Removal of fluoride ions from zinc sulfate solution by mesoporous alumina: Performance and mechanism

In a flowsheet of sulfide roasting, leaching, purification and electrowinning, fluoride ions in zinc sulfate (ZnSO4) solution significantly impair the subsequent electrowinning process for zinc production. The effective removal of fluoride ions is essential for the smooth and effective operation of the entire production workflow. In this study, mesoporous alumina was synthesized using a template method and applied in fluoride ion removal from ZnSO4 solution. The maximum fluoride ion removal efficiency (94.0 %) was achieved under optimum conditions (adsorbent dosage = 2 g L-1, pH = 3.0, adsorption time = 20 min) in simulated fluoride ion solutions. Furthermore, the performance of mesoporous alumina in industrial ZnSO4 solutions was evaluated, showing a fluoride ion removal efficiency of 80.9 % and a minimal zinc loss of 0.58 %. The adsorption process of fluoride ions on mesoporous alumina was examined using adsorption kinetics and isotherms, which conformed to the pseudo-second-order kinetic model and Langmuir isotherm model. Additionally, mesoporous alumina before and after adsorption was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR). The analysis indicated that ion exchange between fluoride ions and Al-OH groups on the adsorbent led to fluoride ion removal. Moreover, the complex ions between zinc and fluoride ions in the ZnSO4 solution occupied active sites on the adsorbent surface and impeded ion exchange.