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High pressure grinding rolls provide mines with energy savings

Published by , Assistant Editor
Global Mining Review,

Across the resources sector, mine owners are looking to high pressure grinding rolls (HPGR) to improve the energy efficiency of their greenfield projects. Favoured for decades in cement production, HPGRs are now being used in hard mineral comminution to offset the ever-increasing energy required to process declining ore grades and maintain profits amid falling commodity prices.

Studies and on-site data have both found that despite their higher initial cost, HPGRs can offer significant reductions in total ownership cost savings due both to their lower direct energy consumption, and downstream energy savings they create in grinding and mineral recovery.

Additionally, with reduced available groundwater in some key geographies, mines are being built in increasingly arid environments, requiring bold comminution plans to reduce their reliance on expensive water pumped from remote sources.

How HPGRs increase energy efficiency

Depending on the requirements of the circuit, High Pressure Grinding Rolls have been found to consume as much as 35% less power than conventional SAG mill-based circuits. This amounts to between 0.8 and 3.0 kWh per tonne and lower total cost of ownership.

Several factors will determine the magnitude of the energy saving, most prominently the size of the desired final output and the hardness of the ore. In general, harder ores being crushed into smaller sizes at higher volumes will offer the greatest reductions over conventional methods.

HPGRs have greater resilience to ore hardness, coping with harder ore feeds and variation within a single feed better than ball-based mills. With Enduron® HPGRs, the rollers can dynamically skew to maintain even pressure across a feed of mixed sizes, which together with their spring-loaded lateral walls, reducing the quantity of ore that needs to be recycled through the system.

Efficiency also scales with the size of the output, with the additional fines naturally generated by a HPGR’s inter-particle compression reducing the effort that needs to be put into subsequent milling.

HPGR  is mainly used in pellet feed to increase the specific surface area of the particles, in order to allow for strong pellets. This is in contrast to normal comminution applications, where size reduction is the main purpose. In pellet feed application, HPGR is competing with dry ball mills, which consume more energy.

Research conducted by the University of British Columbia found the introduction of a HPGR into Can Huckleberry gold mine’s mill circuit reduced power usage by 21% when grinding to a P80 of 160 μm, but that increased to 34% when milled down to 75 μm.


Micro-cracking and downstream energy savings

The high-pressure HPGRs exert on the feed creates micro-cracks in material, often between grain boundaries, significantly aiding in separating ore from the surrounding waste rock and thus reducing the overall demands on the circuit.

Highly resistant materials such as diamond resist the forces applied, while the surrounding waste rock is disintegrated. Similarly, gold suffers only minimal deformation while the feed’s other material is broken up – contributing to energy savings at the Mt. Todd mine in Western Australia of “over one-third” compared to SABC comminution.

Other circuits proposed include using magnetic rejectors to separate magnetite from surrounding silica at the gangue stage to reduce or eliminate downstream energy use. Processes involving leaching and floatation also require less energy when working with rocks subjected to micro-cracking.

Low water usage – how HPGR can drive dry comminution

In addition to direct power savings, High Pressure Grinding Rolls also offer the potential for mill circuits which consume much less water, a top priority for an industry where the scarcity of opportunities is driving investment in some of the world’s most arid geographies.

Responding to this growing need to conserve resources, we have spent the last decade working with mine operators as well as some of Germany’s leading mineral processing universities (such as the University of Freiberg) to develop systems for dry air classification in tandem with Enduron® HPGR grinding.

Using this method, we’ve been able to achieve lower energy consumption and produce finer end products – ingesting a feed of F80 50 mm and reducing it down to P80 of 150 μm and lower, utilising dry air classification in a closed HPGR circuit.

This makes the Enduron® HPGR ideal for projects in those parts of the world with arid environmental conditions that force the operator to use as little water as possible, such as Australia and the United States, where this technique is already in use.

Together with the strong energy savings the technology offers, High Pressure Grinding Rolls are a vital tool for businesses to counter-act the falling margins imposed by lowered ore grades and reduced commodity prices.

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