714 NEW YOEK STATE MUSEUM 



Per cent passing sieves 

 No. 50 No. 100 No. 200 



• • 



• • 



Vulcanite • 99.6 95.3 



Sandusky 99.6 92.8 



Brooks, Shoobridge & Co 98.8 88.3 ...» 



Aslen 99.7 92.4 68.4 



Aalborg . . . . 100 99.6 Y2 



Condor.. . ., 99.6 88.5 



• • • • 



Ball mills. At many factories a ball mill is used, wbicb con- 

 sists of a revolving cylindric cbamber, divided into segments witli 

 inclined steps. In this chamber is placed the clinker together 

 with a number of flint balls, and, 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 fi.ne 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 

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

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

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

 to 20' mesh. If used in conjunction with millstones, they take 

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

 pends of course on the fineness to which the material is to be 

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

 pacity of these mills under given conditions. 



1 Portland cement, p. 140. 



