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Ultrafine grinding for improved mineral liberation in flotation concentrates Parry, Jennifer Marie


As the minerals industry is required to process increasingly complex, finely-grained ores, stirred mills are replacing ball mills for regrind applications in flotation circuits. Stirred mills are able to produce fine grind sizes in an energy efficient manner and without additional size classification. Laboratory grinding trials were conducted using two highspeed stirred mills; one vertical and one horizontal, to treat three lead-zinc concentrator flotation streams which are currently reground using tower mills. The effect of stirred milling, in particular mill type, stress intensity and grind size, on downstream processing was investigated in terms of energy requirements, particle size distributions, mineral liberation and mineral breakage rates. It was shown that the breakage rates of hard and soft minerals converge at high stress intensities. The high stress intensity and open circuit configuration of high-speed stirred mills allow them to remedy the effects of density and hardness in streams ground in primary ball mills with classifying cyclones. By varying the stress intensity in a mill via the impeller speed it is possible to target either hard or soft minerals for liberation depending on the requirements of a particular flotation stream. A lower impeller speed would be used in order to improve liberation of softer minerals without needlessly grinding harder minerals, while a higher impeller speed would be necessary if liberation of hard minerals were important. The difference in impeller speed requirements reflects the difference in optimal stress intensity for grinding hard and soft minerals. The two high-speed stirred mills had similar energy requirements, and both mills had lower specific energy requirements than full-scale tower mills treating the same flotation streams. The vertical stirred mill products contained a greater proportion of fines compared to the horizontal mill products when compared using the Rosin-Rammler distribution, although this result was not consistent across different means of size distribution characterization. Mineral liberation behavior was similar for the horizontal and vertical high-speed stirred mills. The greatest benefit of regrinding using high-speed stirred mills was improved quartz liberation.

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