Abstract

Sequestration of atmospheric CO₂, currently a necessary process, occurs within the 0-30 cm depths of drying bauxite (red mud) (BR) lakes. However, at greater depths, carbonation occurs at less than half the rate that it does in the 0-30 cm zone. Maximizing atmospheric exposure can increase the rates of carbonation in bauxite wastes. This study aimed to identify the restrictions on carbonation operating at greater depths in such waste. With carbonation hardening calcium-rich red muds, the strength of bauxite waste was used as a surrogate for inferring carbonation. Samples were taken from two depths (cm): 15-25 and 25-35. There were three groups of samples. Samples taken from 25-35-cm depth were placed subaerially (outdoors) for 12 months (OBR), while others were located indoors (IBR) at room temperature, while controls remained in situ underground (UBR), unmoved and undisturbed in the ground in the natural semi-viscous state at a depth of 15-25 cm. All were revisited and re-tested after 12 months. Strength tests were done on the samples (a) in the fresh state (b) after 12 months of subaerial exposure, using a hand-held penetrometer. For strength of samples, the Chi square test revealed a significant difference between treatments and controls. Samples stored subaerially and those stored in the laboratory exhibited a 7-fold and 5-fold increase in strength respectively, above that of the in-situ controls, while 12 extra months in the ground failed to measurably increase the strength of the in-situ BR anywhere at 15-25- or 25-35-cm depth. As carbonation does occur in all bauxite red muds, all BR originally contains some free calcium ions. After the 12-month subaerial exposure, X-ray diffraction (XRD) tests revealed significant levels of calcite in the subaerially stored samples. The large increase in the strength of these samples could be due to atmospheric carbonation to produce calcium carbonate. This result shows that transferring viscous BR from underground to subaerial locations could substantially increase the rate of sequestering atmospheric CO₂ in red mud residues. Carbonation also decreases the high levels of alkalinity in such residues.in such residues.

This work is licensed under a Creative Commons Attribution 4.0 License.