A new approach to sustainable management of industrial phosphogypsum waste: mechanism exploration and industrial application

A novel inexpensive modified calcium oxide (MCA) was prepared using commercial calcium oxide (CCO) and n-butanol and applied to curing phosphogypsum (PG) waste. With the MCA, the soluble phosphorus (SP), soluble fluoride (SF) and pH of PG changed from 230.6 mg/L, 180.7 mg/L and 2.89 drastically to 0.46 mg/L, 8.3 mg/L and 8.58 after 3 days, meeting the GB8978–1996 (SP≤0.5 mg/L, SF≤10 mg/L, pH = 6∼9), remaining stable even after 90 days. Our mechanism study revealed that the MCA reacted with SP to form amorphous CaHPO₄ which further tranformed to Ca₅(PO₄)₃OH. Ca₅(PO₄)₃OH combined with SF to produce Ca₅(PO₄)₃F (K_sp=2.1 × 10^–59). Compared to CaF₂ and CaHPO₄, the K_sp of Ca₅(PO₄)₃F decreased markedly by approximately 10⁴⁸ and 10⁵² times. More importantly, in this work MCA has been industrially applied to cure 790,000-tons PG. Moritoring for >500 days showed that the SP, SF and pH values met the GB8976–1996 standard continuously. This study demonstrated a promising new approach to sustainable management of industrial PG waste by treating PG with the novel inexpensive MCA to solidify SF and SP into precipitates of Ca₅(PO4)₃F with greatly improved long-term stability.