|
The actual comminution of the material is as result of the pressure exerted by the grinding media and the impact generated by the weight and movement of the charge.
At every phase of grinding operations, there should be a balance between the comminution resulting from pressure and that resulting from impact.
In primary mills, large particles of feed material are crushed by impact. A minimum size of grinding body is needed to generate the required impact. However, this means that a small number of large grinding bodies occupy a large proportion of mill volume. The degree of medium and fine grinding is reduced, and mill throughput is low.
Smaller grinding bodies can be used if their kinetic energy can be increased through greater drop height. This can be achieved by installing higher lifter bars, as shown in figs. E65 and G65.
A comparison of these two pictures shows that the grinding bodies drop off the lifter bars at a greater height in Fig. G65, in which the toe of the charge is also thinner. This indicates that total droop height has increased and that impact force is consequently greater.
The pressure within the charge is very uniform in both cases. Figs. F65, F75 and F85 illustrate how charge pressure decreases as mill speed is increased.
The quantity of grinding bodies is identical in each of these illustrations, although the charge level appears to be higher in F85.
The lining profile affects the relative movement of the grinding bodies. This in turn is another important factor with respect to the utilization of mill capacity.
The height of the lining profile thus affects charge pressure, impact force and the relative movement of the grinding bodies.
|
