Chapter 30

POWDER SURFACE AND POLYMER STRUCTURAL CHANGESIN ULTRAFINE GRINDING IN STIRRED MEDIA MILLS

Jie Zheng, Colin C. Harris and P. SomasundaranHenry Krumb School of Mines, Columbia University, New York, NY

ABSTRACT

Powder surface and polymer structural changes during stirred milling have been studied as a function of grinding time. solid con-centration and polymer molecular weight. It was found that specific surface area of limestone particles can be increased over 100%by using polymer additives. Particle surface and polymer structure were also found to change during grinding. Powder surface andpolymer properties have been determined using Surface Tension. Fluorescence. TOC and HPLC,GPC analysis. and surface and poly-mer properties are correlated with operating variables and the mechanisms involved are explored. Particularly. the fragmentation ofthe polymer molecules was found to occur during bng term grinding and this was beneficial for ultrafine grinding.

INTROOUC nON

The effects of chemical additives on grinding have been explained in the past mainly by two kinds of mechanisms. One is basedupon the alteration of surface and mechanical properties of individual particles (Rehbinder, 1931; Westwood and Goldheim, 1969), whilethe other considers the arrangement of particles and their flow in suspensions (Klimpel, 1987). These mechanisms have been exam-ined during the last few decades (Somasundaran and Un, 1972; EI-Shall and Somasundaran, 1984; Somasundaran and Shrotri, 1995;Fuerstenau, 1995). Grinding, especially ultrafine grinding using grinding aids, has been reported to induce changes not only in par.ticle shape, size distribution and surface area but also in particle chemical decomposition, lattice distortion, polymorphic transforma-tion, surface free energy and conductivity (Lin and Somasundaran, 1972; Un, Nadiv and Grodzian, 1975; Kawashima and Meguro,1975; Lefelshtel and Nadiv, 1978; Senna and Schonert, 1982; Lin and Nadiv, 1985; Senna and Ikeya, 1986: Boldyrev, 1986: Tkacovaand Stevutova, 1987: Heegn, 1989). The materials due to grinding treatment have been reported to be more reactive and active, easierto sinter and 01 higher compressive strength. However. there are very few studies on the powder properties and structural changes ofgrinding aids due to grinding and stirred media milling using chemical additives. It is the objective of this paper to study powder sur-face and polymer structural changes in ultrafine grinding in stirred media mills. In this study. the effect of polyacrylic acid on the grind-ing of limestone was studied as a function of solid concentration and grinding time together with powder surface and polymer proper-

ties.

EXPERIMENTAL

The arrangement of the vertical stirred media mill and instrumentat..,n employed is shown in Figure 1. Detailed description of themill and instrumentat..,n and the procedures for the measurement of torque or power and energy and the control of impeller rotat..,nalspeed using the instrumentation has been given in our earlier papers (Zheng, Harris c:nd Somasundaran, 1995; 1996a).

High purity limestone (96% CaCO3). which is processed from the deposit of natural calcite located in Adams. MA. provided bySpecialty Minerals Inc.. PA. was used for the grinding experiments. Use of limestone permitted comparison with studies in the litera-ture. Since glass media was reported more energy efficient for grinding limestone and lower contamination than steel media (Zheng.Harris and 5'?f!1asundaran. 1996a). technical quality soda-lime silica glass spheres (density 2.5 g/cm3 and monosize 2.05 mm ). ob-tained from Potters Industries Inc.. NJ, were used as grinding media in this study. A cylindrical glass tank was used so that the flowpattern of media/pulp could be observed; otherwise. stainless steel cylindrical grinding chambers were employed. In this experiment.the tank diameter was 11.8 cm and the four-pins impener diameter was 6.5 cm. Poly acrylic acids, obtained from POlysciences, Inc.,PA, were used as the grinding aids. Operating conditions, such as the ratio of media to particle volume and the ratio of media to feedparticle size, were set at optimum values arrived at in our earlier work (Zheng. Harris and Somasundaran, 1996a). namely: volume ratio,2.8; size ratio, 12:1. A constant high impeller speed of 1000 rpm was chosen to obtain relatively high product surface area. After eachtest, all the media and the ground pulp were removed from the mill. the supernatant separated from the pulp, and the media separatedfrom the particles by screen washing. Residual poly acrylic acid concentrat..,n of the test samples were measured using a Total OrganicCarbon (TOC) Analyzer. The specific surface area was determined using the multiple point BET method using a Quantasorb appara-tus. ThQ fluorQSCQncQ spQctra were recorded on a Photon InternatkJnal PTI.lS 100 spectrophotometer. For solid samples, the fluo-

COMMINUTION PRACTICES234

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Fig1Jre 1. Expermenl apparaws attang&menlrescence experiment was conducted in a 2 mm flat quartz cell. High Pressure Liquid Chromatograph (HPLC) was conducted usingPerkin-Elmer Series 400 device with the solvent tray and a refractive index detector. Gel Permeation Chromatography (GPC) modewas used with Shodex OHpak 58-806 column as a packed column. Surface tension was determined using a Wihelmy plate tensiom-

eter.To present the grinding results, three parameters defined below, new specific surface (5), specific energy (Ew) and energy efficiency

(Ef), are used:

(1)5 . Sp' Sf

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

(2)f:w

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

(3)S

EwRESULTS AND DISCUSSION

Energy Efficiency and Partcle Surface Change

One of the most important factors in the use of chemical additives is solids concentration (Zheng. Harris and Somasundaran. 1996b).Effect of polyacry6c acid on grinding at different solid concentrations ranging from 60% to 100% was studied at a fixed molecular weightof 5.000 and an additive concentration of 0.1 weight % in reference to feed weight. Total solid (media and particle) volumetric concen-trations. 65%. 75%. 80%, 85%. 900/0, 95% and 100%. correspond to particle solid volumetric concentrations. 33%. 44%, 51%. 60%,

700/0, 83% a~? 1000/0. The results obtained for specific surface area, energy efficiency, and the corresponding average percentageincrease due to additives are shown in Figure 2 as a function of solid volumetric concentration. In the absence of polymer, the prod-uct surface area was found to increase with increase in solid concentration from 65% to 75% and then to decrease. In the presence of

polymer, the product surface area was found to increase with solid concentration up to 80% and then to decrease. Observation throughthe transparen1 tank during the stirrjng test at the solid concentration of 80% without and with the additives revealed that without addi-tives. only the solids (media and particles) around the center of the impeller are stirred while beyond the impeller pins the solids re-mained almost stationary (Zheng, Harris and Somasundaran, 1995), while with polymer, the solids both around and beyond the im-peller pins are stirred resulting in increased pulp activity and particle grinding. Furthermore, polyacryflC acid resuhs in better grinding in

the entire concentration range studied: at a solid concentration of 900/0 over 1000/0 improvement in both surface area and energy efficiencywas obtained. This result is of commercial w,terest since industrial stirred mills QJ)erate n'KIstly at the highest possible solid concentrati>ns.

In order to explain the effect of grinding aids at higher solid concentration. media/pulp flow is examined in the light of torque drawn

by the mill during operating period. A diagrammatical representation of the variation of torque vs. time for grinding at high solid con-

POWDER SURFACE ABD POLYMER STRUCTURAL CHANGES IN UL TRAFINE GRINDING 235

C4

M;to>u

C

~u

c:'y

:9;3:u

:z

60 65 70 75 80 85 90 95 100

Solid Concentration (%)Fgure2 Effect 01 polyacryllC acid Imw: 5.000) at O"1~ ~ill'" concenwaoon on QflnGng lmes~ at different solids concenwatanMedlatPulp F'- Patterns Char1g.

centrat~ns is given in Fig. 3. The system used was the batch mill w~h the 4.pin impeller immersed in the media and pulp of high sol-ids concentration without any supernatant and operating at constant high impeller speed. In region AB, the torque drops gradually asrotat~n commences, and the largely stat~nary pacj(ing of solids becomes loose and mobile. As milling time increases so does thequantity of solids rotating with the impeller. When all solids are in rotation, the torque increases with grinding time (region BC). andincreasing quantities of fines are produced. Towards the upper end of region BC, a portion of the solid mass begins to form a largely~tationary layer above the impeller pins, resuhing in a" drop in torque. The fOrmation of the layer occurs in the following sequence: sol-ids move toward the wall of the tank, pack there, migrate upwards, then move inwards above the impeller pins. This stage marks alsoan increase in the sound level since the tips of the pins are now exposed as they grind against the packing at the wall. At still longergrinding times. solids form a substantial layer above the impeller pins (region CDE), and torque decreases with grinding time. Towardthe end of this.region. a complete layer of solids forms above the impeller pins while the volume around the center of the impeller pins.IS almost empty.. The torque decreases to its lowest level and tends to remain constant in this region. " there is a substantial distancebetween the tips of the impeller pins and tank wall, the formation of the layer does not occur; instead a packed layer forms around thetank wall beyond the tips of the pins.

The most suitable region for grinding is BC in Fig. 3, and extension of this region is likely to improve grinding. Grinding aids are foundto extend this region and enhance production of fines due to improved fluidity or lubrication and lor decreased capillary forces or sur-face tension resulting in less stationary material. In fact, surface tens~n of water is lower with polyacrylic acid addition than without,as illustrated in Table 1, relating surface tension of supernatant to that with or without PAA addition and as a funct~n of PM molecu-lar weight. Surface tension of supernatant decreases with decreasing polymer molecular weight. This result is in agreement with theconclusion that lower PAA molecular weight results in more dispersion of limestone particle suspension as reported earlier (Zheng,Harris and Somasundaran, 1996b).

236 COMMINUTION PRACTICES

AS LOOSENING REGIONBC ROTATING REGIONCD BEGIN TO FORM A LAYER ABOVE THE IMPELLER PINSDE LAYER ABOVE IMPELLER PINS CONTINUING TO BE FORMEDEf LAYER ABO~ IMPELLER PINS FULLY FORMED

Figwe 3 DlagralmlaDcai ,.,._lalKln 01 die V8iaoOfl of ~ ¥S. ~ far gJ1~ al" ad OOtX8nlraion~f"

I$'"

i

Table 1. ComP-'1On of sultxe *~ 01 - 8Id Iknesane ~sICII1lUpematan1 willo and wittloul poIy-=ryHc acid addidon allemp8faa,te 2e"C

i-i."~

t

t~,,-"!

Examinaton of the internal fk>w patterns of media/pulp and rotating impeDer shows that fk>w patterns change with changing solidconcentration. As illustrated in Figure 4. with increasing solid concentraton. flow patterns usually pass through four regimes: vortexfk>w, rotating. layer formation above the impeller pins, and layer formation adjacent to the tank wall (see Figure 4). Use of polyacrylicacid causes media/pulp flow to behave as though it is at a lower solid concentration corresponding to greater fluidity and improvedgrinding conditions thus resulting in better grinding. Therefore. from a practical view point. additives are best used when solid concen-tration is high. .

Polymer Conformation Change IFluorescence spectroscopy technique was used for determining polymer conformation at interfaces. This method involves the ex-

ploitation of 8xcimer formation of pyrene. Pyrene excimer fluorescence can be observed as a broad spectral band around 480 nm that

~

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vortexflow

rotating layer fonmtion layer fonmtionflow above the impeller ~ tank wail

pins

Increasing solid concentration ~

Fig... 4 Flow p-.ma of meQaIpu~ WlrI ~ solid calcenrailln

m~

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)6$ JI' 405 425 40&5W A VELENOTH, 11m 46S

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238 COMMINUTION PRACTICES

Figure D. The raGO of /ellm as a Nnclion of grinding time (grinding conditiOt!s: impeter speed: 1000'pm; solid (media ~ parlide' concenlra:ion: 65%; PM (5.0001

~nlralion: 0.1% referred to ~ parGete; pyrene concenwaaon: 10'% 'efe"ed to PM)

Rcspor.se ( MV )

FiglXe 7 Comparison of GPC results for PAA (mw: 150.000) before and ater grinding «*Jring grinding lmeslOne

CONCLUSIONS

(1)

(2)

(3)

(4)

(5)

Energy efficiency, powder surface area, media flow patterns, pulp surface tension, polymer conformation and structuralchanges during grinding limestone with and without polyacrylic acid addition have been studied.For grinding at different solids concentrations, results with polyacrylic acid were better than without over the entire range stud-ied: more than 100% increases in both the specific surface and the energy efficiency were obtained at a solid concentrationot"900fo.As solid concentration is increased. media/pulp flow patterns pass through four regimes: vortex flow. rotating flow, layer for-mation above the impeller pins, and layer formation adjacent to the tank wall. Use of poly acrylic acid caused medial pulp athigh solids concentration to flow as though of lower solids concentration. This increased fluidity facilitates improved grinding.

Polymer conformation changes with grinding time. EX1ension of the polymeric species as grinding proceeds. appears to be anadvantage of additives especially for long time grinding.Polymer structure or molecular weight also changes as grinding proceeds. Fragmentation of high molecular weight polymermolecules may result in improved grinding since lower molecular weight polymers are benefICial for grinding.

POWDER SURFACE ABD POLYMER STRUCTURAL CHANGES IN ULTRAFINE GRINDING

ACKNOWLEDGEMENTS

239

Institute Program administered by the United States Bureau of Mines through the Generic Mineral Technology Center for Commi-nution under Grant Number G, , 35249 and G, '45249.

REFERENCES

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EI-Shall H. and Somasundaran P., 1984, "Physico-<:.hemical Aspects of Grinding: a Review of Use of Additives-, Powder Technol-QgX,. 38, 275-293

Fuerstenau, D.W., 1995, "Grinding Aids-, KONA (Powder and Particle). No. 13, pp. 5-17Heegn, H., 1989, "On the Connection between Ultrafine Grinding and Mechanical Activation of Minerals-, Aufbereitunas- Techni<.

30, 10, pp. 635-642Kawashima, N. and Meguro, K., 1975, " Dry Grinding on a Pigment Mixture 1., The Effect of a Copper Phthalocyanine as a lubricant

on the Mechano-Chemical Transformat~n of Calcium Carbonate", Bulletin of the Chemical Societv of Jaoan. 48, 6, pp. 1857-1861

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Lefelshtel, N. and Nadiv, S., 1978, "Production of Zinc Ferric in a Mechano-Chemicai Reaction by Grinding in a Ball Mill-, PowderTechnoklgv. 20, pp. 211-217

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Malbrel, C.A. and Somasundaran, P., 1992, "Effect of Water on the Dispersion of Colklidal Alumina in Cyclohexan Solutions ofAOr, Langmuir. 8, pp 1285-1290

Rehbinder, P.A., 1931, f11x.iih. 72,191Senna, M. and Ikeya, T., 1986, "Amorphization and Phase Transformat~n of Niobium Pentoxide by Fine Grinding", In oreorints First

World Congress on ParticlA Techno Part 11. ComminutK>n (6th Eurooaan svmoosium Comminution). Leschonski, K. ed.,(Nuremberg: Nurnberger Messe and Ausstellungsgesellschaft GmbH, April 16-18, pp. 571-581

Senna, M. and Schonert, K., 1982, "Change in the Entralpy and Structure of PbO2 by Grinding and Pressing-, Powder Technoloav.31. pp. 269-275

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Mineral Processing, Pradip and Kumar R., eds., New Age International Publishers, pp. 47-70Tkacova, K. and Stevulova, N., 1987, "Change in Structure and Enthalpy of Carbonates and Quarts Accompanying Grinding",

Powder Techno~~. 52, pp. 161-166Westwood, A.R.C. and Goldheim, D.L., 1968, J. ADOI. Phvs., 39, 3401Zheng, J., Harris, C.C., and Somasundaran, P., 1995, -Power Consumption of Stirred Media Mills-, Minerals and Metanurdical

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Technoloov. 86, pp 171-178Zl1eng, J., Harris, C.C., ,.nd Somasundaran, P., 1996b, -The Effect of Additives on Stirred Media Milling of Limestone-, Preorint 96-

151. SME Annual MAetino, Phoenix, Arizona, March 11-14