71-1 raw tobk i rM 



Per oent imssing sieves 



No. 50 N... 100 No. 200 



Vulcanite 99. G ..3 



99. G 92.8 



- Loobridj 9S.8 8S.3 .... 



99.7 92.4 68.4 



Aal! 100 99. G 72 



Condor 99. G 8S.5 



• • 



; mills. At many factories a ball mill is used, which con- 

 sists of a revolving cylindric chamber, divided into segments with 

 incline s. In this chamber is placed the clinker together 



with a number of flint balls, ami, owing to the rotating action, 

 the clinker is pulverized. The surface on which the balls travel 

 is of hard metal. It is also perforated so that, as soon as the 

 clinker is ground fine enough, it falls through on a metal screen, 

 which retains the coarser grit, the finer particles passing on 

 through gauze. PI. 19, 20 show a sectional view of a ball mill. 



Tube mills (pi. 21). These are also used for the reduction of 

 the clinker, and consist of an iron cylinder about 15 feet long 

 and 4 feet in diameter, half filled with flint balls. The chief 

 object of the tube mill is to complete the grinding of the cement, 

 the preliminary grinding of the clinker taking place in some 

 other machine. 



The cylinder rotates at a speed of 25-30 revolutions a minute, 

 and the material, which is charged at one end, gradually works 

 way out to the other, though the mill is horizontal. The 

 lining of the mill is either of cast iron strips or specially prepared 

 brick. The material fed to it should have been previously crushed 

 to L'<> mesh. If used in conjunction with millstones, they take 

 the heaviest part of the wear off the latter. Their capacity de- 

 pends "f murse on tin; Oneness to which the material is to be 

 ground. Butler 1 gives the following figures illustrating the ca- 

 pacity r.f these mills under given conditions. 



i I'm tl&nd cement, p. 140. 



