Developments in centrifugal millingby A. A. BRADLEY*,B.Sc. (Rhodes) (Visitor),

A. J. FREEMANTLE*, B.Sc. Hons. (Wits) (Visitor), andP. J. D. LLOYD*, B.Sc. (Chem. Eng.), Ph.D. (Cape) (Fellow)

SYNOPSISThe theoretical and experimental results of a study of centrifugal mills are reviewed. Means for the scaling up of

such mills are suggested, and tests on a prototype mill are described. Despite the high lining wear in the centrifugalmachine, which reduces its productive availability, its other advantages suggest that there is merit in pursuing itsfurther development.

SAM EVATTI NGDie teoretiese en eksperimentele resultate van 'n studie van sentrifugale meule word in oenskou geneem. Metodes

vir die vergroting van sodanige meule volgens skaal word aan die hand gedoen en toetse op 'n prototipe meul wordbeskryf. Ten spyte van die hoe voeringslytasie in die sentrifugale toestel wat sy produktiewe beskikbaarheid vermin-der, dui sy ander voordele daarop dat dit die moeite werd is om met die verdere ontwikkeling daarvan voort te gaan.

INTRODUCTION

The concept of centrifugal millinghas long been known!, and manytypes of batch machine have beendeveloped2. In addition, work hasbeen carried out on a number ofcontinuous machines using the sameconcept3,4, but as yet there have beenno successful full-scale industrialapplications.

In 1968, work was initiated in theChamber of Mines of South AfricaResearch Organisation on the con-cept, because there seemed to bemerit in attempting to reduce thesheer bulk of conventional comminu-tion machines. Calculations indicatedthat the volume taken up by themachine should be a simple functionof the acceleration that was imposedon the grinding charge, and thataccelerations of the order of 103m.s-2 (102 g) should be readilyattainable in machinery of moderatesize operated at rotation speeds wellwithin the range of conventionalrotary-drive and bearing systems.Accordingly, a study of centrifugalmills was initiated, and it is thepurpose of this paper to review boththe theoretical and experimentalresults of this study.

THEORY OF CENTRIFUGALMILLING5

The centrifugal mill comprises agrinding tube that is rotated aboutits own axis, while that axis is inturn rotated about an axis of gyra-tion parallel to the tube axis, inorder to impose a centrifugal forceupon the tube and its contents. This

*Metallurgy Division, Chamber of Minesof South Africa Research Organisation.

is indicated in Fig. 1. Plainly, thegrinding tube may be rotated aboutits own axis by a suitable arrange-ment of gears, of which sun-and-planet or sun-idler-planet arrange-ments are particularly convenient ifthe axis of the sun gear is the sameas that of gyration, and the axis ofthe planet gear is the same as thatof the mill.

It may be thought that there is a

limit to the speed of rotation of suchan assembly above which the chargein the grinding tube will centrifuge.It transpires that this limit is set bythe gear ratio, not by the speed, asthe following argument indicates.

Consider a sun-to-planet gear ratioof R, taken to be positive when thedirection of rotation about the tubeaxis and the direction of rotationabout the gyration axis are the

~

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'6

Planetary gear

Grinding tube

11::J

I-

-',\---Idler gear. I--_\ I

" ,~ // ~\.....-/ \ --\

@I J~--""\ I

/1/' , /'I \ --

"-III

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Axis of gyration

Stationary sun gear

~0

~Fig. I-Configuration used for the analysis of the tumbling action in a centrifugal

mill.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JUNE 1974 379

same (negatIve R resuitIng, forexample, when there is an idler gearbetween the sun and the planetgears). Then, it can be easily shownthat, for a frequency of gyration N,the frequency of rotation of the milland tube is (R+l)N.

Then, if the force due to gravityis neglected, a small particle on themill wall will 'centrifuge'; that is, themill will be operating at the criticalspeed, when the forces experiencedby that particle just tend to keep theparticle on the wall. The force dueto rotation of the tube about its ownaxis results from the acceleration2172D[(R+l)N]2, where D is thediameter of the mill. The force due togyration of the tube about thecentral axis similarly results fromthe acceleration 2172GN2, where G isthe diameter of the circle of gyration.The critical speed will be reachedwhen these two accelerations bal-ance, i.e., when

2172D(R+ 1)2N2=2172GN2,or when

(R+l)2=GID.Thus, critical operation of the mill

is determined by its configuration,as set by the ratio GID, and by thegear ratio R. For a fixed ratio GID,there exists a critical gear ratio Rcgiven by

Rc = -1 :t yGID,and it will be noted that there arein fact two such ratios. It is also ofinterest to note that, at R = - 1,

GID = 0 for the above equation tobe valid, which implies that there isno mill configuration of finite sizethat can be operated in a criticalstate at this gear ratio.

It is also important to note that

.....--------.

Fig. 2-Couple formed by the displace-ment of the mill load by friction.

380 JUNE 1974

frequency does not enter Into theequation for the critical behaviourof the centrifugal mill. Thus, the millcan be operated at a frequency setonly by the limitations on thestrength of the materials employedand on the available power.

The power drawn by the mill canbe estimated approximately as fol-lows. The mill charge can be regardedas being due to a mass M with centreof mass a distance Z from the centreof the mill. Because of frictionalforces between the wall of the milland the mill charge, and because ofthe rotation of the mill, the centreof the mass will not lie on the linethrough the axis of gyration and theaxis of the mill rotation, but will bedisplaced by an angle 8 from thatline (see Fig. 2). Thus, a couple willbe set up that will adsorb power ata rate proportional to M. Z sin 8.In addition, the frequency of rotationof the mill is, as before, (R+l)N,and, because the mill charge mustfollow the main rotary field causedby gyration about the central axis,the mill charge will lag behind themill, and thus the effective frequencyof rotation of the charge will be

N - (R+l)N = -NR.Finally, the acceleration impressedupon the mill charge will be closeto 2172GN2, and thus the mill power,P, will be given by

P a M.Z sin 8 (-NR) .2172GN2.Now, for a given proportional

filling of the mill tube, Z a D. Also,it seems reasonable to assume that8 will not vary greatly, so that sin8 will be effectively constant. For aconstant level of mill charge, themass of the charge will be propor-tional to D2L and thereforeP a D2L.D. (-NR) . (2172GN2),

i.e., P a D.3L.R.G.N3.Thus, to a first approximation for

a mill of chosen configuration andgear ratio, the power drawn by themill should be proportional to thecube of the speed of rotation.

It seems reasonable to suggest thatscale-up of a centrifugal mill shouldbe based on a constant ratio GID, aconstant shape of the mill tube (i.e.,constant LID), a constant gear ratioR, and a constant acceleration im-posed upon the charge (i.e., constant2172 GN2). If this is done, then thepower drawn by the mill is propor-tional to K3.5, where K is the ratio

between a dImensIon of the smalimill and the same dimension of thescaled-up mill. This indicates thatvery small mills can consume largeamounts of power. For instance, if alaboratory-scale mill with a diameterof gyration of 20 cm were found torequire 2 kW for efficient milling, amill with a diameter of gyration ofonly 60 cm would demand nearly100 kW when operated under identic-al conditions, and a 1,44 m millwould demand 1000 kW. Plainly,therefore, it should be possible inprinciple to design very compact,high-capacity machines by the useof this principle.

LABORATORYEXPERIMENT ATI ON

The theory of the critical be-haviour of the centrifugal mill waschecked on a single-tube transparentmodel in which the effective gearratio was infinitely variable and inwhich the ratio GID could be variedby changes in the mill diameter. Themill was charged with small, vari-coloured plastic spheres to approxi-mately 50 per cent by volume.High-speed, colour motion pictureswere taken while the mill was inoperation and the gear ratio wasbeing varied.

With smooth liners, so much slipcould be generated between thecharge and the mill wall that it wasimpossible to determine preciselywhen the mill became critical. How-ever, when small lifter bars wereinserted, slip was prevented and theonset of critical behaviour could thenbe determined exactly. These studies,which were carried out with GIDratios of 1,5 and 0,5, and with bothpositive and negative gear ratios,confirmed the theory of criticalbehaviour to within the limits ofexperimental error. Most of thiserror arose from the determinationof the exact value of R at the pointwhere critical behaviour was firstobserved.

A laboratory-scale mill was con-structed for preliminary batch tests.Plate I shows this mill, which hastwo tubes each of inside dimensions82,5 mm long and 63,5 mm diameter.These tubes are driven by a sun-and-planet arrangement of gears about acentral axis of gyration, and theradius of the circle of gyration is

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

fixed at 76 mm. Drive is via ashunt-wound variable-speed d.c.motor. Accelerations of up to 2 x 103m/s2 can be imposed on the tubes,which occur at a speed of rotation ofabout 26 Hz (1560 rev/min).

Grinding tests were carried outwith a feed of minus 2,38 plus 1,19mm quartzite and steel balls. Anumber of tests were undertaken tofind the optimum conditions ofloading, and a charge consisting of500 g of balls, 125 g of dry feed, and62,5 g of water, which filled about70 percent of the mill volume, wasfound to be optimal. Studies werethen carried out on the effects ofball size and speed of rotation. Theresults are shown in Figs. 3 and 4.

In Fig. 3, the fraction of the chargereduced to minus 74 /Lm is shown asa function of ball size for three speeds

of rotation. The time of milling wasreduced as the speed was increasedto ensure identical net energy inputsat each speed, on the assumptionthat the power varied as the cube ofthe speed. The results shown indicateclearly that, at each speed, thereexists an optimum ball size, and thatthis optimum size decreases as thespeed is increased. This may implythat, at anyone speed, as the ballsize is reduced the grinding improvesbecause of the increase in the numberof impacts, but that at a certainpoint the force delivered by eachimpact drops to the point wherebreakage becomes inefficient, andthe rate of comminution accordinglydrops off markedly. From Fig. 3, itcan be estimated that the optimumball size varies approximately as(speed)-O,5. Thus at the higher

Plate I-The batch centrifugal mill. Two tubes driven by a sun-idler-planet arrange-ment of gears are shown installed in the circular frame, which is rotated round the

stationary sun gears. A dismantled tube is shown in the foreground.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

speeds, the ball diameter may bereduced markedly without reducingthe efficiency of grinding. While thismay not be of importance when steelgrinding media are used, there maybe significant advantage whenpebbles are used because the availa-bility of pebbles of adequate sizeis often limited in conventionalmilling.

The results shown in Fig. 4indicate the importance of usinggrinding medium of the correct sizeto obtain the minimum energyconsumption in the grinding of agiven feed. They also indicate that,over a limited range of speeds, thegrinding mechanism in the centri-fugal mill is probably constant,because the net energy used doesnot vary greatly with speed. Itshould be noted that the results at16,6 Hz may be biased because ofthe difficulties of estimating thepower consumption during a 10-second run. The net energy wasestimated in these tests by monitor-ing the armature voltage and cur-rent, and subtracting the energyneeded to run the mill empty andat the same speed. A small correctionwas also made for the losses inarmature resistance.

It can be noted that, from Fig. 3,at 4,2 Hz for 640 seconds the feedwas reduced to approximately 60per cent minus 74 /Lm, at 8,3 Hz for80 seconds to approximately 60 percent minus 74 /Lm, and at 16,6 Hz for10 seconds to approximately 55 percent minus 74 /Lm. If the difficultiesof experimentation at the fastestspeed are borne in mind, it seemsreasonable to suggest that theseresults confirm the cube law ofpower as a function of speed, becausedoubling the speed reduces the timetaken to produce a product of con-stant composition by a factor of 23.

PROTOTYPE TESTING

On the basis of the laboratoryresults, a continuous centrifugal millwas designed (Fig. 5). The mill hasthree grinding tubes each of insidedimensions 200 mm diameter and350 mm length. These tubes aredriven by a sun-and-planet geararrangement giving a value ofR = - 1. The diameter of the circleof gyration is 330 mm, giving a ratioG/D=1,65.

JUNE 1974 381

382 JUNE 1974

EE

~cg

70

60UIUI~§

:0:; 50u::J~0~0.-0 40~

305 10 15

<:> ',2 Hz, 640 s

c:J 8.3 Hz, 80 s

A 16,6 Hz, 10 s

20

Fig. 3- The effect of rotation speed and ball size on the milling process.

EE

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Ball size (mm)

28

26

24

22

0 4,2 Hz, 640 s

G 8.3 Hz, 805

~ 16.6 Hz, 105

20

Fig. 4- The effect of rotation speed and ball size on the net consumption of energy.

20

18

165 10 15

Ball size (mm)

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Rotation speed, Hz 3,3 5 6,7 8,3 10 11,7Net power, kW 4,1 13,7 32,5 63,5 110 174Centripetal acceleration, 10 m 2/S 2 0,9 2,0 3,5 5,6 8,0 11,0

- 74 /Lm material, t/h . . . , , . 0,15 0,50 1,28 2,3 4,0 6,4

PREDICTED PERFORMANCE OF THE PROTOTYPE MILL

TABLE I

The feed enters a conical rotatinghopper having three 30 mm outletsat its base. These outlets dischargeinto a spiral feeder 6 attached toeach grinding tube; this arrange-ment was chosen because it obviatesthe need for any seal between thehopper outlets and the grinding tube.The product of the mill is dischargedby overflowing down a trunnion andis caught in a slurry tray. A screenmay be fitted in each grinding tubeas shown. The whole assembly isdriven by a hydraulic motor, anarrangement that was used to allowvariable-speed operation withoutgreat loss in efficiency.

The mill is operated vertically.This does not mean that there is anyrisk of discharge due to gravity,because, even at the lowest speedsused, the centripetal acceleration isover 90 m/s2, or nine times that dueto gravity. The upper level of thecharge thus lies effectively verticalduring operation.

The predicted performance of themill is given in Table 1. Tests of theperformance were carried out onbatches of a crushed quartzite con-taining 15 per cent by mass of plus4,76 mm material and 14 per centof minus 0,074 mm material.

The grinding medium was initially17,5 mm steel balls, but, duringrepeated tests on successive batchesof feed, the ball charge was worntowards an equilibrium size distribu-tion that varied with the speed atwhich the tests were carried out.Fig. 6 shows the results of thesetests, and it is clear that, as the ballcharge was worn towards the equili-brium size distribution, the grindingefficiency increased. This was alsoindicated by the energy consump-tion, which was about 40 kW hit otminus 74 /Lm material with the initialball charge, but which fell to between23 kW hit at 6 Hz and 29 kW hit at10 Hz at the end of the test period.

The equilibrium capacity of themill appears from Fig. 6 to beslightly over 1 t of minus 74 /Lm

material per hour at 6 Hz, about 2tlh at 8 Hz, and 4 tlh at 10 Hz. Thisis in gratifyingly close agreementwith the performance predicted fromthe results of small-scale batch tests.

The mill has been installed on amine where it is now being operatedin closed circuit. Circulating loads ofup to 500 per cent can be achievedby the use of one or more 76 mmcyclones in parallel. The mill dis-charge can contain up to 25 t of

solids per hour. Fresh feed is drawnfrom an ore bin independent of themain mill supply, and is fed to themill by a belt feeder at rates of up to12,5 t/h. The feed, which is nominallyminus 7 mm, can be diverted intoa bin mounted on a weightometerto calibrate the feed rate. Fresh feedcan go either direct to the mill ordirect to the cyclones. The mill dis-charge, which contains 60 to 70 percent moisture, passes through a flow-

Fixedhopper

Rotatinghopper

'Spiralfeeder'

Mill,tube

,Gearbox

Hydraulicmotor

Spray ring

Flexible probe

Upper tube bearing

,

Screen

Discharge

Lower.tube bearing

Slurry tray

~rmrill

Fig. 5-Diagram of the three-tube prototype centrifugal mill.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JUNE 1974 383

meter into an agitated sump 1,8 mdiameter by 1,8 m high. The sumplevel is maintained constant by theaddition of water. The grindingmedium is steel of 16 mm diameterby 19 mm and is fed to the mill by avibratory feeder. The rate of feed iscontrolled manually by observationof the power drawn by the mill.Because of the short residence timein the mill (about five seconds), thismethod of control is very satis-factory, and changes in the feed ratecan be detected virtually instantane-ously. Any medium that escapesfrom the mill is caught by permanentmagnets installed in the dischargelaunder.

.

Much of the work on the mill in itspresent form has gone into gainingexperience and confidence in its useas a continuous production tool.Various features can be summarizedas follows.Productivity

It has proved difficult to determinethe productivity with any degree ofprecision, but instrumentation in-stalled recently has indicated a highprobability that 2,5 t of minus 75/Lm material per hour is exceededwhen the cyclone overflow is approxi-

~~~:i 20EE..'"~ ~1;..g~ I ~

Qo ID

Relativo time of operation

Fig. 6- The performance of the proto-type centrifugal mill in long-term

continuous open-circuit operation.

31;14 JUNE 1974

mately 70 per cent by mass of minus75 /Lm material and the mill isoperated at 8,3 Hz.Throughput

When the mill is operated as anoverflow mill, throughputs of 25 t ofsolids per hour have been achieved.Discharge screens to retain thegrinding medium have proved tolimit the throughput to about 15 tjhat 8,3 Hz operation.Energy Consumption

Because of the difficulties experi-enced in the precise determination ofmill productivity, it has proveddifficult also to estimate the powerdemand. However, the figure of2,5 t of minus 75 /Lm material perhour at 8,3 Hz indicates a net energyconsumption of about 26 kW h pertonne, which is in good agreementwith the energy consumption inconventional milling on the mine atwhich the work is being undertaken.The net energy in this case includescorrections for the efficiency of thehydraulic-drive system employed onthe mill and for the power demandof the pump to feed the cyclones.Consumption of Grinding Media

The steel being used is by nomeans optimized for wear perform-ance. Accordingly, the estimatedconsumption of about 4 kg pertonne of minus 75 /Lm materialproduced seems reasonable.

Liner WearMild-steel liners appear to be worn

in the centrifugal mill at approxi-mately I kg per tonne of minus 75/Lm material produced. Because ofthe small mass of the wear partswithin the mill, this indicates a linerlife of only 12 hours at 10 Hzoperation, which may seem cata-strophic were it not for the fact thatthe mill tubes can be replaced inabout 3 hours with the mill in itspresent configuration. Rubber linersappear to be more successful, andthe mill is currently being operatedwith a serrated rubber liner. A wearrate of only 0,13 kg per tonne ofminus 75 /Lm material has beenindicated in some tests, and there isconfidence that this can comfortablybe exceeded. The implications ofliner wear in the centrifugal millare discussed more fully below.

The Wear of Screens and OtherInternals

Discharge screen wear has been

very rapid. Cast polyurethanescreens lost up to 50 per cent of theirthickness (20 mm) in 4 hours ofoperation at 8,3 Hz, mild-steelscreens lost up to 64 per cent of theirthickness (12 mm) in 8 hours, andwear-resistant steels lost up to 70per cent of their thickness (12 mm)in 11 hours, most of this wearoccurring at the centre of the screen.As noted above, however, screenshave been found to limit the through-put of the mill severely, and,accordingly, the preferred method ofoperation is without dischargescreens, i.e., as an overflow mill.Little grinding medium escapes fromthe mill whilst it is in operation,and, because of the small size of themedium, it is possible to pump themedium to the cyclones, where itreports to the underflow and is re-cycled to the mill. Wear of otherparts of the mill has been slight.Mild-steel spiral-feeders worethrough after the passage of about1000 t offresh feed, but rubber linersin this position are showing noapparent wear after the passage ofa similar tonnage. The liner endpieces and the discharge trunnionliner, all made of rubber, appear tobe behaving satisfactorily.Feeding

No particular difficulty has beenexperienced in the feeding of oreup to 15 mm in size, but choking wasexperienced at 22 mm during a testof autogenous grinding. Grindingmedium was found to choke thetubes leading from the hopper to thespiral feeders, but these tubes havebeen enlarged and straightened toovercome this difficulty.Discharge

No difficulty has been experiencedwith the discharge system.

OperationStartup and shutdown have

proved to be relatively simple,although, when the mill is operatedvertically and without dischargescreens, the contents of the tubesare released on shutdown and duringstartup this charge must be built upagain. In future designs, however,horizontal operation would over-come this difficulty. The mill hasoperated in reasonable balance pro-vided no chokes have occurred.Some self-balancing is expected be-cause tubes with an excess of charge

,JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

will draw more power and thus wearthe charge more rapidly. In one test,of 124,4 kg fed, the 64,8 kg of grind-ing medium remaining was found tobe distributed among the three tubesas follows: 20,9 21,6 22,3 kg. Anychoking rapidly puts the mill outof balance, but stopping the feedand operating at reduced speedusually clears the choke quickly andbrings the mill back into balance.During prolonged operation, the oilin the hydraulic-drive system heatedup more than was expected, and ithas been necessary to install a watercooler to prevent overheating.Maintenance

The maintenance necessary hasbeen mainly the replacement of wornparts as noted above. However, theupper bearings failed when dirtpassed through incorrectly installedseals, and examination of the gear-box revealed excessive wear, whichwas traced to underdesign of thegears. In other respects, the proto-type has required no significantmaintenance.

Thus, at the present stage, confi-dence has been generated that theprototype centrifugal mill can beoperated continuously and for rela-tively extended periods. Presentwork is aimed at a demonstrationof the availability of the mill as aproduction tool, because it is believedthat availability is a better measureof liner performance than the periodbetween relinings, particularly as thetime taken to reline the mill com-pletely is measured in minutesrather than in days.

DEVELOPMENTS FROMTHE PROTOTYPE

At the present stage, it is believedthat an 80 per cent availability canbe demonstrated for the prototypemill, and that this is the only para-meter in its performance that inany way puts it at a disadvantagein a comparison with conventionalmills, which have an availability ofabout 95 per cent. Two lines ofattack are being mounted on thisproblem. The first has arisen froman assessment of the time taken tochange liners on the prototype. Notonly is the design of the prototypevery cramped, but three tubes mustbe removed during the liner change.Patently, the time taken to replace

the liners could be drastically re-duced if there were only a single tube,and if a less compact design wereadopted.

Studies have shown that a singletube design is not only practical butthat it can offer a number of advan-tages in addition to reducing thetime taken for relining. Plate IIshows a working model of sucha design, with a large-diameter spiralfeeder above, a short grinding tubecounterbalanced by a single weight,discharge ports just above thecounterbalance arm, and a maindriving motor that turns the millvia a two-gear gearbox. The wholeassembly, including the main drivingmotor, is turned by a low-poweredsubsidiary motor that is merelyrequired to provide the centripetalacceleration. The feeding of this millis simple, because the spiral feedercan be made so that there is anopening which is stationary at theaxis of gyration, and there is thus noneed for a rotating hopper as on themulti tube mill. The single tube canbe removed and replaced extremelyrapidly, as required for minimizingthe time taken to reline. The drivesystem is extremely simple and per-mits very smooth start-up, becausethe main driving motor effectivelydraws no power until the subsidiarymotor starts and, if the subsidiarymotor is of variable speed, the rateof build-up of power demand by themain motor can be controlled by therate at which the subsidiary motoris brought up to speed. In addition,with a mill of large diameter relativeto the amplitude of gyration (i.e.,with a small aiD ratio), a machineof a throughput equal to that of amulti tube machine can be designed,which occupies no greater volume.

A prototype based on this prin-ciple has been designed and is beingbuilt for the testing of new linersystems. It is expected that thismachine will make it possible todemonstrate that availabilities ofover 90 per cent are feasible withoutany further improvements in linerperformance.

The second line of attack on theproblem of improving the availabilityof the centrifugal mill is in the direc-tion of new liner systems. Becausethe area of the mill is so small rela-tive to its throughput, compara-

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

tively exotic materials can be con-sidered for the liner. Compoundssuch as boron carbide and tungstencarbide have been studied, and,while they do indeed show promiseof increasing the availability of themill, they do not appear to do so toan extent that would nullify theirhigh cost. A more promising solutionappears to lie in the direction ofpneumatic rubber liners7.

This development is based on thefact that a minimum thickness ofsolid rubber is required to resist wearduring impact. Thicknesses of solidrubber less than this minimum haveinsufficient energy absorption toresist impact and therefore wearrapidly. However, a thin layer ofrubber backed by a fluid has anenergy absorption at least as goodas a thick layer of solid rubber, evenwhen the total thickness of thepneumatic system is less than thatof the solid rubber. In addition, ofcourse, abrasive wear can take placein the mill, but again there is reasonto suppose that the pneumaticsystem is superior to solid rubber,because the rubber in the pneumaticsystem can more readily be placedin compression within the mill, inwhich condition rubber is far moreresistant to abrasive wear than it iswhen unstressed or in tension. Thisdevelopment is being pursued inassociation with an internationalrubber company that is to manufac-ture liners for test in the single-tubeprototype.

THE FUTURE FOR THECENTRIFUGAL MILL

From what has been said in theprevious sections, it is obvious thatthe centrifugal mill is at present byno means a fully developed machine.However, there are most cogentreasons for believing that develop-ment should continue, and it is thepurpose of this section to brieflyreview those reasons.

Firstly, there is every prospectthat the centrifugal mill will offerthe conventional mill strong com-petition in normal fine-grindingapplications. The centrifugal millshould cost not more than 15 percent of the cost of a conventionalmill of equal capacity, and should beable to be installed for far less thana conventional mill. It should be far

JUNE 1974 385

Plate II-A working model of a single-tube continuous mill. A description of thecomponents is given in the text.

easier to drive, because the powermust be transmitted at relativelyhigh rotational speeds, and driveshafts and gears can thus be reducedin size. As described earlier, smoothstart-up can be expected withoutthe need for expensive switchgearor clutches. Control of the millingoperation may be simplified becauseof the rapid response to changes inthe inputs. There may be new possi-bilities for autogenous or pebblemilling because of the freedom fromlimitations on the availability of

pebbles of adequate size. The chiefdisadvantage may be the need formore frequent maintenance, but thismay be offset by a reduction in thesize of the inventory of wear parts.

Secondly, there is a possibility thatthe centrifugal mill can be used inan unconventional role in the systemfor the recovery of metals from ores.Some possible applications in open-pit operations were reviewed pre-viously8, when it was shown that asystem comprising a mobile crushingand centrifugal milling plant within

386 JUNE 1974

the pit, and hydraulic hoisting ofthe milled ore from the pit, couldoffer significant economies. There ismerit in the consideration of such astep in some detail, because theconventiong,l means of transportingore from an open-pit mine caninvolve costs as high as 40 per centof the total cost of winning the metal.A hydraulic-hoisting system wouldoffer advantages over the conveyorsystems already used because ofgreater flexibility and lower capitalcost.

The possible implications of asimilar rock-transportation systemin underground mines have also beenreviewed9. In that study, it wasfound that, in deep-level mines, theremight be great merit in making moregeneral use of incline shafts for thetransport of men and materials, andmilling of the ore followed by hy-draulic hoisting. If undergroundmilling were to be practised, itwould be necessary to use very smallmills, i.e., either a multiplicity ofsmall conventional mills or a rela-tively few centrifugal mills.

There would also be great meritin performing some metallurgicaltreatment underground on the milledore in order to concentrate the metalvalues and generate a waste suitablefor hydraulic backfill. The implica-tions of such a step are considerable,and require far more assessment thanhas thus far been possible. However,it can safely be said that it wouldoffer the following:(1) a marked reduction in the ton-

nage to be hoisted, and conse-quent release of hoisting capa-city for other purposes;

(2) an improvement in strata con-trol, which might make practicalan increase in the amount of orethat could be mined at greatdepths;

(3) a reduction in the quantity ofmaterials needed for support,and a consequent reduction inthe fire hazard; and

(4) a means of mining a series ofreefs that are too closely spacedto be mined by present methods.

A constraint on the use of millingequipment underground would bethe additional heat load on theventilation system. This effect might,however, be minimized by appropri-ate siting of the equipment, and by

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

the reduction in heat pick-up fromworked-out areas once they werefilled.

At present, the study is beingdirected at means for the concentra-tion of gold and other minerals intoa concentrate that is 20 to 40 percent by mass of the feed and thatcontains more than 95 per cent of thegold in the feed. A possible 5 percent loss of gold may appear veryhigh at present gold prices, but itshould be remembered that, belowcertain depths, it is currently beingfound necessary to leave up to 20per cent of the reef in situ for supportpurposes. In addition, if lower ton-nages are fed to existing reductionworks, an increase in their efficiencymay be expected, which would leadto a recovery of some of that 'lost'gold.

CONCLUSIONS

In this paper, the theory of centri-fugal milling has been presentedbriefly and it has been demonstratedthat the theory is obeyed in practice.Means for the scaling up of centri-fugal mills have been suggested, anda test of those means has indicatedthat the results of small-scale batchtests can be applied to the design oflarge continuous machines that willgive predictable performance.

Tests of a prototype machine haveindicated that continuous centrifugal

mills are powerful, compact machineson a practical scale, and that theycan be operated for long periodswithout difficulty. Liner wear, how-ever, reduces the productive avail-ability ofthese mills, and, while thereare a number of possible solutionsto this problem (such as increasingthe number of machines for a givenduty, or reducing the number ofgrinding tubes), it has not beendemonstrated that centrifugal millscan match the availability of con-ventional mills, the best expectedavailability being about 90 per cent.

New liner systems may offer ameans for improving the availabilityof centrifugal mills, and pneumaticliners have particular promise forachieving this end.

The advantages of the centrifugalmill over the conventional mill havebeen outlined. These advantages aresuch as to suggest that there is greatmerit in pursuing the further de-velopment of the mill. In particular,the availability of centrifugal millswould permit an integration ofmilling and rock transport in mines.In both open-pit and undergroundmines, this integration offers con-siderable financial advantages. Inaddition, it is possible to considersimple metallurgical procedures toreduce the tonnage of ore to betransported to surface, and this holdspromise of permitting a new method

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

of mining to be developed for theSouth African gold-mining industry.

ACKNOWLEDGMENTS

The authors wish to acknowledgethe assistance of many individuals,particularly the staff of DurbanRoodepoort Deep, Limited, andMessrs G. R. Jennery, R. Neilson,D. G. Sheppard, and P. Willows. Inaddition, they acknowledge the per-mission granted by the Chamber ofMines of South Africa to publish thispaper.

REFERENCES1. HERZFELD, A. U.S. Pat. 569 828. 20th

Oct., 1896.2. For example, STEEL and COWLISHAW,

LIMITED, Henley, Staffs., England.Mark lA high-speed planetary ballmill.

3. DUBROWIN, B. 'N. Bergakademie, vo!.21, no. 12. 1969.

4. EpPERS, W. 'Po U.S. Pat. I 951 823.20th Mar., 1934.

5. BRADLEY, A. A. Proe. 3rd EuropeanSymp. Comminution, Vo!. 2. 1972. pp.781-803.

6. CHAMBER OF MINES SERVICES (PTY)LIMITED. S.Afr. Pat. 71/5740. 27thAug., 1971.

7. CHAMBER OF MINES SERVICES (PTY)LIMITED. S.Afr. Pat. 73/1076. 15thFeb., 1973.

8. BRADLEY, A. A., LLOYD, P. J. D.,WHITE, D. A., and WILLOWS, 'Po W.S.A. Meek. Engr, vo!. 22, no. 4. 1972.pp. 129-133.

9. LLOYD, P. 'J. D., HINDE, A. L., andBRADLEY, A. A. Int. Coni. HoistingMen, Materials, Minerals. Johannes-burg, Oct., 1973. Paper 39.

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