Evaluation of the effectiveness of Cd stabilization by a low-temperature sintering process with kaolinite/mullite addition

This study first examined the phase transformation in the reactive sintering systems of cadmium-laden industrial sludge and Al-Si-rich precursors with different Cd/Al/Si molar ratios under various temperatures. X-ray diffraction results indicated that the Cd started to be incorporated at 750 °C by kaolinite or mullite (calcined from kaolinite). Three hours of processing at 950 °C can effectively incorporate Cd into Cd-Al-Si or Cd-Si materials. The amount of CdO in the reactive systems had significant influences on the Cd incorporation behavior into crystalline phases. With a small amount of CdO, product phase CdAl₂Si₂O₈ dominated in the systems. Systems with considerable CdO produced notable amounts of Cd₂SiO₄ and Cd₃SiO₅. The production of Cd₂SiO₄ and Cd₃SiO₅ from CdO + mullite was more significant than that using kaolinite due to the preferred reaction between CdO and SiO₂. To assess the effect of metal stabilization, single-phase products that host Cd (namely, CdAl₂Si₂O₈, Cd₂SiO₄, and Cd₃SiO₅) were obtained, maintained at pH 4.0, and subjected to a constant-pH leaching test (CPLT) for 120 min. CPLT results evidently indicated that these phases were remarkably resistant to substantial acid (nitric acid) attack; the leaching behavior of CdAl₂Si₂O₈ was incongruent dissolution. Finally, cadmium can be effectively incorporated into CdAl₂Si₂O₈, Cd₂SiO₄, or Cd₃SiO₅ by using sludge ash from a secondary sewage treatment works, suggesting that precursors enriched with Al and Si can be promising materials in a cleaner production process for treating cadmium-laden industrial sludge.