PHOSPHORUS FERTILIZERS 



137 



are shown in Fig. 7, where a rotary drum is enclosed in a double 

 envelope supplied with sieves. 

 The balls, the number of which is 

 variable, run on trituration plates 

 forming segments of several 

 chords to the circle of the sieves, 

 so that by dropping off the edges, 

 the balls hammer the material to 

 pieces by shock, as well as grind 

 it to powder by rolling. The sub- 

 stances ground in the ball mills 

 pass through holes in the tritura- 

 tion plates, and fall on to a coarse 

 sieve, and then meet the finer 

 sieves. There is always some 

 considerable wear and tear of the 

 balls, depending on the nature of 



the phosphates and the kind of A shows the axle on which the 



balls employed ; forged steel balls 

 last much longer than cast-iron 

 balls. What may be described 

 as a compromise between a ball 

 mill and an edge runner is also 

 employed in some factories, wheie 

 a flat stone with deep grooves ro- 

 tates on a vertical axis, and four 

 heavy balls, of about i cwt. each, 

 roll round in these grooves, the 

 mill being completed with a cover g 

 and a series of sieves in a similar 

 way to the common ball mill. The 

 phosphates should be made to pass 

 through a 6o-mesh sieve, when a 

 large fraction of the material is 7 

 considerably finer than that. 

 In practice 95 % of the final 

 powder passes a sieve with 80 meshes to the inch, 

 and 85 % goes through a sieve with 100 meshes to the 



FIG. 7. Section of ball mill 

 elevator. 



moving parts rotate, in 

 the direction shown by 

 the arrowhead. 



B are the steel balls which 

 pound the material by 

 falling by their own 

 weight on to it. 



T are the trituration plates, 

 on to which the balls fall ; 

 the trituration plates 

 of necessity are made of 

 strong material firmly 

 fixed. The trituration 

 plates are set in steps, 

 so that the balls must 

 fall off one trituration 



in number, an inner cy- 

 linder of coarse material 

 and an outer cylinder 

 of fine material. The 

 dusty material passes 

 through these sieves into 



fine powdered material is 

 delivered. 



