Simulation study for the formation of alkaline efflorescence on bauxite residue disposal areas following the phosphogypsum addition

Bauxite residue is a solid, highly alkaline byproduct generated in the alumina refining process. The process that forms the fine white salt deposits on the surface of bauxite residue disposal areas (BRDAs) following prolonged dry periods, and which also forms on products made from reused residues (sintered bricks, etc.), is known as efflorescence. It not only seriously affects the utilization of bauxite residue in building materials, but also creates potential dust pollution around the BRDAs, which means it would require sustained and intensive resources to manage and transform its alkalinity. Through a laboratory simulation experiment using soil leaching columns, we show that the extent of bauxite residue efflorescence is affected by many factors, including porosity, initial water content, bauxite residue particle size. The main soluble mineral precipitates of efflorescence are Na₂CO₃ and NaHCO₃. Addition of phosphogypsum, a common amendment to reduce dispersion and promote removal of Na⁺ through leaching, shifted the composition of effloresced salts to favor Na₂SO₄·10H₂O. SEM and Na K-edge X-ray absorption near-edge structure (XANES)spectroscopy demonstrated that free alkali migrates to the residue surface to precipitate as a uniform agglomerate in the efflorescence process, which also changes the chemical form of Na-containing minerals in the bauxite residue and the mesoscale spatial distribution of Na (soft X-ray scanning transmission microscopy imaging). Free alkali migration with water is the main driver of efflorescence, and this work also demonstrated that phosphogypsum can reduce the generation of alkaline efflorescence for ameliorating alkaline dust pollution from bauxite residue.