Rheological Behavior of Smectite Dispersions: The Influence of Suspension Concentration and Exchangeable Cation (original) (raw)
Related papers
Advances in Colloid and Interface Science, 2008
Smectites are swelling clay materials with pronounced colloidal properties that are widely used in industry. These properties originate in the electrokinetic properties of the smectite layers and their linkage capacities. Thin layers may be dispersed or aggregated according to many parameters, such as concentration, particle size and morphology, exchangeable cation nature and chemical environment (pH, ionic strength). The literature usually provides general rules, like the sodium dispersion contains a lot of small units whereas the calcium dispersion contains a few large units. A volume of water molecules bound to the clay surface is considered as the immobile water phase that behaves like the solid phase obstructing the flow. The water immobilized around layers and trapped inside aggregates cannot participate to the flow. In this study, we evaluated the volume occupied by calcium and sodium units inside the dispersion containing the immobile water phase. First, the smectite was cautiously extracted from a raw bentonite and its physicochemical properties were determined. A large quantity of extracted and saturated smectite (Na-smectite and Ca-smectite) was obtained. Second, the unit size and a shape factor for each sample were evaluated using granulometry and scanning transmission electron microscopy on wet samples (Wet STEM) and some flow curves. Na-smectite dispersions contain 0.13 µm 2 surface units with a shape factor of 50. Ca-smectite dispersions contain 0.32 µm 2 surface units with a shape factor of 3.3. Finally, rheometry allowed us to evaluate the unit occupancy using an adaptation of the Krieger-Dougherty law. We used shape factors and evaluated the concentration from which the entire immobile volume was connected (6.4% for Na-smectite and 11.9% for Ca-smectite). This study explains the evolution of flow properties with increasing concentrations by the evolution of layer interactions at the microscopic scale for homoionic smectite particles in diluted dispersions.
Experimental transformation of Na,Ca-smectite under basic conditions at 150 °C
Applied Clay Science, 2004
The aim of this study is to examine the effect of various high pH solutions on smectites. The starting materials were a Na,Casmectite and two homoionic Na-and Ca-smectites. The experimental solutions were 0.01 m NaOH (pH 12), 1 m K 2 CO 3 and mixtures of 1 or 3 m KCl with 10 À 4 or 10 À 2 m KOH (pH 10 or 12). To enhance the reaction rate, all experimental investigations were carried out at 150 jC ( F 5jC max.), during 2 months. The internal pressure was 5 bars (liquid -vapour equilibrium pressure) when experiments were conducted in warm seal autoclaves and fixed to 150 bars, using standard cold seal vessels. The liquid/clay mass ratio was fixed to 10/1. Integrated data obtained by a multitechnique analytical approach [XRD, electron microprobe, transmission electron microscopy (TEM), scanning electron microscope (SEM) on run products, and inductively coupled plasma atomic emission (ICP-AES) and mass spectrometry (ICP-MS) on experimental solutions] show a distinct behaviour of smectite as a function of experimental fluid composition. In NaOH solution, run products remain low-charge smectites. In KCl + KOH solutions, crystal chemical changes concern mostly the interlayer. A significant replacement of Na by K, and a partial substitution of Ca by K are observed. The tetrahedral Si content does not change, but the total interlayer charge in smectite increases, balancing the changes occurring in the octahedral occupancy (release of Al and Fe). Low-charge smectite transforms into high-charge smectite and byproducts are quartz and feldspars. In the presence of 1 m K 2 CO 3 , homoionic Na-and Ca-smectites are unstable and in part replaced by zeolites (merlinoite), feldspars and calcium silicate hydrates (CSH), the latter being dominated by a tobermorite-like phase. The main consequences of the cement or concrete like pore fluid -bentonite contact at elevated temperature induce dissolution of smectite (at various rates), cation exchange and the formation of byproducts (quartz, feldspars, zeolites and CHS). These mineralogical changes affect the expandability and surface area of minerals, and therefore may significantly change the porosity and fluid flow diffusion in clay barriers exposed to high pH fluids. D
Proton saturation and rheological properties of smectite dispersions
Applied Clay Science, 2001
Proton-saturated dispersions of the -2-mm fractions of five smectites were prepared. Potentiometric titration data of freshly saturated and autotransformed samples were analysed by calculating proton affinity distribution curves to distinguish different proton interacting sites. Sites with p K values of ; 2.8 and 11.3 were assigned to protons exchanged for sodium ions and deprotonisation of silanol groups, respectively. Hydrated aluminum ions in freshly proton-saturated dispersions were characterized by p K ; 6. This group of weakly acidic centers also included oligomeric hydroxoaluminum cations because the amount of these sites increased during autotransformation and was accompanied by a shift to p K ; 5.5. The Ž . freshly prepared proton-saturated dispersions showed low pH values ; 2.6 , and the particles interacted by Ž . Ž . edge q rface y contacts. This increased the viscosity in comparison to the sodium forms at pH ; 7, and, depending on the smectite, yield values between 10 and 400 mPa were observed. Autotransformation removed all sites with p K ; 2.8, Ž . reduced the viscosity, the yield value mostly -10 mPa , and the flow behaviour approached that of the sodium smectite dispersions at pH ; 7. The cause is enrichment of aluminum andror oligomeric hydroxoaluminum cations in the solution and Stern layer, which leads to fragmentation of the networks into smaller stacks of more densely packed particles. q
Physico/chemical stability of smectite clays
Engineering Geology, 1996
There is convincing evidence from field data that smectite clay undergoes conversion primarily to illite and chlorite if it is fully water-saturated and heated. The conversion may take place through mixed-layer formation with increasing illite/smectite ratio at higher temperatures and pressures. This process requires dehydration of the interlamellar space, for which either an external pressure or drying are needed. An alternative mechanism that takes place without dehydration, is dissolution of smectite and neoformation of illite. Both processes imply reorganization of the smectite crystal lattice for which the activation energy is fairly high, meaning that the conversion is negligible at temperatures lower than about 60°C. At elevated temperatures the conversion rate is controlled by the access to potassium for either mechanism. An ongoing detailed investigation of this subject has led to a tentative model for the smectite-to-illite conversion in natural sediments and in canister-embedding clay in high-level radioactive waste (HLW) repositories.
Smectite suspension structural behaviour
International Journal of Mineral Processing, 2009
Smectite suspensions, at low solids contents, are known to be naturally high in volume with diverse structural properties. The changing structural properties of smectite aqueous suspensions in the absence and presence of calcium ions were investigated using an acoustosizer and an advanced cryo-SEM technique to further understand and thereby control their environmental impact. In the absence of Ca(II) ions, smectite particles are present as a colloidally stable sol due to electrical double layer repulsion of the negatively charged platelets. The smectite network is observed to be extended throughout the suspension via clay platelets networking with an edge-edge (EE) orientation due to high basal surface repulsion. After the initial addition of Ca(II) ions, the smectite negative zeta potential reduces and the smectite platelets coagulate forming 2 µm aggregates. The platelets are randomly orientated, lettucelike, coagulated aggregates with a high presence of both edge-edge (EE) and edge-face (EF) orientations. After equilibration, the smectite platelets forming an orientated honeycomb cellular structure comprised of face-face (FF) multiply sheet aggregates. The voids in the cellular structure are larger than prior to Ca(II) addition, measured at 2-8 µm. The changing structural properties of a smectite suspension in the absence and presence of Ca(II) greatly influence smectite stability and in turn, mineral processing and/or environmental management. Adequate time is required to allow suppression of the initial swelling of the smectite, full Ca(II) exchange and platelet aggregation.
Effect of Heating on Swelling and Dispersion of Different Cationic Forms of a Smectite
Clays and Clay Minerals, 1996
The effect of heat treatments on the swelling, dispersion, particle charge and particle aggregation of Li-, Na-, K-, Mg-, Ca-and Al-Wyoming bentonite was investigated. Before thermal treatment, unheated (25 ~ Li-, Na-and K-clays showed increased d001 spacing on glycerol solvation and dispersed spontaneously in water. Mg-, Ca-and Al-clays did not disperse spontaneously in water, but the d0o~ spacing increased upon glycerol solvation. After heating at 300 ~ or above, none of these clays dispersed spontaneously. However, swelling varied with the type of cation and the temperature of heating. The results generally suggested that swelling and dispersion of homoionic Wyoming bentonite after heating at various temperatures depended upon the nature of bonding between clay particles and the cations. Enhanced swelling and dispersion of clays indicated the more ionic character of the cationic bonding than cases where heating resulted only in swelling, with polar covalent bonding of cations to clay surfaces allowing limited hydration. It is also suggested that, when both swelling and dispersion as a result of thermal treatment are absent, a covalent bond is formed between cation and clay surface. Thermal treatment apparently affects the bonding in different ways. It appears that the smaller cations (ionic radius <0.7 A) Li, Mg and A1 migrate to octahedral vacant sites and form covalent bonds after heating at 400 ~ this drastically reduces the negative charge. This process for Li-clays occurred even at 200 ~ The larger cations (ionic radius > 0.9 A) Na, K and Ca apparently did not migrate into the lattice sites after heating to 400 ~ a high proportion of them were exchangeable. The data for exchangeable cation, particle charge and clay particle size were consistent with the postulated effect of the nature of cationic bonding upon swelling and dispersion properties.
2006
The influence of layer charge and charge distribution of dioctahedral smectites on the rheological and swelling properties of bentonites is examined. Layer charge and charge distribution were determined by XRD using the LayerCharge program . Determination of layer charge characteristics of smectites. Clays Clay Miner. 51, 644-655.]. The rheological properties were determined, after sodium exchange using the optimum amount of Na 2 CO 3 , from free swelling tests. Rheological properties were determined using 6.42% suspensions according to industrial practice. In smectites with layer charges of − 0.425 to − 0.470 per half formula unit (phfu), layer charge is inversely correlated with free swelling, viscosity, gel strength, yield strength and thixotropic behaviour. In these smectites, the rheological properties are directly associated with the proportion of low charge layers. By contrast, in low charge and high charge smectites there is no systematic relation between layer charge or the proportion of low charge layers and rheological properties. However, low charge smectites yield more viscous suspensions and swell more than high charge smectites. The rheological properties of bentonites also are affected by the proportion of tetrahedral charge (i.e. beidellitic charge), by the existence of fine-grained minerals having clay size, such as opal-CT and to a lesser degree by the ionic strength and the pH of the suspension. A new method for classification of smectites according to the layer charge based on the XRD characteristics of smecites is proposed, that also is consistent with variations in rheological properties. In this classification scheme the term smectites with intermediate layer charge is proposed.
Hydromechanical effects: (I) on the Na-smectite microtexture
Clay Minerals, 2000
Changes in particle organization and pore-spaces with applied mechanical and hydraulic stresses were followed using TEM, SAXS mercury porosimetry and gas adsorption for two Na-smectites, Laponite and hectorite, with similar structural formulae but different particle sizes. The TEM images show that hectorite has particles larger and more anisotropic than those of Laponite. The particles order perpendicularly to the direction of axial mechanical stress and become disoriented under hydraulic stress. According to the SAXS results, Laponite is composed of 1 – 3 small layers and hectorite of more compact (10 – 80 layers) particles. In Laponite, mechanical stress strongly reduces the amount of macropores but does not affect micropores and mesopores; hydraulic stress increases the macropores. In hectorite, the pore-volume is lower than in Laponite. The different techniques used yield complementary results and show the considerable effect of layer dimension on the behaviour and microtexture ...
Characterization and properties of treated smectites
Journal of the European Ceramic Society, 2012
Two natural smectite clays named BC and AC were thermal and chemically treated. Apart from smectite, quartz and kaolinite, BC clay also contains calcite, whereas illite and higher quartz content is found in AC. Treatment at 823 K leads to a collapse of the smectite structure. Treating with H 2 SO 4 or NaOH also leads to the elimination of calcite and Al and Si ions. The higher swelling capacity of BC clay is in accordance with its higher cation exchange capacity. γ d S values for both smectites decreases with the applied treatments. The surface acid-base constants determined for BC clay are higher than those obtained for AC independently of the applied treatment. This result together with the higher nanorugosity index, has been attributed to the higher quartz of the AC clay. Moreover, it has been observed that the cationic exchange capacity increases in both clays with the acidity of the surface.