The viscous behaviour of aqueous goethite-containing suspensions

While of technological importance in many industries, the viscous behaviour of aqueous suspensions of goethite, α-FeOOH, has received little attention in the scientific literature. Recent interest by miners in the recovery of nickel and cobalt from laterite ores by hydrometallurgy has brought the subject to the forefront, and so motivated this study of the viscous behaviour of aqueous suspensions of goethitic laterite ore and pure, synthetic goethite. The ore sample primarily consisted of sub-micron-sized goethite rods, and slurries of the ore were prepared in a manner that simulated mineral dressing for hydrometallurgical processing. These modestly concentrated slurries, with volume fractions of between 6 and 18%, possess high viscosities that could not be accounted for by the shape of the goethite particles. The presence of flocculation in the slurry clearly indicated the presence of attractive interparticle forces, and these forces may be responsible for the high viscosity. However, since laterite is mineralogically complex, detailed consideration of this implication was reserved for the study of the pure goethite system. When solid volume fractions were greater than about 01, slurry viscosity was highly dependent on time. Although time dependence has been noted before for laterite slurries, it has not been characterised. The present work demonstrated that the time dependence is well characterised by the Maxwell model, and that slurry relaxation times are of the order of 10 s. The synthetic sample consisted of capsule-like colloidal particles that also flocculated, at least in solutions of neutral pH. Viscometry showed that these suspensions were even more viscous than the laterite slurries at equivalent solids concentrations, and highly shear thinning. Studies conducted at different concentrations of HCl or NaOH showed that both the conformation of the flocculated structure and the viscous behaviour are determined by the pH: a pH that yields a more dispersed structure also yields a lower viscosity and a lower degree of shear thinning. This correspondence suggested that the forces responsible for the structure are also responsible for the rheology. An analysis to determine the important forces was performed, for which the zeta potential and the titrateable charge were measured. Flocculated structure and high viscosity were found to persist at unexpectedly high surface potentials, and the persistence could not be predicted using classical Deryaguin-Landau-Verwey-Overbeek (DLVO) theory. An analysis of crystal structure indicated that different crystal faces of goethite may possess different charges, and this theory was supported by an electron microscopy study of mixtures of charged spheres and goethite particles. It is therefore surmised that the structure and the rheology of goethite-containing suspensions are governed not only by the strengths of attractive van der Waals forces and repulsive electrostatic forces—which is in accordance with classical DLVO theory—but also by the strength of attractive electrostatic forces. The practical implications of this mechanism are clear once it is realised that the rheology of goethite-containing suspensions may be controlled by chemical modification of the surface charge.