MANUFACTURE OF NATURAL CEMENTS. 



241 



idea is to prevent unequal wear, for in an ordinary millstone the rim 

 wears much faster than the central area. These rock-emery millstones 

 are listed by the manufacturers at the following prices per pair, f. o. b. 

 Boston : 



30-inch $100 



36- " 150 



42- " 200 



48- " 250 



54- " 300 



The Sturtevant rock-emery mill itself, which uses the stones above 

 described, is made both as a vertical and horizontal mill, the latter being 

 preferable for harder work. The horizontal mill is shown in Fig. 45, 

 and a section of it is given in Fig. 47. It is made only in 42-inch size, 

 with under-runner stone. The running stone, the under one in this 

 mill, is rigidly fixed to the head of the large vertical shaft, which is 

 made to revolve by a pulley placed on it below the millstones. 



A mill of this size will grind from 15 to 20 barrels of cement per hour, 

 taking 15 to 20 H.P. in doing so. 



FIG. 47. Section of Sturtevant horizontal mill. 



The Cummings grinder * crushes material between two vertical 

 chilled-iron discs, one of which is stationary, while the other is revolved 

 at high speed. The faces of both discs are cut into a series of bands 



* Ball, C. M. The Cummings ore-granulating mill. Trans. Amer. Institute 

 Mining Engrs., vol. 21, pp. 516-519. 1893. 



