Utilization of Bayer red mud by a calcification–carbonation method using calcium aluminate hydrate as a calcium source

Abstract

Bayer red mud accounts for the largest proportion of solid waste produced in the non-ferrous metallurgy industry and is usually handled by stockpiling because of its high alkalinity. The calcification–carbonation method (CCM) was proposed to extract alumina and sodium oxide from red mud using a fully hydrometallurgical process in order to scale the valuable utilization of Bayer red mud. In this study, a modified CCM was applied, using calcium aluminate hydrate as the calcium source to recover alumina from a low-NaOH-concentration digested solution. Thermodynamic analysis revealed that the stable product of the calcification process was hydrogarnet with a high silicon saturation coefficient (x, 3CaO·Al2O3·xSiO2·6-2xH2O), and a higher reaction temperature was beneficial for the formation of hydrogarnet with a high silicon saturation coefficient. Experimental results indicated that: (1) the calcification temperature was the most important determinant of the coefficient x of the hydrogarnet, which varied from 0.35 to 0.98 over a temperature range of 60–280 °C; (2) the x value and Na mass content in the calcified slag were 0.98 and 0.35%, respectively, under calcification conditions with an NaOH concentration of 200 g/L, a temperature of 260 °C, and a C/S of 2.5:1; (3) the Na mass content and A/S decreased to 0.03% and 0.52, respectively, in the new red mud structure due to the carbonation and digestion process. The Na mass content in the new red mud structure satisfies the raw material requirements for the cement industry, and approximately 74% of the alumina in the Bayer red mud was recovered by the CCM treatment. The primary phase in the new red mud structure was CaCO3, which can be used for preparing cement or other building materials.