Improved removal of phosphorus from incinerated sewage sludge ash by thermo-chemical reduction method with CaCl2 application

Abstract

Incinerated sewage sludge ash (ISSA) can be used as raw materials for producing construction materials. However, high content of phosphorus in ISSA distinctly lowers the compressive strength of construction materials. This work presented a new method for phosphorus removal from ISSA with thermo-chemical reduction assisted by calcium chloride (CaCl2) based on carbon reduction reaction and converted phosphate to gas P. It was shown that 82 wt% phosphorus was removed under the addition of 40 wt% CaCl2 and 40 wt% C at 1373 K, and the removal rate of P was significantly enhanced by CaCl2. Results showed that the addition of CaCl2 lowered the treating temperature, increased the reactivity and decreased CO2 production as well. CaCl2 molten salt effectively contributed to both solid-state reaction and Ca5(PO4)3Cl formation, acting both as a flux and chlorine source. According to metallographic analysis, phosphorus in Ca5(PO4)3Cl could be easily deoxidized by C into gas phase P2 at 1373K.