus:— The mass (black ash) resulting from this furnace operation was then, after standing for a day or two, extracted with water and the liquor evaporated for the recovery of sodium carbonate. The insoluble part of the black ash formed unsightly heaps of "alkali waste" upon which much research had to be conducted before a satisfactory method of utilizing their valuable sulphur contents was discovered. Salt cake itself is a white powder and usually contains a small proportion, about I %, of free sulphuric acid, with a smaller amount of undecomposed salt.
When the Le Blanc process was the principal source of alkali, salt cake was required in such quantity that the hydrochloric acid produced with it was more than could be absorbed in industry, so that processes were devised by Weldon and Deacon for converting a large proportion to chlorine for subsequent use in the manufacture of bleaching powder. The introduction of the ammonia soda process has completely reversed that state of affairs. Although a certain quantity of salt cake is still required, for purposes specified below, the hydrochloric acid thereby pro duced does not suffice for modern requirements ; the quantity is therefore supplemented by direct synthesis of acid from hydrogen and chlorine, or by manufacturing a further supply of salt cake. As for chlorine, so far from it now being deliberately manufac tured as an outlet for surplus hydrochloric acid, it is a matter of some difficulty to dispose of the quantity produced in the elec trolytic process for making caustic soda.
Salt cake is used to a certain extent in the glass industry, where it serves as a source of sodium ; for the manufacture of sodium sulphide and Glauber's salts (q.v.) ; and, in far larger quantities, as a raw material for the sulphite process of wood pulp and paper manufacture. It is a matter of some difficulty to dispose of all the salt cake now produced ; more especially as it is also obtained as a by-product of the potash industry in Germany, and further occurs naturally in Canada. (A. E. H.)