Recovery of scandium from Canadian bauxite residue utilizing acid baking followed by water leaching.

The current study put the emphasis on developing a novel and environmentally friendly waste valorization process, called "acid-baking water-leaching", to recover scandium from bauxite residue produced by the aluminum industry. In this process, bauxite residue is mixed with concentrated sulfuric acid, baked in a furnace at 200-400 °C, and leached in water at ambient conditions. Compared with direct acid leaching processes, the developed process offers the advantages of less acid consumption, less wastewater generation, and fast kinetics. With fundamental investigation into the reaction mechanism, acid baking temperature was shown to be the controlling factor that dictates the final phases of the process. Baking at 200 °C results in the formation of (H3O)Fe(SO4)2 that leaches in water rapidly (<5 min), but extraction efficiency is low (58% scandium). In contrast, baking at 400 °C results in the formation of Fe2(SO4)3 that leaches at slower kinetics (>45 min), but results in higher extraction efficiency (80% scandium). The acid baking water leaching process proves to be a promising technique as the first step of a potential near-zero-waste integrated process for the sustainable valorization of bauxite residue to help build the circular economy.