KONA Powder and Particle Journal No. 33 (2016) 17–32/Doi:10.14356/kona.2016007 Review Paper Surface Chemistry and Rheology of Slurries of Kaolinite and Montmorillonite from Different Sources † Pek-Ing Au and Yee-Kwong Leong * 1 School of Mechanical and Chemical Engineering, The University of Western Australian, Australia Abstract Zeta potential (ζ)-pH and yield stress (τy)-pH behaviour of a number of kaolinite and montmorillonite slurries, including CMS-sourced materials, were compared. The CMS KGa-2 and Crown kaolin with very similar elemental composition displayed almost identical ζ-pH and τy-pH behaviour. Both displayed a pHζ=0 at 3–4 where the maximum τy was located. This pHζ=0 was higher at higher ionic strength as the pH-dependent charge increased with ionic strength and the permanent structural charge being invariant. The other kaolinite slurries (Reidel and Unimin) with different composition showed different behaviour. The surface chemistry and rheological properties of CMS-sourced SWy-2 (Na-) and STx-1b (Ca-) montmorillonite and bentonite slurries were also compared. They all displayed a negative zeta potential that was insensitive to pH. STx-1b slurries required a much higher solid concentration to form a gel. Maximum τy occurred over a broad range of pH. This pH is ~5 for SWy-2, 8 for STx- 1b, <2 for bentonite and 12 for API bentonite. Their difference in clay mineral composition such as impurities and exchangeable cations were highlighted. The point of zero charge (pzc) of kaolinite and montmorillonite slurries obtained via Mular-Roberts pH-salt addition method did not correlate well with the pHζ=0 except for the KGa-2 and Crown kaolin. Keywords: drilling fluids, yield stress-volume fraction, zeta potential, clay mineralogy, fractal dimension, Mular- Roberts, KGa-2, SWy-2, STx-1b 1. Introduction of kaolinite slurries (Au P. and Leong Y., 2013). Bentonite composed of mainly sodium montmorillonite Kaolin mineral slurries from different sources or de- (Na+Mt) is another important commercial clay mineral posits display different rheological and surface chemical because of its many uses, for example in paper coating, properties (Au P. and Leong Y., 2013; Au P. et al., 2014; catalysts, pharmaceutical products, drilling muds, as an Lagaly G., 1989; Rand B. and Melton I.E., 1977; Teh E. et impermeable slurry wall and nuclear waste storage bar- al., 2009). Kaolin is a very important commodity mined rier. Slurries of bentonite displayed highly complex all over the world. The ability to use its mineral and ele- time-dependent rheological properties. Na+Mt in water mental composition to predict its slurry rheological be- swelled considerably. Factors affecting its rheological be- haviour from its surface properties would be extremely haviour are thus more numerous such as impurities, ex- useful and beneficial when processing this clay mineral. changeable cations and salt concentration (Chang W. and However not all of the important factors responsible for Leong Y., 2014; Goh R. et al., 2011; van Olphen H., 1955). the variation in rheological behaviour have been identified However, despite the numerous studies, the knowledge and their effects understood. Attempts have been made to available in the open literature is still quite confusing. identify these factors (Rand B. and Melton I.E., 1977; Teh Gels of Na+Mt from different sources displayed different E. et al., 2009). Factors such as Ca(II) or CaO content or rheological properties. According to Lagaly G. (1989), the Ca/Na ratio (Avadiar L. et al., 2014; Avadiar L. et al., important factors are particle structure and texture. Upon 2015; Lagaly G., 1989) have been identified. The presence swelling different Na+Mt will disintegrate to different ex- of a relatively small amount of smectite content can also tents giving rise to stack-layered particles of different have a very significant effect on the rheological behaviour thickness, different particle concentration and different shape flow units producing gels. A better evaluation of the † Received 26 May 2015; Accepted 29 June 2015 difference would be to compare the rheological properties J-STAGE Advance published online 25 July 2015 over a wide pH range. Like kaolinite, the comparison 1 35 Stirling Highway, Crawley WA 6009, Perth, Australia should also include zeta potential-pH behaviour. Such a * Corresponding author: Yee-Kwong Leong; E-mail: [email protected] comparison would allow for a more effective evaluation TEL: +61-8-6488 3602 FAX: +61-8-6488 1084 of the factors responsible for the rheological variation. ©2016 Hosokawa Powder Technology Foundation 17 Pek-Ing Au et al. / KONA Powder and Particle Journal No. 33 (2016) 17–32 Rheological and surface properties data from slurries pre- constant AH and is independent of the particle size a and pared from standard or well-characterised Na+Mt and concentration of the dispersion. This equation has been kaolinite are required for comparison and evaluation. used to determine the Hamaker constant of pristine oxide Such clay minerals can be sourced from the Clay Mineral in water (Leong Y. and Ong B., 2003; Teh E. et al., 2010). Society USA (CMS) and will have well-characterised Laxton P. and Berg J. (2006), however, observed a posi- 2 compositional, physical and surface properties data (CMS, tive slope for the linear relationship between τy and ξ of 2015). The surface chemistry and rheological property kaolinite–bentonite composite slurries. They attributed data obtained for these clay mineral slurries will form the this to positive–negative charge attraction between the baseline or benchmark data for other kaolin and montmo- clay platelets. In their study, the zeta potential was varied rillonite slurries to compare in this search to identify the by ionic strength instead of pH. important factors and understand their effects. CMS- Apart from zeta potential, there are several techniques sourced kaolin KGa-2 and Na+Mt SWy-2 and Ca2+Mt for characterizing the pzc of a mineral powder. One such STx-1b were used in this study. technique is the Mular–Roberts (MR) salt addition–pH The complete characterization of the surface properties method (Mular A. and Roberts R., 1966). This method of the clay minerals may require the value of several com- has been shown to accurately determine the pzc of pure ponents: point of zero charge (pzc) or isoelectric point inorganic oxides such as, Fe2O3, Al2O3, SiO2 and TiO2 (IEP) (Pradip et al., 2015), point of zero net proton charge, (Alvarez-Silva M. et al., 2010; Mular A.L. and Roberts zeta potential-pH-ionic strength behaviour, charges (layer, R.B., 1966). Its accuracy with clay minerals such as ser- tetrahedral, octahedral, unbalanced and extra Si) and pentine and chlorite phyllosilicate minerals is still ques- cation exchanged capacity or CEC. For CMS-sourced tionable (Alvarez-Silva M. et al., 2010). For a pristine clay minerals, most of these parameters are known and inorganic oxide such as rutile TiO2, the pzc is independent listed on their database (CMS, 2015). For correlation with of the ionic strength of an indifferent electrolyte (Hunter rheological properties, normally only the zeta potential, R., 2003). The MR method exploited this property to de- including pzc, is required because the magnitude of the termine the pzc. At pzc, the pH of the mineral suspended rheological parameters such as yield stress and viscosity, in 0.001 M sodium chloride concentration will remain is governed by the nature and strength of the predominant unchanged when the salt concentration is increased to interparticle force. Zeta potential is a measure of the 0.01 M. At other pH levels, a difference in the pH be- strength of the interparticle repulsive force and is often tween these two ionic strength states will be present. used to define the state of the slurries; flocculated or dis- With pure oxides, such as alumina and silica, the surface persed. A high magnitude is normally associated with no charges are all pH-dependent. In contrast, clay minerals or a very low yield stress. This is often not the case with contain permanent structural negative charges which are some clay mineral slurries (Leong Y. et al. 2012; Au P. pH-independent (Bolland M. et al., 1980). and Leong Y. 2013). Many kaolin slurries do obey the Alvarez-Silva M. et al. (2010) found that the MR yield stress-DLVO model (Hunter R. and Nicol S., 1968; method for pzc determination was suitable for serpentine Teh E. et al., 2009; Au P. and Leong Y, 2013). but not for chlorite phyllosilicate mineral. In this study, Among the many yield stress τy-DLVO force or inter- we extended their work to other clay minerals and in- action energy models (Hunter R. and Nicol S., 1968; Leong cluded rheological data to correlate with the surface Y. and Ong B., 2003; Teh E. et al., 2010) is one based on chemistry data. In addition, we also aim to develop an constant surface potential for interactions between spher- understanding of the applications and limitations of the ical particles and is given by (Scales P., et al., 1998): MR method for kaolinite and montmorillonite. 2 −κDo φκsHA 2 e τ yo≈−2πε εξ (1) aD12 2 1e+ −κDo o 2. Materials and methods 2 2 The number of particles per unit area is scaled to ϕs /a . The model predicts a linear relationship between τy and 2.1 Materials and mineral characterisation zeta potential squared ξ2 at a fixed ionic strength. The critical zeta potential ξcri characterizes the point of transi- The CMS KGa-2 kaolin was sourced from a deposit tion from flocculated to dispersed state where τy = 0 located in Warren County, Georgia, USA. KGa-2 is a high (Leong Y.K. and Ong B.C., 2003). Thus Eqn.