HYDROMETALLURGICAL PROCESSING OF BAUXITE RESIDUE (RED MUD) FOR RECOVERY OF METALLIC VALUES

Tanvar, Himanshu

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HYDROMETALLURGICAL PROCESSING OF BAUXITE RESIDUE (RED MUD) FOR RECOVERY OF METALLIC VALUES

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Metal extraction processes often result in waste stream production, which must be responsibly processed and correctly disposed of. Red mud, more frequently termed bauxite residue, is an industrial waste generated during bauxite ore processing using the Bayer process. More than 95% of the estimated 2020 global alumina production of 134 million tons is from bauxite refined in the Bayer process and will probably remain so for decades. Approximately 0.7 to 2 tons of bauxite residue is generated per ton of alumina, and the annual waste generation is over 160 million tons, with a total of 4-5 billion tons accumulated by 2022 globally. The alumina industry’s global objective is the safe disposal of the residue to maintain low environmental and social impacts. The industry continues searching for technically and economically viable options for value extraction and direct utilization, with the ideal future of disposing of no residue in landfills. Bauxite residue contains iron oxide as the major constituent, along with oxides and hydroxides of aluminum, titanium, silicon, calcium, and sodium. There are often significant quantities of rare earth elements, specifically scandium, in bauxite residue. The presence of multiple elements presents both an opportunity and a challenge for recovering different metal values with high market demand. This dissertation focuses on developing a hydrometallurgical process to recover metal oxide products from bauxite residue. Different metallic oxides are sequentially recovered in a multistage process with high recovery and purity of final products. The process stages are critically evaluated based on kinetic and thermodynamic factors to develop a deep understanding of the mechanism and physiochemical changes. An effort is made to recycle waste generated in the process, such as liquid solution for reagent regeneration and solution recycling. The developed approach is also tested with bauxite residue samples from different alumina refineries to assess the process flexibility. Bauxite residue is first neutralized after leaching with mild hydrochloric acid, followed by recovery of dissolved aluminum, silicon, and calcium values. The neutralized bauxite residue is further leached with oxalic acid for selective iron leaching and photochemical reduction of leach liquor to obtain ferrous oxalate precipitate, which is converted to high-purity magnetite after thermal decomposition. The iron-depleted bauxite residue is processed through a sulfation baking process using sulfuric acid to recover titanium and rare earth values. The major products recovered from the developed process include high-purity magnetite, titanium dioxide, alumina, calcium carbonate, aluminum sulfate, silica, and rare earth (scandium) concentrate.

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