The effect of NaOH on the direct calcification-carbonation method for processing of Bayer process red mud

The Bayer red mud generated from the alumina industry is a hazardous solid waste. In our team, a green calcification-carbonation process is proposed for its disposal. Red mud is treated with lime to convert the silicon phase in solution into hydrogarnet, which is then decomposed by CO2 to recover alumina. In order to simplify the process flow, the direct carbonation process is employed, in which the NaOH-containing solution resulting from calcification is sent directly to carbonation without prior liquid-solid separation. The discrete and direct carbonation processes gave 34.9% and 35.5% alumina recoveries, respectively, with Na2O contents in the final red muds of 0.15 %wt and 0.21 %wt, respectively. The optimum NaOH concentration in the whole calcification-carbonation process liquor was 40 g/l. Under this alkali condition, alumina recovery reached 40.5% and the Na2O content in the processed red mud was reduced to <1 %wt.