Migration and transformation of phosphorus and toxic metals during sludge incineration with Ca additives

The recycling and utilization of phosphorus resources in sludge is becoming increasingly important. In this study, we compared the conversion of phosphorus and toxic metal passivation effects of different Ca additives under oxygen-rich combustion conditions and elucidated their specific mechanisms of action. The experimental results indicated that four Ca-based additives improved the recovery rate of total phosphorus, and promoted the generation of stable apatite phosphorus (AP). The effect of CaCl₂ and CaO was greater than that of Ca(OH)₂ and CaSO₄. CaCl₂ promoted the formation of Ca₃(PO₄)₂ and Ca₂P₂O₇, and CaSO₄ improved the conversion of AlPO₄ to Ca(H₂PO₄)₂ with increasing temperature. The conversion capacity of CaO on non-apatite inorganic phosphorus to AP was greater than that of Ca(OH)₂, and more CaH₂P₂O₇, Ca(PO₃)₂, and Ca-Al-P minerals were found. Toxic metal percentages decreased after sludge incineration with CaCl₂. Compared with CaO and Ca(OH)₂, the toxic metal adsorption effect of CaSO₄ was more significant. The influence of Ca additives on the conversion of Zn into stable components was as follows: CaCl₂ > Ca(OH)₂ > CaO > CaSO₄. Ca additives reduced the toxic metal contamination level and ecological risk index values, and the order of toxic metal contamination levels was Ni > Zn > Cr > Cu > Mn. The experiment confirmed the conversion of phosphorus and the toxic metal passivation effect of Ca additives during oxy-fuel combustion of sludge, which is beneficial for its resource utilization.