MANUFACTURE OF NATURAL CEMENTS. 243 



burned portions make a cement which has a much higher tensile strength 

 than the normally burned product. 



Edge-runners. Dry-pans are in use at only one natural-cement 

 plant, where they take a moderately soft clinker from the crusher and 

 prepare it for the tube mill. Two dry-pans are in use, taking about 

 15 H.P. each and handling about 400 barrels each per day of ten hours, 

 equivalent to a power requirement of about 0.4 H.P. hour per barrel 

 of cement. 



Centrifugal grinders. Two machines of this type, the Grffiin mill 

 and the Clark pulverizer, are in limited use in the natural-cement 

 industry. 



The Clark anti-friction pulverizer has been supplied to two of the 

 plants of the Louisville district for use as an intermediate reducer. 



The Griffin mill, described with illustrations on pages 440-443, is, to 

 the writer's knowledge, little used in the natural-cement industry. 

 The only instances of its use that have come to his acquaintance are in 

 grinding the hard portions of the clinker produced at a natural-cement 

 plant in western New York and as an intermediate reducer in one 

 of the mills of the Louisville district. 



Ball grinders. This group includes three very important and effi- 

 cient machines, the kominuter, the ball mill, and the tube mill. All 

 three of these are now in operation in one important natural-cement 

 district, and all have proven satisfactory, though in different degrees. 

 As these mills are described in detail and figured on pages 447 to 465, 

 nothing need be said here in regard to their construction or general 

 methods of operation. 



In handling natural-cement clinker, certain things seem to have 

 been firmly established by the experience of the district above alluded 

 to. These facts may be summarized as follows. 



The tube mill, applied to the reduction of natural cement, is an 

 unqualified success. When working on this material its output is 

 very large, while its repairs and wear of pebbles are trifling. The 

 kominuter, highly efficient in actual grinding, formerly gave some trouble 

 in regard to repairs of certain parts, but these defects have now been 

 remedied. The ball mill is also efficient as regards output and power, 

 but is much worse than either kominuter or tube mill as regards repairs. 



Two Lindhard kominuters were run for 13^ months on a rather 

 hard-burned natural-cement clinker, during which time they turned 

 out "considerably over" 400,000 barrels of cement. The repair 

 expenses for the same period amounted to $300.18, which includes 

 the cost ($182.70) of one extra set of unused screen frames. 



