ALKALI INDUSTRY BY-PRODUCTS 67 



reverberatory furnace. Revolving black ash furnaces are 

 now very largely employed. The first part of the process 

 probably consists in the reduction of sodium sulphate by 

 carbon to sodium sulphide, according to the equation 



Na 2 S0 4 + 4C = Na 2 S + 4CO 



The second part of the process consists of the conversion 

 of sodium sulphide into sodium carbonate and calcium 

 sulphide by interaction with calcium carbonate, as shown 

 by the equation 



Na 2 S + CaC0 3 = CaS + Na 2 CO 3 



The soluble products are washed out of the black ash by 

 water, about 48 hours being needed for this washing, and 

 the residue still contains a little sodium carbonate. In 

 modern works much of the sodium carbonate produced is 

 at once converted into caustic soda by boiling with excess 

 of lime, when the changes that occur may be represented 

 by the equation 



Na 2 CO 3 + Ca(OH) 2 ^ 2NaOH + CaCO 3 



Air is blown in to oxidize any iron compounds, which in 

 the ferrous state would remain in solution and contaminate 

 the caustic soda, but which in the ferric state settle to the 

 bottom with the precipitate of calcium carbonate. The 

 calcium carbonate formed in this way is washed free from 

 caustic soda, and, after drying by exposure to air, can be 

 used directly as a lime fertilizer. It consists chiefly of 

 calcium carbonate in a fairly dense condition, and the small 

 amounts of sodium and iron compounds left are of no con- 

 sequence. The physical condition is good, and the percentage 

 of calcium carbonate about 80 %. 



The alkali waste consists of the crude calcium sulphide 

 left as a residue after washing out the sodium carbonate from 

 the black ash ; it is now generally used for the recovery of its 

 sulphur. Carbon dioxide from lime kilns, in the form of 

 waste gas, containing about 28 % of carbon dioxide, is 

 forced by powerful pumps through a series of tall closed 

 cylinders containing a cream of alkali waste and water. 



