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MEI Online: MEI Conferences: Gravity Concentration 06: Conference Report

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If it true that responsible environmental management is no longer considered a soft issue by the modern day miner, then surely the same could be said about metallurgists. Like their mining counterparts, the rock processing lab arts are coming up with a multitude of operational innovations designed to not only extract the most value from the ore taken from the ground, but also to work within what should be regarded as an acceptable and sustainable footprint.

This was just one of the conclusions reached during Minerals Engineering International's recent Gravity Concentration '06 Conference, which was once again held at the Sheraton Hotel in the West Australian capital of Perth, and sponsored by Falcon Concentrators, Gekko Systems, Roche Mining, and the Gold & Minerals Gazette.

Attended by around 50 people, the one-and-a-half day event attracted an international audience, with professional metallurgists coming from the UK, Iran, South Africa, Namibia, Canada, Turkey and, of course, Australia.

As usual the mood was relaxed, with organisers MEI putting on extended coffee and lunch breaks so delegates had time to take part in a peer review - to chat and exchange notes on the papers being presented. While most of the talks were dominated with issues pertaining to improved grade and recovery, some of the speakers highlighted the fact that a number of their pragmatic decisions regarding minerals processing were in fact motivated by an environmental agenda.

Nigel Grigg from the Victorian-based Gekko Systems, for example, said a combination of gravity, flotation, intense cyanidation, solution recovery and then detoxification during the extraction of gold would ultimately result in the production of mostly benign tailings. Although gravity is one of the oldest minerals processing methods available, its use in gold recovery has been declining due to the availability of more effective chemical means such as flotation and leach/carbon-in-pulp.

The intense cyanidation system developed by Gekko, Grigg explained, was an intense in line leach reactor, which was design to eliminate carbon and minimise the solution volumes being used. "It's a very ore specific process - sometimes the flow rate (of the feed going through the reactor) has to increase to increase the kinetics (and) sometimes we might need a couple of stages of counter current thickening - it really depends on how the ore settles," Grigg explained. "The tailings are then rejected out to the tailings dump - and that can be a separate contained system, with all your sulphides and your cyanide in it. "So it doesn't necessarily have to go to the main tailings dam - it can actually be treated to detoxification and sent underground if required. "Depending on the ore type and site specific issues, the combination of gravity flotation followed by intensive cyanidation can give you the best return on investment - it can get higher recoveries and it really has a lot better environmental outcomes.

"I think gravity recovery systems are advancing in leaps and bounds - you can see all the technology here over the last couple of days and I think its going to be the continuing focus of a lot of mining companies around the world."

During an earlier presentation Francis Nwafor from the University of Nottingham's School of Chemical, Environmental and Mining Engineering said he and his fellow researchers were using the basic principles behind the traditional mineral processing technique of dry jigging to explore the possibility of separating heterogeneous waste streams.

A literature search, he explained, had indicated that little (if any) work had been conducted on the use of pneumatic dry jigging for waste separation. However, current legislative and market conditions had placed much demand on the development of efficient and environmentally friendly waste separation techniques which guaranteed the production of higher quality recyclates. "Over the past two decades the common concern in the recycling industry has gradually shifted," Nwafor explained. "Current emphasis is no longer about the achievement of high recycling rates for products whose market cannot be guaranteed, but about value creation and quality improvement for recycled products.
"The development of sustainable and efficient recycling systems that have the capacity to improve the market value of recycled products is one of the biggest challenges facing the recycling industry."

Although previous attempts to generate quality products had been achieved through a complex arrangement of traditional mineral separation techniques (like magnetic, electrostatic and gravity separators), the common problem with many of these was the fact they were mainly binary or wet separators, whose application under current levels of legislative and market conditions generally demanded a re-modification of an operation's activities to improve not only separation efficiency, but also environmental friendliness. Nwafor said a specially constructed jig compartment was instrumented and used in monitoring the contributory effects of the variables that affect the separation process. Initial results - acquired through feeding the jig with materials of known physical properties (like density tracers) - had shown that airflow characteristics significantly influenced segregation of a given set of feed material. "This work is developing a deeper understanding of the inter-relationship between the feed and airflow characteristics, which will hopefully inform the design of an efficient dry separator fore waste materials," Nwafor added.

Meanwhile the WA-based ConSep Pty Ltd was mindful of its environmental duty while developing a novel system to enable sites to reliably predict the impact of gravity leaching upon the performance of an existing processing plant. The ConSep ACACIA is a packaged processing unit designed to treat high grade gravity gold concentrates such as those generated from a Knelson concentrator. The plant consists of an up flow fluidised reactor for the leaching process, which has been developed and optimised to produce the ideal solid-liquid interaction for maximised gold leaching reaction kinetics without mechanical agitation.
According to the company's Barrie Watson, the ConSep ACACIA typically achieves overall recoveries in excess of 95% - and mostly over 98%. The use of intensive cyanidation in flotation plants, Watson noted, presented a major challenge for cyanide to contaminate and depress the flotation circuit. One of the earliest installations of the ConSep ACACIA was at the Porgera gold plant in Papua New Guinea, which required the WA technology supplier to develop a novel solution to eliminate cyanide in solid tails. This led to the use of an aggressive washing step at the end of the pregnant solution recovery stage of the process and had proven to generate tailings with less than five parts per million cyanide.

The ConSep ACACIA also uses a low oxygen leach environment which has generated the side benefit of a low arsenopyrite dissolution.
Watson said a survey of a site using the ConSep ACACIA CS2000, conducted over 22 leaches, revealed that despite an average gold recovery to solution of 99.4%, the arsenic recovery in solution was only an average of 2.26%. This was despite the fact the operation had high arsenopyritic grades (averaging 2010 parts per million arsenic) within the Knelson concentrate.

The Proceedings of the conference are available on CD-ROM from MEI (www.min-eng.com/gravityconcentration06/paps.html) and selected papers will be published in a special Magnetic, Electrical and Gravity Separation special issue of Minerals Engineering later in the year.

Mark Fraser, Gold & Minerals Gazette, Australia. Email: mfraser@riu.com.au

For more on the Gravity Concentration'06 Conference, see the April edition of Gold & Minerals Gazette)



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