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

Abstract

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.