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A golden new method

By: Paul Hayes
10 March, 2010

The practice of in-situ leaching has traditionally been largely exclusive to the uranium industry.

In simple terms, in-situ mining is the extracting of minerals from the ground without digging holes and crushing the ore when it is brought to the surface.

In-situ leaching involves drilling holes in a mineralised deposit, pumping through liquids in order dissolve or attract the target mineral, and then recovering the liquid for further processing.

It is estimated that up to 20% of the world's uranium is recovered through this process, including the Beverly uranium operation in South Australia.

Despite the method being most often used in uranium mining, the CSIRO's Minerals Down Under Flagship is conducting in-situ leaching research involving the use of a number of chemicals to successfully recover gold from near surface oxide ores.

Given the differing mineral properties of gold and uranium, in-situ leaching for gold presents inherent challenges, including finding a chemical other than the traditionally used cyanide in order to target underground gold.

In addition, researchers have had to address the permeability of oxide gold ores, which are less permeable than the rocks in typical uranium in-situ leach operations.

The CSIRO said that without artificial permeability enhancement gold extraction rates are likely to be too low to remain financially viable.

In order to combat the first problem, the CSIRO successfully trialled the use of sodium thiosulfate and ferric EDTA in weathered oxide deposits in Western Australia.

The oxide deposits were important because they do not contain iron sulfide, also known as pyrites, or 'fool's gold', which breaks down the chemicals used to recover the gold.

Focusing on the oxide deposits saw gold recoveries of between 60 and 95% achieved in laboratory tests, the CSIRO reported.

According to financial modelling, the in-situ method could potentially prove very cost effective; with results showing that after some permeability work gold could be extracted at an operating cost of between $400 and $650 per ounce, which is approximately half the current gold price.

Studies into improving permeability enhancement for in-situ gold extraction have only recently begun, but the CSIRO said it is considering methods consisting of either mechanical rock breakage via hydraulic fracturing methods or explosives and/or chemical dissolution of gold-hosting minerals.

The CSIRO said that the immediate plan is to identify several deposits in order to conduct further studies.

The next phase of work has also been granted $657,000 in CSIRO funding, with an additional $150,000 being sought from industry sponsors.

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