Picking diamonds in the rough

Diamond sorting has taken another step forward in precision with the field testing and release of a new X-Ray Transmission machine, currently in use only in South Africa.

Sorting solutions manufacturer Steinert has made some rapid advances in precision diamond sorting over the past year thanks to continuing development of XTS technology.

The new XST machines are capable of reliably detecting diamonds of different sizes, from large diamonds locked within kimberlite, all the way down to diamonds as small as 3mm in diameter, a previously improbable achievement.

Steinert Business Development Manager Kai Bartram said the XTS unit is designed to recover diamonds with a size of 4mm and larger, but even 3mm large diamonds can be recovered efficiently due to the sophisticated algorithms paired with a high pixel resolution of 0.5mm.

“The Steinert XTS is outstanding for the recovery of large diamonds but precise and reliable while finding the small diamonds, which are harder to detect,” he said.

Currently in use in South Africa, the testing phase demonstrated the XRS unit’s capacity to achieve 100 per cent recovery.

During the test work, separation algorithms were further developed for detecting and ejecting diamonds and hardware optimizations were carried out at the same time, which meant the equipment had to be re-designed to fit the client’s footprint requirements. 

During this process, Steinert integrated the latest developments in the detection equipment as well as improvements in the algorithms developed during the test work. 

Steinert chief technology officer Dr Uwe Habich said the result of the algorithm development was that they were able to customize the sorter to fit the required footprint, while retaining the maintainability through using standard components.

“We have improved all areas of performance including detection and ejection into a secure area, all while obtaining the lowest possible yield to increase the diamond by weight ejected,” he said.

During commissioning, a 2-carat diamond was recovered from tailings material that was fed to the machine, while shortly after a 50-carat GNT type 2, 75 per cent unbroken diamond was recovered, demonstrating the wide versatility of the XTS system.

Steinert Australia general manager Johan van Zyl said the machines were specifically built to ensure recovery of large diamonds without the risk of value loss through destructive crushing.

“If your ore body does contain large free diamonds, or even large diamonds encased within pieces of kimberlite, if your crusher is set to 20 or 30mm, quite often a larger diamond will be broken up in shards in the crusher,” he said.

“That’s not good for anybody, once its broken its worth very little.

“The X-Ray transmission sorter, before you go into DNS and downstream and the final concentration processes, will be able to pick up these large free diamonds and remove them from the crushing stream, so they don’t go into the crusher.”

Steinert now lays claim to being at the cutting edge in diamond recovery and diamond sorting technology, with its main advantage being that it detects different x-ray absorption levels of different material types.

The resulting x-ray images relate to the atomic make-up of each particle going through the detection zone of the machine.

Therefore, a diamond, which consists of carbon atoms, appears as a light-grey x-ray image when compared to denser waste rock and, because this is sorting on an atomic level, all diamonds, no matter what their physical characteristics, can be detected using the XTS technology.

“The difference with the current X-ray machines for fines recovery is what’s called an x ray luminescence machine, whereby the diamond is lit with florescence under X-ray radiation, and then you have a colour camera basically picking up a flash,” van Zyl explained.

“With the XTS sorter you have an X-ray source on one side of the material and a detector on the opposite side, and you measure the amount of radiation transmitted through the material to determine what each particle is made up of.

“The major benefit of XTS over X-ray luminescence sorting is that not all diamonds luminesce; it’s dependant on the nitrogen content of the diamond.

“But because our XTS sorters are looking for the carbon signal, we don’t miss any diamonds the way an X-ray luminescence machine could do.”

From a practical perspective, the diamond ore is presented to the sorter via a vibrating feeder which feeds it onto a belt as a single particle layer. 

The ore travels on the sorter belt and the individual particles are scanned as they pass through the x-ray beam.

The particles attenuate the transmitted x-rays which are detected and recorded.

The signals are processed through an advanced algorithm that enables the program to differentiate between the less-dense carbon of diamonds and the denser minerals of kimberlite and other waste rock.

Steinert sensor sorting technology is most relevant to today’s mining conditions where, increasingly, challenges such as low-grade ore reserves, high-energy costs, water shortages and stringent environmental legislation need to be overcome. 

Sensor-based sorting has brought about significant innovation in mineral processing technology and upfront beneficiation and it's time has certainly come in South Africa.