Viability of optical sorting of gold waste rock dumps - · PDF fileVIABILITY OF OPTICAL...

VIABILITY OF OPTICAL SORTING OF GOLD WASTE ROCK DUMPS 271

IntroductionMining activities around the Witwatersrand complex havebeen going on for over 120 years. Waste rock from shaftsinking and development operations has been dumped onmassive waste dumps throughout the area. Inherently, thesedumps contain a fair amount of gold which was misplacedduring the tramming and hauling operations.

In recent years, gold mines reclaimed some of this goldby screening off the fines, 16 mm and blending it with therun of mine as a feed to the processing plant. These finesusually have higher 0.8 to 1.0 g/t gold content compared tothe dump’s overall grade which normally ranges between0.2 to 0.9 g/t. Some mines even send the entire waste rockdump to the plant to make sure that all the gold getsrecovered. This is typically done at Anglo Gold Ashanti’sVaal Reefs operations and Goldfield’s Driefontein Mine.

A number of mines have tried hand picking of gold ore,for example, at Buffelsfontein Gold Mine and also at KloofGold Mine. Usually, these operations failed due to theinconsistencies and inefficiencies of such an operation.Particularly in the finer size ranges of 50 mm hand pickingis labour intensive and difficult to produce adequatetonnages.

In some Witwatersrand ore, a relationship between goldand uranium, related to a particular reef has been exploitedby using radiometric sorting to selectively pre concentratecoarse gold (and uranium) bearing rock from waste rock1.In the 1970’s and 1980’s, radiometric sorters were used atBuffelsfontein Gold Mine. Density separation techniqueshave not worked for this application where all the rocktypes have a similar specific gravity.

Optical sorting, using colour and brightness properties ofthe different rock types, was seen as a potentialbeneficiation technology for this waste rock sortingapplication. Extensive test work was conducted at anoptical sorting test plant at Mintek before a pilot plant waserected and operated at Kloof Gold Mine.

Sensor based sorting technologyOre sorting itself is not a new concept, with hand sortingbeing one of the first methods of minerals processing.Electronic ore sorting equipment was first produced in the

late 1940’s (Wills 1992)2. Although still a relatively smallindustry, ore sorting equipment can be applied to a varietyof different applications. ‘Ore Sorting involves the appraisalof individual particles and the rejection of those particlesthat do not warrant further treatment’ (Wills 1992)2. Salterand Wyatt (1991)3 discuss that the sorting process can bedivided into four interactive sub-processes:

• Particle presentation• Particle examination• Data analysis• Particle separation.Feed preparation is more critical for sorters due the

importance of surface characteristics and physical size ofthe particles. Most sorters need a 3:1 or 2:1 ratio betweenthe largest and smallest particle to be efficient. Once theparticles have been properly prepared for sorting, they mustbe presented to the sensor. To operate efficiently, the sensormust be able to analyse each single particle. As a result,feed rate and the materials handling methods are the criticalcomponents, with this most commonly being done by aconveyor belt or chute (Wotruba, 2006)4.

The critical stage of examining the particle anddetermining whether material is valuable or barren, is doneby a combination of sensor and processing unit. Once thedecision of has been made as to accept or reject a givenparticle, a mechanical device is required to physically makethe sort. High pressure jets of air, or water, and mechanicalarms or paddles are generally used to make this separation.Of all the components in a sorter, it is the choice of sensorthat controls the design of a sorter (Weatherwax, 2007)5.

VON KETELHODT, L. Viability of optical sorting of gold waste rock dumps. World Gold Conference 2009, The Southern African Institute of Mining andMetallurgy, 2009.

Viability of optical sorting of gold waste rock dumps

L. VON KETELHODTCommodasUltrasort, South Africa

During the period October 2003 to June 2004, a containerized optical sorting pilot plant wasoperated at Kloof Gold Mine on a waste rock dump. In this paper, we describe the test work andits results leading to the decision to operate this pilot plant. This paper presents the operationaldata and financial figures achieved during this project, in particular, for the month of June 2004when the plant was run at full capacity on three shifts, 24 hour operation. These results are alsoextrapolated and presented to show the profitability of such an operation at today’s gold pricelevels and cost structure.

Figure 1. Two types of sorting systems: Chute vs. Belt (Harbeck,Kroog, 2008)6

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WORLD GOLD CONFERENCE 2009272

A multitude of different sensors are available, and thechoice is generally driven by the mineralogy of a given ore.Optical sensors are the most common sensor type, whichhas been very successfully used in the industrial mineralsindustry (Wotruba, 2006)4.

Test work7

Preliminary test on small sample

The material supplied for the test work comprised of sevensamples of the most common rock types found in the dumpas well as a 1/2 ton randomly selected bulk sample. Therandom sample was taken from the <75 mm >30 mmstockpile using a front end loader. The various rock typesamples were used to formulate a sorting program as wellas emulate a sweetened synthetic feed.

Table II shows the Au grades of the seven rock types.Note that the dolomite, lava, green and grey quartzite wereassigned the same grade, since, these samples wereidentified as waste and were submitted as a single sample.

Table IThe electromagnetic spectrum and the different sensors available for sensor based sorting in mineral processing (Harbeck, Kroog, 2008)6

Figure 2. Kloof Gold Mine: Gold-bearing rock types

Table IIAu head grades of the various rock types

Rock types Au (g/t)VCR 14.50Cobble 3.70Marginal 0.55Dolomite <0.08Lava <0.08Green Quartz <0.08Grey Quartz <0.08

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Prior to submitting the rock types for assaying, theclassified rocks were used to establish a sorting algorithmand a synthetic feed for optimizing the optical sorter

Program set-up and designClassification of the most common rock types:

• VCR• Cobble• Marginal reef• Dolomite• Lava • Green quartzite• Grey quartzite.Line scan images were then taken of each rock type, after

which the colour and brightness were analysed, therebyallowing distinct colour classes to be defined. Thedolomite, lava, green and grey quartz were defined as wasteand given the accept function in the programme.

The VCR, which was in the minority, was defined as theproduct and given the reject function in the program. Thecobble and marginal reef were also included in the VCRcolour cloud. All particles defined as product in the feedstream, would then be rejected by the sorter to the productstream.

Figure 4 shows the classification of the different colourclasses in the designed program.

The following colour classes were defined:1.BG (background): ‘Back’

(background colour)2.RE (reject): ‘VCR’

(VCR, cobble & marginal reef)3.RE (reject): ‘Brown’

(Oxidized reef)4.AC (accept): ‘Lava’

(Lava and dolomite)5.AC (accept): ‘Grey_qrtz’

(Grey quartz)6.AC (accept): ‘Waste’

(Green quartz)

7.FGDC (foreground) ‘REST’(All other undefined pixels)

The focus of the test work was to maximize VCRrecovery as well as maximizing waste rejection. Onedistinct colour used in the sorting algorithm was rustybrown which is related to the oxidized sulphide which isassociated with the gold reefs. The positive side effect ofsorting for reef also removes the sulphides which cause aciddrainage.

Method of sortingFigure 5 shows the flow diagram for a single pass sortingmethod. The aim of the single pass method was to obtainmaximum VCR recovery to the concentrate stream withsmall to moderate dilution by waste particles.

The synthetic feed’s head grade was calculated to be 1.77g/t and is higher than expected in the actual dump. Thediscard stream comprised of 81% of the synthetic feed massto the sorter, which was calculated to be 0.59 g/t. The Aulost to tailings amounts to 27% of the optical sorter feed.The cobble reef contributed 21% to the loss of Au in thetailings.

97%, 10% and 100% of the VCR, cobble and marginalreef in the sort feed reported to the concentrate resulting ina calculated grade of 6.95 g/t. Only 9% of the waste in thefeed to the sorter reported to the concentrate stream,majority of which being the grey quartzite.

Bulk test workAfter a successful test conducted with 500 kg of Kloofwaste, a 6 ton sample was tested to establish both the headgrade and the reproducibility of the first test.

Table IV shows the results of a single stage of sorting inwhich both VCR and Cobble was recovered.

Recovering both VCR and Cobble resulted in a 13.5%mass pull to concentrate, which assayed at 1.06 g/t Au. Thediscard contained 0.12 g/t Au and this amounts to 40.1% ofthe gold in the feed to the sorter. A representative sample ofthe fines adhering to the coarse material was assayed at0.98 g/t.

Figure 3. Kloof Gold Mine: Waste rock types



Pilot plant operationThe primary focus of this campaign was to evaluate theeconomic viability of optically sorting the +16 mm sizefractions that were produced at Kloof’s rock dumps. Thismaterial was generated by screening the waste dumps torecover the +16 mm fraction, which has sufficient gold

content to be processed further by the mine. In order toevaluate the sorting option, two issues were investigated:

• The range of variability of the head grade in the +16 mm fractions

• The throughput and efficiency of the test sorter in orderto specify a large-scale plant performance.

A pilot optical sorting plant was brought onto site fromSeptember 2003 until June 2004. This period can be dividedinto 2 phases:

Phase 1: From September 2003 until February 2004 theplant was operated in its initial design and construction.After the first week of commissioning using one day shiftonly, the plant was ramped up to be operational on a 3 shiftbasis during the week and one shift on Saturdays.

Phase 2: After the completion of the first phase, a detailedevaluation in terms of production performance, goldrecoveries and profitability of the operation was conducted.After a period of repairs and maintenance and modificationsto the plant, it was decided to run the sorter for one moremonth during June 2004 at a 3 shift full capacity.

Description of operation and plant:

• The feed to the sorting plant was contracted to Saldanhaplant hire (SPH) who were already operating the dumpscreen at 16mm to produce a supplementary feed to themine’s process plant

• The plant includes a feed hopper with variable speedfeeder, conveyors, washing screen and water recirculationsump, sorter, power generator and loading and haulingequipment for feed, product and waste handling

• Site establishment took approximately one week,including wet commissioning of all equipment

• The sorter feed was loaded directly into a feed hopperequipped with an oversize grizzly and a variable speedfeeder

• The water circulation system around the sorter initiallyconsisted of a waste skip overflowing into a portablemetal sump. The degritted water was circulated to thewashing screen by means of a submersible pump.Intermittently, the skip will be drained and the fines willbe collected and sampled for analysis

Table IIIOptical sorter results for the synthetic feed

Stream Stream description Mass (%) Au (g/t) % Au number recovery1 Feed 100.0 1.77 100.02 Tails (accept class) 81.4 0.59 27.03 Concentrate (reject class) 18.6 6.95 73.0

Figure 4. Programme with colour classes

Figure 5. Flow diagram for a single pass sorting method

Table IVOptical sorter results recovering VCR and cobble reef

Stream description Mass (%) Au (g/t) % Au recovery

Feed 100.0 0.24 100.0Single stage concentrate 13.5 1.06 59.9Single stage discard 86.5 0.11 40.1



Figure 6. Flow sheet of optical sorter pilot plant at Kloof Gold Mine

Figure 7. Optical sorter pilot plant at Kloof Gold Mine

Figure 8. Sorted product (left) discarded waste (right)



• During the first month of operation, it turned out that thisskip water treatment system was inadequate to handle thelarge volumes of slimes that have been washes off thefeed material. A conical settling tank, including aflocculation system, was installed. A sludge pumpunderneath the cone transferred the slimes to a slimesdam close to the sorter plant. Later, the dry slimes weredug up and added to the sorter product

• 4 operators were employed and trained for this project.Production was run at one operator per shift. The site wasmanaged by the subcontractor SPH, the Mine and thegeneral Manager of TollSort

• In order to accommodate the dual purpose of treatingsufficiently large bulk samples and to obtain performancedata, it is proposed that the 8hr day shifts be run on asingle sorting algorithm. This effectively fixed the masspull and throughput for that shift. As a rule, feed to theplant should also come from a single source/area from thedump

• Product and waste was sampled intermittently during theshift, and then combined to form one composite productsample (1 ton) and one composite tailing sample (1 ton)per day

• During the course of the period, the product and tailingssamples were kept in 1 t bags and transported to Mintekwhere they were crushed, sub sampled, pulverized andanalysed for gold content. The 1t samples weretransported weekly to Mintek to minimize cost oftransport and setup of crusher facilities at Mintek

• Kloof took control samples to confirm the assay results.• The waste produced by the sorting process was loaded

and redumped onto the waste stockpile. The product fromall the sorter runs was stockpiled separately before takento the Kloof gold plant for further processing.

Production data and operational results

During the period of operation a total of close to 110 000tons was processed through the optical sorter. The resultsare shown in Tables V and VI:

Table VOperating data – plant tonnages and availability

Table VIOperating data – grades and recovery

Table VIIOperating costs for Kloof pilot plant

Cost structure USD

Depreciation (60mnths) ($/day) 846 Finance cost ($/day) 402 Operating cost (SPH) ($/day) 1.633 Accommodation and S&T ($/day) 70 Kloof processing/transport ($/day) 415 Salaries ($/day) 204 Assays ($/day) 114 Maintenance ($/day) 379 Hire of ablution ($/day) 3 Hire of office ($/day) 30 Total ($/day) 4.095

Total cost per ton feed ($/t) 2,47 Total costs per day ($/day) 4.095 Total costs per month ($/month) 81.902

Table VIIIProfit/loss evaluation Kloof pilot plant

Gold price June 2004 $/ounce 380

R/$ exchange rate R/$ 6,6

Gold Price R/g 81

Feed rate (t/hr) 82

Mass percent to sorter concentrate (%) 2,78

Head grade of feed (g/t) 0,27

Sorter concentrate grade (g/t) 5,7

Fines grade (g/t) 1,5

Overall concentrate grade (g/t) 3,9

Value of product per month ($/month) 72.474

Total costs per month ($/month) 81.902

Loss per month ($/month) (9.428)



Operating results June 2004Even though it was only a pilot plant, the participatingparties were expecting this operation to be a profitableentity. Unfortunately, in 2004 the low Gold price $380 peroz made this operation unprofitable. It was for this reasonthat the project was stopped.

The overall operating cost per ton was 2.47 US$ per t.The upgrade from the feed grade of 0.27 g/t to a sorter

product of 5.7 g/t shows a 20 fold improvement. The wastestream had negligible gold losses.

Estimated operating results 2009Over the last year, the price of Gold has improvedsignificantly. When predicting a scenario for 2009, we havetaken these 2004 figures as a basis and increased the costsby 60% and used a Gold price of $800 per oz to get thefollowing estimated results Table IX):

This may be a very simplistic approach, but it highlightsthe potential of optical sorting in a gold waste dumprecovery application.

ConclusionSensor based sorting is still not widely recognized and usedas a beneficiation process in mineral applications (otherthan industrial minerals). The pilot work at Kloof GoldMine in 2004 has shown that misplaced gold reef caneffectively be separated from waste rock at low mass pull toconcentrate (5% to 10%) and at a gold recovery rate of70%.Rapid advances in the development and improvementof sensors open up more and more application fields.Previously uneconomical resources, such as shown herewith waste rock gold dumps, can now be exploited at goodprofit margins.

References1. MARSDEN, J.O. and HOUSE, C.I. Published by

SME in 2006.

2. WILLS, B.A. Camborne School of Mines, Cornwall,UK, ‘Mineral Processing Technology–AnIntroduction to the Practical Aspects of OreTreatment and Mineral Recovery’ 5th Edition,Pergamon Press. 1992.

3. SALTER, J.D. and WYATT, N.P.G. Sorting in theMinerals Industry: Past, Present and Future, MineralEngineering, vol. 4, nos. 7–11, 1991. pp. 779–796,Pergamon Press, Great Britain.

4. WOTRUBA, H. Sensor Sorting Technology – is theminerals industry missing a chance?, XXIIIInternational Mineral Processing Congress, Istanbul,Turkey, 2006. pp. 21–30.

5. WEATHERWAX, T.W. Integrated Mining andPreconcentration Systems for Nickel Sulphide Ores,The University of British Columbia. 2007.

6. HARBECK, H. and KROOG, H. New Developmentsin Sensor Based Sorting, Montan University Loeben,Austria, January 18, 2008.

7. BERGMANN, C. Kloof Test Report, 2003, Mintek,TollSort.

Table IXProfit/loss prediction at 2009 gold price and cost levels

Gold price June 2004 $/ounce 800

R/$ exchange rate R/$ 8,0

Gold price R/g 206

Feed rate (t/hr) 82

Mass percent to sorter concentrate (%) 2,78

Head grade of feed (g/t) 0,27

Sorter concentrate grade (g/t) 5,7

Fines grade (g/t) 1,5

Overall concentrate grade (g/t) 3,9

Value of product per month ($/month) 152.577

Total costs per month ($/month) 108.111

Loss per month ($/month) 44.466

Lütke von KetelhodtGeneral Manager, Commodas (Pty) Ltd

Lütke was born and raised in Johannesburg, South Africa. He is a Mining Engineer with aHigher National Diploma in Metalliferous Mining. For 4 years he worked in the Deep-LevelGold Mines in the Witwatersrand area as well as Coal (Douglas Mine) and Chrome (WinterveldChrome Mine). He also holds a Bachelor of Commerce degree from the University of theWitwatersrand.He moved from South Africa to Germany and then Barcelona Spain, where he held variouspositions in Financial Controlling and Financial management within the AEG Group.He returned to South Africa where he got back into the mining game as managing director ofWIMICO, which was a company, specialized in underground Rock-Consolidation.Further Financial management positions followed within the Mannesmann Demag Group beforehe joined IMS Engineering as the General Manager for the MikroSort Division. This is where he

worked closely together with his current employer Commodas. It is thus since 2002 that Lütke is involved in introducing Sensor Based Sorting equipment into the Southern African miningand minerals market. Apart from sorting equipment he also managed the IMS sales department for crushing and screeningequipment.July 2007 he joined Commodas as General Manager – North America. He is setting up and managing the newly formedCommodas Inc which is based in Toronto, Canada to serve the North American mining market with the Commodas sortingtechnology. In July 2009 Lütke returned to South Africa to manage Commodas (Pty) Ltd which have their office in Sandton.


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