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:: Collaboration with Gravity Man Yields Gold
Over the last five and a half years, Associate Professor André Laplante from McGill University in Canada and Parker Centre gold researchers have worked together to increase gold recoveries in the gold industry and run educational workshops for industry professionals.
Professor Laplante is an internationally recognised expert in gravity methods for recovering gold from its ores. The collaborative research between the Centre and Professor Laplante has highlighted the advantages of intensive cyanidation over traditional tabling methods for the treatment of gold concentrates obtained from gravity devices.
This work has also delivered a computer model which allows gold plants to estimate how much gold they can recover by gravity from their particular ore.
Professor Laplante’s relationship with the Parker Centre began in 1999 when he led a Centre workshop on gravity gold recovery. He subsequently joined the AMIRA P420B 'Gold Processing Technology' project team when this project started in April 2001. His regular visits to the Centre since then earned him the inaugural title of Parker Centre Honorary Visiting Researcher in recognition of his contributions. During his visits, he has participated in surveys of gravity circuits at gold mines in Australia, South Africa and Tanzania and run three further gravity gold workshops.
Other than picking nuggets off the ground, gravity concentration is the oldest method of recovering gold, and has been used for thousands of years. Impure gold in an ore is 16 to 18 times heavier than water while the associated waste rock (gangue) is less than three times as heavy as water. "So gold is much denser than the gangue minerals, which means you can recover it by gravity methods that use that difference in densities," says Professor Laplante. For example, gold panning uses shaking and washing with water in a shallow pan to sluice away the lighter gangue while the denser gold stays in the bottom of the pan.
Advanced equipment and economic and environmental pressures have increased the popularity of gravity concentration. "In the past twenty years, some new machines that can very effectively concentrate gold by gravity have been developed," Professor Laplante says.
"Centrifuge units and jigs produce small masses of high-grade gravity gold concentrate," says Professor Laplante. He says that traditionally, these concentrates have been further physically concentrated by a shaking table in the gold room, before smelting and pouring into bullion bars. "Part of our work has investigated the link between gravity recovery and hydrometallurgy and how gravity can assist the overall gold recovery."
Since 1999, two new commercial devices have been increasingly used for processing gravity concentrates via a hydrometallurgy route rather than the pyrometallurgy route of smelting. Both Gekko System’s InLine Leach Reactor and the Consep ACACIA Reactor use intensive cyanidation, with gold recoveries of more than 95%. Gold from the resulting gold solution can then be directly recovered by electrowinning.
As gravity gold concentrates are a fraction of the original ore, only a small amount of cyanide is needed to treat these concentrates at high cyanide concentrations.
Professor Laplante says that one of the significant findings of the AMIRA P420B project was the impact of utilising intensive cyanidation for gravity concentrates. "We found that intensive cyanidation increases gravity gold recovery as it is much more effective than the traditional gold room."
"Gravity circuit audits by the P420B project provided some recommendations for improvements but showed that the traditional gold room was the weak link with usual recoveries of 50 to 60% or even lower. We then said we know of intensive cyanidation systems which give recoveries of 95 to 99%, and suggested one or other be tried."
He says that more and more gold rooms are being converted into intensive cyanidation units with electrowinning, whilst retaining a furnace to pour gold from the electrowon gold cathode. "So, the gold room is changing dramatically from what it used to be six years ago and P420B facilitated that change."
"Some of the plants that have switched from the classical gold room [with shaking tables] to the chemical gold room so to speak have seen increases in overall plant recovery of 1% or more, which is quite significant," says Professor Laplante. Security is also improved by eliminating manual handling of gold concentrates. In addition, intensive cyanidation has environmental benefits and low operating costs.
Intensive cyanidation reactors for gravity concentrates have now been installed at over 45 gold operations in Australia, Africa, Canada, South America and Asia. The completion of the AMIRA P420B project in June last year has not ended Professor Laplante’s association with the Centre.
He will also be involved in the new AMIRA P420C extension project: in gravity circuit surveys at gold plants, further development of the gravity computer model, technology transfer and training.
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