Simultaneous Extraction of Fe, Al, Sc and Cr from Red Mud by Ammonium Sulfate Calcination in the Range 250–400°C

Red mud is a strong alkaline waste from the Bayer process for Al production, which is known as a promising polymetallic secondary resource. The stockpiling and landfill disposal of red mud have produced environmental pollution and resource waste. A novel, promising, environmentally friendly technology for simultaneous extraction of valuable metals from red mud was proposed by ammonium sulfate calcination at temperatures of 250–400°C and water leaching. The phase change in the calcination process and various parameters such as the amount of ammonium sulfate, calcination temperature, calcination time, leaching temperature, leaching time and solid-to-liquid ratio on element extraction were investigated. The results indicated that Fe, Cr, Sc and most Al in the red mud were first transformed into (NH4)3(Fe, Cr, Sc, Al)(SO4)3 during the sulfation calcination process and then into NH4(Fe, Cr, Sc, Al)(SO4)2 at 400°C, while the Al in diaspore was not. More than 98.23% Fe, 70.32% Al, 95.38% Sc and 88.07% Cr were extracted under the optimal conditions, while Ti mainly remained in the leaching residue.