Abstract:Red mud is a bulk solid waste with strong alkalinity discharged during the alumina extraction process. It contains a large amount of undigested alumina, especially from the low-temperature Bayer process for gibbsite, which results in a great waste of resources. The red mud from low-temperature Bayer process was used as raw materials to recover alumina by high-temperature digestion. The effects of lime addition, caustic alkali concentration, reaction temperature, solid content and reaction time on Al2O3 leaching performance were systematically studied, and the phase transformation behavior and the leaching kinetics during leaching process were investigated. The results show that the Al2O3 leaching efficiency rises with the increase of caustic alkali concentration, leaching temperature and time, while drops with the increase of lime addition and solid content. The optimal conditions for Al2O3 leaching are as follows: the solid content is 500 g/L, the caustic alkali concentration is 200 g/L, the leaching temperature is 270 ℃, the time is 60 min, and no lime is added. The corresponding Al2O3 leaching efficiency and the silica index of the leaching solution are 50.36 % and 260, respectively. The kinetic analysis indicates that the leaching process of Al2O3 is controlled by internal diffusion, and the apparent activation energy is 33.46 kJ/mol. The kinetic equation is 1-2α/3-(1-α)2/3=0.00101×e-3346/RT×t. During the leaching process, the boehmite is dissolved, and the Al-goethite is transformed into hematite, which leads to an increase in Al2O3 leaching efficiency.