Direct Calcification-Carbonation Method for Processing of Bayer Process Red Mud

The highly-alkaline red mud, which is the Bayer process residue generated from the alumina industry, is a severe environmental problem. In this study, a new calcification-carbonation process was proposed for red mud disposal. Red mud was processed by lime to convert the aqueous silicon phase into hydrogarnet, which was then decomposed by CO2 to recover alumina. In the direct carbonation process, the NaOH-containing solution after calcification was directly carbonated without prior liquid-solid separation. The discrete and direct carbonation processes had alumina recovery rates of 34.9 and 35.5%, respectively, with 0.15 and 0.21 wt % Na2O in the final red muds, respectively. The optimum NaOH concentration in the calcification liquor was 30 g/L. Under these conditions, alumina recovery was increased to 44.5% and the Na2O concentration in the processed red mud was reduced to < 1 wt %. The final red mud can be used as a construction material.