Enhanced phosphorus recovery from incinerated sewage sludge ash as vivianite through calcination with sodium-based additives

Phosphorus (P) recovery from Incinerated Sewage Sludge Ash (ISSA) is a promising way to alleviate P shortage concerns. An enhanced P recovery strategy from ISSA through calcination, water leaching, and crystallization was proposed to obtain high value-added vivianite. The influence of sodium-based additives (NaCl, NaOH, Na₂CO₃, and NaHCO₃), calcination temperature (700∼1000 °C), additives dosage, and residence time on the P transformation was examined with theoretical and experimental approaches. The effect of filtrate recirculation on the vivianite crystallization was further assessed for water conservation. Results indicate that alkaline sodium-based additives (NaOH, Na₂CO₃, and NaHCO₃) and elevated calcination temperature facilitate the conversion of apatite phosphorus (AP) and non-apatite inorganic phosphorus (NAIP) into water-soluble P (H₂O-P) as Na₃PO₄. At 1000 °C, the conversion of AP to water-insoluble CaNaPO₄ is observed. The highest proportion of H₂O-P to total phosphorus (TP) of 90.53 % is yielded. The optimal P leaching rate from calcined ISSA reaches 90.12 % at 50 % NaHCO₃ dosage, 800 °C, and 60 min. Following the purification and crystallization of P-rich leachate, the vivianite product is collected with a typical flower-like morphology and practical Fe/P molar ratio of 1.40. The influence of filtrate recirculation on the P recovery rate from the leachate is negligible. The proposed strategy can yield the highest total P recovery rate of 78.03 % from ISSA. This study provides an appealing way to recover P from ISSA as high value-added vivianite.