High grade ores have largely been depleted and those currently being treated are low grade, complex and sometimes finely disseminated, requiring fine grinding to liberate valuable minerals. For fine grinding applications, conventional tumbling mills are energy intensive. More energy efficient technologies such as stirred mills have been developed and widely used for fine and ultra-fine grinding. In this study, the effects of residence time, solids concentration, impeller speed, impeller type, media size and media density on energy consumption in a batch vertical stirred mill were investigated. The effect of energy on mill performance was assessed using the perfect mixing mill model. In addition, the effect of media stress intensity on grind and energy efficiency at constant residence time was also investigated. It was found that irrespective of the method of altering the energy input, the fineness of grind improved with increase in the specific energy input. This suggests that energy is the key driver for size reduction. The perfect mixing model can be used to assess mill performance and the breakage rates generally increased with increase in the specific energy input, impeller speed and solids concentration. The media stress intensity approach is useful in assessing mill performance in stirred mills at constant residence time. The fineness of grind improved when the media stress intensity was varied from 4.41×10-3 to 27.41×10-3Nm. In addition, the specific energy required to produce material below 25μm and 38μm decreased with an increase in the media stress intensity. When slurry density effects were considered, an optimum stress intensity was observed with respect to specific energy required to produce material below 25μm and 38μm. It was recommended that additional test work be carried out to investigate the effect of media size in the range -6.7mm + 2mm on energy efficiency. It was also recommended that tests be carried out at impeller speed between 600rpm and 1500rpm to assess how mill performance increases even at relatively high impeller speeds. In addition, a model predicting the specific energy using the impeller speed and solids concentration can also be developed.

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