Recent advances in the automated sensor-based sorting set of technologies enable diamond producers to enjoy the environmental and economic advantages that are becoming increasingly helpful in a harsh economic climate.

Sensor-based sorting is a rock-by-rock analysis system that scans each individual rock processed at a diamond mine with a sensor to determine if the rock is waste material or mineral bearing.

“The sensor detects an image of a particle and makes a decision based on differences in the images of each individual rock. After detection has taken place, the physical sorting process takes place, where the rock is ejected by means of compressed air nozzles,” says TOMRA Sorting Solutions GM Lütke von Ketelhodt.

He explains that a series of tests conducted in recent years by TOMRA Sorting Solutions in South Africa has shown that a combination of sensor-based sorting technologies – one using colour and near-infrared spectroscopy (NIR) scanning and the other X-ray transmission (XRT) – significantly improves diamond recovery, reduces diamond breakage and reduces overall energy consumption.

“Waste-rock sorting from kimberlitic ores was a challenging task in the past, owing to the complex variety of kimberlitic ores and their associated waste-rock types,” Von Ketelhodt notes.

He adds that traditional diamond-processing flow sheets make no provision for the preconcentration of kimberlitic ore as such and, as a result, the plant processes all material, including much avoidable waste.

“Once in the plant, the kimberlitic ore is processed using complex crushing, screening and preconcentration through dense-media separation (DMS), after which it is moved into traditional X-ray luminescence sorters,” states Von Ketelhodt.

The disadvantage of this material flow and liberation process is that large diamonds between the various secondary and tertiary crushing stages could be damaged.

Also, if diamonds are coated or have low-luminescent properties, the traditional XRT sorters will have difficulty in recovering the diamonds.

With the introduction of an NIR waste-sorting step prior to processing, producers can increase the overall diamond grades by sending only diamond-bearing kimberlite to the recovery plant, which will improve the recovery efficiencies.

“This sorting step significantly reduces plant wear and improves the energy consumption of diamond plants,” he says.

NIR scanning creates a ‘finger- print’ of each sample that is directly related to the sample’s mineralogical composition, and can sort according to signals that are a direct result of each sample’s mineralogical composition.

With the addition of XRT sorting, the need for a coarse DMS circuit is removed, as large free diamonds between secondary and tertiary crushing can now be directly recovered.

This increase in the recovery of large diamonds between the crushing stages and the reduction in large-diamond breakage will increase the value extracted by a diamond operation.

“Another important advantage of XRT sorting is that it specifically targets carbon-based materials. The XRT sensor detects and distinguishes diamonds on the basis of X-ray image processing, which correlates with the atomic densities of the particles,” notes Von Ketelhodt.

He adds that it is, therefore, irrelevant whether diamonds are coated or type II low-luminescent diamonds, as 100% of all free diamonds will be detected.

Recent pilot work done by TOMRA Sorting Solutions in respect of colour, NIR waste-rock sorting and XRT sorting conclusively showed how sensor-based sorting can be used to provide a revolutionary new flow sheet, from preconcentration of kimberlitic ore for upfront waste removal to final recovery.
“The colour and NIR tests were conducted at the TOMRA Sorting Solutions test facility, at Mintek, in Johannesburg, on kimberlitic samples provided by the Good Hope diamond mine, near Barkley West. The samples were scanned and sorted using the prosecondary colour and NIR sorter, a combined sensor technique developed by TOMRA, which uses a high-resolution line-scan camera and an NIR scanner to accurately detect the mineral footprints of each rock,” notes Von Ketelhodt.

During the tests, an overall kimberlite recovery rate of more than 95% was achieved with the two-stage sorting approach, which included a low-sensitivity primary rougher and a secondary high-sensitivity scavenger step on the tail of the first pass.

XRT

The XRT tests were conducted at the Letšeng diamond mine, in Lesotho, where there is a need to recover large diamonds prior to the secondary and tertiary crushing stages to prevent large-diamond breakage.

“The tests were aimed at establishing not only whether the sorter could detect and recover all types of diamonds, including stones with low luminescence, but also whether XRT technology could be used as an alternative to process the coarse material.

“High-capacity XRT sorters are essential to replacing DMS with a dry sorting process, in which the diamonds are detected and recovered directly,” Von Ketelhodt says.

Tests were conducted using tracers comprising cubical polyurethane particles, which have an atomic density similar to that of diamonds. For the bulk testwork, kimberlite material was crushed and delivered to the plant where the pilot XRT tests were conducted.

The purpose of the bulk test runs was to quantify the performance of the XRT sorting technology on kimberlite, running at maximum capacity.

“The results were overwhelmingly positive. High throughputs in both the larger fraction (–65 mm to 20 mm) size range of up to 100 t/h and in the midrange fraction (–20 mm to 10 mm) of as much as 66 t/h were achieved at 100% to 98% tracer recovery,” he says.

This means that orebodies with a large percentage of heavy waste-rock minerals will no longer suffer from high mass concentrations, as was the case during traditional DMS processing. High-capacity XRT sorters can process the same tonnages and recover the diamonds in one step.

Von Ketelhodt states that the implications for diamond companies are clear.

“Mines that produce large diamonds also face a huge risk of diamond breakage when the final diamond recovery only starts after three stages of crushing,” he says, adding that these inefficiencies no longer need to exist.

“The new sensor-based sorting flow sheet, which incorporates colour and NIR sensor technology to remove waste rock prior to the main processing stage, coupled with XRT technology to recover free diamonds after crushing from the sized subfractions, prevents most of these problems,” he says.

The increase in recovery and the reduction in breakage and energy consumption give diamond producers a distinct advantage in the tough new business climate in which they need to operate.