PREPARING THE MIXTURE FOR THE KILN. 427 



by simply placing separators between each of the steps of the series. 

 The cost of crushing per ton of product would incidentally be decreased, 

 but not in quite the same ratio. 



Disadvantages of separators. In view of the enormous gain in out- 

 put that would be secured by the use of separation, as noted in the 

 preceding paragraph, it is evident that the disadvantages attending 

 their use must also be very great, for otherwise every cement-mill 

 would now be fully equipped with them. The principal disadvantages 

 attending the use of separators in a Portland-cement plant are two : 



1. Separators destroy the uniformity of the product. 



2. Separators prevent the attainment of very great fineness. 

 These two objections will be discussed in the order named. It is 



probable that the second is in actual practice the more important. 



Uniformity of product destroyed by separators. Throughout the 

 entire course of cement manufacturing processes, from the moment 

 the raw materials enter the will until the finished cement is packed 

 the object of the manufacturer is to attain a product as nearly homo- 

 geneous as possible. The quality of the cement depends in large part 

 on the extent to which his attempts to secure absolute uniformity have 

 been successful. In the opinion of the present writer, one great objec- 

 tion to the introduction of separators into cement practice is that their 

 use will tend to destroy this uniformity. 



The action of separators is based commonly on one of two principles. 

 They take advantage either (a) of differences in the weight of particles 

 or (6) in differences of size of particles. In the first case, the separator, 

 other things being equal, will take out both the finer particles and the 

 particles of lowest specific gravity. In dealing with raw mixtures they 

 will tend to separate the clay particles from the limestone particles. 

 Separators depending for their action on the differences in size between 

 the particles will take out the finer, which, other things being equal, 

 will, of course, be particles of the constituent which is most readily 

 pulverized. Similar effects will be observed if separators be used in 

 the progress of clinker-grinding, for the lighter-burned, more readily 

 ground portions will be separated from the rest and the uniformity 

 of the product will be destroyed. 



Great fineness prevented by separators. If in a plant not using sepa- 

 rators the finest grinders be adjusted to give a product of which 95 

 per cent passes a 100-mesh sieve, a very large proportion of that prod- 

 uct (say 70 to 75 per cent) will pass a 200-mesh sieve. If separators 

 were installed throughout the plant, and the same adjustment of the 

 fine grinders maintained, a product passing the same 95 per cent through 





