Research

Energy Efficient Liberation and Comminution Program

Program Leader: Prof Malcolm Powell, University of Queensland

This program investigated energy-efficient equipment and processes to reduce the amount of energy consumed in liberating valuable minerals from ores.

Energy-efficiency in this context encompasses all energy based process inputs, not simply the electrical power required to run comminution equipment, but also energy-intensive process inputs such as grinding media and wear-resistant mill liners, as well as ancillary inputs like materials handling and net water consumption. Size reduction of an ore to achieve mineral liberation is a major consumer of process energy. To liberate the valuable minerals from an ore, it is necessary to break it down (comminute) to fine sizes, usually to below 200 microns. This is very energy intensive and accounts for over half the electrical power requirement of Australia's minerals industry. In addition, the millions of tonnes of steel grinding media used to crush and grind the ore already have high embedded energy – energy used in their manufacture– equivalent to over half of the direct energy usage.

Investigating ways to achieve better liberation with less comminution is synergistic with a quantitative evaluation of the current commercially available comminution devices which claim to improve energy-efficiency. Some of this work was conducted under the auspices of the AMIRA P9 project. Achieving "best practice" in the design and operation of mineral processing plants is done, principally though not exclusively, through the development of modelling and simulation techniques and measurement tools that will enable mining companies that sponsor the project to design better plants and operate them more efficiently.

Other comminution research took a more fundamental approach by developing a Virtual Comminution Machine (VCM) that uses computationally intensive techniques and allows customised optimisation of existing equipment design and accelerated development of new devices. The VCM development was a collaborative initiative between CSIRO, JKMRC (University of Queensland) and the University of Cape Town.

This program included projects in the areas of High Pressure Grinding Rolls (HPGR), banana screens, mill charge monitoring and coarse liberation. A high level of success has been achieved in applying advanced computational techniques (such as DEM and SPH) to modelling of comminution devices – the tools in use are at the forefront of the technology worldwide. Demonstration projects were used to take the most promising outputs of the program into pilot or industrial scale trials that tested, validated and quantified the predicted benefits. Industry buy-in has been demonstrated through their funding of the demonstration projects that have arisen from the research.

Achievements

World-leading computational modelling techniques developed for comminution equipment, known as the Virtual Comminution Machine (VCM). The VCM also incorporates of the effect of slurry on breakage and transportation in mills.
A framework for a Unified Comminution Model (UCM) that will integrate all comminution models under one structure has been developed and reported to industry sponsors. This model is a description of breakage processes, in which particle interactions are calculated according to particle type, interaction energy and impact mode. The model includes incremental damage and two major modes of abrasion. The UCM is a key component of the VCM.
A new ore breakage characterisation device – the JK Rotary Breakage Tester – developed and 7 units deployed worldwide to be used on active mine sites. The JKRBT is enabling the collection of the most detailed and complete breakage database we have ever had.
With CSRP's support, the world's most sensitive Positron Emission Particle Tracking (PEPT) camera has been donated to the University of Cape Town in South Africa. CSRP has also contributed funds to the establishment of the resultant PEPT facility.
A world first slurry model has coupled fluid flow (SPH) and solids flow (DEM) and has been used to analyse the slurry flow in the tower mill. The same technique has been applied to the pilot mill, with outcomes of direct relevance to mill operation and slurry flow control.
Techniques to save direct energy and media consumption through the use of HPGR technology and to save energy through better use of existing equipment.
Test work showed that there is a limit to the number of times that an ore can be crushed efficiently by HPGRs in series. Following a reasonably large size reduction ratio on the first pass and second passes, a drop in grinding efficiency is experienced on the third HPGR pass – indicating a limit of two HPGR passes for efficient grinding of hard ores.
PhD student Zeljka Pokrajcic won the "2010 Vittorio de Nora Prize for Environmental Improvements in Metallurgical Industries" awarded by The Minerals, Metals, and Materials Society.
Water storage design features offer mining operations the greatest immediate opportunity to reduce water losses by evaporation from mine site process water storages. Other high impact opportunities were identified for dry processing of minerals and processing with much less water consumption.
Generic screen modelling techniques that can be used to realistically scale-up screen performance with changes in feed and screening parameters – for optimising screen design for maximising screening efficiency.
Significant advances in the understanding and modelling of tumbling mills, both in the area of particle breakage and in the transport of material through the mill. Development of a ruggedised continuously powered AG/SAG mill vibration monitoring system for improved mill control.
Fine ore particles have been included in the simulation of a laboratory-size ball mill to explore what these do to the flow and what breakage environment they experience. This is computationally extremely challenging and the first study of its type. Results will open the door to fundamental grinding simulations and studies of the micro-environment in milling devices – getting to the core of energy transmission.
Coarse particle liberation is a function of the geological and structural properties of the rock, as well as the methods of blasting and rock crushing. The mean fragment size has been predicted using JKRBT testing. Results indicate that fragment size is directly proportional to the amount of energy utilised per blast hole.
A new concept developed to improve the computational fluid dynamics (CFD) modelling of erosion. Current models are not material specific, whereas the new model incorporates the erosion for specific materials.
Mineral processing students at the University of Queensland have been trained in the latest concepts and technology of comminution and liberation. The course was rewritten to make it topical, with the latest equipment such as HPGRs and fine grinding stirred mills included. Sustainable development assessment was introduced for the first time, and made an assessable part of the course.

Impacts

This program aimed to reduce total comminution energy at existing sites by 20 percent or more. Achieving this target will typically require taking advantage of more than one of the opportunities which have been identified:

Savings of 30-40 percent in direct energy and 90 percent of media consumption have been demonstrated at the laboratory scale through the use of High Pressure Grinding Roll technology.
Application of the models will improve existing operations and allow new machines and flowsheets to be developed more cheaply and quickly than currently possible.
Techniques to save 20 percent of circuit energy through better use of existing equipment have been shown in simulations studies.
Circuit simulation work has demonstrated the capability of using existing equipment to reduce conventional circuit energy requirements by 10-15 percent at two existing sites.
HPGR research has clearly demonstrated at laboratory scale the capability of saving 20-40 percent of the energy over ball mill circuits. Research has also resulted in minimised risk and improved confidence in the design of closed HPGR circuits.
Reducing energy consumption through reduction in steel ball wear has been clearly demonstrated through the contribution of ball wear to about 40 percent of energy use in milling via the embedded energy of ball manufacture.
Deeper water storage areas reduce the surface area to volume ratio and thereby reducing the evaporative loss per litre of water stored. The costs and benefits of implementing deeper storages or multi-celled storages are relatively easy to quantify without the need for extensive field investigations. In the longer term, physical covers have the potential to achieve the greatest reductions in evaporation, followed by windbreaks and chemical covers.
Excellent data from the HPGR shows a direct correlation of product temperature to HPGR performance – the degree of grinding, fineness of product, and over-pressurising of the ore leading to a waste of energy.
On average, a measured energy saving of 25-40 percent was achieved through a hybrid HPGR/ball milling circuit when compared with the conventional ball mill circuit.

Completed Projects

Contact

Prof Malcolm Powell
Julius Kruttschnitt Mineral Research Centre
University of Queensland
p. 07 3365 5893
e. malcolm.powell@uq.edu.au