Fig. 5.—Revolving Black Ash Furnace—Elevation (after Lueger)
A Firing hearth; B Furnace; C Dust box.
Crystal soda is obtained from well-purified tank liquor by crystallising in cast-iron vessels.
Caustic soda is obtained by introducing lime suspended in iron cages into the soda liquor in iron caustic pots, heating with steam, and agitating by blowing in air.
The resulting clear solution is drawn off and evaporated in cast-iron pans.
As already mentioned, the tank waste in the Leblanc process, which remains behind—in amount about equal to the soda produced after lixiviation of the raw soda with water—constitutes a great nuisance. It forms mountains round the factories, and as it consists principally of calcium sulphide and calcium carbonate, it easily weathers under the influence of air and rain, forming soluble sulphur compounds and developing sulphuretted hydrogen gas—an intolerable source of annoyance to the district.
At the same time all the sulphur introduced into the industry as sulphuric acid is lost in the tank waste. This loss of valuable material and the nuisance created led to attempts—partially successful—to recover the sulphur.
The best results are obtained by the Chance-Claus method, in which the firebrick ‘Claus-kiln’ containing ferric oxide (previously heated to dull redness) is used. In this process calcium sulphide is acted on by carbonic acid with evolution of gas so rich in sulphuretted hydrogen that it can be burnt to sulphur dioxide and used in the lead chambers for making sulphuric acid. Sulphur also as such is obtained by the method.
These sulphur-recovery processes which have hardly been tried on the Continent—only the United Alkali Company in England employs the Chance-Claus on a large scale—were, as has been said, not in a position to prevent the downfall of the Leblanc soda industry. Before describing briefly the Solvay method a word is needed as to other processes for manufacture of sulphate and hydrochloric acid.