Chemical and Electrochemical Processing of Aluminum Dross Using Molten Salts

A novel molten salt process was investigated, where Al, as metal or contained in Al₂O₃ and AlN, was recovered from Al dross by chemical or direct electrochemical reduction in electrolytic cells. Electrolysis experiments were carried out under argon at temperatures from 1123 to 1243 K. In order to better understand the reduction behavior, the as-received Al dross was simulated using simplified systems, including pure Al₂O₃, pure AlN, an Al₂O₃/AlN binary mixture, and an Al₂O₃/AlN/Al ternary mixture. The reduction of the as-received dross was also studied experimentally. The studies showed that solid Al₂O₃ was chemically reduced by the Ca in a Ca-saturated Ca-CaCl₂ melt to form Al₂Ca or electrochemically reduced to Al-rich Al-Ca alloys and that the Al value in the Al₂O₃ was easily recovered from the Al drosses. It was found experimentally that solid AlN in the drosses could not be calciothermically reduced to any extent, consistent with thermodynamic evaluations. It was also found that the direct electrochemical reduction of the AlN in the drosses was confined to three phase boundaries (3PBs) between the AlN, the electrolyte, and the current collector and could not be enhanced by using the LiCl-containing chloride melt or the chloride-fluoride melts studied. The presence of Al powder in the Al₂O₃/AlN mixture facilitated the direct electrochemical reduction of both Al₂O₃ and AlN. The reduction mechanisms are discussed based upon the present experimental observations. Flow sheets for recovering the metallic Al and the Al in the Al₂O₃ and AlN from Al dross are finally proposed.