RHEOLOGICAL RESPONSE TO CA(II) CONCENTRATION THE SOURCE OF KAOLIN SLURRY RHEOLOGICAL VARIATION

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1 RHEOLOGICAL RESPONSE TO CA(II) CONCENTRATION THE SOURCE OF KAOLIN SLURRY RHEOLOGICAL VARIATION Lavanya Avadiar 1, Yee-Kwong Leong 1*, Andy Fourie 2, Tutun Nugraha 2 The University of Western Australia 1 School of Mechanical and Chemical Engineering 2 School of Civil and Resource Engineering Crawley 6009, Australia * leong@mech.uwa.edu.au ABSTRACT Kaolin has been widely researched especially for its rheological and dewatering properties within mineral waste tailings. While kaolin from different sources displayed different rheological responses to ph, many studies have failed to fully appreciate the reasons of these differences. In this paper, variations in behaviours of kaolin slurries from two sources, Riedel and Unimin, having different elemental compositions are investigated using zeta potential, yield stress and sedimentation methods. Riedel displayed a maximum yield stress at a low ph of ~ 2.2 in contrast to ~ 8.6 within Unimin slurries. At any given ph, Riedel particles are generally more negative as reflected by higher negative zeta potentials. X-ray fluorescence and cation exchange capacity results showed that Riedel carries lower amounts of cations in particular calcium, Ca(II). The amount of Ca(II) cations on Unimin is ~11.1 times more than that on Riedel, i.e., 7.8 mequiv(h + )/100g vs. 0.7 mequiv(h + )/100g. Addition of Ca(II) cations into Riedel slurries at high enough concentrations caused magnitudes of negative zeta potentials to decrease and maximum yield stress peaks to shift to high ph regions, reproducing behaviours displayed by Unimin. This is due to hydrated Ca(II) ions adsorbing onto deprotonated edge sites and alumina faces on kaolin particles at high ph, allowing unlike charge attractions between edge sites and silica faces of kaolin particles that causes increments in yield stress along these ph regions. Ca(II) concentration has been attributed to be the source of difference in the rheological responses between Riedel and Unimin slurries at high ph. Keywords: Kaolin, Calcium, Rheology, Yield stress, Zeta potential, Sedimentation, ph, Charge attraction INTRODUCTION Kaolin is a valuable commodity with many industrial applications as well as a waste found in mine tailings. It is used as additives and fillers in paper coating formulations, ceramics, refractories, plastics, rubber, adhesives, sealants, pharmaceuticals and cosmetics (Greenwood et al., 2007). On the other hand, kaolin found in mine tailings poses profound detrimental effects on the consolidation of these waste tailings (McFarlane et al., 2008, Mpofu et al., 2003, Nasser & James, 2006). Microstructures formed by these negatively charged colloidal kaolin particles has led to poor dewatering behaviours of tailings requiring huge impoundment areas to confine these wastes. This poses environmental and water recycling challenges that needs to be solved effectively (Mpofu et al., 2003, McFarlane et al., 2008). Due to its wide range of applications and continuing problems with consolidation, extensive research concerning kaolin is being carried out. Chemeca 2012 Conference,Wellington, New Zealand, September

2 However, it has been observed that characterisation of kaolin powders used in researches are inadequate and inconclusive. Most studies primarily carried out X-ray diffraction (XRD) on kaolinites or plainly stated their origin, failing to fully appreciate differences that might exist between kaolin types. Melton and Rand (1977) observed that kaolin slurries from different sources (or studies) displayed different rheological responses to ph. The maximum Bingham yield stress was shown to be located at different ph for different kaolinites. No composition data were presented to correlate with the different rheological responses to ph. Presence of aluminium in kaolin was suggested to result in the maximum yield stress of kaolin slurries at high ph (Flegmann et al., 1969). Our group has observed similar differences in rheological responses to ph displayed by Sigma, Fluka and Unimin kaolin slurries (Leong et al., 2012, Shankar et al., 2010, Teh et al., 2009). Sigma and Fluka kaolin slurries displayed a maximum yield stress at ph ~ 2 where the magnitude of zeta potential was near minimum (Teh et al., 2009). However, Unimin kaolin displayed a maximum yield stress at ph ~ 8, where the zeta potential was significant in magnitude at ~ -35mV (Shankar et al., 2010). This suggests the presence of positively charged sites on Unimin kaolin particles at high ph that undergoes strong charge attractions with negatively charged sites on kaolin particles to produce maximum yield strengths. This maximum yield stress of Unimin kaolin slurries at high ph was completely eliminated by the adsorption of negatively charged phosphate additives such as triphosphate and polyphosphate (Shankar et al., 2010); further supporting the presence of positively charged sites on Unimin kaolin particles at high ph from ~ >8 that allowed unlike charge attractions. Kaolin clay comprises of 1:1 tetra octahedral aluminosilicate sheets having a general formula of Al 2 Si 2 O 5 (OH) 4 (van Olphen, 1963).One of the sheets or basal faces consists of tetrahedral siloxane (-Si-O-Si-) species and the other consists of octahedral alumina (Al 2 O 3 ) sheets (van Olphen, 1963). At the edges of kaolin particles, tetrahedral silica and octahedral alumina sheets are disrupted, exposing broken bonds comprising of aluminol (Al-OH) and silanol (Si- OH) groups (van Olphen, 1963). Basal faces comprising of tetrahedral silica sheets carry permanent negative charges due to isomorphous substitution of Si 4+ by Al 3+ groups (Du et al., 2009, Gupta et al., 2011, Mpofu et al., 2003, van Olphen, 1963). Octahedral alumina sheets and Al-OH and Si-OH edge sites on kaolin particles are ph-dependent, where surface hydroxyl groups protonate and deprotonate according to slurry ph (Du et al., 2009, Gupta et al., 2011, Nasser & James, 2006, van Olphen, 1963). The point of zero charge of alumina sheets should be at ph ~ 9.5 (Shankar et al., 2010) and that of edge sites is between ph 5 to 7 (Du et al., 2009). As a result, from ph ~ 8, numbers of positively charged sites on kaolin surfaces should be decreasing to a great extent with increasing ph. However, the adsorption of phosphate additives at high ph suggest the presence of positively bound sites on Unimin particle surfaces at those ph levels. This could be due to the presence of positively charged cations such as Ca(II) on Unimin kaolin surfaces that allows positive-negative charge attractions. Unlike the other multivalent ions such as Al(III) and Mg(II) identified on Sigma, Fluka and Unimin kaolinites, Ca(II) ions exist on Unimin particle surfaces in higher concentrations to hydrolyse and form complex hydrated ions at high ph that leads towards the high yield stress of Unimin slurries. While hydrated Al(III) cations have shown negligible effects on the rheological behaviours of Sigma and Fluka kaolinites at high ph, i.e., where Sigma and Fluka kaolinites displayed low yield stresses along the high ph regions, Mg(II) ions do not tend to hydrolyse easily to form complex ions (Baes & Mesmer, 1976). In this study, Unimin kaolin with high Ca(II) contents and Riedel kaolin with low Ca(II) contents were analysed. The objective was to resolve if Ca(II) cations caused variations in the rheological responses of kaolin types to ph. Zeta potential, yield stress and sedimentation behaviours of these kaolin slurries were evaluated and compared to express differences in 2

3 rheological responses between kaolin types. X-ray fluorescence (XRF) and cation exchange capacity (CEC) analyses of these kaolin types were carried out to substantiate these differences. MATERIALS AND METHODS Two as-received kaolin powders sourced from Riedel-de Haën and Unimin Australia Ltd were used. XRD analysis showed trace amounts of muscovite and quartz in Riedel and quartz in Unimin. Anhydrous CaCl 2 used was provided by Ajax Finechem Pty Ltd. All kaolin slurries were prepared by adding appropriate amounts of respective kaolin powders and CaCl 2 powders (when required) with distilled water and then sonicated using the Branson digital Sonifier with a 1-inch probe over 20-40seconds. For zeta potential and yield stress measurements, the initial ph of slurries was adjusted to ~ 12 with 1-5M NaOH solutions. Subsequently, the ph of these slurries was decreased to ~ 2 in a stepwise manner with the addition of M HCl solutions. For zeta potential measurements, low solids concentrations of 5wt% kaolin slurries were used. Zeta potential was measured using the Colloidal Dynamics ZetaProbe. For yield stress measurements, high solids concentrations of 40wt% slurries that will result with flocculated network structures were used. Yield stress was measured with the Brookfield LVDV-II and RVDV-II vane viscometers, each having a spring constant of mNm and mNm respectively. The vane rotational speed on both viscometers was 0.6rpm and slurries were left to equilibrate over 5min before ph was constant and after each yield stress reading to eliminate past shear histories within slurries. For sedimentation characterization, low solids concentrations of 8wt% kaolin slurries at ph ~ 8 were used. This corresponds to the ph of mineral waste tailings. Unimin s natural ph in water is ~ 8; however Riedel s is ~5. 1M NaOH solutions were added into Riedel slurries before sonication. Upon sonication, slurry contents were emptied into 250ml beakers. At regular time intervals, mud-lines representing suspension-solution interfaces were measured to calculate solids content with respect to time. Particle size distributions were measured using the Malvern Mastersizer Microplus particle size analyzer. RESULTS AND DISCUSSION Significant variations were observed with zeta potential, yield stress and sedimentation behaviours between Riedel and Unimin slurries. Figure 1 shows that the magnitudes of negative zeta potential is higher for Riedel particles compared to Unimin particles at similar ph values. This difference was increasingly significant from ph > ~4.5 to 12, where magnitudes of zeta potentials increased to greater extents for Riedel particles as ph increased. From ph ~4.5 to 12, zeta potentials of Riedel particles ranged from -30.4mV to -53.7mV respectively; where it was lower at -24.5mV to -39.4mV for Unimin particles. No isoelectric points (IEP) were observed for both kaolin types. The absence of IEP even at very low ph is an indication of the greater effect of negatively charged silica basal faces over edge sites and alumina faces on the zeta potentials of Riedel and Unimin particles (Johnson et al., 1999). As ph decreases, greater amounts of hydrogen ions (H + ) will be adsorbed onto edge sites and alumina faces, causing zeta potentials of particles to be increasingly positive (Teh et al., 2009). Increments in positive charges of edge sites and alumina faces could equalise negative charges on silica basal faces, reaching IEP of particles. The absence of IEP indicates that Riedel and Unimin comprise of particles which have high face-edge ratios. This observation was supported by Shankar et al. (2010) who reported similar structures for Unimin particles. High face-edge ratios cause particles to have 3

4 more negative than positive charges even when ph is low due to the permanent negative charges on basal silica faces (Teh et al., 2009). From particle size distributions of Riedel and Unimin shown in Table 1, it was observed that these particles also have very similar face-edge ratios. The equivalent volume mean diameters or D (4, 3) of Riedel and Unimin are 5.8 µm and 5.4µm respectively. Variations between the Hinckley indexes (I H ) of the two kaolin types are also insignificant with Riedel s I H at 1.2 and Unimin s at These minor differences in I H and particle sizes between Riedel and Unimin particles indicate that variations in rheological properties between the kaolin types cannot be attributed to the differences between their microstructures. In this case, the smaller negative zeta potential of Unimin particles could be due to i) the adsorption of positively charged ions such as multivalent cationic species and/or ii) a higher fraction of particle area with positively charged sites. Unimin has shown to have a higher content of Ca(II) cations adsorbed on its particle sites. Fig.1: Zeta potential-ph behaviours of 5wt% Riedel and Unimin slurries Tab. 1: Particle size distributions of Riedel and Unimin kaolinites (All measurements are in microns) Kaolin type D(4,3) D(3,2) D(v,0.9) D(v,0.5) D(v, 0.1) Riedel Unimin From Figure 2, yield stress of 40wt% Riedel and Unimin slurries were also seen to deviate significantly from ph > ~4.5. Yield stress of Riedel was zero at ph >~8. The high negative zeta potential in this ph region results in strong repulsive interactions between the Riedel particles, causing the slurry to be completely dispersed. Below ph 8, yield stress began to appear. Upon further reductions in ph, yield stress initially increased gradually and then sharply to a peak value of ~102.5Pa at ph ~2.2. From ph ~4.5 to ~8, the gradient of Riedel s zeta potential ph curve was higher than between ph ~8 to ~12. This means that the decrease in the magnitudes of negative zeta potentials were higher from ph ~ 8 to ~ 4.5 than from ph ~ 12 to ~8. The sharper decrease in zeta potential corresponded to the region of 4

5 increasing yield stress where electrostatic repulsive forces progressively became weaker, allowing unlike charge attractions between positively charged edge sites and negatively charged basal silica faces to dominate, resulting in the formation of flocculated card-house network structures. Below ph ~4.5, these attractive inter-particle forces became stronger, strengthening the network structure which was reflected by a smaller (negative) zeta potential and a larger yield stress. However, the yield stress-ph trend was very different for Unimin slurries which showed large yield stress in the high ph region of >8. Yield stress was observed to increase from ph ~ 11.5 to ~8.6, where it peaked at ~143.1Pa. Subsequently, it gradually decreased to reach a minimum value of ~63.4Pa at ph ~2.0. With decreasing ph, kaolin particles exhibit reductions in magnitudes of negative zeta potentials where edges sites begin to protonate from ph ~7. This increases edge-face interactions between edge sites and basal silica faces where correspondingly repulsive forces between kaolin particle sites are reduced, increasing inter-particle network strengths and thus the yield stress of the slurries. There was no ph region where the slurry was completely dispersed. The maximum yield stress occurred at the condition of high negative particle zeta potentials. The inter-particle attractive forces must be very strong to overwhelm the strong repulsive forces as reflected by the high negative zeta potential. Unimin has a much higher CaO content, which may be responsible for a strong attraction such as a Ca(II) mediated unlike charge attraction Yield stress (Pa) Unimin 20 0 Riedel Fig. 2: Yield stress-ph behaviours of 40wt% Riedel and Unimin slurries. The error bar indicates an average deviation of ±5Pa in yield stress values measured where these error margins could be attributed to past shear histories within slurries Tab. 2: Compositions of Riedel and Unimin kaolinites. Highlighted values represent elements with significant concentration differences between the two kaolinites (All values are in %) Elements Detection limit Riedel Unimin (DL) Al2O SiO TiO ph 5

6 Fe2O MnO CaO K2O MgO Na2O 0.1 <DL 0.36 P2O SO <0.01 The chemical compositions of Riedel and Unimin kaolinites determined via XRF are shown in Table 2. It shows that Unimin kaolin has higher amounts of Ti(IV), Ca(II) and Na(I) and smaller amounts of K(I) and P 2 O 5, where multivalent Ti(IV) and Ca(II) cations could have contributed to the unlike charge attractions between Unimin particles. However, TiO 2 is soluble in water only at ph <7 and that is not in the ph range of interest that could have led to the rheological differences between Riedel and Unimin slurries (Baes & Mesmer, 1976). Table 3 has shown higher amounts of exchangeable Ca(II) and Mg(II) cations on Unimin particle surfaces where Riedel has a higher content of exchangeable Al(III) cations. The latter is due to Riedel s low natural ph that causes the dissolution of Al 2 O 3. Al(III) ions hydrolyse from ph >3 and this explains that even with the presence of hydrolysed aluminium ions at high ph within Riedel slurries, yield stress was negligible within Riedel (Baes & Mesmer, 1976, Bezak-Mazur et al., 2001). This eliminates the contribution of Al(III) to the yield stress peak observed within Unimin slurries along the high ph region. Even though concentrations of Mg(II) was quite similar between Riedel and Unimin particles as observed from Table 2, Unimin showed much higher amounts of exchangeable Mg(II) from Table 3. This explains that Unimin has higher amounts of Mg(II) cations present on particle surfaces than within crystal structures unlike Riedel particles. Mg(II) cations show reducing signs of hydrolysis with decreasing ph, where this reduction becomes more significant from ph ~ 6 (Baes & Mesmer, 1976). Thus Mg(II) could have contributed towards the higher yield stress of Unimin along the high ph region. However, Mg(II) ions carry low tendencies towards forming complex ions compared to Ca(II) (Baes & Mesmer, 1976). Calcium compounds are soluble in water at high ph and forms hydrated complex ions. The hydrolysis and formation of complex ions and the eventual adsorption of these ions onto kaolin particle surfaces brings about the formation of face-face or edge-face interactions between kaolin particles that result in high yield stresses within Unimin slurries. Unimin has 11.1 times more Ca(II) content than Riedel on its exposed faces and edge sites according to the CEC analysis on Table 3. Ma and Eggleton (1999) explained that exchangeable cations are primarily present on edges and alumina faces of kaolin particles. These cations present on exposed surfaces affect surface charges and interactions of particles, resultantly affecting their rheological and flocculation behaviours. Tab. 3: Compositions of exchangeable ions on Riedel and Unimin kaolinites at their natural ph in water (All values are in mequiv(h + )/100g) Ex Al Ex Ca Ex K Ex Mg Ex Na Natural ph Unimin < Riedel

7 Figure 3 shows reductions in magnitudes of negative zeta potentials of Riedel particles with increasing concentrations of CaCl 2 added. The zeta potential-ph behaviour of Riedel approached that of Unimin s. Yukselen and Kaya (2003) made similar observations to zeta potentials of kaolinite with Ca(II) added. These increments in zeta potentials were especially obvious from ph > ~4.5. This corresponds to the ph level where increasing deviations were observed with zeta potential and yield stress behaviours between Riedel and Unimin slurries. With CaCl 2 pellets added into water, the partial positive and negative charges on water molecules cause these ionic compounds to dissolve, liberating Ca(II)ions into the slurry. Ca(II) ions in water have large hydration energies and thus each Ca(II) ion bonds with 6 water molecules to exist in the hexahydrate form, Ca(H 2 O) 6 2+ (Konan et al., 2007). According to Konan et al. (2007), these hydrated calcium ions adsorb onto the Lewis donors of silica faces on kaolin particles through water molecules. However, adsorptions of these hydrated calcium ions are inferred to take place on alumina faces and edge sites of kaolin particles instead. Isoelectric points of alumina faces may be at ph ~9.5 and that of edge sites is between ph 5 to 7. As ph increases to ~5, edge sites begin to deprotonate and gradually allow adsorptions of hydrated calcium ions onto Lewis donors on edge sites. This explains the increments in zeta potentials observed from ph ~4.5 with CaCl 2 added. Further increments in ph to 9.5 causes deprotonation of alumina faces as well, where they exist as (Al-O) instead of (Al-OH). This increases adsorptions of hydrated calcium ions, causing zeta potentials to increase significantly over the high ph range. Adsorptions of hydrated calcium ions onto silica faces alone would not result in such variations with ph as silica faces are negatively charged throughout the ph range of 2 to 12. Zeta potential behaviours of Riedel were most similar to Unimin at CaCl 2 concentrations of between 0.2dwb% and 0.35dwb%. Fig. 3: Effect of CaCl 2 on zeta potential-ph behaviours of 5wt% Riedel slurries. The plot for Unimin was included for comparison Yield stress-ph behaviours of Riedel slurries were also similar to Unimin slurries when CaCl 2 was added. This result is shown in Figure 4. Yield stress peaks are now located at high ph and the previous yield stress peak of Riedel at a low ph of ~ 2.2 decreased in value. With 0.2dwb%, 0.35dwb% and 0.5dwb% CaCl 2 added, yield stress peaks were observed at ph ~8.1, ~8.5 and ~10.7 respectively. Yield stress-ph behaviour of Riedel was most similar to 7

8 Unimin s at similar CaCl 2 concentrations observed from zeta potential results on Figure 3; of between 0.2dwb% and 0.35dwb% CaCl 2. As Ma and Eggleton (1999) had observed, exchangeable Ca(II) cations are inferred to be primarily on alumina faces and edge sites on Unimin kaolin particles. As ph increases, these ions could be tightly bound to these surfaces through adsorptions between hexahydrate forms of these calcium ions and deprotonated alumina faces and edge sites. This increases unlike charge attractions of positively charged alumina faces and edge sites with negatively charged silica faces, allowing F-F interactions and card-house or E-F interactions between Unimin particles at high ph levels. Lagaly (1989) had categorised the microstructures of kaolin slurries to be card-house at low ph and over-lapping coins configurations comprising of E- E interactions that form band-like structures at high ph. This explains the yield stress trend of Riedel slurries; where extensive microstructures formed by kaolin particles with card-house structures at low ph yield at higher stresses than flatter and weaker bonds of kaolin particles adopting the over-lapping coins configurations at high ph. The formation of card-house structures within Unimin slurries at high ph due to the presence of Ca(II) cations increased yield stress along those ph regions Riedel 0.2dwb% CaCl 2 Yield stress (Pa) dwb% CaCl 2 0.5dwb% CaCl 2 Unimin ph Fig. 4: Effect of CaCl 2 on yield stress-ph behaviours of 40wt% Riedel slurries. The plot for Unimin was included for comparison. The error bar indicates an average deviation of ±5Pa in yield stress values measured where these error margins could be attributed to past shear histories within slurries The effect of Ca(II) on the sedimentation of Riedel and Unimin slurries is shown in Figure 5. Sediment solids concentration vs time behaviours of Riedel slurries were most comparable to Unimin slurries with 0.35dwb% CaCl 2 added into Riedel slurries. Negligible flocculation and consolidation were observed due to inter-repulsive forces between highly negatively charged Riedel particles at ph ~8. However, due to positive-negative charge attractions between kaolin particles, aggregation and sedimentation of kaolin flocs were observed within Unimin slurries at a similar ph. The addition of Ca(II) cations into Riedel slurries initiated possibilities of unlike charge attractions between Riedel particles, which could have led to similarities in microstructures of flocs formed within Unimin and Riedel slurries. 8

9 Fig. 5: Settling test: Sediment solids concentration (wt%) vs. t (min) of Unimin and Riedel slurries with CaCl 2 added into Riedel. ph of slurries at ~8 CONCLUSION Differences in the rheological responses to ph between Riedel and Unimin kaolin slurries has been shown to be due to the difference in Ca(II) concentrations between the two kaolin types. Unlike other multivalent ions such as Al(III) and Mg(II), Ca(II) ions exist on kaolin particle surfaces in higher concentrations to hydrolyse and form complex hydrated ions at high ph that leads towards the high yield stress of Unimin. Addition of Ca(II) cations to the low calcium Riedel kaolin caused its yield stress (rheological)-ph, zeta potential-ph and sedimentation responses to be similar to that displayed by the high calcium Unimin kaolin. Strong attractive interactions between kaolin particles with adsorbed Ca(II) cationic species increased the yield stress of Riedel kaolin slurries at high ph. ACKNOWLEDGEMENTS We wish to thank ARC for funding this project via DP REFERENCES BAES, C.F. & MESMER, R.E The Hydrolysis of Cations. New York, London, Sydney and Toronto: A Wiley-Interscience Publication. BEZAK-MAZUR, E., WIDLAK, M. & CIUPA, T A Speciation Analysis of Aluminium in the River Silnica. Polish Journal of Environmental Studies, 10, DU, J., PUSHKAROVA, R.A. & SMART, R.S.C A cryo-sem study of aggregate and floc structure changes during clay settling and raking processes. International Journal of Mineral Processing, 93, FLEGMANN, A.W., GOODWIN, J.W. & OTTEWILL, R.H Rheological Studies on Kaolinite Suspensions. Proceedings British Ceramic Society, 13, GREENWOOD, R., LAPCIKOVA, B., SURYNEK, M., WATERS, K. & LAPCIK, L The zeta potential of kaolin suspensions measured by electrophoresis and electroacoustics. Chemical Papers, 61,

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