MANUFACTURE OF SUPERPHOSPHATE 69 



cording as it is the upper or lower stone that is 

 made to revolve. Mills of the first kind are used 

 for crushing very hard phosphate, those of the 

 second kind for soft phosphates. The foundation 

 consists of cast-iron columns or a hollow cast-iron 

 support, on which the cage of the mill is fitted up. 

 The shaft of the mill is sustained by a movable 

 bearing with collar; its lower part rests in a socket. 

 The bearing with collar is screwed to the bottom 

 of the mill cage and completely protected from 

 dust. The adjustment of the revolving millstone 

 is done by means of an endless screw or by lever 

 transmission with screw, and hand fly-wheel. The 

 number of revolutions is about 120 per minute for 

 mills five feet in diameter, under which conditions 

 a mill can grind about five tons of phosphate per 

 hour with 20 horse power. The mill is fed by a 

 cup elevator and shaking hopper; a single workman 

 with an assistant can attend to two pairs of stones 

 placed side by side. The mills of the same group 

 are generally driven by a single main shaft by 

 direct cog-wheel gearing. Generally three pairs 

 of stones are in use, two of which are at work and 

 the other pair being faced. A great many other 

 kinds and classes of mills are used among which are 

 the ball mills, Pfeiffer's Mill with combined air 

 separator, which is a kind of ball mill, and the jaw- 

 breaker mill. 



The phosphate was formerly rendered soluble 

 in pits where the sulphuric acid and phosphate 

 were hand-mixed with suitable tools; in other 



