MORPHOLOGY OF MINERAL FINES PRODUCEDUNDER DIFFERENT GRINDING CONDITIDNS

R. ROUSSEV Bnd P; SOMASUNDARANHenry Krumb School of Mines,Columbia University, New York, NY 10027

~BSTRAC

Minerals can be expected to fracture differently depending upon thegrinding mechanism used and the mineralogical characteristics of the particl.Inthta study the effect of different types of comminution (ball milling,abrasion, and ~acting) on the 8>rphology, surface area, size distributionand packing wre examined with quartz, l~8tone, graphite, and calcite.Differences obtained particularly with respect to morphology were significanfor quartz and limestone.

INTRODUCTION

Behavior of mineral po~rs during various interfacial processes isdependent on size, surface chemical coaposition, zeta potential, as well asshape and mo~holO9Y of the surface. While paraaeters such as size distri-

bution and zeta potential of .inerals have received considerable attention,

properties related to the Shape and their role in determining interfacialprocesses have not been studied in detail. Phenomena such as wetting and

adhesion, packing, flocculation, flotation, etc., can, however, be expected tobe influenced by the shape and roughness of the particles. SUch inforaationin literature is limited partly because of the difficulties involved in

acquiring quantitative information on shape and roughness and partly becauseof the difficulties in controlling the grinding process to achieve size re-duction by a single desired mechanisa. ~ researchers have att-.pted tostudy the grinding techniques and others the properties of ground -terial(1). An examination of the variety of crushing and grinding equipment employed,mineral types and experimental techniques used easily reveals the difficultiesinvolved in developing an understanding of the -chani-. responsible for thegenesis of surface regularities or irregularities.

In this study. an attetlpt is ~ to obtain infonlation on the nature ofsurface morphology as a function of the qrindinq mechanism for selected hardand soft materials. The emphasis was on determininq the effect of ~ct andabrasion qrindinq on the shape and ~rphology of the particles produced.

305Particulate Science and Tectv1Oiogy 4:305-320. 1986Copyright @ 1986 by Hemisphere Publishing Corporation

R. Roussev and P. Somasundarsn306

EXPERIMENTAL

Gr inding tests were performed using a 7.5" x 8" stainless steel ball millwith a ball loading of 69 one-inch diameter stainless steel balls. Mill ~was varied frcm 50 to 65 to 8~ of critical speed in an attempt to force thegrinding mechanism towards abrasion or .impact. All wet and dry grinding testawere conducted at 65' solid. ,

In addition to ball mill grinding, powders were produced by an impac'

tester, ~rtar and pestle and by hand rubbing in a B>rtar.

Characterization of the ground products was done by sieving to obtainthe size distribUtion, quantasorb surface area analyzer, scanning electronmicroscopy and by packing in a cylinder (2) to obtain porosity of the poWer

assembly.

The above measurements were made for selected size fractions produced bydifferent methods as well as those produced by a given technique for differentgrinding times. For hard mineral, dArkansasd quartz and for soft minerals"New York" limestone and dMadagascar" graphite were used in all the tests.

RESULTS AND DISCUSSION

The process of grinding in a ball aill is determined by

Mill characteristics: dimensions, speed, volume and sizedistribution of balls, type of liners and lifters.

a.

feed size, product size,b. Materials characteristics:qrindability

Operating conditions: batch/continuous grinding, dry/wetgrinding, pulp density, residence time, circulatingload.

Most of the above factors will influence the interaction of balls with themineral and the resulting breakage. The ball motion inside the mill is con-sidered here to be one of the main factors influencing the pattern of breakage.

Ball milling in this study is limited to batch grinding under constantvolume, ball size, feed amount and size and pulp density. Variables selectedfor cqanging grinding pattern were mill speed and residence time. While usinglow mill speed resulting in cascading of ball loading, grinding by abrasion isexpected to be dOBlinant. This can be due to both sliding or ball loading onthe mill walls and sliding between different ball layers. At the high mill~eeds, the ball loading should cataract and grinding by ~acting can be ex-pected to be significant.

Gaudin-Schuhmann size distribution of prOduct obtained at 50 and 65'critical speeds for grinding for 5 and 15 minutes are given in Figure 1. Itcan be seen that more material is ground at 65~ speed. Which is considered tobe in the range of optimum speed. There is no significant difference in theslopes of the distributions obtained at the two speeds suggesting that themechanimn of grinding itself has not altered substantially between the optimum65~ speed and the less efficient 50' speed even though more abrasion and there-fore a smaller distribution modulus could have been expected at the lower

307Morphology of Mineral Fines Produced under Different Grinding Conditions

Y19ur.l Gaudin-Schuhaann size distributions of quartz produced

by wet ball .illing at different percent critical

speeds and grinding times.

speed. The effect of critical speed on the amount of -400 mesh material pro-duced is more clearly shown in Figure 2. It is seen that there is a sub-stantial decrease in grinding at the lower speed and possibly also at thehigher speed. It is to be noted that this decrease is not an effect limitedto the ultrafine size range but occurs with respect to all sizes (see Figure1) solely due to a decrease in grinding efficiency at the lower and higherspeeds. Size distribution of products obtained from mortar grinding andiIPact tester are shown in Figure 3. The product from the i]lpact tester can befound to obey the Gaudin-Schuhmann behavior to a greater extent than those fromwet grinding (Figure 1) or mortar grinding. Mortar grinding as expected isfound to produce more -400 mesh fines. It is not clear, however, as to Why theslope of the impact tester product is lower than that of the ball mill product.

Surface area and porosity as measured by packing density of various sizefractions are given in Tables 1 and 2. Both porosity and surface area arefound to increase with increase in fineness. Also surface area of the productfrom 65% critical ~ed is higher for all size fractions except -200 + 270mesh than of the product from 50% critical speed. There is also a measurablevariation in packing density between fractions obtained at the two speeds.This could resul t either from a lower median diameter of fractions or presence

of more cracks and roughness on particles obtained at the higher speed.

308R. Roussev and P; Somasundaf8n

Fi9ure2 -400 mesh quartz produced by wet ball mi11169 ...

a function of percent critical speeds.

60

:I:VI\oJ~

00~

50

40

aet-

~&.J3:

30

2050 60 70 80 90

MILL SPEED. % ~S

Figure 3 GaUdin-Schuhmann size distributions o~ quartz produced

u8inq impact tester and mortar and pestle.

309Morphology of Mineral Fines Produced under Different Grinding Conditions

Table 1

Surface area of fractions obtained by abrasion grinding (50\ CritiealSpeed) and by impact grinding (65\ Critical Speed)

Method of BreakageAbrasion Impact

m2/q m2/q

Fraction,mesh

1.6901.7251.8142.475

1.3841.70,4Z..4102.432

3.197

-100 .-150 '-200 .

-270.-400

Table 2

Porosity of fractions obtained by Abrasion (5~ Critical Speed) and

by Inpact (65' Critical speed)

Fraction,mesh

Method of BreakageAbrasion Impact

,p 'P

53.9541.3656.1442.2355.9244.3260.6746.3668.0552.14

54.6043.9556.6144.0557.5644.1961.6444.9665.0348.13

-100 + 150 a.b.a.b.a..b.a.b.a.b..

-150 + 200

-200 + 270

400

400

loosest packinq of the samplevibration packinq J

a.b.

The morphology of various saq>les was examined in detail usinq scanninqelectron miCroscopy and typical micrographs are presented in Figures 4-9.Evidently interpretations and canparisons of these micrographs are qualitativeand different conclusions miqht be reached dependinq upon the nature of theemphasis received. It is obvious, however, that there are distinct diff-

erences between the morphology of different particles.

Comparison using Figures 4a-d of mo~holO9Y of -400 mesh quartz, limestone,~cite, and graphite particles all prepared by abrasion grinding using mortarand pestle shows clearly that there are distinct differences between differentparticles of different materials. Similar differences are also seen by com-paring Figures 5a-c of -400 mesh quartz, limestone and graphite particles using

.150

.200

.270

.400

310 R. ROUSS8V 8nd P; Somssundarsn

Figure 4. Scanninq electron micrographs of -400 mesh mineral fractionsprepared by abrasion using mortar and pestle.

Figure 4 (a) quartz

FilJ\lre 4(b) limestone

Morphology of Mineral Fines Produced under Different Grinding Conditions 311

Figure 4(c) calcite

Figure 4 (d) graphite

R. Roussev and P. Somasundaran3.12

Scanning electron micrographs of -400 mesh mineral fractions

prepared using an impact tester.Fiqure 5

Figure 5 (a) quartz

Figure 5 (b) lilEstone

313Morphology of Mineral Fines Produced under Different Grinding Conditions

Figure 5 (c) graphite

an impact tester. These differences laight be iUI)Ortant in determining theefficiency of processes such as flotation and flocculation. Flotation processis described usually on the basis of surface chemistry of aineral particles.1fhile differences in surface properties have to be considered to be priJaarilyresponsible for selectivity of the flotation process, the rate of flotationitself will depend among other factors on the probability of adhesion ofparticles to bubble during collision which in turn will be influenced by theirshape. It is probable that with equally hydrophobic quartz and graphiteparticles, higher flotation rate ~uld be obtained with the former due to itsangular shape in coaparison to the flaky appearance of graphite.

R. Roussev and P; Somasundaran314

Even with the same mineral, morphological differences can be expected ifparticles are prepared using different techniques. Scanning electron micro-graphs of quartz particles prepared using the impact tester (Figure Sa) can becompared with those prepared using mortar and pestle. The particles from thelatter sample appear to have undergone erosion in comparison to the one fromthe impact tester. Particles prepared by manual rubbinq of a single piece ofquartz (Figure 6), on the other hand, appear similar to that prepared byimpact tester (Figure Sa). In comparison to that of quartz, effect of thegrinding mechanism is observed to be larger on the softer limestone (compareFigure 4b with Sb) and almost nil on the flaky graphite (compare Figure 4d and

Sc).

Scanning electron micrographS of -400 _sh ~tz prepared by

rubbing.Figure 6.

Morphology of Minersl Fines Produced under Different Grinding Conditions 315

Fiqure 7 Comparison of coarse -100 + 150 .esh quartz (Figure 7 (a» with afine -400 mesh sample (Figure 7(b». Note the difference inmagnification ~d.

Figure 1 (a)

It has been suggested in the past that ultrafine particles of a .81neralmight have different roughness than coar ser particles of the same mineral andthat this might be the reason for the marked difference between the flotationefficiency of ooarse and fines (3,4). In Figure 7,a coarse particle is ~pared with a particle that is about 10 times smaller, but magnified 10 timesIK>re so that their ~rpbo1ogies ~ld be CO8Ipared. At least in this casethere is no significant difference in the angularity of the particles eventhough the coarse particle exhibits a typical crack with even finer particlesinside the crack. It would appear frca a cCDparison of the fine in Figure 7bwith the coarse in 7a that it will be easier to further comminute the latterthan the former as is normally found to be the case. Indeed in the case of anore particle with measurable mineralogical heterogeneity, it is possible toget particles of totally different shape in various size fraction. This wasfound to be the case with ground hematite (4) which can be seen from Figure 8where the fine fraction bad many IK>re elongated particles possibly due to thepresence of clay type material in the fraction.

R. Roussev and P. Somasundaran316

Fi~e 7 (b)

Comparison of unwashed (Figure 8(a» and washed (Figure 8(b»coarse hematite with hematite slime obtained from washing(Figure 8(c» (4).

Figure 8.

Figure 8(a)

Morphology of MInerai Fines Produced under Different Grinding Conditions 317

Figure 8(b)

Figure 8 (c)

318 R. Roussev and P; Somasundaran

The possibility of generating particles of different 8:lrpbology by

grinding for longer periods was tested by examining the -400 mesh fractionsobtained by ball milling quartz at low critical speed for 5 minutes and15 minutes. Typical micrographs of these series are shown in Figure 9.It is seen that ~t ~ther particles are produced by lwager te%agrinding suCJgesting that the residence time of the mineral in the mill canhave implications with respect to shape in addition to size. It has beenobserved earlier that prolonged grinding of _terials can produce Amorphous-ness,polymorphic transition and even solid state reactions involving thea (5).

F1qure 9. Scanning electron micrographs of -400 _sh quartz prepared by dryball milling for 5 minutes (Figure 9(a» and 15 minutes(Figure 9(b».

Figure 9(a)

Morphology of Mineral FInes Produced under Different Grinding Conditions 319

Fi9ure 9 (b)

CONCLUSIONS

1. Fines obtained by grinding, ~ct1ng and abrasion of quartz, l~st<Xle,graphite and calcite are found to produce particles of different .orpho1ogy.With quartz, all the above operations produced irregular particles withcracks on the coarse particles Whereaa in the case of limestone round shapedparticles with a tendency to a991~ate _re obtained. With graphite, on theother hand, flaky particles with a coating of a 81ch finer material --observed.

2. Some differences in morphology resulted from the differences in the~tion -thod eaployed particularly for limestone. PlCNinq of the sur-face could be clearly seen in the ca.e of quartz particles subjected toabrasion.

3. At least for the case of quartz which was tested, no .ajor difference wasobserved between the morphology of CO8r.e particles and fines. Particles(-400 mesh) produced by ball milling for different periods, however, exhibitedBaBe variations in shape.

4. Size distribution and porosity characteristics of the above particlesystems sho_d S(88 minor differences.

5. lBIIlication8 of the above ob_rvations on the efficiency of such processesas flotation and flocculation Which depend on efficient collision betweenparticles and bubbles or particles and particles should be noted. Additionalwork to fully establish differences in particulate morphology produced byc~nution type. residence time or ainera),oqy will prove beneficial for theefficient processing of 8inera1s.

...

R. Roussev and P; Somasundarsn320

ACKNOWLEDGEMENTS

Authors wish to acknowledge the support of the International Di.v.b!on ofthe National Science Foundation (rNT-So-04329).

REFERENCES

A Review,C.B. Holt, The Shape of Particles Produced by Caainution.Powder Tech., Vol. 28, pp. 59-63, 198L

10

C.C. Harris and H.G. Smith, The Moisture Retention Properties of FineCoal. A Study by Pe~ability and suction potenti~l Methods. Part 2.The University of Leeds. Department of Mining Second s~sium on CoalPreparation, pp. 1-81, 21st-25th October, 1957.

1

3. J.A. Finch and G.W. Smith, Contact Angle and Wetting, Minerals Sci.EDgin_ring, Vol. 11, pp. 36-63, 1979.

4. P. Somasundaran, Role of Surface Chemistry of Fine Sulphides in TheirFlotation, in COmPlex Sulphide Ores, M.J. Jones, ed., Inst. of Min. and

Met. (London),pp. 118-127, 1980.

I.J. Lin and P. Somasundaran, Alterations in properties of Sa&p1es DuringTheir preparation by Grinding, Powder Tech., Vol. 6, pp. 171-180, 1972.