68 MANUFACTURE OF FERTILIZING MATERIALS 



the present day ball mills continuously fed 

 and discharged are in general use, which owing 

 to their strong construction and stable working 

 answer well for the purpose. In older factories 

 flatstone mills are frequently used. Griffing crusher 

 with a walking beam has likewise some rare par- 

 tisans, but it is costly and requires repairs which 

 become heavy in the end. The material to be 

 introduced into this grinding machine ought pref- 

 erably to be reduced, and for that purpose edge 

 runners are suitable. The crusher, with blades- 

 disintegrator is likewise used, but only to crush 

 phosphate in large pieces or rock phosphate. 

 These machines will now be considered. 



Edge runners consist generally of two stones 

 turning on a circular plate round a vertical shaft; 

 at the same time each stone turns round its own 

 horizontal axis, and grinds the material both by 

 crushing and rubbing. The horizontal axes of the 

 two mills are independent of one another and each 

 connected with the vertical shaft by means of a 

 hinged crank. The stones can thus be raised or 

 lowered independently of each other. The ma- 

 terial is fed into the mill directly, by shovel or by 

 an elevator; it is drawn continually under the 

 stones by collectors, and when it is sufficiently 

 ground, it is evacuated by the automatic discharge, 

 sifted and bagged up. Two men are sufficient to 

 attend to such a mill. 



Flatstone mills are built either with the upper 

 stone stationary or the lower stone stationary, ac- 



