The physicochemical mechanism of iron removal of aluminum sulfate solution by a novel synthesis process

Abstract

Titanium dioxide is widely used as a pigment in industrial processes and life, and the mechanical strength of the coating of titanium dioxide is of vital importance for its quality. The content of iron impurities in aluminum sulfate coatings directly affects their quality; therefore, the removal of iron from aluminum sulfate solutions is of paramount importance. In this study, an aluminum sulfate solution containing iron (18 mg/L) was used as a raw material to explore a composite process to reduce iron content to< 5 mg/L for coating high-quality titanium dioxide products. A novel process consisting of sodium diethyldithiocarbamate (DDTC) complexation–precipitation, flocculation by cationic polyacrylamide, and ultrafiltration have been proposed in this study. The reactant structure and thermodynamic equilibrium were determined by quantum chemical calculations to clarify the mechanism of selective iron removal using organic precipitants. The effects of DDTC dosage, reaction temperature, reaction time, stirring speed, and ultrafiltration membrane specifications on iron removal were systematically investigated. The physical and chemical properties of the precipitated particles were analyzed by zeta potential, scanning electron microscopy (SEM), and atomic force microscopy (AFM) before and after the addition of flocculants. Before adding the flocculant, the surface zeta potential of the precipitated particles was negative, and the cationic flocculant was added to neutralize the surface charge, eliminate the electrostatic repulsion, and promote the flocculation of precipitated particles for growth; this is beneficial for ultrafiltration. Furthermore, the experimental conditions were optimized to achieve the lowest iron content of 3.8 mg/L in the solution. Finally, the reaction mechanism of the organic precipitation—cationic flocculation—ultrafiltration complex was elucidated.