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Stabilization of kaolin clay slurry with sodium silicate of different silicate moduli
Applied Clay Science 146 (2017) 147–151
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Note
Stabilization of kaolin clay slurry with sodium silicate of different silicate moduli
MARK
A. Stempkowskaa, J. Mastalska-Popławskab,⁎, P. Izakb, L. Ogłazac, M. Turkowskad a
AGH University of Science and Technology, Faculty of Mining and Geoengineering, 30-059 Krakow, Poland AGH University of Science and Technology, Faculty of Materials Science and Ceramics, 30-059 Krakow, Poland c Rudniki S.A. Chemical Plant, Rudniki, 42-240 Częstochowa, Poland d Inorganic Chemistry Division, New Chemical Synthesis Institute, 44-101 Gliwice, Poland b
A R T I C L E I N F O
A B S T R A C T
Keywords: Depleted stabilization Ion exchange Kaolin Silicate modulus Sodium water glass
In the ceramic technological processes, the suspensions with high concentration of ceramic particles (> 60%) play a very important role because this let to obtain inter alia reasonable casting rates what is one of the most important steps during the ceramics manufacturing. The article shows the attempts to modify the rheological properties of concentrated ceramic slurries, based on kaolin KOC. Different alkali sodium silicate based stabilizers, i.e. sodium water glasses ((Na2SiO2)nO) of silicate moduli in the range 1.74–3.25, were used for the research. The flow curves were analyzed and the technological parameters of slurries were described. Special attention was paid to the stabilization of slurries with the use of sodium water glasses of higher silicate moduli (> 2). It was found that the best results of the stabilization of the kaolin slurry can be obtained while using sodium water glass of silicate moduli 2–2.5. Sodium water glasses of the moduli smaller than 2 precipitated free silica in the suspension and increased the alkalinity of ceramic slurry while those larger than 2.5 created independent silicate micelles, co-existing with the dispersed kaolin grains. The mechanisms appearing during the stabilization of ceramic slurry with the use of sodium water glass were ion exchange and, the so called depleted stabilization.
1. Introduction Ceramic slurries consist of inorganic components (clay minerals, quartz, feldspar), water and most frequently organic modified additives (plasticizers, fluidizers, antifoams, lubricants, etc.). Regardless the method of forming, the rheological properties of slurries are always a matter of great interest. These properties are often out of reach by normal mixing of ceramic mass with water. That is why it is an usual practice to add various modifiers to obtain the most beneficial rheological properties with the minimum use of water. The polar liquids, mainly water, cause the dispersion of clay minerals grains. The polarity of water determines its physicochemical properties. The easiness of proton transportation from one water molecule to another and from one hydroxide ion to another allows, within a certain mass (volume) of water, the determined number of hydroxide and hydronium (oxonium) ions to be obtained. For its large number the balance state is established, and as a result, the ordered one. Among the charged grains or ions producing the electric field in the suspension, the ordered water molecules produce their own electric field, oriented oppositely. That is why the presence of water weakens significantly the
⁎
Corresponding author. E-mail address: [email protected] (J. Mastalska-Popławska).
http://dx.doi.org/10.1016/j.clay.2017.05.046 Received 21 February 2017; Received in revised form 30 May 2017; Accepted 31 May 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
electrostatic interaction of ions or charged molecules in the suspension. What is more, the water dipoles take part in solvation (hydrophobic forces). In these conditions, the ceramic slurry becomes unstable and sediments easily. The addition of stabilizer results in a suspension not only with low viscosity but also high density (Horn, 1990; Iskece et al., 1999; Hamley, 2000; Cheng, 2003). Due to their construction, the stabilizers used to modify the ceramic suspensions are divided into inorganic and organic. These in turn may be of natural, synthetic or artificial sources. They must also meet the following requirements: wettability of grains- by lowering the surface tension; breakage of aggregates and dissipation of the grains in a liquid phase- by means of the grain-stabilizer intermolecular interactions; reduction of viscosity and consolidation of dispersion- by changing the physicochemical interactions between the particles of the solid phase. Depending on the type of the system (water suspension, non-water suspension), and the type of modifiers (organic, inorganic) four kinds of chemical stabilization can be listed: - electrostatic- caused by the change of pH or ion exchange, - polymeric (steric)- caused by the specific adsorption of organic
Applied Clay Science 146 (2017) 147–151
A. Stempkowska et al.
polymers (so called, spatial effect or antiflocculation effect), - electro-spatial- caused by the specific adsorption of organic polymers with the functional groups capable of dissociating, or - depleted- caused by the dispersion of other fractions.
Table 1 Chemical analysis and mineralogical composition of kaolin KOC. Chemical analysis [wt%] SiO2 51.60
The most effective mechanism of the suspension stabilization is the electro-spatial one connected with the activities of the organic macroparticles with the functional groups capable of dissociating. The organic polymer particles, the so called protective colloids, of big molecular weight, may undergo the multipoint adsorption- i.e. specific adsorption, and limit the access to the attractive van der Waals-London forces and in this way they may compensate the surface charge. The stabilization (fluidization) of these can be done by:
Al2O3 34.20
Fe2O3 0.49
Mineralogical content [wt%] Kaolinite Mica 84.21 4.62
TiO2 0.47
CaO 0.08
MgO 0.13
K2 O 0.71
Na2O 0.32
Quartz 11.17
mineral structure, kaolin KOC used for the study was calcined at 600 °C. Its chemical analysis and mineralogical composition is shown in Table 1. The sodium water glasses used in the study were filtered and its chemical analysis was performed with the use of the factory standards (Factory standard, 2016) (Table 2). Silicate moduli (M) were calculated on the basis of the following formula:
- adsorption of the protective organic colloid on the surface of the grains from the solid phase and possible deactivation of the multivalent compensating cations, by creating the complex compounds; - creating the so called potential tunnel around the functional groups of the adsorbed polymer (Bergaya and Lagaly, 2001; Dinger, 2002; Mpofu et al., 2003).
M=
xSiO2 ⋅1.0323 yNa2 O
(1)
where: xSiO2, yNa2O- weight percentage content of silica and sodium oxide respectively; 1.0323- conversion factor from weight units to molar units. The performed 29Si resonance measurements (Bruker 300 MHz spectrometer, D2O as a solvent) revealed that spatial structures of applied silicates were comparable with the literature data in the range of signal intensities (Harris et al., 1980; Harris and Knight, 1983; Ray and Plaisted, 1983; Svensson et al., 1986; Wijnen et al., 1990; Uchino et al., 1992; Bass and Turner, 1997; Schneider and Mastelaro, 2003). The signal intensity was calculated as the signal surface area by integration. For example, resonance spectrum of sodium water glass R-137 showed presence of some silicate polymer structures (Fig. 1).
In the case of inorganic stabilizers the stabilization mechanism is the ion exchange. Reinforcement of negative charges on the grains surface results in the removal of the positive coagulation charges. In this process, Mg2 +, Ca2 + and Al3 + ions, characterized by a small ionic radius and high potential, are preferred. In the suspension, these cations are attracted to the negatively charged surface of grains, thereby reducing the intermolecular repulsive forces. If multivalent cations are combined with, for example, the silicate anion, they are immediately neutralized and removed to the medium, so that the viscosity of the suspension is getting lower (Otterstedt and Brandreth, 1998; Chi and Eggleton, 1999). Izak et al., 2003 and Stempkowska et al., 2011 revealed that silicate fluidizers are not sensitive to overdosing in the suspension, unlike other inorganic stabilizers. That is why they are friendly in the use for application. This is characteristic for the organic fluidizers which work basing on specific adsorption and shielding of the van der Waals attractive forces. But there were no literature data about the fluidization mechanism of sodium water glasses of different silicate moduli, what could be very useful for the ceramic industry applications. Only articles about rheological properties of such slurries were found (Yildiz et al., 1998; Penner and Lagaly, 2001; Pacheco-Torgal et al., 2008; Penkavova et al., 2014). Because of this, the authors are trying to answer a question: How can the silicate modulus of the sodium water glass affect the mechanism of stabilization of the ceramic slurry? It is believed that these studies will help to understand the fluidization mechanism of sodium water glasses and will contribute to the development of knowledge about the structure and properties of sodium water glasses.
2.2. Rheological measurements The rheological measurements were conducted with the use of the rotary viscometer (Brookfield DV-III), equipped with a cylinder-andspindle (R-29) measuring system. Flow curves were made for increasing and decreasing shear rates in the range of 2.5–50 s− 1 and measuring points related to the change of shear rate were collected every 30 s. Additionally, after each flow curve measurement, thixotropy was measured. It relied on stopping the rotation for 10 min, whereupon viscosity was measured at a constant shear rate of 5.6 s− 1 after the time of 2 s, 3 s, 4 s, 5 s, 10 s, 20 s and 30 s. 3. Results and discussion 3.1. Rheological characterization of kaolin KOC slurries modified with sodium water glasses
2. Materials and methods The results showed that slurries based on kaolin KOC and modified with sodium water glasses possess thixotropic properties, which was probably associated with the intermolecular interaction mechanism. They exhibited c flows both at increasing and decreasing shear rates. It can be assumed that decreasing apparent viscosity (Fig. 2) at the increasing shear rate was the result of the gradual destruction of the internal structures of the slurry and their arrangement along the flow direction. Disintegration of the internal structure at high shear rates was not immediate, reflecting the dynamic (non-rheostable) and reversible properties. Increase the amount of fluidizer caused faster stabilization of the dispersion structure. Above the 0.3 wt% concentration of sodium water glasses with the silicate moduli in the range of 2.0–2.5 there was no visible improvement in stabilization of the ceramic slurry. Therefore, it can be concluded that the best fluidization properties were possessed by sodium water glasses with the silicate moduli in the range of 2.0–2.5
2.1. Preparation of kaolin KOC slurries The research was conducted with the use of concentrated kaolin KOC slurries which consisted of 60 wt% of kaolin KOC (Surmin-Kaolin, Poland; enriched in quartz and low in titanium and iron white-burned raw material, designed for the use in ceramic industry because if its stable rheological properties), 40 wt% of distilled water and sodium water glass ((Na2SiO2)nO) (Rudniki S.A. Chemical Plant, Poland) of different silicate moduli in the range of 1.74–3.25, which was added to the ceramic slurry in the amount of 0.1–0.5 wt% to the kaolin KOC dry mass. The sample was stirred for 5 min in such a way as not to aerate it. After obtaining a homogenous slurry, it was matured for 10 min and then again stirred before the measurement to destroy possible thixotropic structure. To remove water from the kaolin and thereby to stabilize the clay 148
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A. Stempkowska et al.
Table 2 Physicochemical analysis of used sodium water glasses (Factory standard, 2016). Sample number
Type
Na2O [wt%]
SiO2 [wt%]
Silicate modulus
Specific gravity [g/cm3]
Viscosity (20 °C) [mPa]
1 2 3 4 5 6 7 8 9 10 11 12
R-151-1.7 50 SH R-150S R-145 a R-145S R-145 R 47/48 R-149 R-140 R-146 2.9 R-137 3.1 R-137 E
16.6 13.8 13.6 12.7 13.5 12.1 12.0 11.7 10.3 10.7 9.3 9.2
28.0 27.4 30.2 29.4 31.9 29.3 31.1 32.4 28.9 31.3 28.1 28.9
1.74 2.05 2.29 2.39 2.44 2.50 2.67 2.86 2.89 3.02 3.09 3.25
1.550 1.484 1.523 1.485 1.528 1.472 1.487 1.505 1.427 1.460 1.394 1.381
80 157 530 610 500 189 363 750 141 518 100 114
and concentration of approximately 0.3 wt% to the kaolin dry mass. Silicates with higher moduli (above 3.0) had weaker stabilization properties which can be associated with the reduction of pH of the slurry and the existence of independent micelles, caused by heterocoagulation (Fig. 3). On the basis of the prepared analyses and literature data (Garcia et al., 2002; Papo et al., 2002; Falcone et al., 2010) it can be assumed that sodium silicates with higher moduli created their own independent structures that formed stable micelles, occurring independently of the dispersed kaolinite grains. As a result of this, the so-called depleted stabilization mechanism was created. The more developed sodium silicate structure, the larger independent micelle in the suspension. In contrast, sodium silicates with lower moduli formed insoluble silicates by ion exchange with cations compensating the surface charge and thereby they increased alkalinity of the medium. Probably in this way the developed spatial structure of sodium silicates reduced the efficiency of fluidization of ceramic slurries. This was evidenced by the increase of apparent viscosity of the kaolin suspension together with the increase of the silicate modulus regardless of the existence of the thixotropic structure. In this case it was assumed that at a shear rate of 50 s− 1 the thixotropic structure of the slurry was completely destroyed, while at a shear rate of 2.5 s− 1 it still existed. Basing on 29Si NMR, IR and Raman tests (not presented here) it can
Fig. 1.
29
Fig. 2. Fluidization curves of kaolin slurries modified with sodium water glasses.
be stated that depending on the silicate modulus there were several space structures in the solution formed from SiO44 − tetrahedrons. This “arrangement” could block (via a strong link between oxygen ions at
Si NMR spectrum of sodium water glass R-137.
149
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A. Stempkowska et al.
modified slurries together with the values of silicate modulus, a significant difference in dependence of measurement methods was seen. In the case of the assignment of internal energy and rigidity of thixotropic structure the correlation power was about 70%, whereas in the case of the thixotropy coefficient this fitting was below 20% (R2 = 0.1854). Generally, the low fitting of the measurement results to the determined trend (function of the best fitting was always chosen) was related to the type of the created thixotropic structure. It was valid because two stabilization mechanisms were putting on, i.e. ionic exchange and increment of the mutual repulsion of the clay mineral grains, as well as by the depleted stabilization in which micelles of the alkaline polymer silicates created a separate dispersed fraction. It can be justified by the determination of the work used for destruction and reconstruction of the thixotropic structure during shearing (Izak, 2015). The tests showed that at the specific shear rates, for example at 10 or 20s− 1 etc., this work was higher and probably associated with the reconstruction or destruction of the specified thixotropic structure. At this point it was hard to say if this mechanism was associated with interactions between grains or micelles. In the case of sodium silicates a clear repetition of this phenomenon was observed, even though the height of the peaks depended on the silicate modulus, thus the larger the modulus value the smaller height of the peaks.
Fig. 3. The pH changes of pure sodium water glass and kaolin slurry after the addition of 0.3 wt% of sodium water glass depending on the silicate modulus.
the silicon atom) the ion exchange with compensation cations on the surface of clay minerals and thus created a stable structure arrangement of electric charges, i.e. an independent micelle in the kaolin slurry. Such a mechanism was proved by a steady increase of the consistency coefficient (k) in the power law of Ostwald de Waele model k (η = γ1 − n ) as the increase of the silicate modulus value. In this case, the ceramic slurry modified with silicates behaved during shearing as if it had a higher solids concentration. The value of the flow index was stabilized at low values of the silicate modulus which was related to the thixotropic structure between the kaolinite grains. At higher values of the silicate modulus larger micelles begun to reduce the dilution properties of the suspensions. In this case, the values of the flow index increased.
3.3. Rheological models of the slurries modified with sodium silicates Tests showed that kaolin slurries modified with alkaline silicates fulfill best the Ostwald de Waele power model. Material constants of this formula for 0.3 wt% of sodium water glasses with different silicate moduli were placed in Table 3 (data for 0.33 h). The flow index remained stable (about 0.14) up to the modulus value of 2.86 and it increased for a higher modulus. This confirmed the impact of additional silicate micelles on the slurries flow mechanism during shearing. The values of the material constants, according to the above model, changed during aging of the suspensions (Fig. 5). This showed also the increase of the number or size of the slurry aggregates occurring during shearing of the suspensions. This phenomenon was due to an opposite ion exchange on surfaces of the clay minerals in a suspension. Polycations compensating the surface charge had a higher charge and a smaller size. The increase of number of polycations in the aging slurry was due to partial solubility of the calcium and magnesium (dolomite) silicates founded in the kaolin.
3.2. Thixotropic properties of kaolin slurries modified with sodium water glasses The tests showed that thixotropic properties of kaolin slurries depend upon the fluidization degree (Reeds, 1986; Pierre and Ma, 1997; Sjoberg et al., 1999). Because of it, the smallest thixotropy field loops were at lower values of the silicate modulus (Fig. 4). This relationship was best described by the quadratic function at the correlation level of approximately 78%. Independently from the fact of growing thixotropic properties of the
4. Conclusion Summing up, silicate modulus of sodium water glass has a significant impact on the rheological properties of the slurries based on kaolin KOC. The best results of the stabilization of kaolin slurry can be obtained while using sodium water glass of silicate moduli 2–2.5, what is associated with the ion exchange mechanism and the so called Table 3 Material constants of the Ostwald de Waele power model.
Fig. 4. Energy of thixotropy as a function of silicate modulus.
150
Silicate modulus
Consistency index (k) [–]
Flow index (n) [–]
R2 (CoF) [%]
1.74 2.05 2.29 2.39 2.44 2.50 2.67 2.86 2.89 3.02 3.09 3.25
14,510 15,140 16,501 15,881 15,447 16,363 17,238 16,169 18,789 17,310 20,032 18,918
0.14 0.13 0.14 0.14 0.13 0.13 0.12 0.14 0.15 0.16 0.23 0.18
92.1 90.4 87.6 89.7 91.3 89.3 87.2 88.4 85.0 87.4 89.1 86.2
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174–180. Dinger, D.R. (Ed.), 2002. Rheology for Ceramists. Morris Publishing, New York. Factory standard, 2016. ZN-02/Z.Ch. Rudniki SA/257. Falcone, J.S., Bass, J.L., Angelella, M., Schenk, M.R., Brensinger, K.A., 2010. Characterizing the infrared bands of aqueous soluble silicates. J. Phys. Chem. A 114, 2438–2446. Garcia, N.J., Ingram, M.D., Bazan, J.C., 2002. Ion transport in hydrated sodium silicates (water glasses) of varying water content. Solid State Ionics 146, 113–122. Hamley, I.W. (Ed.), 2000. Introduction to Soft Matter: Polymers, Colloids, Amphiphiles and Liquid Crystals. Wiley, Hoboken. Harris, R.K., Knight, C.T.G., 1983. Silicon-29 nuclear magnetic resonance studies of aqueous silicate solutions. Part 6. -Second-order patterns in potassium silicate solutions enriched with silicon-29. J. Chem. Soc. Faraday Trans. 279, 1539–1561. Harris, R.K., Jones, J., Knight, C.T.G., Pawson, D., 1980. Silicon-29 NMR studies of aqueous silicate solutions: part II. Isotopic enrichment. J. Mol. Struct. 69, 95–103. Horn, R., 1990. Surface forces and their action in ceramic materials. J. Am. Ceram. Soc. 73, 1117–1135. Isikece, Ő., Gűngőr, N., Alemdar, A., 1999. Influences of electrolytes, polymers and o surfactant on rheological properties of bentonite-water system. J. Incl. Phenom. Mol. Recognit. Chem. 33, 155–168. Izak, P. (Ed.), 2015. Rheology in Ceramics. AGH Publishing House, Krakow. Izak, P., Lis, J., Izak, A., Cioch, A., 2003. Modification of rheological properties of ceramic suspensions based on kaolin KOC. Ceramics 76, 35–47. Mpofu, P., Addai-Mensah, J., Ralston, J., 2003. Investigation of the effect of polymer structure type on flocculation, rheology and dewatering behaviour of kaolinite dispersions. Int. J. Miner. Process. 71, 247–268. Otterstedt, J.-E., Brandreth, D.A. (Eds.), 1998. Small Particles Technology. Springer Science, New York. Pacheco-Torgal, F., Castro-Gomes, J., Jalali, S., 2008. Alkali-activated binders: a review. Part 2. About materials and binders manufacture. Constr. Build. Mater. 22, 1315–1322. Papo, A., Piani, L., Ricceri, R., 2002. Sodium tripolyphosphate and polyphosphate as dispersing agents for kaolin suspensions: rheological characterization. Colloids Surf. A Physicochem. Eng. Asp. 201, 219–230. Penkavova, V., Guerreiro, M., Tihon, J., Teixeira, J.A.C., 2014. Deffloculation of kaolin suspensions-the effect of various electrolytes. Appl. Rheol. 25, 1–9. Penner, D., Lagaly, G., 2001. Influence of anions on the rheological properties of clay mineral dispersions. Appl. Clay Sci. 19, 131–142. Pierre, A.C., Ma, K., 1997. Sedimentation behaviour of kaolinite and montmorillonite mixed with iron additives, as a function of their zeta potential. J. Mater. Sci. 32, 2937–2947. Ray, H.R., Plaisted, R.J., 1983. The constitution of aqueous silicate solutions. J. Chem. Soc. Dalton Trans. (3), 475–481. Reeds, J.S. (Ed.), 1986. Introduction to the Principles of Ceramic Processing. John Wiley & Sons, New York. Schneider, J., Mastelaro, V.R., 2003. Qn distribution in stoichiometric silicate glasses: thermodynamic calculations and 29Si high resolution NMR measurements. J. NonCryst. Solids 325, 164–178. Sjoberg, M., Bergstrom, L., Larsson, A., Sjostrom, E., 1999. The effect of polymer and surfactant adsorption on the colloidal stability and rheology of kaolin dispersions. Colloids Surf. A Physicochem. Eng. Asp. 159, 197–208. Stempkowska, A., Izak, P., Ogłaza, L., 2011. Rheological properties of ceramic slurries modified with sodium, potassium and lithium silicates. Ceram. Mater. 63, 547–551. Svensson, I.L., Sjoberg, S., Ohman, L.-O., 1986. Polysilicate equilibria in concentrated sodium silicate solutions. J. Chem. Soc. Faraday Trans. 182, 3635–3646. Uchino, T., Sakka, T., Ogata, Y., Iwasaki, M., 1992. Mechanism of hydration of sodium silicate glass in a steam environment: silicon-29 NMR and ab initio molecular orbital studies. J. Phys. Chem. 96, 7308–7315. Wijnen, P.W.J.G., Beelen, T.P.M., De Haan, J.W., Van De Ven, L.J.M., Van Santen, R.A., 1990. The structure directing effect of cations in aqueous silicate solutions. A 29SiNMR study. Colloids Surf. A Physicochem. Eng. Asp. 45, 255–268. Yildiz, N., Sarikaya, Y., Calimli, A., 1998. Modification of rheology and permeability of Turkish ceramic clays using sodium silicate. Appl. Clay Sci. 13, 65–77.
Fig. 5. Changes of consistency coefficient according to the Ostwald de Waele power model during aging of the slurries modified with sodium silicates.
depleted stabilization. This is related to the lack of presence of the silicate structures of SiO44 − tetrahedrons which can block the ion exchange. Sodium water glasses of the moduli higher than 2.5 are worse fluidizers because they create independent silicate micelles co-existing with the dispergated kaolin grains in the slurry, while those of the moduli smaller than 2 precipitate free silica in the suspension and increase the basicity of ceramic slurry. Using different spectroscopy techniques such like 29Si NMR and IR spectroscopy, we do research on the better understanding of the structures of the sodium water glasses described in the article, but obtained by different synthesis methods. We also plan to extend our research on the fluidization mechanisms on other alkali silicates aqueous solutions, i.e. lithium and potassium water glasses with different silicate moduli. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References Bass, J.L., Turner, G., 1997. Anion distributions in sodium silicate solutions. Characterization by 29Si NMR and infrared spectroscopies, and vapour phase Osmometry. J. Phys. Chem. B 101, 10638–10644. Bergaya, F., Lagaly, G., 2001. Surface modification of clay minerals. Appl. Clay Sci. 19, 1–3. Cheng, D.C.H., 2003. Characterization of thixotropy revisited. Rheol. Acta 42, 372–382. Chi, M., Eggleton, R., 1999. Cation exchange capacity of kaolinite. Clay Clay Miner. 47,
151
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Note
Stabilization of kaolin clay slurry with sodium silicate of different silicate moduli
MARK
A. Stempkowskaa, J. Mastalska-Popławskab,⁎, P. Izakb, L. Ogłazac, M. Turkowskad a
AGH University of Science and Technology, Faculty of Mining and Geoengineering, 30-059 Krakow, Poland AGH University of Science and Technology, Faculty of Materials Science and Ceramics, 30-059 Krakow, Poland c Rudniki S.A. Chemical Plant, Rudniki, 42-240 Częstochowa, Poland d Inorganic Chemistry Division, New Chemical Synthesis Institute, 44-101 Gliwice, Poland b
A R T I C L E I N F O
A B S T R A C T
Keywords: Depleted stabilization Ion exchange Kaolin Silicate modulus Sodium water glass
In the ceramic technological processes, the suspensions with high concentration of ceramic particles (> 60%) play a very important role because this let to obtain inter alia reasonable casting rates what is one of the most important steps during the ceramics manufacturing. The article shows the attempts to modify the rheological properties of concentrated ceramic slurries, based on kaolin KOC. Different alkali sodium silicate based stabilizers, i.e. sodium water glasses ((Na2SiO2)nO) of silicate moduli in the range 1.74–3.25, were used for the research. The flow curves were analyzed and the technological parameters of slurries were described. Special attention was paid to the stabilization of slurries with the use of sodium water glasses of higher silicate moduli (> 2). It was found that the best results of the stabilization of the kaolin slurry can be obtained while using sodium water glass of silicate moduli 2–2.5. Sodium water glasses of the moduli smaller than 2 precipitated free silica in the suspension and increased the alkalinity of ceramic slurry while those larger than 2.5 created independent silicate micelles, co-existing with the dispersed kaolin grains. The mechanisms appearing during the stabilization of ceramic slurry with the use of sodium water glass were ion exchange and, the so called depleted stabilization.
1. Introduction Ceramic slurries consist of inorganic components (clay minerals, quartz, feldspar), water and most frequently organic modified additives (plasticizers, fluidizers, antifoams, lubricants, etc.). Regardless the method of forming, the rheological properties of slurries are always a matter of great interest. These properties are often out of reach by normal mixing of ceramic mass with water. That is why it is an usual practice to add various modifiers to obtain the most beneficial rheological properties with the minimum use of water. The polar liquids, mainly water, cause the dispersion of clay minerals grains. The polarity of water determines its physicochemical properties. The easiness of proton transportation from one water molecule to another and from one hydroxide ion to another allows, within a certain mass (volume) of water, the determined number of hydroxide and hydronium (oxonium) ions to be obtained. For its large number the balance state is established, and as a result, the ordered one. Among the charged grains or ions producing the electric field in the suspension, the ordered water molecules produce their own electric field, oriented oppositely. That is why the presence of water weakens significantly the
⁎
Corresponding author. E-mail address: [email protected] (J. Mastalska-Popławska).
http://dx.doi.org/10.1016/j.clay.2017.05.046 Received 21 February 2017; Received in revised form 30 May 2017; Accepted 31 May 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
electrostatic interaction of ions or charged molecules in the suspension. What is more, the water dipoles take part in solvation (hydrophobic forces). In these conditions, the ceramic slurry becomes unstable and sediments easily. The addition of stabilizer results in a suspension not only with low viscosity but also high density (Horn, 1990; Iskece et al., 1999; Hamley, 2000; Cheng, 2003). Due to their construction, the stabilizers used to modify the ceramic suspensions are divided into inorganic and organic. These in turn may be of natural, synthetic or artificial sources. They must also meet the following requirements: wettability of grains- by lowering the surface tension; breakage of aggregates and dissipation of the grains in a liquid phase- by means of the grain-stabilizer intermolecular interactions; reduction of viscosity and consolidation of dispersion- by changing the physicochemical interactions between the particles of the solid phase. Depending on the type of the system (water suspension, non-water suspension), and the type of modifiers (organic, inorganic) four kinds of chemical stabilization can be listed: - electrostatic- caused by the change of pH or ion exchange, - polymeric (steric)- caused by the specific adsorption of organic
Applied Clay Science 146 (2017) 147–151
A. Stempkowska et al.
polymers (so called, spatial effect or antiflocculation effect), - electro-spatial- caused by the specific adsorption of organic polymers with the functional groups capable of dissociating, or - depleted- caused by the dispersion of other fractions.
Table 1 Chemical analysis and mineralogical composition of kaolin KOC. Chemical analysis [wt%] SiO2 51.60
The most effective mechanism of the suspension stabilization is the electro-spatial one connected with the activities of the organic macroparticles with the functional groups capable of dissociating. The organic polymer particles, the so called protective colloids, of big molecular weight, may undergo the multipoint adsorption- i.e. specific adsorption, and limit the access to the attractive van der Waals-London forces and in this way they may compensate the surface charge. The stabilization (fluidization) of these can be done by:
Al2O3 34.20
Fe2O3 0.49
Mineralogical content [wt%] Kaolinite Mica 84.21 4.62
TiO2 0.47
CaO 0.08
MgO 0.13
K2 O 0.71
Na2O 0.32
Quartz 11.17
mineral structure, kaolin KOC used for the study was calcined at 600 °C. Its chemical analysis and mineralogical composition is shown in Table 1. The sodium water glasses used in the study were filtered and its chemical analysis was performed with the use of the factory standards (Factory standard, 2016) (Table 2). Silicate moduli (M) were calculated on the basis of the following formula:
- adsorption of the protective organic colloid on the surface of the grains from the solid phase and possible deactivation of the multivalent compensating cations, by creating the complex compounds; - creating the so called potential tunnel around the functional groups of the adsorbed polymer (Bergaya and Lagaly, 2001; Dinger, 2002; Mpofu et al., 2003).
M=
xSiO2 ⋅1.0323 yNa2 O
(1)
where: xSiO2, yNa2O- weight percentage content of silica and sodium oxide respectively; 1.0323- conversion factor from weight units to molar units. The performed 29Si resonance measurements (Bruker 300 MHz spectrometer, D2O as a solvent) revealed that spatial structures of applied silicates were comparable with the literature data in the range of signal intensities (Harris et al., 1980; Harris and Knight, 1983; Ray and Plaisted, 1983; Svensson et al., 1986; Wijnen et al., 1990; Uchino et al., 1992; Bass and Turner, 1997; Schneider and Mastelaro, 2003). The signal intensity was calculated as the signal surface area by integration. For example, resonance spectrum of sodium water glass R-137 showed presence of some silicate polymer structures (Fig. 1).
In the case of inorganic stabilizers the stabilization mechanism is the ion exchange. Reinforcement of negative charges on the grains surface results in the removal of the positive coagulation charges. In this process, Mg2 +, Ca2 + and Al3 + ions, characterized by a small ionic radius and high potential, are preferred. In the suspension, these cations are attracted to the negatively charged surface of grains, thereby reducing the intermolecular repulsive forces. If multivalent cations are combined with, for example, the silicate anion, they are immediately neutralized and removed to the medium, so that the viscosity of the suspension is getting lower (Otterstedt and Brandreth, 1998; Chi and Eggleton, 1999). Izak et al., 2003 and Stempkowska et al., 2011 revealed that silicate fluidizers are not sensitive to overdosing in the suspension, unlike other inorganic stabilizers. That is why they are friendly in the use for application. This is characteristic for the organic fluidizers which work basing on specific adsorption and shielding of the van der Waals attractive forces. But there were no literature data about the fluidization mechanism of sodium water glasses of different silicate moduli, what could be very useful for the ceramic industry applications. Only articles about rheological properties of such slurries were found (Yildiz et al., 1998; Penner and Lagaly, 2001; Pacheco-Torgal et al., 2008; Penkavova et al., 2014). Because of this, the authors are trying to answer a question: How can the silicate modulus of the sodium water glass affect the mechanism of stabilization of the ceramic slurry? It is believed that these studies will help to understand the fluidization mechanism of sodium water glasses and will contribute to the development of knowledge about the structure and properties of sodium water glasses.
2.2. Rheological measurements The rheological measurements were conducted with the use of the rotary viscometer (Brookfield DV-III), equipped with a cylinder-andspindle (R-29) measuring system. Flow curves were made for increasing and decreasing shear rates in the range of 2.5–50 s− 1 and measuring points related to the change of shear rate were collected every 30 s. Additionally, after each flow curve measurement, thixotropy was measured. It relied on stopping the rotation for 10 min, whereupon viscosity was measured at a constant shear rate of 5.6 s− 1 after the time of 2 s, 3 s, 4 s, 5 s, 10 s, 20 s and 30 s. 3. Results and discussion 3.1. Rheological characterization of kaolin KOC slurries modified with sodium water glasses
2. Materials and methods The results showed that slurries based on kaolin KOC and modified with sodium water glasses possess thixotropic properties, which was probably associated with the intermolecular interaction mechanism. They exhibited c flows both at increasing and decreasing shear rates. It can be assumed that decreasing apparent viscosity (Fig. 2) at the increasing shear rate was the result of the gradual destruction of the internal structures of the slurry and their arrangement along the flow direction. Disintegration of the internal structure at high shear rates was not immediate, reflecting the dynamic (non-rheostable) and reversible properties. Increase the amount of fluidizer caused faster stabilization of the dispersion structure. Above the 0.3 wt% concentration of sodium water glasses with the silicate moduli in the range of 2.0–2.5 there was no visible improvement in stabilization of the ceramic slurry. Therefore, it can be concluded that the best fluidization properties were possessed by sodium water glasses with the silicate moduli in the range of 2.0–2.5
2.1. Preparation of kaolin KOC slurries The research was conducted with the use of concentrated kaolin KOC slurries which consisted of 60 wt% of kaolin KOC (Surmin-Kaolin, Poland; enriched in quartz and low in titanium and iron white-burned raw material, designed for the use in ceramic industry because if its stable rheological properties), 40 wt% of distilled water and sodium water glass ((Na2SiO2)nO) (Rudniki S.A. Chemical Plant, Poland) of different silicate moduli in the range of 1.74–3.25, which was added to the ceramic slurry in the amount of 0.1–0.5 wt% to the kaolin KOC dry mass. The sample was stirred for 5 min in such a way as not to aerate it. After obtaining a homogenous slurry, it was matured for 10 min and then again stirred before the measurement to destroy possible thixotropic structure. To remove water from the kaolin and thereby to stabilize the clay 148
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Table 2 Physicochemical analysis of used sodium water glasses (Factory standard, 2016). Sample number
Type
Na2O [wt%]
SiO2 [wt%]
Silicate modulus
Specific gravity [g/cm3]
Viscosity (20 °C) [mPa]
1 2 3 4 5 6 7 8 9 10 11 12
R-151-1.7 50 SH R-150S R-145 a R-145S R-145 R 47/48 R-149 R-140 R-146 2.9 R-137 3.1 R-137 E
16.6 13.8 13.6 12.7 13.5 12.1 12.0 11.7 10.3 10.7 9.3 9.2
28.0 27.4 30.2 29.4 31.9 29.3 31.1 32.4 28.9 31.3 28.1 28.9
1.74 2.05 2.29 2.39 2.44 2.50 2.67 2.86 2.89 3.02 3.09 3.25
1.550 1.484 1.523 1.485 1.528 1.472 1.487 1.505 1.427 1.460 1.394 1.381
80 157 530 610 500 189 363 750 141 518 100 114
and concentration of approximately 0.3 wt% to the kaolin dry mass. Silicates with higher moduli (above 3.0) had weaker stabilization properties which can be associated with the reduction of pH of the slurry and the existence of independent micelles, caused by heterocoagulation (Fig. 3). On the basis of the prepared analyses and literature data (Garcia et al., 2002; Papo et al., 2002; Falcone et al., 2010) it can be assumed that sodium silicates with higher moduli created their own independent structures that formed stable micelles, occurring independently of the dispersed kaolinite grains. As a result of this, the so-called depleted stabilization mechanism was created. The more developed sodium silicate structure, the larger independent micelle in the suspension. In contrast, sodium silicates with lower moduli formed insoluble silicates by ion exchange with cations compensating the surface charge and thereby they increased alkalinity of the medium. Probably in this way the developed spatial structure of sodium silicates reduced the efficiency of fluidization of ceramic slurries. This was evidenced by the increase of apparent viscosity of the kaolin suspension together with the increase of the silicate modulus regardless of the existence of the thixotropic structure. In this case it was assumed that at a shear rate of 50 s− 1 the thixotropic structure of the slurry was completely destroyed, while at a shear rate of 2.5 s− 1 it still existed. Basing on 29Si NMR, IR and Raman tests (not presented here) it can
Fig. 1.
29
Fig. 2. Fluidization curves of kaolin slurries modified with sodium water glasses.
be stated that depending on the silicate modulus there were several space structures in the solution formed from SiO44 − tetrahedrons. This “arrangement” could block (via a strong link between oxygen ions at
Si NMR spectrum of sodium water glass R-137.
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modified slurries together with the values of silicate modulus, a significant difference in dependence of measurement methods was seen. In the case of the assignment of internal energy and rigidity of thixotropic structure the correlation power was about 70%, whereas in the case of the thixotropy coefficient this fitting was below 20% (R2 = 0.1854). Generally, the low fitting of the measurement results to the determined trend (function of the best fitting was always chosen) was related to the type of the created thixotropic structure. It was valid because two stabilization mechanisms were putting on, i.e. ionic exchange and increment of the mutual repulsion of the clay mineral grains, as well as by the depleted stabilization in which micelles of the alkaline polymer silicates created a separate dispersed fraction. It can be justified by the determination of the work used for destruction and reconstruction of the thixotropic structure during shearing (Izak, 2015). The tests showed that at the specific shear rates, for example at 10 or 20s− 1 etc., this work was higher and probably associated with the reconstruction or destruction of the specified thixotropic structure. At this point it was hard to say if this mechanism was associated with interactions between grains or micelles. In the case of sodium silicates a clear repetition of this phenomenon was observed, even though the height of the peaks depended on the silicate modulus, thus the larger the modulus value the smaller height of the peaks.
Fig. 3. The pH changes of pure sodium water glass and kaolin slurry after the addition of 0.3 wt% of sodium water glass depending on the silicate modulus.
the silicon atom) the ion exchange with compensation cations on the surface of clay minerals and thus created a stable structure arrangement of electric charges, i.e. an independent micelle in the kaolin slurry. Such a mechanism was proved by a steady increase of the consistency coefficient (k) in the power law of Ostwald de Waele model k (η = γ1 − n ) as the increase of the silicate modulus value. In this case, the ceramic slurry modified with silicates behaved during shearing as if it had a higher solids concentration. The value of the flow index was stabilized at low values of the silicate modulus which was related to the thixotropic structure between the kaolinite grains. At higher values of the silicate modulus larger micelles begun to reduce the dilution properties of the suspensions. In this case, the values of the flow index increased.
3.3. Rheological models of the slurries modified with sodium silicates Tests showed that kaolin slurries modified with alkaline silicates fulfill best the Ostwald de Waele power model. Material constants of this formula for 0.3 wt% of sodium water glasses with different silicate moduli were placed in Table 3 (data for 0.33 h). The flow index remained stable (about 0.14) up to the modulus value of 2.86 and it increased for a higher modulus. This confirmed the impact of additional silicate micelles on the slurries flow mechanism during shearing. The values of the material constants, according to the above model, changed during aging of the suspensions (Fig. 5). This showed also the increase of the number or size of the slurry aggregates occurring during shearing of the suspensions. This phenomenon was due to an opposite ion exchange on surfaces of the clay minerals in a suspension. Polycations compensating the surface charge had a higher charge and a smaller size. The increase of number of polycations in the aging slurry was due to partial solubility of the calcium and magnesium (dolomite) silicates founded in the kaolin.
3.2. Thixotropic properties of kaolin slurries modified with sodium water glasses The tests showed that thixotropic properties of kaolin slurries depend upon the fluidization degree (Reeds, 1986; Pierre and Ma, 1997; Sjoberg et al., 1999). Because of it, the smallest thixotropy field loops were at lower values of the silicate modulus (Fig. 4). This relationship was best described by the quadratic function at the correlation level of approximately 78%. Independently from the fact of growing thixotropic properties of the
4. Conclusion Summing up, silicate modulus of sodium water glass has a significant impact on the rheological properties of the slurries based on kaolin KOC. The best results of the stabilization of kaolin slurry can be obtained while using sodium water glass of silicate moduli 2–2.5, what is associated with the ion exchange mechanism and the so called Table 3 Material constants of the Ostwald de Waele power model.
Fig. 4. Energy of thixotropy as a function of silicate modulus.
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Silicate modulus
Consistency index (k) [–]
Flow index (n) [–]
R2 (CoF) [%]
1.74 2.05 2.29 2.39 2.44 2.50 2.67 2.86 2.89 3.02 3.09 3.25
14,510 15,140 16,501 15,881 15,447 16,363 17,238 16,169 18,789 17,310 20,032 18,918
0.14 0.13 0.14 0.14 0.13 0.13 0.12 0.14 0.15 0.16 0.23 0.18
92.1 90.4 87.6 89.7 91.3 89.3 87.2 88.4 85.0 87.4 89.1 86.2
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Fig. 5. Changes of consistency coefficient according to the Ostwald de Waele power model during aging of the slurries modified with sodium silicates.
depleted stabilization. This is related to the lack of presence of the silicate structures of SiO44 − tetrahedrons which can block the ion exchange. Sodium water glasses of the moduli higher than 2.5 are worse fluidizers because they create independent silicate micelles co-existing with the dispergated kaolin grains in the slurry, while those of the moduli smaller than 2 precipitate free silica in the suspension and increase the basicity of ceramic slurry. Using different spectroscopy techniques such like 29Si NMR and IR spectroscopy, we do research on the better understanding of the structures of the sodium water glasses described in the article, but obtained by different synthesis methods. We also plan to extend our research on the fluidization mechanisms on other alkali silicates aqueous solutions, i.e. lithium and potassium water glasses with different silicate moduli. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References Bass, J.L., Turner, G., 1997. Anion distributions in sodium silicate solutions. Characterization by 29Si NMR and infrared spectroscopies, and vapour phase Osmometry. J. Phys. Chem. B 101, 10638–10644. Bergaya, F., Lagaly, G., 2001. Surface modification of clay minerals. Appl. Clay Sci. 19, 1–3. Cheng, D.C.H., 2003. Characterization of thixotropy revisited. Rheol. Acta 42, 372–382. Chi, M., Eggleton, R., 1999. Cation exchange capacity of kaolinite. Clay Clay Miner. 47,
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