Removal of Fluoride from Aluminum Industrial Wastewater by Secondary Aluminum Ash for the Preparation of Fluoride-Containing Al2O3 for Low-Temperature Aluminum Electrolytes

Secondary aluminum ash was modified with malic acid to prepare the adsorbent for a fluoride absorption. The effects of contact time, adsorbent dose, and pH on the defluorination effect of adsorbent were investigated. The structure and properties of the adsorbent were characterized by SEM, EDS, N₂ adsorption–desorption isotherm, XRD, FTIR, TEM, XPS, and removal rate. The results show that the surface roughness of the secondary aluminum ash modified by sintering and malic acid gradually increased. The BET specific surface area and pore volumes of secondary aluminum ash modified by sintering and malic acid were 42.691 m²/g and 0.128 cm³/g, respectively. The peak strength of F in the secondary aluminum ash was enhanced after adsorption, and the main phase in the ash was γ-Al₂O₃. The –OH peak of the modified secondary aluminum ash was enhanced and the –OH peak strength was weakened after adsorption of fluoride. The spacing between the interplanar distance of the adsorbent material was 0.24 nm. The fluoride adhered to the secondary aluminum ash surface by mainly substituting the hydroxyl group (–OH) on the surface of the adsorbent and forming an Al-F complex. When pH = 2, the maximum adsorption capacity was 92.8 mg/g. The adsorption process accorded to the pseudo-second-order kinetic model, the correlation coefficient, R² = 0.9995, indicating that the defluorination was chemisorption. The adsorption mechanism is mainly ion exchange and the formation of complexes. The liquidus temperature of the fluoride-containing Al₂O₃ industrial electrolyte system was 913 °C. The Al₂O₃ solubility in fluoride-containing Al₂O₃ electrolyte increased by 0.9 wt% compared to traditional aluminum industry electrolytes.