MERIWA project M390 set out with the primary project objectives:
The consistent measurement of zeta potential in a full strength Bayer liquor was achieved. The zeta potential for aluminium hydrate, haematite, titania, silica and sodium oxalate were all positive. The zeta potential ranged from approximately 2 mV to 7 mV depending on the solid and liquor used. These non-zero values at such high electrolyte levels contrast with the classical view which would predict a zeta potential of zero for such systems. Monte Carlo modelling indicated that the positive non-zero zeta potential could be caused by the different interaction of ions with the structured water layers near the particle surface.
Positive zeta potentials were also obtained for solids in concentrated caustic and synthetic liquor solutions. These samples as well as those in industrial liquors were all recorded under ambient conditions. A water jacket for the electroacoustic probe was constructed to protect the probe and allow higher temperature measurements. The water jacket did not sufficiently protect the probe from damage at elevated temperatures but it did help to stabilise the measurements under ambient conditions. There was evidence that the zeta potential change in concentrated caustic between ambient and 55°C was negligible.
The techniques used to measure the zeta potential using the currently available DT1200 electroacoustic probe were improved over the course of the project. The improvements in both accuracy and precision involved several factors. These included incorporating the viscosity, density and sound speeds for each of the background solutions, minimising the time between background and sample measurement, taking multiple measurements and accounting for any probe drift. The standard deviation for a single point reading of the zeta potential in high caustic solutions could be as low as 0.1 mV, but more typically was 0.3 mV.
A correlation between zeta potential and rheological changes for numerous systems was found. Systems investigated predominantly involved either a fine aluminium hydroxide or iron oxide at high pH. In many cases an increase in the zeta potential magnitude was associated with a decrease in the “solid-like” nature of the slurry (i.e. lower elasticity, more “fluid-like”). The degree to which these changes occurred seemed to be influenced by the sign of the zeta potential. Other factors influencing the zeta potential and rheology included pH, ionic strength, temperature (at lower ionic strength) and certain surface active species.
The aluminate and organics present in a Bayer liquor have an impact on the zeta potential and rheology. Organic species that were found to have an influence on either the rheology or zeta potential of solids in caustic solutions include humate, dihydroxybenzene species, quaternary ammonium compounds and palmitic acid. Of these species the humate, quaternary ammonium and aluminate shift the zeta potential to more positive values while the dihydroxybenzene species cause a shift to slightly less positive values. All these species had a similar effect on the rheology making the slurry more “fluid-like”. Many of the organic species shown to affect the zeta potential and rheology also had an impact on the degree of aluminium hydroxide crystallisation.
The effect of both monovalent (Li, Na, K and Cs) and divalent (Ca, Sr and Ba) cations on the zeta potential and rheology of solids in caustic solutions was determined. For the monovalent ions lithium showed the greatest effect on the zeta potential and rheology with increasing caustic concentration. Lithium ions also enhanced the flocculation of goethite under certain caustic conditions. The addition of calcium ions enhanced the adsorption of humate onto red mud solids, haematite and aluminium hydroxide. The adsorption of a polyacrylate onto haematite was also enhanced. The addition of the calcium shifted the iron oxide zeta potential from negative to positive under moderate sodium hydroxide concentrations.
SAMUEL Monte Carlo simulation was used to model surfaces in water and in a number of high caustic aqueous solutions containing different cations. All of the aqueous simulation results indicated strongly structured layers of water near the solid interface. SAMUEL calculations indicated that water in contact with a negatively charged surface had the lowest energy, while water in contact with a positively charged surface had a slightly higher energy. Under the same conditions, water near an uncharged surface had the highest energy.
SAMUEL Monte Carlo modelling of concentrated caustic solutions (LiOH, NaOH, KOH, and CsOH) provided useful information on effects occurring in the double layer region. Moving from a negatively charged surface in concentrated aqueous alkali, an initial cation band was followed by an anion band and then a variable intermediate region which lasted until 0.8 nm from the surface. By 0.8 nm from the surface, the structured water layers were considerably weaker. A band of high hydroxide concentration occurred 1 nm from the surface. A cation-rich diffuse region was observed beyond 1 nm, and possibly out to 4 nm from the surface. This cation-rich region appears to be responsible for the observed positive zeta potentials in concentrated caustic liquors. Using a relative measure of the local field, an approximate correlation was observed between the calculated excess of cations 2 to 3 nm from the surface, and the measured zeta potential for haematite in 5 M caustic solutions obtained using the DT1200.
A virtual instrument was established to measure the mean direct current, voltage, or magnetic field resulting from the application of asymmetric electric and acoustic fields between electrodes. Work towards measuring the zeta potential of solids in slurries through the use of asymmetric radio frequency waves showed that temperature fluctuation, solution conductivity, cell capacitance, and associated circuit resonances interfered with attempts to measure zeta potential. In addition, the cell design required further optimisation and there was a need to more effectively remove the electrical field when measuring current from asymmetric acoustic signals. More work would be needed in this area to establish a working technique for zeta potential measurement.
from: “Measuring particle surface charge and particle interactions in process liquor” MERIWA Report 282, E. Karakyriakos, C.J. Patrick, and V.A. Patrick, May 2010. ISBN 1920981438.