Exploring dealkalization and Cr removal from red mud through freeze-thaw and acid washing: An experimental approach

Red mud (RM) is an industrial solid waste produced during the extraction of alumina from bauxite. The strong alkaline and heavy metal leaching issues are the primary factors limiting its utilization. This paper proposes a method for dealkalization and chromium (Cr) removal by repeated freeze and thaw to enhance the comprehensive utilization rate of RM. This study focused on the Bayer RM and investigated the effects of freeze-thaw (FT)-acid washing (AW) for dealkalization and Cr removal. The variables were the eluent concentration and FT cycles. The results showed that the synergistic action of FT-AW significantly improved the efficiency of dealkalization and Cr removal. After five FT cycles with 0.5 mol/L oxalic acid, the dealkalization and Cr removal rates reached 97.5% and 65.38%, respectively, 16.1% and 7.40% higher than those achieved at room temperature. The repeated FT disrupted the structure of the RM particles, leading to an increase in the pore space between the soil particles. This enables complete eluent contact and reaction with Cr and alkali, thereby enhancing the removal rate. The FT-AW process is suitable for practical engineering applications. It offers a novel method for RM dealkalization and Cr removal by using the FT alternation phenomena in seasonally frozen regions.