Phase characterization and thermochemical simulation of (landfilled) bauxite residue (“red mud”) in different alkaline processes optimized for aluminum recovery

In coherent studies three different processes of hydrometallurgical, alkaline extraction of aluminum from bauxite residue (BR) are examined and benchmarked regarding their efficiency on aluminum recovery. The employed processes include a direct second caustic pressure leaching, a caustic pressure leaching of the slag produced by reductive smelting of BR with simultaneous pig iron recovery (adapted “Pedersen-process”) and leaching of BR after a sintering stage with sodium carbonate and further additives including lime and coke. In order to ensure a direct comparability, all experiments are conducted using the same homogenized BR from an old industrial landfill. After detailed characterization of the used BR using XRF chemical analysis, XRD phase analysis, SEM optical analysis and Qemscan® phase analysis/distribution, occurring phase formations during the employed processes are also calculated and predicted by FactSage® simulation software. The actual phase formation and leachability of the formed aluminum phases are verified during experiments and a special focus is set on the dissolution of silicon as major impurity in all processes. It can be shown that aluminum extraction efficiencies of 90% are still possible but interlinked with massive silicon dissolution due to the almost complete dissolution of aluminum silicates.