MECHANISMS AND KINETICS OF HIGH-TEMPERATURE CADMIUM SORPTION BY PACKED BED OF CALCINED KAOLIN

Abstract

Sorption experiments by passing CdCl₂-carrying flue gas through the packed bed of calcined macro-porous kaolin particles were performed over a temperature range of 973–1173 K and a CdCl₂ partial pressure range of 8–16.1 Pa. The observed structural change of the sorbent mineral at the stage of sorption and the results of desorption tests revealed the characteristics of an irreversible chemical reaction as a major cadmium capturing mechanism. In the fully saturated kaolin sorbent, CdO·Al₂O·2SiO₂ is present as a sorption reaction product together with a smaller amount of 2CdO·Al₂O·2SiO₂. The increase in sorbent bed temperature resulted in an increase in the rate of sorption, but it had no effect on maximum cadmium uptake. The gas-phase CdCl₂ diffusion into the macro pores of calcined kaolin had a negligible effect on the overall sorption rate. The reaction between gaseous CdCl₂ and solid Al₂O₃.2SiO₂ is very sensitive to the concentration of CdCl₂ but relatively insensitive to the temperature of the sorbent bed. The order of reaction with respect to the CdCl₂ vapor concentration was determined to be 3.26. The activation energy, E_a , was estimated as 5.56 kcal/mol according to the Arrhenius relationship.

Keywords:

• Incineration
• Hazardous metal
• Chemisorption
• Kinetic model
• Kaolinite

ACKNOWLEDGMENT

This study has been carried out under the Nuclear R & D Program by the Korean Ministry of Science and Technology.