338 LECTURES TO SCIENCE TEAC3EES. 



saturated liquors come in contact with the fresh black-ash. 

 In this way with the least quantity of water the complete 

 lixiviation of the black-ash is effected. The average 

 time needed for working off a vat is about forty-eight 

 hours. 



The residue remaining in the vats after the soluble 

 matter has been extracted constitutes what is known as 

 the alkali-maker's waste. 



It is impossible, however, by this process of lixiviation 

 completely to wash out the soluble carbonate of soda, so 

 that we find about 3 per cent, of soluble alkali left in the 

 vat waste. 



I need scarcely remind you that this waste contains the 

 whole of the sulphur from the salt-cake ; and this as a rule 

 is in the insoluble state of sulphide of calcium. This waste 

 is now thrown away, being either made into heaps which 

 often become a nuisance in the neighbourhood of the alkali- 

 works, carried out to sea, or otherwise made away with. 



A considerable excess of limestone is used in the manu- 

 facture of black-ash, and this in the course of the reaction 

 becomes converted into caustic lime ; on treating the mass 

 with water this caustic lime transforms a considerable 

 quantity of the carbonate of soda into caustic soda, and 

 carbonate of lime is formed ; thus : 



Na 2 C0 3 + CaO + H 2 = 2NaOH + CaC0 3 . 



Hence it happens that as a rule about one -third of the 

 total amount of soluble soda present in the black-ash liquor 

 is caustic. Besides this, the black-ash liquor usually con- 

 tains small quantities of sulphide as well as sulphocyanide 

 of sodium. 



In order to evaporate the black-ash liquors the waste 

 heat of the black-ash furnace is employed. Large pans 

 (D D, Fig, 14) are filled with the liquor, the water is boiled 

 off, and the crystals which are deposited are scraped 

 out into the drainers, E E, Fig. 14. These crystals are then 

 heated, and yield common soda-ash, as it is termed. In 

 order to separate the sulphide of sodium as well as the 

 caustic soda which the black-ash liquors contain, these 

 liquors are frequently oxidized and carbonated by the liquid 

 being allowed to fall down towers, filled with coke, up which 

 hot air mixed with carbonic acid passes. 



