Anais do 43º Congresso Brasileiro de Cerâmica
2 a 5 de junho de 1999 - Florianópolis – S.C.
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SLIP PROPERTIES AND PRESSURE FILTRATION OF ALUMINA.
L.B. Garrido
CETMIC (Centro de Tecnología de Recursos Minerales y Cerámica). Cno. Centenario y
506. C.C.49 (1897) M.B. Gonnet. Prov. de Buenos Aires. ARGENTINA.
FAX:54-
221-4710075.
E-mail:garridol@netverk.com.ar
ABSTRACT
Pressure filtration of oxide and advanced ceramic materials has recieved a great
attention in recent years.
In this work, the results of a pressure filtration experiments using aqueous alumina
suspensions (A-16SG, Alcoa Inc., USA) up to 50 vol% solid concentration are
presented. Concentrated suspensions of different degree of dispersion were prepared
to study the effect of the applied pressure on the relative density ( % of the theorical) of
dried compacts obtained at 4 to 20 MPa.
Slip properties were evaluated by rheological measurements and correlated with
the characteristics of the compacts.
Key words: Alumina, pressure filtration, rheological properties.
INTRODUCTION
Highly loaded and stable suspensions are used to produce green ceramic bodies
that showed high density and homogeneous and fine microestructures. However,
previous studies reported that low viscosity suspensions provided low filtration rates
and same mass segreggation during the consolidation step of ceramic multicomponent.
To overcome these problems either solid concentration is usually increased or surface
potentials between particles can be controlled or manipulated to produce a high density
consolidates. This is the case of concentrated suspensions in a flocculated or
Anais do 43º Congresso Brasileiro de Cerâmica
2 a 5 de junho de 1999 - Florianópolis – S.C.
coagulated state
(1)
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. Thus, optimization of the slip properties is required for suitable
control of the casting process.
In this work, results of pressure filtration experiments using concentrated aqueous
alumina suspensions up to 20 MPa are presented. The effect of solid volume
concentration, state of dispersion and the desaggregation method on the relative
density of alumina green compacts is studied.
MATERIALS AND METHODS
Materials. Alumina powder used is a fine and pure -Al2O3 (A-16 SG, Alcoa Inc., USA)
and contains Na2O and MgO as main impurities (2,3).
The mean particle size determined was 0.4 m and the specific surface area 9.5 m 2/g
(4)
The isoelectric point has been determined to be 9
(4)
.
Suspension preparation and characterization. Dispersed 20-50 vol% suspensions were
prepared by adding the powder to aqueous solutions containing HCl or the dispersant.
The suspensions were ultrasonicated for 25 min.
The state of dispersion was varied using the procedure described elsewhere(1) .
Dispersed 20 vol% were flocculated by changing the pH to 9 or coagulated by 1.8 M
NH4OH.
Dispersed suspensions containing 50 vol % solid concentration were prepared by
attrition milling to compare the effect of the desaggregation method on the compact
characteristics.
Flow curves of the suspensions were obtained using a rotational viscometer
Haake RV3 of coaxial cylinders with NV and MVIP measure systems at 25°C.
Pressure filtration and characterization of the compact. Pressure filtration was done on
a metallic sintered porous filter with a wet filter paper placed on it. The pressure was
applied from the top and the liquid moved through the porous filter. Pressure was
manually controlled until no change in the pressure indication was observed and kept at
a constant value for a minimum of 25 min.
Cylindrical compacts were dried 24 hs at room temperature and then at 110 C for 24
hs. Relative density (% of the theorical) was measured by Hg inmersion.
Anais do 43º Congresso Brasileiro de Cerâmica
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Particle size distribution was used to examine the tendency of the particles to
segregate. Some selected compacts dried at room temperature were sliced in three
parts. Various grames of solid were resuspended at pH=4 at a solid concentration
adequate for Sedigraph analysis.
RESULTS AND DISCUSSION
Figure 1 shows the relative density (% of the theorical) as a function of the applied
pressure for green compacts obtained from 20 vol% alumina suspensions deflocculated
R e la tiv e d e n s ity ,% th e o r ic a l
(pH=4), flocculated (pH=9) and coagulated with 1.8 M NH4OH.
70
65
60
55
50
0
5
10
15
20
Applied pressure, MPa
pH=4
pH=9
coagulated
Figure 1: Relative density of green compacts obtained from alumina suspensions with
different degree of dispersion vs applied pressure.
Density is dependent of the applied pressure for coagulated and flocculated
suspensions. At 16 MPa the maximum density is reached for the unstable suspensions.
In pressure filtration the suspension is subjected to a compressive stress. Previous
studies reported that strongly attractive interactions resulted in a particle network that
supported some stress up to a critical value. Once this critical stress value was exceded
the network consolidated to a higher volume fraction with a higher critical stress.
Figure 2 shows the flow curves for alumina suspensions (20 vol %) with different
degree of dispersion. Low viscosity suspension of Newtonian flow behaviour was
obtained at pH=4 where the particle had a high surface charge at low electrolyte
concentration (high zeta potential).
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Anais do 43º Congresso Brasileiro de Cerâmica
2 a 5 de junho de 1999 - Florianópolis – S.C.
At high electrolyte concentration, zeta potential decreased due to double layer
compression and particles adhered weakly as attraction was significative.This adhesion
is potentially reversible. Hence, the suspension showed a typical shear thinning
behaviour of coagulated suspensions.
120
pH=9
S h e a r s tr e s s , P a
100
80
60
40
coagulated
20
pH=4
0
0
100
200
300
400
500
600
Shear rate, s-1
Figure 2: Shear stress vs shear rate curves for alumina suspensions (20 vol%) with
different degree of dispersion.
An strong atractive potential is obtained by changing the pH to 9 due to the
reduction of the charge of alumina particle and produced a flocculated suspension. The
flow curve exhibited high shear stress values and a hysteresis loop between the two
branches of the flow curve that are related with the formation of unstable structures.
The liquid is immobilized in the flocs resulting in a higher effective solid content and
therefore the viscosity was higher than the one of the deflocculated suspension. Shear
thinning of unstable suspensions was attributed to the break down of the agglomerates
and the release of the inmobilized liquid.
Flow curves were fitted with the Casson model:
 1/2=  c 1/2+ ( c D)1/2
where  is the shear stress [Pa], D is the shear rate [s -1] and 
c
and 
Casson yield stress and viscosity respectively. In agreement with Guo et al.
c
indicates strong flocculation whereas low  c shows low solid content.
c
are the
(5) a
high 
Anais do 43º Congresso Brasileiro de Cerâmica
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Tabla 1:Viscosity and yield stress Casson model parameters (best fit of flow curves of
Fig 2 and 4)
Suspension
Viscosity yield stress Correlation
mPas
mPa
0
Coeficient
Defl., pH=4, 20%
1.539
0.985
Coagulated
3.9
8095
0.920
Flocculated, pH=9
13.75
60410
0.987
Defl., pH=4, 27%
2.544
43.8
0.986
Defl., pH=4, 36%
4.751
216
0.987
Defl., pH=4, 50%
22.72
8367
0.998
Density behaviour correlated well with slip properties (viscosity and yield stress
Casson parameters). Dispersed suspensions of low viscosity and small yield stress
could be packed to a higher density compact than the unstable suspensions.
Futhermore, the effect of the degree of dispersion on the density was similar to that
previously reported (1).
Figure 3 shows relative density of green compacts obtained from alumina
suspensions deflocculated (pH=4) for solid volume concentrations up to 50 vol% as a
function of the applied pressure. Green densities of the compacts were nearly constant
with the applied pressure. Values close to 64 % at 8 MPa
were obtained with
increasing solid concentration from 20 to 37 vol%. A decrease of 3% was obtained
with the 50 vol% suspension. No deformation or cracking was observed after drying.
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R e la tiv e d e n s ity ,% th e o r ic a l
Anais do 43º Congresso Brasileiro de Cerâmica
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65
60
55
50
0
5
10
15
20
Applied pressure, MPa
20 v ol%
27 v ol%
37 v ol%
50 v ol%
Figure 3: Relative density of green compacts obtained from deflocculated at pH=4
alumina suspensions with different solid volume concentrations vs applied pressure.
40
S h e a r s tr e s s , P a
50
30
20
10
37
26
20
0
0
100
200
300
400
500
600
Shear rate, s-1
Figure 4: Shear stress vs shear rate curves for alumina suspensions with different solid
concentration ( vol%).
Figure 4 shows the flow curves of alumina suspensions with different solid volume
concentration. The apparent viscosity at 500 s-1 of suspensions increased from 8 to 79
mPas with increasing solids content from 37 to 50 vol%.
The concentration of HCl in solution required to electrostatically stabilize the
suspension increased from 0.04, 0.1, to 0.2 M for solid volume concentrations of 20, 37
and 50 % respectively. An increase in the concentration of counterions (high ionic
strength of the solution) decreased the zeta potential and consequently increased the
viscosity of the suspension.
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Shear thinning behaviour was observed for suspensions containing more than 20
vol% of alumina.
Shear thinning of stable concentrated suspensions was previously attributed to the
perturbation of the suspension structure at rest by the viscous forces at high shear
rates.
The Casson model showed a good fit of the flow curves (Table 1). Pressure
filtration of high solid content suspension of low viscosity give compacts with the
highest green density. Green densities decreased as the viscosity and yield stress
Casson parameters increased .
Compacts prepared from atrittion milled 50 vol% suspensions (pH=4) showed
poor characteristics than the obtained by ultrasonication. Lower densities and
deformation after demoulding were found. Significative differences occurred in the
particle size diistribution curves of the powders. A decrease in the content of fine
particles was seen from the top to the bottom of the compact with respect to the
distribution of the ultrasonicated alumina. The mean diameter d 50 increased from 0.38,
0.4 and 0.46 m to 0.42, 0.5 and 0.7m for the top to the bottom. Higher d 50 than
0.4m for the milled suspension was due to a decrease in efficiency of the attrition as
the viscosity of the suspension increased. Stabilized suspensions containing high
amount of agglomerate particles (not completely desaggregated prior to the pressure
filtration experiment) were loosely packed at the bottom of the compact where the
content of coarse particles was high. As expected, at high solid concentration
deflocculated suspensions of coarse particles showed a strong dilatant behaviour.
Dispersed 50 vol% suspensions were dificcult to prepared with HCl and the effects
of adding a polyelectrolyte (ammonium polyacrylate AP) and citric acid AC to achieve
stability by an electrosteric stabilizing mechanism were examined. Figure 5 shows the
relative density of green compacts prepared from 50 vol % deflocculated suspensions
at pH=4 and with different amounts of dispersants for applied pressures up to 20 MPa.
Comparison of green density values with those obtained from electrostatically stabilized
suspension showed that the use of CA was more effective. The additon of AP caused a
small effect on the density.
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R e la tiv e d e n s ity ,% T h e o r ic a l
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65
60
55
50
0
5
10
15
20
Applied pressure, MPa
pH=4
0.5%AP
0.44%AP
0.3%CA
Figure 5: Relative density of green compacts obtained from 50 vol% suspensions at
pH=4 and deflocculated with 0.44-0.5 wt% of ammonium polyacrylate and 0.3 wt%
citric acid vs applied pressure.
Figure 6 shows the shear stress vs shear rate curves for 50 vol% suspensions with
deflocculant.
pH=4
40
S h e a r s tr e s s , P a
0.5%AP
30
0.4%AP
20
0.3%CA
10
0
0
100
200
300
400
500
600
Shear rate, s-1
Figure 6: Shear stress vs shear rate curves for 50 vol % alumina suspensions with
different dispersant.
The addition of 0.44 wt% of AP slightly improved the degree of dispersion of the
particles as lower apparent viscosity values (63 mPas at 500 s -1) than the ones of the
suspension deflocculated with HCl ( 80 mPas) were obtained. The flow curve exhibited
a hysteresis loop indicating that the suspension was not fully deflocculated. Further
Anais do 43º Congresso Brasileiro de Cerâmica
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additions of AP increased the apparent viscosity to 77 mPas probably due to the
presence of an excess of polyelectrolyte. At pH=9.45 (above iep pH ) the surface
developed AlO- groups which enhanced electrostatic repulsion between the alumina
surface and the polyelectrolyte. Then, low adsorption of polyacrylate anion on the
alumina surface groups and consequently a higher amount of free polyelectrolyte
produced a decrease in the degree of dispersion.
Low viscosity suspension of shear thinning behaviour was obtained with 0.3 wt %
of citric acid (pH=8). At pH 8 more Al OH2+ groups existed on the alumina surface that
increased the COO- adsorption and improved the degree of dispersion. Hibder et al. (6)
proposed that citrate stabilized alumina suspensions by a combined electrostatic and
steric mechanism at very small interparticle separations.
Green density values of 65 % were obtained by pressure filtration 50 vol % slip with
viscosity of 29 mPas. Salomoni et al.(7) reported densities of green bodies cast at 3.4
MPa of 65.2%.
CONCLUSIONS
Low viscosity suspensions with a small
yield stress due to greater repulsive
interparticle potentials and an optimum particle size distribution produced green
compacts with the highest relative density. Green densities decreased as the viscosity
and yield stress Casson parameters increased .
Compacts obtained from stabilized suspensions containing high amount of
agglomerate particles (not completely desaggregated prior to the pressure filtration
experiment) were loosely packed at the bottom where the content of coarse particles
was high as indicated by the particle size distribution measurements. Some of them did
not retain their initial shape after demoulding.
Compacts resulting from weakly and strongly flocculated suspensions were less
dense and the relative density increased with increasing the applied pressure.
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REFERENCES
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(1991) 2201.
(2) C.M. Incorvati, D.H. Lee, J.S. Reed, R.A. Condrate, Am. Ceram. Soc. Bull. 76, 9
(1997) 65.
(3)J.F. Kelso, J.A. Ferrazolli, J. Am. Ceram. Soc. 72, 4 (1989) 625.
(4) P.A. Smith, R.A. Haber, J. Am. Ceram. Soc. 78, 7 (1995) 1737.
(5)L. Guo, Y. Zhang, N. Uchida, K. Uematsu, J. Am. Ceram. Soc. 81, 3 (1998) 549.
(6) P.C. Hibder, T.J. Graule, L.J. Gauckler, J. Am. Ceram. Soc. 79, 7 (1996) 1857.
(7)A. Salomoni, C. Galassi, E. Roncari, J. Stamenovik, Fourth Euro Ceramic, Ed. C.
Galassi. Gruppo Editoriale Faenza Editrice S.p.A, Italia (1995) ,Vol.I, p453.