Ball Mills Mineral Processing Metallurgy13/02/2017· If finer grinding is wanted two or three stage grinding is advisable as for instant primary rod mill with 75—100 mm 3″—4″ rods secondary ball mill with 25—40 mm 1″—1½ balls and possibly tertiary ball mill with 20 mm ¾ balls or cylpebs To obtain a close size distribution in the fine range the specific surface of the grinding media should be as high as possible Thus