42 



The further reduction of the mixture is usually carried on in 

 two stages, the material being ground to say 30 mesh in a ball 

 mill, komminuter, Griffin mill, etc., and finally reduced in a tube 

 mill. At a few plants, however, single stage reduction is prac- 

 ticed in Gnffn or Huntington mills, while at the Edison plant 

 at Stewartsville, N. T., the reduction is accomplished in, a series 

 of rolls. 



The majority of plants use either the Griffin mill and tube 

 mill or the ball andi tube mills, and there is probably little differ- 

 ence in the cost of operating these two combinations. The ball 

 mill has never been quite as much of a success as its companion, 

 the tube mill, and has been replaced at several plants by the 

 komminuter. 



FINENESS OF MIXTURE. After its final reduction, and when 

 ready for burning, the mixture will usually run from 90 to 95 

 per cent, through a loo-mesh sieve. In the plants of the Lehigh 

 district the mixture is rarely crushed as fine as when limestone 

 and clay are used. Newberry* has pointed out in explanation 

 for this that an argillaceous limestone (cement rock) mixed 

 with a comparatively small quantity of purer limestone, as in 

 the Lehigh plants, requires less thorough mixing and less fine 

 grinding than when a mixture of limestone and clay (or marl 

 and clay) is uesd, for even the coarser particles of the argil- 

 laceous limestone will vary so little in chemical composition 

 from the proper mixture as to affect the quality of the resulting 

 cement but little, should either mixing or grinding be incom- 

 pletely accomplished. 



A very good example of typical Lehigh Valley grinding of 

 raw material is afforded by a specimen examined* by Prof. E. 

 D. Campbell. This specimen of raw mix ready for burning was 

 furnished by one of the best of the eastern Pennsylvania ce- 

 ment plants. A mechanical analysis oif it showed the following 

 results : 



Mesh of sieve. 



50 100 200 



Per cent, passing 96.9% 85.6% 72.4% 



Per cent, residue 3.1% 14.4% 27.6^ 



The material, therefore, is so coarsely ground that only a 

 trifle over 85 per cent, passes a loo-mesh sieve. 



"Twentieth Ann. Kept. U. S. Geol. Surv., Pt. 6, p. 545. 

 "Journal Amer. Chem. Soc., vol. 25. 



