Phosphogypsum stabilization of bauxite residue: Conversion of its alkaline characteristics

Reduction of the high alkalinity of bauxite residue is a key problem to solve to make it suitable for plant growth and comprehensive utilization. In this study, phosphogypsum, a waste product from the phosphate fertilizer industry, was used to drive the alkaline transformation of the bauxite residue. Under optimal water washing conditions (liquid/solid ratio of 2 mL/g, 30°C, 24 hr), the impact of quantity added, reaction time and reaction mechanism during phosphogypsum application were investigated. Phosphogypsum addition effectively lowered pH levels and reduced the soluble alkalinity by 92.2%. It was found that the concentration of soluble Na and Ca ions in the supernatant increased gradually, whilst the exchangeable Na⁺ and Ca²⁺ in solid phase changed 112 mg/kg and 259 mg/kg, respectively. Ca²⁺ became the dominant element in the solid phase (phosphogypsum addition of 2%, liquid/solid ratio of 2 mL/g, 30°C, 12 hr). X-ray diffraction data indicated that cancrinite and hydrogarnet were the primary alkaline minerals. SEM images suggested that phosphogypsum could promote the formation of stable macro-aggregates, whilst the content of Ca²⁺ increased from 5.6% to 18.2% and Na reduced from 6.8% to 2.4%. Treatment with phosphogypsum could significantly promote the transformation of alkalinity cations by neutralization, precipitation and replacement reactions. This research provided a feasible method to promote soil formation of bauxite residue by phosphogypsum amendment.