450 



CEMENTS, LIMES, AND PLASTERS, 



BALL MILL. 



Rejected materials return to the mill 

 by gravity through perforated plates at 

 each angle of the polygon. These plates 

 extend the length of the mill. Assuming 

 one third of the interior area to be cov- 

 ered with balls and material, this pro- 

 portion of the return plates is of course 

 under an outward pressure. Another 

 third of the interior area is moving up- 

 ward, and through this portion alone 

 the rejected material must pass back 

 into the mill. As the remaining third is 

 moving downward, a portion of the ma- 

 terial may move downward with it and 

 thus remain on the screen. Consequently 

 a considerable proportion of rejected ma- 

 terial is carried around indefinitely at 

 the periphery. If the mill is overfed, 

 the insufficiency of the return openings 

 results in choking the screens and limit- 

 ing, if not altogether stopping, the screen 

 action. 



The discharge area in the ball mill is 

 limited by the size and number of 

 the holes in the perforated grinding- 

 plates. The ball mill is not supplied with 

 means for enlarging or reducing this dis- 

 charge area. If the ball mill is fed with 

 a material relatively fine and difficult of 

 reduction, too large an amount will pass 

 the perforations, overloading the screens 

 and leaving too little material in the 

 drum to be acted upon by the balls. 

 Here the exit area is too large, and it 

 cannot be readily reduced. On the other 

 hand, if the ball mill is fed with a ma- 

 terial easy of reduction, the balls will 

 reduce more than the perforations and 

 sieves can pass, resulting in an accumu- 

 lation of an excess of finished material 

 in the grinding-drum. Here the dis- 

 charge area is too small, and it cannot 

 be enlarged. 



A very large percentage of material 

 which passes through the perforated 

 grinding-plates has been ground small 

 enough to pass the screens, but owing 

 to the limited screen area it is returned 

 to the mill and the power for regrind- 

 ing is wasted. Twenty to twenty-five 

 per cent of the material is thus reground 

 unnecessarily. 



KOMINUTER. 



All materials fed to the kominuter 

 must pass the full length of the drum 

 under the grinding action of the balls, 

 and is discharged through ports at the 

 outlet end. The conical shape of the 

 screens forces the material to move from 

 the outlet end back to the inlet end, 

 where the material rejected by the 

 screens is caught by return buckets and 

 returned to the inlet end of the drum 

 by gravity. The buckets empty at the 

 center of the mill around the stiaft and 

 above the level of the balls. It will tims 

 be seen that it is impossible for any of 

 the material to reenter the screens with- 

 out repassing the grinding chamber, as 

 may easily happen in the ball mill. 



On the kominuter adjustments of the 

 feed, of the discharge ports, and of the 

 screens, are easily and quickly made. 

 The total area of the discharge ports is 

 greater than is required by the maxi- 

 mum charge of balls. It is an easy mat- 

 ter to insert a sufficient number of port 

 closers, or covers, to allow the discharge 

 of only sufficient material to utilize the 

 extreme capacity of the sieves. With 

 these three adjustments, the highest 

 possible efficiency upon a given material 

 may be promptly and easily secured. 



The material discharged from the 

 grinding-drum must pass over screens 

 extending from the outlet end to the in- 

 let end. The material refused by the 

 screen in this travel is automatically re- 

 turned to the inlet end of the drum. 

 About one half of one per cent only of 

 such returned material would have passed 

 the screen. 



A kominuter running on raw mix a fairly soft, shaly limestone 

 gave a product of 15,046 Ibs. per hour. The feed was coarse, varying 

 from 4 inches down to dust, and would probably average 1^ inches. 

 The product gave the following sieve test: 



