LIME AND CEMENT INDUSTEIES 689 



The emery millstones of Sturtevant type are seen at many 

 cement works. They consist of two millstones, with emery on 

 the surface. The millstones are in each mill and are set in a 

 horizontal plane. The bedstone is bolted in place, while the 

 upper stone is set to revolve with the minimum amount of vibra- 

 tion. The lower stone is adjustable (pi. 9, 10). 



The size of the mill is expressed in the diameter of the stone. 

 The mills come in 36 and 42 inch size. A 42 inch size will grind 

 on the average 5-6 barrels of Portland an hour, when working 

 on crushed clinker. About 95$ of the product from the mill- 

 stones will pass a 100 mesh sieve. 



The same machines grind from 18-20 barrels an hour of rock 

 cement. These machines require from 15-18 horse power to 

 operate them. -'"•* 



The Stedman disintegrator represents a type of mill in which 

 the pulverization of the particles is produced by their being 

 thrown violently against each other as the machine revolves. 

 The machine which is shown in pi. 11, consists of several con- 

 centric drums, alternate ones revolving in the same direction, at 

 a high rate of speed and represents a similar disintegrator of 

 Sturtevant make. 



These machines are said to be very efficient till the unequal 

 wear of parts disturbs their balance, after which they deteriorate 

 rapidly unless the worn parts are renewed. 



Chemical composition 



The amount of carbonate in each hydraulic limestone must 

 not exceed 75$ and preferably is under 70$, and, where the 

 quarry contains several strata of different richness, it is possible 

 to bring about the proper composition by mixing. Hydraulic 

 limestones free from magnesia, it is claimed, give the best results 

 provided they contain enough clay. The Maryland rock with 

 €8.44$ of lime carbonate and only 4.58$ of magnesia, but with 

 silica and -clay amounting to 29.66$, is an example of this kind. 

 In any single rock the amount of magnesia should not exceed 



