design andoperation ofdense-medium cyclone ... · design ofaheavy-medium cyclone plant at first...

Design and operation of dense-medium cycloneplants for the recovery of diamonds in Africa *

by I. R. M. CHASTONt, B.Sc. (Met. Eng.) London, A.R.S.M. (Fellow), andT. J. NAPIER-MUNN:j:,B.Sc.(Eng), A.R.S.M.(Visitor). »

SYNOPSIS

A full and detailed description is given of the dense-medium cyclone plants used in Africa for the recovery ofdiamonds. The criteria used in plant design are discussed with reference to type and size of separator and to theassociated screening and medium-recovery circuits.

The use of automatic controls and instrumentation in the operation of the plants is discussed, with examples frompresent practice.

Characteristics of the various types of media used are given, and the effects of changes in the quality and sizeof the medium are noted.

A break-down and comparison of the operating costs of various working plants are given.

SAMEVATTING

Daar word 'n volledige en uitvoerige beskrywing gegee van die digtemediumsikloonaanlegginge wat in Afrikavir die herwinning van diamante gebruik word. Die maatstawwe wat by die aanlegontwerp toegepas word, wordbespreek met verwysing na die keuse van die soort en grootte van die skeier en die geassosieerde sif- en medium-herwinningsbane.

Die gebruik van outomatiese kontroles en instrumentasie in die bedryf van die aanlegginge word bespreek metvoorbeelde uit die hedendaagse praktyk.

Daar word kenmerke vir die verskillende soorte media wat gebruik word, gegee en die uitwerkingvan veranderinge in die gehalte en grootte van die medium word aangegee.

Daar word ook 'n ontleding en vergelyking van die bedryfskoste van verskillende werkende aanlegginge gegee.

INTRODUCTION

For nearly a century, the diamondpan has been the basis of diamond-recovery plants. Long experience hasshown that close control of diamond-pan operation is difficult, even withthe most skilled and conscientiousoperators. Diamond losses, particu-larly when the finer sizes of groundare treated, can be considerable.

Given adequate feed preparation,heavy-medium processes can be con-trolled automatically; they canhandle wide fluctuations in feedconditions and consistently givevirtually complete recovery of freediamonds in the feed. Heavy-medium plants are more expensiveto run than pan plants, but, wherethe potential losses can justify thisextra operating cost, they are moreattractive than diamond pans.

A heavy medium was first usedin diamond processing at PremierMine in 1946 in the form of heavy-medium cones, which were intro-duced to treat the coarser, plus 3mm feed materiaP. The success ofthese cone installations led to experi-ments in heavy-medium cyclone

*Based on a paper by 1. R. M Chastonthat was presented at the InternationalMineral Processing Congress, London, in1973.

tAnglo American Corporation of SouthAfrica Limited.

tDe Beers Industrial Diamond Division.

120 DECEMBER 1974

operation for the recovery of thesmaller diamonds down to thelowest size at which diamond re-covery is usually attempted-! to1 mm.

At that time, the early 1950s,most heavy-medium cyclone plantsused magnetite media, and so itwas that a magnetite system wasinstalled in 1955 to recover diamondsfrom the minus 6 mm ground atWilliamson Diamond Mine in Tan-zania. An almost identical plant wasinstalled at Bakwanga, in what isnow Zaire, in 1959. As describedbelow, these plants used small,high-angle cyclones.

Following these successful installa-tions, work was carried out in 1961at Premier Mine with a small,narrow-angle cyclone using a ferro-silicon medium. This was followedin 1963 by the installation of a 525mm rubber-lined 20° cyclone using100 D ground ferrosilicon. Thiscyclone was subsequently replacedby a 600 mm cyclone and a 750 mmcyclone for test purposes, and theplant was finally operated with a600 mm rubber-lined 20° cyclone.

A 600 mm DSM (Dutch StateMines) heavy-medium cyclone wasinstalled to treat the fine materialat the No. 4 plant of the ConsolidatedDiamond Mines of South WestAfrica Limited in 1965. In the sameyear, a 450 mm rubber-lined cyclone

was installed to reconcentrate thefine material at Finsch. A 450 mmrubber-lined cyclone plant was in-stalled on the De Beers mine atDreyers Pan in Namaqualand during1966 to treat the whole gravel sizerange from minus 20 mm to 2 mm.Standard DSM 350 mm heavy-medium plants were built at Kim-berley in 1968 for reconcentration.All these installations had pump-fedcyclones.

Gravity-fed 600 mm heavy-medium cyclones were introducedwith the construction of the No. 1plant at Consolidated DiamondMines in South West Africa in 1968for treating all the feed in the sizerange minus 20 mm plus 2 mm.

The success of the gravity-fedsystem was such that it was followedby similar installations at the No. 4plant at Consolidated DiamondMines in 1969, the Koffiefontein(Orange Free State) plant in 1971,the Orapa (Botswana) plant in1971, and at the Kleinzee (N amaqua-land) plant in 1972.

Four pump-fed 600 mm cycloneswere successfully installed at PremierMine in 1970 to take the place ofjigs in the retreatment section. Itwas necessary to use a pump-fedsystem to fit the plant into theexisting building.

Details of some of these heavy-medium cyclone plants are given in

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Tables I and n. Table I lists theoperating conditions in the majorcyclone plants used for the recoveryof diamonds. Tables Il and III listthe equipment installed in theseplants under the headings that areuseR in the following discussion.

Heavy-medium cyclones nowrepresent the major primary con-centrating process used in diamondrecovery, and all the diamond plantsat present being planned are beingdesigned round this system.

DESIGN OF A HEAVY-MEDIUMCYCLONE PLANT

At first sight, the separation ofdiamonds of specific gravity 3,5from a gangue that mostly has aspecific gravity of less than 2,7 doesnot seem to present any greatproblem. The gap between the twois wide in terms of many separationsthat involve heavy media in thecoalfields. Separations in which thedifference is only 0,1 specific grav-ity units are referred to in the lit-erature2-4.

However, in those operations theemphasis is not on completerecovery, whereas, because of thevery high value of the individualdiamonds, it is usually necessary toplan for a total recovery of diamonds.Paradoxically, the value of theground as mined is often extremelylow because of the very smallnumber of diamonds in the ground.The Premier Diamond Mine, sourceof the Cullinan and many otherfamous diamonds, extracts ore fromunderground that has an averagediamond content worth less thanore containing 0,2 per cent copper.It is therefore also necessary tooperate the diamond concentrationprocesses as cheaply as possible.

A typical flowsheet for a heavy-medium cyclone is shown in Fig. 1.

A. Preparation of Feed

Great emphasis is usually placedon the need for careful feed prepara-tion, and in all the diamond plantsconsiderable trouble is taken toremove fines and water from thefeed to the heavy-medium cyclones.If fines are allowed to enter themedium in excessive quantities, theycan accumulate in the circuit untilit becomes difficult to maintain thedensity. The increase in viscosity

resulting from excessive contamina-tion by fines will lead to an increasein the power drawn by the mediumpumps, and may also affect theseparation achieved by the cyclone.Some authorities believe that theviscosity of the medium has littleeffect on cyclone operation, but arecent incident at the KoffiefonteinMine indicated that a fall-off incyclone performance could be partlyattributed to high viscosity caused bycontamination of the medium byfines.

The build-up of contamination isrelated to the effectiveness of themedium-cleaning circuit. If all themedium were cleaned every time itpassed through the cyclone, therewould be little need to remove finesfrom the feed. However, this wouldinvolve increasing the cleaning cir-cuit more than threefold to handlethe extra volume of pulp, and theextra cost of the extended medium-cleaning circuit would need to bebalanced against the cost of thefeed-preparation installation. At pre-sent, the balance of capital andoperating costs is thought to favourcareful feed preparation, exceptwhen the feed contains a largeamount of slowly disintegrating clay.

A more important objective offeed preparation is to reduce thewater content of the feed to aconstant low figure. Uneven drain-age in surge bins ahead of thecyclones can result in wide variationsin the moisture content of the feed.The effect of free water in the feedto the cyclone is to dilute themedium and so reduce its effectivedensity. A fluctuating moisture con-tent in the feed can result in un-desirable variations in the densityof separation. To overcome thisbin-drainage effect, the final feedersare now fitted with dewateringwedge-wire panels.

B. Method of Feeding

As mentioned above, the feed tothe early cyclone plants was addedto the medium in a pump sump,and the mixture was pumpedthrough the cyclones. Fine materialis easily introduced in the mediumflow but, with coarser material, therafting of flat, light particles inthe feed sump and feed surgingpresent problems. These problems


have been overcome by gravityfeeding the heavy-medium cyclones,A constant head is maintained ina feed-mixing box above the cyclone,and a prepared feed is then addedto the mixing box and gravitatesinto the cyclone. In this gravity-fedsystem, only the finely dividedparticles of the medium have to bepumped. Ferrosilicon particles areextremely abrasive and can makeshort work of ordinary unprotectedsteel pumps and piping. Rubber-lined pumps are, however, easilydamaged by stray steelwork, weldingrods, bolts, etc., and, when damaged,can cause catastrophic failure of theheavy-medium system by chokingthe cyclone inlets, spigots, etc., withlarge pieces of detached rubber.Ni-hard cast-iron pumps aretherefore preferred on many of therecent diamond plants. The extracost of spares for the pump iscovered by the reduced risk ofsudden and expensive failure.

A further advantage of thegravity-fed plant is that the pressuredrop across the cyclone is keptconstant by the constant head (onthe assumption that the density ofthe medium is constant) and is notdependent upon pump efficiency,which may change as a result ofwear or other factors. A disadvantageis the additional height required,resulting in increased capital costs.

The optimum value of head re-quired for any application is amatter for conjecture. Extensivetestwork by the DSM established anoptimum of 9D for coal separation,where D is the diameter of thecyclone; for a 600 mm cyclone,the required head would thereforebe 5,4 m. However, it was believedthat greater heads (and hence pres-sures) would be required for high-density separations, and all gravity-fed cyclone plants within De Beershave been constructed with a headof 8 m. Recent testwork at theDiamond Research Laboratory, how-ever, has shown that, at least forores from the Consolidated DiamondMines, a head as low as 5,1 m givesa satisfactory separation; indeed,the quality of separation was mar-ginally improved. Clearly, the feedpressure should not be so low as tocause the vortex inside the cycloneto collapse, but it may be that excess-

DECEMBER1974 121

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122 DECEMBER 1974 ~OURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

A

I

B C GMine Preparation of food Method of feeding Cyclones Density control

Williamson's Four 0,9 m sieve bends, I mm Two 8/6 Warman Six 300 mm nickel-hard Manualaperture pumps Inlet: 75 X 28 mm

Four 1,8 X 3,9 m a.c. step- Vortex: HO mm dia.deck screens, 1 X 6 mm Spigot: 35 mm dia.mesh Cone angle: 60 0

,

Premier Mine, One DID 1,5 X 4,3 m a.c. One 8/6 Warman One 600 mm ManualNo. 9 Unit low-head screen pump with stand- Inlet: 280 X 55 mm

Top deck: 8 X 32 mm mesh by Vortex: 230 mm dia.Bottom deck: 3 mm square Spigot: HO mm dia.mesh Cone angle: 200

One standby cyclone

Dreyers Pan Feed milled and screened at One 6/4 Warman One 450 mm rubber-lined steel Manual2mm pump Inlet: 75 X 75 mm

Vortex: 150 mm dia.Spigot: 50 mm dia.Cone angle: 200

Consolidated One 1,5 X 4,9 m a.c. low-head Gravity One 600 mm nickel-hard Ramsay magneticDiamond Mines, screen, 1,5 X 7 mm mesh Inlet: HO X HO mm coilNo. 1 Plant Vortex: 250 mm dia.

Spigot: 180 mm dia.Cone angle: 200

One standby cyclone

Consolidated Feed milled and screened at Gravity Four 600 mm nickel-hard Ramsay magneticDiamond Mines, 2mm Inlet: HO X HO mm coilNo. 4 Plant Vortex: 250 mm dia.

Spigot: 180 mm dia.Cone angle: 200

Premier Mine, Four DID 1,5 X 4,3 m Vibrex Four 8/6 Warman Four 600 mm nickel-hard Ramsay magneticRetreatment screens pumps Inlet: HO X HO mm coilPlant Top deck: 5 X 45 mm Vortex: 250 mm dia.

Bottom deck: 1,7 X 30 mm Spigot: 150 mm dia.Cone angle: 20 0

Orapa Five 1,9 X 4,9 m a.c.low-head Gravity Four 600 mm nickel-hard Ramsay magneticscreens, 1,6 X 4mmmesh Inlet: HO X HO mm coil

Vortex: 250 mm dia.Spigot: 180 mm dia.Cone angle: 20 0

Koffiefontein Five DID 1,9 X 6 m Ripflo Gravity Seven 600 mm nickel.hard Ramsay magneticscreens Inlet: HO X HO mm coilTop deck: 12 mm mesh Vortex: 250 mm dia.Bottom deck: 3 mm mesh Spigot: 130 mm dia.

Cone angle: 20 0

Kleinzee Two 1,9 X 4,9 m a.C. Ripflo Gravity Three 600 mm nickel-hard Ramsay magneticscreens, 2,0 X 5 mm mesh Inlet: HO X HO mm coil

Vortex: 250 mm dia.

ISpigot: 180 mm dia.Cone angle: 200

'TABLE 11DETAILS OF EQUIPMENT FOR SOME MAJOR HEAVY-MEDIUM CYCLONE DiAMOND i>tAN'tS

A B c 0 E F

~ECOVERY OFMEDIUM

CLEANING OFMEDIUM

II

PREPARATION OFI

MEDIUM

I

FEEDPREPARATION

METHOD OFFEEDING

CYCLONE

,-

FLOAT

FEEDSINK

FINES

FINES

Fig. I-Typical flowsheet for a heavy-medium cyclone

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY DECEMBER 1974 123

TABLE III

DETAILS OF EQUIPMENT FOR THE TREATMENT OF MEDIA ON SOME MAJOR HEAVY-MEDIUM CYCLONE DIAMOND PLANTS

DRecovery of medium

ECleaning of medium

FPreparation of mediumMine

Williamson's Two duplex 0,9 Sieve bends,1 mm aperture

Two 1,8 X 3,9 a.c. step-deckfloat screens, 1 X 6 mm mesh

One 0,6 X 3,0 m a.c. step-decksink screens, 1 X 6 mm mesh

Two-stage drum magneticseparatorsTwo: 0,75 m dia. X 1,5 mTwo: 0,75 m dia. X 1,5 m

One thickener unit, 4,9dia. X 1,8 m

One 75 mm Dorrco pump

m

Premier Mine,No. 9 Unit

One 0,9 m float sieve bend,0,8 mm aperture

One 1,9 X 4,9 m a.C. low-headfloat screen, 1,0 mm wedgewire

One 1,9 X 4,9 a.c. low-headfloat screen standby

One 0,6 X 3,0 m a.c. low-headsink screen, 0,5 X 4,5 mmmesh

Two-stage drum magneticseparatorsTwo:0,9mdia. X 1,9mEriez

Medium thickened by decanta-tion from the feed sump

Floor drainage pumped tomagnetic separator

One 0,9 m sieve bend, 2,0 mmaperture

One 1,5 X 3,0 m a.c. low-headsplit screen: 1,2 m for float

0,3 m for sink2,5 mm aperture

One-stage drum magnetic Spiral densifier, 0,6 m dia.separator - .One: 0,75 medium X 1,2 mRapid

Dreyers Pan

ConsolidatedDiamondMine,No. 1 Plant

One 1,5 m wedge wire drainagepanel, 2 mm aperture

One 1,9 X 4,9 m a.c- low-headfloat screen, 1,5 X 7 mm mesh

One 0,6 X 4,9 m a.c. low-headsink screen, 1,5 X 7 mm mesh

Two-stage drum magneticseparatorsTwo: 0,9 m dia. X 1,9 m Eriez

Spiral densifier, 1,7 m dia.

ConsolidatedDiamondMines,No. 4 Plant

Four 1,5 m sieve bends, 2 mmaperture

Four 1,9 X 4,9. m a.c. low-headfloat screens, 1,5 X 7 mmmesh

Two 0,6 X 4,9 m a.c. low-headsink screens, 1,5 X 7 mmmesh

Spiral densifier, 1,7 m dia.Two-stage drum magneticseparatorsFour: 0,9 m dia. X 1,9 mEriez

Eight DID 1,5 X 4,3 m a.c.low-head float screensTop deck: 5 X 45 mm meshBottom deck: 0,7 X 30 mmmesh

Four 0,5 m sieve bends, 1,2 mmaperture

Four 0,6 x. 3 m Aerovibe sinkscreens, 1 X 5 mm mesh

Premier Mine,Retreatment Plant

Two-stage drum magneticseparatorsEight: 0,9 m dia. X 1,9 mEriez

Four cyclones, 200 mm dia.

Orapa Four 1,5 m sieve bends, 2 mmaperture

Four 1,9 X 4,9 m a.c. low-headfloat screens, 1,6 X 4 mmmesh'

Two 0,6 X 4,9 m a.c. low-headsink screens, 1,6 X 4 mmmesh

Spiral densifier, 1,7 m dia.Two-stage drum magneticseparatorsFour: 0,9 m dia. X 1,9 mSala

..

Two-stage drum magneticseparatorsFourteen: 0,9 m dia. X 1,9 mSala

Koffiefontein Seven DID 1,9 X 6 m Eliptexfloat screensTop deck: 12 mm meshBottom deck: 0,5 X 4,5 mmmesh

Seven 0,9 X 3,7 m a.c. low-headsink screens, 0,5 X 4,5 mmmesh

Seven cyclones, 200 mm dia.

K1einzee Three 1,5 m sieve bends, 2 mmaperture

Three 1,9 X 4,9 m a.c. low-headfloat screens, 2,0 X 5 mmmesh

On~ 0,6 X 4,9 m a.c. low-headsmk screen, 2,0 X 5 mm mesh

Spiral densifier, 1,8 m dia,Two-stage drum magneticseparators

Two: 0,9 m dia. X 1,9 m Sala

124 DECEMBER 1974 JOU~NAL QF TH!: ~QUTH AFRICAN INSTITUTE OF MINING A~[) METALLURGY

ively large heads are also detri-mental, possibly owing to turbulenceeffects. It seems, therefore, that theDSM estimate of 9D may also beapplicable to diamond separations.

The problem of light particlesfloating in the feed-mixing chamberis avoided by use of the DSMStamicarbon design of feed box,which creates a suction condition inthe feed section and effectively drawsall the feed material down into thecyclone feed pipe.

The Orapa Mine is an open-castoperation starting literally at grassroots. In addition to grass roots,there are many tough bush rootsthat penetrate deep into the groundin search of moisture. On the start-up of the plant, these long roots inthe feed were dragged through thescreens by the gravel and causedchoking of the cyclone feed inlets.Once in the cyclone, these rootstended to be pinned in the outerdense layer and to appear in theconcentrate. This caused choking inthe concentrate chutes and in thereconcentrating system. Thesechokes had to be cleared by handand led to some diamond theft. Toeliminate these roots, which had adensity of about 1,5, the feed-mixingboxes were quickly redesigned inthe form of inverted settling pyra-mids with continuous overflow.These effectively separated the rootsbefore they were drawn into thecyclone, and the settling-box over-flow, which had a density of 1,9,together with the roots and somelight particles, was safely fed ontothe cyclone tailing-product screenfor immediate disposal of the rootsand for recovery of the medium.

This shape-factor effect, by whichsome of the light particles that areflat or elongated can be entrappedwith the heavy fraction in theheavy-medium cyclone, has beenencountered with feed containingshell fragments in coastal operationsand shale slabs inland. These lightflats cause problems in the subse-quent treatment of the diamondconcentrates, both on the grease beltsand in the X-ray separators. Theeffect seems to be related to thecyclone cone angle and is worse with20 ° cyclones than with 40 ° cyclones.

The ratio of medium to ore fedto the cyclone is important. Obvi-

ously, it is necessary to preventcrowding and interference with freemovement of particles in the brieftime that they are in the cycloneand the actual separation is takingplace. To use an excessive quantityof medium is, however, wasteful ofpower, and requires extra screeningand cleaning capacity. For normalore separation, the DSM designershave found that the pulp of themedium should be at least three tofour times the quantity of ore. How-ever, over 99 per cent of the feedhas to be rejected as float in diamondseparations, and it has been foundthat a low ratio of medium to oreleads to excessive sink fractionsand to unstable operations. It isnecessary to use a medium-to-oreratio of between 5 and 7 to achievethis high degree of concentrationand total diamond recovery with aclean separation.

For a gravity-fed cyclone, theratio of medium to ore is controlledentirely by the feeding rate of oreto the mixing box. Recent testworkon the 600 mm cyclone plant atthe Diamond Research Laboratoryusing ore from Consolidated Dia-mond Mines, has shown that ex-cellent separations can be achievedat feed rates up to 135 tjh with amedium flow of approximately 220m3/h (800 gal/min), a medium-to-oreratio of 4,4 to I; feed rates of morethan 160 t/h were also maintainedfor long periods (a medium-to-oreratio of 3,7 to 1), and, although thequality of the separation was notassessed accurately, observation sug-gested that the separation was notimpaired. At these feed rates, largeproduct screens are required to givea good recovery of medium.

C. Type of Cyclone

The Williamson cyclones, usinga magnetite-base medium, are300 mm in diameter, have a widecone angle (60°), and are operatedat fairly high pressure (290 kPa).As discussed later, it has been foundthat high-angle cyclones can rejectcoarse, heavy particles larger than6 mm, and so this type of cyclone isnot suitable for coarse feed.

The bulk of the diamond opera-tions now use standard DSM-designheavy-medium cyclones with 20°cones. There is a limited application


for 40 o cones in reconcentration, buttestwork has shown that thesecyclones, too, can reject heavyparticles larger than 18 mm.

The cyclones are nearly all con-structed of Ni-hard cast iron.Rubber-lined cyclones are probablycheaper to maintain, but, as withrubber-lined pumps, the lining hasan unfortunate tendency to tear offin patches as it becomes worn ordamaged. Unless immediatelynoticed, the pieces of rubber caninterfere catastrophically with theseparation in the cyclone and cancause serious losses. Wear on themetal cyclones causes no more thana gradual deterioration in perform-ance, and periodic planned inspect-ions are then sufficient to determineat what point the cyclone partsshould be replaced.

Worn vortex finders are particu-larly dangerous since even a smallhole in the vortex finder has beenfound to result in significant diamondlosses.

The testwork at Premier Minewith their No. 9 unit encompassed avariety of cyclone designs rangingfrom a 20 o cyclone of 525 mm dia-meter with an inlet of 270 mm by70 mm, to a cyclone of 750 mmdiameter with an inlet of 420 mmby 45 mm. The final cyclone used isa 600 mm 20° rubber-lined cyclonewith an inlet of 280 mm by 55 mm,a vortex finder of 230 mm diameter,and a spigot of no mm diameter.This cyclone is fed with upwards of150 t of feed per hour and with8200 litres of medium per minute.It is evident that many designs ofheavy-medium cyclone perform ade.quately.

The choice of cyclone size is nota decision that can be based onfeed tonnage alone. If the feed con-tains large particles, the cyclonechosen will need to have an inletlarge enough to prevent choking.It has been found that standardDSM cyclones of 350 mm diametercan accept feed material up to 20mm without choking, and have evenoperated successfully with feeds upto 25 mm with only occasionalblockage.

Apart from particle size, the choiceof cyclone size may be governed bythe associated pumping or screeningequipment. Thus it has been found

DECEMBER1974 125

that screening of the product andrecovery of the medium from a600 mm cyclone can be achievedsatisfactorily on a standard 1,8 mby 4,9 m low-head screen.

The use of cyclones of largercapacity would then require eithermultiple product screens or largerproduct screens. To split the feedto multiple screens would result ina loss of head-room and wouldrequire more floor area. Largerscreens have, to date, not beenfound as reliable as those used atpresent.

Until the recent test work at theDiamond Research Laboratory, thecapacity of the standard range ofDSM heavy-medium cyclones forthe treatment of diamonds fromgravel was taken to be as follows:

14 inch16 inch20 inch24 inch

Cyclone diametermm350400500600

Feed capacityt/hO. 25

25- 5050- 7575-100

However, it seems probable that,for each type of plant and for eachtype of feed condition, there willbe one particular design that willhave particular advantages inachieving an accurate split at minim-um cost. The Diamond ResearchLaboratory is at present undertakingresearch on a full-scale test rig topermit the development of optimumdesigns for particular conditions.

D. Recovery of Medium

After the separation in the cyclone,it is usual to recover as much of themedium as possible by drainage.This medium is promptly recycled.If the feed has been effectivelyprepared, there is little chance offine, light solids accumulating inthe recycling material. However, iffines do enter the medium circuit,a controlled amount of this recycledmedium must be bled off and fedto the medium-cleaning circuit.

Many plants do not have thisbleed facility, and rely on theinefficiency of the medium-drainagesection to push a sufficient quantityof contaminated medium onto thewashing screen and so keep thecontamination of the medium withinacceptable limits. However, exist-ing evidence is that the major lossof medium is from the medium-

126 DECEMBER 1974

cleaning circuit. It would thereforeseem sensible to keep to a minimumthe quantity of medium being fedto the magnetic separators by havingan efficient medium-drainage sectionand controlling the quantity ofmedium to the cleaning circuit tothe minimum required for efficientoperation.

Usual practice on diamond plantsis to have a sieve bend before theproduct screens to provide prelimin-ary drainage of media, and then touse the first quarter of the productscreen to complete the drainage.Williamson Mine has found that adouble-stage sieve bend is mosteffective in draining the mediumfrom the float product, and theythen use the total-product screen forwashing. The double sieve bendrequires an extra 4 ft of head-roombetween the cyclones and the screen.This may present a problem ingravity-fed cyclones, where head-room is usually at a premium,but should not be difficult whenpump-fed cyclone plants are beingdesigned. The authors' view is thata double-stage sieve bend, combinedwith a drainage section on the screen,would pay in terms of reducedconsumption of the medium whenthe feed preparation has beeneffective.

E. Cleaning of Medium

The dilute medium from thewash sections of the product screensand the bleed, if any, from thedense-medium circuit are cleaned bybeing passed through drum magneticseparators.

Losses of medium from the mag-netic separator form a major partof the total loss from the systemwhere adequate screen washing areahas been supplied for the cycloneproducts. General experience sug-gests that some three-quarters of thetotal physical loss of the medium,as opposed to loss in chemical solu-tion, can be attributed to the mag-netic separators. The finer particlesof the medium are more likely tobe lost from the separators, bothfor mechanical reasons and becauseof their magnetic properties, andthis results in a constant depletionof fine medium from the circuit,which is not usually balanced suffi-ciently by the continuous degrada-

tion of the remaining medium bypumping and attrition. For thisreason, the medium in the cyclonecircuit is nearly always found to becoarser than the fresh mediumadded to the circuit. In extremecases, this loss of fines has contri-buted to a significant drop in theefficiency of the cyclone. However, aserious loss can usually be detectedthrough an increase in the differen-tial in specific gravity between theoverflow and underflow medium.

Electro-magnetic separators wereused at Williamson, but these havebeen superceded in the later plantsby a modern type of improvedpermanent-magnet drum separatorsemploying ceramic magnets5.

These magnetic separators areusually fed concurrently, two inseries, with the second machinescavenging the magnetics from thetailings of the first machine, althoughsome designers have suggested theuse of the two separators in parallel,thus reducing the load to eachseparator.

Drum speed is held at around 28m/min at the surface of the drum,since this corresponds to themanufacturers' recommendations.Whether this is the optimum speedfor all solid loadings is doubtful, andexperiments are being carried outto determine whether there is anyreduction in losses of medium ifthe second separator is run moreslowly than the first.

In the earlier plants, the feed tothe magnetic separators was pumpedto the machine header box, whichdistributed the feed across the fullwidth of the magnet. The designfeed rate was about 1500 l/min permetre of separator width for theusual 'O,9m-diameter drum separa-tors. It was found that pumpsurging led to fluctuations in theflow rate of the magnetic separators,and that this led to increased lossof magnetics.

Recent plants have been fittedwith a header cone, to which thedilute medium is pumped. The coneoverflow is led to flood boxes overthe float screens prior to the watersprays. The steady flow of coneunderflow is led to the magneticseparators. Because of the settlingaction in the cones, very little


medium overflows the cones and isreturned to the circuit.

F. Preparation of Medium

Once the medium has been re-covered and cleaned, it is necessaryto return enough over-dense mediumto absorb the free moisture contentof the feed and to permit finaladjustment of the density of thefeed medium by addition of avariable amount of water.

At the early Williamson plant,this preparation of the medium wascarried out by a small 16 ft-diameterthickener. The underflow from thethickener was withdrawn by aDorrco duplex diaphragm pump.The density was controlled byadjustment of the variable strokeof the diaphragm pump to give therequired feed density as measuredby manual samples. Thickener over-flow was returned as wash water onthe product screens. To assist in thethickener operation, the feed to thethickener was passed through amagnetizing coil to help the magnet-ite particles to flocculate. Thissystem has been found clumsy tooperate, and any build-up of fines inthe circuit can lead to great diffi-culties in obtaining sufficient densityin the thickener underflow.

With the introduction ofthe denserferrosilicon medium, two methods ofdensification have developed.

One, 'settlement densification',uses a spiral classifier to accept themagnetic fractions from the mag-netic separators. The ferrosiliconsettles in the classifier and is rakedup at high density for return to thedense-medium pump sump. Theclassifier overflow feeds back to thedilute-medium sump, and returnsto the magnetic separators. 'Withthe spiral raised, all the medium inthe circuit can be stored in theclassifier, and this prevents themedium from settling out in thesumps during close-downs.

It can be very difficult to re-agitate media that have been allowedto settle out in sumps. Corrosion ofthe fcrrosilicon in the sump cancause it to form agglomerates largeenough to be lost from the circuitwith the cyclone products when theplant is restarted. However, a med-ium that has settled out in a spiralclassifier can be gradually returned

to the circuit in a dispersed formby gentle lowering of thc spiral ontothe settled material.

The evolution of bubbles of hydro-gen as a result of the corrosionprocess is a common sight in bothspiral densifiers and sumps duringshut-down. Recent laboratory andplant work at the Diamond ResearchLaboratory6 has established thatthe regular addition of a powderedmixture of sodium nitrite and boraxto the circuit, in the approximateproportions of 0,3 to 0,5 kg of eachreagent per tonne of ferrosilicon incircuit per day (for a closed circuit),causes corrosion to cease almostentirely. A dramatic reduction inhydrogen evolution was observedduring the test of this inhibitoron the full-scale cyclone pilot plantat the Diamond Research Labora-tory, and it was found that theferrosilicon could be allowed tostand in the storage sump for overtwo weeks without any seriousdifficulty being encountered on start-up after this period. A full test ofthe inhibitor is to be carried outat Consolidated Diamond Minesshortly.

The second system is 'cyclonicdensification'. Here, part of themedium is pumped at high pressurethrough small cyclones. The densifiedunderflow fraction is fed to thedense-medium sump, and the over-flow is fed to the dilute-mediumsump and through the magneticseparators. This system gives a morecompact plant of lower capital cost.

With cyclone densifiers, the quan-tity of medium in the circuit isgenerally less than an equivalentspiral densifier plant. As a result, thecircuit is more sensitive to suddenchanges in feed conditions andtherefore requires a higher standardof operating attention. There is alsono alternative to storing the mediumin the pump sumps during shut-down.

G. Plant Control

DensityInitially, heavy-medium cyclone

plants were controlled by the opera-tors, who measured the density ofthe media manually.

The basis of automatic densitycontrol is the production of anover-dense medium to which water


Is added in respOI'lse to measurementsfrom a density-determining device,either on the pipe that feeds themedium to the mixing tank abovethe cyclone or on the pipe that feedsthe mixed feed and medium into thecyclone.

Two tried and reliable methodshave been developed for this densitymeasurement.

The first is a radiation attenuationsystem. This employs a radio-activesource on one side of the pipe anda detecting system on the oppositeside of the pipe that determines thedegree of radiation passing throughthe pipe and its contents. This typemeasures all the solids in the pipe(ferrosilicon, slime, and ore) andwill, at a given setting, react todilute the medium if there is anincrease in slime content of thecirculating medium, or if there isan increase in quantity of feed.When viscosity is not a problem,this dilution will reduce the densityof separation and may be un-desirable. Where viscosity problemsexist because of inadequate feedpreparation or breakdown of thefeed in the cyclone, this device willensure that the system fails to&afety and that increasing viscositywill not lead to loss of diamonds.

The second system uses wire coilsround the pipe. Magnetic materialpassing through the coil generatesa current indicative of the quantityof magnetic material present. Be-cause of the variation in magneticproperties, this second system can-not be used satisfactorily for mixturesof magnetite and ferrosilicon, orfor flows of mixed ore and mediumwhere the ore contains an appreciableproportion of magnetic material.

The capital costs of both thesesystems are of the same order -approximately R4000 per installa-tion.

The simple differential-pressuredevices used for the measurementof medium density in coal prepara-tion have not found favour indiamond plants.Viscosity

At present, no automatic meansof measuring or controlling theviscosities of media are in use onheavy-medium cyclone plants.Several devices based on measuringthe force required to rotate drums

DECEMBER 1974 127

or vanes in the medium have beenproposed, but none has gainedacceptance.Efficiency

All the more recent diamondplants have been designed with anintegral tailing-sampling unit thattreats a continuous sample cut fromthe tailings screens.

Typically, a cut of 1 to 3 per centis made of the tailing stream, andthis is treated in a separate smallheavy-medium cyclone installation.By observation of the quantity ofheavy mineral recovered by thetest plant, the operator of the maincyclone plant can quickly detectany defect in the operation. Dailyrecovery of diamonds from theconcentrate of the test cyclone givesa continuous figure for the operatingefficiency of the main cyclone plant.

A statistical-significance techniquehas recently been introduced thatallows the operator to determinewhether the number of diamondsrecovered by the test plant cansimply be attributed to normalprocess fluctuation or is indicativeof a significant drop in the efficiencyof the main planV.

It is accepted that the use of aheavy-medium cyclone test plantto check the efficiency of anotherheavy-medium cyclone plant is notideal, since any basic inefficiencyof the system will be duplicated.Past alternatives have, however,presented cumbersome problems inoperation and certain securityhazards. Consideration is now beinggiven to the use of X-ray separatorsfor this policing operation, but this,in turn, presents certain operatingproblems because of the vibrationand humidity associated with mostlarge-scale treatment plants.

In some plants a continuous checkis kept of the operating efficiency ofthe cyclone plant by the circulationof radio-active isotopes of diamonddensity, which are made by theirradiation of particles fabricatedfrom aluminium and cobalt powder.These isotopes are detected byscintillators next to the concentrateproduct stream, which operate hy-draulic deflectors to return the portionof the concentrate containing theisotope back to the feed. The isotopethen circulates continuously throughthe system and triggers a counter

128 DECEMBER 1974

that records the number of trips. Ascintillator on the tailings belt de-tects any isotope lost from thesystem, and sounds a hooter toalert the plant staff to the plantfailure. The artificial isotope particlesgradually wear away in the system,and are eventually lost through thescreens.

An alternative type of isotopethat has been used consists of anactual diamond with a hole drilledin it. This hole is plugged by a pieceof radio-active cobalt wire.Pressure

All heavy-medium cyclones arefitted with pressure gauges to indi-cate the feed pressure, but few ofthese operate reliably for any periodof time. In theory, these gaugescould be used to deduce pulp den-sities on gravity-fed cyclones andto indicate pump wear on pump-fedcyclones, but in practice the usualtype of pressure gauge cannot beexpected to provide reliable informa-tion for more than a short period.

However, experience on the pilotplant of the Diamond ResearchLaboratory has shown that, if thenarrow-bore pipe feeding the gaugediaphragm is extended verticallyby about 20 cm, the solids in thestatic pulp present in the pipe tendto settle rapidly, preventing block-age of the pipe and damage to thediaphragm. One standard coil-springgauge has now operated successfullyon this plant for some weeks.

Good correlation has been ob-tained on this plant between thereading of actual pressure and thetheoretical pressure to be expectedfrom a consideration of the constanthead and the pulp density. Thissuggests that accurate pressuremeasurement could be used as amethod for estimating the densityof the medium.Analyses of the Product

The use of analyses of productdensity and Tramp curves is almostthe only aspect of heavy-mediumplant operation that is well coveredin the literature2-4,8. Criteria de-rived from these curves, such as the'split point density' or S.g'50 andthe 'probable error' or Ep, are veryuseful in the determination ofoptimum operating conditions, but,in diamond concentration, wherecomplete recovery is aimed for, it

is the shape of the upper and lowertail end of the Tromp curve thatis of major importance. A sharpupper cut-off will indicate the likeli-hood of complete diamond recovery,and a sharp lower cut-off will ensurethat a relatively large quantity oflight material will not accompanythe small percentage of heavymineral usually present. Thestandard Ep limits of 75 per centand 25 per cent cannot always beguaranteed to reveal the tail shapesof the Tramp curves, and may notbe sufficient for the comparisonbetween different diamond heavy-medium separators.

The importance of this tail shapeof the separation curves has beencommented upon by Krijgsman3and Dreissen2, but no simple methodof defining these tail shapes has yetbeen advanced.

Plans are well advanced for theintroduction of a specially manu-factured plastic material, colour-coded in a range of accurately knowndensities, that can be added inparticulate form to a cyclone feed,recovered from the products, sortedby hand, and counted or weighed,thus providing a rapid method ofestimating the Tromp curve of theseparation; present methods ofheavy-liquid analysis at high den-sities are messy and time-consuming.Magnetite

The early Williamson cycloneplant8 used a ground Swedishmagnetite, which was substantiallyminus 65 mesh. It was found thatthe magnetite wore down in thecircuit and that the optimum grad-ing of new medium added to thecircuit was 40 per cent coarser than200 mesh.

This resulted in a circulatingmedium that was only 30 per centcoarser than 200 mesh. A finermedium was found to increase theviscosity of the medium circuit andto give a poorer separation, whilea coarser medium classified toomuch in the cyclone and createdunstable separating conditions. Witha feed density of 1,9, the spigotdensity was 2,7 and the overflowdensity 1,88. Feed pressure was330 kPa, ore-to-medium ratio 1 :7,percentage concentrate 0,44 per cent,and medium consumption 700 g ofmagnetite per tonne of cyclone feed.


This two-cyclone plant treated47 tjh. It was later increased to afour-cyclone plant treating 100 tjh.The feed pressure was dropped to280 kPa and the ore-to-medium ratioreduced to 1 :5. With a feed densityof 2,05, the percentage concentratefell to 0,3 per cent and the loss ofmedium increased to 900 gjt.

Density analyses of the productsfrom the cyclones gave irregularTromp curves. This suggested thatan improvement in separation witha reduced percentage concentratecould be expected from an increasein the feed density. With magnetitealone, tests showed that a higherfeed density ga ve an increasedviscosity, which led to an increasedretention time of diamond tracersand a loss of tracers of up to 5 percent to the cyclone overflow.Mixture of Magnetite and Ferrosilicon

Tests were then carried out atWilliamson's with mixtures of Knap-sack atomized ferrosilicon and mag-netite. These pulps showed a rapidfall in viscosity when the percentageof ferrosilicon was above 20 percent for mixtures of magnetite andnormal fine-grade ferrosilicon andalso with cyclone 60 grade atomizedferrosilicon.

Mixtures of magnetite and milledferrosilicon showed the same fallonly with ferrosilicon contents ofover 40 per cent. One unexpectedresult of using the low viscositymixtures was that large diamondtracers (2 carats) were lost to thecyclone overflow. Scintillometersgrouped round the test cycloneindicated that the tracers wereappearing at the outer shell beforebeing lost. Smaller tracers wererarely lost with feeds of low viscosity.This suggests that the loss of thelarger tracers was not due to anyturbulence in the high-angle high-pressure cyclones.

A similar effect has been noted inlarge-angle desliming hydro cyclones,where large stones are held in thecyclone or are rejected with theoverflow but do not come out ofthe spigot. It is suggested that thishappens when the thickness of thedownward-spiralling wall flow isappreciably less than the diameterof the particle at some part of thecyclone wall, i.e. the large particleprojects through the zero flow en-

velope into the inner rising-spiralflowstream. Under these circum-stances the large particles may easilybe held up in the cyclone, or evenpulled into the vortex flow anddischarged with the overflow.

At Williamson's the effect wasascribed 8 to the large particlesricocheting off the inner wall andwas avoided by limiting the per-centage of ferrosilicon in the feedto 20 per cent of the magnetite.The pulp density of the feed wasincreased to 2,15, and the totalcyclone capacity, six cyclones operat-ing, was increased to 155 tjh. Thepercentage concentrate was reducedto 0,2 per cent.

One result of mixing ferrosiliconwith magnetite was an increasedconsumption of medium. This wasprobably due to the abrasive natureof the hard ferrosilicon, whichrapidly grinds away the softermagnetite. The fact that ferrosiliconis an effective grinding mediumshould not be overlooked. One ofthe early advantages ascribed togravity-fed heavy-medium cyclonesover pump-fed heavy-medium cy-clones was the reduced pump wearthat would result through not havingto pump abrasive ore particles.However, it would seem that forkimberlite the reverse may be true.F errosilicon

Ferrosilicon is now used by allthe diamond plants in SouthernAfrica. Because of the nature of theseparation required, the use of themore expensive grades of atomizedferrosilicon cannot normally be justi-fied, and most of the cyclone plantsoperate on the finer grades of milledferrosilicon. The manufacturers'specifications for the various gradesavailable in South Africa are givenelsewhere9.

Previous operating experience hassuggested that any given grade (sizedistribution) of ferrosilicon would cutat a fixed specific gravity regardlessof the specific gravity of the mediumor the feed pressure. However, recenttests at the Diamond ResearchLaboratory on a 600 mm cycloneindicated that relatively wide varia-tions in s.g.so can be achieved byvarying the specific gravity of themedium - typically variations of2,4 to 3,0 in the specific gravity ofthe medium produced variations of


2,9 to 3,35 in the s.g.so over anarrow range of ore characteristics.Small changes in the size distributionof the medium were also found toaffect the quality of the separationsignificantly. However, no directrelationship was found between thes.g.so and the specific gravity ofthe underflow medium, as suggestedin the literaturelo, since the specificgravity of the underflow mediumwas found to vary only slightlywith specific gravity of the feed fora given grade of ferrosilicon.

The sharpness of the split achievedhas been found to be influenced bythe grade of milled ferrosilicon used,being higher for the finer ferrosilicon,particularly in the finer sizes of ore.In some diamond applications, how-ever, the proportion of feed ore ofmiddlings density is small, and it isnot necessary to strive for thesharpest possible split. It is thenpossible to operate the cyclone witha low feed density, so as to reducethe probability of diamond losswithout increasing the amount ofconcentrate unduly. Under a givenset of operating conditions, this alsoleads to a reduction in the amountof ferrosilicon in the cleaning circuit,and hence to lower ferrosiliconlosses.

Corrosion of the milled ferrosiliconcan lead to high losses, and massivecorrosion can be initiated, even ata pulp pH as high as 10, by a varietyof causes that as yet are not wellunderstood. However, as noted ear-lier, the introduction of a promisingcorrosion inhibitor may succeed inreducing corrosion significantly.

Some losses of ferrosilicon canoccur if new medium is added tothe circuit without pre-wetting, theunwetted ferrosilicon being carriedout as a film on the products of theseparation; a separate circulatorypump and sump have been founduseful to pre-wet the added ferro-silicon. Laboratory tests have sug-gested that atomized ferrosilicon isconsiderably less prone to thisloss than milled ferrosilicon.

Costs

Although the subject of costs mustbe paramount in the consciousnessof all practising engineers, it ispossibly the most difficult area inwhich to obtain exact and com-

DECEMBER 1974 129

No. 4 Plant PremierC.D.M. Hetreatment Koffiefontein Orapa \VilliamSOll's

eft cft eft cft cft1,1 2,5 2,0 1,9 1,31,6 1,.5 1,4 1,9}3,4 2,8 6,8 5,4 5,30,4 0,2 0,1 0,10,8 0,1) 0,2 0,41,6 2,7 1,5 2,6 2,64,0 3,8 3,5 6,6 6,22,3 2,8 2,1 1,.5 1,5

15,2 16,8 17,6 20,4 16,9

360 340 560 350 155

parable figures, not only from oneoperation to another, but even frommonth to month within one givenoperation. The most detailed com-puterized system still relies on thefallible individual for the conscienti-ous logging of time spent on differentsectors by maintenance and engineer-ing personnel. The cost of spareparts mayor may not includetransport costs, storagecosts, and stores overheads.Amortization allowances are usuallyincluded in power costs, but notin water-supply costs or in generalplant costs. The cost of replacingmajor equipment may be in-cluded as it arises, or may becarried in an accumulating suspenseor contingency account. To thedespair of the engineer, accountingseems to be much more of an artthan a science, and its findings mustbe applied with considerable circum-spection. The figures in Table IVmust then be viewed in the light ofthe above. The basic data have beensupplied in good faith by the minesconcerned but may not be strictlycomparable between mines. Theauthors have arranged the data inroughly similar areas and havereduced the figures to cost per tonneof units of feed to the cycloneoperation concerned. The tonnagesare generally corrected for moisturecontent. The figures for screening,pump parts, and cyclone partsrefer solely to the cost of replace-ment parts and, together with thecost of ferrosilicon, represent themost accurately costed areas.

Dpspite all the above reservations,Table IV shows remarkably closeagreement between the operatingcosts of the various plants. Theslightly higher cost of spare parts

at Consoiidated Diamond Mines canbe related to the corrosive effect ofthe sea water used in this plant,while the higher use of medium atKoffiefontein may be due to thelack of storage facilities in thecircuit as discussed above. Thehigher power costs relate to themines that generate their own power,while the lower figures are frommines that use cheaper power fromthe national grid.

A reasonable conclusion from thesefigures is that the operating costoffairly large heavy-medium cycloneplants in the diamond field isabout 15 to 20 cents per tonne offeed to the heavy-medium cyclone.Capital Cost

The capital cost of heavy-mediumplants is increasing rapidly withinflation, but at the time of writingit is estimated that the budgetprice of a heavy-medium cycloneplant with feed preparation is ofthe order of R150 000 per 100 t/h ofcyclone capacity.

CONCLUSIONS

Heavy-medium cyclones are nowwidely used in the diamond industry,and it is hoped that the figuresgiven in this paper will enableother sections of the minerals in-dustry to assess the economic worthof this technique. As a primarygravity-separation system, it cannotrival the low cost of, say, the jigsused for the recovery of tin. How-ever, the heavy-medium cyclone iscapable of making a much morecomplete separation of minerals thathave only a small difference inspecific gravity than can a jiggingprocess, and it is capablc of handlinga coarser feed.

This aspect of the technique

. . . ,

makes it an ideal primary separatorfor the removal of clean, light ganguefrom partially liberated heavyminerals, with a consequent reduc-tion in the amount of ground to betreated by further expensive grind-ing and flotation methods. Heavy-medium cyclones are already beingsuccessfully used for primary con-centration in the recovery of uran-ium, tin, fluorspar, coal, and ironore in South Africa, and of tungstenin Portugal. The expansion of theunselective open-cast mining of low-grade ore bodies makes it seem likelythat there will be a growing field ofapplication for heavy-medium cy-clones in the mineral-processingindustry.

REFERENCES

1. Symposium of modern practices indiamond mining in South Africa.Johannesburg, South African Insti-tute of Mining and Metallurgy, Mar.1961.

2. DREISSEN, H. H. The contribution ofStamicarbonfDSM to the technologyfor the concentration of minerals andcoal. Koninklijk Institut van In-genieurs, November 1970.

3. KRIJGSMAN,C. The Dutch State Minesdense medium cyclone washer. Col-liery Engineering, vol 37, 1953, pp.328-332, 383.386.

4. LEEMAN, J. N. J. Cyclone washerprocess for the separation of coal.Institute of Mines, Metal and Fuels,Special Ins. 1964.

5. HAWHER, T. G. Recovery of magnetic-ally susceptible dense media materials.Mining and Minerals Engineering,1971.

6. JAMES, P. First interim report onferrosilicon corrosion studies. DiamondResearch Laboratory, Report No.E3.5f2.01. Nov. 1973. (Unpublished).

7. NAPIER-MuNN, T. J. The applicationof statistical techniques to the moni-toring of diamond plant performanceby tailings auditing. Diamond Re-search Laboratory, Report No.E8.4f3.01. Jul. 1974. (Unpublished).

8. INAMDAR,A. P. Heavy media systemsat Williamsons Diamonds Ltd Treat-ment Plant. Mwadui Engineering

TABLE IVOPERATING COST OF HEAVY-MEDIUM CYCLONE PLANTS IN SOUTH AFRICAN CENTS PER TONNE OF FEED TO THE CYCLONE

Labour and supervision. . . . . . . . . . . . . .Screening ........................Medium. . . . . . . . . . . . . . . . . . . . . . . . . .Cyclone parts ......................Pump parts ,...............Maintenance ......................Power and water ,.....Overheads and sundries. . . . . . . . . . . . . .

TOTAL"""""""""""'"

.

Tonnes per hour toheavy-medium cyclones. . . . . . . . . . . . . .

130 DECEMBER 1974 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Association, vol. 1, no. 6. Mar. 1971.9. COLLINS, B., NAPIER-MuNN, T. J.,

and SCIARONE, M. The production,properties and selection of ferrosiliconfor heavy media separation. J. S. Afr.Inst. Min. Metall., Dec. 1974 (thisissue) pp. 103-119

10. DAVIES, D. S., DREISSEN, H. H., andOLIVER, R. H. Advances in hydro-cyclone heavy media separation tech-nology for fine ores. Proc. 6th Int.Min. Proc. Cong., Cannes 1963.

DISCUSSION OF THE ABOVEPAPER

In introducing the above paper,Mr Chaston mentioned that it wasbased on a paper that he hadpresented to the InternationalMineral Processing Congress in Lon-don in 1973. However, as a resultof the recent work of his co-author,Mr Napier-Munn, several additionshad been made to the originalpaper. For example, this work hadled to a re-appraisal of the optimumhead required for gravity-fedcyclones. It now appeared that ahead of only 5 m was required tooperate standard 600 mm cyclonesand that, on the type of graveltreated at Consolidated DiamondMines, the quality of separation wasmarginally improved at this lowerhead. The consequent saving inconveyor length and structural steelin future plant designs would bevery considerable. The work alsoindicated that higher feed rates ofup to 160 tonnes per hour percyclone of this type of gravel couldbe handled by the standard 600mm cyclone without any appreci-able effect on the quality of theseparation.

The development of the mixtureof sodium nitrite and borax as acorrosion inhibitor could ce ofgreat interest to many operatorswho had problems during hot-weather shut-downs with cakingand gassing of corroding media.

The costs given in the paper wereapplicable to 1972 and had risenconsiderably since then. The averageoperating cost of a dense-mediumcyclone plant on the diamond minescould now be reckoned to lie in therange 20 to 25 cents per tonnetreated, rather than the 15 to 20cents given in the paper, and thecapital cost of recent plant hadbeen of the order of R200 000 toR250 000 per 100 tonnes per hourof cyclom capacity.

Mr Chaston wished that he hadas much faith in the high efficiencyof magnetic separators as had beenindicated by Dr Beeton in hiscontribution (pp. 115-116). If thiswas so, then it would seem possible,as he had suggested in the paper,to extend dense-medium cycloneoperation down in size to below the0,5 mm that was the present limitimposed by the difficulty andexpense of fine screening, and alsoto eliminate the expense of carefulfeed preparation. If the feed wascycloned to remove the bulk of theslime, it could be mixed with re-cycled ferrosilicon and passedthrough the cyclone. The productscould then be treated directly bymagnetic separators for the recoveryof medium without screening or,at most, the coarser material thatmight interfere with the operationof the magnetic separator could beremoved. Although the expense inmagnetic-separation equipmentmight be high, the operating cost ofmagnetic separators was very lowand such a plant could be extremelycompact. The major considerationwould be the very high recoveryefficiency required of the magneticseparators. At present, the authors'experience suggested that the ferro-silicon loss from the magnetic separa-tors at present in use would makesuch a system impractical, but itwould not seem impossible to hopethat, with improved magneticseparators, such a system would beoperable.

The table on page 133, which showsan analysis of the products of acyclone separation of a uraniumore, is of considerable interest.

It can be seen that most of theground is in the density range 2,6to 2,65. Nevertheless, after separa-tion in the cyclone, the sink productin this very narrow density rangeassayed three times the float pro-duct in the same density range. Itseems probable that this was dueto some fortuitous shape factor thatcauses the uranium-rich material tocollect in the sink. Such an effectwould not have shown up in apreliminary density analysis of thefeed material.

Mr Chaston closed his introductionby emphasizing the worth of thegravity-concentration process as a


cheap method of primary concentra-tion for the increasingly low-gradeore bodies of today and the future,and hoped that more researchstudents would continue theexcellent investigational work car-ried out by Professor Richards at theend of the last century into thescience of gravity concentration.This had been largely overlooked inthe rush of research work into thenew science of flotation, which haddeveloped in the early part of thecentury. It was now time to directsome of this energy into the develop-ment of improved methods of gravityconcentration, since this permitteda concentration to be made at acoarse size before the great expensein cost and energy that was involvedin fine grinding was incurred.

R. J. ADAMSON*

The paper is a comprehensivedescription of past and currentpractice, and it is therefore difficultfor me to offer a contribution onnormal lines-such as filling in gapsor elaborating on methods currentlyemployed or pending-the latterparticularly as I have not been inclose touch with dense-medium sepa-ration for the last five years. How-ever, it has been said, and I thinkjustifiably, that the history of ascience is in fact the science itself.By providing a history of heavy-medium separation as applied todiamond recovery, the authors haverendered a valuable addition to itsgeneral application III mineral-dressing techniques.

There is, however, one emendationand one explanation that I wish toadd to their presentation. Firstly Imust correct an impression conveyedby them that dense-medium pro-cesses were introduced to displacewashing pans in the treatment ofdiamondiferous ore. Up to the lastWorld War, methods of diamondrecovery in Southern Africa weremore or less evenly divided betweenpans and jigs. At Kimberley, atJagersfontein, and on the alluvialdiggings, the concentration of dia-mond ore depended on washing pansbut, at Premier Mine, at the StateAlluvial Diggings in Namaqualand,

*Chamber of Mines of South Africa.

DECEMBER)974 131

and in South West Africa, pans hadnot been successful and primaryconcentration was effected by jigs.At Premier Mine, so'called 'pulsionjigs' displaced pans from 1909 on-Wfti'ds, and, in Namftqualand andSouth West Africa; both Plietz andSchiechel jigs held Sway until 19501.

When heavy-medium separation~also the11 termed 'high D' andlater 'sink and float'-was inventedby Wuensch and his collaboratorsat Eagle Picher for the beneficiationof iron ore, the process depended oneither galena or iron pyrites asthe medium, with flotation the meansof medium recovery. Subsequently,Henry Wade and his staff at Minne-sota University developed a mediumof 15 per cent ferrosilicon, whichcould be recovered and cleanedby magnetic separators. The higherdensity of ferrosilicon (close to 7)greatly extended the scope of heavy-medium separation, as a separatingdensity of 2,8 to 3,0 could be sus-tained with a low viscosity in themedium.

As a result, great interest wasaroused in mineral-dressing circlesby this new process, and soon aslogan appeared propagating theadoption of heavy-medium separa-tion, claiming that 'whatever jigscan do, sink and float can do better'.

It was at this stage of heavy-medium separation that we in theAnglo American Corporation werestudying the best means oftreatmentfor the re. opening of Premier Mine,which had ceased operations in1932. As it appeared possible thatheavy-medium separation wouldoffer an improvement on the pre-vious jigging plant, a pilot plant of100 tons per hour capacity waserected in 1947, using ferrosiliconimported from the U.S.A. Followingsuccessful results, the main plantof 13 000 tons per day capacity wascommissioned in 19501. It was notonly the sole heavy-medium separa-tion plant in Africa, but tonnage-wise was the largest in the world.The 100 tons per hour pilot plantwas then transferred to ConsolidatedDiamond Mines in South WestAfrica, where it proved an excellentproduction unit, replacing the jiggingplant and meeting demands forextended output from 1951 on-wards. In both cases no difficulties

132 DECEMBER 1974

were experienced regarding patentrights and royalties-the latter, be-ing reasonable, were readily paid.

It would seem anomalous, there-fore, that, after heavy-mediumseparation by means of cones hadbee11 pioneered by diamond plantsin South Africa in the late forties,hydro cyclone separation units, usingeither magnetite or ferrosilicon as amedium, were not in operation inSouth Africa until 1961. The authorsmention the installation of hydro-cyclones during 1955 in Tanzaniaand in 1959 at Bakwanga. Presum-ably these installations were nothandicapped as we were in SouthAfrica and South West Africa bythe imposition of uneconomic royal-ties by the holders of the patentrights for the use of hydrocyclonesas concentrators employing magnet-ite or ferrosilicon media. The un-realistic royalty demands-particu-larly in application to the smallestand least valuable constituents ofthe diamondiferous ores-delayedthe use of hydro cyclones at PremierMine until 1961, the year in whichpatent rights were expiring. Thisresulted in the installation of hydro-cyclones with ferrosilicon media inthe re-treatment pla11t commissionedin 1962 to recover industrial dia-monds from the tailings dump.

From that stage onwards, as theauthors have clearly demonstrated,marked advances in the applicationof dense-medium cyclones have beenmade in diamond recovery over allsize ranges.

References

1. J. S. Afr. Inst. Min. Metall., Aug. 1959.2. Ibid., Sep. 1963.

R. J. MACGREGOR* and

C. B. PARKER*

Much published information isavailable to permit the precisedesign of 'classifying' cyclones forany particular application, but todate there has been a paucity ofpublished data on the design, andeven application, of the cyclone asa heavy-medium separator. Mostdesign criteria and the effects of thetype of medium used remain purely

*An~lo American Corporation of SouthAfrica Limited.

empirical. It is hoped that thepublication of Messrs Chaston's andNapier-Munn's paper will provokewider international interest, andwill lead to a general disseminationof the information already availableso that needless duplication ofresearch work is prevented.

It is interesting to compare theconcentration ratios for the variousplants as reflected in Table I.Dreyers Pan, Consolidated DiamondMines, and Kleinzee, all treat alluvialgravels and give the lowestconcentration ratios. Better ratiosare achieved when only kimberliteis treated. Is this effect solely dueto the lower ratio of heavy mineralcontent in kimberlite (generally,less than 0,2 per cent mass has aspecific gravity of more than 3,0),or is the separation affected byinherent slime generated in themedium circuits of the latter?

Feed preparation is correctlyshown to be an economic considera-tion balanced against the amountof medium to be cleaned per cycle.At the Premier Mine RetreatmentPlant, the quality of the water usedin circuit was found to have Itsignificant effect on separation overa period of time. The spray waterand make-up circuits consist entirelyof reclaimed water containing 8 to12 per cent suspended solids, whichare all in the minus 200 mesh sizerange and mainly colloidal in nature.Possibly owing to the decreasedscreening efficiency, a build-up ofslime in the medium circuit can bemeasured. This in effect displacesthe equivalent amount of mediumbeing recycled, i.e., the ratio ofmedium to ore (as mentioned insection B of the paper) graduallydecreases and results in a sharp dropin efficiency of separation, visible asa sudden increase in spigot productwith entrainment of lighter floatparticles. This effect is minimized inpractice by the 'bleeding off' ofmedium to the magnetic cleaningcircuit during 'slack' periods torestore the ratio.

It is felt that this unbalance ofthe medium-to-ore ratio by en.trained slimes is the main reasonfor an upset in separation efficiency,rather than the increased viscosityof the medium due to the slime.High shear rates in pump and

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND MI:TAL,I.U~GY

Float SinkDensity

I

range Mass % DaOs D.Os Mass % D"Os %of feed kg/t % of feed kg/t of feed

-2,6 2 0,03 - - - --2,65+2,6 55 0,06 17 19 0,20 19-2,7 +2,65 13 0,27 17 4 0,48 10-2,9 +2,7 - - - 4 1,00 20+2,9 - - - 3 1,20 17

cyclone tend to overcome the effects

of increased viscosity.

Experience with the earlier pump-

fed plants emphasized the necessity

of maintaining a constant head for

efficient separations. Only slight

variations, or rather fluctuations, in

medium levelin the pump sumpwere sufficient to reject radio-active

tracers to tailings.

Particle shape has been mentioned

as resulting in the entrainment of

flat, light particles in the spigot

product. In this regard it may be

worth mentioning a problem en-

countered with the Premier No. 9

cyclone unit treating old dump

tailings, in which layers of ash and

particles of unconsumed coal are

often found.

Like the grass roots of Orapa, a

considerable amount of coal reported

with the final concentrates, fouling

the grease surfaces in the final

recovery process and leading to

mechanical loss of diamonds.

Because it was not a gravity-fed

unit, there was no easy solution as

at Orapa. A measure of success (but

not a complete solution) wasachieved by altering the geometry of

the inlet to the cyclone, which at the

time was a semi-ribbon feed flush

with the top of the inlet chamber.

Lowering the top of the inlet below

the top of the inlet chamber and

using a wider and shorter inlet

aperture with substantially the

same inlet area appeared to have

the desired effect on the separation

by minimizing the amount of coal

entrained in the spigot product.

Further investigation on these

lines may be profitable. In this

regard, also worthy of mention

may be the wear pattern of the

vortex finders in the Premier retreat-

ment nickel-hard cyclones, which

are now routinely rotated by 900 or

1800 to extend their useful life.

Wear shortens the effective length

of the vortex finder by as much as

20 to 25 per cent before a decrease

in diamond recovery efficiency is

noted.

The most effective use of a heavy-

medium cyclone in older established

diamond plants, where jigs are part

of the concentration circuit, is as a

secondary concentrator treating the

very dilute jig concentrate. Under

these circumstances, the reconcen-

trating heavy-medium cyclone may

make a spigot product of 10 per

cent or upwards, i.e.,a concentration

ratio of 1 :10. Slight variations in

day-to-day efficiency are of no

import if the float product from the

cyclone is recycled to the jig feed.

AUTHORS' REPLY

Messrs MacGregor and Parker areto be thanked for their addition tothe information on the operationof the heavy-medium cyclones atPremier Mine. A sudden change inrheological properties of the mediummay present a very real danger toefficient separation. Recent work atthe Diamond Research Laboratorysuggests that, under high shearforces, ferrosilicon suspensions forma dilatent mixture where viscosityincreases under shear. This couldlead to some very odd effects in thecentral regions of high shear in thecyclone. The effect of a pseudo-plastic slime on the dilatent ferro-silicon mixture is being investigatedat the Diamond Research Labora-tory.

The reason for the lowerconcentration ratios in the treatmentof clean gravels as compared withthe treatment of kimberlites issolely related to the percentage ofheavy minerals, which is higher inthe gravels than in kimberlite.

Products from a heavy-medium cyclone treating a uranium ore

70 34 30 0,44 660,10

Electrochemistry Conference

The Fourth Australian Electro-chemistry Conference will be heldat The Flinders University of SouthAustralia, Adelaide, from 16th to20th February, 1976. The centraltheme will be 'Electrochemistryfor a Future Society', and thesessions will emphasize electro-chemical contributions in the follow-

ing areas:Energy Conversion and StorageMaterials Science and CorrosionUtilization of Natural ResourcesEnvironmental SciencesWaste Treatment and RecyclingBio-electrochemistry.

Other fields of electrochemistry willbe included in a general session.


Papers are now invited on theabove specific topics and for thegeneral session. The final submissiondate for titles is 1st August, 1975,and for abstracts is 1st October, 1975.

The Conference Secretary is Dr T.Biegler, CSIRO, Division of MineralChemistry, P.O. Box 124, PortMelbourne, Victoria 3207, Australia.

DECEMBER 1974 133

design andoperation ofdense-medium cyclone ... · design ofaheavy-medium cyclone plant at first...

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