CRUSHING AND PULVERIZING MACHINERY. 437 



clinker. For this last purpose they are specially well adapted, because 

 they can handle hot clinker with less injury than any other form of grind- 

 ing machinery. 



Class 4. Millstones. 



This class of crushing machinery includes those types in which the 

 material is ground between two flat or grooved discs, one of which 

 revolves. These discs are usually set horizontally, though they may 

 also be arranged vertically. The class includes the millstones and buhr- 

 stones proper, and several patented devices such as the Sturtevant 

 rock-emery mill and the Cummings mill. As these machines are exten- 

 sively used in grinding natural-cement clinker, they have been already 

 described under that head (pp. 239-243). In Portland-cement practice 

 they are now rarely used, though in the old-style wet-process plants 

 millstones were almost invariably adopted. 



At one small Portland-cement plant seven runs of 48-inch upper- 

 runner French buhrstones were employed in taking |-inch clinker from 

 the rolls and reducing it to about 20-mesh. Each run of stones required 

 8 H.P. and handled about two barrels of clinker per hour. A sieve 

 test showed the following results: 



. Mesh of sieve 20 30 50 100 



Per cent passed 98 96 92 65 



Per cent residue 2 4 8 35 



The seven mills required about one man's time for dressing. The 

 particular economy of the installation lay in the fact that the mills 

 cost only about $10 per run, having been purchased from old flour-mill 

 plants. New mills of similar type would cost about $250 each. 



Class 5. Edge-runners. 



The class of edge-runner mills includes those types in which the 

 material is crushed in a pan under a cylinder, the cylinder having two 

 motions turning on itself on a horizontal axis and also revolving as 

 a whole about a fixed vertical axis. The familiar " dry-pans" and 

 " wet-pans" of brick-plants are here included, a modern dry-pan of 

 improved type being shown in Fig. 92. In the Portland-cement indus- 

 try, particularly in wet-process plants, dry-pans are much used in grind- 

 ing the clay or shale. Elsewhere they are of little service. 



Clay is ground in a dry-pan, at one marl-plant, at the rate of 6 tons 

 per hour, the dry-pan requiring about 30 H.P. for operation. This 



