THE EFFECT OF ADDITIVES ON STIRRED MEDIA MILLING OF LIMESTONE

J. ZhengC. C. Harris

P. Somasundaran

Columbia UniversityNew York, New York

For presentation at the SME Annual MeetingPhoenix, Arizona March 11-14,1996

Permission is hereby given to publish with appropriate acknowledgments, excerpts orsun'VT\aries not to exceed one-fourth of the entire text of the paper. Permission to print in moreextended form subsequent to publication by the Society for Mining, Metallurgy, and Exploration(SME), Inc. must be obtained from the Executive Director of the Society.

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Abstract ~~jIbed. Scale-up guidelines ~th respect to power~on have also been proposed The best operatingconditions, including impeller rotational speed with andwithout supernatant, total solid ( media and panjcles )concentration. ratio of media to panjcle volume, mediasize, media density, impeller and tank dimensions,impeDer design and p.llp viscosity, for grinding limeStonein the laboratory StilTed media mill have been identified.Side effects of additives have to be noted: complexationby the polymer during uJtrafine grinding of yttriastabilized mconia in the stirred mills was found recentlyto cause extraction of yttrium into solution withsignificant changes in the composition of the product [Lartiges and Somasundaran, 1992 ] From the resultsobtained, it is clear that there is a need for fundamentalunderstanding of effects of additives on grinding. productcharacteristics and power consumption in Stirred mediamills.

The effects of additives, sodium hydroxide,sodium carbonate, sodium oleate, oleic acid andpolyacrylic acid, on stirred media milling of limestonehave bea1 investigated. The results are evaluated in termsof speci.6c surface area., specific energy and energyefficiency as a function of additive dosage, pol)1Dermolecular weight, solids concentration and relevantoperating variables Use of additives generally results inchanges in specific surface ar~ and energy efficiency,while unda- certain coDditions a more than 1000/0 increasein these parameters can oCGUr. Relevant interfacialproperties have been measured and milling mechanismsinvolved are explored pania1larly in terms of the effect ofadditives on the flow patterns at higher solidsconcentration.

Introduction In this study, the effeCt of seleCted additives onthe grinding of limestone was studied as a function ofadditive type and dosage, polymer molecular weight,solids concentration and other operating variables along~th zeta potential, adsorption and other interfacialproperties. Also, the effeCts of these variables on millmeGhanisms were examined

Experimental

The effects of chemical additives on grinding-(including beneficial and detrimental effects) have beenexplained mainly by two kinds of m«.hanisms. One isbased upon the alteration of surface and mechanicalproperties of individual panicles, such as reduCtion ofsurface energ:-' [ Rehbinder, 1931 ] and modification ofsurface hardness [ Westwood and Goldheim, 1968 ],while the other considers the arrangement of particles andtheir Bow in suspensions, for example, improvement ofpulp Buidity [Klirnpei. 1987] These m«.hanisms havebeen examined in the case of tumbling mills [Somasundaran and Lin, 1972; EI-Shall andSomasundaran. 1984; Somasundaran and Shrotri, 1995 ]However, there are very few stUdies concerning stirredmedia mills .-\Jso, findings from tumbling mill grindinginvolving the use of grinding aids ~il1 not be whollyapplicable to the stirred media milling case, in whichRnal)er granular grinding media are used, and the patternand velocit), of rotation are different,

Stirred media mills have anraCted attention inrecent times because of their reponed high energyefficiency, ability for grinding into the mjcron and sub-mjcron range, and lower produCt contamination, Stirredmedia mills are used for fine panicle produCtion in manyindustries such as mjneral, metallurgical, ceramic,electronic, pigments, paint and lacquer, chemical, bio-technology, rubber, agricultural, pharmaceutical,photographic, coal and energ:-' [ TlIDenez, 1981; Stehr,1988; Zheng. et ai" 1995c ] We have recently studiedpower and operating behavior in stirred media mills [Zheng, et ai" 1994, 1995a; 1995b, 1995c ], The torque~ to rowe impeDers immersed in dense particulatemedia with supernatant versus impeDer rotational speeddisplays four regions marked by sharp transitionstranSition &om static to dynamic friCtion; chaMeling,dispersing; and centrifuging Equations, includingdimensionless group correlations of power and modifiedReynolds oornber, for relating power, speed, impeller andtank dimensions and design, media size and density, solidconcentration. and other relevant variables have been

The venical stirred media mill employed wasdesigned to accommodate a ~ide range of impellerdesigns by interchangeable fittings on the drive shaft[Zheng, 1994; 1995a]. Torque is obtain~d by a torquepick.-up and an indicator, and speed ~'as controlled bymeans of a dc motor with a rectifier and voltageregulator. The measured net torque t ( gross torque lessidling torque) and speed N give power ( P = 27t~': ).The energy input ( E ) was calculated from torquereadings or power values taken at one minute intervalsduring grinding ( E = I.P~t ). A water bath is used fortemperature control.

High purity limestone ( 96% CaCO) ) processedfrom natural calcite deposit of Adams, MA and obtainedfrom Specialty Minerals Inc was used for the grindingexperiments Average feed particle size was 1 06 micronsT~tuUcaI quality soda-lime silica glass spheres ( 2.5 g/cc) with mono size 2 05 mm obtained from PonersIndustries Inc, NJ were used as grinding media Atransparent glass cylindrical tank was used to observe theflow pattern of media/pulp; otherwise, Stainless Steelcylindrical grinding chambers were employed The tank~'3S 11.8 cm in diameter and the four-pins impeller usedwas 6.5 cm in diameter

Sodium hydrox:ide, sodium carbonate, sodiumoleate, oleic acid and polyacrylic acid were used asadditives during grinding Operating conditions, such as,the ratio of media to particle volume and the ratio ofmedia to feed particle size, are set at the optimum values~nunended by Zheng eI aI., namely: volume ratio, 28;size ratio, 12:1 [Zheng el aI., 1995b]. A constant highimpeDer speed of 1 000 rpm was chosen to obtain

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PAA wu dX>sen for further study, and molecular weightand concentration effects were studied in detail

Effect or oolvac:rvlic: ac:id C:OD~eDtnrion

relatively high product surface area. After each test, allthe media and ground pulp were removed uom the mill,the supernatant separated uom the pulp, and the mediaJeParated uom the panicle by screen washing Residualpolyacrylic acid concentrations of the test samples weremeasured using a Total Carbon Analyzer Zeta potentialof the panicles wa£ determined using a Zeu MC:lcr Thespecific surface area was determined using the multiplepoint BET method by a QuantASOrb apparatus Theparticle size distribution was anai)"zed by a Microtracparticle size analyzer utilizing low-angle, forward-scattering light uom a laser beam.

To present the grinding results, three parametersdefined below, new soecmc surface ( S ), sDecmc ener~(Ew ) and ener~ efficien~ (Ef), are used

S . S, - S, (1)

where E is the energy input during grinding and W is theweight of the ground material.

FI--=- (3)

During the firSt series of tests, the only change inoperating conditions was the polyacf)uc ~idconcentration while the molecular weight was keptconstant at 5,000. The results for produCt fineness,specific energy and eI)el'gJ efficiency are shown in Figure2 as a function of additive concentration. Both productsJrface area and energy efficiency increase with additiveconcentration from 0.005 to 0 1 %, and then decrease,while energy consumption gradually d«:reases withadditive concentration. The best energy efficiency andproduct fineness are obtained at the additive dosage of0.1 % by weight of feed limestone. Dosages below orabove that value result in both lower produCt surfacearea and lower energy efficiency even though grinding"unprovements are obtained using P AA at all levels exceptat excessively high or low dosages.

That an excessively low dosage of P A."- may notbe effective for grinding can be explained with the ajd ofFigure 3 (a) and (b), which show adsorption densit)° andZeta potentiaJ as a funCtion of P AA concentration,respectively. Both adsorption density and Zeta potentia]increase ",ith polymer concentration and then quicklyreach plateau values, but a vef')' low concentration of 10ppm results in both a low adsorption densit)' and a lownegative Zeta potential vaJue

Polyacrylic acid aJso results in a lower energyconsumption as compared ",ith tests done withoutpoiyacrylic acid Higher the additive concentration, .loweris the energ:-' consumption Since torque or energyco~tion indicates "fluidity" of media/pulp during theoperation, it can be concluded that P AA addition resultsin both "fluidit)," and grinding improvements However,it may be noted that an excessively high tluidit), mayresult in less contact and interaCtion between media andparticles, and thus in lower grinding Higher fluidiryresuhing in lower product fineness has been reponed inour previous papers on grinding of limestone withwpematant but without grinding ajds ( Zheng. el ai.,1995b] Therefore, in order to optimize use of chemicaladditives, additive concentration should be ",ithin aspecific range in the case of polyacrylic acid, the bestadditive concentration for the current grinding conditionsis 01% by ""eight This optimum dosage was selected forthe study on the effect of polymer molecular weight on

grinding.

s

Results and Discussion

r.ff~ct of .dditiv~s tvR~

Chemical additives chosen were representativeinorganic electrol)1eS, surfactant, and organic polymer:sodium hydroxide, sodium carbonate, sodium oleate,oleic acid, and polyacrylic acid ( P A.~ ). Total solids (media and panicles) concemrarion was fixed at 75%, andpanicle solid concentration at 44% by volume. Theresults of average energ:-' efficiency YS. additiveconcentration are shown in Figure I. Compared withgrinding without additives, use of sodium carbonate andsodium oleate results in decreased energy efficiency overthe additive concentration range studied while use ofsodium hydroxide results in increased efficiency only at alower concentration of 0 008% but decreased efficiency81 higher concentrations. However, use of oleic acid andPM ( molecular weight 4,000,000 ) produced abeneficial effect at a higher concentration of 0.1% buteither no effects or detrimental effects at lowerconcentrations It can be concluded that use of grindingaids can result in beneficial or detrimental effectsdepending on the additive type and the concentration.PoIyacryIic acid was found to be among the best additivesfor improving the grinding energy efficiency Therefore,

Effect or Dolvacrvlic acid mol~c:ular w~i&ht

Effect of polyacrylic acid molecular weight on thegrinding of limestone was studied and the results givenin Fig 4 show energy efficiency, specific energy andproduct fineness as a function ofPAA molecular weightfor grinding at 75% and 65% solid concentrations Inthese tests. P AA concentration was fixed at 0.10/0 byweight of f~ particles; other operating conditions were

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also kept constant. Both product fineness and energyefficierx:y inaease with increasing molecular weight from2,(XX) to 5,(XX), and then decre.ase. It is DOted that higherproduct fineness was obWned 81 higher solidoo;x;g.-uAcion (75%) over the full range StUdied. Specific«!Ia'8Y was also found to increase with molecular weightat both SCllic! concentrat;"n~ uith higher t:nergyoonsumprion oc.oJring 81 the higher solid concentration of750/.. The molecular weight ofPAA for the best energyefficiency and product fineness at both solidCOIMl-W-.nOns was found to be 5,000 Higher molecularWCght resuhs in greater ~gy input in both cases. whichcould be due to incr~g panicle flocculation caused bythe polymer. In fact, higher PAA molecular weight wasfound to result in more fJocculation, as illustrated inFigure 5, relating settling rate of limestone panicles toP AA molecular weight. From the above results, solidconcentration emerges as a very important factor whichwill be discussed next. The optimum polymer molecularweight of 5,000 was selected for the tests.

concentration. Reasons as to why more than 1000/.increase is obtained at 900/. solid concentration by usingthe P AA additive will be examined next.

Effect orPAA on now or media/oulo in tbe mill

Effect orp.,,-~ at different solid concentrations

In order to explain the effect of the solidconcentration in using grinding aids, media/pulp Bow isexamined in the light of torque drawn by the mill duringope'3ring period. An example of torque vs. grinding timecurve is given in figure 7 at 85% solids concentration forcases with and without P AA. The curves show that forboth cases torque increases with time, reaches amaximum and then decreases and finally tends to beconstant. However, after the initial increase, while thetorque in the absence of polyacrylic acid begins todecrease with time, in the case with P AA, torquecontinues to increase and reaches a higher maximumvalue at a longer time Therefore, average torque ( orpower) drawn by the mill during I 5 minutes grinding ishigher with P AA than without. Grinding aids resulting inhigher average torque have been reponed for tumblingmill grinding as well [Fuerstenau et aI., 1985, Katzer etaI.; 1981 ] However, the increased torque due to thepoi)mer in the case of stilTed media milling remains to be

explainedA diagrammatica.I representation of the variation

of torque vs time for grinding at high solid concentrationis given in Fig 8 There are a number of distinct regionsimplying a change in the mill behavior; observationthrough a transparent cover allowed a description of thesequence of events associated with the various regions ofthe curve. In this case. the system was a batch mill ~itha 4-pin impeller immersed in a media and pulp of highsolids concentration ~ithout any supernatant andoperating at constant impeller speed

Tlming began when the impeDer speed reached itsoperating value. The torque dropped gradually as rotationcommenced ( region AB ) In this region the impeller isloosening largely stationat). packing of solids ( media andpulp) by chaMeling or dispersing. depending on theimpeller speed, and as time increases so also does thequantity of solids rotating with the impeller When allsolids are in rotation. the torque increases with grindingtime ( region BC ), and increasing quantities of fines areproduced then Towards the upper end of region BC, aportion of the solid mass begins to fonn a largelystationary layer above the impeller pins. resulting in adrop in torque The fonnation of the layer occurs in thefollowing sequence solids move toward the wall of thetank. pack there, migrate up wards, then move in wardsabove the impeller pins. This stage is marked also by anincrease in the sound level since the tips of the pins arenow exposed as they grind against the largely stationarysolids packed against the wall At still longer grindingtime, solids fonn a substantial layer above the impeDerpins ( region CDE ). and torque decreases with grindingtime. Toward the end of this region, a complete layer ofsolids is formed above the impeller pins while thevolume around the center of the impeller pins is almost

One of the moS1 important factors in the use ofchemical additives is solid concenmtion. In the follo\l,ing.effect of P .-\.-\ on grinding at different solidconcentrations ranging from 6O~0 to 1000/0 was studiedat a fixed molecular weight of 5,000 and an additiveconcentration of 0.1% The changes in solidconcentration were produced only by means of waterdilution, while the media and particle weight or volumeranained conStant Tow solid volumetric concentrations,65%, 75°'0, 80~'o, 85%, 90%, 95% and 1000,'0,correspond to panicle solid volumetric concentrations,33~0, 44~o, 51~o, 600/0, 700/0, 83% and 1000,/0,respectively The resuhs for specific surface, energ'}'efficiency, and the corresponding average percentageincreases due to additives are sho\lo11 in Figure 6 as afunction of solid concentration. In the absence ofpolymer, the product surface area was found to increasewith increase in solid concentration from 650/. to 75%and then to decrease. In the presence of polymer, theproduct sulface area was found to increase with solidconcentration up to 800/'0 and then to decrease.Observation through the transparent tank during thestirring test at the solid concentration of 800/0 without and\\ith the additives has revealed that without additives,only the solids ( media and panicles) around the centerof the impeller are S1irred while beyond the impeller pinsthe solids remained almoS1 stationary [ Zheng el al.,1995b ], while \l,ith polymer, the solids both around andbeyond the impeller pins are stirred resuhing in increasedpanicle grinding P .A.A thus enhances media/pulp flowand resuhs in better grinding

Furthem1ore, polyacrylic acid results in bettergrinding in the entire concentration range studied at asolid concentration of 900/0 over 1000/0 improvement inboth sulface area and energy efficiency was obtained ,This result could be of commercial use since induS1rialStirred mills mostly operate at the higheS1 possible solid

.causes media/pulp Bow to behave as though it is at alower solid concentration, thus improving grindingconditions and resulting in better grinding

CoDclusioDS

(1) The eff«:t of additives on mrred media millingof limestone has been studied as a function of additivetype and dosage, polymer molecular weight, solidsconcentration and relevant operating variables.

(2) Use of additives results in beneficia! ordetrimemaJ eff«:ts depending on the additive type and thedosage. Polyacrylic acid is found to be the best additiveamong those tested. P AA addition results in lower energyconswnption.

(3) For the present system, the optimumconcentration of polyacrylic acid was 0.1 % by weight.Higher additive concentrations, lower energy

consumption.(4) The optimum molecular weight ofPoiyacrylic

acid was 5,000 for the grinding of limestone Highermolecular weight resulted in greater energy consumptiondue to increased particle flocculation caused by thepolymer

(5) For grinding at different solids concentrations,results in the presence of polyacrylic acid are better forthe full range studied as compared v.ith no P .": morethan l00D/o improvements in both the specific surface andthe energy efficiency were obtained at a solidconcentration of 900/0

(6) As solid concentration increases, media/pulpflow patterns pass through four regimes vortex flow,rotating flow, layer formation above the impeller pins,and layer formation adjacent to the tank wall. Use ofpolyaaylic acid caused media/pulp flow to move to\J,'ardslower solid concentration situation, thus improvinggrinding conditions and resulting in better grinding

Acknowledgement

This research has been supported by theDepartment of Interior's l\.fineral Institute Programadministered by the United States Bureau of Minesthrough the Generic Mineral Technology Center forComminution under Grant Number G1135249.Gl14S249

References

snpty. The torque d«:reases to its lowest level and tendsto remain constant in this region If there is a substantialdistance betWeen the tips of the impeller pins and tankwan. the formation of the layer will not occur; inStead apaciced layer is formed around the tank wall beyond thetips of the pins.

To summarize, in the case of high solidscolx:entration. the solids are gradually expelled ttom theIdiYe volwne betw~ the impeller shaft and the pin tipsIf there is sufficient volume betWeen the pin tips and thetank wall, the expelled solids form a layer at the wall; ifDOt, the material is expelled upwards fonning a Stationarylayer above the pins

The most suitable region for grinding is BC inFigure 8, and ex1ension of this region is likely to improvegrinding. Grinding aids are proved to extend this regiondue to improved fiuidity resulting in less stationarymaterial. and this explains the enhanced production offines with additives.

Inspection of the impeDer-pin wear profile, showsthat the top and base pins display the greatest wear,which suggests that grinding takes place mainly at thebase and top of the impeller, with less grinding occurringnear the middle and center pins. This is consistent withthe way in which solids migration occurs setting upconcentration profiles within the pulp mass

The grinding process is different ttom thatdescn"bed above when solid concentration reaches 900/0There is no region AB in the torque vs. time curve, in~ the torque d~eases ~ith time. Also, only the partof solids around the center of the impeller is stirred whilesolids beyond the impeller pin tips move upwards alongthe wall of the tW As stirnng continues, more and moresolids move to~"&rds the ~'aD and then up~'ards until the)'reach the cover of the tank During this process, thetorque increases ~ith grinding time, reaches the highestvalue, and then begins to decrease with grinding timeThe movement of solids during grinding with and withoutadditives is diStinctly different Without additives, solidsmigrate up~"&rd to reach the cover of the tank and remainStationar)', and attach to the wall due to capillary forcesAs the process continues, more and more solids movetowards the wall and remain attached, while less and lesssolids are Stirred This explains as to why the torquedecreases with time When no more solids move towardthe wall, and the volume of Stirred solids remainsconStant, torque also tends to be constant. With additives,on the other hand, after the solids reach the tank cover,and torque reaches the highest point, the migrated solidsbegin to descend due to increased lubrication or fluidityand/or decreased capillary forces owing to the action ofthe additives

In summary, examination of the internal flowpatterns of media/pulp and rotating impeller shows thatflow patterns change with changing solid concentrationAs illuStrated in Figure 9. with increasing solidconcentration. flow patterns usually pass through fourregimes: vonex flow, rotating flow, layer formationabove the impeller pins. and layer formation adjacent tothe tank wall ( see Figure 9 ). Use of polyacrylic acid

1. El-Shall H. and Somasundaran P., 1984,"Physico-ChemicaJ Aspects or Grinding a Re..;ew oruseor Additives", P0\4.'der Technolog)', 38,275 - 293

2. Fuerstenau, D.W., Venkataraman, KS andVelamakanni, B.V., 1985, "EffCCt ofchcmical additiveson the dynamics of grinding media in wet ball millgrinding", International Journal of Mineral Processing,

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15,251 - 267

3. Ttmenez. J.L.S., 1981, "A Detailed Study onStirred Ball MiD Grinding", Ph.D. Dissa'Wion, UniversityofUuh. Salt Lake City

~ Katzer, M., Klirnpel, R.R and Sewell, J., 1981,"Example of the laboratory characterization of grindingaids in the wet grinding ores", Min. Eng., 33, 1471 -1476.

s. KIimpel, RR., 1987, "Grinding Aids Based onSlurry Rheology Control", Reagents in MineralTechnology, Somasundaran, P. and Moudgil. B.M. ed.,Marcel Dekker, Inc., New York, pp. 179-194

6. Laniges, B., and Somasundaran, P., 1992,"Ultrafine Grinding of yttria Stabilized Zirconia inPolyacrylic Acid Solutions", Special SymposiumProceedings: Comminution - Theory and Practice,SME Annual ."-feeting, Phoenix, AZ, Feb 24-27

Rehbinder, P.A., 193 Physik, 72, 191

8. Somasundaran, P. and Lin 1.1., 1972,/aIwlECProcesses Des. Dev., 11,321

9. Stehr. N.. 1988, "R~ Developments in StirredBall MIlling", Int. J. Miner. Process., 22, W.431-444

10. WestWood, ARC and GoJdheim, D.L., 1968, JAppJ. Phys., 39, 3401

11. Zheng, J, Harris, CC, and Somasundaran, P.,1994, "Power CharacteriStics of StilTed Media Mills",Firsllmem:Itional Particle Technology FOnIm, Denver,USA, Pan 2, August 17-19, pp 135-141

12. Zheng, J., Harris, CC. and Somasundaran, P.,1995.. "Power Consumption of Stirred Media M111s",Miner. And ,l.,feta//urgicaJ Process., 34, pp. 34-40

13. Zheng, J., Harris, CC., and Somasundaran, P.,1995b, .. A Study on Grinding and Energy Input in

StilTed Media Mills", Preprint Number 95-175, SMEAnnual Meeting. Denver, Colorado, March 6-9

14. Zheng, J. and HarTis, C.C., 1995c, "Power andOperating Behavior in StilTed Media Mills", ~-"-Ig.)of the XI);," International Mineral Processing Congress,San Francisco, USA. Vol. 4, October 22-27, Chapter 9,00.47-51

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Figure 3 (b) Zeta potential of lift stone \'Sopolyacrylic acid (mw: 5,000) concentration0 0.05

% AdditivesFigure 1 Effect of additives on energy efficiencyin grinding limestone at 75% solids concentration

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Figure 2 Effect ofP AA (tn\\°:5,OOO) concentration ongrirxiing li~sto~ at 75% ~lids concemration

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Figure S Effect of polyacrylic acid nX>lecularweight on senling rate ofli~g.o~

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Grinding Time, MinutesFillltt"i Comparison O(lorqllC durin. ,,;ndin@ I~one al ~1i~eonCC1lU211On 8$% ~"ilh and ~ito'OUI poly~lic acid (m~ ~.':IOO) a"0 I', .o~ce:llnlion

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FiFf g Dia~~al ~prtsenlallOn of th~ variation oftorqu~ \ s tunc for Inndlnl a: hllh solid ;onccmnt,on

rotating layer formationflow abo..-e the impeller

pinsIncreasing solid concentration -

la:-'er formationnear lank ",-all

vonexflow

Figure 9 Flow patterns of~dia/pulp \\ith changing solid concentration