Sustainability in the mineral processing industry is guaranteed by the efficient operation of unit processes to maximize profitability within the prevailing economic conditions at the time. Efficient operation can be measured based on cost, metallurgical and energy efficiency of the operation or as a combination of the three, with energy efficiency becoming increasingly more important in the world and current South African context even more so. With ore bodies becoming more complex and finely disseminated the need for ultra fine grinding to maximise mineral recoveries in PGM concentrating operations by means of mainstream grinding has become more important. Conventional grinding technology was found to be inefficient to address this challenge. Introduction of new stirred milling technology for mainstream grinding using ceramic media successfully addressed the challenge posed by finely disseminated and complex PGM ore bodies. Introduction of new technology also brings about optimisation opportunities on various levels to ensure that the new technology is performing optimally in order to maximise profitability. As relatively new technology, several potential optimisation areas for the horizontal stirred mill has been identified for study. Operating costs for IsaMillsTM horizontal mills can be classified into 3 groups i.e. media, maintenance and energy cost29. Ceramic media type used can potentially play a role in determining the cost contribution of each of these groups to the overall mill operating costs. An area found very important to study, taking into consideration the enormous strain on the South African electricity supply system, was the impact of ceramic media properties on the energy efficiency of the horizontal stirred mill. Historical ceramic media test data5, 14, 17 indicated general trends between ceramic media properties and energy consumption. However the datasets available contained little data on the physical properties of the media to conclusively define the relative role of the media properties on energy usage. Five (5) different ceramic media samples from various sources were selected for further subsequent tests in an attempt to decouple the effect of the various media properties on mill power draw. Various physical properties of the media evaluated were measured during the study. Physical properties that were measured were: friction coefficient, size, density, and aspect ratio of every media. M4 IsaMillTM tests were also conducted in water and slurry environments to determine the relative contribution of the media properties on energy consumption. Analysis of historical data and results from the water-media study indicates that media mass and density contributes minimally if at all to the energy consumption of the M4 IsaMillTM. Energy consumption in the M4 IsaMillTM, operating in a water ceramic media environment, was found to be driven by a combination of media load and machine contributing factors. Machine contributing factors can be defined as the No Load Power (NLP) of the mill. Media load factor was found to be proportional to the product of the friction coefficient of the media, the total surface area of the grinding media charge and the aspect ratio of the media. From the results, a methodology was developed to predict the power draw of the M4 IsaMillTM in a water-ceramic media system that incorporates the media properties contributing to energy consumption. The model was successfully applied to a separate set of tests, with the average error in the predicted M4 IsaMillTM power draw being less than 2% of the measured mill power. The model developed in a water-media system was modified to incorporate the friction coefficient measured in slurry and used to predict the power draw of the M4 IsaMillTM during slurry tests. The prediction of power using the modified model was inaccurate because ore particle properties and the energy requirements for particle breakage were not incorporated in the modified water model. Stress Intensity and stress number is proportional to specific energy required for breakage in horizontal stirred mills33. As the ore and ceramic media properties will influence the stress intensity and the stress number the slurry model was expanded to incorporate a breakage energy component After fitting breakage (kb) and media charge (kc) constants to the model fairly accurate prediction of the power draw could be accomplished. Attempts were made to validate the model at lower volumetric media filling degrees with varying success. At very low media loads (50% less than model development data), the prediction was very inaccurate and based on the work from other authors43, it was clear that the model requires further development. Results of the study and the methodologies developed serves as a good foundation for future research into defining and quantifying the impact of ceramic media properties on all areas of horizontal stirred mill operation. The methodology also indicates promise in decoupling the factors contributing to the energy consumption of horizontal stirred mills. Further work is required to define the impact of particle size on media friction coefficient, the overall impact of ore to media ratio, and the effect of slurry rheology on energy consumption of horizontal stirred mills. It also serves as a good foundation for future scale up studies to test the wider application of the methodology on pilot and industrial scale mills. If scale up is reasonable the models could be potentially incorporated into a process control system to provide online monitoring of mill load conditions in the mill.

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