The effect of pH and ionic strength on starch–kaolinite interactions

Abstract

Starch–kaolinite interactions are of critical importance for the successful removal of kaolinite from iron ore in selective flocculation and froth flotation processes. In this work, the adsorption behaviour of starch on kaolinite surfaces and the flocculation/dispersion of the kaolinite particles coated with starch were investigated through adsorption and turbidity measurements. It was found that starch adsorption on kaolinite is pH dependent, with starch adsorption decreasing with increasing pH from 7 to 10.5. At pH 10.5, increasing ionic strength significantly enhances the adsorption density of starch on kaolinite. The enhanced starch adsorption at lower pH and higher ionic strength was attributed to the reduced electrostatic repulsion between the anionic starch molecules and the negatively charged kaolinite particles as pH and ionic strength change.The steric stability of kaolinite particles was found to depend on the adsorption behaviour of starch on kaolinite surfaces. In the presence of starch, bridging flocculation of kaolinite particles is rather weak in distilled water at pH 10.5, since starch shows an extremely low affinity towards kaolinite. However, at lower pH or in the presence of electrolytes, when starch shows a higher affinity towards kaolinite surfaces, starch starts to play a significant role in the aggregation of kaolinite particles through bridging flocculation of the mineral particles at low starch dosages and by inducing steric stability of kaolinite at higher starch dosages. When the ionic strength decreases to 0.00001M NaCl, no effect of ionic strength on starch–kaolinite interactions was observed.