Gamma rays on gold - new method for gold detection

Materials World magazine
,
24 Sep 2013

While an Australian gold concentrator plant typically produces around AUS$1bln of gold each year, this often represents just 65–85% of the gold present in mined rock. The net result is hundreds of millions of dollars worth of gold left unrecovered.

To address this issue, a collaborative team at CSIRO and Canadian X-ray sourcing company, Mevex, has devised a gold detection technique using gamma activation analysis (GAA) that it claims can offer two to three times more accuracy than traditional chemical analysis.

Lead researcher at CSIRO, Dr James Tickner, explains, ‘Gold has always been a measurement challenge, because it is mined commercially at such low levels (typically around one gramme of gold per tonne of rock), so the analysis technique has to be very sensitive. GAA was used industrially in the 1980s by the former Soviet Union to measure gold. However, they struggled to achieve the necessary sensitivity.’

The technique is straightforward. A sample of material – typically about 500g of crushed rock – is placed in a plastic jar. The sample is then irradiated using high-energy X-rays produced by an accelerator. The X-rays activate the gold in the sample, making it mildly radioactive for a few seconds. The sample is then moved quickly across to a sensitive detector that picks up the radiation emitted by the gold. By counting the number of gamma rays detected, it is possible to determine the number of gold atoms in the sample and, therefore, the concentration of gold.

Tickner explains, ‘The main challenges of the existing analysis fire assay method are that it measures the gold in a small mass of material, typically 20–30g. Gold can be quite coarsely and irregularly distributed and this is a big issue, as reducing a large mass of material, often many tonnes of ore, down to a small sample in a representative way can be difficult. Secondly, fire-assay is a destructive method, with the sample being fused in a furnace as part of the extraction of the gold. This means that samples cannot be re-analysed or cross-checked and the method is slow and laborious, involving several steps. So measurements have to be carried out in centralised labs, often leading to a turnaround time of several days.’

GAA overcomes these hurdles because it works with larger sample masses of up to 500g, and fundamentally counts atoms to determine gold content independent of the chemical or physical form of the gold or the sample matrix. The method is nondestructive and samples can be re-analysed. The technique can also be automated, with samples analysed in as little as three minutes.

But Tickner adds that it is the economic benefits that stand out. ‘A certain portion of incoming gold will never be economically recoverable. However, anecdotal feedback from professionals in the industry is that real-time data on recovery can reduce losses by about one third. Even a 5% improvement in recovery would be worth around AUS$500m a year to Australia. Last year, Australia produced about AUS$10bln of gold, less than 10% of the world’s total gold output, meaning the benefit from a global application of the technology could be significant.’

Furthermore, the technique can be deployed locally, which opens up further applications in real-time data for exploration or mapping of existing deposits.

Tickner adds, ‘We have also carried out tests to establish that the GAA can be used for platinum, palladium and rhodium, and we have done preliminary studies on tin, copper, zinc, silver and lead, all of which could be analysed using this method.’

The prototype GAA facility will be set up in Australia within the next two years.