Rapid wet grinding carbonation of sintering red mud for highly efficient CO2 sequestration and Cr solidification

Sintering red mud (SRM) is an alkaline and aqueous solid waste from bauxite sintering for alumina production. The challenges in the effective utilization of SRM are strong alkalinity, aqueous state and leaching toxicity of the heavy metals. Herein, a rapid wet grinding carbonation technique of simultaneous injection of CO2 in wet grinding process was proposed. The rapid carbonation of alkaline metal ions and the efficient solidification of heavy metal ions were achieved in the wet grinding carbonation. After 40 min of short-term carbonation, the CO2 uptake and carbonation degree of SRM reached 30.25 g CO2/100 g SRM and 79.31 % respectively. The high-efficiency CO2 uptake of SRM was attributed to the continuous stripping of the attached surface CaCO3 layer by wet grinding, the constantly exposed new surface area after refinement and the largely increased Ca2+ concentration, which was 12.5 times the initial value at 10 min. Furthermore, the simultaneous carbonation realized the solidification of dissolved chromium ion (Cr(III)/Cr(VI)), which was transformed into CaCrO4, Ca3Cr2(OH)12 or Cr(OH)n(CO3)n(3n-3)- complexes under the weak acid environment of CO2. After 40 min of wet grinding carbonation, the Cr concentration was notably decreased by 85.71 %, which was lower than the limit of groundwater standard requirement.