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Effect of Surface Structure of Kaolinite on Aggregation, Settling Rate, and Bed Density Jianhua Du,*,†, Gayle Morris,‡ Rada A. Pushkarova,§ and Roger St. C. Smart† ACeSSS (Applied Centre for Structural and Synchrotron Studies), §Ian Wark Research Institute, and CRC CARE (Cooperative Research Centre for Contamination Assessment and Remediation of the Environment), University of South Australia, Mawson Lakes, SA 5095, Australia, and ‡Institute for Sustainability and Innovation, Victoria University, P.O. Box 14428, Melbourne MC, VIC 8001, Australia )
†
Received January 8, 2010. Revised Manuscript Received April 18, 2010 The flocculation and solid/liquid separation of four well-characterized kaolinites (2 well, 2 poorly crystallized) have been studied for comparison of surface structure (SEM), aggregate structure during flocculation (cryo-SEM), settling rate, and bed density (with raking). It is shown that major differences in these properties are largely due to crystallinity and consequent surface structure of the extensive (larger dimension “basal”) face. Well-crystallized kaolinites, with higher Hinckley indices and lower aspect ratios, have relatively smooth, flat basal surfaces and thicker edge planes promoting both effective initial bridging flocculation (largely edge-edge) and structural rearrangement to face-face during the raking process. This results in faster settling rates and more compact bed structures. Poorly crystallized kaolinites, with low Hinckley indices and high aspect ratios, exhibit ragged, stepped structures of the extensive face with a high proportion of nanosized islands forming cascade-like steps (i.e., multiple edges) contributing up to 30% of the specific surface area and providing flocculant adsorption sites (hydroxyl groups) across this extensive face. This leads to bridging flocculation taking place on both edge and extensive (“basal”) planes, producing low-density edge-face structures during flocculation which leads to slow settling rates and poor bed densities. In particular, the complex surface morphology of the poorly crystallized kaolinites resists the transformation of edge-face structures to dense face-face structures under shear force introduced by raking. This results in low sediment density for poorly crystallized kaolinites. The studies suggest that the main influence on settling rates and bed densities of kaolinites in mineral tailings is likely to be related to the crystallinity and surface morphology of the kaolinite. They also suggest that interpretation of kaolinite behavior based on models of a flat (001) basal plane and edge sites only at the particle boundaries is not likely to be adequate for many real, less-crystallized kaolinites.
1. Introduction Kaolinite is a common clay mineral present in many industrial mineral tailings. The solid-liquid separation of clay-based mineral tailings in industrial-scale thickeners generally requires the addition of a large volume of high molecular weight polymeric flocculant. Flocculation is a multistep process involving the following stages or components: (a) particle-polymer mixing, (b) adsorption of the polymer molecules onto the particle surface, (c) reconformation of the polymer molecules on the particle surface, (d) particle flocculation, and (e) floc breakup due to application of shear. These processes can take place concurrently and are often competing.1 A full understanding and control of the flocculation and dewatering processes during mineral tailing disposal is not yet achieved. Lack of control of the flocculation and dewatering process can result in poor solid-liquid separation in industrial thickeners along with excessive operational and capital costs. There have been extensive studies of the chemical and physical properties of kaolinite-based mineral tailings to gain a better understanding of their influences on the solid-liquid separation and consolidation. Most researchers have focused on the chemical properties of the tailings, for example, solution pH, ionic strength, polymer types, polymer charges, and polymer adsorption. *Corresponding author. Tel.: þ618 83023520; fax: þ618 83025545. E-mail address:
[email protected]. (1) Elimelech, M.; Gregory, J.; Jia, X.; Williams, R. A. Particle Deposition and Aggregation: Measurement, Modelling and Simulation; Butterworth-Heinemann: Oxford, 1998; Part 1.
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Zhou et al.2 studied by atomic force microscopy the effect of polymer charge on the interaction between silica surfaces and cationic polymeric flocculant. They found that the magnitude of the adhesive force was more significant in determining the compressive yield stresses of the silica particle sediments than the aggregate size and structure. Mpofu et al.3 investigated adsorption of Mn (II) and Ca (II) onto the surface of kaolinite at pH 7.5 and 10.5 and found that the optimum flocculation conditions involve a partial, rather than complete, particle surface coverage by both metal ions and anionic polyacrylamide (PAM) flocculant. At pH 7.5, the particle interactions produced by the metal ions and flocculant were more conducive to higher settling rates and greater consolidation. Three different types of high molecular weight flocculants have been studied4-6 to investigate the influence of shear conditions on floc characteristics, including anionic polyacrylamide (PAM-A), nonionic polyacrylamide (PAM-N), and polyethylene oxide (PEO). These studies found that the settling rates of PAM-A and PEO-based slurries were considerably higher than that of PAM-N-based slurry, proposed to be due to the more expanded interfacial conformation of PAM-A and PEO polymer (2) Zhou, Y.; Gan, Y.; Wanless, E. J.; Jameson, G. J.; Franks, G. V. Langmuir 2008, 24, 10920–10928. (3) Mpofu, P.; Addai-Mensah, J.; Ralston, J. J. Colloid Interface Sci. 2003, 261(2), 349–359. (4) McFarlane, A.; Bremmell, K. E.; Addai-Mensah, J. Powder Technol. 2005, 160(1), 27–34. (5) Farrow, J. B.; Johnston, R. R. M.; Simic, K.; Swift, J. D. Chem. Eng. J. 2000, 80(1-3), 141–148. (6) Sengupta, D. K.; Kan, J.; Al Taweel, A. M.; Hamza, H. A. Int. J. Miner. Proc. 1997, 49(1-2), 73–85.
Published on Web 07/14/2010
DOI: 10.1021/la100088n
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Figure 1. Schematic depiction of kaolinite particles aggregation mode with increasing polymer concentration.13
chains. McFarlane et al.7,8 investigated orthokinetic flocculation of kaolinite and smectite dispersions at pH 7.5 and 22 °C and concluded that the optimum flocculation settling rate conditions did not have a significant impact on the final sediment solid content of 20-22 wt %, while the further application of shear to presediment pulps improved consolidation by 5-7 wt % solid. These studies, focusing on the actions of the flocculant and solution conditions, have not addressed the surface structure and morphology of different kaolinites and their effect on the flocculation. This will be the focus of this paper. The flocculant adsorption mechanisms have also been extensively studied over many years. Heller and Keren9 suggest that the edge surfaces play a major role in polyacrylamide adsorption; The adsorption of polyacrylamide on the different surfaces of kaolinites has been studied,10,11 and Lee et al.10 concluded that 94% adsorption of polyacrylamide occurs primarily on the edge surface of kaolinite through hydrogen bonding. Nabzar et al.12-14 proposed an aggregation model as shown in Figure 1, with edge-edge (E-E) flocculation through polymer bridging occurring at moderate polymer concentration. In all cases, relatively perfect model kaolinite particles with flat basal surfaces and edges have been assumed. Our results will challenge these assumptions. With flocculant adsorption proposed to occur only onto the edge surface of kaolinite, it is important to have more detailed understanding of the physical properties (particle size, specific surface area, aspect ratio, surface charge, kaolinite surface morphology, and crystallinity) of kaolinite. The ideal structure of kaolinite is a pseudo hexagonal platelet composed of a layer of Al-OH octahedra linked to a layer of Si-O tetrahedra, where the basal surface carries a permanent pH-independent negative charge assumed to be a result of either isomorphous substitution of Si4þ by Al3þ groups15 or positioning of the electrodependent charge (7) McFarlane, A.; Bremmell, K.; Addai-Mensah, J. J. Colloid Interface Sci. 2006, 293(1), 116–127. (8) McFarlane, A.; Yeap, K. Y.; Bremmell, K.; Addai-Mensah, J. Colloid Surf., A: Physicochem. Eng. Aspect 2008, 317(1-3), 39–48. (9) Heller, H.; Keren, R. Clay Clay Miner. 2003, 51(3), 334–339. (10) Lee, L. T.; Rahbari, R.; Lecourtier, J.; Chauveteau, G. J. Colloid Interface Sci. 1991, 147(2), 351–7. (11) Ray, D. T.; Hogg, R. J. Colloid Interface Sci. 1987, 116(1), 256–68. (12) Nabzar, L.; Pefferkorn, E. J. Colloid Interface Sci. 1985, 108(1), 243–8. (13) Nabzar, L.; Pefferkorn, E.; Varoqui, R. J. Colloid Interface Sci. 1984, 102(2), 380–8. (14) Nabzar, L.; Pefferkorn, E.; Varoqui, R. Colloids Surf. 1988, 30(3-4), 345–53. (15) Bolland, M. D. A.; Posner, A. M.; Quirk, J. P. Clay Clay Miner. 1980, 28(6), 412–418. (16) Gerson, A. Computational Chemistry and Chemical Engineering, In Proceedings of the UNAM-CRAY Supercomputing Conference, 3rd, Cisneros, G., Cogordan, J. A., Castro, M., Wang, C., Eds.; World Scientific Press: Mexico City, 1997; pp 227-235.
13228 DOI: 10.1021/la100088n
above the surface16 while the edge surface has pH-dependent charge determined by protonation or deprotonation of the edge surface aluminol (Al-OH) and silanol (Si-OH) groups.17,18 Hence, due to the different basal/edge surface characteristics, edge-edge (E-E), edge-face (E-F), and face-face (F-F) aggregate structures models have long been proposed.12-14,19-21 In 1959,22 the first SEM image of a “card house” E-F structure was obtained from marine clay sediment using freeze-drying techniques and later was confirmed by O’Brien23 who also revealed F-F flake orientation in a porous clay network in salt-coagulated sediment. Zbik et al.24-27 have examined the morphology of the wellcharacterized, high-crystallinity KGa-1b and poorly ordered North Queensland (Weipa) kaolinites using AFM and SEM. The AFM and high-resolution SEM images show a more complex surface structure in poorly ordered North Queensland kaolinite with subhedral flakes locked or attached on the basal surface of larger particles. The high frequency of steps and nanometer-scale irregularities at crystallite edges were estimated to contribute over 30% of the kaolinite total surface area. As a result, for the case of poorly ordered kaolinite, it cannot be assumed that the exposed extensive surfaces are flat, pH-independent, negatively charged basal surfaces. Instead, they are composites of small areas of basal surface between steps and edges of OH-exposed edge sites. As these previous studies have shown that the edge surface area contributes significantly to the total surface area, any underestimates which may occur due to inaccurate measurement of the edge thickness and inability to measure the edge steps on the extensive faces, can have a significant impact on interpretation. The studies reported here show that the physical morphology of the kaolinite surface and crystallinity play a decisive role in the particle interaction and flocculation. This paper aims to correlate kaolinite morphology and crystallinity with flocculated kaolinite settling rates and sediment density. SEM and cryo-SEM images were used to analyze kaolinite morphology and flocculated kaolinite aggregation structure. A particle aggregation mechanism of well or poorly crystallized kaolinites is proposed. (17) Castro, E. S.; Martins, J. L. Int. J. Quantum Chem. 2005, 103(5), 550–556. (18) Nasser, M. S.; James, A. E. Sep. Purif. Technol. 2006, 51(1), 10–17. (19) Van Olphen, H. An introduction to clay colloid chemistry; John Wiley & Sons: New York, 1977. (20) Lambe, T. W. Proc. Am. Soc. Civil Eng. 1953, 1–49. (21) Rand, B.; Melton, I. E. Nature 1975, 257(5523), 214–216. (22) Rosenqvist, I. T. Proc. Am. Soc. Civil Eng. 1959, 31–53. (23) O’Brien, N. R. Clay Clay Miner. 1971, 19, 353–359. (24) Zbik, M. Colloid Surf., A: Physicochem. Eng. Aspect 2006, 287(1-3), 191–196. (25) Zbik, M.; Smart, R. Miner. Eng. 2002, 15(4), 277–286. (26) Zbik, M.; Smart, R. Clay Clay Miner 1998, 46(2), 153–160. (27) Zbik, M.; Smart, R. S. C. In Clays for our Future, Kodama, H., Mermut, A. R., Torrance, J. K., Eds.; Canada, 1997; pp 361-366.
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2. Materials and Methods Four kaolinite samples were used in this study: Q38, Snobrite, KGa-1b and KGa-2. The “crystallinity index”, termed by Hinckley,28 is the ratio of the sum of the heights of (110) and (111) diffraction peaks measured from the interpeak background to the height of the (110) peak measured from the background of the whole X-ray diffraction record. As the crystal perfection improves, the 110 and 111 resolution in the XRD pattern also improves, giving a direct indication of kaolinite crystal lattice defection concentration. This crystallinity index, now called the Hinckley index (IH). Kaolinite Q38 is a high-purity (99% purity) kaolinite, with a crystallinity or Hinckley index (IH)28 of 0.49; Snobrite has a IH of 0.92 and contains 3.8 wt % quartz as determined by XRD mineralogy. Both kaolinites, Q38 and Snobrite, were obtained from Granville, New South Wales, provided by Unimin Australia Limited. Two extensively characterized and studied kaolinites, Georgia kaolinite KGa-1b and KGa-2 purchased from Clay Minerals Society, USA, have been studied in parallel as model systems to compare with the Snobrite and Q38 samples. KGa-1b is a well-crystallized kaolinite obtained from Washington County Georgia, USA, with a IH of 1.22; KGa-2 is a poorly crystallized kaolinite, with a IH of 0.40 obtained from Clay Mineral Society, Warren County, Georgia, USA. XRD mineralogy shows KGa-1b contains traces of halloysite, and KGa-2 contains traces of anatase.29 Procedures for settling rate and bed density tests have been reported previously.30,31 In summary, 50 wt % kaolinite slurry was prepared in 0.01 M KCl solution and stored in a refrigerator overnight. The concentrated slurry was diluted to 2 wt % in 0.01 M KCl. 2.25 L of the slurry was added to an acrylic cylinder with flocculant added to the top of the slurry and mixed by plunging four times. The settling rate was calculated by recording the mud line (suspension/clear solution interface) as a function of time. After 2.5 min settling, a single rectangular rake (6 cm width 25.5 cm length) was inserted into the settled bed and raked at a constant speed of 3 rpm for 1 h, the final bed height was recorded to calculate the bed density. Anionic polyacrylamide copolymer FLOPAM AN-910 powder (10 mol % anionic, 1.2 107 Da molecular weight) was provided by SNF, USA. The flocculant stock solution (0.5 wt %) was prepared in Milli-Q water and rotated gently for 24 h using a bottle tumbler at 12 rpm. The stock solution was then stored in a refrigerator for at least 24 h before dilution into a 0.025 wt % working solution at 12 rpm in a bottle tumbler for 1 h. Normally, flocculant dosage in industry ranges from 10 to 50 g/t with an exception to 100 g/t. Analytical-grade KCl and highpurity Milli-Q water were used in all the sample preparations.
3. Materials and Characterization 3.1. Snobrite and Q38 Surface Morphology by Scanning Electron Microscope. The surface morphology of KGa-1b and KGa-2 studied using both SEM and AFM have been reported previously.24-26 A Philips XL30 field emission gun scanning electron microscope (FESEM) was used to study the surface morphology of Snobrite and Q38. Samples were coated with 3 nm carbon film. The surface morphologies at the same magnification are compared in Figure 2. SEM images in Figure 2 show that the extensive kaolinite faces, (001) basal planes, have dimensions ranging from 100 nm to 2 μm (28) Hinckley, D. N. In Proceedings of 11th National Conference of Clays and Clay Minerals; Pergamon: New York, 1963; pp 229-235. (29) Sutheimer, S. H.; Maurice, P. A.; Zhou, Q. Am. Mineral. 1999, 84, 620–628. (30) Zbik, M. S.; Du, J.; Pushkarova, R. A.; Smart, R. S. C. J. Colloid Interface Sci. 2009, 336(2), 616–623. (31) Du, J.; Pushkarova, R. A.; Smart, R. S. C. Int. J. Miner. Proc. 2009, 93(1), 66–72.
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Figure 2. High-resolution SEM images show relatively smooth (001) basal surfaces of Snobrite (A), but microislands and individual crystallites on the extensive (basal) surfaces for Q38 kaolinite (B).
for both Snobrite and Q38, but their surface nanomorphology is quite different. Sutheimer et al29 found that KGa-1b particles tend to be larger in diameter and thicker than KGa-2 particles; our examination of many areas and particle orientations has shown that the well-crystallized Snobrite has relatively smooth areas of (001) surface and thicker edge dimensions compared with Q38. Poorly crystallized Q38 has more complex surface structure on the extensive (basal) surface as seen in Figure 2B. Together with micropits (
