A novel process and its mechanism for recovering alumina from diasporic bauxite

A novel process involving microwave roasting of slurry mixtures of diasporic bauxite and alkaline solutions followed by dilute alkaline leaching, is proposed for extracting alumina from low grade diasporic bauxite ores. It was found that when the mole ratio of mNaOH/(mAl + mFe) and mCa/mSi in the slurry mixture was close to 1 and 2, the Al and Si contained in the bauxite were completely liberated into soluble NaAlO2 and insoluble CaSiO4 after roasting using a 1.0 kW microwave source at 800 °C for 20 min. The roasting process significantly improved the performance of the downstream leaching processing. The extraction of Al reached 97% while that of Si was as low as 7%, on leaching in low-concentration alkali solution at 80 °C under atmospheric pressure for 20 min. A new mechanism involving flashing, layering and homogeneous reaction is proposed for the new microwave roasting process. The key steps proposed are: creation of a hot core at the peak location of the electric field, preheating of the outer layer facilitated by heat conduction and vapor from the hot core, and finally microwave heating of the outer layer due to increasing dielectric loss and decreasing penetration depth.