SECTION IV. ALKALI INDUSTEY 

 BY-PRODUCTS 



The Alkali Industry Wastes. The Leblanc soda industry 

 provides some useful by-products for fertilizing purposes. 

 The lyeblanc soda industry consists of two processes : the 

 first is the salt cake process, in which sulphuric acid and 

 sodium chloride are treated together in large cast-iron pots, 

 and the action finished in furnaces. The charge consists of 

 about 1 6 cwts. of common salt, with 23 gallons of sulphuric 

 acid. As soon as heat is applied, large quantities of hydro- 

 chloric acid gas are given off, and the temperature rises. 

 During the first part of the process the reaction that takes 

 place is represented by the equation 



NaCl + H 2 S0 4 = NaHS0 4 + HC1 



After the mixture has been heated for about an hour, and the 

 salt has become solid, it is pushed on to the hearth of the 

 furnace. Here the hot air and flame from the fire complete 

 the decomposition between the sodium hydrogen sulphate 

 and the sodium chloride, according to the equation 



NaHS0 4 + NaCl => Na 2 SO 4 + HC1 



The resulting material usually contains 95 % of sodium 

 sulphate, with a little sodium hydrogen sulphate, a little 

 sodium chloride and a few impurities introduced accidentally. 

 Muffle furnaces often replace the open furnaces, as conden- 

 sation of hydrochloric acid is more economically per- 

 formed when waste fuel gases are not admixed. Mechanical 

 furnaces are also used, which have revolving beds. 



After the sodium chloride has been converted into 

 sodium sulphate by the salt cake process, it is then treated 

 by the black ash process, which consists in mixing the salt 

 cake with about its own weight of limestone and rather more 

 than half its w r eight of coal, the mixture being heated in a 



