Evaluation of the Effectiveness of Cd Stabilization by a Low-temperature Sintering Process with Kaolinite/Mullite Addition

Abstract

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.