Efficient and selective recovery of iron phosphate from the leachate of incinerated sewage sludge ash by thermally induced precipitation

Phosphorus recovery from incinerated sewage sludge ash (ISSA) is important but hindered by low selectivity. Here, a novel strategy of acid leaching followed by thermally induced precipitation was proposed for the efficient and selective recovery of FePO4 from ISSA samples. A high phosphorus leaching efficiency of ∼ 99.6% was achieved with 0.2 mol/L H2SO4 and liquid to solid (L/S) ratio of 50 mL/g. Without removing various co-existing ions (Al3+, Ca2+, SO42-, etc.), high-purity FePO4 of ∼ 92.9% could be facilely produced from this highly acidic H2SO4 leachate (pH = 1.2) by simple addition of Fe(III) at a molar ratio of 1:1 to the phosphorus and reacted at 80 °C for thermally induced precipitation. The remained acid leachate could be further reused for five times to continue leaching phosphorus from the ISSA samples and produce the FePO4 precipitates with a high phosphorus recovery efficiency of 81.1 ± 1.8%. The selective recovery of FePO4 from the acid leachate was demonstrated more thermodynamically favorable compared to other precipitates at this acidic pH of 1.2, and elevated temperature of 80 °C towards thermally induced precipitation. The estimated cost of this strategy was ∼$26.9/kg-P and lower than that of other existing technologies. The recovered FePO4 precipitates could be used as a phosphate fertilizer to promote the growth of ryegrass, and also as a precursor to synthesize high-value LiFePO4 battery material, demonstrating the high-value application potential of the phosphorus from the ISSA.