secondary crushing), the reduction ratio in each stage being in the range 3 to 6. Mineral
crushing is accomplished by compression against rigid surfaces or by impact against surfaces
in a rigid constrained motion [1]. Figure 17.2 presents equipment used to accomplish
crushing: (a) jaw crushers, in which a large jaw toggles back and forth to crush lumps
against a hard, rigid surface; (b) gyratory crushers, which use a gyrating cone to compress
lumps against a rigid surface; (c) roll crushers, in which the ceramic lumps are squeezed
between rotating rolls; and (d) hammer mills, which use rotating hammers impacting the
material to break up the lumps.
Grinding, in our context here, refers to the operation of reducing the small pieces
after crushing to a fine powder. Grinding is accomplished by abrasion and impact of the
crushed mineral by the free motion of unconnected hard media such as balls, pebbles, or
rods [1]. Examples of grinding include (a) ball mill, (b) roller mill, and (c) impact grinding,
illustrated in Figure 17.3.
In a ball mill, hard spheres mixed with the stock to be comminuted are rotated inside
a large cylindrical container. The motion causes the balls and stock to be carried up