Performance of the vertical roller mill in a mineral processing application when coupled with internal and external classifiers

Little, Warren

Performance of the vertical roller mill in a mineral processing application when coupled with internal and external classifiers

Doctoral Thesis

2021

Abstract

Comminution is applied in all mineral processing plants where mineral liberation is needed for separation and concentration of valuable minerals. The process is energy intensive and is predominantly carried out in tumbling mills which are known to be highly inefficient. Comminution technologies utilised in other industries, such as the vertical roller mill (VRM), a high compression dry grinding device, have the potential to contribute towards enhancing the energy efficiency of comminution and the sustainability of mineral processing. The vertical roller mill is used extensively in cement and coal grinding applications, where it is known to be more energy efficient than traditional tumbling mills. The VRM has a larger reduction ratio and greater flexibility in terms of product quality control and throughput. However, in the majority of operations where the vertical roller mill is applied, the device is coupled with an internal dynamic air classifier. These classifiers are significantly more energy intensive than other classifiers typically used in the mineral processing industry. Operating the vertical roller mill with less energy intensive classification devices could have the potential to further reduce the total comminution circuit energy. Reducing the comminution energy usage in mineral processing is important, however there are other factors which also need to be considered before the vertical roller mill can be considered as a viable alternative to tumbling mills. The effect of grinding with the vertical roller mill on product quality and mineral liberation, and their impact on downstream processing is of critical importance and has not yet been extensively investigated. This study investigates the grinding of the platinum group mineral (PGM) bearing Platreef ore, in a pilot scale vertical roller mill. The research explores the effects of milling variables and methods of classification on the mill performance, energy consumption, throughput, product characteristics and flotation response. The energy efficiency of the vertical roller mill and product flotation response is also compared with that of a traditional ball milling circuit. Vertical roller mill products of three target grinds, typical to primary, secondary and tertiary grinds in PGM circuit, were generated with the VRM operated in the standard airflow mode, with the internal dynamic air classifier, for a variety of grinding pressures (an online control used for maintaining product quality), and dam ring heights (a design variable affecting residence time on the grinding table). The specific grinding energy for the vertical roller mill was found to increase as the target grind became finer, conforming to general comminution theory. Vertical roller mill specific grinding energies were lower than those for a ball mill at all product sizes, and when estimates of classification energy and scale-up of the VRM are included, the specific energy for the vertical roller mill is up to 35% lower than for a ball mill in closed circuit. Grinding at higher pressures yielded greater throughputs and was found to generate products of less steep particle size distribution (PSD), but did not affect specific grinding energy. The effect of varying dam ring height was found to be target grind dependent. Higher dam rings yielded products with a greater proportion of fines, for higher specific grinding energies with the differences more pronounced at coarser target grinds. The grinding component of the vertical roller mill was operated in conjunction with three classification devices - the internal dynamic air classifier, external hybrid air classifier and an external vibrating screen. The external air classification circuit had a lower throughput than the internal air classification circuit, and operated with a 20 - 40% higher specific grinding energy. When including pilot scale estimations for classification energy, the external air classification circuit overall specific energy is lower at intermediate and coarse grinds, and comparable at finer target grinds, to that of the internal air classification circuit. The specific grinding energy in the VRM - external screening system was higher than both air classification circuits, however when considering the lower energy intensity of screening, this circuit may prove more efficient. The flotation response of the tumbling mill product and the vertical roller mill product prepared under different operating conditions in circuits with different classification devices was assessed using a standard reagent suite, in terms of platinum, palladium, gold (3E PGE), and copper, nickel (Cu, Ni) concentrate grades and recoveries. Both vertical roller mill and tumbling mill products yielded greater recoveries as the products became finer and valuable mineral liberation increased. The vertical roller mill products however had substantially higher recoveries, which was linked to higher froth recoveries and differences in oxidation potential within the flotation pulp. Increasing the compressive force in the VRM through the application of higher grinding pressures led to an increase in fines causing a decrease in froth stability during flotation, which corresponded with lower valuable metal recoveries. Variation in dam ring height caused small decreases in discrete PGM liberation but an increase in effective PGM liberation (considering PGMs liberated and locked in floatable base metal sulphide minerals). Flotation recoveries followed changes in froth stability with products prepared at the highest dam ring height, which had greater fines contents, yielding lower valuable metal recoveries. An optimised reagent suite for VRM products should be developed to address issues with froth stability caused by the increase in fines component at higher grinding pressure and dam ring heights. This will allow for the improvements in valuable mineral liberation with these grinding conditions to be realised as increased flotation recoveries. When operating the VRM grinding components with different classification devices, flotation performance was influenced by variation in the particle size distributions of products. External and internal air classification products with closely comparably particle size distributions yielded similar 3E PGE flotation recoveries. Flotation recoveries were however lower for the finest external air classifier product, which had a less steep PSD. A similar difference in PSD was observed for the screening vertical roller mill circuit products, which also had lower flotation recoveries than products generated with the internal dynamic air classifier. The research indicates that the vertical roller mill is able to efficiently prepare material for valuable mineral concentration through flotation. The effects that the vertical roller mill variables grinding pressure and dam ring height have on operation and the comminution product has been investigated. Furthermore the VRM has been successfully coupled with different classification devices to generate a product with flotation response better than that of tumbling mill products.

Keywords

Chemical Engineering

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Collections

PhD / Doctoral

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