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Rheology of concentrated alumina suspension to improve the milling output in production of high purity alumina powder
~
"
COLLOIDS -(:olloid~.ndS.ra~e~
ELSEVIER
S~FAC~S
i
A: Physicochernicaland EngineeringAspects 133( 1998) 41 47
Rheology of concentrated alumina suspension to improve the milling output in production of high purity alumina powder R. M . A n k l e k a r , S. A . B o r k a r , S. B h a t t a c h a r j e e , C. H . P a g e , A. K . C h a t t e r j e e * The ,4ssochtted (2"mt'l~t (~mp~tnies Lhnited, R~e~#'l'h and ('OtlSldl~ltl~T Dircvtorare, L. B. S. Marg, Thane-4fX)604. India
Accepted 10 February 1997
Abstract
In production of high purity alumina, milling is an important processing step during which particle size has to be reduced to sub-micron size using minimum energy input and avoiding contamination from the milling system. In this investigation, to increase the efficiency of the milling process, the effect of two anionic dispersants were studied in concentrated alumina slurry. The optimized slurry was granulated using freeze drying and the properties of granules and sinters were compared with that using spray drying. ~ 1998 Elsevier Science B.V. K~trword~': Alumina: Rhcology: Dispersant; Spray drying; Freeze drying: Simering
1. Introduction
High purity sub-micron size alumina powders are widely used in electronic and structural applications [I-4], e.g. ~ls substrates, cutting tools, nozzles, grinding media, bio-implants, coatings, etc. Each of the above applications demands specitic physieo-chcnrical characteristics of the powders, which in turn directly depends on the processing route employed [5,6]. The Research and Development Division of the Associated Cement Companies Limited (ACC) has put up a semi-commercial pilot plant for producing high purity alumina, zirconia and barium titanatc powders using wet chemical processes such as sol-gel, hydrothermal, co-precipitation, etc. The optimization of the process parameters becomes relevant l¥om the cost-effectiveness view point for commercialization of each of the above powders. In the manufacture of sub-micron high purity * Correspondingauthor. 11927-775798s19.011~' It198ElsevierScienceB.V.All rightsreserved. PII S0927-7757(971001111-6
alumina powder, "milling" is one of the important processing step to achieve the desired particle size distribution tbr specific applications. However, to achieve this in a cost-effective way, it is essential to optimize the mill output by maximizing the solid loading and minimizing the milling time, which in turn reduces energy consumption [7,8] and contamination I¥om the grinding system. In the present investigation, the theological behavior of concentrated slurries of alumina powder I HP grnde) has been studied using two dispersants with the aim of increasing the throughput of the mill by maximizing the solid loading but maintaining the slurry pumpable. The alumina slurry, using optimum dispersant concentration and powder loading, was subjected to freeze drying to obtain ready-to-press granules which could be sintered to higher density with improved microstructure compared with that obtained by spray drying.
42
R. M, Ankh'kar t't a£
Colloids S.rj~wes A: Phyxiea,t'hem. Eng. A,qwcts 133 (1998) 4l- 47
2. Experimental High purity alumina powder (ACC's H P grade) with alumina content of > 99.9% and having impurities Na,O, Fe_,O3, SiO_, and CaO typically 340. 130. 250 and 14{)ppm, respectively, was employed. This powder was wet milled for 10 h in a Sweco M-18 vibro-mill using 75 kg of 1/2in~xl/2 in alumina cylpebs maintaining 20 vol.% slurry concentration without any dispersant. The average particle size (~/~0) and BET surface area of the ground powder was found to be 0.72 l,tm and 9.5 m-'/g, respectively. Two anionic dispersants viz. tri-ammonium citrate (TAC) from s.d. Fine Chemicals, India, having molecular weight ~250 and Darwin C {Ammonium polymcthacrylate) from R. T. Vanderbilt Company, LISA, witb average molecular weight ~ 10 000 were used. To determine the optimum dispersant concentration, the alumina slurry was prepared by mixing 125 g of the powder in 75 ml of DM water ( ~ 3 0 vol.%), containing different amounts of the above dispersants, in a roller mill for 8 h and measuring the viscosity of the slurry as a function of shear rate using a concentric cylinder rotational viseometer (Haake VT 5011l. The solid loading experiments were done by increasing the powder content in the slurry at the optimum dispersant concentration until the slurry was found to be pumpable 1< 500 mPa s at 11.7 s ~). The relative viscosity 01,~0 for the particle liquid suspension was obtained using the relationship 'llre[=q~.=/qllq
quantity of alumina (1 kg) was equilibrated at 25:C with additive solutions of different initial concentrations in a ball mill for 8 h. The suspension was centrifuged and the additive concentration in the supernatant was determined gravimetrieally. The zeta potential measurements were carried out using Zeta-Meter 3.0+ {Zeta-Meter Inc., USA ). The electrokinefic mobilities of the powders were determined by making the powder suspension (4 wt%) and milling for 6 h followed by centrifugation, and the supernatant dilute suspension wets taken for measurement. The zeta potentials were determined from electrophoretic mobility values using the Helmhohz Smoluchowski equation [9]. The slurry with optimum solid loading wets spray dried using a laboratory Buchi 190 mini spray-dryer to study the effect of the granulation technique on the sinterability of the ahtmina. The same slurry was also freeze dried under a pressure of <0.2 mbar and a temperature of - 4 0 C using a Life Science Laboratories" Lyolab F-3052 Lab freeze-dryer. The granules obtained by spray-drying and freeze-drying techniques were compacted at 150MPa and sintered at 1550 C for 2h. The nmrphology of the spray-dried and freeze-dried granules and sinters were observed using Jeol's JSM-5400 scanning electron microscope (SEM). The density of the sintcred pellets was measured by the displacement method using Archimedes" principle employing Mettlcr bahmcc and density determinatiou kit ME-33360.
{I)
where 0~,s is the suspension viscosity and iltiq is Ibe viscosity of suspension liqtfid medium. The dispersion characteristics of the above slurries were studied by measuring the pH using a Systronics System 361 pH meter. The particle size distribution of the vibro-milled slurry was measured using the Sedigraph 5100, Micromeritics Inc., USA. The adsorption isotherm of the additive on alumina particle wets determined by monitoring the change in concentration of the adsorbate in the suspension bclbrc and after equilibration with the adsorbent. For this purpose, a sallicient
3. Results and discussion The effect of concentration of TAC and Darvan C dispersants on the theology of the alumina slurry is shown in Fig. 1. As the dispersant concentration increases, the pseudoplasticity of the slurry was observed to decrease proportionately, resuhing in close to Newtonian behavior at higher concentrations, thereby suggesting good dispersion of the slurry. The optimum concentration o f T A C dispersant beyond which there is no change in the viscosity-shear rate curve for maintaining the pumpability of the slurry {viscosity <5(10 mPa s
R. M .,Inklekar et ol. , Colhfid~ St,J~wes A." Phys'icochem. Eng.
son
8oo
r
-.
E
As;pt'ct~133 (1998) 41-47
.E
t,13 0
son\ ~, \
750
500
2oo
~--
=< 2sopr-
1250
\
I I I)111|| J J IIIII 10 1 O0 SHEAR RATE
.1000
(S -] )
0 J 1
~.
,,,\
\~
~-
0
43
•
J } l l l l ~ 'I ! 0 SHEAR
I
J lll[l~'~I~ 1 O0 RATE IS
1000
-1)
Fig. I. The etl~ct of dispcrsant concentration o. the rheolog.~of the alumina slur~ 1311vol.%}: [al Ti&Cand (b) Darvan C. The .,,ahleShldic~llcdzlre ill ppm, NI) denoles no dispersant, and Ihe doned line is oblaincd by extrapolation. at 11.7 s ') was found to be 2000 ppm. However, when Darvan C was used instead of TAC, the wdue of optimum dispersant concentration was observed to be 1250 ppm. These optimized values have also been corroborated from adsorption isotherms (Fig. 2). The difference in the optimized values for the two anionic dispersants may be attributed to its chemical nature, i.e. TAC is a low molectdar weight dispersant while Darvan C is a long-chain polymer. Thus, both electrostatic and steric stabilization mechanisms are expected to
IE
....i
DISPERSANT CCI%K~EN TRATION
Fig. 2. Adsorpti.n i:,olhennsof alumina slurry employingTAC and Darvan ('.
operate in the case of Darvan C, while it should be predominantly electrostatic in the case of TAC dispersant. The effect of solid loading on the rheology of the alumina slurry using optimized dispersant concentrations of TAC and Darvan C is illustrated in Fig. 3. Although as high as 50 vol.% solid loading has been reported [10], not much detail on the slurry preparation, optimum concentration, etc. has been mentioned. Our studies indicate that at the optimized concentrations of TAC and Darvan C, the maximum solid loading obtained was 50 and 40vo1.%, respectively, keeping the slurry pumpable (viscosity < 500 mPa s at 11.7 s- t). This may net be due to bridging flocculation as it generally occurs in the case of very high molecular weight polymers at much lower concentrations in a poor solvent medium [11]. However, the zeta potential vs pH curve for alumina slurry (Fig. 4) without dispersant, and with optimum TAC and Darvan C concentrations indicates that the difference between the slurry pH and the effective pHil=, (ApHtEp) is more than 2.5 for both the dispersants. This suggests that both the dispersants yield well dispersed slurries [12]. Further, the higher wdue of ApHIEr for TAC (4.0) as compared with Darvan C (2.7) suggests more electrical
R, ;%I ,4n.Uekt#" ~'1 a/
44
('oll,#dr Sl#filce,* el." I'hrsicodwm. F,n¢, ,la?e¢t.~" 133 1998~ 41 47
.-.g,4.ao~-
(a)
15O0
(b)
~ 200 >-
--
~-
50
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\
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1201--
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, &0
600 30
L ' ' ' ' ' ' ~ ~ A0
_~
,.y
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300
d_ 10
o
1 oc 11000 SHEAR ForTE ( S - )
10
100
11000
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Fig. 3. The ett~cl of mlid loadingusingoptimizeddi~pcrsanlconccntralkm on Ihc rhcolog~ of the alamina slum: a) FA("(2000 ppm), and (b) Darvan (" 11250ppml. The valncs indicated nre in ",ol c~.
,z
a
Fig. 4. The ZClapOlelllialof attllllitl;Ipox~derils zt ftmclit}n~.1 pll: (a) no di~pcrsant: (b) 1250ppm of I)dlx;lll ('; iltld (C) 211{10ppm ol 'rA(.'. double-layer repulsion [13] resulting in better dispersion. The specific energy f interaction betweert powder surface and dispersant was also evaluated using the following equation [13], A p H . v = 1.(1396(' o ex p( -- a ~ p
R 1" )
(2 )
where ApHu: P is the shill in the iso-elccric point at the dispersion conccnlrmlou C,, and -Af~,~p
represents the corresponding specific energy o f irReractiol'l between the ceramic powder surface and the dispersam. R mid T arc the standard gasconstant and temperature in Kelvin, respectively. The inthtencc or dispersams on the suspension characteristics such as pit, shift in the iso-elecric point at the optirnurn dispersion concentration, ~lnd specilic energy of interaction between powder surface and dispersant are given in Table I. h ix evident rrom the table that the computed specific interaction energy indicates strong adsorption of both the dispersauts on alunlirla powder surface. But the slightly higher value of - AG'p iu the ease of the TAC system as compared with Darvan C supports the more ctl'cctivcness of TAC leading to higher solid loading. It was found that when the optimally solidTaNc L The alun'tif4il shlrr~ chard¢leristi~;~ v,'hh and ;'*ithotit dispcrs;mts N) ~il~nl
( 'OlICC'nII'll[IOn
{ ppm) No dispcr~lnl lA("
2tlUt}
l)ar~ an (2
1250
pll
Apll. p A(g~ I R F Illlil~,}
g.~) ~.92 41) 41.23 27
625 6.17
R. AL ,.lllkl~.~-io-c! uL
¢~'olloh~ SttJ?htt~..s +l: Phvsil'ochem. EII~. +lspgcts 133 ¢ /99,'¢) 41=-47
loaded dispersed alumina-slurry was granulated by freeze-drying and spray-drying techniques, the former resulted in compact granules compared with that of the latter. ~ls illustrated in Fig. 5. This distinct characteristic of the granules has been
45
reflected in their green and sintered densities (Table 2). The sinlers of freeze-dried granules indicated dense and uniform mierostructure compared with that of spray-dried granules (Fig. 6). The microstructure of the sintered freeze-dried sample
Ib) Fi~. "~ Sc~mning el¢clron micrt~graphs of ahunma powder: (~1) f~¢zc dried ~md I b) s nra', dried.
46
R. M. Anklekar el aL / Colloids Sur]~t~z's A: Phlwicochem. Eng. Aspects 133 (199,~) 41 47
{a)
(b)
Fig. 6. Sc;ptmingelectron micrographs of ~llulllina sinters: (aJ freezedried and ('ol spra3 dried. indicated cqniaxed grains, which are slightly larger compared with those of spray-dried sinlers, probably due to enhanced densilication and grain growth. The micrograph also indicates lower porosity in the case of the freeze-dried sample which was intcrgranular in both cases.
4. Conclusions
( I } For ACC alumina-HP powder, the optimu!'n concentration of TALC and DtLI+WLI+IC we:Is found to be 2000 and 1250 ppm, respectively. (2) The optimun+~ solid loading was found to be
R. ,11 ,.tnkh.kaJ" el uL ? ('olloid~ SlaJ~t~'es.,t: Physi~'odliw~ /:.'n~. A~7~ect~ 133 (1998) 41 47
Table 2 Grc~n and sintercd densities of spray dri0d and I¥ccze dried grannies Technique Freeze dr$ing Spray drying
(ircen density I g tin "~)
t g em ~)
Sintered dei'~sity
2.31 2.ll6
3.92 386
50 v o l . % using T A C , ag~tinst 40 vol.% in the c a s e o f D a r v a n C , still m a i n t a i n i n g the p u m p a bility o f the a l u m i n a slurry. T h e b e t t e r effectiveness o f T A C c o m p a r e d w i t h D a r v a n C is d u e to a h i g h e r shift in A p H , p vahte. ( 3 ) T h e f r e e z e - d r y i n g tcchniqtte yicldcd c o m p a c t g r a n u l e s c o m p a r e d with spray d r y i n g w h i c h resulted in h i g h e r sintered densities with a m o r e unit\'~rm UliCrostrl.lCltlr¢.
Acknowledgment T h e a u t h o r s g r a t e f u l l y a c k n o w l e d g e the technical assistance p r o v i d e d by C G C R I , C a l c u t t a , for zel~l p o t e n t i a l nle~tsurenlents.
References [ I ] S. Somiya (Ed,). Adwmced fechnic~d ('er~m~ics. Aclldcmic Press. Japall. 1989. 121 N. Icliinoxe ted.), Inlrl)duclion tn Fine Uenimics Applications hi Engineering. Wiley. Chicheslcr. 1983.
47
131 R. Morrell, Handbook of Properties of Technical and Engineering Ceramics. Part 2: Data Reviews, Section I High Alumina Ceramics. Her Majesty's Stationery Office, London, 1987. [41 R.W. Cahn, P. Haasen. E.J. Kramer tEals.), Materials Science and Technology: A Comprehensi'~e Treatment, M.V. Swain ted.), Vol. II. Slrtlcture and Properties of Ceramics, VCH Publishers. Weinheim. Germany. 1994. [5] WH. Rhodes. S. Natansohn, Pounders for Advanced Structural Ceramics. American Ceramic Society Bulletin 68 / 19891 18114. [6 IJ.S. Reed, Introduction to the Principles of Ceramic Processing. Wile~. New York, 1988. [7] M. Berg, in: G.Y. Ont~da. Jr.. L.L. Hench ( Eds.h Ceramic Processing Before Firing, Wile5, New York. 1978. [8] C,L Brayman. R.A.L Drew. Comparison of Milling Methods Ik~r Reactive Aluminas, ill: (i I,. Messing. E.R. Fuller, Jr.. t1 Hausner (Eds,). Ceramic Tn~.nsaetions. Vol. I, Ceramic Powder Science II B. The American Ceramic S~cicty, Ohio. 1988. [Y]J.T.G. Ovcrbeek, The Interaction Between Colloid Particles. in: HR. Kruyt I Ed.). Colloid Science. Elsevier. Amsterdam. 1052. I Ill] M.D. Sacks. tI.W. Lee. O.E. Rt~ias. Suspension Processing of SiC Whiskcr-Reinlbrced Ceramic Ct~mposiles. in: G.L. Messing. E.R. Fuller, Jr., H. Hausner (Eds.). Ceramic l'ransaciions. Vol, I. Ceramic Powder Science II A. The American Ceramic Society, Ohio, 1988. [11] R.G. Fh~rn, Surfiice Ft,rces and their Action in Ceramic Materials. Journal of the An~erican Ceramic Society 73 ( 19901 1117 [I 21 S.G. Malghan. R.S. Prcmachandran, P.T, PCI, Mechanistic Indcrstanding of Silicon Nilride Dispersion using Czltionic and Anitlnic Polyeleclrolytes, Powder Technology 79 ( 19941 43. [131 Pradip. On the Intcrprel;ition of Elcctroklnetic Behaviour Of Chenlisl)rbdlg Nurl~lclanl System. Transactions of hldian Institute of Melals 41 119881 15.
"
COLLOIDS -(:olloid~.ndS.ra~e~
ELSEVIER
S~FAC~S
i
A: Physicochernicaland EngineeringAspects 133( 1998) 41 47
Rheology of concentrated alumina suspension to improve the milling output in production of high purity alumina powder R. M . A n k l e k a r , S. A . B o r k a r , S. B h a t t a c h a r j e e , C. H . P a g e , A. K . C h a t t e r j e e * The ,4ssochtted (2"mt'l~t (~mp~tnies Lhnited, R~e~#'l'h and ('OtlSldl~ltl~T Dircvtorare, L. B. S. Marg, Thane-4fX)604. India
Accepted 10 February 1997
Abstract
In production of high purity alumina, milling is an important processing step during which particle size has to be reduced to sub-micron size using minimum energy input and avoiding contamination from the milling system. In this investigation, to increase the efficiency of the milling process, the effect of two anionic dispersants were studied in concentrated alumina slurry. The optimized slurry was granulated using freeze drying and the properties of granules and sinters were compared with that using spray drying. ~ 1998 Elsevier Science B.V. K~trword~': Alumina: Rhcology: Dispersant; Spray drying; Freeze drying: Simering
1. Introduction
High purity sub-micron size alumina powders are widely used in electronic and structural applications [I-4], e.g. ~ls substrates, cutting tools, nozzles, grinding media, bio-implants, coatings, etc. Each of the above applications demands specitic physieo-chcnrical characteristics of the powders, which in turn directly depends on the processing route employed [5,6]. The Research and Development Division of the Associated Cement Companies Limited (ACC) has put up a semi-commercial pilot plant for producing high purity alumina, zirconia and barium titanatc powders using wet chemical processes such as sol-gel, hydrothermal, co-precipitation, etc. The optimization of the process parameters becomes relevant l¥om the cost-effectiveness view point for commercialization of each of the above powders. In the manufacture of sub-micron high purity * Correspondingauthor. 11927-775798s19.011~' It198ElsevierScienceB.V.All rightsreserved. PII S0927-7757(971001111-6
alumina powder, "milling" is one of the important processing step to achieve the desired particle size distribution tbr specific applications. However, to achieve this in a cost-effective way, it is essential to optimize the mill output by maximizing the solid loading and minimizing the milling time, which in turn reduces energy consumption [7,8] and contamination I¥om the grinding system. In the present investigation, the theological behavior of concentrated slurries of alumina powder I HP grnde) has been studied using two dispersants with the aim of increasing the throughput of the mill by maximizing the solid loading but maintaining the slurry pumpable. The alumina slurry, using optimum dispersant concentration and powder loading, was subjected to freeze drying to obtain ready-to-press granules which could be sintered to higher density with improved microstructure compared with that obtained by spray drying.
42
R. M, Ankh'kar t't a£
Colloids S.rj~wes A: Phyxiea,t'hem. Eng. A,qwcts 133 (1998) 4l- 47
2. Experimental High purity alumina powder (ACC's H P grade) with alumina content of > 99.9% and having impurities Na,O, Fe_,O3, SiO_, and CaO typically 340. 130. 250 and 14{)ppm, respectively, was employed. This powder was wet milled for 10 h in a Sweco M-18 vibro-mill using 75 kg of 1/2in~xl/2 in alumina cylpebs maintaining 20 vol.% slurry concentration without any dispersant. The average particle size (~/~0) and BET surface area of the ground powder was found to be 0.72 l,tm and 9.5 m-'/g, respectively. Two anionic dispersants viz. tri-ammonium citrate (TAC) from s.d. Fine Chemicals, India, having molecular weight ~250 and Darwin C {Ammonium polymcthacrylate) from R. T. Vanderbilt Company, LISA, witb average molecular weight ~ 10 000 were used. To determine the optimum dispersant concentration, the alumina slurry was prepared by mixing 125 g of the powder in 75 ml of DM water ( ~ 3 0 vol.%), containing different amounts of the above dispersants, in a roller mill for 8 h and measuring the viscosity of the slurry as a function of shear rate using a concentric cylinder rotational viseometer (Haake VT 5011l. The solid loading experiments were done by increasing the powder content in the slurry at the optimum dispersant concentration until the slurry was found to be pumpable 1< 500 mPa s at 11.7 s ~). The relative viscosity 01,~0 for the particle liquid suspension was obtained using the relationship 'llre[=q~.=/qllq
quantity of alumina (1 kg) was equilibrated at 25:C with additive solutions of different initial concentrations in a ball mill for 8 h. The suspension was centrifuged and the additive concentration in the supernatant was determined gravimetrieally. The zeta potential measurements were carried out using Zeta-Meter 3.0+ {Zeta-Meter Inc., USA ). The electrokinefic mobilities of the powders were determined by making the powder suspension (4 wt%) and milling for 6 h followed by centrifugation, and the supernatant dilute suspension wets taken for measurement. The zeta potentials were determined from electrophoretic mobility values using the Helmhohz Smoluchowski equation [9]. The slurry with optimum solid loading wets spray dried using a laboratory Buchi 190 mini spray-dryer to study the effect of the granulation technique on the sinterability of the ahtmina. The same slurry was also freeze dried under a pressure of <0.2 mbar and a temperature of - 4 0 C using a Life Science Laboratories" Lyolab F-3052 Lab freeze-dryer. The granules obtained by spray-drying and freeze-drying techniques were compacted at 150MPa and sintered at 1550 C for 2h. The nmrphology of the spray-dried and freeze-dried granules and sinters were observed using Jeol's JSM-5400 scanning electron microscope (SEM). The density of the sintcred pellets was measured by the displacement method using Archimedes" principle employing Mettlcr bahmcc and density determinatiou kit ME-33360.
{I)
where 0~,s is the suspension viscosity and iltiq is Ibe viscosity of suspension liqtfid medium. The dispersion characteristics of the above slurries were studied by measuring the pH using a Systronics System 361 pH meter. The particle size distribution of the vibro-milled slurry was measured using the Sedigraph 5100, Micromeritics Inc., USA. The adsorption isotherm of the additive on alumina particle wets determined by monitoring the change in concentration of the adsorbate in the suspension bclbrc and after equilibration with the adsorbent. For this purpose, a sallicient
3. Results and discussion The effect of concentration of TAC and Darvan C dispersants on the theology of the alumina slurry is shown in Fig. 1. As the dispersant concentration increases, the pseudoplasticity of the slurry was observed to decrease proportionately, resuhing in close to Newtonian behavior at higher concentrations, thereby suggesting good dispersion of the slurry. The optimum concentration o f T A C dispersant beyond which there is no change in the viscosity-shear rate curve for maintaining the pumpability of the slurry {viscosity <5(10 mPa s
R. M .,Inklekar et ol. , Colhfid~ St,J~wes A." Phys'icochem. Eng.
son
8oo
r
-.
E
As;pt'ct~133 (1998) 41-47
.E
t,13 0
son\ ~, \
750
500
2oo
~--
=< 2sopr-
1250
\
I I I)111|| J J IIIII 10 1 O0 SHEAR RATE
.1000
(S -] )
0 J 1
~.
,,,\
\~
~-
0
43
•
J } l l l l ~ 'I ! 0 SHEAR
I
J lll[l~'~I~ 1 O0 RATE IS
1000
-1)
Fig. I. The etl~ct of dispcrsant concentration o. the rheolog.~of the alumina slur~ 1311vol.%}: [al Ti&Cand (b) Darvan C. The .,,ahleShldic~llcdzlre ill ppm, NI) denoles no dispersant, and Ihe doned line is oblaincd by extrapolation. at 11.7 s ') was found to be 2000 ppm. However, when Darvan C was used instead of TAC, the wdue of optimum dispersant concentration was observed to be 1250 ppm. These optimized values have also been corroborated from adsorption isotherms (Fig. 2). The difference in the optimized values for the two anionic dispersants may be attributed to its chemical nature, i.e. TAC is a low molectdar weight dispersant while Darvan C is a long-chain polymer. Thus, both electrostatic and steric stabilization mechanisms are expected to
IE
....i
DISPERSANT CCI%K~EN TRATION
Fig. 2. Adsorpti.n i:,olhennsof alumina slurry employingTAC and Darvan ('.
operate in the case of Darvan C, while it should be predominantly electrostatic in the case of TAC dispersant. The effect of solid loading on the rheology of the alumina slurry using optimized dispersant concentrations of TAC and Darvan C is illustrated in Fig. 3. Although as high as 50 vol.% solid loading has been reported [10], not much detail on the slurry preparation, optimum concentration, etc. has been mentioned. Our studies indicate that at the optimized concentrations of TAC and Darvan C, the maximum solid loading obtained was 50 and 40vo1.%, respectively, keeping the slurry pumpable (viscosity < 500 mPa s at 11.7 s- t). This may net be due to bridging flocculation as it generally occurs in the case of very high molecular weight polymers at much lower concentrations in a poor solvent medium [11]. However, the zeta potential vs pH curve for alumina slurry (Fig. 4) without dispersant, and with optimum TAC and Darvan C concentrations indicates that the difference between the slurry pH and the effective pHil=, (ApHtEp) is more than 2.5 for both the dispersants. This suggests that both the dispersants yield well dispersed slurries [12]. Further, the higher wdue of ApHIEr for TAC (4.0) as compared with Darvan C (2.7) suggests more electrical
R, ;%I ,4n.Uekt#" ~'1 a/
44
('oll,#dr Sl#filce,* el." I'hrsicodwm. F,n¢, ,la?e¢t.~" 133 1998~ 41 47
.-.g,4.ao~-
(a)
15O0
(b)
~ 200 >-
--
~-
50
\
\
N 24o s
1201--
'\.+900
, &0
600 30
L ' ' ' ' ' ' ~ ~ A0
_~
,.y
\\_
300
d_ 10
o
1 oc 11000 SHEAR ForTE ( S - )
10
100
11000
SHEAR RATE ( S - )
Fig. 3. The ett~cl of mlid loadingusingoptimizeddi~pcrsanlconccntralkm on Ihc rhcolog~ of the alamina slum: a) FA("(2000 ppm), and (b) Darvan (" 11250ppml. The valncs indicated nre in ",ol c~.
,z
a
Fig. 4. The ZClapOlelllialof attllllitl;Ipox~derils zt ftmclit}n~.1 pll: (a) no di~pcrsant: (b) 1250ppm of I)dlx;lll ('; iltld (C) 211{10ppm ol 'rA(.'. double-layer repulsion [13] resulting in better dispersion. The specific energy f interaction betweert powder surface and dispersant was also evaluated using the following equation [13], A p H . v = 1.(1396(' o ex p( -- a ~ p
R 1" )
(2 )
where ApHu: P is the shill in the iso-elccric point at the dispersion conccnlrmlou C,, and -Af~,~p
represents the corresponding specific energy o f irReractiol'l between the ceramic powder surface and the dispersam. R mid T arc the standard gasconstant and temperature in Kelvin, respectively. The inthtencc or dispersams on the suspension characteristics such as pit, shift in the iso-elecric point at the optirnurn dispersion concentration, ~lnd specilic energy of interaction between powder surface and dispersant are given in Table I. h ix evident rrom the table that the computed specific interaction energy indicates strong adsorption of both the dispersauts on alunlirla powder surface. But the slightly higher value of - AG'p iu the ease of the TAC system as compared with Darvan C supports the more ctl'cctivcness of TAC leading to higher solid loading. It was found that when the optimally solidTaNc L The alun'tif4il shlrr~ chard¢leristi~;~ v,'hh and ;'*ithotit dispcrs;mts N) ~il~nl
( 'OlICC'nII'll[IOn
{ ppm) No dispcr~lnl lA("
2tlUt}
l)ar~ an (2
1250
pll
Apll. p A(g~ I R F Illlil~,}
g.~) ~.92 41) 41.23 27
625 6.17
R. AL ,.lllkl~.~-io-c! uL
¢~'olloh~ SttJ?htt~..s +l: Phvsil'ochem. EII~. +lspgcts 133 ¢ /99,'¢) 41=-47
loaded dispersed alumina-slurry was granulated by freeze-drying and spray-drying techniques, the former resulted in compact granules compared with that of the latter. ~ls illustrated in Fig. 5. This distinct characteristic of the granules has been
45
reflected in their green and sintered densities (Table 2). The sinlers of freeze-dried granules indicated dense and uniform mierostructure compared with that of spray-dried granules (Fig. 6). The microstructure of the sintered freeze-dried sample
Ib) Fi~. "~ Sc~mning el¢clron micrt~graphs of ahunma powder: (~1) f~¢zc dried ~md I b) s nra', dried.
46
R. M. Anklekar el aL / Colloids Sur]~t~z's A: Phlwicochem. Eng. Aspects 133 (199,~) 41 47
{a)
(b)
Fig. 6. Sc;ptmingelectron micrographs of ~llulllina sinters: (aJ freezedried and ('ol spra3 dried. indicated cqniaxed grains, which are slightly larger compared with those of spray-dried sinlers, probably due to enhanced densilication and grain growth. The micrograph also indicates lower porosity in the case of the freeze-dried sample which was intcrgranular in both cases.
4. Conclusions
( I } For ACC alumina-HP powder, the optimu!'n concentration of TALC and DtLI+WLI+IC we:Is found to be 2000 and 1250 ppm, respectively. (2) The optimun+~ solid loading was found to be
R. ,11 ,.tnkh.kaJ" el uL ? ('olloid~ SlaJ~t~'es.,t: Physi~'odliw~ /:.'n~. A~7~ect~ 133 (1998) 41 47
Table 2 Grc~n and sintercd densities of spray dri0d and I¥ccze dried grannies Technique Freeze dr$ing Spray drying
(ircen density I g tin "~)
t g em ~)
Sintered dei'~sity
2.31 2.ll6
3.92 386
50 v o l . % using T A C , ag~tinst 40 vol.% in the c a s e o f D a r v a n C , still m a i n t a i n i n g the p u m p a bility o f the a l u m i n a slurry. T h e b e t t e r effectiveness o f T A C c o m p a r e d w i t h D a r v a n C is d u e to a h i g h e r shift in A p H , p vahte. ( 3 ) T h e f r e e z e - d r y i n g tcchniqtte yicldcd c o m p a c t g r a n u l e s c o m p a r e d with spray d r y i n g w h i c h resulted in h i g h e r sintered densities with a m o r e unit\'~rm UliCrostrl.lCltlr¢.
Acknowledgment T h e a u t h o r s g r a t e f u l l y a c k n o w l e d g e the technical assistance p r o v i d e d by C G C R I , C a l c u t t a , for zel~l p o t e n t i a l nle~tsurenlents.
References [ I ] S. Somiya (Ed,). Adwmced fechnic~d ('er~m~ics. Aclldcmic Press. Japall. 1989. 121 N. Icliinoxe ted.), Inlrl)duclion tn Fine Uenimics Applications hi Engineering. Wiley. Chicheslcr. 1983.
47
131 R. Morrell, Handbook of Properties of Technical and Engineering Ceramics. Part 2: Data Reviews, Section I High Alumina Ceramics. Her Majesty's Stationery Office, London, 1987. [41 R.W. Cahn, P. Haasen. E.J. Kramer tEals.), Materials Science and Technology: A Comprehensi'~e Treatment, M.V. Swain ted.), Vol. II. Slrtlcture and Properties of Ceramics, VCH Publishers. Weinheim. Germany. 1994. [5] WH. Rhodes. S. Natansohn, Pounders for Advanced Structural Ceramics. American Ceramic Society Bulletin 68 / 19891 18114. [6 IJ.S. Reed, Introduction to the Principles of Ceramic Processing. Wile~. New York, 1988. [7] M. Berg, in: G.Y. Ont~da. Jr.. L.L. Hench ( Eds.h Ceramic Processing Before Firing, Wile5, New York. 1978. [8] C,L Brayman. R.A.L Drew. Comparison of Milling Methods Ik~r Reactive Aluminas, ill: (i I,. Messing. E.R. Fuller, Jr.. t1 Hausner (Eds,). Ceramic Tn~.nsaetions. Vol. I, Ceramic Powder Science II B. The American Ceramic S~cicty, Ohio. 1988. [Y]J.T.G. Ovcrbeek, The Interaction Between Colloid Particles. in: HR. Kruyt I Ed.). Colloid Science. Elsevier. Amsterdam. 1052. I Ill] M.D. Sacks. tI.W. Lee. O.E. Rt~ias. Suspension Processing of SiC Whiskcr-Reinlbrced Ceramic Ct~mposiles. in: G.L. Messing. E.R. Fuller, Jr., H. Hausner (Eds.). Ceramic l'ransaciions. Vol, I. Ceramic Powder Science II A. The American Ceramic Society, Ohio, 1988. [11] R.G. Fh~rn, Surfiice Ft,rces and their Action in Ceramic Materials. Journal of the An~erican Ceramic Society 73 ( 19901 1117 [I 21 S.G. Malghan. R.S. Prcmachandran, P.T, PCI, Mechanistic Indcrstanding of Silicon Nilride Dispersion using Czltionic and Anitlnic Polyeleclrolytes, Powder Technology 79 ( 19941 43. [131 Pradip. On the Intcrprel;ition of Elcctroklnetic Behaviour Of Chenlisl)rbdlg Nurl~lclanl System. Transactions of hldian Institute of Melals 41 119881 15.