PREPARING THE MIXTURE FOR THE KILN. 



425 



TABLE 175. 

 FINENESS OF RAW Mix AT VARIOUS PLANTS. (BLEININGER.) 



At present, however, the "one-stage" type of fine-reduction prac- 

 tice has fallen into disuse. It has been gradually modified out of exist- 

 ence, so that cement-mills now are all using a gradual reduction system. 



The change from " one-stage" to "gradual" has been effected in 

 two opposite ways, according to whether Griffin or ball mills were in 

 use at the original plant. In plants using the Griffin (or Huntingdon) 

 mill, it has finally become the general practice to place either a fine 

 gyratory crusher or a set of fairly fine rolls between the coarse crusher 

 and the Griffin mill, thus insuring that the material fed to the Griffin 

 mill shall not be over | or even % inch in size. In plants using ball 

 mills, the change has taken place at the latter end of the series by put- 

 ting in a tube mill after the ball mill. The ball mill, instead of taking 

 a coarse product from the crusher and reducing this product to its ulti- 

 mate fineness, has been made an intermediate reducer, the final reduc- 

 tion taking place in the tube mill. 



Present-day systems. Present-day practice in the reduction of 

 dry raw materials may therefore be summarized as in the following 

 two schedules. The two schemes differ from each other mainly in 

 the mills used. 



TYPE A. GRIFFIN (OR HUNTINGDON) MILL TYPE. 



1. Gyratory crusher to about 2\ inches 



2. Crusher or rolls " " i to J inch 



3. Griffin or Huntingdon mill " " 85 to 95% through 100-mesh 



TYPE B. BALL (OR WILLIAMS) MILL TYPE. 



1. Gyratory crusher to about 2 J inches 



2. Kominuter, ball mill or 



Williams mill " " 20- to 30-mesh 



3. Tube mill " " 85 to 95% through 100-mesh 



