The need of light weight alloys for future transportation industry puts Sc and Ti under a sudden demand. While these metals can bring unique and desired properties to alloys, lack of reliable sources brought forth a supply problem which can be solved by valorization of the secondary resources. Bauxite residue (red mud), with considerable Ti and Sc content, is a promising resource for secure supply of these metals. Due to drawbacks of the direct leaching route from bauxite residue, such as silica gel formation and low selectivity towards these valuable metals, a novel leaching process based on oxidative leaching conditions, aiming more efficient and selective leaching but also considering environmental aspects via lower acid consumption, was investigated in this study. Combination of hydrogen peroxide (H2O2) and sulfuric acid (H2SO4) was utilized as the leaching solution, where various acid concentrations, solid-to-liquid ratios, leaching temperatures and times were examined in a comparative manner. Leaching with 2.5 M H2O2: 2.5 M H2SO4 mixture at 90 °C for 30 min was observed to be the best leaching conditions with suppressed silica gel formation and the highest reported leaching efficiency with high S/L ratio for Sc and Ti; 68% and 91%; respectively.
A new process is suggested in the present work turning bauxite residue (BR) into a ready-made mortar for the synthesis of inorganic polymers, effectively transforming the Bayer process into a zero-waste process. This was achieved by firing BR at 1100°C which supports the formation of liquid phase and results after subsequent fast cooling in a semivitreous material. Based on thermodynamic calculations, the process was subsequently improved by adding minor quantities of C and silica to BR before firing which leads to the carbothermic reduction of ferric iron into ferrous iron; the new blends demonstrated an increase in the melt formation and eventually in the content of amorphous phase after solidification. The resulting material was activated using a K-silicate solution and was cured at 60°C for 72 h. The synthesized materials were water insoluble and dense, demonstrating compressive strength exceeding 40 MPa for the silica-rich blend. These inorganic polymers can find applications in construction, such as pavement tiles or floor/roofing tiles. A possible implementation of the process to transform BR within the alumina refinery is also represented.
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