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Slip casting of zircon by using an organic surfactant
Ceramics International 17 (1991) 37-40
Slip Casting of Zircon by Using an Organic Surfactant R. M o r e n o , J. S. M o y a & J. Requena Instituto de Cer/tmica y Vidrio, CSIC, Arganda del Rey, Madrid, Spain (Received 4 October 1989; accepted 28 May 1990)
Abstract: The effect of an organic surfactant on slip casting of a zircon powder has been studied. Zeta potential measurements have been made for zircon suspensions with different surfactant concentrations and pH values. Rheological properties have been determined in order to achieve the best casting conditions. Drying behaviour, densification evolution, and microstructural observations of the sintered compact are also reported.
1 INTRODUCTION
0"16..Its specific surface area (N 2 adsorption, BET method) is 7.4 m 2 g-~. Particle size distribution has been determined by X-ray sedimentation analysis (Sedigraph, Micromeritics Inc., USA); the powder has a mean particle size of 1"1 pm with 97% below 4pm. Suspensions have been prepared by the addition of the proper amount of deionized water and deflocculant (Dolapix PC 33, Zschimmer-Schwarz, WG) to give a solids content of 60 and 70% wt. Electrophoretic behaviour has been measured on concentrated s.uspensions (30% wt) using a mass transport analyser (Micromeritics, Inc., USA). Two measurements were performed for different conditions of pH and surfactant concentration. Viscosity measurements have been made for a broad shear rate domain ranging up to 1000s -~ with a rotational viscosimeter (Haake Rotovisco RV 20, Karlsruhe, WG) coupled to a personal computer by means of a Haake rheocontroller RC 20. The temperature of the suspensions was maintained constant at 25 _+0.1°C with a .Haake circulator. The slips have been cast into plaster of Paris moulds (water to plaster ratio = 70/100) as both drain cast crucibles and solid cast bars. Observations have been made on the crucibles concerning drain properties such as wall formation rate. Sintering behaviour has been studied on solid cast bars.
Zircon is an abundant raw material with low impurities content, relatively low thermal expansion (4-10-6°C -~) and very high chemical stability versus glassy phase and molten slag. Because of these properties zircon-dense bodies are being considered as excellent candidates for structural applications in severe conditions (i.e. continuous steel casting, glass fibre technology, etc.). The slip casting of zircon powders and the influence of iron impurities on rheology and casting were studied by the present authors in previous investigations. ~'2 In these works the deflocculation of zircon suspensions was controlled by adjusting the pH. In the present work the deflocculation has been reached by using an organic surfactant. Also the control ofpH by adding NaOH has been studied, considering both electrostatic (pH) and steric (surfactant) interactions for the obtention of a welldispersed suspension.
2 EXPERIMENTAL A commercial zircon powder (Opacir S, Quiminsa, Spain) has been used as starting material. Chemical analysis revealed the presence of the following main impurities (% wt): AI20 a, 0.9; TiO2, 0.2 and Fe203, 37
Ceramics International 0272-8842/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
38
R. Moreno, J. S. Moya, J. Requena
Once the cast bodies have been removed from the moulds, they have been dried in air. Green densities have been measured by the Hg immersion method. Dried cast bodies have been treated at different temperatures ranging from 1400 to 1600°C for 2 h with a heating and a cooling rate of 5°Cmin -~. Densification evolution has been studied with solid cast bars by measuring the relative density by Hg immersion. 3 RESULTS
AND
DISCUSSION
Zeta potential measurements have been made in order to determine the colloidal stability of zircon aqueous suspensions in the presence of an organic surfactant. Figure 1 shows the zeta potential values of zircon suspensions as a function of the surfactant concentration. The increase of the surfactant concentration leads to a higher fluidity and an increase of pH. The zeta potential increases with surfactant concentration up to a maximum of 6% wt; from this value, the zeta potential decreases again. From Fig. 1 it can be thought that a concentration > 4 % wt is the best in order to obtain a well-dispersed slip for casting. The rheological measurements have been performed for 60 and 70% wt solids loading with a deflocculant concentration of 4 % wt. In all cases thyxotropic behaviour is observed, and the hysteresis cycle is higher for increasing solids content. The suspensions with 60% wt solids loading were slip cast. However, when the cast bodies were exposed to air for drying, spontaneous cracking appeared (Fig. 2). A layer configuration is observed from the outer part of the piece (that in contact with the mould) to the inner part (in contact with air). This fact is due to a heterogeneous distribution of organic c o m p o u n d in the bulk as a consequence of the liquid removement through the mould, as can be deducted from the thermogravimetric curves plotted in Fig. 3. In
Fig. 2. Spontaneous cracking which appears during drying of zircon green bodies obtained from suspensions with 4%wt surfactant concentration. this figure the outer part presents a higher weight loss than the inner one (2-8% versus 1-8%). The difficulties in removal of the organic compound requires the use of lower surfactant concentration for the preparation of castable zircon suspensions. Temperature ( *C ) 100 i
200 i
300 i
¢00 i
~0 i
600 i
700 i
800 i
90O
1.0
o 1,5 .Jc -5O
--X
X
>
E i
2.0-
-t,I]
f
e
~
O
2.5-
2 -20 ¢J N -1(]
. . . . l
2
~ 3
Surfactont
Fig. !.
~ content
'
,
6
7
.,
(wt%)
Zeta potential of zircon suspensions versus surfactant concentration.
3.0-
Fig. 3. Weight loss versus temperature for both the inner x) and the outer (O) parts of the zircon green body obtained with 4% wt surfactant content.
Slip casting of zircon by using an organic surfactant
39 30
/
1°1,/~ 4.0-
2o E E
v
N_.I ~,a
-20.
lo
-10'
;
;
;
,o
,',
pH
/_...-~--,~-'7 0
,
,
;
,0
T (rain)
Fig. 4. Zeta potential values versus pH for surfactant concentrations of 0.5% (A), I% (O), 2% (x), 4% (&) and 6% wt ( 0 ) .
Fig. 6. Square of the wall thickness versus casting time for
For this reason the effect of pH variation on the colloidal stability has been studied in order to obtain well deflocculated suspensions with lower surfactant content by considering both steric and electrostatic stabilization mechanisms. Figure 4 shows the zeta potential values for different surfactant contents as a function of pH. As it can be seen, the addition of NaOH allows us to obtain significantly higher zeta potential values for lower surfactant contents ( < 2 % w t ) . Rheological measurements of zircon suspensions with 1 and 2% wt deflocculant concentrations and a solids loading of 60% wt reveal that viscosity decreases with increasing pH and the thyxotropy effect disappears. The apparent viscosity versus pH curves for those suspensions are plotted in Fig. 5 for a shear rate of 500 s- 1. Very low viscosities are obtained for pH > 10. Drain casting properties have been studied from the measurement of the wall thickness formation rate. The slip casting process is rate-controlled by the
diffusion of the suspension medium through the layer already cast. The cast thickness L is related to the casting time Tby L 2 = kT. 3 - 5 The wall thickness formation rate of 60% wt suspensions with 1 and 2 % w t deflocculant concentration and pH = 11 is shown in Fig. 6. These curves agree with those o f other authors who have studied slip casting kinetics 6- 8 and indicate a diffusion controlled process. H e r m a n n and Cutler 6 found an effective cast thickness at zero time for clay casting, whereas it is not observed in this case. The zero time thickness reported elsewhere is caused by flocculation of calcium ions from the plaster. 7 The strong effect of the surfactant concentration on casting rate can be seen: the higher the surfactant content, the lower the wall thickness rate. This is due to the contribution of the steric mechanism to the suspension stability. Neither cracking nor spalling are observed in the pieces obtained from zircon suspensions stabilized with 2% wt surfactant concentration and pH = 11. According to the previously reported data, high zeta potential, low viscosity and low wall thickness rate, the optimal conditions for slip casting zircon are those in which a 2% wt surfactant concentration and pH = 11 are employed. The green density of the dried pieces is reported in Table 1 for the differential casting conditions studied in this work. As expected, the higher green densities correspond to those conditions selected as being more favourable. This fact supports the criterion for the selection of the optimal conditions for casting. Once the best conditions for slip casting are known, the dried cast pieces have been heated at different conditions up to 1600°C for 2h soaking time. The relative density evolution is plotted in Fig. 7. A maximum density of 96% is reached at 1600°C.
50.
lZ
t,0.
O
0_ 3 o 2 Z
& 20,
10
6
';
1'o
,',
pH Fig. 5. Apparent viscosity versus pH of 60%wt zircon suspensions with surfactant concentrations of 1 and 2% wt.
60% wt zircon suspensions with 1 and 2%wt surfactant concentration and pH = 11.
R. Moreno, J. S. Moya, J. Requena
40 T a b l e 1.
G r e e n d e n s i t i e s o f slip cast z i r c o n bodies at different casting conditions
o•'100 x
cE Deflocculant conditions Per cent deflocculation
pH
4 2 1
8 11 11
Solids content 60% wt
51 "2 +_0'6 53-0 _+0"3 47"5 _+0"1
70% wt
52"7 + 0"1 51 "3 + 0-1 49-1 _+0"6
~
90
c •
80
0
7O
o/o/°-
/
o
J
I 1400
I 1500
I 1600
Temperature ( °C )
In spite of the the firing shrinkage being high (22%) no cracking was observed in any case. In a previous investigation 2 the slip casting of zircon was studied by controlling pH, considering also the effect of iron impurities on rheology and casting. In the present work a parallel route has been studied for slip casting zircon in which the powder washing is not required. The results obtained in this work show that an excess of organic surfactant can be deletereous for the obtention of slip cast bodies. Furthermore, a base addition improves the rheological properties of the suspension, and deflocculation results as a combination of electrostatic and steric mechanisms. In aqueous media the adsorption of ionogenic surface active agents leads to an increase in the double layer capacity provided that the adsorption process does not destroy the ionisation. In this way, anionic surfactants tend to increase the negative value of the stabilizing potential. For this reason, anionic surfactants are more effective in alkaline medium and cationic surfactants in acid medium. 9 The sintering studies reveal that no differences are observed for the zircon samples obtained for slip casting from pH controlled or surfactant stabilized suspensions. In both cases a similar trend is observed for the densification and microstructural evolution. A final relative density of 96% (theoretical) is obtained at 1600°C. At higher temperatures
Fig. 7.
Relative density of slip cast zircon versus temperature for soaking times of 2 h.
zircon decomposition takes place and Z r O 2 peaks appear in the corresponding X-ray diffraction spectra, probably as a consequence of the small amount of impurity present in the starting zircon powder. 1°
REFERENCES 1. MORENO, R., MOYA, J. S. & REQUENA, J., Slip casting of zircon. J. Mat. Sci. Let., 5 (1986) 127. 2. MORENO, R., MOYA, J. S. & REQUENA, J., Slip casting of zircon. Effect of iron-impurities. Ceramics International, 16 (1990) 115-19. 3. ADCOCK, D. S. & McDOWALL, I. C., The mechanism of filter pressing and slip casting. J. Am. Ceram. Soc., 40 (1957) 355. 4. TILLER, F. M. & TSAI, C., Theory of filtration of ceramics: I, Slip casting. J. Am. Ceram. Soc., 69 (1986) 882. 5. HAMPTON, J. H. D., SAVAGE, S. B. & DREW, R. A. L., Experimental analysis and modeling of slip casting. J. Am. Ceram. Soc., 71 (1988) 1040. 6. HERRMANN, E. R. & CUTLER, I. B., The kinetics ofslip casting. J. Am. Ceram. Soc., 61 (1962) 207. 7. WORRALL, W. E. & RYAN, W., The mechanism of slip casting. J. Br. Ceram. Soc., 1 (1964) 269. 8. DEACON, R. F. & MISKIN, S. F. A., Gamma-ray adsorption for the continuous observation of slip casting kinetics. Trans. Brit. Ceram. Soc., 63 (1964) 473. 9. PARFITT, G. D., Dispersion of Powders in Liquids, 3rd edn, Applied Science Publishers, Englewood, N J, USA, 1981. 10. PENA, P. & DE AZA, S., The zircon thermal behaviour: effect of impurities. Part I. J. Mat. Sci., 19 (1984) 135.
Slip Casting of Zircon by Using an Organic Surfactant R. M o r e n o , J. S. M o y a & J. Requena Instituto de Cer/tmica y Vidrio, CSIC, Arganda del Rey, Madrid, Spain (Received 4 October 1989; accepted 28 May 1990)
Abstract: The effect of an organic surfactant on slip casting of a zircon powder has been studied. Zeta potential measurements have been made for zircon suspensions with different surfactant concentrations and pH values. Rheological properties have been determined in order to achieve the best casting conditions. Drying behaviour, densification evolution, and microstructural observations of the sintered compact are also reported.
1 INTRODUCTION
0"16..Its specific surface area (N 2 adsorption, BET method) is 7.4 m 2 g-~. Particle size distribution has been determined by X-ray sedimentation analysis (Sedigraph, Micromeritics Inc., USA); the powder has a mean particle size of 1"1 pm with 97% below 4pm. Suspensions have been prepared by the addition of the proper amount of deionized water and deflocculant (Dolapix PC 33, Zschimmer-Schwarz, WG) to give a solids content of 60 and 70% wt. Electrophoretic behaviour has been measured on concentrated s.uspensions (30% wt) using a mass transport analyser (Micromeritics, Inc., USA). Two measurements were performed for different conditions of pH and surfactant concentration. Viscosity measurements have been made for a broad shear rate domain ranging up to 1000s -~ with a rotational viscosimeter (Haake Rotovisco RV 20, Karlsruhe, WG) coupled to a personal computer by means of a Haake rheocontroller RC 20. The temperature of the suspensions was maintained constant at 25 _+0.1°C with a .Haake circulator. The slips have been cast into plaster of Paris moulds (water to plaster ratio = 70/100) as both drain cast crucibles and solid cast bars. Observations have been made on the crucibles concerning drain properties such as wall formation rate. Sintering behaviour has been studied on solid cast bars.
Zircon is an abundant raw material with low impurities content, relatively low thermal expansion (4-10-6°C -~) and very high chemical stability versus glassy phase and molten slag. Because of these properties zircon-dense bodies are being considered as excellent candidates for structural applications in severe conditions (i.e. continuous steel casting, glass fibre technology, etc.). The slip casting of zircon powders and the influence of iron impurities on rheology and casting were studied by the present authors in previous investigations. ~'2 In these works the deflocculation of zircon suspensions was controlled by adjusting the pH. In the present work the deflocculation has been reached by using an organic surfactant. Also the control ofpH by adding NaOH has been studied, considering both electrostatic (pH) and steric (surfactant) interactions for the obtention of a welldispersed suspension.
2 EXPERIMENTAL A commercial zircon powder (Opacir S, Quiminsa, Spain) has been used as starting material. Chemical analysis revealed the presence of the following main impurities (% wt): AI20 a, 0.9; TiO2, 0.2 and Fe203, 37
Ceramics International 0272-8842/91/$03.50 © 1991 Elsevier Science Publishers Ltd, England. Printed in Great Britain
38
R. Moreno, J. S. Moya, J. Requena
Once the cast bodies have been removed from the moulds, they have been dried in air. Green densities have been measured by the Hg immersion method. Dried cast bodies have been treated at different temperatures ranging from 1400 to 1600°C for 2 h with a heating and a cooling rate of 5°Cmin -~. Densification evolution has been studied with solid cast bars by measuring the relative density by Hg immersion. 3 RESULTS
AND
DISCUSSION
Zeta potential measurements have been made in order to determine the colloidal stability of zircon aqueous suspensions in the presence of an organic surfactant. Figure 1 shows the zeta potential values of zircon suspensions as a function of the surfactant concentration. The increase of the surfactant concentration leads to a higher fluidity and an increase of pH. The zeta potential increases with surfactant concentration up to a maximum of 6% wt; from this value, the zeta potential decreases again. From Fig. 1 it can be thought that a concentration > 4 % wt is the best in order to obtain a well-dispersed slip for casting. The rheological measurements have been performed for 60 and 70% wt solids loading with a deflocculant concentration of 4 % wt. In all cases thyxotropic behaviour is observed, and the hysteresis cycle is higher for increasing solids content. The suspensions with 60% wt solids loading were slip cast. However, when the cast bodies were exposed to air for drying, spontaneous cracking appeared (Fig. 2). A layer configuration is observed from the outer part of the piece (that in contact with the mould) to the inner part (in contact with air). This fact is due to a heterogeneous distribution of organic c o m p o u n d in the bulk as a consequence of the liquid removement through the mould, as can be deducted from the thermogravimetric curves plotted in Fig. 3. In
Fig. 2. Spontaneous cracking which appears during drying of zircon green bodies obtained from suspensions with 4%wt surfactant concentration. this figure the outer part presents a higher weight loss than the inner one (2-8% versus 1-8%). The difficulties in removal of the organic compound requires the use of lower surfactant concentration for the preparation of castable zircon suspensions. Temperature ( *C ) 100 i
200 i
300 i
¢00 i
~0 i
600 i
700 i
800 i
90O
1.0
o 1,5 .Jc -5O
--X
X
>
E i
2.0-
-t,I]
f
e
~
O
2.5-
2 -20 ¢J N -1(]
. . . . l
2
~ 3
Surfactont
Fig. !.
~ content
'
,
6
7
.,
(wt%)
Zeta potential of zircon suspensions versus surfactant concentration.
3.0-
Fig. 3. Weight loss versus temperature for both the inner x) and the outer (O) parts of the zircon green body obtained with 4% wt surfactant content.
Slip casting of zircon by using an organic surfactant
39 30
/
1°1,/~ 4.0-
2o E E
v
N_.I ~,a
-20.
lo
-10'
;
;
;
,o
,',
pH
/_...-~--,~-'7 0
,
,
;
,0
T (rain)
Fig. 4. Zeta potential values versus pH for surfactant concentrations of 0.5% (A), I% (O), 2% (x), 4% (&) and 6% wt ( 0 ) .
Fig. 6. Square of the wall thickness versus casting time for
For this reason the effect of pH variation on the colloidal stability has been studied in order to obtain well deflocculated suspensions with lower surfactant content by considering both steric and electrostatic stabilization mechanisms. Figure 4 shows the zeta potential values for different surfactant contents as a function of pH. As it can be seen, the addition of NaOH allows us to obtain significantly higher zeta potential values for lower surfactant contents ( < 2 % w t ) . Rheological measurements of zircon suspensions with 1 and 2% wt deflocculant concentrations and a solids loading of 60% wt reveal that viscosity decreases with increasing pH and the thyxotropy effect disappears. The apparent viscosity versus pH curves for those suspensions are plotted in Fig. 5 for a shear rate of 500 s- 1. Very low viscosities are obtained for pH > 10. Drain casting properties have been studied from the measurement of the wall thickness formation rate. The slip casting process is rate-controlled by the
diffusion of the suspension medium through the layer already cast. The cast thickness L is related to the casting time Tby L 2 = kT. 3 - 5 The wall thickness formation rate of 60% wt suspensions with 1 and 2 % w t deflocculant concentration and pH = 11 is shown in Fig. 6. These curves agree with those o f other authors who have studied slip casting kinetics 6- 8 and indicate a diffusion controlled process. H e r m a n n and Cutler 6 found an effective cast thickness at zero time for clay casting, whereas it is not observed in this case. The zero time thickness reported elsewhere is caused by flocculation of calcium ions from the plaster. 7 The strong effect of the surfactant concentration on casting rate can be seen: the higher the surfactant content, the lower the wall thickness rate. This is due to the contribution of the steric mechanism to the suspension stability. Neither cracking nor spalling are observed in the pieces obtained from zircon suspensions stabilized with 2% wt surfactant concentration and pH = 11. According to the previously reported data, high zeta potential, low viscosity and low wall thickness rate, the optimal conditions for slip casting zircon are those in which a 2% wt surfactant concentration and pH = 11 are employed. The green density of the dried pieces is reported in Table 1 for the differential casting conditions studied in this work. As expected, the higher green densities correspond to those conditions selected as being more favourable. This fact supports the criterion for the selection of the optimal conditions for casting. Once the best conditions for slip casting are known, the dried cast pieces have been heated at different conditions up to 1600°C for 2h soaking time. The relative density evolution is plotted in Fig. 7. A maximum density of 96% is reached at 1600°C.
50.
lZ
t,0.
O
0_ 3 o 2 Z
& 20,
10
6
';
1'o
,',
pH Fig. 5. Apparent viscosity versus pH of 60%wt zircon suspensions with surfactant concentrations of 1 and 2% wt.
60% wt zircon suspensions with 1 and 2%wt surfactant concentration and pH = 11.
R. Moreno, J. S. Moya, J. Requena
40 T a b l e 1.
G r e e n d e n s i t i e s o f slip cast z i r c o n bodies at different casting conditions
o•'100 x
cE Deflocculant conditions Per cent deflocculation
pH
4 2 1
8 11 11
Solids content 60% wt
51 "2 +_0'6 53-0 _+0"3 47"5 _+0"1
70% wt
52"7 + 0"1 51 "3 + 0-1 49-1 _+0"6
~
90
c •
80
0
7O
o/o/°-
/
o
J
I 1400
I 1500
I 1600
Temperature ( °C )
In spite of the the firing shrinkage being high (22%) no cracking was observed in any case. In a previous investigation 2 the slip casting of zircon was studied by controlling pH, considering also the effect of iron impurities on rheology and casting. In the present work a parallel route has been studied for slip casting zircon in which the powder washing is not required. The results obtained in this work show that an excess of organic surfactant can be deletereous for the obtention of slip cast bodies. Furthermore, a base addition improves the rheological properties of the suspension, and deflocculation results as a combination of electrostatic and steric mechanisms. In aqueous media the adsorption of ionogenic surface active agents leads to an increase in the double layer capacity provided that the adsorption process does not destroy the ionisation. In this way, anionic surfactants tend to increase the negative value of the stabilizing potential. For this reason, anionic surfactants are more effective in alkaline medium and cationic surfactants in acid medium. 9 The sintering studies reveal that no differences are observed for the zircon samples obtained for slip casting from pH controlled or surfactant stabilized suspensions. In both cases a similar trend is observed for the densification and microstructural evolution. A final relative density of 96% (theoretical) is obtained at 1600°C. At higher temperatures
Fig. 7.
Relative density of slip cast zircon versus temperature for soaking times of 2 h.
zircon decomposition takes place and Z r O 2 peaks appear in the corresponding X-ray diffraction spectra, probably as a consequence of the small amount of impurity present in the starting zircon powder. 1°
REFERENCES 1. MORENO, R., MOYA, J. S. & REQUENA, J., Slip casting of zircon. J. Mat. Sci. Let., 5 (1986) 127. 2. MORENO, R., MOYA, J. S. & REQUENA, J., Slip casting of zircon. Effect of iron-impurities. Ceramics International, 16 (1990) 115-19. 3. ADCOCK, D. S. & McDOWALL, I. C., The mechanism of filter pressing and slip casting. J. Am. Ceram. Soc., 40 (1957) 355. 4. TILLER, F. M. & TSAI, C., Theory of filtration of ceramics: I, Slip casting. J. Am. Ceram. Soc., 69 (1986) 882. 5. HAMPTON, J. H. D., SAVAGE, S. B. & DREW, R. A. L., Experimental analysis and modeling of slip casting. J. Am. Ceram. Soc., 71 (1988) 1040. 6. HERRMANN, E. R. & CUTLER, I. B., The kinetics ofslip casting. J. Am. Ceram. Soc., 61 (1962) 207. 7. WORRALL, W. E. & RYAN, W., The mechanism of slip casting. J. Br. Ceram. Soc., 1 (1964) 269. 8. DEACON, R. F. & MISKIN, S. F. A., Gamma-ray adsorption for the continuous observation of slip casting kinetics. Trans. Brit. Ceram. Soc., 63 (1964) 473. 9. PARFITT, G. D., Dispersion of Powders in Liquids, 3rd edn, Applied Science Publishers, Englewood, N J, USA, 1981. 10. PENA, P. & DE AZA, S., The zircon thermal behaviour: effect of impurities. Part I. J. Mat. Sci., 19 (1984) 135.