effects of physical properties of powder particles on binder liquid requirement and agglomerate...

European Journal of Pharmaceutical Sciences 14 (2001) 135–147www.elsevier.nl / locate /ejps

Effects of physical properties of powder particles on binder liquidrequirement and agglomerate growth mechanisms in a high shear mixer

*Anita Johansen, Torben SchæferDepartment of Pharmaceutics, The Royal Danish School of Pharmacy, 2 Universitetsparken, DK-2100 Copenhagen, Denmark

Received 12 March 2001; received in revised form 29 May 2001; accepted 5 June 2001

Abstract

A study was performed in order to elucidate the effects of the physical properties of small powder particles on binder liquidrequirement and agglomerate growth mechanisms. Three grades of calcium carbonate having different particle size distribution, surfacearea, and particle shape but approximately the same median particle size (4–5 mm), were melt agglomerated with polyethylene glycol(PEG) 3000 or 20 000 in an 8-l high shear mixer at three impeller speeds. The binder liquid requirement was found to be very dependenton the packing properties of the powder, a denser packing resulting in a lower binder liquid requirement. The densification of theagglomerates in the high shear mixer could be approximately predicted by compressing a powder sample in a compaction simulator. Withthe PEG having the highest viscosity (PEG 20 000), the agglomerate formation and growth occurred primarily by the immersionmechanism, whereas PEG 3000 gave rise to agglomerate growth by coalescence. Powder particles with a rounded shape and a narrow sizedistribution resulted in breakage of agglomerates with PEG 3000, whereas no breakage was seen with PEG 20 000. Powder particleshaving an irregular shape and surface structure could be agglomerated with PEG 20 000, whereas agglomerate growth becameuncontrollable with PEG 3000. When PEG 20 000 was added as a powder instead of flakes, the resultant agglomerates became rounderand the size distribution narrower. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Melt agglomeration; High shear mixer; Binder viscosity; Powder particle properties; Agglomerate growth mechanisms; Binder liquidrequirement

1. Introduction mixers. However, systematic examinations of the effects ofpowder particle size are complicated due to limitations in

In agglomeration processes, many problems in choosing the grades of excipients available from suppliers, and onlythe right formulation and process conditions as well as a few systematic studies have been published (Schæfer etdifficulties in controlling the process arise because of al., 1992; Keningley et al., 1997; Knight et al., 1998;variations in the physical properties of the starting materi- Johansen and Schæfer, 2001).als. Ideally, a comprehensive knowledge of the starting In spite of the central role of the mean particle size inmaterial properties should make it possible to predict the the agglomeration process, it cannot solely explain therequired amount of binder liquid and the process con- effects of starting materials observed. Other properties,ditions necessary for successful agglomeration without such as particle size distribution, particle shape, surfacehaving to perform costly and time-consuming preliminary structure, surface area, and wettability also play a signifi-experiments. However, knowledge of the effects of starting cant role with regard to binder liquid requirement andmaterial properties on the binder liquid requirement and agglomerate growth. Systematic investigations of the ef-agglomerate growth is still sparse, and the need of fects of these starting material properties are difficult,additional research in this area is obvious. however, because it is practically impossible to vary one of

The mean particle size of the powder is the starting these properties without changing other particle properties.material property that has been most commonly examined These complex interactions make it difficult to distinguishin wet and melt agglomeration experiments in high shear between the effects of different particle properties.

Generally, a smaller size of the powder particles hasbeen found to increase the binder liquid requirement in*Corresponding author. Tel.: 145-3530-6000; fax: 145-3530-6031.

E-mail address: [email protected] (T. Schæfer). order to obtain agglomerates of a similar size (Tapper and

0928-0987/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0928-0987( 01 )00164-6

136 A. Johansen, T. Schæfer / European Journal of Pharmaceutical Sciences 14 (2001) 135 –147

Lindberg, 1986; Schæfer et al., 1992; Knight et al., 1998; coalescence (Eliasen et al., 1998, 1999; Knight et al.,Johansen and Schæfer, 2001). Presumably, the liquid 1998). When using a highly viscous binder liquid, agglom-requirement in the nucleation phase of the process is erate formation and growth might occur by immersion ofprimarily dependent on the surface area of the primary powder particles in the surface of the binder dropletsparticles. Further growth depends on the liquid saturation (Schæfer and Mathiesen, 1996a; Johansen and Schæfer,of the agglomerates (Kristensen et al., 1984), and the 2001).amount of liquid required will depend, therefore, on the Powders having a mean particle size below |10 mm willpacking properties of the powder (Newitt and Conway- normally be difficult to agglomerate, because their cohe-Jones, 1958). Hancock et al. (1994) stated that the binder siveness causes agglomerates of a high strength (Kristen-liquid requirement also depends on the ability of the binder sen et al., 1985b; Schæfer, 1996a). Such agglomerates willliquid to fill the void space of the powder particles and not have a reduced deformability because of the high strength,only on the packing properties of the powder. The packing and more binder liquid is required in order to render theproperties of a powder depend on the particle size dis- agglomerates sufficiently deformable for agglomeratetribution but also on the shape, surface structure, and growth by coalescence. This increases the risk of overwet-surface area of the particles. ting and uncontrollable agglomerate growth.

Interlocking between particles of an irregular shape The agglomerate strength is also affected by the particleincreases the agglomerate strength and might reduce the size distribution of the powder. Litster et al. (1998)need for binder liquid (Schæfer and Mathiesen, 1996b). A describe agglomerates of coarse and narrowly sized par-rounded particle shape, on the other hand, reduces the ticles as moderately weak agglomerates that easily deformagglomerate strength (Holm, 1987). The particle shape and coalesce. Fine, widely sized particles, on the otheralso affects the surface structure of the agglomerates. hand, are mentioned as resulting in strong, non-deformableAqueous agglomeration of needle-shaped particles has agglomerates with slow rates of consolidation. Studies inbeen shown to result in agglomerates of high porosity drum granulators showed that a narrow particle size(Juppo and Yliruusi, 1994). Likewise, melt agglomeration fraction resulted in a low agglomerate strength, whereas aof plate-like and needle-shaped particles produced agglom- wide particle size distribution gave very strong granuleserates of an irregular and loose surface structure (Schæfer (Linkson et al., 1973). Further, a narrower size distributionand Mathiesen, 1996c). The liquid requirement will also be has been found to result in more porous agglomeratesaffected by differences in the surface porosity of the owing to lower densification (Adetayo et al., 1995).particles since the binder liquid might enter pores in the The aim of this work is to obtain further knowledge ofsurface thus increasing the binder liquid requirement the effects of the physical properties of powder particles on(Smith and Nienow, 1983; Schæfer and Mathiesen, binder liquid requirement and agglomerate growth by1996b). comparing starting materials having different particle

Attempts have been made to predict binder liquid properties but approximately the same median particlerequirement from starting material properties. Some ex- size. The work is based upon the hypothesis that smallperiments have shown the optimum amount of binder powder particles having different properties can be ag-liquid to be approximately proportional to the surface area glomerated in a controllable way provided that the agglom-of the powder mass (Pendharkar et al., 1990; Schæfer et erate strength and deformability are controlled by anal., 1993), whereas other experiments showed that the appropriate choice of the amount and the viscosity of theamount of binder liquid per surface area had to be binder liquid.increased with a larger powder particle size (Schæfer,1996b). In previous work (Johansen and Schæfer, 2001), adifferent liquid requirement of powders with a similar 2. Materials and methodsspecific surface area was seen, and this was ascribed to adifferent shape of the powder particles. An attempt to 2.1. Materialscorrelate the interparticular porosities, i.e. the bulkporosities obtained by tapping, with the binder liquid Three different grades of calcium carbonate powdersrequirement showed no general correlation, probably be- were used as starting material. Durcal 5 (Omya, France)cause the densification in the tapping apparatus is much and Eskal 500 (KSL Staubtechnik, Germany) were statedlower than in the high shear mixer. to be manufactured from a natural white marble and

The strength of the agglomerates determines the ag- produced by comminution and classification. Sturcal Fglomerate growth mechanisms. Coalescence will be the (Rhone Poulenc, France) was said to be produced bydominant growth mechanism if the agglomerates possess precipitation. The same batch of Durcal 5 was applied inhigh agglomerate strength (Tardos et al., 1997). However, previous experiments (Johansen and Schæfer, 2001). Poly-if the agglomerates are too weak to resist the impact and ethylene glycol (PEG) 3000 S (flakes), 20 000 S (flakes),shearing forces in the mixer, a marked agglomerate or 20 000 P (powder) (Clariant, Germany) was used asbreakage will occur simultaneously with growth by meltable binder. The same batches of the flakes were

A. Johansen, T. Schæfer / European Journal of Pharmaceutical Sciences 14 (2001) 135 –147 137

applied in previous experiments (Johansen and Schæfer, 2.2. Equipment2001). Butylated hydroxyanisole (BHA) (Merck-Schuch-ardt, Germany) was used as an antioxidant in order to The agglomeration experiments were performed in anprevent thermal decomposition of the binder during ag- 8-l Pellmix PL 1/8 laboratory scale high shear mixerglomeration (Schæfer and Mathiesen, 1996b). (Niro, Denmark) (Schæfer et al., 1993).

The size distribution by volume of the calcium carbon-ates was determined in triplicate by a Malvern Mastersizer 2.3. Mixing procedureS laser diffraction particle sizer (Malvern Instruments,UK). The span was calculated as the difference between The heating jacket was preheated to 508C. Samples ofthe diameters at 90 and 10 percentage points relative to the 1500 g of Durcal, 1500 g of Eskal, or 1000 g of Sturcal,median diameter, D(v;0.5). the amount of PEG (% m/m of calcium carbonate), and

A Gemini 2375 Surface Area Analyzer (Micromeritics, 3% BHA (% m/m of PEG) were dry mixed at an impellerUSA) was used for the determination of the BET multi- speed of 1300 rpm. Because of formation of frictional heatpoint surface area of the calcium carbonates. Analyses caused by the impeller rotation, the product temperaturewere performed in duplicate. increased during mixing to a temperature exceeding the

The true density of the calcium carbonates and of the melting point of the PEG. The melting point was observedsolid PEGs was determined in duplicate by an Accupyc as an inflection point on the recorded product temperature1330 gas displacement pycnometer (Micromeritics, USA) curve. This inflection point was defined as the start ofusing helium purge. The poured and tapped densities of the massing time. At 2 min after the melting point wascalcium carbonates were determined in duplicate according observed on the temperature curve, the impeller speed wasto the test for apparent volume (European Pharmacopoeia, lowered to 500, 800, or 1100 rpm. After 8 min of1999), and the interparticular porosities obtained during additional massing time, the agglomeration procedure wastapping were calculated. A compaction simulator described terminated. The mixing was interrupted at 2, 4, 6, and 8previously (Pedersen and Kristensen, 1994) was applied min after melting, and small samples (5–6 g) werefor determination of the compressibility of the calcium randomly withdrawn from the mass with a spoon.carbonates. Samples of |500 mg were compacted as At the end of each experiment, the agglomerates weredescribed previously (Sonnergaard, 1999), and the upper sieved on a 4-mm Jel-Fix 50 vibration sieve (J. En-punch pressure and the distance between the punches were gelsmann, Germany) for |10 s, until the fraction finer thanrecorded. The samples were compacted in triplicate. The 4 mm had passed. The agglomerates were then spread outvolume between the punches and the true density of the in thin layers on trays allowing them to cool at ambientcalcium carbonates were used for calculating the inter- temperature. The adhesion of mass to the bowl wasparticular bulk porosity as a function of the applied estimated as previously described (Schæfer, 1996a) andpressure. corrected for the weight of the samples withdrawn during

The size distributions of the PEGs were estimated in an experiment.duplicate by sieve analysis with a series of 12 ASTMstandard sieves in the range of 75–4000 mm. A sample of 2.4. Agglomerate characterisation|100 g was sieved for 5 min at low vibration level (flakes)or 10 min at high vibration level (powder) by a Fritsch 2.4.1. Size distributionanalysette 3 vibrator (Fritsch, Germany). The mass median The size distributions of the agglomerates were esti-diameter and the span were calculated. mated by sieve analysis of a sample of |100 g prepared

The melting range and the peak temperature of the PEGs from the cooled fraction finer than 4 mm by a Laborette 27were estimated in duplicate by a Perkin-Elmer DSC 7 automatic rotary cone sample divider (Fritsch, Germany).differential scanning calorimeter (Perkin-Elmer, USA) as A series of 14 ASTM standard sieves in the range ofpreviously described (Schæfer and Mathiesen, 1996b). 75–2000 mm was vibrated by a Fritsch analysette 3

The densities of the molten PEGs were estimated at 70, vibrator (Fritsch, Germany) for 10 min. The mass median80, and 908C as previously described (Eliasen et al., 1998). diameter and the span were calculated.The analyses were performed in triplicate.

The viscosities of the molten PEGs were estimated in 2.4.2. Intragranular porosityduplicate at 70, 80, and 908C by a Rotovisco RV 12 The intragranular porosity of the agglomerates wasrotation viscometer (Haake, Germany) as previously de- estimated by a mercury immersion method similar to thatscribed (Schæfer and Mathiesen, 1996c). described by Strickland et al. (1956). A sample of 3–4 g

All data on material properties presented here are the from the agglomerate size fraction 250–2000 mm wasmean values of the repeated estimations. placed in a glass pycnometer with an approximate volume

Photographs of the calcium carbonates were taken by a of 30 ml having a calibrated scale. Mercury was sucked upscanning electron microscope (SEM) (Jeol JSM 5200, into the pycnometer by means of vacuum. The apparentJapan). volume of the sample was estimated by displacement of


Table 1mercury after increasing the intrusion pressure to 98.7 kPaPhysical properties of the calcium carbonates(740 mmHg). At this intrusion pressure, mercury willCalcium Particle size True Specific Poured Tappedpenetrate into pores greater than |20 mm in diameter. Thecarbonate density surface density densitycorrected intragranular porosity and the liquid saturation D(v;0.5) Span 3grade (g /cm ) area (g /ml) (g /ml)were calculated as described by Eliasen et al. (1998). (mm) 2(m /g)

These calculations are supposed to result in approximateDurcal 4.8 2.9 2.80 2.31 0.79 1.12values only since they are based upon some assumptionsEskal 4.6 1.5 2.74 1.07 0.75 1.10

that might not be completely correct. The corrected Sturcal 4.4 1.7 2.74 4.55 0.32 0.46intragranular porosity is an estimate of the packing of thesolid particles within the agglomerates when the binder isin the molten state, i.e. the corrected intragranular porosity median particle size but must be due to differences in othercorresponds to the pore volume of the agglomerates with physical properties. Table 1 and Fig. 1 show that there is athe binder removed. All porosity analyses were performed significant difference in the span of the size distributions.in duplicate. Durcal has the widest size distribution, whereas the size

The binder concentrations (% m/m of the calcium distributions of Eskal and Sturcal are narrower. However,carbonate) of the fractions (250–2000 mm) were estimated Sturcal is seen to contain a considerably larger amount offrom a quantitative determination of the content of calcium fine particles compared to Eskal. Fig. 2a illustrates that thecarbonate by titration (Pharmacopoea Nordica, 1963). The Durcal particles are angular and rough, whereas the Eskaltrue density of the agglomerate size fractions was calcu- particles in Fig. 2b are rounded and smoother. Accordinglated from the true densities and the estimated contents of to the supplier, the rounded shape of the Eskal particles iscalcium carbonate and PEG and was used in the calcula- due to ‘‘a special method of manufacturing’’. Fig. 2ctions of the intragranular porosity. reveals that the Sturcal particles are agglomerates consist-

ing of needles. SEM photographs of Sturcal that had been2.4.3. Photographs dry mixed in the high shear mixer for 12 min at 1300 rpm

Photographs of selected agglomerates were taken by a showed that the Sturcal particles were unaffected by thescanning electron microscope (SEM) (Jeol JSM 5200, mixing indicating a high particle strength. It is most likely,Japan). therefore, that no breakage of the particles occurs during

the melt agglomeration procedure.2.5. Experimental design The differences in particle shape and surface structure of

the calcium carbonates are reflected in Table 1. The EskalEskal and Sturcal were agglomerated at three impeller particles have the lowest surface area because they are

speeds (500, 800, and 1100 rpm) applying three types of smooth and rounded, whereas the needle-shaped surfacePEG (PEG 3000 and 20 000 flakes, and PEG 20 000 structure of the Sturcal particles results in the largestpowder). Durcal was agglomerated at 800 rpm in a surface area. The angular and rough Durcal particles haveprevious experimental series (Johansen and Schæfer, an intermediate surface area.2001), and results from these experiments are included in The poured and tapped densities (Table 1) show a looserthe present paper. In the present series, supplementary packing of Sturcal compared with the packing of Durcalexperiments with Durcal were performed at two impeller and Eskal which is similar. This is why the mixer load hadspeeds (500 and 1100 rpm) using two types of PEG (PEG to be lower (1000 g) with Sturcal than with Durcal and3000 and 20 000 flakes). Eskal (1500 g) as mentioned in Section 2.3. The difference

All data on the agglomeration experiments presented inthis paper are mean values of two experiments, unlessotherwise stated. The range of the repeated experiments isindicated as 6 in Tables 4–6 and as error bars in Figs. 4,5, 6, 7, 9 and 10.

3. Results and discussion

3.1. Starting material properties

It is seen from Table 1 that the volume mediandiameters of the calcium carbonate grades are almostidentical. This means that different effects of the calcium Fig. 1. Cumulative particle size distributions by volume of the calciumcarbonate grades on binder liquid requirement and agglom- carbonates: bold continuous line, Durcal; dashed line, Eskal; continuouserate growth cannot be ascribed to differences in the line, Sturcal.


Fig. 3. Interparticular bulk porosity of the calcium carbonates determinedby (a) tapping according to the test for apparent volume and (b)compression in a compaction simulator. Calcium carbonates: (j) Durcal,(♦) Eskal, and (m) Sturcal.

tion of angular particles, while needle-shaped particles hadthe opposite effect. Previously, interparticular tappedporosities were found to be markedly higher than theintragranular porosities of agglomerates produced in a highshear mixer (Schæfer, 1996b), because the shearing andcompaction forces were much lower in the tapping ap-paratus than in the mixer. In order to estimate the powderpacking at pressures related to the forces imposed by thehigh shear mixer, the powders were compacted in acompaction simulator (Fig. 3b). Due to the small particlesize applied it is presumed that the calcium carbonateparticles were not being crushed during the compression.The results in Fig. 3b show that the bulk porositiesobtained with the compaction simulator are markedlylower than those obtained with the tapping apparatus. Still,Sturcal gives rise to the loosest packing. The packing ofthe Durcal particles is slightly denser than that of the Eskalparticles, probably due to the wider particle size dis-

Fig. 2. SEM photographs of the calcium carbonates: (a) Durcal, (b) tribution of Durcal.Eskal, and (c) Sturcal. The physical properties of the PEGs are shown in Table

2. The initial median diameter and the span of the sizein the packing properties is further illustrated in Fig. 3a distribution are shown. It has been demonstrated, however,showing the interparticular bulk porosities as a function of that the particle size of the flakes becomes reduced duringthe number of taps. The higher bulk porosity obtained with dry mixing (Schæfer and Mathiesen, 1996a). The initialSturcal is ascribed to the irregular surface structure of the size of the molten binder droplets, therefore, will beparticles. In agreement with these findings, it was found smaller than the values in Table 2. The density values of(Podczeck and Sharma, 1996) that the packing properties the molten PEGs at the final product temperature wereof binary powder mixtures became denser with the addi- estimated for each experiment by means of extrapolation


Table 2Physical properties of the PEGs

Type Median Span True Density (molten) Melting point Viscosity

of diameter density708C 808C 908C Range Peak 708C 808C 908C

PEG (mm) (solid)(g /ml) (g /ml) (g /ml) (8C) temp. (mPa?s) (mPa?s) (mPa?s)3(g /cm )

(8C)

3000 Flakes 1175 1.3 1.229 1.086 1.080 1.068 47–58 56 196 148 121

20 000 Flakes 1340 1.4 1.231 1.089 1.081 1.072 55–63 62 24382 19051 13955

20 000 Powder 218 1.2 1.223 1.083 1.081 1.068 52–63 61 21364 15843 11499

of the regression line. This value was used for calculation 0.5% increase in the binder concentration resulted inof the liquid saturation of the agglomerates. overwetted agglomerates. Consequently, experiments with

The solubility of calcium carbonate in PEG was previ- Sturcal and PEG 3000 had to be omitted from the finalously found to be less than 0.1% at 908C (Johansen and series. This uncontrollable growth indicates that the mech-Schæfer, 2001). anisms of agglomerate formation and growth occurring

with PEG 3000 are different from those occurring with3.2. Preliminary experiments PEG 20 000, as will be discussed in Section 3.4.

Preliminary agglomeration experiments were performed 3.3. Binder liquid requirementin order to establish the proper experimental conditions.The binder concentrations were generally chosen as |0.5% Table 3 shows the binder liquid requirement estimatedbelow the amount of PEG resulting in overwetted agglom- on the basis of the preliminary experiments. Experimentserates at an impeller speed of 1100 rpm. However, for were carried out in duplicate at the experimental conditionsDurcal and Sturcal it was not possible to obtain agglomer- in Table 3 giving a total of 42 experiments. The experi-ates by applying the same binder concentration at 500, ments were performed in a randomized order. Previously,800, and 1100 rpm. Consequently, the binder concen- it was found that melt agglomeration with PEG 1500 of atrations being applicable had to be estimated at each of the calcium carbonate powder of approximately the samethree impeller speeds. median particle size at an impeller speed of 150 rpm in a

Preliminary experiments with Sturcal revealed that 30-l high shear mixer required a binder concentration inagglomeration with PEG 3000 flakes was not possible in a the range of 17–19% (Knight et al., 1998).controllable way. The resulting product was powdery with As can be seen from Table 4, it was difficult to producea binder concentration of 38, 37, or 35% at 500, 800, or agglomerates of a similar size at different experimental1100 rpm, respectively. For all three impeller speeds, a conditions, because the agglomerate growth was sensitive

Table 3Binder liquid requirement of the calcium carbonates expressed as % m/m of the amount of calcium carbonate

Impeller PEG 3000 flakes PEG 20 000 flakes PEG 20 000 powderspeed

Eskal Durcal Eskal Durcal Sturcal Eskal Sturcal(rpm)

500 17.0 16.5 17.0 16.5 35.0 17.0 35.0800 17.0 15.5 17.0 15.5 31.5 17.0 31.5

1100 17.0 14.5 17.0 14.5 29.0 17.0 29.0

Table 4Mass median diameter (mm) of the agglomerates produced with the binder concentrations in Table 3



a500 250 (628)500 430 (6102) 227 (622) 235 (60) 264 (66) Overwetted 414 (63) 354 (63)800 560 (677) 459 (6105) 282 (64) 314 (69) 248 (65) 507 (65) 457 (64)

1100 379 (613) 148 (61) 418 (626) 229 (62) 291 (65) 981 (670) 567 (612)b1100 240

a Binder concentration: 34.0%.b Binder concentration: 15.0%.


to variations in binder concentration, type of binder as well The packing of the particles within the agglomerates isas impeller speed. At most of the experimental conditions, expressed by the corrected intragranular porosities (Fig. 4).it was impossible to produce large agglomerates without The corrected intragranular porosities are seen to beoverwetting. The Sturcal agglomerates prepared with PEG similar to the interparticular bulk porosities estimated with20 000 flakes were clusters of small agglomerates at 800 the compaction simulator (Fig. 3b) within the pressureand 1100 rpm and became completely overwetted at 500 range of |20–100 MPa. From Fig. 3b, the interparticularrpm. The same binder concentrations resulted in agglomer- bulk porosities within this pressure range are read to varyates with PEG 20 000 powder. Two supplementary experi- between 32 and 41.5% for Durcal, 36 and 46% for Eskal,ments were performed at a binder concentration of 34.0% and 42 and 55% for Sturcal. The corrected intragranularfor Sturcal and PEG 20 000 flakes at 500 rpm in order to porosities of the agglomerates (Fig. 4) vary between 32avoid overwetting. and 42.5% for Durcal, 33 and 40.5% for Eskal, and 44.5

The agglomerates formed with Durcal and PEG 3000 and 55% for Sturcal. This indicates that the compactionflakes at 1100 rpm became very small and contained a lot simulator might be useful for prediction of the densifica-of unagglomerated powder. Consequently, a single supple- tion of agglomerates in a high shear mixer.mentary experiment was performed at a binder concen- It was investigated, therefore, whether the compactiontration of 15.0% in order to obtain sufficient agglomera- simulator data could be used to predict the binder liquidtion. requirement. It was assumed that the forces in the mixer

The span of the agglomerate size distributions is shown correspond to a pressure range of 20–100 MPa in thein Table 5. It is seen that the narrowest size distributions compaction simulator. The amount of PEG necessary toare obtained with PEG 20 000 powder. saturate agglomerates, having corrected intragranular

The adhesion of mass to the bowl was found to vary porosities corresponding to the range of interparticularbetween 4 and 9% with Eskal, 4 and 15% with Durcal, and bulk porosities mentioned above, to a liquid saturation of0 and 4% with Sturcal. The amount of agglomerates larger 100% were calculated. A liquid saturation of 100% wasthan 4 mm was 7.3 and 2.1% with Eskal agglomerated chosen, because experiments have shown that liquidwith PEG 3000 flakes at 500 and 800 rpm, respectively. In saturation has to approach or even exceed 100% in order tothe rest of the experiments, the amount of agglomerates obtain a sufficient agglomerate growth of small, cohesivelarger than 4 mm was less than 0.5%. particles (Kristensen et al., 1985b; Schæfer, 1996a). By

From the specific surface areas of the calcium carbon- this calculation, the liquid requirement was predicted to beates (Table 1) it would have been expected that Durcal in the range of 18–27% for Durcal, 22–33% for Eskal, andrequired more binder liquid than Eskal, if the surface area 28–48% for Sturcal. Compared to the binder concen-alone affected the liquid requirement at an identical trations found experimentally (Table 3), the prediction ismedian particle size. However, the data in Table 3 show reasonable for Sturcal, whereas the predicted binder con-that the liquid requirement is slightly higher for Eskal than centrations are too high for Durcal and Eskal. The esti-for Durcal. This indicates that the surface area cannot be mated PEG concentrations found in the agglomerate sizeused to predict the binder liquid requirement. fractions 250–2000 mm (Table 6) are seen, however, to be

The interparticular bulk porosities of the powders (Fig. higher than the nominal PEG content (Table 3) and closer3) are seen to be more suitable for predicting the liquid to the concentrations predicted from the compactionrequirement (Table 3). Sturcal has the highest interparticu- simulator data. This can be explained by an observedlar bulk porosity as well as the highest liquid requirement adhesion of unagglomerated powder below the impeller(29–35% m/m). Durcal, which has a slightly lower during agglomeration, as well as by an inhomogeneousinterparticular bulk porosity than Eskal, also has a slightly distribution of binder in the product with agglomerates lesslower liquid requirement (14.5–16.5% m/m) compared to than 250 mm having a lower binder content. Previous meltEskal (17% m/m). agglomeration experiments with PEGs have shown that

Table 5Span of the agglomerates produced with the binder concentrations in Table 3



a500 1.2 (60.1)500 1.3 (60.1) 2.3 (60.3) 2.3 (60.3) 2.1 (60.0) Overwetted 0.9 (60.0) 0.7 (60.0)800 1.6 (60.1) 1.8 (60.5) 1.9 (60.0) 2.1 (60.0) 1.1 (60.0) 0.7 (60.0) 1.0 (60.1)

1100 2.7 (60.2) 3.4 (60.0) 1.2 (60.0) 1.9 (60.0) 1.1 (60.1) 0.6 (60.2) 0.9 (60.1)b1100 2.3

a Binder concentration: 34.0%.b Binder concentration: 15.0%.


PEG content in different agglomerate size fractions mightvary (Knight et al., 1998; Johansen and Schæfer, 2001).The binder concentrations in Table 6 have been applied forcalculation of corrected intragranular porosities and liquidsaturations. Thus the deviation between the predicted andthe nominal binder concentration can be explained partlyby an uneven binder distribution and partly by the fact thatthe assumption that agglomerate growth occurred at aliquid saturation of 100% was not correct, as the liquidsaturation was below 100% in some of the experiments(Fig. 5).

For Eskal (Fig. 5a) agglomerated with PEG 20 000flakes or powder, the liquid saturation approximates 100%with a higher impeller speed due to the increased densifica-tion. For Eskal agglomerated with PEG 3000 flakes, theliquid saturation is only |80% at all three impeller speedsbecause of the higher porosity (Fig. 4a). This liquidsaturation, however, is sufficient for agglomerate growth,probably due to a satisfactory deformability as a result ofthe rounded Eskal particles giving rise to weak agglomer-ates. It has further to be mentioned that the values of liquidsaturation are average values. This means that although theestimated value is below 100%, the saturation at thesurface of the agglomerate might be |100% if intragranu-lar voids filled with air are present. The potential foragglomerate growth is supposed primarily to be dependenton the liquid saturation of the agglomerate surface.

For Durcal (Fig. 5b), the liquid saturation is |100% withPEG 3000 but markedly lower with PEG 20 000. This isbecause the more viscous PEG 20 000 reduces the densifi-cation (Fig. 4b). For Sturcal (Fig. 5c), PEG 20 000 flakesgive rise to a liquid saturation of |100%, whereas aconsiderably lower liquid saturation and a correspondinglyhigher intragranular porosity (Fig. 4c) are obtained whenapplying PEG 20 000 powder. Nevertheless, PEG 20 000powder results in the largest agglomerates (Table 4). Thisindicates different mechanisms of agglomerate growth aswill be discussed in Section 3.4.

Normally, a lower intragranular porosity and a higherliquid saturation are to be expected at a higher impellerFig. 4. Corrected intragranular porosity (%) of size fractions (250–2000

mm) of agglomerates produced from (a) Eskal, (b) Durcal, and (c) Sturcal. speed due to the higher collisional forces promoting aBinder type: (♦) PEG 3000 flakes, (j) PEG 20 000 flakes, and (m) PEG densification of the agglomerates (Ennis et al., 1991). This20 000 powder. For binder concentrations cf. Table 6. (y) 17.5% PEG is seen for most of the agglomerates in Fig. 4 and explains3000 flakes.

why the binder concentration had to be lowered at a higher

Table 6Estimated PEG concentrations (% m/m of the amount of calcium carbonate) of the agglomerate size fractions 250–2000 mm



500 37.4 (61.3)500 20.4 (60.4) 19.1 (60.4) 19.0 (60.2) 20.8 (60.3) – 19.2 (60.3) 39.0 (60.2)800 20.5 (60.9) 17.8 (60.4) 18.3 (60.2) 17.2 (60.7) 34.6 (60.4) 19.1 (60.5) 35.0 (60.5)

1100 20.6 (60.1) 19.3 (60.8) 18.8 (60.2) 18.5 (60.6) 31.3 (60.4) 19.6 (60.1) 33.3 (60.2)1100 17.5


Fig. 6. Size distributions of agglomerates produced at different impellerspeeds. Powder: Eskal; binder: 17.0% PEG 3000 flakes. Impeller speed:(a) 500 rpm, (b) 800 rpm, and (c) 1100 rpm.

3.4. Agglomerate formation and growth mechanisms

From the agglomerate size distributions obtained withEskal and PEG 3000 flakes (Fig. 6), a slight increase in theamount of large agglomerates with an increasing impellerspeed can be seen. At the same time, the amount ofparticles below 180 mm increases markedly with anincrease in impeller speed. This is further illustrated by thelarger span values in Table 5. The bimodal size dis-tributions seen (Fig. 6b and c) are an indication of abreakage of agglomerates (Knight et al., 1998; Eliasen etal., 1999). It is surprising that breakage occurs whenagglomerating particles of such a small size since particleshaving a mean particle size below 10 mm are generallyFig. 5. Liquid saturation (%) of size fractions (250–2000 mm) ofexpected to result in agglomerates of a very high strengthagglomerates produced from (a) Eskal, (b) Durcal, and (c) Sturcal. Binder(Kristensen et al., 1985b; Schæfer, 1996a). The results intype: (♦) PEG 3000 flakes, (j) PEG 20 000 flakes, and (m) PEG 20 000

powder. For binder concentrations cf. Table 6. (y) 17.5% PEG 3000 Fig. 6 indicate, however, that a rounded particle shape andflakes. a narrow size distribution might give rise to a marked

lowering of the agglomerate strength. Photographs of theagglomerates sampled during the process showed large and

impeller speed for Durcal and Sturcal. However, for Eskal rounded agglomerates as well as smaller fragments of(Fig. 4a) agglomerated with PEG 3000 flakes, a higher agglomerates clearly indicating that coalescence and break-impeller speed causes a slight increase in the corrected age are the dominant mechanisms involved with Eskal andintragranular porosity. This is explained by increased PEG 3000.breakage of agglomerates at a higher impeller speed The agglomerate strength is increased by increasing(Eliasen et al., 1999). For Durcal (Fig. 4b) and Sturcal binder viscosity (Keningley et al., 1997; Johansen and(Fig. 4c), the effect of impeller speed on densification is Schæfer, 2001). Therefore, when the Eskal particles aresupposed to be reduced due to the lower binder con- agglomerated with PEG 20 000 flakes or powder (Fig. 7),centration applied at the higher speed since a lower amount the agglomerate size distributions become unimodal in-of binder liquid results in a decreased lubrication effect dicating that no breakage occurs. An increase in impeller(Kristensen et al., 1985a). speed gives rise to a larger agglomerate size and to a


Fig. 7. Size distributions of agglomerates produced at different impellerspeeds. Powder: Eskal; binder: 17.0% PEG 20 000 flakes (h) or PEG20 000 powder (j). Impeller speed: (a) 500 rpm, (b) 800 rpm, and (c)1100 rpm.

narrower size distribution (Fig. 7, Tables 4 and 5) due tothe increased liquid saturation (Fig. 5a). PEG 20 000powder is seen to result in larger agglomerate size andnarrower size distribution than PEG 20 000 flakes. It wasindicated by photographs of samples of the agglomeratestaken after 2 and 4 min that immersion of powder particles Fig. 8. SEM photograph of unfractionated Eskal agglomerates after ain the surface of the molten binder was the dominant massing time of (a) 4 min and (b) 10 min. Binder: 17.0% PEG 20 000

powder; impeller speed: 500 rpm.agglomerate formation mechanism for PEG 20 000 flakesas well as the powder. After the initial immersion, thebinder liquid becomes slowly squeezed to the agglomeratesurface. The molten PEG 20 000 powder will become ates produced with Eskal and PEG 20 000 powder becamecovered by a thinner layer of calcium carbonate particles fairly rounded with a narrow size distribution.compared with the flakes because of the larger surface area The agglomerate size distributions obtained from Durcalof PEG 20 000 powder. Therefore, the binder liquid is agglomerated with PEG 3000 flakes (Fig. 9) are notsupposed to become squeezed to the agglomerate surface directly comparable, as the binder concentration is differ-more rapidly when using PEG 20 000 powder, and the ent at the three impeller speeds applied. The differences inincreased surface wetness will increase the potential for agglomerate size and size distribution (Fig. 9, Tables 4 andsome agglomerate growth by coalescence causing a larger 5), therefore, cannot be ascribed to the impeller speedagglomerate size. alone. Contrary to the experiments with Eskal (Fig. 6),

Fig. 8a illustrates that after 4 min of massing the Durcal results in unimodal size distributions indicating thatagglomerate growth with Eskal and PEG 20 000 powder no breakage occurs. The wider size distribution and theoccurs primarily by immersion of particles or small more irregular shape of the Durcal particles give rise to aagglomerates in the surface of larger agglomerates but higher agglomerate strength, and this is why no breakage issome coalescence between smaller agglomerates is also seen.seen. These agglomerates were seen to be larger compared Although the binder concentration was the same withto the size of the agglomerates with PEG 20 000 flakes PEG 3000 and PEG 20 000, the agglomerate size dis-where growth by coalescence was not evident. After 10 tributions for Durcal agglomerated with PEG 20 000 flakesmin of massing (Fig. 8b), all the powder particles and the were found to contain a large amount of particles below 75small agglomerates have been immersed into the surface of mm at all impeller speeds applied. The agglomerate sizethe larger agglomerates. Consequently, the final agglomer- distribution at 1100 rpm is shown in Fig. 10a. The large


amount of unagglomerated particles is ascribed to thedifficult distribution of the highly viscous PEG 20 000 inthe cohesive Durcal powder. These results confirm thatagglomeration occurs primarily by immersion with Durcaland PEG 20 000 flakes, while coalescence is the dominantmechanism by agglomeration of Durcal with PEG 3000flakes (Johansen and Schæfer, 2001). The oblong and flakyappearance of the agglomerates made from Durcal andPEG 20 000 flakes (Fig. 11a) supports the assumption thatthe agglomerates are formed by immersion of powderparticles in molten fragments of flakes. The high binderviscosity results in deformability of the agglomerates thatis so low that the agglomerates cannot become spheronizedeven at the highest impeller speed. With Durcal and PEG3000 flakes, the final agglomerates were found to be morerounded.

As mentioned in Section 3.2, it was impossible toagglomerate Sturcal with PEG 3000 flakes in a controllableway because of an extremely high sensitivity to variationsin binder concentration. Further, the binder concentrationhad to be 3–6% higher, dependent on the impeller speed,with PEG 3000 compared with PEG 20 000 before over-wetting occurred. This is ascribed to the lower viscosity of

Fig. 9. Size distributions of agglomerates produced at different impellerPEG 3000 making it possible for the binder liquid tospeeds. Powder: Durcal; binder type: PEG 3000 flakes. Binder con-penetrate to a larger extent into the surface irregularities ofcentration and impeller speed: (a) 16.5% and 500 rpm, (b) 15.5% and 800

rpm, and (c) 15.0% and 1100 rpm. the Sturcal particles (Fig. 2c). Consequently, a high binderconcentration is necessary in order to saturate the surfaceof the Sturcal particles. When the particles have becomesaturated, agglomerate formation and growth by coalesc-ence will take place. Since the binder concentration isextremely high with PEG 3000, the surface of the agglom-erates will have a high deformability. This will make theagglomerate growth extremely sensitive to random varia-tions in the densification of the agglomerates duringmassing, and the agglomerate growth, therefore, will beuncontrollable. With PEG 20 000, the Sturcal particlesbecome immersed in the surface of the molten binderparticles. This makes it possible for agglomeration to occurat a lower binder concentration. Immersion results in alower surface wetness because of the high concentration ofpowder particles in the agglomerate surface, and this willreduce the risk of uncontrollable growth.

There is considerable difference in the agglomerate sizedistributions of Sturcal agglomerated with PEG 20 000powder (Fig. 10b) and flakes (Fig. 10c). The mass mediandiameters (Table 4) are significantly higher with PEG20 000 powder than with flakes, and the size distributionsare slightly narrower (Table 5). This is similar to theresults obtained with Eskal and is ascribed to someagglomerate growth by coalescence caused by PEG 20 000powder as was discussed above. The structure of theagglomerates formed with PEG 20 000 powder (Fig. 11b)indicates that some coalescence between agglomerates hasoccurred. The oblong and flaky appearance of the agglom-Fig. 10. Size distributions of agglomerates produced from (a) Durcal,erates produced with PEG 20 000 flakes (Fig. 11c) indi-14.5% PEG 20 000 flakes, (b) Sturcal, 29.0% PEG 20 000 powder, and

(c) Sturcal, 29.0% PEG 20 000 flakes. Impeller speed: 1100 rpm. cates that immersion has been the dominant mechanism of


viscous binders causing a low deformability might result ininternal holes in the resulting agglomerates.

4. Conclusions

The present work documents that the binder liquidrequirement for agglomeration of starting materials havingdifferent particle properties but approximately the samemedian particle size varies considerably. It is shown thatthe packing properties of the powders are of great impor-tance for liquid requirement. The results indicate that thecompaction simulator might be useful as a method topredict the densification of powders and thus the binderliquid requirement during agglomeration process in a highshear mixer. There is a need for additional research usingother powders in order to decide whether the compactionsimulator will be generally applicable for such a predic-tion.

Agglomerate formation by immersion of powder par-ticles in droplets of a highly viscous binder liquid andsubsequent agglomerate growth by immersion of particlesin the surface of the agglomerates have been shown to be afavourable method for agglomeration of powders with aparticle size being so small that agglomerate growth bycoalescence is difficult to control. In order to producepellets from small powder particles, it will be preferable toadd a highly viscous meltable binder as a powder insteadof flakes since a powder results in agglomerates of a morespherical shape and a narrower size distribution. It hasfurther been demonstrated that the high agglomeratestrength that is generally supposed to be associated with asmall powder particle size becomes markedly reduced by arounded shape and a narrow size distribution of the powderparticles. For such particles, a highly viscous binder liquidmight also be favourable in order to obtain an agglomeratestrength sufficiently high to prevent breakage of theagglomerates.

Acknowledgements

The authors wish to thank Omya, France, for supplyingthe Durcal 5 and Clariant GmbH, Germany, for supplyingthe PEGs. Jørn Møller-Sonnergaard is thanked for per-forming the compaction simulator experiments.

Fig. 11. SEM photographs of agglomerates produced from (a) Durcal,14.5% PEG 20 000 flakes, (b) Sturcal, 29.0% PEG 20 000 powder, and(c) Sturcal, 29.0% PEG 20 000 flakes. Size fraction: 250–2000 mm.

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