246 



CEMENTS, LIMES, AND PLASTERS. 



not hold with regard to natural- cement mills, for in grinding natural 

 cement separation does not destroy the homogeneity of the product. 



The most effective separating system seen in operation is the Berthe- 

 let, first used at the mills of the Milwaukee Cement Company and now 

 installed at many other plants. Sections of a Berthelet separator are 

 shown in Fig. 49, while examples of installation of this separator are 

 shown in both Figs. "48 and 49. 



Power required in grinding. The total horse-power hours required 

 for crushing and grinding a barrel of natural cement will depend on. 

 (a) the hardness of burning, (6) the fineness of the product, and (c) 

 the character of the machinery used. 



In the table below are given the H.P. hours required to mill a bar- 

 rel (280 Ibs.) of natural cement at eight leading plants. The condi- 

 tions which affect the power are also briefly noted. 



TABLE 104. 

 POWER REQUIRED IN GRINDING NATURAL CEMENT. 



* Excellent separating system, 

 t Not run to full capacity. 



Fineness actually attained. -Until quite recently the grinding of 

 an American natural cement was rarely carried further than was neces- 

 sary to pass 95 per cent of the material through a 50-mesh sieve. There 

 was no particular demand from the engineers for a more finely ground 

 natural cement, and most of the manufacturers merely lived up to 

 the requirements of the average specification. These latter varied 

 considerably, as may be seen from the following table, but in only a 

 few cases was a greater fineness demanded than 85 per cent through, 

 a 100-mesh sieve. 



The average requirements, then, were low, and the average cement 

 just about passed the requirements. ' In some cases, however, a higher 

 standard was maintained by the cement manufacturer than by the 



