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Effect of some salts on the viscosity of slip casting
Applied Clay Science 13 Ž1998. 213–218
Effect of some salts on the viscosity of slip casting C¸ etin Guler ¨ a
a,)
, Ender Balci
b
¨ Fen Fakultesi, ˙ ˙ Turkey E.U. Kimya Bolumu, ¨ ¨ ¨ ¨ BornoÕa, Izmır, b ˙ ˙ Turkey Dekoser A.S¸., Halkapınar, Ismır, Received 21 October 1997; accepted 23 March 1998
Abstract In this study, the effect of salts such as FeCl 3 , KCl, NaCl, FeSO4 , K 2 SO4 and Na 2 SO4 on the viscosity of the slip casting was investigated. It was observed that the viscosity increases with an increasing amount of salt added Žwith the exception of sodium sulfate.. Moreover, as far as the anions are concerned, the effect of chloride on the viscosity is greater than that of sulfate. q 1998 Elsevier Science B.V. All rights reserved. Keywords: viscosity; slip casting; rheology; salt effect; ion effect
1. Introduction Forming, by means of casting, is an important process which has been used in traditional and modern ceramic production for more than 200 hundred years ŽScharrer, 1994. . Rheological properties of slip casting are of importance because they are one of the significant parameters which control the production. The rheological properties depend on physical and chemical properties of the raw material and on the conditions under which the slip casting is prepared. Since the properties of the raw material are variable, depending on the area where the raw material is produced, the rheological properties of the slip casting change depending on the raw material. Therefore, the rheological properties must be redetermined for each new raw material in the casting process. The wall thickness of the product and the casting speed are mainly controlled by the viscosity of the slip in a casting process. The viscosity of the slip must, )
Corresponding author.
0169-1317r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 1 8 - 0
214
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
therefore, always be constant within statistical limits. The viscosity of the slip depends on particle size, shape and type of the raw material, and the viscosity of the medium, the speed of mixing and the purity of the water used. The slip should not precipitate during the process. The water used in preparing the slip must not contain excess Naq, Kq, Fe 2q, Ca2q, Mg 2q Cly, SO42y PO43y and CO 32y ions; they cause precipitation of the slip and spoil its stability. It is well known that some ions effect the colloidal clay suspension ŽSchulle et al., 1983; Brandenburg and Lagaly, 1988; Singh et al., 1992. . The zeta Ž z . potential of colloidal clay decreases with adding counter ions ŽHunter, 1992.. In the meantime, the clay particles aggregate due to surface–surface, edge–surface and edge–edge interactions Ž Demiralp et al., 1987. . Variations in the shape and size of the colloidal particles cause changes in their rheological properties. In this work, the effect of some salts on the viscosity of a casting slip with a viscosity of 400 mPa s was studied.
2. Methods and materials The composition of the slip used in sanitary ware production is approximately as follows: feldspar 20%, quartz 15%, kaolin 32% and ball clay 33% by weight. The density and the viscosity of the slip are about 1800 g ly1 and 400 mPa s, respectively. These viscosity values are rearranged by adding Na 2 CO 3 and Na 2 SiO 3. The viscosity was measured by using LVTD Model Brookfield Digital Viscometer and the particle size was measured by using Sedigraph S100 V3.03 model. Chloride and sulfate salt solutions were prepared as 2% and 4% weight by weight, respectively. The viscosity was measured by adding salt solutions in varying amounts into the slip with a constant viscosity.
3. Results and discussion The solid particle size distribution in the slip used is shown in Table 1. It is observed from Table 1 that 97.5% of the particles are smaller than 80 m m and 50% of them are smaller than 3.6 m m. As it is shown in Table 1, 16% of all particles are over the range of 2–3 m m and 16.9% are smaller than 1 m m. The salt solutions were firstly added with increasing amounts into the slip and the viscosity of which was prefixed by means of Na 2 CO 3 and Na 2 SiO 3. The viscosity of the suspension was finally measured. In case of FeCl 3 addition, it was observed that the viscosity had increased rapidly with increasing amount of FeCl 3. As far as KCl and NaCl are concerned, the salt effect on the viscosity of
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
215
Table 1 Particle size distribution of slip Diameter Ž m m.
Cumulative mass finer than Ž%.
100.00 80.00 60.00 50.00 40.00 30.00 25.00 20.00 15.00 10.00 8.00 6.00 5.00 4.00 3.00 2.00 1.50 1.00
97.1 97.5 97.2 96.6 95.6 93.3 91.5 89.0 86.7 79.7 76.5 69.2 61.5 53.1 44.4 28.3 21.1 17.9
the suspension increases with increasing concentration whilst it is very low at lower salt concentrations. The order of effect of the chloride salts on the viscosity of the suspension is FeCl 3 4 KCl ) NaCl as shown in Fig. 1. The effect of the sulfates such as
Fig. 1. The effect of salt additives on the apparent viscosity of a given slip.
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C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
FeSO4 , Na 2 SO4 and K 2 SO4 on the viscosity of the suspension is less than that of the chlorides ŽFig. 1.. The viscosity of the suspension varies slightly with the increasing amount of Na 2 SO4 while it does remarkably with FeSO4 and K 2 SO4 . The comparative order of the effect is K 2 SO4 ) FeSO4 ) Na 2 SO4 . The clay particles have a negative surface charge. Therefore, they have the capacity to attract and adsorb cations from the surrounding suspension. In general, the greater the change and the higher the atomic weight of the cation, the more readily it will be adsorbed. The tendency of the different cations to be adsorbed is again predicted by a lyotropic series Ž Grimshaw, 1971; Hunter, 1993. . The series is: Hq) Al 3q) Ba2q) Sr 2q) Ca2q) Mg 2q) NH 4q) Kq) Naq) Liq Monovalent cations, such as potassium and sodium, are large and more readily hydrated. For this reason, the number of oriented water molecules is much greater and the hydrosphere of particles with sodium or potassium as the counter-cations is extremely large Ž Grim, 1968. . The z-potential of clays in this state is comparatively very large and consequently, the force of repulsion between particles is such as to produce maximum separation, or, in other words, a deflocculated system. At high cation concentrations, the z-potential decreases because, if the number of cations is increased excessively, they crowed into the diffuse double layer, with the result that the field of influence of the negatively charged surface is diminished. The thickness of the double layer is reduced and, in the limiting case, cations may invade the surface layer itself and reduce the surface charge. The reduction in the z-potential produces a lower degree of dispersion or deflocculation and will eventually cause coagulation ŽGrimshaw, 1971. . When some anions are introduced into a suspension, there will be a tendency for them to be adsorbed by the clay particles provided they are in excess. Particles which are capable of negative adsorption, preferentially select different anions according to the lyotropic series ŽGrim, 1968; Hunter, 1993. . 2y OHy) CNSy) Iy) Bry) Cly) Fy) NOy 3 ) SO4
Other anions may preferentially replace hydroxyl units on the surface of clay particles. The effect of such replacements depends on the ionic size: if the dimensions of the anion are large, a relatively small number will be able to surround the particle. Consequently, the charge Ž e . on the particle will be reduced, the number of counter-cations which are able to approach the surface will be less, hence the thickness Ž d . of the double layer will also be reduced. Although the dielectric constant Ž D . may be reduced also, the usual effect of adding an anion in the form of an acidic radicle to the clay suspension is to
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
217
reduce the z-potential Ž z s 4 P edrD . and increase the rate and amount of flocculation. The effect of increasing the flocculation of clay is on increase the of apparent viscosity Ž Wu, 1993. . The ball clay in the slip casting contains some organic substances Ž humic acids. and it has plastic properties. During prefixing the viscosity of the slip an alkaline structure is firstly obtained by adding sodium carbonate solution into the slip. Thus, the alkaline ions facilitate this structure to become colloidal reacting with the organic substances in the slip when sodium silicate is added. As a result of the precipitation of the anions of the alkaline salts Žsuch as sulfate by Ca2q and Mg 2q . in the slip, the clay particles which form the slip become colloidal. If the amounts of Na 2 CO 3 and Na 2 SiO 3 used as deflocculant are insufficient, such a slip is not convenient for casting because the solid particles precipitate. The material which has been produced from a slip casting with insufficient amount of deflocculant has a soft and porous structure. Therefore, it has a low strength. Strength of the structure increases with increasing amount of deflocculant. The clay suspension is flocculated in the presence of higher valence cations such as Ca2q, Fe 2q and Mg 2q, and of the anions such as SO42y and Cly. The excessive amount of this kind of ions is harmful for the slip casting, no matter if it originates either from the water or raw material used. Therefore, both the amount of ions in the water used for preparing the slip and the amount of soluble salts in the raw material are of importance.
4. Conclusions High amounts of soluble salts containing di- or trivalent cations or SO42y or Cl anions are injurious to the casting slip. It does not matter whether they are introduced in the water or come from the raw materials. Such ions make deflocculation difficult and the properties of the resultant slip inferior. y
References Brandenburg, U., Lagaly, G., 1988. Rheological properties of sodium montmorillonite dispersions. Appl. Clay Sci. 3, 263–279. Demiralp, C., Sarıer, N., ve Guler, ¨ C¸ ., 1987. Kaolinin notral ¨ poliakrilamid ile flokulasyonuna iyon etkisi. Ceram. Tech. Congress Proc., August 24–28, Ankara, Turkey, 59 pp. Grim, R.E., 1968. Clay Mineralogy. McGraw-Hill, New York, 214 pp. Grimshaw, R.W., 1971. The Chemistry and Physics of Clays, 4th ed. Ernest Benn, London, 467 pp.
218
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
Hunter, R.J., 1992. Adsorption from solution. Foundation of Colloid Science, Chap. 12, Vol II. Clarendon Press, Oxford, p. 709. Hunter, R.J., 1993. Double layer interaction and particle coagulation. Foundation of Colloid Science, Chap. 7, Vol. 1. Clarendon Press, Oxford, p. 395. Scharrer, K., 1994. Pressure casting in the ceramic industry. CfirBer. DKG 71 Ž4., 157–159. Schulle, W., Dudolph, W., Pluscke, R., 1983. Beeinflussung des Verflussigung-und Gressverhal¨ tens silikatisch-tonkeramischer Schicker durch die Anionen im Suspensionwasser. Silikattechnik 34, 195–198. Singh, P.K., Sharma, V.P., Caennr, R., 1992. Effect of additives and temperature on the flow behaviour of sodium bentonite suspensions. J. Petrol. Sci. Eng. 17, 349–355. Wu, K., 1993. Ceramic slurry control in manufacturing. Ceram. Eng. Sci. Proc. 14, 41–56.
Effect of some salts on the viscosity of slip casting C¸ etin Guler ¨ a
a,)
, Ender Balci
b
¨ Fen Fakultesi, ˙ ˙ Turkey E.U. Kimya Bolumu, ¨ ¨ ¨ ¨ BornoÕa, Izmır, b ˙ ˙ Turkey Dekoser A.S¸., Halkapınar, Ismır, Received 21 October 1997; accepted 23 March 1998
Abstract In this study, the effect of salts such as FeCl 3 , KCl, NaCl, FeSO4 , K 2 SO4 and Na 2 SO4 on the viscosity of the slip casting was investigated. It was observed that the viscosity increases with an increasing amount of salt added Žwith the exception of sodium sulfate.. Moreover, as far as the anions are concerned, the effect of chloride on the viscosity is greater than that of sulfate. q 1998 Elsevier Science B.V. All rights reserved. Keywords: viscosity; slip casting; rheology; salt effect; ion effect
1. Introduction Forming, by means of casting, is an important process which has been used in traditional and modern ceramic production for more than 200 hundred years ŽScharrer, 1994. . Rheological properties of slip casting are of importance because they are one of the significant parameters which control the production. The rheological properties depend on physical and chemical properties of the raw material and on the conditions under which the slip casting is prepared. Since the properties of the raw material are variable, depending on the area where the raw material is produced, the rheological properties of the slip casting change depending on the raw material. Therefore, the rheological properties must be redetermined for each new raw material in the casting process. The wall thickness of the product and the casting speed are mainly controlled by the viscosity of the slip in a casting process. The viscosity of the slip must, )
Corresponding author.
0169-1317r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 1 8 - 0
214
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
therefore, always be constant within statistical limits. The viscosity of the slip depends on particle size, shape and type of the raw material, and the viscosity of the medium, the speed of mixing and the purity of the water used. The slip should not precipitate during the process. The water used in preparing the slip must not contain excess Naq, Kq, Fe 2q, Ca2q, Mg 2q Cly, SO42y PO43y and CO 32y ions; they cause precipitation of the slip and spoil its stability. It is well known that some ions effect the colloidal clay suspension ŽSchulle et al., 1983; Brandenburg and Lagaly, 1988; Singh et al., 1992. . The zeta Ž z . potential of colloidal clay decreases with adding counter ions ŽHunter, 1992.. In the meantime, the clay particles aggregate due to surface–surface, edge–surface and edge–edge interactions Ž Demiralp et al., 1987. . Variations in the shape and size of the colloidal particles cause changes in their rheological properties. In this work, the effect of some salts on the viscosity of a casting slip with a viscosity of 400 mPa s was studied.
2. Methods and materials The composition of the slip used in sanitary ware production is approximately as follows: feldspar 20%, quartz 15%, kaolin 32% and ball clay 33% by weight. The density and the viscosity of the slip are about 1800 g ly1 and 400 mPa s, respectively. These viscosity values are rearranged by adding Na 2 CO 3 and Na 2 SiO 3. The viscosity was measured by using LVTD Model Brookfield Digital Viscometer and the particle size was measured by using Sedigraph S100 V3.03 model. Chloride and sulfate salt solutions were prepared as 2% and 4% weight by weight, respectively. The viscosity was measured by adding salt solutions in varying amounts into the slip with a constant viscosity.
3. Results and discussion The solid particle size distribution in the slip used is shown in Table 1. It is observed from Table 1 that 97.5% of the particles are smaller than 80 m m and 50% of them are smaller than 3.6 m m. As it is shown in Table 1, 16% of all particles are over the range of 2–3 m m and 16.9% are smaller than 1 m m. The salt solutions were firstly added with increasing amounts into the slip and the viscosity of which was prefixed by means of Na 2 CO 3 and Na 2 SiO 3. The viscosity of the suspension was finally measured. In case of FeCl 3 addition, it was observed that the viscosity had increased rapidly with increasing amount of FeCl 3. As far as KCl and NaCl are concerned, the salt effect on the viscosity of
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
215
Table 1 Particle size distribution of slip Diameter Ž m m.
Cumulative mass finer than Ž%.
100.00 80.00 60.00 50.00 40.00 30.00 25.00 20.00 15.00 10.00 8.00 6.00 5.00 4.00 3.00 2.00 1.50 1.00
97.1 97.5 97.2 96.6 95.6 93.3 91.5 89.0 86.7 79.7 76.5 69.2 61.5 53.1 44.4 28.3 21.1 17.9
the suspension increases with increasing concentration whilst it is very low at lower salt concentrations. The order of effect of the chloride salts on the viscosity of the suspension is FeCl 3 4 KCl ) NaCl as shown in Fig. 1. The effect of the sulfates such as
Fig. 1. The effect of salt additives on the apparent viscosity of a given slip.
216
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
FeSO4 , Na 2 SO4 and K 2 SO4 on the viscosity of the suspension is less than that of the chlorides ŽFig. 1.. The viscosity of the suspension varies slightly with the increasing amount of Na 2 SO4 while it does remarkably with FeSO4 and K 2 SO4 . The comparative order of the effect is K 2 SO4 ) FeSO4 ) Na 2 SO4 . The clay particles have a negative surface charge. Therefore, they have the capacity to attract and adsorb cations from the surrounding suspension. In general, the greater the change and the higher the atomic weight of the cation, the more readily it will be adsorbed. The tendency of the different cations to be adsorbed is again predicted by a lyotropic series Ž Grimshaw, 1971; Hunter, 1993. . The series is: Hq) Al 3q) Ba2q) Sr 2q) Ca2q) Mg 2q) NH 4q) Kq) Naq) Liq Monovalent cations, such as potassium and sodium, are large and more readily hydrated. For this reason, the number of oriented water molecules is much greater and the hydrosphere of particles with sodium or potassium as the counter-cations is extremely large Ž Grim, 1968. . The z-potential of clays in this state is comparatively very large and consequently, the force of repulsion between particles is such as to produce maximum separation, or, in other words, a deflocculated system. At high cation concentrations, the z-potential decreases because, if the number of cations is increased excessively, they crowed into the diffuse double layer, with the result that the field of influence of the negatively charged surface is diminished. The thickness of the double layer is reduced and, in the limiting case, cations may invade the surface layer itself and reduce the surface charge. The reduction in the z-potential produces a lower degree of dispersion or deflocculation and will eventually cause coagulation ŽGrimshaw, 1971. . When some anions are introduced into a suspension, there will be a tendency for them to be adsorbed by the clay particles provided they are in excess. Particles which are capable of negative adsorption, preferentially select different anions according to the lyotropic series ŽGrim, 1968; Hunter, 1993. . 2y OHy) CNSy) Iy) Bry) Cly) Fy) NOy 3 ) SO4
Other anions may preferentially replace hydroxyl units on the surface of clay particles. The effect of such replacements depends on the ionic size: if the dimensions of the anion are large, a relatively small number will be able to surround the particle. Consequently, the charge Ž e . on the particle will be reduced, the number of counter-cations which are able to approach the surface will be less, hence the thickness Ž d . of the double layer will also be reduced. Although the dielectric constant Ž D . may be reduced also, the usual effect of adding an anion in the form of an acidic radicle to the clay suspension is to
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
217
reduce the z-potential Ž z s 4 P edrD . and increase the rate and amount of flocculation. The effect of increasing the flocculation of clay is on increase the of apparent viscosity Ž Wu, 1993. . The ball clay in the slip casting contains some organic substances Ž humic acids. and it has plastic properties. During prefixing the viscosity of the slip an alkaline structure is firstly obtained by adding sodium carbonate solution into the slip. Thus, the alkaline ions facilitate this structure to become colloidal reacting with the organic substances in the slip when sodium silicate is added. As a result of the precipitation of the anions of the alkaline salts Žsuch as sulfate by Ca2q and Mg 2q . in the slip, the clay particles which form the slip become colloidal. If the amounts of Na 2 CO 3 and Na 2 SiO 3 used as deflocculant are insufficient, such a slip is not convenient for casting because the solid particles precipitate. The material which has been produced from a slip casting with insufficient amount of deflocculant has a soft and porous structure. Therefore, it has a low strength. Strength of the structure increases with increasing amount of deflocculant. The clay suspension is flocculated in the presence of higher valence cations such as Ca2q, Fe 2q and Mg 2q, and of the anions such as SO42y and Cly. The excessive amount of this kind of ions is harmful for the slip casting, no matter if it originates either from the water or raw material used. Therefore, both the amount of ions in the water used for preparing the slip and the amount of soluble salts in the raw material are of importance.
4. Conclusions High amounts of soluble salts containing di- or trivalent cations or SO42y or Cl anions are injurious to the casting slip. It does not matter whether they are introduced in the water or come from the raw materials. Such ions make deflocculation difficult and the properties of the resultant slip inferior. y
References Brandenburg, U., Lagaly, G., 1988. Rheological properties of sodium montmorillonite dispersions. Appl. Clay Sci. 3, 263–279. Demiralp, C., Sarıer, N., ve Guler, ¨ C¸ ., 1987. Kaolinin notral ¨ poliakrilamid ile flokulasyonuna iyon etkisi. Ceram. Tech. Congress Proc., August 24–28, Ankara, Turkey, 59 pp. Grim, R.E., 1968. Clay Mineralogy. McGraw-Hill, New York, 214 pp. Grimshaw, R.W., 1971. The Chemistry and Physics of Clays, 4th ed. Ernest Benn, London, 467 pp.
218
C¸ . Guler, ¨ E. Balci r Applied Clay Science 13 (1998) 213–218
Hunter, R.J., 1992. Adsorption from solution. Foundation of Colloid Science, Chap. 12, Vol II. Clarendon Press, Oxford, p. 709. Hunter, R.J., 1993. Double layer interaction and particle coagulation. Foundation of Colloid Science, Chap. 7, Vol. 1. Clarendon Press, Oxford, p. 395. Scharrer, K., 1994. Pressure casting in the ceramic industry. CfirBer. DKG 71 Ž4., 157–159. Schulle, W., Dudolph, W., Pluscke, R., 1983. Beeinflussung des Verflussigung-und Gressverhal¨ tens silikatisch-tonkeramischer Schicker durch die Anionen im Suspensionwasser. Silikattechnik 34, 195–198. Singh, P.K., Sharma, V.P., Caennr, R., 1992. Effect of additives and temperature on the flow behaviour of sodium bentonite suspensions. J. Petrol. Sci. Eng. 17, 349–355. Wu, K., 1993. Ceramic slurry control in manufacturing. Ceram. Eng. Sci. Proc. 14, 41–56.