IMPLEMENTING STRATEGIES TO IMPROVE MILL CAPACITY AND EFFICIENCY 85

IntroductionThe purpose of this paper is to present an analysis of thetypical grinding circuits used in the processing of basemetals with emphasis on classification. This paper presentshistorical references to past classification practices,practices used presently, and the most up to date technologyavailable. The overall intention is to provide end users themeans to increase production capacity while realizing otherbenefits such as improved flotation recovery and reducedchemical consumption, improved filtration, better control ofcircuit product size distribution, etc.

Early mineral processing techniques employed the use ofgrinding mills as the first phases of liberating and isolatingdesirable minerals from gangue. In the early phases it wasalso recognized that some form of size classificationassociated with the grinding mill would allow for moreefficient operation of these grinding mills. We will look atthese classification technologies as they have evolvedthrough the years with attention paid to physical andeconomic limitations of the technologies. Accuracy of theclassification is paramount in efficient grinding circuitoperation; however, accuracy comes at a price. Of coursewe all recognize that economic considerations mustultimately rule in selecting appropriate technologies.

HistoryTraditionally the classification systems used in closedgrinding circuits comprised rake classifiers, spiralclassifiers, hydrocyclones, sedimentation cones, and insome cases rudimentary commodity style vibratoryscreening machines with steel wire surfaces.

The classification of particles in rakes, spirals, cyclones,and sedimentation cones is based on settling rate velocities,which take into account both particle size and density.Vibratory screening machines utilize particle size only inthe classification process. It is this fundamental differencebetween hydraulic classifiers and vibratory screens whichcreated the potential for increased capacity in closedgrinding circuits.

In 1925 E.W. Davis, see Figure 1, conducted tests whichcompared rake classifiers, spiral classifiers and vibratoryscreens in the closing of grinding circuits. He concludedthat vibratory screens provided higher capacity and bettercontrol over grind size than either rake or spiral classifiers.Grinding mills at the time were comparatively small,therefore some attempts were made to utilize vibratoryscreens in full-scale operations based on this testing. Due tolimitations in the weaving of stainless steel wire at the timeand the inherent problems with wire cloth relating toblinding, high wear rates and replacement costs, these full-

BARKHUYSEN, N..J. Implementing strategies to improve mill capacity and efficiency, with platinum references and case studies . The 4th InternationalPlatinum Conference, Platinum in transition ‘Boom or Bust’, The Southern African Institute of Mining and Metallurgy, 2010.

Implementing strategies to improve mill capacity andefficiency, with platinum references and case studies

N.J. BARKHUYSENDerrick Corporation, South Africa

Recent advances in fine screening technology allow for the efficient classification of mill productsby means of particle size separation only. The advantages are numerous, including improvedthroughput, reduced power consumption, coarser grind and reduced reagent consumption. Thepaper will review the history of fine screening from the early 20th century to modern daytechnology and will detail the economic benefit obtained through modern day fine classificationtechniques by means of the patented Stack SizerTM technology.

Figure 1. Title page of article published in 1925

PLATINUM IN TRANSITION ‘BOOM OR BUST’86

scale operations proved impractical and uneconomical. Inaddition the screening machines produced at the time hadextremely low capacity, requiring huge amounts of floorspace. This floor space requirement increased installedcosts tremendously, thus making vibratory screensimpractical.

After WWII, hydrocyclones became the norm in theclosing of grinding circuits. As mill sizes increased, rakeand spiral classifiers became less economically viable dueto their large footprint requirements and capital costs. Thehydrocyclone on the other hand required very little space,provided high capacity with relatively low capital costs. Forthese reasons and the increased demand for refinedminerals, processing plant designs started catering forgreater tonnages. With these larger processing facilitiesbecoming the norm, hydrocyclones became the standarddevice for closing of grinding circuits.

Operational experiences have indicated that theseparation efficiencies of hydrocyclones fall between 45%and 65%. Throughout the years, cyclone manufacturersmade a series of modifications in the design includinggeometry, materials of construction, cone angles, vortexfinder penetration and diameter, etc. These changes weremade in efforts to improve performance, and in some casesprovided marginal improvements, but most processingplants never surpassed the 60% separation efficiency level.The end result of closing grinding circuits with thisrelatively low separation efficiency device is that mostmills in the base metals industry operate with circulatingloads in excess of 200%.

Other studies concluded with statements such as ‘lowergrinding costs are possible with screen circuits’ (Albert1945) and ‘the master key for great improvements incapacity and in energy consumption in closed grindingcircuits is in improved sharpness of classification’ (Hukkiand Eland 1965). A recent presentation on flowsheetdevelopment for the planned Essar steel iron oreconcentrator project in Minnesota, USA, which includedresults from pilot-scale tests and simulation work,demonstrated a significant coarsening of the grind-graderelationship through the use of screens instead of cyclones(Murr, Wennen and Nordstrom 2009).

Stack sizer screening machineIn 2000, Derrick Corporation designed and introduced theStack Sizer™ multiple deck fine wet screening machine asillustrated in Figure 2. In review of Figure 2 we see that asingle feed is introduced to the screening machine at fiveseparate feed points by means of a standard flow divider.This is done in order to take advantage of the well knownfact that width of screen surface is the most importantfactor in determining the capacity and efficiency of wetscreening machines vs. length or area. It is understood thatonce free water leaves the mineral slurry on a screensurface, sizing ceases to occur. Therefore feeding thinuniform layers of slurry across the entire width of each ofthe five feed points provides maximum separationefficiencies and tonnage rates. A linear high frequencyvibratory motion is supplied to all five screen decksuniformly throughout the entire length and width of eachscreen deck by means of a pair of TENV (totally enclosednon ventilated) vibratory motors with a sealed for lifebearing design. This vibratory motion induces fluid andundersize solids throughput while also extending theeffective fluidized zone for sizing. The vibratory motion isalso extremely effective in conveying oversize material to

the end of each screen deck-making way for the newoncoming feed. A unique hopper and launder assemblyallows all undersize to discharge from one outlet and theoversize to discharge from another single outlet.

The introduction of the Stack Sizer provided end userswith an exceptionally high tonnage, wet screening systemwhich was previously not available. The Stack Sizer incombination with the high open area urethane screensurfaces basically eliminates all of the prior concerns aboutthe applicability of fine screening technology in closedcircuit grinding. The high tonnage rate capabilities reducefootprint requirements dramatically while the patentedurethane screen surfaces eliminate wear and blindingissues. It is this combination of technologies from DerrickCorporation that makes screening a viable optionparticularly in closing grinding circuits for base metalapplications. At this point it should also be mentioned thatthis is not experimental technology as over 900 Stack Sizershave been produced and delivered around the world.

Figure 3. Photograph of 5 deck Stack SizerTM

Figure 2. Schematic diagram of 5 deck Stack Sizer

IMPLEMENTING STRATEGIES TO IMPROVE MILL CAPACITY AND EFFICIENCY 87

Polyurethane screen surfacesThe availability of high open area, long wearingpolyurethane screen surfaces with openings as fine as 75microns (200 mesh) has made fine wet screening morefeasible than previously thought possible. The open area ofurethane screen surfaces has traditionally been one of theirbiggest drawbacks, particularly when dealing with the finerend of the screening spectrum. The conventional 305 × 305pin and sleeve panel, typically has a relative open area ofabout 10% for a 500 micron aperture panel. In contrast thetotal open area of Derrick urethane panels range from 35 to45%, similar to conventional woven wire screen panels,resulting in significantly higher capacities or smaller,lighter equipment being required. Remarkably, theseurethane screen surfaces typically last 4 to 12 months,depending upon the abrasiveness of the material and thepanel opening. Moreover, the slotted openings are designedwith a tapered relief angle to minimize blinding. Figure 4shows how the urethane panels are side-tensioned over aslightly crowned deck.

Grinding circuitsGrinding is the process of reducing the size of a mineral oreto a point where the desirable mineral is liberated and thenpresented to further process technologies for efficientseparation from the undesirable gangue material. The millsthemselves have evolved in size, power draw and durabilityfor over a century but the technology has remainedbasically the same. It is in the classification stages thattechnology has evolved into functionally different forms inrecent years. We identify the following four basic types ofgrinding circuits, commonly used in base metals (Barrios2006):

Until recently, the majority of classifiers used in theclosing of base metals grinding circuits have beenhydrocyclones of varying models and installation methods.These include vertical, horizontal and inclined installations,with varying materials of construction and a myriadgeometrical adaptations. Irrespective of the mode anddesigns applied, the separation efficiency seldom surpasses60%. The end result is that most grinding circuits closed byhydrocyclones operate with high circulating loads thatrange between 230 and 450%. (See Figures 5–8 for circuittypes.)

Picture 4. Typical crowned deck configuration

Figure 5. Primary rod mill followed by secondary ball mill

Figure 6. Two ball mills in series

Figure 7. Single ball mill

Figure 8. SAG mill (semi-autogenous) followed by secondary ball mill

PLATINUM IN TRANSITION ‘BOOM OR BUST’88

The grinding operation in a mineral processing facilitytypically consumes the majority of the energy used. Inaddition to energy, these mills consume special steel rods orballs as well as the steel mill linings. Combining all of thesefactors we see why grinding is one of the highest costsinvolved in mineral processing. Keeping these costs inmind, it is extremely important that the mills carry out theiroperation in an efficient manner. Basically we want toensure that once a particle has been ground to its designatedliberation size, it is effectively removed from the millingprocess as soon as possible by an efficient classificationmethod. In other words, let us consume energy, rods, ballsand linings only on the particles which need to be groundfurther and remove liberated particles for downstreamseparation and refining.

Classification, cyclone versus Stack SizerTM

The main objective of a classifier in the closing of agrinding circuit is to remove liberated mineral particlesfrom the mill as coarse as possible in order for them to berecovered via mineral concentrating method, such asflotation, spirals, etc. A liberation point for any ore isdefined by a specific particle size; therefore a classificationsystem which makes the most efficient size separation willbe the one that most efficiently reaches the objective ofgetting liberated particles on their way for recovery. Wementioned earlier that cyclones typically operate withseparation efficiencies in the range of 45% to 65%. TheStack Sizer on the other hand typically operates withseparation efficiencies in the range of 85% to 99%. Thedifference between the two technologies does not end atsize separation efficiency. Cyclones make their separationsbased on differences in settling rate velocities, particle sizeand mass, whereas screening machines ignore all othervariables with the exception of particle size. Complicationswith cyclones in grinding circuits are further exacerbatedwhen there are large specific gravity differentials betweenthe desirable mineral and the gangue material. The heavierfine, liberated minerals report to the underflow and ofcourse back to the grinding mill, whereas lighter, middlingparticles report to overflow and on for concentrationwithout the liberation of the desirable mineral. The screenon the other hand recognizes the particle size only andallows for accurate determination of liberation. Below is alisting of the main economic advantages of screens versuscyclones in grinding circuits.

• Typically require less water for operation• Reduction in slimes generation allows for reduced

reagent consumption in flotation• Reduced energy consumption due to reduced

circulating loads• Increased production rates (typically between 10 to

30%)• Reduces grinding media consumption• Reduced mill maintenance costs• Increased efficiency in downstream classification

devices such as spirals, flotation cells and jigs as theyprocess a narrower size range of particles moreefficiently

• Increased mineral recovery rates• Reduced concentrate dewatering costs due to coarser

liberation and reduction of slimes.

Efficiency calculationsMany different efficiency calculations exist and it isimportant to clarify the method used. The formulaemployed in this paper is a simple one, as defined below.

The oversize efficiency is the amount of coarse in theoversize divided by the amount of coarse material in thefeed.

The undersize efficiency is the amount of fines in theundersize divided by the amount of fine material in thefeed.

The overall efficiency is the total amount of correctlyplaced material, coarse in the overs plus the fines in theunders divided by the total amount of material in the feed.

We define (all values expressed as percentage %)Measured valuesA = oversize in feedB = undersize in feedC = coarse in oversizeD = fines in undersize Calculated valuesR = fraction of feed in undersO = fraction of feed in oversEo = oversize efficiencyEu = undersize efficiencyE = overall efficiencyConsidering that:A + B always equals 100R + O always equals 100We calculate R by writing a material balance around the

coarse fraction. The coarse in the feed equals the coarse inthe oversize plus the coarse in the undersize.

100 (A) = O × C + R (100 – D), therefore100 (A) = (100 – R) × C + R (100 – D), from this follows R = 100((C – A) / (C + D – 100))Therefore:Eo = O × C / AEu = R × D / BE = (O × C + R × D) / 100

Applications in platinumIn UG2 processing plants, there is a particular problematicseparation that can best be solved by screening; where theseparation relies on particle size, and particle size only,density does not play a role. This application is theseparation of Cr2O3 from PGMs as well as SiO2, and thisfunction is best performed by replacing the secondarymill’s (typically Ball Mill) cyclone closed circuit with theStack SizerTM technology.

Conventionally these closed mill cyclone circuits willoperate with high circulating loads, between 200 and 400%and will generally produce milled product at a specificationof 80% less than 75 micron, which is required to liberatethe PGMs. This mill product will typically contain 20 to24% Cr2O3, about 25% SiO2 and 4 grams per ton PGMs.The problem arises largely with the Cr2O3, which isovermilled as the heavier Cr2O3 will continue to report tothe cyclone underflow and returned to the mill. During thisprocess the PGM fraction is typically milled much finerthan 80% less than 75 microns, to around a D50 of 20micron.

By replacing the cyclone circuit with Stack SizerTMtechnology, the following advantages are attained:

• Coarser grind, PGMs are removed at liberation point of75 micron

• Continue to mill to 75% less than 75 micron, but lessslimes (ultra fines) are generated

• Improve Cr2O3 recovery in downstream circuits by 15to 20%

IMPLEMENTING STRATEGIES TO IMPROVE MILL CAPACITY AND EFFICIENCY 89

• Decrease the mill circulating load to around 100%, incases where multiple mills are employed, entire millscould be made redundant

• Huge reduction in• Energy consumption• Grinding media consumption • Mill liners and mill maintenance • Flotation reagents, due to decrease in surface area

• Increase production rates, recoveries, and efficiency ofdownstream processes, due to narrower particle sizerange.

Case studies

Mina Colquijirca—Sociedad Minera El Brocal S.A.A. The plant is located at Tinyahuarco, Pasco Province Peru,high up in the Andes mountain range. It produces lead,zinc, copper and gold (Aquino and Vizcarra 2007). Asshown in Figure 9, the concentrator flowsheet originallyconsisted of three rod mills operating in parallel and inopen circuit followed by three ball mills operating inparallel in closed circuit with a bank of 10 inch cyclones.The rod mill discharge and ball mill discharge arecombined in the same sump and pumped to the cyclones.The circulating load is about 350% and overgrinding of thehigh specific gravity minerals such as galena results insignificant slime losses before flotation.

Brocal wished to increase capacity and was consideringthe addition of a fourth ball mill. As an alternative theyconsidered ways to improve grinding efficiency andconducted full scale Stack Sizer tests. Encouraging resultsled to the installation of two 5 deck Stack Sizer screensfitted with 0.50 mm urethane panels in place of the 10 inchcyclones. The effect was immediate as production increasedsignificantly by 11% and lead recovery increased by 9%.The circulating load decreased to about 60% and Brocalshut down two of the three operating ball mills withsignificant savings in operating costs.

Subsequent to the initial screen installation and to takeadvantage of the increased grinding capacity, Brocal addedadditional crushing, flotation and filtration capacity, as wellas three additional Stack Sizers. The current grinding circuitwith screen classification is shown in Figure 10. One of thetwo ball mills originally shut down was put back into

operation and rod mill rotational speed was increased.Production increased from 138 tph with the cyclone circuitto 245 tph with the screen circuit, an increase of over 75%with less total grinding energy. The circulating load wasmaintained at about 60% and overgrinding has beenminimized; the slime content (particles less than 10microns) in the flotation feed reduced from 18% to 10%,resulting in significant increases in metal recovery.

OJSC KMAruda KMAruda operates an iron ore mine in Russia and run ofmine ore contains about 34% total iron (Pelevin andLazebnaya 2009). The primary grinding circuit consists oftwo ball mills in parallel operating in closed circuit withspiral classifiers. Similarly, the secondary circuit has twoball mills in parallel in closed circuit with hydrocyclones.Product is fed to multiple stage magnetic separators.

Two Stack Sizer screens fitted with 100 micron urethanepanels were fitted in place of the cyclones on oneconcentrator production line. The circulating load in thescreen circuit was reduced and significantly coarser. Whilethe goal was to maintain the same final product grade, thefinal concentrate from the screen circuit was also coarserand they experienced reduced iron losses in the magneticseparation stages.

Minera Cerro LindoMinera Cerro Lindo is located south-west of Lima Peru andproduces copper, lead and zinc concentrates (Ticse 2009).The grinding circuit consists of ball mill operating in closedcircuit with a bank of 26 inch cyclones, with circulatingload of about 260%.

Following the success of screen classification at Brocaland other polymetallic mining operations in Peru, CerroLindo initiated a programme to improve concentratorperformance. Following full-scale testing, four Stack Sizerscreens were installed in place of the 26 inch cyclones,fitted with 0.23 mm and 0.18 mm aperture panels. As perthe previous case studies, the circulating load decreased to108% and line capacity increased by some 14%. The P80was coarser in the screen circuit at 160 microns comparedto 141 microns with the cyclone circuit. Steel consumptionper kwh decreased and tailings filtration capacity increasedas the tailings are now coarser and more homogeneous.

Figure 9. Original Brocal grinding circuit with hydrocyclones Figure 10. Current Brocal grinding circuit with screens

PLATINUM IN TRANSITION ‘BOOM OR BUST’90

Ongopolo TsumebOngopolo Tsumeb is located in northern Namibia, adjacentto the world famous Etosha Game Reserve. The companyproduces primarily copper with lesser quantities of gold,zinc and silver. They conducted tests on a Stack Sizerscreen to split the mill feed sample into two products withthe specific requirement of achieving a screen undersizespecification of 80% less than 115 micron.

The objective was achieved with 125 micron panels,resulting in a recovery of 97.2% of plus 125 micron in theoversize at an overall efficiency of 91.7%.

Ongopolo was delighted with the results. Unfortunatelythe world economic crisis intervened and they placed theplant on care and maintenance at the end of 2008.

ConclusionsThe introduction of the Stack SizerTM technology allows forefficient and effective screening with vibratory equipmentthat was unattainable before and remains unattainable withany conventional screening technology. To summarize, thisis due to the following unique attributes of the StackSizerTM:

• Introduction of the long life, high open areapolyurethane screen panels, the 75 micron aperturepanels have a total open area of 35%

• The compact nature of the five deck Stack SizerTM,where effectively five times the 1.2 metre wide screensare employed on a 1.5 metre wide base, effectively a6.0 meter wide screen

• Test work on various UG2 ores are in progress andresults are very promising

• Huge savings in capital and consumable costs can beattained; payback periods of a couple of months forentire turkey projects can be achieved.

References

DAVIS, E.W. Ball Mill Crushing in Closed circuits withScreens. Bulletin of University of Minnesota, MinesExperiment Station Bulletin No. 10. 1925.

ALBERT, E. Characteristics of Screen-Circuit Products.Min. Tech. T.P. 1820. 1945.

HUKKE, R.T. and ELAND, H. The Relationship BetweenSharpness of Classification and Circulating Load inClosed Grinding Circuits. Transactions, Society ofMining Engineers. 1965.

MURR, D.L., WENNEN, J.E., and NORDSTROM, W.J.Essar Steel Minnesota, Concentrator FlowsheetDevelopment. 70th Annual University of MinnesotaMining Symposium. Duluth, Minnesota. 2009.

BARRIOS, G.F. Increasing the Capacity of the GrindingCircuits without Installing more Mills. Goldex S.A.2006.

AQUINO, B. and VIZCARRA, J. Re-engineering theMetallurgical Process at Colquirjirca Mine, SocidadEl Brocal S.A.A. Mineria. 2007.

PELEVIN, A.E. and LAZEBNAYA, M.V. Application ofDerrick Screens in Locked Grinding Circuit at theKMAruda Mining Complex Concentrating Plant.Mineral Processing Journal (Russia). 2009.

TICSE, F. Milpo-Cerro Lindo. Information provided duringplant visit. 2009.

Nic BarkhuysenRegional Manager, Derrick Corporation

Metallurgist by professionStudied at WITS & University Durban WestvilleMember SAIMM (South African Institute of Mining & Metallurgy)Past Chairman Zululand branch SAIMMCareer:Mintek, 4 years, in training.Anglo Vaal Reefs, 1 year, Senior Plant Metallurgist.Richards Bay Minerals, 13 years, technical development, Metallurgical Superintendent.Spectrum Technical, 9 years, marketing, consulting, Managing Director.Derrick Corporation USA, 4 years, marketing, current position.Current position:Regional Manager Derrick Corporation responsible for marketing in Africa, Australia and Englishspeaking countries in between.