Phase transformation and its role in stabilizing simulated lead-laden sludge in aluminum-rich ceramics

This study investigated the mechanisms of stabilizing lead-laden sludge by blending it into the production process of aluminum-rich ceramics, and quantitatively evaluated the prolonged leachability of the product phases. Sintering experiments were performed using powder mixtures of lead oxide and γ-alumina with different Pb/Al molar ratios within the temperature range of 600-1000 °C. By mixing lead oxide with γ-alumina at a Pb/Al molar ratio of 0.5, the formation of PbAl2O4 is initiated at 700 °C, but an effective formation was observed when the temperature was above 750 °C for a 3-h sintering time. The formation and decomposition of the intermediate phase, Pb9Al8O21, was detected in this system within the temperature range of 800-900 °C. When the lead oxide and γ-alumina mixture was sintered with a Pb/Al molar ratio of 1:12, the PbAl12O19 phase was found at 950 °C and effectively formed at 1000 °C. In this system, an intermediate phase Pb3(CO3)2(OH)2 was observed at the temperature range of 700-950 °C. Over longer leaching periods, both PbAl2O4 and PbAl12O19 were superior to lead oxide in immobilizing lead. Comparing the leaching results of PbAl2O4 and PbAl12O19 demonstrated the higher intrinsic resistance of PbAl12O19 against acid attack. To reduce metal mobility, this study demonstrated a preferred mechanism of stabilizing lead in the aluminate structures by adding metal-bearing waste sludge to the ceramic processing of aluminum-rich products.