the effect of powder size on induction behaviour and binder distribution during high shear melt...

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(2008) 298–312www.elsevier.com/locate/powtec

Powder Technology 184

The effect of powder size on induction behaviour and binder distributionduring high shear melt agglomeration of calcium carbonate

Wei-Da Tu a, Shu-San Hsiau a,⁎, Andy Ingram b, Jonathan Seville b

a Department of Mechanical Engineering, National Central University, 32001 Taiwan, ROCb Centre for Formulation Engineering, Department of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK

Received 28 November 2006; received in revised form 17 August 2007; accepted 4 September 2007Available online 8 September 2007

Abstract

Growth mechanisms in high shear mixer granulation were observed over a wide range of particle size and liquid-to-solid (L/S) ratio. The materialsused were calcium carbonate (CaCO3; size fractions in the range 1.5 to 85 μm) with a binder of polyethylene glycol 6000 (PEG 6k). The binder, solidat room temperature, was added by the “melt-in” method. A 10 L vertical-axis granulator was used, with a chopper and a four-bladed impeller.

The mean granule size and granule size distribution were measured at regular intervals during the agglomeration process by careful samplingand sieving. The uniformity of binder distribution among the granules was also measured.

The growth behaviours of each grade of primary particles were classified and compared. An induction type mechanism was observed with aninitial period of slow growth in mean particle size that lasted 2 to 3 min (the induction period). This was followed by a short rapid growth phaselasting 1 to 2 min. The final stage was a plateau of more or less zero growth. Interestingly, the end of the induction period and the onset of rapidgrowth corresponded to a change in the granule size distribution from bimodal to monomodal and a similar change in the distribution of binder.Induction period growth rate tended to be lower for granules of finer particles, but these grew more rapidly during the rapid growth stage andproduced larger granules than the coarser primary particles.

The liquid-to-solid (L/S) ratio had a significant effect on the growth rate during the rapid growth stage but a minor effect on the granule sizedistribution and binder distribution. Primary particle size had a significant effect on the final average size of granules, the growth rate during therapid growth stage and the distribution of granule size and binder.© 2007 Elsevier B.V. All rights reserved.

Keywords: Induction behaviour; Binder distribution; High shear melt agglomeration

1. Introduction

Wet agglomeration (or “granulation”) of powders is afundamental technique in a variety of industries, such as foodand pharmaceutical manufacture, mining and manufacture ofagricultural products. The aim of agglomeration is to producelarger, more easily handled particles (granules) from finepowders by the addition of binder materials during some formofmechanical action such as high shear mixing [1]. The resultinggranules are agglomerates of the finer particles held together bythe binder. The size and size distribution of the granules is of

⁎ Corresponding author. Tel.: +886 3 426 7341; fax: +886 3 425 4501.E-mail address: [email protected] (S.-S. Hsiau).

0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2007.09.001

great practical importance but is not easily determined a priori.It is dependent on (at least) the size of the primary particles, themagnitude of the mechanical forces and the quantity of binder.

During practical operation, different amounts of binderaddition will be required according to product properties,operating conditions and the type of manufacturing device. Theliquid-to-solid (L/S) ratio is typically between 10% and 20%(weight liquid/weight of solid). The important consideration forrobust granules is that there is sufficient binder present in thevoids and forming bridges between the primary particles. It isnoted that the L/S ratio required for successful granulation willvary with different operating conditions or material properties,such as the size and porosity of particles or binder viscosity.

In recent years, research into agglomeration growth behav-iour [2–5] has been focused on the growth regime map in which

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299W.-D. Tu et al. / Powder Technology 184 (2008) 298–312

the mechanism of growth is related to the material properties, theL/S ratio and the growth rate. Induction type behaviour dividesthe growth process into two parts: the induction stage followedby the rapid growth stage. In the former, the average granule sizeincreases slowly but steadily with time. In this period thegranules are deformed easily and binder is progressivelysqueezed to the surface of the granules. The induction periodis followed by the rapid growth stage. After thorough mixing ofthe binder and powder materials during the induction stage, themean size of the granules suddenly increases over a shortinterval, known as the rapid growth stage. Following this, theaverage size becomes constant or increases slowly. It is oftendesirable in industry to produce granules with a monomodal sizedistribution for controllable dissolution time, avoidance ofsegregation and good flow properties. However, in practice it hasproved difficult to achieve this, since the growth behaviour isdifficult to predict [3,6–10], and lack of such understanding

Fig. 1. High shear mixer granulator. (a) Schematic of high shear m

leads to low yields in the desirable fraction and hence highrecycling rates.

The equipment most frequently employed in granulation isthe high shear mixer granulator. In principle, the rapidly rotatingblades (typical tip speed of 5–10 m/s [10]), distribute the binderuniformly within the powder, more rapidly than the alternativesof the rotating drum or fluidized bed. This results in moreuniform and stronger granules.

Melt agglomeration is a type of wet agglomeration in whicha solid binder is introduced into the powder and then meltsduring the process and becomes solid upon cooling. Thismethod has several advantages. For example, water-sensitivepowders can be processed. Also, since such binders tend to berelatively insoluble the tablets formed from the granules willdisintegrate more slowly in vitro, providing controlled releaseof actives [11]. The dissolution time can be regulated bycontrolling the amount of binder.

ixer granulator, (b) four-bladed impeller and the (c) chopper.

Fig. 2. Mean diameter of different particle size for L/S ratio=20% (ignore the 1stmin of 82.5 μm).

Fig. 4. Comparison of final average granule size with different L/S ratio.

300 W.-D. Tu et al. / Powder Technology 184 (2008) 298–312

In addition to the speed and geometry of the high shear mixergranulator, the primary particle size is another critical parameterthat affects the outcome of the granulation process. The particle/droplet size ratio determines whether the particles are coated byor immersed in the binder droplets [12]. Furthermore, powderswith diameters smaller than about 10 μm are subject to strongvan der Waals forces, which can cause the initial formation ofdry aggregates which are not easily penetrated by the binder.However, when wet agglomerates are formed, they are noteasily consolidated, because of resistance to flow of the binderthrough the small inter-particle spaces. Few studies have beenpublished concerning the effect of initial powder diameter onthe growth mechanisms. In this paper, induction type behaviourwas examined by measuring the growth rate of granules, thechange in granule size distribution and the change in distri-

Fig. 3. Mean diameter of different particle size for L/S ratio=17% (ignore the 1stmin of 82.5 μm).

bution of the binder material during granulation as a functionof primary particle size and L/S ratio.

2. Experiment

2.1. Material

Narrow size fractions of CaCO3 powders were used asparticulate materials with average diameters (d50) of 1.5 μm,3.5 μm, 5.75 μm, 8 μm, 12 μm, 14 μm, 35 μm, 50 μm and82.5 μm. They were produced from white marble, and theaverage diameters were measured using the Mastersizer 2000(Malvern Instruments, UK).

It has been reported that optimum agglomerate growthoccurs when the granules are of intermediate deformability [12].Hence, in order to achieve a monomodal granule sizedistribution, a binder of moderate viscosity is required.

Fig. 5. Comparison of size growth rate vs. different primary particle size.

Fig. 6. Size distribution of granules composed of CaCO3 and PEG 6k, L/S=20%, 1–10 min. The primary particle sizes are: (a) 1.5 μm, (b) 3.5 μm, (c) 5.75 μm,(d) 8 μm, (e) 12 μm, (f) 14 μm, (g) 35 μm, (h) 50 μm, (i) 82.5 μm (solid points: bimodal or multimodal distribution; hollow points: monomodal).


Fig. 6 (continued).


Accordingly, PEG 6k was selected. The melting range of thebinder was between 50 °C and 61 °C [13] and it behaved as aNewtonian fluid in the molten state. The viscosity was 1.1 Pa·sat 60 °C [14]. The binder is applied in the form of flake shapedparticles that are melted during agglomeration.

2.2. High shear mixer granulator

Agglomeration experiments were performed in a vertical-axis high shear mixer (Ya-Chen Industry, Taiwan, ROC) asshown in Fig. 1(a). The internal diameter was 25 cm and thedepth 40 cm giving a maximum working capacity of 10 L. Thetemperature range of the heating jacket was controllable fromambient temperature to 100 °C. A four-bladed impeller wasmounted on the vertical axis at the centre line of the bowl whilea chopper was installed on a horizontal axis on the inner side.The axis of the chopper was 15 cm higher above the base. The

diameters of the impellers were 25 cm for the longer blades and21 cm for the shorter blades, respectively. The diameter of thechopper was 13 cm. Both the impeller and the chopper areshown in Fig. 1(b) and (c).

2.3. Experimental processes

The temperature of the heating jacket was set at 60 °C, whichis higher than the melting point of PEG 6k (55.5 °C). 2000 g ofCaCO3 was weighed and freely poured by hand into theagglomerator. In order to achieve reproducibility of the initialpowder bed structure, the impeller and the chopper wereswitched on for 10 s to pre-mix the poured powder. After thepre-mixing was stopped, the required quantity of PEG 6k wasthen added by the melt-in method [15]. This meant that thesolid-state binder was poured onto the top surface of CaCO3

layer by layer and the agglomerator lid was firmly closed

Fig. 7. Size distribution of granules composed of CaCO3 and PEG 6k, L/S=17%, 1–10 min. The primary particle sizes are: (a) 1.5 μm, (b) 3.5 μm, (c) 5.75 μm,(d) 8 μm, (e) 12 μm, (f) 14 μm, (g) 35 μm, (h) 50 μm, (i) 82.5 μm (solid points: bimodal or multimodal distribution; hollow points: monomodal).


Fig. 7 (continued).


afterwards to allow the contents to heat up. The agglomeratorwas left closed for 90 min, after which time the binder had fullymelted.

The impeller, chopper and the stopwatch were synchronizedand triggered. A sample of granules was collected every 60 swith an automatic sampling tube mounted on the top of the lid,without suspending the agglomeration process. In order tominimize the effect of sampling on subsequent granulation, thetotal mass withdrawn during an experiment was maintainedbelow 10% of the starting mass. The samples were removedcarefully from the sampling tube and spread onto a tray forcooling. They were then placed in a vibrated sieve stack (RX-29, W. S. Tyler, Canada) in order to measure the granule sizedistribution of each sample.

In order to measure the binder content, the agglomerates wereplaced within an oven (DCM 452, Channel Co. Inc., Taiwan,ROC) for 6 h at 200 °C. It has been established that PEG 6k

under such heating conditions can be removed to an extent of95% to 99% so that the loss of weight could be assumed equal tothe weight of binder contained within the samples.

2.4. Calculation method

The average diameter of the particles collected on each sievewas taken to be the arithmetic mean value of the collecting sieveand the one above it, e.g., the diameter of the samples betweensieves 90 and 106 μm was 98 μm. The mass mean diameter (da)in each sample was calculated according to:

da ¼Xn�1

i¼1

wi � diwt

� �ð1Þ

where n is the number of sieves including a receiver (sievei=1),for which the aperture size is 0 μm. wi is the weight of granules

Fig. 8. Binder distribution among granule sizes, CaCO3, PEG 6k, 20%, 1–10 min. Primary particle sizes are: (a) 1.5 μm, (b) 3.5 μm, (c) 5.75 μm, (d) 8 μm, (e) 12 μm,(f) 14 μm, (g) 35 μm, (h) 50 μm, (i) 82.5 μm (solid points: uneven distribution; hollow points: uniform distribution).


Fig. 8 (continued).


collected by sievei and wt is the total weight summed up fromsievei=1 to sievei=n−1. di represents the mean diameter of thecollecting sieve and the one above. The granules collected by thelargest sieve (sievei=n) will be ignored in calculation, because themean granule diameter of those granules is unknown.

3. Results and discussion

3.1. Mean diameter of the granules

3.1.1. L/S=20%Fig. 2 shows the increase ofmean granule sizewith time for the

various sizes of primary particle. It shows that particle diametersranging from 1.5 to 8 μm (defined as Group I) behaved similarly.In this group, agglomeration showed a typical induction typebehaviour. The induction stage, which was completed by the third

minute, was characterised by a slow but steady increase in granulesize. This was followed by the start of the rapid growth regime.The rapid growth period lasted for another 3–4 min and wasfollowed by a slow, almost zero, growth period until the end of theexperiments. Primary particles in the 12 to 35 μm range behaveddifferently, defined as Group II behaviour. In Group II, inductiontype behaviour was also observed; however, in this case theinduction period lasted 1 min longer. The powder particles ofdiameter 50 μm displayed some growth; however, the growthbehaviour did not correspond with the typical induction type, sothat the curve was classified as showing steady growth behaviour.The coarsest primary particles, 82.5 μm, did not agglomeratesatisfactorily, producing only crumb.

During the induction period for both Groups I and II typematerials, it was observed that the larger primary particlesproduced larger agglomerates. However, the opposite was

Fig. 9. Binder distribution among granule sizes, CaCO3, PEG 6k, 17%, 1–10 min. Primary particle sizes are: (a) 1.5 μm, (b) 3.5 μm, (c) 5.75 μm, (d) 8 μm, (e) 12 μm,(f) 14 μm, (g) 35 μm, (h) 50 μm, (i) 82.5 μm (solid points: uneven distribution; hollow points: uniform distribution).


Fig. 9 (continued).


observed during the rapid growth stage: the rate of granulegrowth and the ultimate granule size were seen to decreasewith increasing primary particle size. Clearly, the granulescomposed of finer particles had higher yield strength and werebetter able to resist breakage than those formed from coarserparticles.

3.1.2. L/S=17%As shown in Fig. 3, induction behaviour is seen for six

different grades of powders. The induction stages for thepowders below 10 μm in size were affected by the reduction inthe quantity of binder added, in that the induction times wereextended from 3 to 4 min. This implies that increasing theamount of binder shortens the time for induction in a high shearmixer, as found byWauters et al. [16] in rotary drum granulation.Regarding the growth behaviour of 35 and 50 μmparticles, these

showed a typical steady growth and displayed a transition stagebetween induction behaviour and crumb behaviour. In the caseof 82.5 μm powders, the average size of granules at the firstminute was ignored, since many lumps exceeded the sievingresolution. The average size seemed independent of time fromthe secondminute as the particles were too coarse to agglomeratesatisfactorily. Agglomerates remained almost the same size at500 μm, implying that for this case, growth no longer took placeor that the rates of growth and breakage were in balance.

From Fig. 3, with the reduced L/S ratio, 35 μm powders nolonger displayed induction type behaviour and were shifted tothe group of steady growth. This implies that either moreviscous or a threshold amount of binder was needed to providesufficient adhesion forces between particles [12].

12 and 14 μm particles with 17% L/S ratio required moretime to finish the induction stages which suggests that either less


binder requires more time to be adequately transferred anddistributed or binder transfer is slower with a low L/S ratio.

3.1.3. Comparisons between L/S=20% and L/S=17%During the induction stages, both Figs. 2 and 3 show that

coarser primary particles produced larger agglomerates thanfiner ones. Reducing the quantity of binder did not have asignificant effect on the average granule diameter duringinduction. This is presumably because at the beginning of themixing process, the granules were continuously reconstructing,so that the effect of the L/S ratio was not obvious until the laterstage of the experiments, at which the binder was spread evenlyamong the granules and the mean granule sizes were becomingclearly different.

Comparing the steady growth behaviours in Figs. 2 and 3, itwas found that the average of the starting and final diameters oftwo cases had deviations of 0.51% and 0.57%, so that thequantity of binder had a limited effect on the granule size at thisstage. The comparison between the groups showing crumbbehaviour was excluded since the comparison of the effects ofbinder content has been limited to induction mode granulation.

Fig. 4 plots the final average granule diameter againstprimary particle size. From this figure, it can be seen thatreducing binder content resulted in smaller final mean diametersfor primary particles below 50 μm. For primary particlesexceeding 50 μm, there is no effect of L/S ratio. It could beconcluded that final average granule size is a function of bindercontent until the primary particle is too coarse (50 and 82.5 μmin this study).

The onset of the rapid growth stage is here defined as thepoint at which the slope of the curve of granule size vs. time(Figs. 2 and 3) is steepest. This maximum rate of growth isplotted against primary particle size in Fig. 5, in which thegrowth rate can be divided into three regimes as a function ofprimary particle size: fast growth, transition and slow growth.Furthermore, Fig. 5 suggests a descriptive method to determineboundaries between induction, steady growth and crumbbehaviours.

In the case of L/S=20%, the growth rates of the primaryparticles from 1.5 to 35 μm (fast growth regime) were 4.0, 3.7,3.4, 3.3, 3.2, 3.0 and 2.7 μm/s, respectively. Therefore theseprimary particles could be agglomerated more efficiently thanthose primary particles with diameters above 35 μm since theyformed stronger agglomerates. From 35 to 50 μm, a steepdecrease in the growth rate occurred which was named thetransition regime. The diameters of primary particles from 50 to82.5 μm grew at relatively slower rates, below 0.6 μm/s. Inaddition, the granules composed of 82.5 μm particles did notagglomerate well, the agglomerate mean size remaining atapproximately 500 μm as shown in Fig. 6(i).

From Fig. 5, the growth behaviour for L/S=17% shows similarbehaviour, except for the transition zone (the steady growth in Fig.3). The differences imply that either coarser particles require morebinder to promote the rate of agglomeration due to the lack of inter-particle adhesion forces, or that the larger particles are subject tolarger forces in the mixer so that they are unable to agglomeratesatisfactorily as well as being more easily broken up.

3.2. Size distribution measurement

3.2.1. L/S=20%Fig. 6 shows the size distribution of granules from each

grade of primary particles at intervals of 1 min from the start ofmixing for L/S=20%. Comparison with the growth in averagegranule diameter from Fig. 2 shows that the onset of the rapidgrowth period corresponds with the transition from a bimodal(or a multimodal) to a monomodal size distribution. In the casesof primary particles of 1.5 and 5.75 μm, there was a 1 minutedifference between the start of the rapid growth period and theend of the induction period. Since this is the same as the timeinterval, it is expected that the time difference would beminimized with real time sampling or online granule sizemeasuring techniques.

For as long as the size growth continues, for all primaryparticle sizes, the size distribution curves in Fig. 6 becomenarrower with time and gradually move towards a larger mean.It is interesting to follow the change in population of largegranules (N1090 μm): those formed early (during induction) areweak, probably due to uneven binder distribution and poorconsolidation; these break down during the early stages of therapid growth regime. After this, as consolidation progresses andgranules become stronger, then growth of larger particlesresumes. At the same time, the fraction of smaller granules(b300 μm) decreases due to agglomeration.

The 82.5 μm primary particles formed granules withirregular size distribution and this did not change with time.From Fig. 2, it was found that 82.5 μm particles coalescedquickly at the beginning of the experiment but the averagegranule size remained constant thereafter. Taken together withthe irregular but relatively unchanging distributions shown inFig. 7(i), this suggests equilibrium between coalescence andbreakage.

3.2.2. L/S=17%Figs. 6 and 7 show that reducing the L/S ratio did not

significantly affect the shape of the granule size distributionproduced from the primary particles ranging from 1.5 to 14 μm.As with L/S=20%, it seems that the end of the induction periodin Fig. 3 corresponds to onset of monodistribution in Fig. 7. Forprimary particles less than 14 μm and also for the 50 μmparticles (Fig. 7(a) to (e) and Fig. 7(h)), it was found that thesize distribution remained monomodal during the growthperiod following induction. For the other primary particles(Fig. 7(f), (g) and (i)), the distribution was only monomodal fora brief period and reverted to multi- or bimodal towards the end.This is probably due to the non-uniformity of distribution of thesmaller binder amount. Compared to L/S=20%, L/S=17% wasnot enough to bind coarser powder particles so that breakagecould be expected and the mean diameter would be smaller, asshown in Fig. 3. The coarsest primary particles, 82.5 μm, whichbelonged to the non-induction behaviour group, displayed amultimodal size distribution and remained irregularly distrib-uted between size classes, due to equilibrium betweencoalescence and breakage, as observed with L/S=20%. In thiscase, 82.5 μm is too coarse to be agglomerated. In summary,

Fig. 10. Standard deviation of binder content among granules, L/S=20%.

Fig. 11. Standard deviation of binder content among granules, L/S=17%.


with L/S=17%, it was difficult to maintain a monomodal sizedistribution with the coarser particles.

Mackaplow et al. [17], who worked with primary lactoseparticles ranging from 39 to 127 μm, found that primary particlesize had a strong effect on growth rate when granulating withwater as the binder; increasing the size of primary particlesresulted in larger granules. In the present study, particle size ofCaCO3 powders in the range 1.5 to 82.5 μm also had a strongeffect on growth rate when granulated with PEG 6k as binder,but, in this case increasing the size of the primary particlesresulted in smaller granules as a result of breakage, as shown inFigs. 6 and 7.

3.3. Binder distribution

Binder distribution measurements were performed bycombining certain ranges of granules together into broaderclasses as follows: b196; 196–550; 550–1090; 1090–1500;N1500 μm. This gave larger masses to minimize discrepanciesfrom the effect of weighing errors and absorbed moisture fromthe air [18]. It is noted that the binder content in thesemeasurements is expressed as concentration (weight of liquid /weight of liquid and solid) and plotted on the y-axis.

3.3.1. L/S=20%Figs. 8 and 9 show the distribution of binder amongst the

different sizes of granules as a function of granulation time forL/S=20% and 17% respectively. From Fig. 8, it was found thatbinder distributions among all grades of granules graduallybecame more uniform with time. Furthermore it was observedthat for primary particles under 50 μm (induction typebehaviour), the binder distribution became uniform (except forthe smallest size class) by the time the induction stage hadfinished. In other words it appears that after uniform binderdistribution was achieved, a rapid growth stage could beexpected and a monomodal size distribution could be obtained.

Thus the binder distribution could be a key parameter inachieving monomodal size distribution.

In the group of smaller granules (b196 μm), it was found thatbinder content was roughly proportional to the primary particlediameter. Binder penetration is easier with coarser primaryparticles, because of the greater inter-particle pore dimensions,enabling uniform binder distribution to be achieved morerapidly. Nevertheless, induction type behaviour was notobserved with particles greater than 35 μm (50 and 82.5 μm)due to the weaker granule structure and subsequent breakage.So while coarser particles promote binder spreading, theresulting granules are weaker and breakage dominates overgrowth for particles larger than 35 μm (50 and 82.5 μm).

It was also found that in each powder grade, granules smallerthan 196 μm always contained less binder, even after theinduction time. This was presumably because these granuleswere from the stage of nucleation as well as the stage of breakagewhich would intrinsically contain a lesser amount of binder.

Mackaplow et al. [17] found that for lactose powder above39 μm, the size did not have a measurable effect on thedistribution of liquid during mixing with water in a high shearmixer. The current study shows a very similar behaviour: thatfor powders coarser than 35 μm (50 and 82.5 μm), size has nomeasurable effect on the distribution of liquid.

3.3.2. L/S=17%As shown in Fig. 9, it was found that for each grade of

CaCO3, the granules smaller than 196 μm contained less binderthan the larger granules. Coarser granules carried a higherpercentage of binder. This trend is independent of the changesof L/S ratio from 20 to 17%.

Comparing Figs. 9 and 3, for the powders classified inGroup I, it can be seen that the time needed to achieve uniformbinder distribution is the same as the induction time or even lessin the cases of 8, 12 and 14 μm. This again implies that uniformbinder distribution is a requirement for commencement of rapid

Table 2Comparison of induction time and stable time for L/S=17%

aSD: Standard deviation.

: Coincide with each other.


growth. Furthermore, mapping the groups of transition and non-induction behaviour from Figs. 3–9, it was found that forpowders smaller than 35 μm, uniform binder distribution wasrequired before the onset of rapid growth and monomodal sizedistribution; however, for those larger than 35 μm, no inductiontype behaviour was observed even though uniform binderdistribution was achieved. It was concluded that, under thecondition of L/S ratio=17%, 35 μm could be regarded as thethreshold for determining whether a uniform binder distributionwould be required for the appearance of induction typebehaviour.

3.3.3. Comparisons between L/S=20% and L/S=17%It appears that, under the same operating conditions, the

distribution of binder among the granules is unaffected by L/Sratio in these experiments. For both L/S ratios investigated, thebinder content in the largest granules was markedly higher thanthat in the smaller granules and persisted until the end of theinduction stage. These results correspond with those obtainedby other workers in the rotary drum [18] and the high shearmixer granulator [14].

In this study, homogeneous binder distribution sometimesoccurred earlier than the induction time as shown in Fig. 9(d),(e) and (f), which indicated that efficient mixing was critical forgranule size enlargement. Moreover, use of coarser particlescould facilitate the binder spread more efficiently than with finerparticles, regardless of the L/S ratio, as shown in Fig. 8(h) and(i) and Fig. 9(g), (h) and (i). It was observed that induction typebehaviour did not appear with coarser particles due to thedomination of breakage.

The standard deviation (SD) of binder content among thegranules decreased and then became constant with time asshown in Figs. 10 and 11. It can be observed that, in general,granules generated from smaller primary particles had higherSD values than those generated with larger ones, whichindicated that the degree of binder distribution could affectthe average diameter of granules. In addition, by the time the SDvalues became stable, the induction period had also ended inmost of the experiments as seen in Tables 1 and 2, except fortwo cases which had 1 minute difference in Fig. 11. It could be

Table 1Comparison of induction time and stable time for L/S=20%

aSD: Standard deviation.: Coincide with each other.

concluded that based on the stability of the SD, the appearanceof the induction time could be predicted.

In general, use of the higher L/S ratio (20%) resulted in morestable growth. The onset of the rapid growth stage and thedevelopment of a constant SD value also coincided with eachother more closely at the higher L/S ratio.

4. Conclusions

High shear mixer granulation of calcium carbonate with amelt binder shows a range of agglomeration behaviour, frominduction to rapid growth, followed by equilibrium betweengrowth and breakage. Different L/S ratios have a measurableeffect on the duration of the induction stage and the final meandiameter, a higher L/S ratio shortening the induction time andfinally leading to larger granules. More binder addition leads toa shorter duration of the induction stage, which corresponds tothe results obtained by Wauters et al. in a rotary drum [15]. Thecurrent study found that for primary particles from 1.5 to82.5 μm, size had a strong effect on the growth rate, increasingthe size of the primary particles resulting in smaller granules,probably due to breakage.

It appears that in this system and for smaller primaryparticles, the successful development of uniform binderdistribution coincides with the onset of rapid growth; whetherthe former is a precondition for the latter remains to bedetermined. Primary particle diameter also has a strong effect onthe rate of binder distribution, since a bed of coarser particles ismore easily penetrated by the binder. Homogeneous binderdistribution has been shown elsewhere to be a pre-requisite forrapid growth in a rotary drum [15]. Uniform liquid distributionand use of a primary particle diameter below the critical valueare key parameters in promoting higher yield rates and reducingthe subsequent recycle rate.

Acknowledgements

The authors would like to acknowledge theGraduate StudentsStudy Abroad Program of NSC (National Science Council),Taiwan, ROC, and the Department of Chemical Engineering,University of Birmingham, UK, which kindly sponsored the first


author's stay in the Department of Chemical Engineering,University of Birmingham, UK. Mr. Nien-Huang Tsou, theCEO of Ryh-Shuo Inc., Taiwan, ROC is also gratefullyacknowledged for the help in technical problem solving.

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the effect of powder size on induction behaviour and binder distribution during high shear melt...

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