Electrokinetic behaviour of tin tailings slurries in solutions of lanthanum and thorium salts

Abstract

The electrokinetic behaviour of clay particles in tin tailings slurries in the presence of lanthanum(III) and thorium(IV) salts were studied. The clay minerals in the two slurries chosen for this work were mainly kaolinite with some minor compositional variations. An interesting feature of the electrophoretic mobility-pH curves at fixed lanthanum(III) concentrations is the occurrence of two constant mobility regions, one in the acidic pH range and the other at alkaline pH. It is known that lanthanum(III) at different concentrations forms hydrolysed species of different charges with different pH. Thus at low lanthanum(III) concentrations, the concentration of hydrolysable species present was too low to cause any charge reversal and the clay particles remained negatively charged. The constant negative mobility was thus the result of a delicate balance between the charge behaviour of the clay particles and the adsorptivity of cationic hydrolysed mononuclear species with pH. In contrast, the effect of the clay surface on metal ion hydrolysis via surface nucleation was manifested in the formation of the insoluble hydroxide at a much lower pH compared with bulk solution hydrolysis. The interplay of this with the charge behaviour of the clay particles culminated in the second constant negative mobility region at alkaline pH. Results show that at fixed thorium(IV) concentrations, the higher the electrolyte concentration, the higher the pH at which charge reversal occurred. Adsorption of mononuclear hydrolysed species at low pH and ion concentration was sufficient to impart positive charges to the clay particles. The negative mobilities of the clay particles at different thorium(IV) concentrations all converged to about the same magnitude and remained constant above pH 7. The mobility of the clay particles results from a complex interplay of the hydrolysis of the cations under different conditions of concentration and pH, the charge behaviour of the clay particles as a function of pH and the presence of the clay surface itself.