Experimental investigation of phosphorus and potassium recovery from chicken manure incineration ash (CMIA) using a mixed nitric acid–oxalic acid system

Traditional acid leaching of chicken manure incineration ash (CMIA) often shows poor selectivity and may generate phosphogypsum. To address these limitations, this study developed a nitric–oxalic acid process for recovering phosphorus (P) and potassium (K) from CMIA with a high Ca/P ratio. The influence of thermal-treatment temperature on the transformation of P-bearing phases in chicken manure was investigated. At 1073 K, thermal treatment promoted the transformation of operationally defined organic P into crystalline calcium phosphate (Ca-P) phases. The leaching behavior under different mixed acid conditions was systematically studied. Under optimal mixed acid conditions, P and K leaching efficiencies reached 91.24% and 94.45%, respectively. Kinetic fitting, the apparent activation energy of 10.81 kJ/mol, and particle-size dependence were consistent with a mixed-control process involving an appreciable diffusion-related contribution. Under these conditions, XRD identified hydrated calcium oxalate phases in the residue, while FTIR and SEM-EDS corroborated oxalate groups and Ca-C-O-rich deposits. Thermodynamic calculations indicated that concurrent Ca precipitation during Ca-P dissolution was feasible. The process produced a nitrate-containing, P- and K-rich leachate. A 35-day cherry-radish pot experiment provided preliminary evidence of its fertilizer application potential, while commercial formulation and storage stability require further validation. Because no sulfate reagent was introduced, the process may help reduce phosphogypsum generation at the source.