The effect of ball mill operating parameters on mineral liberation

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Virginia Polytechnic Institute and State University

In previous studies, the analysis of ball mill operating parameters and their effects on breakage phenomena has been limited to homogeneous materials. Though these studies have proven to be an asset in predictions of product size distributions and mill scale-up, they have not addressed the primary role of grinding, i.e. liberation.

The present investigation analyzes the effect of ball mill operating parameters on the breakage rates of both liberated and composite material. The operating parameters studied include mill rotational speed, ball size, mill charge, and wet versus dry grinding. Breakage rates have been determined experimentally utilizing a SEM-IPS image analyzer. The mineral sample used was acquired from ASARCO's Young Mine which is located in Jefferson City Tennessee. It was a binary ore consisting of sphalerite and dolomite.

Batch grinding experiments were conducted to provide breakage rates for the various composition classes. Breakage rates were then normalized with respect to energy to see if the changes in breakage rates associated with mill operating parameters were due to changes in breakage kinetics, or simply a function of energy input.

The energy normalized data indicates that the free dolomite breakage rates tend to normalize with respect to energy in the case of varying interstitial fillings. Furthermore, changes in mill rotational speed tend to provide energy normalizable breakage rates for both free dolomite and sphalerite. In all other cases, analysis of the breakage rates and energy-specific breakage rates indicate that a change in breakage kinetics may be occurring.

In general, particles containing a high proportion of sphalerite are more apt to break under impact conditions. On the other hand, particles containing a large proportion of dolomite were found to prefer attrition breakage conditions.