Mechanisms of Neutralization of Bauxite Residue by Carbon Dioxide

Bauxite residue (red mud), an alkaline slurry from alumina refining, is produced in large volumes and disposed of in large surface impoundments. The objectives of this study were to measure the extent of neutralization of bauxite residue by carbon dioxide as a function of CO2 partial pressure and to determine the geochemical reactions responsible for carbon sequestration. Bauxite residue was exposed to carbon dioxide (CO2) at partial pressures ranging from 10−3.5to1atm and the residue pore-water pH was measured until a steady state pH was achieved. Using pure CO2(PCO2=1atm), a steady state pH of 7.7 was established in 1day while it took 9days to reach a pH of 9.8 at PCO2=10−3.5atm. The pH rebounded to 9.9 after 1day when bauxite residue first neutralized at PCO2 of 1, 0.1, or 0.01atm for 10days was subsequently exposed to PCO2=10−3.5atm, indicating that the pH change during short-term carbonation (t⩽10days) was due to reactions of carbonic acid and OH− in the pore water. A longer reaction time (30days) at PCO2=1atm indicated carbonation of tricalcium aluminate (C3A solid) and conversion to calcite. This was confirmed in aged field samples that had been carbonated in air (PCO2=10−3.5atm) for 30years. The maximum solid-phase CO2 sequestration (as calcite) potential of red mud can be realized at PCO2=10−3.5atm; however, the sequestration rate is limited by slow dissolution of C3A. The CO2 sequestration potential is small (0.029–0.057 MMT CO2∕year) compared to annual CO2 emissions (110MMT∕year) for bauxite refining and red mud production of 30MMT∕year.