340 LECTURES TO SCIENCE TEACHERS. 



After the carbonate of lime has separated out, the liquor is 

 run off, the lime-mud remaining behind being well washed 

 to extract the soluble caustic soda, and these washings being 

 used for diluting fresh quantities of the red-liquor. The 

 dilute -caustic liquors are then either boiled down in ordinary 

 boilers or evaporated in open pans, and afterwards concen- 

 trated in cast-iron pots sufficiently large to hold from eight 

 to ten tons of finished caustic soda, being frequently 9 feet 

 in diameter and 5^ feet deep. These pans are built into 

 brickwork and are heated by means of a furnace placed 

 either below or on one side. At 120 C. the liquor in the pots 

 begins to boil and ammonia gas is evolved ; the temperature 

 gradually rises until it reaches 260 C. The pots are next 

 loosely covered, and the fire urged until the contents of 

 the vessel are raised to a dull red-heat. It is now neces- 

 sary to oxidize the small quantities of sulphides and 

 cyanides which the red-liquors contain. This is effected 

 by throwing in a few handfuls of nitre, or in some in- 

 stances by pumping air through the hot liquid. When nitre 

 is employed torrents of ammonia gas are given off, and a 

 black deposit of graphite, first observed by Dr. Pauli, occurs. 

 The white caustic soda contains 70 per cent, of Na.jO, or 

 nearly 90 per cent, of the hydrate NaOH ; a less concen- 

 trated product containing 60 per cent, of ]STa 2 O is made, and 

 this is termed cream caustic. When the operation is 

 finished the molten mass is ladled out by means of copper 

 ladles into iron drums, where it is allowed to cool ; and thus 

 preserved from the action of the air it may be transported 

 to any distance. This of course is the best form of concen- 

 trated alkali, and caustic is therefore largely exported to 

 districts where soap is needed, and where fats can be 

 obtained in quantity but where carriage is a matter of 

 consequence. 



We next come to consider the alkali waste. For every 

 ton of soda-ash produced, from one-and-a-half to two tons of 

 waste is formed ; and those of you who have been in the 

 neighbourhood of alkali-works know in what enormous 

 quantity this waste accumulates. This waste contains, as I 

 have said, the whole of the sulphur burnt in the pyrites 

 .kilns, amounting to from 15 to 20 per cent, of the 

 weight of the waste. The importance of recovering this 

 sulphur, not only for the sake of the manufacturers them- 



