Effectiveness of municipal sewage sludge (MSS) ash application on the stabilization of Pb-Zn sludge from mining activities

The feasibility of immobilizing Pb-Zn sludge by sewage sludge ash under thermal conditions was investigated. Sewage sludge ash was found to crystallochemically incorporate Pb into crystalline apatite (Ca5.5Pb4.5(PO4)6(OH)2) and stabilize Zn into spinel (Zn(Al0.5Fe1.5)O4). The influences of temperature, treatment time and Ca/Pb molar ratio on phase transformation of Pb and Zn were examined to determine the optimal incorporation conditions. The amount of Pb incorporated into Ca5.5Pb4.5(PO4)6(OH)2 peaked at 1000 °C, and then decreased with increasing temperature. The immobilization of Zn into Zn(Al0.5Fe1.5)O4 was enhanced with increased treatment temperature from 800 °C to 1000 °C and became steady with a further increase of temperature. The most effective treatment time was determined to be 3 h before Ca5.5Pb4.5(PO4)6(OH)2 decomposed with prolonged treatment time. The growth of Ca5.5Pb4.5(PO4)6(OH)2 and Zn(Al0.5Fe1.5)O4 initially increased with an increase in the Ca/Pb molar ratio, reached maximum at a Ca/Pb molar ratio of 9.2/4.5 and dramatically decreased with a further increase of Ca/Pb molar ratio. Prolonged leaching experiments demonstrated the superiority of the sintered mixture of Pb-Zn sludge and MSS ash in stabilizing Zn and Pb. Overall, the present study hinted the feasibility for the beneficial use of sewage sludge ash in immobilizing hazardous Pb-Zn sludge.