By Dr Kurt PetersenRockwell Diamonds, Douglas, Northern Cape

kurtp@rockwelldiamonds.com

Minerals processing atRockwell Diamonds

Rockwell Diamonds is a junior alluvial diamond producer that is committed to maintaining high recovery efficiencies, lowering operating costs and to process at rates that make sustainable business sense. This document outlines the key technology that Rockwell is using and the advances that have been made in the last three years. It also outlines the value proposition of Rockwell and how a culture of continual improvement is contributing to our business success.

IntroductionRockwell Diamonds is a junior diamond mining company that is primarily focused on alluvial deposits. These deposits are characterised by low grades (< 1cphm3), high quality stones and relatively easy mining. Rockwell presently has three mines under its management, processing a total of 340 000m3 per month (714 000 tonnes).

The challenge is to remove unwanted size fractions, leaving a correct size, typically +5-35mm, for further bulk reduction and final diamond recovery. Figure 1-1 shows a fairly standard processing flowsheet, which consists of:

• Oversizescalpingof+75mmusingaBarrelscreen

• Removalofremaining+35mmoversizeand-5mmsandusing standard screens and dry screening

• WashingofmaterialusingaScrubber

• Bulkreduction/concentration

• Finalrecoveryofdiamonds

Processing technologyThe primary business of alluvial diamond processing is:

• Earthmoving

• Screening

• Washing

Examplesofscreeningaregiveninfigure2.1,whichincludes Barrel screen that scalps oversize, standard screens that also further reduce unwanted oversize material and the Bivitec (right), which removes -5mm material (dry screening). Rockwell is committed to minimising water usage as much as possible.

Figure 2.1: Screening at Rockwell, including Barrel, conventional screen and Bivitec dry screening (right)

2.1 Concentration/recoveryThe traditional concentration and recovery steps are showninfigure2.2.Pans(left),adensitybasedprocessingtechnology, has been the mainstay of alluvial processing for more than 150 years. This is a particle density separation technique that relies on diamondiferous material being heavier than most of the gangue material. It typically reduces the incoming amount of material by 90%, concentrating heavy minerals. Pan efficiency is pinned at approximately75%.

OntherightisastandardFlowsortdiamondrecoveryX-ray machine that uses X-rays to produce luminescence. Diamondsandarangeofothermineralsexhibitthisproperty. The final reduction of Pan concentrate is down to only a few kilograms per shift.

Figure 2.2: Typical Pan/flowsort combination that reduces material bulk and then recovers luminescent material

2.2 Dense Medium Separation (DMS)RockwelldoesuseaDMSononeofitssites. It is a technology that is known to be more efficient than Pan technology, however, its use is inhibited by its costs, whichareprimarilyduetoFeSilosses.Calcrete,amainconstituent in alluvial gravels, is notorious for soaking up considerableFeSiasitisprocessedthroughthesystem.

2.3 High throughput X-ray sortingSince2012,Rockwellhasembarkeduponaprogrammeto perform both concentration and recovery using Bourevestnik high throughput X-ray technology. It works on a similar principle to that of standard X-ray machines, exceptthatitisabletoprocessatextremelyhighrates,viz. +50tph. This technology has been in use in Russia for more than four (4) decades, but has only recently been employedinSouthernAfrica.Therearenewcompetitorsto this machine emerging. The main benefits of this type of technology are:

• Doesnotdifferentiateondensityofminerals

• SmallfootprintcomparedtoPanplantorDMSplant

Rockwell is committed to using this type of technology on all new operations. Its efficiency is higher than that of both PansandDMS.

Figure 2.3: Example of high throughput X-ray technology

Processing efficiencyThe important aspect about alluvial diamond processing is that most of the revenue is located within the coarse stone sizes. Table 3-1 makes this point by showing the recovery efficiency of the three mines, where the carat recovery rangesfrom64%to82%(datafrom2013).However,allrevenue recoveries are above 85%, with the high throughput X-raytechnologyatSHCindicatingaveryhigh93%.

It has to be noted that Rockwell recovers stones no smaller than 5mm (1 carat). This decision has been based upon the revenue deportment per size fraction, where it has been shown that it makes business sense to “chase” the larger size fractions at an appropriate throughput rate.

Table 3.1: Approximate carat and revenue recoveries at Rockwell mines

Efficiency SAX SHC NJKCarat recovery 64% 82% 68%

Revenue recovery 85% 93% 87%

Rockwell’s value propositionTable4-1showscontextforRockwell’sbusinesscomparedto that of a typical kimberlite mine. The important points to make here, which have a direct impact upon processing requirements:

• Lowinsitugradesinalluvials

• Lowminecostofalluvials,butequallyrelativelylowinsiturevenues to be recovered

• Revenuerecoveryefficiencyequaltothatofanymajorkimberlite mine

Table 4.1: Comparative processing data for Rockwell mines and a kimberlite mine

EfficiencyInsitu

gradeMined

costRevenue

efficiency RevenueRDI (traditional) 0.7 $8/m3 85% $11.5/m3

RDI (X-ray plant) 0.7 $7/m3 93% $16/m3

Revenue recovery (%) 70-200 $22/m3 95% +$120/m3

The major point is that the minerals processing function at Rockwell is committed to:

• Highestpossiblerevenuerecoveriesatthethroughputsrequired

• Loweringprocessingcosts

• ContributingtothevaluepropositionofRockwellasasustainable alluvial diamond producerFigure 1.1: Standard processing flowsheet

Uncovering the facets of our potential… ACKNOWLEDGEMENTWith thanks to the people at Rockwell for their support and encouragement.

420 x 594 Kurt.indd 1 2014/09/03 9:51 AM