Role of Chemical Additives in Stirred ~Iedia Mill Grinding

Jie Zheng, Colin C. Harris and P. SomasundaranHenry Krumb School of~fines, Columbia l~niversity

New i"ork, ~"y 10027

Abstract

Chemical additives can produce marked elevation or depression of grinding efficiency In this work on therole of chemical additives in stirred media mill ultrafme grinding, polymer additives \vere found to enhancegrinding efficiency by up to 1000/0- The effects of tj'Pe and dosage of additives, polymer molecular weig.'n.t.solids concentration and other rele\-ant variables on ground product surface area, operating energ:.-consumption and efficienCj-. and the pulp flow pattern are discussed- Optimum operating conditions for 1..:.seof additives are identified and mechanisms are proposed.

Introduction

The role of chemical additives m grinding ( beneficial and detrimental effects) have been explained ma.ir..iyby tWo ty-pes of mechanisms. One is based upon the alteration of particle surface and physical proper!~essuch as surface energ:' [ Rehbmder, 1931] and surface hardness [ Westwood and Goldheim, 1968 ], ",,':"i:ethe other considers the distribution ot' particle suspensions and changes in pulp fluidity [Klimpel, 193 - ]Ti-.ese mechanisms have been examined for several systems in the case of tumbling mills [ Somasunda:3.:1and Lin, 19i2; EI-Shall and Somasundaran, 1984; Somasundaran and Shrotri, 1995, Fuerstenau, 1995 ].-1..cdiu\'e stUdies using a variable speed drop weight mill with ability to provide accurate measurements ;)::'energy and power mput have also been reported m the early literatUre [Frangiskos and Smith, 195-:~ Ghcs:-',Harris and Jo\vett, 1960 ]. However. work \\ith stirred media mills has been ver-." limited FindinQ;s r..:':':1- -sr..:dies on tumbling mill grinding inv;)lving the use of grinding aids will not be wholly applicable to st~-:-:~.imedia milling. tor \vhich sri'.aller granular grinding media are used and the patterns and velocities ('!' rota::;::;-.are different In this \\'ork. the role of selected additives in the grinding of limestone was stt;.died 3.S 3.fur.ction or additive t)-pe ar:d dosage. polymer molecular weight, solids concentration and other opera::::gva:iables .-\1 so. the effects of those \:ariables on mill mechanisms were examined

Experimental

.~ \Oertical stirred media mill equipped with phototachometer, torque pick-up and indicator \vas used a:-..dtrus equipment has been described else\vhere [Zheng, Harris and Somasundaran, 1996 ]. The measured r.ett\)rque 't ( gross torque less idling torque) and speed ~ \\'ere used to gi\Oe the net po\ver ( P = 2:-:~: ) T:-'enet energ:o input ( E ) \vas calculated from net torque readings or po\ver values taken at one minute inter\'~~sduring grinding ( E = ~P ...\t ) A water bath \vas used for temperature control

Hign purity limestone ( 960,'0 CaCO) of average feed panicle size of 1 06 microns was used tor 'allthe present experiments Use of limestone also permitted comparison with studies in the literatt:':eTechnical quality soda-lime silica glass spheres (2.5 g/cc) of mono size 2.05 mm obtained from PottersIndustries Inc, NJ were used as the grinding media. The tank was 118 cm in diameter and the four-pinimpeller used was 6.5 cm in diameter. The ratio of media to panicle volume and the ratio of media to feed

particle size were set at the optimum values found in the earlier work, namely: volume ratio, 2.8; size ratio,12:1 [Zheng, Harris and Somasundaran, 1995 ]. A high impeller speed of 1000 rpm was chosen to obtainrelatively high product surface area. After each test, the media and ground pulp were removed from the mill,the supernatant separated from the pulp by filtration, and the media separated from the particles by screenwashing. lni.tial and residual polyacrylic acid concentrations of the test samples were measured using aTotal Carbon Analyzer. The specific surface area was detennined using the multiple point BET method bya Quantasorb apparatus. The particle size distribution was analyzed by a Microtrac particle size analyzerutilizing low-angie, forward-scattering light from a laser beam.

To assess the grinding results, three parameters, new sQecific surface ( S ), sQecific energyand ener~ efficienc~ ( Ef), were used:

Ew

(1)

where Sp and Sf are the specific surface area of the product and feed, respectively.

Ew . EIW (2)

where E is the net energy input during grinding and W is the weight of the ground material

Ef 8 SlEw (3)

Optimization in this study was based on ma.ximum energy efficiency

Results and Discussion

Additive ttne

Chemical additives chosen were representative inorganic electrolytes, surfactants, and organic polymer:sodium hydroxide, sodium carbonate, sodium oleate, oleic acid, and polyacrylic acid ( P.M): Total solids(media and,particles) concentration was fixed at 75%, corresponding to a particle solid concentration of44% by volume. The results obtained for average energy efficiency vs. additive concentration are shownin Figure 1, These results for stirred media mill operation correspond to conclusions drawn in earlierliterature for other types of mills, namely that chemical additives can result in beneficial or detrimentaleffects depending on the t)'Pe and concentration. Polyacrylic acid was found to be among the best grindingaids for improving the energy efficiency, P AA was therefore chosen for funher study of molecular weightand concentration effects.

Polvacn'lic acid concentration

The effect of polyacrylic acid concentration was tested for a polymer of 5,000 molecular weight keepingall other variables constant, The results for product fineness, specific energy and energy efficiency areshown in Figure 2 as a function of the additive concentration, Both product surface area and energ'y'

efficiency increase with additive concentration from 0.005 to 0.1%, and then decrease, while energyconsumption continuously decreases with additive concentration in the tested range. Both energy efficiencyand product fineness peak at an additive dosage of 0.1 % by weight of feed limestone. This optimum dosagewas selected for the tests to determine the effect of polymer molecular weight on grinding.

Polyacnlic acid molecular wei2ht

Effect of polyacrylic acid molecular weight on the grinding of limestone illustrated in Figure 3 show energyefficiency, specific energy and product fineness dependence at 75% and 65% solid concentrations. In thesetests, P A.~ concentration was fixed at 0.1 % by weight of feed particles~ other operating conditions werealso kept constant. Both product fineness and energy efficiency increase with increasing molecular weightfrom 2,000 to 5,000, and then decrease. The molecular weight ofP AA for the maximum energy efficiencyand product fineness at both solid concentrations was found to be 5,000. Larger polymer causes greaterenergy input in both cases, which could be due to increased particle flocculation caused by the polymer. Infact, larger P AA was found to produce more flocculation, as illustrated in Figure 4 for settling rate oflimestone particles. From the above results, solid concentration emerges as a very important factor which\\'ill be discussed next. The combination ofpol~er molecular weight of 5,000 and concentration of 0.1%was selected for these tests.

Polvacn"lic acid at different solid concentrations- -

One of the most important factors in the use of chemical additives is solid concentration. In the following,effect of P ..1;.A on grinding at different solid concentrations ranging from 60% to 100% was studied at afi.xed molecular weight of 5,000 and an additive concentration of 0.1 %. The changes in solid concentrationwere produced only by means of water dilution, while the media and particle weights or volumes were keptconstant. Total solid (media plus limestone) volumetric concentrations, 65%, 75%, 80%, 85%,90%,95%and 1 OO~'O, correspond to particle solid ( excluding media) volumetric concentrations, 33%, 44%, 51 ~/O,60~/o, 700,0, 83~/0 and 100%, respectively. The results for specific surface, energy efficiency, and thecorresponding average percentage increases due to additives are shown in Figure 5 as a function of solidconcentration. Polyacrylic acid results in better grinding in the entire concentration range studied: at a solidconcentration of 90% over 1000/0 improvement in both surface area and energy efficiency was obtained.This result could be of conunercial use since industrial stirred mills operate mostly at the highest possiblesolid concentration. Reasons why P AA additive leads to such large increases at high solids concentration~;ll be examined next.

Polyacn"lic acid in flow of media/oulo in the mill

Examination of the internal flow patterns of media/pulp and rotating impeller at different solidconcentrations with and without additives shows that flow patterns change with changing solidconcentration. As illustrated in Figure 6, with increasing solid concentration, flow patterns usually passthrough four regimes: vortex flow, rotating flow, layer formation above the impeller pins, and layerformation adjacent to the tank wall. Use ofpolyacrylic acid is proposed to cause media/pulp flow to behaveas though of lo~'er solid concentration, improving grinding conditions and resulting in better grinding.

Conclusions

(1) Use of additives in grinding limestone results in beneficial or detrimental effects depending on theadditive type and the dosage. Polyacrylic acid was found to be the best additive among those tested. Formolecular weight 5,000, the optimum concentration was 0.1% by weight for the present system. PAAaddition resulted in lower energy consumption, and higher the additive concentration, lower was the energyconsumption.

(2) At 0.1% concentration the optimum polyacrylic acid molecular weight was 5,000. Highermolecular weight resulted in greater energy consumption apparently due to increased particle flocculationin the presence of the polymer.

(3) For grinding at different solids concentrations, over the entire range studied results withpolyacrylic acid were better than without: more than 100% increases in both the specific surface and theenergy efficiency were obtained at a solid concentration of 90%.

(4) As solid concentration increases, media/pulp flow patterns pass through four regimes: vortexflow, rotating flow, layer formation above the impeller pins, and layer formation adjacent to the tank wall.Use of polyacrylic acid caused media/pulp to flow as low solid concentration system, facilitating improvedgrinding.

Acknowledgements

This research has been supported by the Department of Interior's Mit1eral Institute Program administeredby the t.:nited States Bureau of Mines through the Generic ~fineral Technology Center for Comminutionunder Grant Number Gl135249, Gl145249. We are grateful to Specialty Minerals Inc. for providinglimestone samples.

References

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EI-Shall H. and Somasundaran P., 1984, "Physico-Chemical .-\spects of Grinding: a Re"ie\vof Use of Additives", Po)i-'der Technology, 38, 275 - 293Frangiskos, AZ. and Smith, H.G., 1957, Trans. Miner. Dressing Congr.,Stockholm, Sweden, pp.67-84Fuerstenau, D.W.. 1995. "Grinding Aids". Powder and Particle. No. 13, pp. 5-17Ghosh. S.K., Harris. C.C. and Jowett A.. 1960. Nature 188. 1182Klimpel, R.R, 1987. "Grinding Aids Based on Slurry Rheology Control", Reagents in ~1ineralTechnology, Somasundaran, P. and Moudgil. 8.M. ed., Marcel Dekker, Inc, New "ork, pp179-194Rehbinder, P.A., 1931, Ph}'sik, 72,191Somasundaran, P. and Lin I.J., 1972. IandEC Processes Des. Dev., 11,321Somasundaran. P. and Shrotri S., 1995. "Grinding Aids: A Review of Their Use, Effects and~Iechanisms", in Selected Topics in Mineral Processing. Pradip and Kumar R., eds.. Ne\\'Age International Publishers. pp. 47-70\'.estwood. A.RC. and Goldheim, D.L., 1968,.l Appl, Phys., 39, 3401Zheng. J., Harris. C.C.. and Somasundaran. P.. 1996, "A Study on Grinding and Energy Input inStirred Media Mills". P~!der Technology. 86, pp. 171-178

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