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

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  • en, RO

    ashigdersulfgleontaf theatior angglo

    2011 Elsevier B.V. All rights reserved.

    the prothe mois useon of can is thps, althe proce

    Powder Technology 211 (2011) 165175

    Contents lists available at ScienceDirect

    Powder Te

    e lsThe interaction between liquids and solids is very important.The repellence or attraction between a liquid and a solid is

    determined 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 ne silica sand. They estimatedthe contact angles and water drop penetration time (WDPT) from themolarity obtained with an ethanol droplet (MED) test, the capillary

    shape 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 ner the size of the particles one would choose. If largeparticles are initially used, the binder viscosity must be increased [6].In contrast, Schfer et al. [7] observed that granule strength increasedwhen ner initial raw-particle sizes or a higher viscosity of binderrise method (CRM) and the sessile drop metmethods have different ranges of measuremtheWDPT increasedwith an increase of the cosharp increase when the contact angle increa

    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.2011.04.014ss of wet agglomeration,s between particles (e.g.,namic viscosity forces).

    PEG 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 roundedparticles are combined due to the liquid forcestatic capillary forces, surface tension, and dy1. Introduction

    Wet granulation is important forenlargement during manufacturing inogy. This type of production processfabrication of granules for the productipharmaceutical industry. Agglomeratioparticles are gathered into larger clumcan still be distinguished [1]. During thcess of powder granuledern industrial technol-d during the large-scalepsules and tablets in thee process whereby smallough the initial powders

    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 inuence of liquid wettability, the physicalproperties of the binder liquid and powders also play an importantrole in wet granulation. Johansen and Schfer [5] looked at theprocess of agglomeration, using three grades of calcium carbonatewith different particle sizes, surface areas, and particle shapes withhod (SDM). All of theseent. They observed thatntact angle. There was ased from 88 to 93. With

    were used.In addition t

    different mechatraditionally beefollowing threemand nucleation, cNucleation is theto collision and

    +886 3 425 4501.

    l rights reserved.Heterogeneous dispersionHomogeneous dispersionInuence of the interaction between bindbehavior in a high-shear mixer

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

    a b s t r a c ta 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 behavior

    The purpose of this study wgranular agglomeration in abinder. Three different powcalcium carbonate, calciummeasured with a contact anthe powder bed and the cdetermining the progress ostate, heterogeneous nucleleads to induction behavionucleation and growth of a

    j ourna l homepage: www.r and powders on melt agglomeration

    C

    to investigate the effects of the different surface properties of powders onh-shear mixer. Polyethylene glycol 6000 (PEG 6000) was used as the meltings, with mean granule sizes of 75150 m were used as the raw materials:ate, and sodium carbonate. The wetting properties of the raw materials wereinstrument. The results indicate that the speed at which the droplets sink intoct angle of binder droplets on the powder surface play important roles inagglomeration process. Several types of agglomeration were found: a slurryn, snowballing, and induction growth behavior. Heterogeneous dispersiond subsequent growth, but a homogeneous dispersion leads to little or nomerate size.

    chnology

    ev ie r.com/ locate /powteco the effects of liquid and powder properties, thenisms related to granulation behavior have alson described. Ennis and Litster [1] distinguished theechanisms during the agglomeration process:wettingonsolidation and growth, and breakage and attrition.mechanismbywhich small particles bind toothers duethe force of adhesion. Observations of possible

  • 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] dened two growth behaviors basedon the deformability of the system: steady growth and induction timebehaviors. They also designed a growth regime map which considered

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

    Schfer 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

    Table 1Physical properties of raw materials.

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

    Calcium carbonate 2.93 75150 800Calcium sulfate 2.96 75150 1450Sodium carbonate 2.54 75150 850

    (a) High shear mixer granulator

    (

    Water Filler Point

    ChopperBowl

    Impeller

    Watermark

    Temperature Probe

    Granule Discharge Point

    Water Discharge Point

    166 H.J. Cheng et al. / Powder Technology 211 (2011) 165175(b) Four-blade impeller

    HeaterFig. 1. Schematic representation of the: (a) high shear mixer granuc) Chopperlator; (b) four- blade impeller; (c) chopper (units are in mm).

  • in pour-on system are bigger and faster comparing with the granulesformed in melt-in system. The materials properties have signicantlyheterogeneity in both ways. Knight et al. [13] studied the sizedistribution of granulation with different material sizes (423 m)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 nal 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].

    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 coefcient 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 inuence of the binder

    (a) Before

    Before

    Before

    After

    After

    After

    (b)

    (c)

    Binder sank into the dry powders

    167H.J. Cheng et al. / Powder Technology 211 (2011) 165175Fig. 2. The photographs of the state of the binder-powder mixture before and after the PEG 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)

    con

    tact

    ang

    le (o )

    0 200 400 600 800 1000 1200 14000

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    110calcium carbonatecalcium sulfatesodium carbonate

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

    168 H.J. Cheng et al. / Powder Technology 211 (2011) 165175properties. 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 m assifted by a shaking sieve. Polyethylene glycol 6000 (Showa, Japan)was used in ake form as the melting binder. The use of the binder inake form could give a narrower size distribution and larger granulesize after agglomeration than using a ne 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 2121400 m. The hydroxylvalue is 13.5, the PH (5 w/v%) is 7.3, themelting point is 58 C, and theliquid viscosity is 665 mPas 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 ake was poured slowly on top of thepowder bed through a funnel to ensure that the binder distributionwas uniform (left-hand column in Fig. 2(ac)). 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 the

    intermediate zone of the granular bed in the vertical direction; (2) in versus time.

  • 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 b90,90125, 125212, 212355, 355425, 425500, 500600, 600710,710850, 8501000, 10002000, 20003350, 33504750, 47505600, 56006700, 67008000, 80009500 and N9500 m 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(ac). On the right hand

    best slow-wetting properties (with the PEG 6000 liquid) of the threematerials. Fig. 4 shows the change in the contact angle of the liquid

    Mea

    n siz

    e (

    m)

    0 2000 4000 6000 8000 10000 12000 14000 160000

    500

    1000

    1500

    2000

    2500

    3000

    3500

    4000

    4500

    5000calcium carbonatecalcium sulfatesodium carbonate

    (a)

    Number of impeller revolutions

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

    II(C.S.)

    II(C.C.)

    Mea

    n siz

    e (

    m)

    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).

    Mas

    s per

    cent

    age

    (%)

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    1002 mm

    III III

    Number of revolutions0 1000 2000 3000 4000 5000

    Fig. 7.Mass percentage versus agglomeration time for the calcium carbonate with low

    169H.J. Cheng et al. / Powder Technology 211 (201...

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