458 CEMENTS, LIMES, AND PLASTERS. 



iron, as may be required, and the tube is about one-half filled with flint 

 balls. The enormous grinding surface thus provided permits of a very 

 slow speed of operation 27 turns per minute. The shaft at the feed 

 end is hollow and a screw conveyor carries in the material. By simply 

 regulating the feed any degree of fineness, even to impalpable powder, 

 may be attained. 



" Careful experiments have proved that for coarse grinding in a non- 

 continuous mill, such as an Alsing drum, the highest efficiency is obtained 

 from a charge consisting of a large mass of the material to be ground 

 with a relatively small amount of flint balls, and that the highest effi- 

 ciency in fine grinding results from the charge of a small mass with a 



1 



FIG. 102. Gates tube mill. (Allis-Chalmers Co.) 



large number of flint balls. Take, for example, a charge in the Alsing 

 drum of a small proportion of balls to a large proportion of material. 

 Up to a certain degree of coarseness the grinding will proceed rapidly 

 with such a charge, but beyond that point to secure a high degree of 

 fineness would require a proportionately very much longer working of 

 the materials. Consequently any non-continuous working machine 

 for pulverizing material is both unsatisfactory and expensive. 



" In the tube mill this principle has been taken advantage of; the 

 flint balls bear a progressive ratio to the mass of material best adapted 

 to the conditions demonstrated in the original experiments. 



"The material to be ground is fed in at the center of one end of 

 the tube mill, and is delivered, ground, at the periphery of the other 

 end. The material consequently travels in a vertical line from the 

 center to the circumference of the mill, and in a horizontal line from 

 one end to the other. The tube mill being mounted horizontally, a 



