CHEMICAL FERTILIZERS 



The Operation of Grinding Phosphates. Rock phos- 

 phates are ground in a crusher, and riddled by cylindrical 

 rotary screens similar to those used for the manufacture of 

 road metal. After having been reduced to gravel by such 

 a process, the phosphates are ground in a roller or ball mill. 

 A popular kind of mill for this purpose is a ball mill, which 

 contains from one to five balls of i cwt. each (see p. 137). 

 The so-called balls are very frequently of ring shape, running 

 in a groove. For some purposes disintegrators are used, in 

 which four or more arms dash the material against a corru- 

 gated side, by which the lumps of phosphate become broken 

 into small pieces or dust (see p. 116). Whether a ball mill 

 or disintegrator be used, the fine dust is blown through 

 screens by a draught of air from a fan, until all the material 

 passes through a sieve containing 60 meshes to the 

 linear inch. Of the powder so produced, nearly 92 % 



