Staged leaching behavior of red mud during dealkalization with mild acid

Abstract

Dealkalization is a major limiting factor for the comprehensive utilization of red mud, and acid leaching is considered an effective and promising way to achieve it. Most studies have focused on the recovery of metals in this extreme alkaline waste by strong acid leaching, ignoring the dealkalization and stabilization of red mud with low concentrations of acid. In this study, the leaching behavior of the major elements in red mud was carefully investigated by the treatment with a range of H+ concentrations from 10−6 to 1.8 mol/L. Analysis of both leachate and residue confirmed a stepwise sequential leaching behavior of Na, Al, and Ca, which was important for dealkalization of red mud. Treatment with an acidic solution at pH 1 removed 17.84% of Na, but almost no other metals. Greater than 60% Na was leached by an acidic solution with H+ concentration of 0.5 mol/L, but the dissolution efficiencies of Al, Si, and Ca were less than 10%, resulting in a relatively simple leaching liquid treatment process. And the pH of the leaching residue was 8.5 which slightly alkaline so the residual RM could be used as soil aggregates. When using an acidic solution with an H+ concentration of 1.6 mol/L, the dissolution efficiency of Na reached at 94.70%, leaving only 0.47% Na in the residue, which makes it possible to meet the demand of application for construction materials. From the results of the phase analysis, it was found that the sequential leaching behavior was due to the dissolution, decomposition, and transformation of the particles containing the main elements, and was compatible with the SEM-EDS inspection. Based on these results, a new multi-stage acid leaching dealkalization process has been proposed to provide an effective and economical way to dealkalize red mud.