96 CHEMICAL FERTILIZERS 



raw material have a very important influence, because the 

 large lumps are difficult to leach out, and the smaller powder 

 blocks up the tanks. During boiling, the insoluble mass 

 packs closely in the tank and prevents free circulation ; 

 attempts have been made by forcing a current of liquid 

 through the tank to overcome this difficulty. The amount of 

 fine material which causes so much blocking up often amounts 

 to 25 % of the whole material ; occasionally the caliche 

 is riddled, and the very fine material actually wasted to 

 avoid this trouble. The larger the amount of insoluble 

 matter, the greater becomes the difficulty in effecting a 

 complete separation. Where the caliche is rich in nitrates, 

 the loss due to imperfect washing and blocking up with 

 deposits of salt may not be very serious, but, with raw material 

 containing from 17 to 20 % of nitrate of soda improvements 

 in manufacture are necessary. 



At present in many of the works only about 40 % of the 

 total nitrate of soda is extracted, and probably 50 % is a good 

 round average figure. The nitrate left in the refuse amounts 

 to about 30 %, and there is about 20 % of loss due to leaks 

 which occur from the drainage liquors in various parts 

 of the process. Many attempts have been made to improve 

 the whole system of working. By means of vacuum filters 

 the nitrate solutions are separated much more perfectly 

 from the insoluble matter, and the remaining pulp can be 

 ground up and further treated. In some of the larger works, 

 all the coarse material is treated by the old system and all 

 the fine material by the new system. Weaker liquors are 

 also employed, together with evaporators, so that the 

 efficiency has been raised from 50 to 64 %. Since the intro- 

 duction of the newer methods of working, the estimate of the 

 possible length of life of the Chili deposits has undergone a 

 remarkable revision. With the old system of working it 

 was only possible to utilize the richer materials, which would 

 have been exhausted by 1930 ; the ability to utilize deposits 

 which do not contain more than 10 % or 20 % of nitrate of 

 soda, has raised the probable remaining duration of these 

 deposits from a few years to several hundreds of years. 



