Spotlight Feature Articles - GOLD PROCESSING IM November-December 2022 - Page 6

process where the elemental PGNAA response is characterised and then tested on real material samples . The analysis technique responds differently for different elements , based on the properties of the elemental nuclei , with some elements responding better than others .
Gold had been predicted to have an excellent response to the technique , however difficulties in the characterisation step have historically rendered the element elusive for online analysis on conveyed ore flows .
Yet , in recent years , advances have been made to measure otherwise difficult elements of interest – directly or via proxies – including mercury ( directly ), which is typically present at low concentrations , lithium ( by proxy ), which has a challenging response to the technique , and even platinum ( by proxy ), which is both challenging and at low concentrations .’
Successes in these industries prompted significant interest in examining the gold measurement possibilities .
Newcrest and Scantech undertook a research project to characterise , test and verify gold analysis using Scantech ’ s high specification GEOSCAN PGNAA analyser , which ultimately resulted in a GEOSCAN unit being installed and successfully functioning at the Telfer gold mine , in Western Australia .
The GEOSCAN on-belt analyser uses thermal neutron capture technology to determine the elemental composition of conveyed bulk materials in real time . A Californium-252 radioactive source located within the instrument below the belt generates a flux of neutrons , which are then absorbed by the conveyed material as it passes through the tunnel of the GEOSCAN . The neutrons are captured by the nuclei of the atoms in the material flowing on the belt and gamma-rays are instantaneously produced at specific energies for each element .
The gamma-ray spectrum is captured by an array of high-performance detectors located at the top of the GEOSCAN where Scantech ’ s signal processing algorithms resolve the signal into a set of individual elemental results .
The measurement technique is completely penetrative , allowing for analysis of the full material stream and entire bed depth and belt width , according to Scantech . It is also independent of ore mineralogy , particle size and belt speed , thereby allowing for fully representative analysis of the conveyed material .
Scantech , with its GEOSCAN GOLD analyser , believes it has got closer than any other company in achieving real-time analysis and direct measurement of gold
The Cadia installation had been set up to provide primarily rapid analysis of copper grade , as well
as other elements , with results being reported every 30 seconds to the plant .
The copper and other elements formed part of a standard , high specification GEOSCAN installation where the intention was to use the GEOSCAN and other analysers for classification and potentially diversion of the material . It was known that the Cadia deposit included gold at low concentrations , and since the GEOSCAN had already been installed and was working successfully , it followed logically to test the newly-developed gold characterisation models on Cadia ’ s material .
Cadia ’ s gold concentration was observed to share reasonable correlation to copper and sulphur , with both elements already calibrated and working on the GEOSCAN . It was determined to undertake two tests for gold measurement , with one being the direct measurement using the characterisation model and the second using a correlation model based on the measurement of copper and sulphur . Both systems would operate in parallel .
Samples of the conveyed flow were obtained , and a comparison made between the laboratory analysis of the samples and results from the GEOSCAN . As well as the usual suite of elements for analysis , gold was included in the laboratory assay data .
A number of calibration curves were set up to assess gold analysis performance by proxy measurement of copper and sulphur , as well as the direct measurement from the GEOSCAN . The process involved determining a suitable analysis period for the GEOSCAN unit to integrate over for the direct gold measurement .
Due to the low concentration of gold in the
ore , the counting statistics for direct gold measurement were significantly different to the statistics of the more abundant elements of copper , sulphur and others . While it was possible to maintain 30-second analysis periods for copper and sulphur , and , thus for the proxy measurement of gold , it was found that longer measurement periods of five to 10 minutes were required for the direct measurement of gold to ensure that measurement repeatability was within the expected bounds determined by the duplicate sampling process .
The response for directly measured gold was improved compared with the proxy measurement , with both improved root mean square deviation performance and better correlation . In addition to the improved robustness of direct measurement over relying on an approximated model , the direct measurement offers a clear advantage , Scantech said . These results were used to justify the next stage of installation and testing a GEOSCAN at Telfer gold mine , which led to a gold analysis module being fully calibrated by the start of 2021 .
Telfer does not have a correlation between copper or sulphur and gold , and , thus , relies only on the direct measurement of gold by the GEOSCAN . The characterisation and verification work at Cadia suggested a good response would be expected , despite significant differences in the orebodies .
“ The results for direct gold measurement across the larger sample set available at Telfer validates the measurement technique and confirms that the GEOSCAN can provide effective accurate direct measurement of gold in ore , even at typically low concentrations when considering the magnitude of sampling errors ,” Scantech concluded .
The company says the Telfer operation is currently using the analyser data for gold grade trend monitoring , with the initial work carried out at the operation leading to “ finetuning calibration work ”.
Test work on gold ores from several other companies has resulted in further GEOSCAN GOLD sales , according to the company , with Scantech having deployed analysers measuring gold directly over 5-10-minutes of conveyed flow and / or via proxy elements every 30 seconds of conveyed flow in the likes of Australia , Canada , Peru and the US .
The company concluded : “ Gold measurement data enables mines and process plants to improve their ore quality and process performance in ways they have been unable to do previously . The availability of the gold grade data provides visibility when they were previously in the dark about ore grades and the variability feeding their
plant .”
International Mining | NOVEMBER / DECEMBER 2022