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    2011 Elsevier B.V. All rights reserved.

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    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: (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


    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.


    ev ie 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




    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








    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) calc


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