94 CHEMICAL FERTILIZERS 



going into any technical details is due to the exceptional 

 conservation of heat, the adoption of automatic calorimetric 

 instruments throughout, and an extensive knowledge of 

 critical temperatures and pressures in the most important 

 stages." 



Nitrate of Soda. The mode of occurrence of nitrate 

 of soda in Chili has been described on p. 23. The crude 

 materials so obtained are treated with water, so as to remove 

 the valuable sodium nitrate, and leave as much as possible 

 of the other materials behind. For this purpose boiling tanks 

 are usually erected on a hill slope, so that the liquors may 

 fall by gravity. The caliche, or crude nitrate of soda, is 

 first broken up in an ordinary crusher to pieces 2 ins. or 3 ins. 

 in diameter. Crushing to fine powder is avoided as much 

 as possible, because very fine material packs too closely in 

 the tanks to permit a proper circulation of water. In some 

 works rolls have been used instead of crushers, to avoid 

 the difficulty of fine dust, which clogs the extracting tanks, and 

 to avoid large lumps, which are not easily penetratedby water. 

 At a higher level than the boiling tanks it is usually necessary 

 to have several large reservoir tanks, one for pure water, 

 one for the weak liquor from washing and one for mother 

 liquor from which nitrate has already been crystallized. The 

 usual kind of boiling tank is 32 ft. long by 9 ft. high, 

 by from 6 to 8 ft. wide, with a perforated plate raised 

 9 or 12 ins. from the bottom, with steam coils, and with doois 

 foi discharging the exhausted material. Owing to the large 

 size of the tanks and the effect of high temperature on the 

 caliche, the pressure on the sides is very considerable, so that 

 cross bars are needed for strengthening. 



In practice one of the tanks which contains the exhausted 

 material from a previous extraction has first to be emptied, 

 which is very trying work, owing to heat and steam ; after 

 this refuse material has been removed, it frequently sets 

 .solid in the waste heaps. The charge of 60 or 70 tons of 

 fresh caliche is then admitted, and the five or six tanks are 

 worked in rotation, the water going into the tank which is 

 nearly exhausted and finally entering the tank which 



