Treatment of Bayer-Process Red Mud through Microbially Induced Carbonate Precipitation

Abstract

Microbially induced carbonate precipitation (MICP) was used to treat Bayer-process red mud, a waste product of the aluminum industry. Changes in the unconfined compressive strength (UCS), calcium carbonate content and distribution, leaching of metal ions, pH value, and stress-strain characteristics were measured, and the effects of MICP on red mud were evaluated. The results showed that MICP was effective and efficient in red mud treatment. This method could overcome the adverse effects of complex mineral composition, fine particle size, strong alkalinity, and the presence of toxic and hazardous materials characteristic of red mud to produce urease to decompose urea, precipitate carbonate, and strengthen red mud. After treatment, the red mud experienced a significant increase in strength, whose UCS increased up to 1,395 kPa, and a transition from strain hardening to strain softening behavior was observed. In the process, the heavy metal ions in the red mud could also be fixed in the form of carbonate precipitation. Mechanistic studies suggested that the red mud changes were entirely due to the calcite, which were in a state of aggregation and had a relatively large volume. In addition, they acted as a framework to agglomerate red mud particles. Through the action of cementation, the red mud particles aggregated and changed from a loosely distributed (scattered) form to a tightly bound block morphology. This had nothing to do with the carbonation reaction of the active calcium minerals contained in the Bayer-process red mud.