MANUFACTUEE OF SUPERPHOSPHATE. 



87 



a single main shaft by direct cog-wheel gearing. Generally three 

 pairs of stones are in use, two of which are at w^ork and the other 

 pair being faced. Fig. 2 shows two mills w^ith the lower mill station- 

 ary arranged in a group driven with a common shaft by conical gear- 

 ing. 



Ball Mills. — The ball mill consists essentially of a rotary drum 

 enclosed in a double cast-iron envelope and sieves. It contains a 

 certain number of balls of different sizes. The inside face of the 



Fig. 3. — Ball Mill on Masonry Foundation. 



bottom of the drum is lined with smooth plates of hardened cast- 

 iron or of cast-steel. The circumference consists of triturated plates 

 of cast- steel, and of such a shape that it creates a fall between two 

 consecutive pieces. It follows that during revolution the balls fall 

 from one plate to the other, and roll during the interval between 

 two falls. Grinding is thus effected, by the shock of the balls and 

 by crushing. The substance ground in the ball mills passes through 

 holes in the triturating plates and falls on a first sieve, called the 



