Behavior of Al and Si in Bayer Red Mud Processed by NaOH Sub-molten Salt

Based on the problem of higher Al2O3/SiO2 (A/S) of Bayer red mud, the separation of Al and Si in red mud from Bayer process in digestion with NaOH sub-molten salt was investigated. The effects of digestion temperature, mass ratio of alkali to red mud from Bayer process and CaO addition on the behavior of red mud in the salt were experimentally investigated. The results show that high temperature and mass ratio of alkali to red mud lead to more alumina into the aqueous phase and the decrease of A/S in brown mud. The recovery rate of Al2O3 can reach 79.22% and A/S in the brown mud is decreased to 0.39 under the conditions of mass ratio of alkali to red mud at 6, digestion temperature at 230℃, and digestion time for 2 h. Further study shows that Si exists in the form of NaCaHSiO4 and Ca3(Al0.13Fe0.87)2(SiO4)1.65(OH)5.4. The recovery rate can not be further raised under the condition of CaO addition in sub-molten salt process when the molar ratio of CaO to SiO2 over 1.2 in the original red mud.