Recent advances in automated sensor-based sorting mean that this revolutionary set of technologies now offers diamond producers environmental and economic advantages that are increasingly relevant in a harsh economic climate.
A brace 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 both colour and NIR (near-infrared spectroscopy) scanning, and the other a large X-ray Transmission (XRT) circuit – can significantly improve diamond recovery, reduce diamond-breakage and reduce overall energy consumption.
Waste rock sorting from kimberlitic ores has been a very challenging task in the past due to the complex variety of kimberlitic ores and their associated waste rock types.
Traditional diamond processing flow sheets make no provision for preconcentration of kimberlitic ROM and, as a result, process all material, including much unavoidable waste, through the plant. Once in the plant, the ROM ore is processed through a complex of crushing, screening and preconcentration through dense media separation (DMS), and then into traditional X-ray luminescence sorters.
The disadvantage of this material flow is that no provision is made for recovery of large diamonds between the various secondary and tertiary crushing stages. Also, if diamonds are coated or have poor luminescence properties, the traditional XRL sorters will have difficulties in recovering the diamonds.
With the introduction of a primary colour/NIR waste sorting step prior to processing, producers can, firstly, increase the overall diamond grades reporting to the plant and, secondly, reduce overall energy consumption. This sorting step significantly reduces plant wear and improves the overall performance of diamond plants.
NIR spectroscopy makes it possible to create a “fingerprint” of each sample that is directly related to the sample’s mineralogical composition. NIR offers the ability to sort on signals that are a direct result of its mineralogical composition.
With the addition of XRT sorting, the need for a coarse DMS circuit is removed. Large free diamonds present between secondary and tertiary crushing steps will now also be recovered. This increase in recovery of large diamonds between crushing steps and the reduction 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 to the atomic densities of the particles. It is therefore irrelevant whether the diamonds are coated or Type II low luminescent diamonds as 100percent of all free diamonds will be detected.
Colour/NIRRecent 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 in providing a revolutionary new flow sheet, from pre-concentration of kimberlitic ore for upfront waste removal, secondary concentration using XRT, through to final recovery.
The colour/NIR tests were conducted at the TOMRA Sorting Test Facility, at Mintek in Johannesburg, on kimberlite ROM samples provided by Good Hope Diamond Mine near Barkley West. The samples were scanned and sorted using the PRO Secondary COLOR/NIR sorter, a combined sensor technique developed by TOMRA that uses a high resolution line-scan camera and a NIR scanner to provide accurate detection of mineral footprints of each rock.
The objective of this two stage COLOR/NIR sorting approach is to reduce to a minimum kimberlite loss to waste while still providing maximum waste rejection. In the tests an overall recovery of kimberlite in excess of 95percent was achieved with a two-stage sorting approach that included a low-sensitivity primary rougher and a secondary high sensitivity scavenger step on the tail of the first pass.
The XRT tests were conducted at Letseng Diamond Mine in Lesotho, where there is a need to recover large diamonds prior to secondary and tertiary crushing stages to prevent large diamond breakage. A containerised XRT sorter was installed for a pilot campaign.
The objective of the tests was to find out 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 +10mm material. To replace DMS by a dry sorting process in which the diamonds are being detected and recovered directly, high capacity XRT sorters are essential.
Tests were conducted using tracers made up of cubical polyurethane particles, which have a similar atomic density to diamonds. For the bulk test work ROM material was crushed and delivered to the pilot XRT sorting plant.
The objective of the bulk test runs was to quantify the performance of our XRT sorting technology on ROM kimberlite, running at maximum capacity. The results were overwhelmingly positive. In the larger fraction (-65 +20mm size) range, more than 100 tons per hour were achieved at 100percent tracer recovery. In the mid-range fraction (-20 +10mm), as much as 66 tph were achieved at 98 percent tracer recovery.
This means that ore bodies with a large percentage of heavy minerals will no longer suffer from high mass concentrations that they do in the traditional DMS processes. High capacity XRT sorters are able to process the same tonnages and recover the diamonds in one step.
The implications for diamond companies are clear. Traditional flow sheets, established in the industry, have inevitable inefficiencies as, for example, in the DMS process, as well as during the final recovery using XRL sorters, which miss low-luminescent and coated diamonds, diamond losses can occur. Mines which produce large diamonds also face a huge risk of diamond breakage when the final diamond recovery only starts after three stages of crushing.
These inefficiencies need no longer be accepted. The new sensor-based sorting flow sheet that incorporates COLOR/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 sub-fractions prevent most of these problems. The increase in recovery, minimisation of breakage and the reduction of energy consumption all give diamond producers a distinct advantage in the tough new business climate in which they need to operate.
TOMRA Sorting offers ore sorting solutions from initial amenability testing and characterization to equipment sales and complete sorting lines. Its global presence with offices in Canada, Russia, Germany, South Africa and Australia guarantees mining operations local ore sorting expertise and service.
For more information contact Geoffrey Madderson, Sales Manager of Tomra Sorting in South Africa on +27 87 941 3840.