Four stage precipitation for efficient recovery of N, P, and F elements from leachate of waste phosphogypsum

The leachate from waste phosphogypsum contains substantial amounts of phosphorus, fluorine, ammonia nitrogen (NH₃-N), and magnesium ions, which cause soil and water pollution and have great resource utilization value. In this study, quicklime and alkali were used to treat waste phosphogypsum leachate using step-by-step precipitation with precisely controlled pH values and selective recovery of valuable precipitates. The addition of CaO and NaOH in a specific order to adjust the pH value helped recover NH₃-N, phosphate (PO₄³⁻), and fluoride (F⁻) in the leachate with a recovery rate above 99%. After the reaction, the residual ion concentration in the post leachate met the first level standard of the “Integrated Wastewater Discharge Standard” (GB8978-1996) (NH₃-N < 5 mg/L, TP < 0.5 mg/L, and F⁻ < 5 mg/L). This study showed that phosphogypsum leachate treatment and standard reuse can be achieved simultaneously. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the composition of the precipitates produced at pH 3, 6, 8.5, and 12 to provide basic data for further experimental research.