Selective leaching and recovery of phosphorus from incinerated sewage sludge ash with CaO addition

Incinerated sewage sludge ash (ISSA) with high P₂O₅ content is a potential phosphorus resource that can replace the non-renewable phosphorus rocks. However, extracting phosphorus from ISSA using hydrometallurgical methods also dissolves a large amount of impurity metals into the leachate. Therefore, this study proposes a new method combining high-temperature reaction with CaO addition, selective leaching, and chemical precipitation for efficient and low-cost phosphorus recovery from ISSA. During thermal treatment at 1050 °C, the addition of CaO significantly influenced the types and amounts of phosphate mineral phases in ISSA. When 20 % CaO was added, the Al/Fe-phosphate phases were completely converted into acid-soluble Ca-phosphate phases, while Al and Fe were retained in acid-insoluble phases (e.g., Al- and Fe-containing oxides and silicates). Subsequently, by controlling the pH of acidic leachate (2.0 to 1.5), Ca-phosphate phases were selectively dissolved. At a pH of 1.5, better selective leaching was achieved, with a leaching efficiency of 94.71 % for P and less than 10 % for Al, Fe and Si. The XRF results showed that the majority of SiO₂ (38.39 %), Al₂O₃ (33.26 %) remained in the post-leaching solid residue, which was expected to be further resource utilization. Without purification treatment to remove Al, Fe and Si, almost all phosphate ions were precipitated at pH = 8.5, with P recovery efficiency of 94.07 %. XRD results showed that the precipitate was mainly composed of HAP, with a P₂O₅ content of 37.51 % and low level of contaminants, thereby realizing the effective recovery of phosphorus from ISSA.