Automated sorting on a South African gold mineby P. J. BARTON*, Pr. Eng., B.Sc. (Min. Eng.), M.S.A.I.M.M., and

N. F. PEVERETTt, Pr. Eng., B.Sc. (Min. Eng.), F.S.A.I.M.M.SYNOPSIS

A brief review of the development and ultimate design of the Model 16 Photometric Sorter is given, including thedifferences between it and its predecessor, the Model 13, which is currently in operation at Doornfontein.

A description ofthe West Driefontein sorting plant is given, together with the rationale of its design. The installa-tion programme for the sorters is described, and current operating and maintenance procedures are discussed.

Operating results are tabulated and compared with the original test results. Operating costs are also discussed.

SAMEVATTINGDaar word 'n kort oorsig oor die ontwikkeling en uiteindelike ontwerp van die Photometric Sorter Model 16

gegee asook die verskille tussen hierdie model en sy voorganger, Model 13, wat op die oomblik by Doornfonteingebruik word.

Die sorteeraanleg by West Driefontein word beskryf en die redes vir sy ontwerp uiteengesit. Die installerings-program vir die sorteerders word beskryf en die huidige bedryfs- en onderhoudprosedures bespreek.

Die bedryfsresultate word getabelleer en met die oorspronklike toetsresultate vergelyk. Verder word die bedryfs-koste ook bespreek.

Introduction

Sorting is the original process for concentratingmineral ores as they occur in the mined state. Until thiscentury, this function was performed only manually,and today is still carried out in this manner where labouris readily available and cheap - for example in Indiaand large parts of the African continent. Manual sortingrequires that there is a visual difference between themineral-bearing rocks and the barren rocks.

During 1966 Gold Fields of South Africa undertook ajoint programme with Rio Tinto-Zinc through theirsubsidiary, Ore Sorters, to develop photometric sortersfor specific use in the South African gold-mining in-dustry. This programme culminated in the installationof a prototype photometric sorter - Model 13 - atDoornfontein Gold Mining Company during November1972, where it has continued to play an important roleto this day!.

As a direct consequence of this installation, seriousenquiries were received from around the world for similarequipment. This encouraged the development of acommercially producible general-purpose machine, whichis now known as the Model 16 Photometric Sorter.

Model16s are now in commercial operation in Greece(magnesite)2, Australia (wolfram), the U.S.A. (reclama-tion of scrap), and South Africa (gold reef). It is thislatter application that is discussed in this paper, specific-ally the sorting operation at the West Driefontein GoldMining Company Limited.

Although the Model 16 has been described fully2,3,perhaps it is of value to discuss the main differencesbetween the prototype Model 13 at Doornfontein (Fig.l)and the production Model 16s now operating at WestDriefontein (Fig. 2). This is most easily shown in tabularform (Table I).

The most significant differences are the introductionof rock stabilization on the Model 16; the scanning ofrocks in free flight against a 'black' background ratherthan on a white background conveyor; and a speeding

*Formerly of RTZ Ore Sorters, Johannesburg.tGold Fields of South Africa, Johannesburg.

up of all operations, viz belt speed, number of mirrorfacets, and number of valves in operation. This hasculminated in a machine capable of sorting up to 200tons of material per hour in the 160 to 180 mm sizerange, and 25 tons per hour in the range 10 to 20 mm.

Before Photometric Sorting

Two reefs are mined at West Driefontein - theCarbon Leader and the Ventersdorp Contact Reef (VCR).Owing to metallurgical-processing considerations, thesematerials are processed in separate crushing installations.In this paper consideration is given only to the CarbonLeader circuit.

At West Driefontein the material from the CarbonLeader is softer and more friable than that from theVCR. It is a material of high gold values and mediumuranium values in which the gold and uranium concen-trate in the finer (minus 3 mm) fraction of the run-of-mine ore. This property was made use of in the originalplant design, which was aimed at the production of auranium upgrade ('high grade') for uranium extractionand preferential gold leaching, and a low-grade oversizefor stockpiling. Testwork indicated that, by screeningand reef picking, some 72 per cent of the mass containing97 per cent of the gold would be milled. The milled

TABLE ICOMPARISON BETWEEN THE MODEL 13 AND THE MODEL 16

PHOTOMETRIC SORTERS

Parameter Model 13 Model 16

Effective sortingwidth

Slide plateStabilization

810 mmMild steelNatural

800 mmStellite

.Accelerator/stabilizerassembly4m/sBlack rubberIn free flightSingle unit20 facets6000 r/min2mm3rd generation40 to 80 air valves

Belt speedBelt colourScanningMirror

2m/sWhite PVCOn beltComposite8 facets2000 r/min5mm2nd generation30 air valves

Scanning intervalElectronicsSepa"ation

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1980 103

Fig. 2- The Model 16sorting plant at West Driefontein

104 MARCH 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

RE JECT

REJECTREEF

TPICKING REEF

PICKING

PICKED PICKEDREEF REEF

WASTEDUMP

material would be upgraded from 24,5 gjt to 33 gjt. Atthe prevailing gold price of $35 an ounce and prevailingworking costs and possible mining rates, there weredistinct economic advantages in restricting the capacityof the milling installation and stockpiling 28 per centof the mined material at a value of some 2,5 gjt. Thecrushing plant was designed to screen at 32 mm andrescreen at 38 mm. The middling fraction was to beintensively waste-sorted to reduce the bulk, while theoversize would be reef-picked to remove all obvioushigh-grade material and to supply pebbles for the twentypebble mills. The original circuit is shown in Fig. 3. Theplant operated on this basis until late in 1974, when anunderground fire and a fall-off in labour resulted inreduced underground production.

For a period at this time while gold prices were high,stockpiled material was returned for milling. As theModel 13 had been operating successfully for someeighteen months at Doornfontein on similar material,consideration was given to the sorting of the stockpilereturn. Tests on the Model 16 prototype indicated a

CARBON LEADER PLANT

jPRIMARY SCREENS

-32mm32mm

1'32~

-38mm +32 mmSCALPING SCREENS

38mm

REEF PICKINGREJECT WASTE

TSORTING

PICKEDREEF REEF

WASTEDUMP

TO SECONDARYCRUSHING ANDMILL! NG

Fig. 3-Before the introduction of photometric sorting atWest Driefontein, the circuit at full capacity

CARBON LEADER PLANT

I-32mm

PRIMARY SCREENS32mm

I'~~

-75mm +32mm ALTERNATIVE, --------ROUTE IF SORTERSNOT FUNCTIONING

PHOTOMETRIC ACCEPTSORTER'

SCALPING SCRE EN75mm

+75mm

TO SECONQARYCRUSHING AND

MILLING

Fig. 4- The circuit at West Driefontein after the introductionof photometric sorting

high financial return from the milling of an upgradefrom the stockpile.

Monitoring of the stockpile return showed that thepickers had preferentially and effectively removed thereef from the plus 75 mm fraction but had only lightlypicked the minus 75 mm fraction. Anticipating a returnto higher underground production before sorters couldbe installed for stockpile sorting, consideration was givento the installation of Model 16 sorters to sort currentmaterial in the size between 75 mm and 32 mm and forexpansion to sort the fraction larger than 75 mm.

Sorting in the fraction between 75 mm and 32 mmwould reduce the values to the stockpile to an acceptablefinal discard value. Sorting in the fraction larger than75 mm would have little effect on the sorting efficiencyand the gold milled, and would be justified only on cost-and labour-saving considerations.

With Model 16 SortersThe modified flowsheet at West Driefontein is shown

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1980 105

UJ

-'a..'"::;: w

::> UJ0 a..UJ -'z.... :::!iDU)

~::<:0

0 ........

UJZ

°-'U-'>--'u:f...;UJrj~

~~

u

106 MARCH 1980

UJ0«a:

'"IU)",0- -'IV;0....

.,u

~[

nu 0..0

r---l<3 ~~ Th I

~~~

[

CD

: ;~-- :U I \ I

I I

~

lL -

J~

u

-- --------U ~ --§ ~ 5 ---

~ ~

0:

--'& ~ ---

'"0

;;

-rJ--B

~~ t",:; 0 ~~~

:;~ ~~~ -------H

~

~~u

~~~>:;rW

~§~

~~-~

~~

~O

~

rozCD

'"0v;

:gg

"g0~

OD

U

rr.

M

U

.,~U

c"a:;....C

~";:Q....VIGI

~~ ~

C0..oS....VI..GI

.s:.VI:I..u..GI

"CoSGI.JC0

~..oSuI

1ft

~ii:

V>W

'"'"W~

~

U'

xW

er

'"

w

~~~~

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Fig. 6-Feed from main storage over final washing screen, Model 16at West Driefonteln

Fig. 7-Model 16 photometric sorter at West Driefonteln, hlShlightlng the scanning bridge

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1980 101

in Figs. 4 and 5. The crusher-plant feed is still screenedat 32 mm, but it is rescreened at 75 mm instead of38 mm. The fraction between 75 mm and 32 mm,amounting to 21 per cent of the total tonnage to thescreening plant, is sorted photometrically, and theplus 75 mm material is reef-picked manually as before.It was anticipated from the start that the machineswould reject a limited quantity of reef, which would haveto be recovered manually by backpicking. The finalreject from the fraction between 75 mm and 32 mm joinsthe plus 75 mm reject for disposal. A new waste dumpwas established to accommodate the final reject.

It was also anticipated that the reliability of theinstallation would equal, if not better, the 97 per centavailability achieved by the Doornfontein Model 13since there had been considerable improvements in themechanical and electronic functions, and there were twomachines with a combined capacity of 140 t/h to sort90 t/h. The installation allowed for the milling of theentire fraction between 75 mm and 32 mm during thevery occasional periods when there might be completefailure of the sorter installation.

The installation allows for the rock between 75 mm and32 mm to be conveyed on a new conveyor system fromthe rescreening screen discharge to the sorter feed binin the new photometric sorter building. In the unlikelyevent of sorter failure, the plant can revert to the orignalflowsheet by the use of retractable shuttle conveyors.

The photometric sorting plant comprises an 80 t feedbin, feed-preparation systems (Fig. 6), sorters (Fig. 7) andassociated control gear (Fig. 8) and air compressors,accept and reject return conveyors, and an undersize-dewatering system. The accept and reject rock streams

are conveyed to the reef-picking plant. The acceptmaterial joins the picked reef, and the reject joins theto-be-picked material for backpicking and final discard.

The main storage bin feeds two sorters operating in thefraction of material ranging from 75 mm to 32 mm.The design adopted is the conventional 'box' constructionwith timber walls. A natural dead cone is achieved aboveeach of the two discharge points, thus giving the requiredprotection against wear.

A Jost feeder (Fig. 6) situated below each dischargepoint feeds material to the respective feed-preparationsystems. These systems each comprise a Dabmarvibrating washing screen (Fig. 9) with 25 mm meshpolyurethane decking for the elimination of washingsand fines. The fines/slimes fraction from these screensis dewatered on a common secondary Dabmar screen of6 mm polyurethane mesh. The oversize from the de-watering screen discharges onto the feed conveyor to themain mill bin. The slimes are pumped via the crusher.plant dewatering screens to classifiers. The Dabmardewatering system was installed when it becameapparent that the quantity of grits generated in thetransportation of material to the sorting plant wouldcreate a pumping problem.

Surges in washing-screen oversize - the sorter feed -are controlled through a small surge bin/chute. Thischute is of very simple design and competes favourablywith the elegant load-cell system used at GrecianMagnesite2. The chute discharges onto tandem feedersthat control the manner in which the material reachesthe main conveyor of the sorter.

Ambient light is kept to a minimum in the sortingplant since it can affect the sorting efficiency. All lighting

108 MARCH 1980

Fig. 8-Model 16 control room at West Driefontein

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

is directed away from the scanning cabinet, and daylightenters only through translucent fibreglass shooting inthe walls of the building.

The feed-preparation systems, the sorter mainframe,and the scanning cabinet that houses the optics systemsare mounted on separate steelwork foundations. This hasbeen done so that no unnecessary vibration will betransmitted to the optical system. This is standardtechnique in the design of Model 16 installations.

The accept and reject streams from each machine arecombined on common conveyor-belt systems. This layoutprovides a simple and inexpensive discharge system, buthas made the sampling and evaluation of individualmachine performance inconvenient.

Nuclear-type weighers have been installed on themain feed belt and the reject belt. Owing to the non-uniformity of the particles, the weighers have not per-formed as well as anticipated, although they are provingto be reliable and need little maintenance.

The control room (Fig. 8) has been positioned to givea commanding view over both sorters and the associatedfeed-preparation systems. It contains the control paneland electronic processor for each sorter, as well as acustom-designed control panel for operating all thesubsidiary systems of the sorting plant. Although thisroom has been constructed of brick and double glazing,noise is still a problem. This noise can be dampened bylining the bare steel 'blasting' compartments of eachmachine with brick, rubber, or cork.

Large double doors and crawl beams allow for quickmechanical service and maintenance. For example, thesystem allows for the changing of a complete stabilizer,

so avoiding dissembling and reassembling of the stabilizerin situ and the resultant loss of valuable productiontime.

Installation of the Sorting Plant

The initial engineering was undertaken in May 1976,when the order was placed for the photometric sorters.This was undertaken by the Consulting Mechanical andElectrical Engineers' Department of Gold Fields ofSouth Africa Limited with assistance from Ore Sorters'engineering staff. Tenders were issued in January 1977,and all the steelwork was completed by July 1977. Thesorters were then installed, and their commissioningstarted in October 1977. Trial running was initiated inNovember 1977 with different feed systems on themachines. After certain modifications, the sorting plantbecame fully operational in February 1978.

Although the total elapsed time was approximately 20months before the project was completed, this must beconsidered in relation to the technology and experienceavailable at that time. The West Driefontein application,the first such installation in South Africa, was operatingin a run-of-mine mode and was based on the experienceof only one other operating sorting plant in the world. Itwas with these considerations in mind that a protractedinstallation was effected.

Two of the early problems that led to subsequentmodification were as follows.

(i) The Feed System on Number 2 SorterIt had been thought that the slide plate might bemade redundant so that the feeding system wouldfeed direct to the stabilizer/accelerator assembly.

Fig. 9-Feed leaving the final washing screen. Model 16 at West Drlefonteln

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1980 109

1974

Value

1-Gold Mass

g/t % %23,15 82,0 13,75

5,17 13,9 21,240,60 2,1 20,820,41 2,0 44,19

6,82 100,00 100,00

1978

Value Goldg/t %

34,39 75,374,88 16,511,20 3,990,59 4,13

6,27 100,00

1974 1978

II

!

Fraction Mass Value Gold Mass Value Gold% g/t % % g/t %

Accept 42,4 15,40I

95,9 34,99 16,47 91,88Reject 57,6 0,49

I

4,1 65,01 0,79 8,12

100,0 6,82 100,0 100,00 6,27 I 100,00

1974I

1978

Fraction Mass Value Gold IMass Value

I

GoldI% g/t % % g/t %

Accept 66,3 9,85 95,8 58,22 10,34 96,83Reject 33,7 0,84 4,2 41,78 0,48 3,17

100,00 6,82 100,0 100,00 6,27 100,00

Feb.--Jun. 1979 I Recent

Fraction Mass Value Gold

I

Mass Value Gold% g/t % % g/t %

Accept 58,97 10,04 94,44 63,47 10,02 96,27Reject 41,03 0,85 5,56 36,53 0,67 3,73

100,00 6,27 100,00 100,00 6,61 100,00

This was found to cause significant instability andwear problems, and was modified within a veryshort time.

(ii) The Fragmentation of Sorter FeedDespite in-line screening prior to transfer to thesorter plant, sufficient grits were generated in thetransportation of the material to the sorter plantto create a problem in the pumping of the washingsand fines from the feeding screens back to the maincrusher plant. A 6 mm dewatering screen, for theelimination of grit, had to be introduced into thecircuit.

Operation and Maintenance

The Carbon Leader crusher plant operates for twoIQ-hour shifts each day. Daily maintenance is carriedout between 08hOOand 12hOOon all sections of the plant,including the sorters.

The operating complement of the Model 16 sorterplant is five per shift, comprising the Shift Sampler, and

four general labourers. The Shift Sampler is responsiblefor all the operating, sample collection, and preparation,as well as for limited maintenance. Operating and main-tenance problems beyond his terms of reference arereferred to the Reduction Official in charge of the crush-ing plant, who in turn reports to the Supervisor of thecrushing plant should mine maintenance staff be re-quired. Ore Sorters' staff are called in only for majorproblems.

The five operators are responsible for the following:(i) clearing the feed-belt magnet of the sorter plant and

removing tramp material such as wood and cloth;(ii) regulating the flow by visual checking of the feed

hopper in the sorter plant (no automation is pro-vided), a function that provides for the startingand stopping of the second sorter and the closingdown of the plant as the feed demands;

(iii) sampling the sorter feed, accept, reject, and totalwaste on an hourly basis;

(iv) general honsekeeping.

TABLE IIACTUAL SORTER PERFORMANCE

Fraction Mass%

Dark reefLight reefDark wasteLight waste

24,118,324,033,6

100,00

TABLE IIISORTER PERFORMANCE AT 100 PER CENT EFFICIENCY

PREDICTED SORTER PERFORMANCE

TABLE IV

ACTUAL SORTER PERFORMANCE SINCE START-UP

TABLE V

110 MARCH 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Further supervision of this plant amounts to less than4 hours each day under normal operating circumstances.

An operational log-sheet is completed for each shift,and a log-book is kept in the control room in which allmalfunctions, maintenance, and other anomalies in theplant are recorded.

The engineering staff keep records of the parts usedand the time spent on engineering activities specificallyassociated with the sorting plant. The engineering timeis normally two hours a day.

Sorter Performance

The run-of-mine Carbon Leader ore in the size fractionbetween 75 mm and 32 mm can be readily separatedinto four fractions: light and dark reef, and light anddark waste. The 1974 test split and the 1978 run-of-minematerial split up to the time of writing are shown inTable 11.

Table III shows what a sort of 100 per cent efficiencywould be on the materials listed in Table H, and Table IVshows the predicted sorts based on the 1974 test results.

The sort achieved from the start-up in February untilthe end of June 1979, based on weightometer readingsand hourly samples of the sorter feed and of the acceptand reject fractions, of which the first three months'results were separated into the four fractions beforeassaying, is shown in Table V, together with the mostrecent results.

The 1974 test results indicated that, at the thenprevailing gold-extraction rate of 98,2 per cent, a goldprice of R3678 per kilogram, and 1974 treatment costs,and allowing for the gold that would have been recoveredby reef-pickers, an additional 101 kg of gold, yielding amonthly working profit of R318 000, would be realized.The capital pay-back period would be less than fourmonths.

On the current sort, at the reduced throughput of theplant, some 63 kg of additional gold is being extracted.At a gold price of R5600 per kilogram and the presenttreatment costs, an additional working profit of R306 000is being achieved. The capital pay-back period is still lessthan four months.

The profit calculations include the normal gold-extraction rate, and the cost of sorting, crushing, milling,

cyaniding and precipitation, smelting, refinery charges,and realization, with a credit for the cost that would havebeen incurred if the tonnage that is now being milledwere dumpcd.

Sorter Costs

The installation cost RI 135 000. The operating costsare as follows:

ROperating and maintenance

labourLabourers including backpickersPower at 1,49c per unitWaterSparesTotal cost at full capacity of90 tlh

800 per month2 820 per month0,067 per ton of feedNo charge0,062 per ton of feed0,225 per ton of feed.

Conclusion

It should be mentioned that the installation of photo-metric sorters at West Driefontein was contemplated ata time when long-term planning was aimed at a reductionof the mining rate in the Carbon Leader. The installationwas planned to recover the maximum amount of goldfrom the fraction larger than 36 mm for the millingcapacity that would become available at the reducedmining rate.

Acknowledgements

The authors thank the West Driefontein Gold MiningCompany Limited, Gold Fields of South Africa Limited,and Ore Sorters (Africa) (Pty) Limited for permission topublish this paper. Thanks are also due to the manyindividuals involved in the compilation of the data.

References1. KEYS, N. J., GORDON,R. J., and PEVERETT, N. F. Photo-

metric sorting of ore on a South African gold mine. PreprintNo. 74-8-311. SME of AIME. Presented at Acapulco, Sep.1974.

2. SCRAPPER, M. A. Beneficiation at large particle size usingphotometric sorting techniques. 2nd IF AC Symposium onAutomation in Mining, Mineral and Metal Processing,Johannesburg, Sep. 1976.

3. BARTON, P. J., and SCRMID, H. The application of laser!photometric techniques to ore sorting processes. Paper 1 ofMeeting 3, XIIth International Mineral Processing Congress,Sao Paulo, Sep. 1977.

ForgingThe 9th International Forging Conference, organized

by the Verein Deutscher Eisenhiittenleute (VDEh) inassociation with the Vereinigung Deutscher Freiform-schmieden (VDF), is to be held in Diisseldorf from 4thto 8th May, 1981. The purpose of the meeting is toshow - as have the previous International ForgingConferences - the most recent advances in the field ofopen-die technology and to extend the internationalexchange of experience.

The following are the main topics envisaged for theConference:

A. Manufacture of forging on heavy open-die forgingpresses using special facilities and tools

B. Spacial processes for the manufacture of forginggrade ingots and their processing

C. Experience gained with open-die forging of slightlyductile materials

D. Measures for rationalization in forgesE. Quality features of highly stressed heavy forgings.

Further information is available from the VereinDeutscher Eisenhiittenleute, Schmiedeausschuss, Post-fa ch 82 09, D-4000 Diisseldorf 1, West Germany.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY MARCH 1980 111