92 CHEMICAL MANUEES. 



room. A greater quantity of dust then collects in the chamber, 

 Avhich then requires more frequent cleaning out. A fan making 

 200 revolutions suffices for two mills. A ball mill 6^ ft. requires 

 20 H.P. ; it grinds 7^ tons of Florida phosphate, so as to pass 

 through a 70 mesh sieve in twentv-four hours. It works contin- 

 uously, and yields in a single operation a sifted and ground substance 

 without other preparation than previous "cracking" up of the 

 phosphate so as to reduce it to lumps the size of the fist. 



In certain cases, especially in grinding basic slag, particles of 

 iron may remain in the mill, which cannot be ground. They are 

 removed through an opening made in the side which in normal 

 work is closed by a lid which can be replaced by a discharge grating. 

 The distance between the bars of this grating being inferior to the 

 normal diameter of the balls, it suffices to give a few turns to the 

 xnill to evacuate the residual matter. In large-sized mills a man- 

 iiole, fixed in one of the cheeks, enables the interior to be inspected. 

 Ball mills have been the subject of many improvements. Those 

 made by the firm of F. Krupp, Grussonwerk, Magdeburg, are of 

 such solid construction as to stand anv test and work for years 

 ^thout any repairs. The construction of this machine requires, 

 in fact, a powerful equipment and select materials, conditions to 

 which sixth-rate constructors can make no pretension. 



P/eiffer's Mill loitli Combined Air Separator. — This ball mill, 

 amongst those just described, is the most interesting owing to its 

 •originality and its great working capacity. But its yield is per- 

 force limited by the simultaneous sifting of the ground material ; 

 it gradually diminishes the fineness of the sieves, the surface of 

 which is, moreover, too small compared with the work of the mill. 

 Moreover, if the mill be overfed, the substance treated penetrates 

 into the cavities of the sieve before being reduced to the desired 

 fineness ; the sifting is then retarded as well as the normal work of 

 grinding. But affairs are radically altered if the powder be sifted 

 separately. There is then no obstacle to the enlargement of this 

 machine. This improvement has been effected by the firin of 

 Pfeiffer Brothers of Kaiserlantern. Bv combining the ball mill 

 with an air separator they have succeeded in creating a system of 

 .grinding remarkable for its great simplicity, and in increasing con- 

 siderably the useful effect and output of the mills. The construc- 

 tion of the machine differs from that of the ball mill in the following 

 details. The grinding-stages are comparatively small, each consists 

 of a single piece of crucible cast-steel made with particular care, 

 they have no slopes nor orifices of any kind to weaken them. 

 Consequently they maintain their mechanical resistance to the 

 last stage of wear and tear. There is no fear with this machine of 

 the balls plugging the escape orifices, besides there is no need to 

 -enlarge these orifices by a rimer. The good construction of the 



