Gayle E. Morris

Polymer depressants at the talc–water interface: adsorption isotherm, microflotation and electrokinetic studies

Abstract The behaviour of polymer depressants at the talc–water interface was investigated as a function of ionic strength and pH. Adsorption isotherms, microflotation and electrokinetic studies were used to examine the surface interactions involved. The polymers examined were carboxymethyl cellulose (CMC) and two synthetic polyacrylamides (PAM-A and PAM-N). The adsorption of the two anionic polymers, CMC and PAM-A, on talc, and hence, talc depression, is greatest when electrostatic repulsion is minimized. At high pH values and low ionic strength, the adsorption density of the anionic polymers on talc is low whilst at either high ionic strength or low pH, the adsorption density increases; talc depression is therefore largely influenced by variations in solution conditions. The adsorption of the nonionic polymers, PAM-N, on talc is not influenced by ionic strength or pH. The polymers exhibit Langmuir adsorption behaviour with adsorption occurring on the talc face surface and, possibly, the edge surface. The adsorbed polymer layer thicknesses, calculated from electrophoretic mobility measurements, corresponded to a monolayer indicating that adsorption of the polymers onto the talc surface occurs in a flat conformation.

Surface chemistry and rheological behaviour of titania pigment suspensions

Abstract The influence of pH and surface chemical state on rheological behaviour was investigated for variations of silica- and alumina-coated, aluminium-doped titania pigment suspensions. The variation in rheological properties correlates with the change in the pigment surface properties, determined from electrophoresis measurements, atomic concentrations, chemical states and modified Auger parameters derived from X-ray photoelectron spectroscopy (XPS). Pigment suspensions exhibited a maximum yield stress and viscosity at or near the isoelectric point (iep). At a pH where the magnitude of the zeta potential of the titania pigment is high, a low-viscosity, dispersed suspension was obtained. The pH of the maximum yield value of the pigment suspension increases with increasing aluminium hydroxyl group density and decreases with increasing silicon hydroxyl group density. Low-viscosity pigment dispersions were obtained with increasing aluminium surface concentration and further reduced with an increase in the silicon surface concentration. Pigment particle attractions are chiefly dictated by van der Waals forces and heteroaggregation. The pigment aggregate strength therefore depends upon the Hamaker constant of the heterogeneous pigment based on the proportion of the respective surface groups.

Kaolinite flocculation structure

Effective flocculation and dewatering of mineral processing streams containing colloidal clays has become increasingly urgent. Release of water from slurries in tailings streams and dam beds for recycle water consumption, is usually slow and incomplete. To achieve fast settling and minimization of retained water, individual particles need to be bound, in the initial stages of thickening, into large, high-density aggregates, which may sediment more rapidly with lower intra-aggregate water content. Quantitative cryo-SEM image analysis shows that the structure of aggregates formed before flocculant addition has a determinative effect on these outcomes. Without flocculant addition, 3 stages occur in the mechanism of primary dewatering of kaolinite at pH 8: initially, the dispersed structures already show edge-edge (EE) and edge-face (EF) inter-particle associations but these are open, loose and easily disrupted; in the hindered settling region, aggregates are in adherent, chain-like structures of EE and stairstep face-face (FF) associations; this network structure slowly partially rearranges from EE chains to more compact face-face (FF) contacts densifying the aggregates with increased settling rates. During settling, the sponge-like network structure with EE and FF string-like aggregates, limits dewatering because the steric effects in the resulting partially-gelled aggregate structures are dominant. With flocculant addition, the internal structure and networking of the pre-aggregates is largely preserved but they are rapidly and effectively bound together by the aggregate-bridging action of the flocculant. The effects of initial pH and Ca ion addition on these structures are also analyzed. Statistical analysis from cryo-SEM imaging shows that there is an inverse correlation of intra-aggregate porosity with Darcian inter-aggregate permeability whereas there is a strong positive correlation of Darcian permeability with settling and primary dewatering rate as a function of pH in suspension. Graphs of partial void contributions also suggest that it is not total porosity that dominates permeability in these systems but the abundance of larger intra-aggregate voids.

Influence of aluminum doping on titania pigment structural and dispersion properties.

The influence of aluminum concentration on the structural properties and rheological behavior of aqueous suspensions of aluminum-doped titania pigment from the chloride process was investigated. The variation in rheological properties correlates with the change in the pigment surface properties, determined from electrophoresis measurements and atomic surface concentrations. Pigment suspensions exhibited a maximum yield stress and viscosity at or near the isoelectric point (iep). The pH of the maximum yield value of the pigment suspension increases with increasing aluminum hydroxyl group density at the particle surface. For pigments with a high aluminum surface concentration, at pH values where the magnitude of the zeta potential was high, a low-viscosity, dispersed suspension was obtained. The pigment with the lowest aluminum concentration, however, retained high yield stresses over a large pH range even when the zeta potential was of considerable magnitude. Pigment particle interactions are chiefly dictated by van der Waals forces and electrostatic repulsive forces, likely to be influenced by heteroaggregation. The aggregate strength would therefore depend upon the proportion and distribution of aluminum and titanium surface groups of the heterogeneous pigment, which will influence both the Hamaker constant and the degree of heteroaggregation. Overall, very small additions to the total aluminum concentration translate to significant aluminum surface concentration disparities and subsequently to large particle interaction differences.

Temperature and Pressure Effects of Desalination Using a MFI-Type Zeolite Membrane

Zeolites are potentially a robust desalination alternative, as they are chemically stable and possess the essential properties needed to reject ions. Zeolite membranes could desalinate “challenging” waters, such as saline secondary effluent, without any substantial pre-treatment, due to the robust mechanical properties of ceramic membranes. A novel MFI-type zeolite membrane was developed on a tubular α-Al2O3 substrate by a combined rubbing and secondary hydrothermal growth method. The prepared membrane was characterised by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and single gas (He or N2) permeation and underwent desalination tests with NaCl solutions under different pressures (0.7 MPa and 7 MPa). The results showed that higher pressure resulted in higher Na+ rejection and permeate flux. The zeolite membrane achieved a good rejection of Na+ (~82%) for a NaCl feed solution with a TDS (total dissolved solids) of 3000 mg·L−1 at an applied pressure of 7 MPa and 21 °C. To explore the opportunity for high salinity and high temperature desalination, this membrane was also tested with high concentration NaCl solutions (up to TDS 90,000 mg·L−1) and at 90 °C. This is the first known work at such high salinities of NaCl. It was found that increasing the salinity of the feed solution decreased both Na+ rejection and flux. An increase in testing temperature resulted in an increase in permeate flux, but a decrease in ion rejection.

Polyphosphate interaction with aluminium-doped titania pigment particles

The interaction of a widely used Calgon™ polyphosphate dispersing reagent with aluminium-doped titania pigment particles has been investigated using electrokinetic and rheological studies combined with adsorption isotherms. The influence of pH, aluminium dopant concentration and polyphosphate concentration is reported. Polyphosphate adsorption density and affinity with the titania pigment surface is highest under acidic solution conditions. This however, does not necessarily transfer to enhanced dispersion properties at low pH values. At pH 9, the polyphosphate adsorption density correlates directly with a reduction in pigment particle interactions making polyphosphate an effective titania pigment dispersant under alkaline conditions. Conversely, at pH 4, polyphosphate adsorption densities less than 0.1 mg m−2 have no effect on the colloidal stability of the titania particles and their Newtonian flow behaviour. At adsorption densities of ∼0.1 mg m−2, approaching the iep (near 0.2 mg m−2), the suspension aggregates. It is not until the polyphosphate adsorption density is greater than 0.3 mg m−2 that the titania pigment suspension begins to restabilise. It is proposed that chemisorption dominates polyphosphate adsorption at pH 9 whilst at pH 4 a combination of chemisorption and electrostatic adsorption occurs. Stabilisation by the polyphosphate present at the pigment surface depends on both electrostatic and steric effects. At high pH, both are effective but at low pH, electrostatic stabilisation is partly neutralised and higher adsorption densities are required for effective stabilisation.