After mining ore has been sufficiently crushed by use of a jaw crusher
, gyratory crusher or impact crusher the next step is to grind that material down to even finer powders and particles for processing. There are several types of used grinding equipment that can be utilized to perform this task including ball mills, rod mills, sag mills and hammer mills. Each type of grinding press has its advantages depending on the mining ore being used. Rod mills are quite similar to ball mills but use long steel rods instead of small balls for the grinding media. Steel rods grind the ore by tumbling within the mill and the rods tumble and spin in roughly parallel alignments simulating a series of roll crushers. Since a rod mill doesn"t cascade it can be operated at a lower peripheral speed than a ball mill and since there is less void space in a rod mill than a ball mill there is usually more grinding contact between metal and ore. Ball mills are cylindrical devices that rotate around a horizontal axis and are partially filled with the material too be ground and the grinding medium. Often ball grinding medium consist of ceramic balls, flint pebbles and stainless steel balls. Ball mills operate via an internal cascading motion where the grinding balls and the ore grind together to reduce the material to a fine powder. Industrial ball mills can operate continuously, being fed at one end with the ore and discharging the fine powder at the other end. Hammer mills, unlike ball and rod mills, act to shred mining ore rather than grind it. A hammer mill consists of a steel drum with a vertical or horizontal rotating shaft on which hammers are mounted. The hammers will swing freely from a fixed point on the central rotor and when material is fed into the mill the rotor is spun at a high speed so the hammers can shred the ore material that is then expelled through screens in the drum.