136 



CHEMICAL FERTILIZERS 



which have already been described on p. 27. It is 

 necessary that the raw phosphates should be very carefully 

 ground, as the degree of fineness to which the phosphates 

 are reduced contributes very largely to the success of the 

 superphosphates. The phosphates are ground in different 

 patterns of mills, according to the nature of the phosphates 

 utilized. Some of the phosphates, on exposure to air, 

 weather down to a certain extent, and when this is the case 

 it is advisable to allow them to powder themselves, but in 

 any case a considerable amount of grinding is necessary, 



M 



/V\ 



un 



FIG. 6. Section of edge runner. 



B is the bevel gearing which transmits power to 'the runner. 



MM are the two stone mill wheels which revolve. 



P is the bed plate on which the grinding takes place. The rotation of the 

 bevel gearing (B) causes (MM) to revolve, and as these rest upon the 

 bed plate they execute a rolling and sliding movement, thus grinding 

 the material to powder. 



and the best mill to use will depend upon the kind of phos- 

 phates to be ground. A vertical edge runner (see Fig. 6), 

 is still in occasional use, and permits of the disintegration of 

 a great variety of substances. The ordinary flat stone 

 mill and roller mill are also used, but the common modern 

 grinding machine is the ball mill. The early forms of ball mill 



