Study on phosphorus recovery from sludge-incinerated bottom ash to produce nitrogen–phosphorus compound fertilizer: Preparation method, transfer mechanism and practical application

Phosphorus (P) is an irreplaceable and non-renewable resource but faces a foreseeable resource shortage. In this study, nitric acid (HNO₃) was employed to extract P from sludge-incinerated bottom ash and nitrogen–phosphorus (N-P) compound fertilizer was synthesized by adding aqueous ammonia to the P-rich extract. The N-P compound fertilizer was continued to be applied to the cultivation of ryegrass, and the impact on plants and soil was analyzed. The results indicated that the optimal concentration of HNO₃ was 5.0 mol/L and 93.1% of P in sludge-incinerated bottom ash could be recovered in HNO₃ extraction. The N–P compound fertilizer prepared after ammonia neutralization exhibited an amorphous complex composed of various substances. The Ca, Mg, Al, and Fe contents of N-P compound fertilizer were 813, 220, 137, and 25 mg/kg, respectively. N–P compound fertilizer enhanced ryegrass growth; after 20 days of planting, the root, stem, and leaf length were 2.0, 1.3, and 1.4 times that of the control, respectively. N–P compound fertilizer did not lead to heavy metal accumulation in plants and soil deterioration after fertilization. This method can fully recover the P source in sludge-incinerated bottom ash and turn waste into treasure.