Leaching behavior of phosphorous compounds from sewage sludge ash based on quantitative X-ray diffraction analysis

Phosphorus (P) is an essential and limited resource. Incineration sewage sludge ash (ISSA) contains a high amount of P, which can be recovered using chemical leaching methods. However, the recovery ratio depends on the speciation of P and the leaching conditions. In this study, hydrochloric acid was used as a leaching agent, and the effects of the hydrochloric acid concentration, leaching time, temperature, and liquid-solid ratio on the P leaching ratio were investigated. Furthermore, the co-leaching of macro-metals Ca, Al, Fe, and Mg was analyzed. The results showed that P leached rapidly within 30 min, where the leaching rate reached more than 80% and then gradually stabilized. The leaching concentrations of Ca and Mg had a significant correlation (correlation coefficient r(2) > 0.90), and both were leached completely. Al and P had similar leaching patterns, where the leaching rates increased initially and then decreased with time at 0.2 mol/L HCl. According to X-ray diffraction analysis and Rietveld refinement, the P in ISSA was mainly present in the forms of Mg3Ca3( PO4)(4) and AlPO4. When leached using 0.2 mol/L HCl at 55 degrees C with a liquid-to-solid ratio of 20 L/kg, the AlPO4 and Fe-3( H2O)(3)(PO4)(2) in ISSA dissolved initially and then precipitated on the surface of the solid phase, thereby impeding further P leaching.