A comparison between sulfuric acid and oxalic acid leaching with subsequent purification and precipitation for phosphorus recovery from sewage sludge incineration ash

Abstract Wet chemical approach is widely applied for P extraction from incinerated sewage sludge ash (ISSA) due to the relatively simple process and low lost. In this study, H 2 SO 4 and H 2 C 2 O 4 were compared to recover P from ISSA through three steps of acid leaching, cation exchange resin (CER) purification and precipitation. Transformations of P speciation and mineral phases in ISSA from 600 to 900 °C were studied. The results showed that the ISSA samples were mainly composed of inorganic P (IP), and part of non-apatite IP (NAIP, mainly AlPO 4 ) would transform to apatite P (AP, Ca 3 (PO 4 ) 2 ) with the increase in temperature. The ratio of NAIP to IP dropped from 71.9% at 600 °C to 53.7% at 900 °C. Effect of acid concentration on the leaching efficiency of P from the ISSA samples incinerated at different temperatures by H 2 SO 4 and H 2 C 2 O 4 was investigated, and the leaching behaviors of key elements of P, Ca, Al and Fe were compared. H 2 C 2 O 4 exhibited a better performance than H 2 SO 4 for the leaching efficiency of P. Severe sintering of ash particles occurred at temperature >800 °C inhibited the P leaching by H 2 SO 4 . During CER purification, the impurity elements in the H 2 SO 4 leachate were easily removed by CER, whereas the Al and Fe elements in the H 2 C 2 O 4 leachate were hardly removed due to the formation of anionic complexes between Al 3+ /Fe 3+ and oxalic ions. Finally, high-purity struvite product was synthesized from the purified H 2 SO 4 leachate, which could be directly utilized as a fertilizer with negligible environmental risk. Amorphous aluminum and iron hydroxyphosphates were obtained from the H 2 C 2 O 4 leachate. This study provides insights for P recovery from ISSA samples by different acid leaching systems.