MANUFACTUEE OF SUPEEPHOSPHATE. 



89 



naturally wear. It has been found that in grinding Florida phos- 

 phate forged steel balls lost 1 kg. (2'2 lb.) in a year working day and 

 night whilst cast-iron balls lost 420 grms. (nearly 1 lb. : 0*924:) in 

 twenty-four hours. Their weight must thus be verified from time 

 to time. To start the mill it is turned empty for half an hour to 

 see whether the bearings heat. Then only is it charged with balls 

 and phosphates. The drum ought always to be sufficiently full of 

 material to be ground so that the falling balls produce a dull or 



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Fig. o. — Ball Mill on Masonry Foundation — the Trituration Plates exposed. 



'deadened sound. Consequently, as soon as a bag of ground material 

 is taken from the mill, it is fed with another through the hopper. 

 When the mill has not sufficient feed its output is small and the 

 workman is tempted to increase the load of balls, which has many 

 •drawbacks. The outside sieves are mounted on marked oak-frames, 

 which prevent them being inverted, for they are not interchangeable. 

 To stop up gaps or tears the simplest way is to glue pieces of calico 

 ■above them with fish glue. During stoppages the inside of the 



