MANUFACTUEE OF SUPERPHOSPHATE. 



91 



condition. If the output of the mill decreases, the inside and out- 

 side of the sieves must be speedily cleaned. For that purpose the 

 double envelope must be removed by a chain and counterpoise. 

 In grinding wet phosphate the sieves get choked and they must be 

 replaced bv new at close intervals. It suffices in order that the 

 phosphates"^ can be rendered soluble that they pass through Nos. 60 

 to 80 mesh sieves, according to their origin and their nature. Thus 

 Algerian phosphates and Carolina phosphates are passed through a 

 No. 60, whilst coprolites, apatites, and Florida phosphates must pass^ 

 through a 70 mesh sieve. Guanos and bone phosphates do not 

 require to be ground so fine, owing to their capillarity, which lets the 

 sulphuric acid penetrate easily into the interior of the granules. 

 It suffices to i^ass them through a No. 50. 



Eist of the separate pieces forming Krupp's Ball ^Mill. 



a 



b 



b' 



n 



c 



ci 



cl 



<1 



e 

 f 



9 

 h 



k 



k^ 



I 



Gearing columns. 



As already mentioned, the ball mill is surmounted by a chimney^ 

 This ends in'' a dust chamber on the floor above ; this chamber is 

 itself provided with a ventilating pipe on the side opposite to that 

 of the mill. When the chamber is not sufficiently well ventilated, 

 the entrained steam condenses, the dust becomes wet, and the 

 wood rots. It is, therefore, well to cover the sides with a coat of 

 tar. In front of the chamber, that is to say on the spot where the 

 dust collects, start two pipes closed by dampers, through which the 

 dust falls into the grinding-room. It is, likewise, advisable to add 

 a fan to the dust chamber, so as to renew the air in the grindmg- 



