The synergistic effect of carbide slag and thermal treatment on immobilizing soluble phosphorus and fluoride in phosphogypsum

The recycling of phosphogypsum (PG) is severely hindered by the presence of soluble phosphorus and fluorine impurities, which present significant environmental risks. Conventional purification techniques such as high-temperature calcination (above 500 °C) or alkaline treatment are often associated with high energy consumption and suboptimal efficiency. This study proposes a novel and synergistic method combining low-temperature calcination with carbide slag (CS) treatment to achieve effective impurity removal. The treatment efficiency was systematically evaluated by calcining PG at different temperatures (ranging from 180 to 600°C) and introducing CS either before or after calcination at 180 °C. Results demonstrate that the proposed synergistic method effectively removes phosphorus and fluorine impurities and enhances the early strength of calcinated PG, outperforming traditional calcination. The soluble phosphorus and fluorine contents were reduced to 0.11 mg/L and 0.92 mg/L, respectively, significantly lower than those treated by traditional calcination (57.4 mg/L and 61.29 mg/L). Moreover, the early (2 h) flexural and unconfined compressive strengths increased from 1.81 MPa to 2.4 MPa and from 3.4 MPa to 4.93 MPa, respectively. Microstructural analysis revealed that adding CS prior to calcination enhanced its reactivity, facilitating more efficient impurity immobilization. The proposed strategy offers an energy-efficient and environmentally friendly pathway for PG purification, promoting its potential reuse in sustainable construction materials.