Page:Manufacture Of Soda by Hou Te-Pang.pdf/25

8 try. In the early salt cake process, hydrochloric acid fumes were allowed to discharge into the atmosphere through a chimney, and heavy damages were collected from these manufacturers. William Gossage in 1836 introduced a coke-packed scrubbing tower for these fumes. The tower was built of sandstone slabs, boiled in tar and clamped together with steel straps. Water was sprayed from the top and trickled through the coke to absorb the upgoing fumes. The apparatus was primarily devised to prevent the escape of acid fumes to the air without any idea of recovering any useful product. But it recovered a valuable product which was one of the factors that prolonged the life of the LeBlanc process in the face of a strong competition from the ammonia soda process.

The market for hydrochloric acid at that time was not very strong, but there was a great demand for bleaching powder. The bulk of the hydrochloric acid had to be converted to bleaching powder. In doing 50 it was necessary to convert the hydrochloric acid first to chlorine gas. The usual method then in use was to oxidize it with manganese dioxide (pyrolusite) according to the following reaction:

It is thus evident that the manganouk chloride resulting from the reaction was no longer useful for the work. Further, one-half of the chlorine was not made available this way. The consumption of manganese dioxide also was a considerable item of expense. Walter Weldon from 1866 to 1870 devised a process for the oxidation of manganous chloride to manganese dioxide by means of lime, thus effecting its recovery. The process which bears his name consisted in treating the still liquor with lime, heating it to 550-60°C. by steam in the presence of excess lime, and blowing in air. Thus, almost all the manganese was recovered in a useful form, and a strong calcium chloride liquor resulted, from which calcium chloride powder could be manufactured rather economically for use in refrigerating plants.

So far as the utilization of chlorine was concerned, oxidation by manganese dioxide made available ouly one-half of the total available chlorine as mentioned above, with an efficiency of only 50 per cent. Besides, manganese dioxide was recovered in the form of calcium manganite, CaMn2O2, which consumed acid to free the MnO2, from CaO. An improvement on this method was discovered by Henry Deacon of the firm of Gaskell, Deacon & Co., Widnes, in 1870, who used oxygen direct from the air for oxidation in the presence of copper chloride (CuCl2) as a catalyst. Though the chlorine gas obtained was rather dilute and demanded a special apparatus for the manufacture of bleaching powder, all the chlorine present in the manganese chloride was made available, and; further, the excessive alkalinity due to CaO was avoided.

The process for making bleaching powder from chlorine gas and lime, which then took so much hydrochloric acid from the LeBlanc process manufacturers, was discovered by Charles Tennant, a Scotch merchant,