Chemical extraction and precipitation recovery performance of phosphorus fromfour kinds of sludge heat treatment products

Sewage sludge contains a large amount of phosphorus, and the recovery of phosphorus can realize the secondary utilization ofphosphorus resources. The phosphorus recovery experiments in the acid leaching solution of four kinds of sludge thermal treatment products, including smoldering ash, mixed ash, incineration ash and pyrolysis coke were conducted by the magnesium ammonium phosphate(MAP) recovery method. The phosphorus recovery performance of each thermal treatment product under the acid leaching-MAP recoveryprocess route was analyzed. The phosphorus precipitation rate, the phosphorus and impurities contents in the products were explored. Theenvironmental safety of the precipitated products was further investigated. The results show that the phosphorus content of the precipitateproduct can reach 19.82-31.71 mg/ g, and the phosphorus recovery rate in the original sludge reaches 49.25%-65.81% by the acid leaching-MAP recovery method. The order of the phosphorus recovery performance is incineration ash > pyrolysis coke > smoldering ash >mixed ash. There are a large number of Ca, Fe, Al and Si impurities in the acid leaching MAP recovery product, and a certain amount ofZn, Pb, Cr, Cu, Cd, As and Ni, with Zn content much higher than other heavy metals. The purity and environmental safety of the products follows the order of incineration ash > mixed ash > pyrolysis coke > smoldering ash. In the terms of the environmental safety of theprecipitated products, the heavy metal content of the acid leaching-MAP recovered products of smoldering ash, incineration ash and pyrolysis coke all meet the " Ecological Index of Arsenic, cadmium, lead and mercury in Fertilizers" written in GB/ T 23349—2009 and the" Control Standard of Pollutants in Agricultural Sludge" written in GB 4284—2018. The content of Cd in the recovered product fromthe mixed ash using acid leaching-MAP is 3.20 mg/ kg, which exceeds the limited value of 3.00 mg/ kg for pollutant control of agriculturalsludge.