Removal of heavy metal from sewage sludge ash (SSA) during calcination process and safety evaluation of calcination residues

Sewage sludge is considered to be the biggest secondary resource of phosphate. Sludge incineration reduces mass of the sludge, leading to concentrated phosphorus in the ash. However, most toxic heavy metals are also concentrated in SSA. This paper focused on the possibility for heavy metal removal from SSA during calcination process. The calcination experiment was carried out by the muffle furnace, with varying treatment temperature, residence time and additive amount of CaCl₂ of 900~1200℃, 10~60 min, and 1.5%~8.0% (specified as Cl), respectively. The removal rate of heavy metals and their chemical speciation in calcination residues were investigated. The result showed that: Pb can be removed well in all experiments; the removal rate of Pb increased with increasing temperature, residence time and additive amount of CaCl₂; the highest removal rate of Pb was 82.4% when the temperature was 1200℃ and additive amount of Cl was 5%; temperature was the main influence factor for Zn with the highest removal rate of 83.9% presented at 1100℃; the removal rate of Cu was lower than Pb and Zn, and its highest value was 68.2% at 1100℃. In terms of the chemical speciation of heavy metals in calcination residues, Pb existed as residual form in all experiments, while other speciation was not detected; Zn also mainly existed as residual form; acid-soluble form of Cu existed obviously in calcination residues, was accounting for 29.2% when the temperature was 900℃ and residence time was 10 min. With the increasing additive amount of CaCl₂, the removal rate and the stability of heavy metals increased. The evaluation result of the index of geo-accumulation showed that the pollution levels of heavy metals in calcination residues were reduced.