Powder Technology 211 (2011) 165–175

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Influence of the interaction between binder and powders on melt agglomerationbehavior in a high-shear mixer

H.J. Cheng, S.S. Hsiau ⁎, C.C. LiaoDepartment of Mechanical Engineering, National Central University, Jhongli 32001, Taiwan, ROC

⁎ Corresponding author. Tel.: +886 3 426 7341; fax:E-mail address: sshsiau@cc.ncu.edu.tw (S.S. Hsiau).

0032-5910/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.powtec.2011.04.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 May 2010Received in revised form 21 March 2011Accepted 16 April 2011Available online 22 April 2011

Keywords:AgglomerationHigh-shear mixerMelting binderInduction growth behaviorHeterogeneous dispersionHomogeneous dispersion

The purpose of this study was to investigate the effects of the different surface properties of powders ongranular agglomeration in a high-shear mixer. Polyethylene glycol 6000 (PEG 6000) was used as the meltingbinder. Three different powders, with mean granule sizes of 75–150 μm were used as the raw materials:calcium carbonate, calcium sulfate, and sodium carbonate. The wetting properties of the raw materials weremeasured with a contact angle instrument. The results indicate that the speed at which the droplets sink intothe powder bed and the contact angle of binder droplets on the powder surface play important roles indetermining the progress of the agglomeration process. Several types of agglomeration were found: a slurrystate, heterogeneous nucleation, snowballing, and induction growth behavior. Heterogeneous dispersionleads to induction behavior and subsequent growth, but a homogeneous dispersion leads to little or nonucleation and growth of agglomerate size.

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1. Introduction

Wet granulation is important for the process of powder granuleenlargement during manufacturing in the modern industrial technol-ogy. This type of production process is used during the large-scalefabrication of granules for the production of capsules and tablets in thepharmaceutical industry. Agglomeration is the process whereby smallparticles are gathered into larger clumps, although the initial powderscan still be distinguished [1]. During the process of wet agglomeration,particles are combined due to the liquid forces between particles (e.g.,static capillary forces, surface tension, and dynamic viscosity forces).The interaction between liquids and solids is very important.

The repellence or attraction between a liquid and a solid isdetermined by the forces of cohesion and adhesion. The cohesive forcecomes from the attraction of the liquid molecules. The adhesive forceis, however, the reciprocal attractive force at the interface between aliquid and a solid [2]. Leelamanie and Karube [3] considered thewettability of liquid and powders of fine silica sand. They estimatedthe contact angles and water drop penetration time (WDPT) from themolarity obtained with an ethanol droplet (MED) test, the capillaryrise method (CRM) and the sessile drop method (SDM). All of thesemethods have different ranges of measurement. They observed thattheWDPT increasedwith an increase of the contact angle. There was asharp increase when the contact angle increased from 88 to 93°. With

the same samples in the previous study they used SDM to determinethe effects of the ambient relative humidity on the contact angle andWDPT [4].

In addition to the influence of liquid wettability, the physicalproperties of the binder liquid and powders also play an importantrole in wet granulation. Johansen and Schæfer [5] looked at theprocess of agglomeration, using three grades of calcium carbonatewith different particle sizes, surface areas, and particle shapes withPEG 3000 and PEG 20000 driven at three impeller speeds. The resultsshowed that particle breakage occurred during agglomeration whenthe binder viscosity was low and when the particles had a roundedshape and narrow size distribution; while a low binder viscosity andirregularly shaped particles resulted in uncontrollable agglomerategrowth. They also studied the relation between particle size andbinder viscosity (as shown in SEM photographs) needed to producespherical pellets. With spherical pellets, the lower the viscosity of thebinder, the finer the size of the particles one would choose. If largeparticles are initially used, the binder viscosity must be increased [6].In contrast, Schæfer et al. [7] observed that granule strength increasedwhen finer initial raw-particle sizes or a higher viscosity of binderwere used.

In addition to the effects of liquid and powder properties, thedifferent mechanisms related to granulation behavior have alsotraditionally been described. Ennis and Litster [1] distinguished thefollowing threemechanisms during the agglomeration process:wettingand nucleation, consolidation and growth, and breakage and attrition.Nucleation is themechanismbywhich small particles bind toothers dueto collision and the force of adhesion. Observations of possible

Table 1Physical properties of raw materials.

Material Particle density (g/cm3) Particle size (μm) Melting point (°C)

Calcium carbonate 2.93 75–150 800Calcium sulfate 2.96 75–150 1450Sodium carbonate 2.54 75–150 850

166 H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

nucleation behaviors in a high shear mixer show three differentdistinguishable nucleation mechanisms: (1) penetration-involvingnucleation and granule breakage; (2) penetration- involving nucleationand absence of granule breakage; (3) dispersion-involving onlynucleation [8]. A large number of particle-nucleation behaviors areobserved to occur in the initial phase of agglomeration, but followingnucleation, the deformability of the granules is affected by othermechanisms. Iveson and Litster [9] defined two growth behaviors basedon the deformability of the system: steady growth and induction timebehaviors. They also designed a growth regime map which considered

(a) High shear mixer granulator

(b) Four-blade impeller (

Water Filler

Choppe

Heater

Fig. 1. Schematic representation of the: (a) high shear mixer granu

maximum pore saturation and deformation number, to explore theinfluence of binder properties and agitation intensity on granulebehavior.

Schæfer and Mathiesen [10] considered there to be two basicmechanisms in the nucleation stage: distribution and immersion. Mort[11] observed that the distribution mechanism occurs when the binderdroplets are smaller than the particles. However, the immersionmechanism can also be found to occur when the binder droplets arelarger than the particles. Scott et al. [12] investigated two differentmethods of binder addition: pour-on and melt-in. The granules formed

c) Chopper

Point

r

Bowl

Impeller

Watermark

Temperature Probe

Granule Discharge Point

Water Discharge Point

lator; (b) four- blade impeller; (c) chopper (units are in mm).

167H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

in pour-on system are bigger and faster comparing with the granulesformed in melt-in system. The materials properties have significantlyheterogeneity in both ways. Knight et al. [13] studied the sizedistribution of granulation with different material sizes (4–23 μm)and liquid content in a high shear mixer. In all cases, a bimodaldistribution of agglomeration size occurred during the mixing periodwhichwas related to thenon-uniformdistributionof liquiddroplets andsolid particles. Braumann et al. [14] developed a stochasticmodel that isapplicable to heterogeneous nucleation over a range of binder dropletsizes. At early stages of agglomeration, themodel predictedhighermeangranule size for small droplets compared to large droplets; but this wasreversed at the final stage of agglomeration. On the other hand, thegranulation time was increased with increase of binder viscosity andbinder particle size, and the granules were easily broken with lowerbinder viscosity and smaller binder particle size [15].

(a) Before

Before

Before

A

(b)

(c)

Binder sank into the dry powders

Fig. 2. The photographs of the state of the binder-powder mixture before and after the P

Let us consider the dispersion mechanism. The viscous Stokesnumber (Stv) and critical Stokes number (St⁎) [11,14,16] areimportant parameters for granule growth. Stv is the ratio of initialkinetic energy to energy dissipated due to liquid adhesion. St⁎ isrelated to the particle coefficient of restitution (e), binder thickness onthe particle surface h, and surface asperities of particles. WhenStvNSt⁎, particles will rebound after collision, but when Stv is less thanSt⁎, successful collision occurs. After the mass of granules becomesrigid, another growth mechanism, snowballing, enters the pic-ture [17]. Snowballing is the mechanism by which larger granulesbind up small particles due to rolling and the adhesive force, with theresult that a dense layer of small particles is deposited on the surfaceof the larger granules.

In these studies of agglomeration the focus has mostly been on thedifferent mechanisms of granule growth or the influence of the binder

fter

After

After

EG 6000 melted: (a) calcium carbonate; (b) calcium sulfate; (c) sodium carbonate.

(a) Calcium carbonate

(b) Calcium sulfate

(c) Sodium carbonate

Start 2 min 5 min 8 min

11 min 14 min 17 min 20 min

Start 0.5min 1 min 1.5 min

2 min 2.5 min

Start 1 sec 2 sec 3 sec

4 sec 5 sec

Fig. 3. Images of liquid droplets on the powder bed over time: (a) calcium carbonate;(b) calcium sulfate; (c) sodium carbonate.

Time (seconds)

cont

act a

ngle

(o )

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calcium carbonatecalcium sulfatesodium carbonate

Fig. 4. Contact angles for three different types of powder beds with liquid dropletsversus time.

168 H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

properties. The aim of present study is to investigate the effect of therelationship between the powder's properties and the interactionbetween the binder and the powder during melt agglomeration andgranular formation.

2. Materials and methods

2.1. Materials

In this study, three different powders were used as raw materials:calcium carbonate (Echo Chemical, Taiwan); calcium sulfate (EchoChemical, Taiwan); and sodium carbonate (Penrice Soda HoldingsLimited, Australia). Table 1 shows the properties of the three powders.The particle sizes of the materials ranged between 75 and 150 μm assifted by a shaking sieve. Polyethylene glycol 6000 (Showa, Japan)was used in flake form as the melting binder. The use of the binder inflake form could give a narrower size distribution and larger granulesize after agglomeration than using a fine form of binder [10]. The PEGassay at room temperature in the initial stage by shaking sieveanalysis with a series of 14 ASTM (American Society for Testing andMaterials) standard sieves in the range of 212–1400 μm. The hydroxylvalue is 13.5, the PH (5 w/v%) is 7.3, themelting point is 58 °C, and theliquid viscosity is 665 mPa·s at 80 °C.

2.2. Analysis of wetting ability

Surface wetting between the powders and the binder can becharacterized using a contact angle measurement system (Sindatek,Taiwan). The powder was poured into small aluminum vessels whichwere placed for 1 day in a drying cabinet. Before the experiment, thevessels were placed into an airtight pedestal with the temperaturecontrolled at 80 °C by a heater. The cartridge was made of aluminum.The PEG 6000 melted easily from a solid to a liquid state inside thecartridge. Some of the liquid was extracted using a syringe (0.5 mm)then dropped onto the surface of the powder bed. At the same time,images of the liquid droplet sinking into the powder bed wererecorded by a CCD camera.

2.3. Equipment

The high shear mixer granulator (Yi-Chen Industry, Taiwan, ROC)used in the experiments (shown in Fig. 1(a)), consists of a bowl, adischarge point, a heater, a temperature probe, an impeller and achopper. The volume of the interior-bowl was 10 L. The insidetemperaturewas controlled by a heater andmeasured by a temperatureprobe. Fig. 1(b) shows a schematic representation of the four-bladedimpeller. The longer blades are 25 cm indiameter and the shorter bladesare 20 cm. Fig. 1(c) shows the chopper dish, which is 10 cm in diameterwith 2 cm wide blades.

2.4. Agglomeration procedure

We next discuss the melt agglomeration process that occurred inthe high shear mixer granulator. First, 1.7 L of powdered material wasplaced into the bottom of the bowl before being covered with 0.7 L ofpolyethylene glycol. The PEG flake was poured slowly on top of thepowder bed through a funnel to ensure that the binder distributionwas uniform (left-hand column in Fig. 2(a–c)). The heating apparatuswas turned on to maintain the temperature at 80 °C for 1.5 h to makesure that the PEG 6000 melted completely. After the binder hadmelted inside the granulator, the impeller and chopper were turnedon and the mixing process started. The impeller speed was controlledat 500 rpm and the simultaneous chopper speed was controlled at1000 rpm. A plastic scoop was used for granular-sampling during theexperiments. The sampling locations were as follows: (1) in theintermediate zone of the granular bed in the vertical direction; (2) in

Number of impeller revolutions

calcium carbonate (L/S=0.3)calcium sulfate (L/S=0.4)

II

(C.S.)

II

(C.C.)

Mea

n si

ze (

µm)

00

1000

1000

2000

2000

3000

3000

4000

4000

5000

5000

Fig. 6. Regime II of calcium carbonate (Vliq/Vsol=0.3) and calcium sulfate (Vliq/Vsol=0.4).

169H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

the intermediate zone between the center and the tip of the impellerin the horizontal direction.

2.5. Analysis of granular size

The granular size distribution was analyzed using a shaking sieve(W.S. Tyler, U.S.A.) following the ASTM standard. Size fractions of b90,90–125, 125–212, 212–355, 355–425, 425–500, 500–600, 600–710,710–850, 850–1000, 1000–2000, 2000–3350, 3350–4750, 4750–5600, 5600–6700, 6700–8000, 8000–9500 and N9500 μm werecollected. After the granules were sieved, an electronic scale (Precisa,Switzerland) was used to determine the weight of the granules.

2.6. Photographs

Photographs of the surface structure of the particles were observedby a Low Vacuum Scanning Electron Microscope (LV-SEM) (HITACHI,Japan).

2.7. The initial stage

The photographs of the state of the binder-powder mixture beforeand after the PEG melted are shown in Fig. 2(a–c). On the right hand

Number of impeller revolutions

Mea

n si

ze (

µm)

0 2000 4000 6000 8000 10000 12000 14000 16000

Number of impeller revolutions0 2000 4000 6000 8000 10000 12000 14000 16000

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5000calcium carbonatecalcium sulfatesodium carbonate

calcium carbonatecalcium sulfatesodium carbonate

(a)

(b)

Fig. 5. Mean particle size distributions for the three different types of particles versusthe number of impeller revolutions: (a) ratio of liquid to solid (Vliq/Vsol) was set to 0.3;(b) ratio of liquid to solid (Vliq/Vsol) was set to 0.4.

side of Fig. 2(a) we observe that the PEG 6000 liquid covers the top ofthe calcium carbonate bed. On the right hand side of Fig. 2(b) it can beclearly seen that small parts of the binder have sunk into the powderbed. On the right hand side of Fig. 2(c) we observe that the binder hasbecome uniformly distributed into the sodium carbonate bed. It lookslike the mixing of sand and water.

3. Results and discussion

3.1. Wetting ability

Fig. 3 shows images of liquid drops on the three different powderbeds with penetration time. In Fig. 3(a), we observe that the PEG 6000liquid has caused the level of the calcium carbonate powder bed todrop, and that the liquid drop has been completely sucked into the drypowder bed by the capillary forces [18]. The penetration time was20 min. The penetration time in the calcium sulfate powder bed was2.5 min as shown in Fig. 3(b). In Fig. 3(c), the liquid drop disappearedvery quickly from the surface of the sodium carbonate powder bed(after about 5 s). This was due to the difference in wetting propertiesbetween the powder and binder. Calcium carbonate powders had thebest slow-wetting properties (with the PEG 6000 liquid) of the threematerials. Fig. 4 shows the change in the contact angle of the liquid

Mas

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tage

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100<125 µm125-355 µm355-600 µm600-1000 µm1-2 mm>2 mm

III III

Number of revolutions0 1000 2000 3000 4000 5000

Fig. 7. Mass percentage versus agglomeration time for the calcium carbonate with lowliquid content (Vliq/Vsol=0.3) with respect to granule size.

170 H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

drop over time for the three different types of powders. It can be seenthat the change in the contact angle was the slowest for the liquiddrop sinking into the calcium carbonate powder bed, although therewas no obvious difference between the final contact angles for thethree types of powders.

3.2. Mean size distribution

The mean particle size distributions for three different types ofparticles as a function of the number of impeller revolutions with liquidto solid ratio (Vliq/Vsol) fixed at 0.3 are plotted in Fig. 5(a). It was foundthat the mean size of the calcium carbonate granules decreased slightlywith time from 690 μm to 540 μmbefore 600 impeller revolutions. Thisis the so-called induction behavior [19,20], in which the surfaces of theparticles are dry and the process of granulation requires the breakage ofa coarse granule (1–2 mm) into fine granules (b125 μm) by materialcollisions. At the same time, dry powders are engulfed into the binderliquid of the inter-granule and some liquids are distributed over the

Granule size (micrometer)

Mas

s pe

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tage

(%

)

101 102 103 1040

10

20

0.5 min

10

20

1 min

10

20

2 min

10

20

30

40

3 min

Fig. 8. Size distribution relates to calcium carbonate with low liquid content (Vliq/Vsol=0.3)respect to induction agglomeration time.

granular surfaces. After 600 impeller revolutions, the granules grewrapidly due to the particle coalescence. The granules in this stage wereweak and easily deformed, so the area of contact was increased by thecollisions.

There was little or no growth in the calcium sulfate and sodiumcarbonate mass for 15,000 impeller revolutions in this situation. Withsodium carbonate granules, the fast-wetting layers of powders areeasily infiltrated by the liquid binder. When operated in a high-shearmixer, the binder liquid is dispersed by the impeller revolutions fromthe bottom of the particles, distributing all of the particles uniformlyinside the system. With calcium sulfate granules, the moderate-wetting layer of powder was almost infiltrated by the liquid binder.Uniform particle and binder distribution also occurred in thiscondition as described by the Stokes criteria [11,14,16].

Fig. 5(b) shows the mean size distributions with the number ofimpeller revolutions when the liquid to solid ratio (Vliq/Vsol) was setto 0.4. It can be seen that the mean sizes of the calcium carbonategranules decreased with time from 1180 μm to 640 μm before270 impeller revolutions. The greater the granulation in the liquidcontent, the shorter the induction behavior was. During thecoalescence time, the mean size of the granules grew more rapidlywith granulation time than was the case for the same particles withlow Vliq/Vsol, due to the greater deformability.

Number of revolutions0 5000 10000 15000

<125 µm125-355 µm355-600 µm600-1000 µm1-2 mm>2 mm

III III

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Number of revolutions0 1000 2000 3000 4000 5000

<125 µm125-355 µm355-600 µm600-1000 µm1-2 mm>2 mm

(a)

(b)

Fig. 9. Mass percentage versus agglomeration time for the calcium sulfate with respectto granule size, ratio of liquid to solid (Vliq/Vsol) set to (a) 0.3; (b) 0.4.

Granule size (micrometer)

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101 102 103 104

Granule size (micrometer)101 102 103 104

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Fig. 10. Size distribution relates to calcium sulfate with respect to agglomeration time, ratio of liquid to solid (Vliq/Vsol) set to (a) 0.3; (b) 0.4.

Number of revolutions

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0 5000 10000 150000

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Fig. 11. Mass percentage versus agglomeration time for the sodium carbonate withrespect to granule size, ratio of liquid to solid Vliq/Vsol=0.3.

171H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

Now let us consider calcium carbonate mixing. The phenomena ofslow-wetting (suggested in Fig. 3(a)) between PEG and the particlesare highly dependent on the agglomeration. This behavior was calledas heterogeneous dispersion. An immersion mechanism which occursdue to local droplets leads to nucleation and subsequent growth bycoalescence or layering. Cheng and Hsiau [21] have investigated thishypothesis by the analysis of the percentage of PEG for each size cut.They found that L/S ratios of the granular materials increased with theincrease of the granule size at the initial stage, and decreased to thesteady values with the agglomeration time. On the other hand, therewas no obvious growth in the sodium carbonate granules, even after15,000 impeller revolutions. The interaction between the particlesand the liquid binder resulted in little or no nucleation. Molten PEG isuniformly absorbed (in pores) or distributed (as a thin surface layer)on sodium carbonate granules (as suggested in Fig. 3(c)), andsubsequently fails to achieve the requisite conditions for nucleationor growth.

There was little or no growth in the calcium sulfate mass before2500 impeller revolutions, but it grew rapidly after that. This can beexplained as follows: the calcium sulfate granules nucleated in theinitial stage when there was a uniform distribution of binder liquid

Granule size (micrometer)

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4 min

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Fig. 12. Size distribution relates to sodium carbonatewith low liquid content (Vliq/Vsol=0.3)respect to agglomeration time.

172 H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

due to the liquid binder almost infiltrating the moderate-wettinglayer of powder. However, after 2500 impeller revolutions, there waslikely some different mechanism that operated between the lowliquid (Vliq/Vsol=0.3) and the high liquid (Vliq/Vsol=0.4) contents inthis system.

Considering the mean size-distribution, the ratio of liquid to solidleads to the occurrence of nucleation and may indicate the existenceof a critical volume ratio (see Fig. 5(a–b)). However themore effectivenucleation-ability has to do with the heterogeneous dispersion ofbinder in powder rather than the influence of the liquid content.

3.3. Size distribution

We defined three regimes to discuss the granule growth:(I) nucleation, (II) steady-growth, and (III) rapid growth. In Fig. 6, thesteady-growth of calcium carbonate system (regime II) is from 600 to1200 revolutions and the steady-growth of calcium sulfate system(regime II) is from 2000 to 3500 revolutions. Fig. 7 shows the results ofthe analysis of size-distribution for the low liquid content (Vliq/Vsol=0.3)using calcium carbonate. We observed that regime I, the nucleationregime, usually occurred in the initial phase of agglomeration. Theintermediate sized granules (355–1000 μm)performed an important roleduring the initial stage of nucleation, because of the immersionmechanism [21]. A molten pool of PEG formed on top of the calciumcarbonate powder (as suggested in Fig. 2(a)). This was dispersed into thepowder by the mixer's shearing. The nucleation size could also beinfluenced by the mechanical breakup of molten PEG into local dropletsthat serves as an immersionmechanism, so the liquid dropletswere easilysurrounded by the small particles [18–20]. During the initial formation atroom temperature, several fine particles might adhere together in theliquid, causing some granules to get bigger than others. Granules rangingfrom 355 to 600 μm made up 33% of the total granule mass after 500revolutions (Fig. 7).At theendof regime I thepercentageof small granules(125–355 μm) increased gradually. Theprobable reasonwas that the highspeed impeller tended to create a well-mixed system. The intermediate-sized granules had aweaker structure sowere split by collisionswith eachother. At the beginning of regime II, which is the steady-growth stage, weobserved that there was a slight increase in the mean size of theintermediate granules (after 1000 impeller revolutions in Fig. 5(a)). Inregime III,which is the rapid-growthstage,most of the intermediate-sizedgranules were transformed into coarse granules but both the fine andsmall granules disappeared and the number of large granules increasedsteadily.

Consider the changes in size-distribution during granule growth ofcalcium carbonate with low liquid content given a mixing period ofabout 3 min (Fig. 8). In regime I (0–1 min), the distribution becamebimodal after mixing for 0.5 min, with the peak sizes of 250 and1500 μm. Immersion behavior is indicated by the right hand peak as aresult of the powder holding a larger amount of liquid. The dispersionwas heterogeneous and several initial particles (75–150 μm) could nottouch the binder during this 0.5 min period. After mixing up to 2 min(regime II), the percentage of granules above 1000 μm decreased as aresult of the consolidation and breakage of coarse granules. For the lefthand peak, the percentage of granules increased a little between 0.5 and2 min due to the breakage of large granule. After regime II, therewas anobvious increase in coarse granules at 3 min. It is thought that there ismore binder at the surface due to a compression of the granules.

Fig. 9(a) shows the results of the analysis of the size-distributionfor the low liquid content (Vliq/Vsol=0.3) calcium sulfate. The initialmass percentage of fine granules was 40–43%, but this increased withincreasing number of impeller revolutions. The probable reason is thatthe liquid PEG 6000 was more uniform in this granules system. Thelocal concentration of droplets could not easily exhibit and be heldwith the low liquid content system. It was difficult for the particles tocoalesce due to the low-deformation materials, so the mean granulesize decreased with time (Fig. 5(a)).

Fig. 9(b) shows the results for the high liquid content calciumsulfate system (Vliq/Vsol=0.4). At the beginning of regime I, both thefine and intermediate sized granules performed an important role, butthe mass percentages of fine granules with the high liquid contentwere 10% less than that with the low liquid content. This was becausethere was more liquid binder to be distributed into particles in thehigh liquid content system (Vliq/Vsol=0.4), and the particles alsonucleated in this stage. In regime II, the steady-growth stage, weobserved a decline of intermediate granules with time, and a gradualincrease of coarse granules. In regime III, these fine granules, smallgranules, and intermediate granules almost disappeared. The numberof coarse granules (1–2 mm) decreased slowly with time, but thenumber of very large granules (N2 mm) increased rapidly with time.

Considering the size-distribution behavior with respect to ag-glomeration (Fig. 10(a–b)) for a calcium sulfate with a low liquid tosolid ratio, the agglomeration also becomes bimodal after mixing for0.5 min (Fig. 10(a)). The percentage of right hand peaks (coarsegranules) decreases with increasing agglomeration time at about1500 μm because of the breakage. The other percentage of particlesremains at about the initial size (75–150 μm).

173H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

The size-distribution related to the calcium sulfate with a highliquid to solid ratio is included in Fig. 10(b). Aftermixing for 1 min, thesize-distribution also becomes bimodal. There is no clear indication ofbreakage or any other consolidation behavior (the size of the righthand peaks does not decrease) of coarse granules. However, prior to4 min, the small-sized granules (left hand peaks) increase with theperiod of mixing but decrease in percentage after. The reason is thatthe dispersion between calcium sulfate and the PEG liquid makesagglomeration difficult during the mixing period, but the higher PEGliquid content can generate enough cohesive and adhesive forces foragglomeration to occur.

Fig. 11 shows the analysis of the size-distribution for the low liquidcontent sodium carbonate system. The results show that there was ahigh percentage of small granules (N60%) at all times. This wasbecause the sodium carbonate powder could absorb the liquid well sothe PEG binder was uniformly dispersed. The percentage of finegranules was 20%. The other granules changed to intermediate-sizedgranules, because of the homogeneous distribution of the PEG liquidin the powder. Considering the analysis in Fig. 12, there is noindication of an increase in the percentage of coarse granules, nor anyindication of the enlargement of small-sized granules.

3.4. SEM images

Fig. 13(a) shows an SEM image of intermediate-sized granules ofcalcium carbonate in the low liquid content system (Vliq/Vsol=0.3) at250 impeller revolutions. The granule surfaces were covered withparticles. However, when the impeller revolution time was 1250, thegranule surfaces became covered with binder, as seen in Fig. 13(b).After that time (regime II in Fig. 7), intermediate-sized particlesdecreased with an increasing number of revolutions. Most particlesgrew rapidly by coalescence giving rise to numerous coarse granules.Fig. 13(c–d) show the images of intermediate-sized granules ofcalcium sulfate in the high liquid content system (Vliq/Vsol=0.4) at

(a)

(c)

Fig. 13. SEM images of granules with 355–1000 μm size fraction and regimes in ×500 imagesVliq/Vsol=0.3, 1250 impeller revolutions; (c) calcium sulfate, Vliq/Vsol=0.4, 500 impellers r

500 and at 2000 impeller revolutions, respectively. The surface wascoveredwith particles (as observed in the image taken at 500 impellerrevolutions). However, after 2000 impeller revolutions, the calciumsulfate granules grew rapidly in size (Fig. 5(b)). The granule surfaceswere no smoother than those of the calcium carbonate granulesproduced. There were a few fine particles distributed on the surfaces.

Fig. 14(a) shows intermediate-sized granules of calcium carbonatein a low liquid content system (Vliq/Vsol=0.3) at 1250 impellerrevolutions. The granules were shaped by the coalescence mecha-nism. As seen in Fig. 15(a), the deformability and forces of cohesionwere strong enough to resist the separating forces of collision, soseveral small-sized granules (125–355 μm) were able to combine tointermediate-sized granules. Fig. 14(b) shows the agglomeration ofintermediate-sized granules of calcium carbonate in a high liquidcontent system (Vliq/Vsol=0.4) at 500 impeller revolutions. In thecalcium carbonate system, it was easier for granules to form bycoalescence with each other when the liquid content was low thanwhen the liquid content was high. This was because a large amount ofcohesive force and deformability between the particles was providedby the high liquid content. The SEM image in Fig. 14(c) showsintermediate-sized granules of calcium sulfate in a high liquid contentsystem (Vliq/Vsol=0.4) at 2000 impeller revolutions. The calciumsulfate granules gradually changed in form and shape due to thesnowballing mechanism [17]. In the beginning, fine particles adheredto each other due to the binder liquid distributed on the powdersurface, but it was difficult for the liquid drop to be surrounded bycalcium sulfate powder due to the greater slow-wetting of the PEGbinder than was the case for the granules of calcium carbonate(Fig. 3). Secondly, nucleation occurred more often when fine particleswere surrounded by the binder liquid to form intermediate-sizedgranules. Finally, Fig. 15(b) shows the intermediate-sized granules towhich fine particles adhered due to the binding force of the liquidbinder on the surface. The cohesive force caused fine particles toadhere to the intermediate-sized granules. The reason is that the

(b)

(d)

: (a) calcium carbonate, Vliq/Vsol=0.3, 250 impeller revolutions; (b) calcium carbonate,evolutions; (d) calcium sulfate, Vliq/Vsol=0.4, 2000 impeller revolutions.

(a)

(b)

(c)

Fig. 14. SEM images of granules with 355–1000 μm size fraction and regimes in ×100images: (a) calcium carbonate, Vliq/Vsol=0.3, 1250 impeller revolutions; (b) calciumcarbonate, Vliq/Vsol=0.4, 500 impeller revolutions; (c) calcium sulfate, Vliq/Vsol=0.4,2000 impeller revolutions.

(a) coalescence

(b) snowballing

Fig. 15. Agglomeration mechanisms: (a) coalescence; (b) snowballing.

174 H.J. Cheng et al. / Powder Technology 211 (2011) 165–175

calcium sulfate granules did not interact much with the PEG liquid,making it hard for small-sized or intermediate-sized granules ofcalcium sulfate to coalesce.

4. Conclusions

The influence of the different properties of particles during themelting of PEG 6000 in a high shear mixer granulator has beenpresented. The results show that the particle properties play a crucialrole in the melt agglomeration behavior. PEG wetting on calciumcarbonate was very slow. During granulation, the mass of calciumcarbonate showed induction behavior, likely due to heterogeneousdistribution of the binder, followed by steady and rapid growthbehavior. This system was weak and deformed easily. The moderate-wetting calcium sulfate mass could not grow when the liquid contentwas low (Vliq/Vsol=0.3), but showed induction and snowballingbehavior with the higher liquid content (Vliq/Vsol=0.4). The fast-wetting particles of sodium carbonate entered a slurry-like state due

to the rapid infiltration of the molten PEG binder; this system had noagglomerate growth at either binder levels. The SEM images showthat the surface of calcium carbonate granuleswas smoother than thatof calcium sulfate granules after several impeller revolutions.

Acknowledgments

This study was financially supported by the National ScienceCouncil of the ROC through grant NSC-97-2628-E-008-036-MY3.

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