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:: Mining Gold Without a Mine
In Australia, extracting minerals without first digging and crushing the ore is largely confined to uranium. Yet in theory, Australia’s remote and marginal gold deposits - not suited to conventional mining - could be made economically viable if a CSIRO study on in situ (or in-place) leaching of gold delivers on its early promise.
Paul Roberts, who leads the in situ leaching research for the Minerals Down Under Flagship, says early laboratory tests with a number of chemicals had successfully recovered gold from near surface oxide (or weathered) ores.
“There are a number of steps we need to take to establish the commercial potential of in situ oxide gold ore leaching, but we can already see a strong financial case to support the science," he says.
In its most simple form, in situ leaching involves drilling holes in a mineralised structure, pumping liquids that dissolve or attract the target mineral down the holes, and then recovering the pregnant liquid for processing. An estimated 20 per cent of the world’s uranium is recovered in this way.
However, gold is different and faces two challenges to in situ leaching: the identification of chemicals other than cyanide that will target gold, and addressing the permeability of oxide gold ores. These are less permeable than the rocks in typical uranium in situ leach operations, meaning that without artificial permeability enhancement, gold extraction rates are likely to be too low.
The first challenge, to find a replacement for cyanide - traditionally used as the lixiviant (a chemical used to extract or attract gold) in gold processing - has led to the successful use of sodium thiosulfate and ferric EDTA (or ferric ethylenediaminetetraacetic acid).
However, as if the hunt for gold scouring chemicals was not tough enough the original in situ work on ore from a Western Australian (WA) goldmine was derailed by the presence of iron sulfide, also known as pyrites, or ‘fool’s gold’, which broke down the lixiviants.
“It was at that point that we changed the approach, looking at what had been achieved; we refocused on gold deposits without pyrite and that led us to look at weathered gold deposits," Mr Roberts says.
Across the southern portion of WA, and over the border in South Australia, there are many potential heavily weathered targets where the pyrite has been leached out over the eons.
Refocusing research work onto oxide gold deposits soon delivered better results, with gold recoveries rising to between 60 per cent and 95 per cent in bottle roll tests, with the higher level considered good even by conventional gold processing standards.
However, the most eye-catching aspect of the work was a financial model which showed that, with the right type of ore deposit, gold might be extracted by an in situ leaching method involving some permeability enhancement at an operating cost of between A$400 and A$650 an ounce - about half the current gold price and enough to have most gold companies thinking carefully about adding a new extraction technique to their armoury of technologies.
The second challenge - to improve permeability - has led the team to consider permeability enhancement methods, consisting of either mechanical rock breakage via hydraulic fracturing methods or explosives and/or chemical dissolution of gold-hosting minerals to improve lixiviant access to the gold itself. Studies on developing these options have only just begun.
Mr Roberts says a number of issues require further detailed study, including a close look at the nature of the rocks in an orebody, the effect of permeability enhancement methods on gold recovery, the time it would take for the lixiviants to pass through the rocks, the number and spacing of drill holes, and other costs such as piping and tanks to hold the liquids.
“The critical targets we have set are to achieve 50 per cent gold recovery with a breakthrough time, which is the time it takes between injecting and extracting the fluids, of 20 days. Widening the hole spacing through permeability enhancement methods is also a key focus of the research, as it offers the potential to substantially reduce capital costs," he says.
The plan now is to identify several deposits on which field-based studies can be carried out, preferably with a number of partners interested in the science involved and prepared to co-fund the next phase of work, which has been earmarked to receive A$657,000 in CSIRO funding, with A$150,000 being sought from industry sponsors.
A number of mining companies and government geological surveys have expressed interest in supporting the next phase of research. Field studies involving the injection of lixiviants are likely to start in 2011, once permits and environmental approvals have been obtained.
This article originally appeared in the February 2010 issue of CSIRO’s Process magazine.
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