Concern regarding grinding media wear is as old as the invention of the tumbling mill itself. Over the years many techniques have been tried in an attempt to minimize costs associated with grinding. These have included experimentation with different metals, non-metals and different shapes. Through all this, steel grinding balls have proved the most effective media for comminution in tumbling mills. / As old as the tumbling mill is, no accurate technique for estimating the wear of the ball charge has been developed. The mining industry still utilizes the Bond Abrasion test that was developed in the early 1960s. This test which is based solely on the ore properties is inadequate giving a standard deviation in excess of 100%. This can results in high degree of error which for greenfield applications can indicate profitability for non-profitable operations and vice versa. / Peter Radziszewski proposed a decoupled total grinding media wear model that could be used to predict grinding media instead of using the Bond abrasion test. This new model had a standard deviation of 56%, which was a great improvement over the bond abrasion wear estimates. This new model also allowed for the first time, the three wear modes, abrasion, corrosion and impact, responsible for grinding media wear to be quantified. This was an important development as the ability to identify the dominant wear mode in any given operation gives operators a chance to target reduction of wear. The current mining trend is towards exploitation of lower grade ores as the higher grades become depleted. These low-grade ores require finer grinding for the liberation of the value minerals. This in turn means higher ball consumption and higher beneficiation costs. The reduction of ball wear to minimise the overall cost of ore processing operation is going to be a primary concern for all concentrators. / The objective of the research that forms the basis of this thesis was to improve the accuracy of the decoupled grinding media wear model. This was achieved by refining the tests and methodologies used to determine the wear components that make up the decoupled grinding media wear model, performing fundamental testwork to determine a more accurate structure of the decoupled model, and validating this new model. / The new model termed the "modified decoupled grinding media wear model", gives better accuracy in predicting grinding media wear than both the Bond abrasion test and the original decoupled model.

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Optimisation of the control of an autogenous grinding circuit requires a better understanding of system dynamics than exists at present. To this end extensive operating data were acquired on an industrial semi-autogenous milling circuit and a model was developed to explain the circuit responses to changes in input variables. This model represents the trends in the data fairly accurately and showed that: • When an excess o f slurry was present in Me load (i.e. more than the volume of the voids) a pool of slurry forms st the toe of the load. • The complexity of the power dependence on the variables describing the mill load defied dynamic modelling with toe limited knowledge avail- • The key parameters determining grinding efficiency vary constantly as the grinding environment is disturbed indicating a need for adaptive con- • The flow through the mili is adequately modelled by a single well mixed Itwasalso clear that maximisation of the power with respect to themass alone was insufficient, and that control of the feed dilution water rate is essential. Mill discharge slurry theology was studied eo gain an insight into conditions in the load. An energy balance using temperature measurements around th t mill discharge sump provided a very reliable estimate of the mill discharge density. This estimate was not successfully used for control although this potential should be exploited. Attempts to commission an on-line measurement of discharge slurry viscosity failed. Several theories and ideas were tested in a production environment subject to a multiplicity of disturbances and conclusive qualitative results were obtained. An improved understanding of milling circuit dynamics and a rationalisation of where controi can be improved has resulted from this work.

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