Crushing, sorting, grinding, leaching. As long as man has been extracting ores from the ground, he has been searching for better, more efficient ways to separate the valuable minerals from the gangue (sand, rock and other impurities). As in other fields, the technology of mineral processing has evolved over time. However, in the face of escalating costs and ever-increasing environmental constraints, the thrust for innovation and technological advances in mineral processing is gaining ever greater momentum and significance.
In the base metals industry, mineral processing most often involves upgrading of the ore and concentrating the valuable mineral fraction via flotation. According to Gus Van Weert, mining consultant and president of ORETOME Ltd., ore sorting has not secured a proper foothold in the base or precious metals industry due to the reliance of most sensors on rock surface characteristics. “It has not been very useful in the case of sulphides, and non-existent in the case of gold,” he said.
A new wave of sorting
It has been in industrial minerals and uranium processing that sorting has been most widely adopted. “Since very early on, radiometric ore sorters have been applied in the pre-concentration of uranium ores,” said Van Weert. He went on to explain that for industrial minerals, liberation occurs on a different scale. “Most industrial minerals are mined from massive deposits, which, in the past, required no upgrading. Many of these operations have stockpiles of earlier discarded below-quality rocks, which are now being reprocessed with the help of sorting based on ore colour.”
Van Weert said that the economics of this are very favourable, from zero or negative value — as stockpiles often cannot remain after mine closure — to full product value for at least a portion of the stockpile. Similarly, in the future, the economics of overburden and cutoff grade ore sorting could be very attractive under the right circumstances, and such treatment could well become an integral part of open pit mine development.
With the support of Barrick Gold Corporation, Van Weert has developed and tested a method by which microwave heating and infrared sensors are used to sort sulphidic ores. Utilizing an infrared camera to gauge the rise in rock temperature, Van Weert subjected a number of ore samples to microwave radiation, measured the rise in temperature and analyzed the results obtained. He found that rocks bearing sulphides respond preferentially to microwave radiation, allowing for identification by an infrared (IR) sensor. Where there are sulphides and carbonaceous material present, the hotter fraction contains both, and could be used as roaster feed. Where there is no carbonaceous material, sorting of rocks by sulphide content appears quite straightforward.
Based on these initial results and subsequent testing, all of which appear promising, Process Research ORTECH (PRO) is building a prototype “sulphide spotter” for use on drill core. This technology builds on earlier work on the recognition of sulphide veins, clusters or stockwork through brief exposure of the drill core to microwaves, followed by infrared (IR) sensing.
The hot spots in the image represent sulphides, and these images can be computer processed to build a mine model of the actual sulphide emplacement. Barrick has allowed all of this work to enter the public domain to encourage innovation in the mining industry, and Van Weert is working with CommoDas, a German sorting machine manufacturer, to apply this technology to their hardware. As a sign of interest, parallel development in this area has begun at the Technical University of Aachen.