Effect of chloride and sulfate on the solidification process of cadmium in clay sintering

The effects of chloride and sulfate on cadmium volatilization characteristics and stability in the sintering solidification process were investigated by adding CdO, NaCl and Na2SO4 to the clay raw materials simulating cadmiumcontaining solid waste. The results showed that the volatilization rate of Cd increased with the increase of chloride content and reached 72.82% at 1150°C and was promoted by NaCl. CdCl2 generated when the reaction temperature was over 600°C and transformed into gaseous form and volatilized at the temperature of 1050°C. The volatilization rate of Cd was only 25.37% with the sulfate content of 2.0% when the sintering temperature was low. CdSO4 generated when the temperature was over 884°C accordings to the thermodynamic calculation, and Na2SO4 promoted its generation at a lower temperature. CdSO4 gradually decomposed with the sintering temperature increasing and completely decomposed at 1050°C. Compared with NaCl, Na2SO4 was more unbenefit to the stability of cadmium in the sintered materials. The maximum leaching concentration of cadmium in sulfatecontained samples was 123.70mg/L, which was higher than those of chloridecontained samples. And the proportions of acid extraction form in sulfatecontained samples were significantly higher than those of the chloridecontained samples. In addition to generation of CdCl2 and CdSO4 by reacted with NaCl and Na2SO4, cadmium also reacted with silicaluminate mineral to generate stable cadmium silicaluminate (CdAl2Si2O8) by the form of Cd2+ ions replacing Ca2+ ions in the silicaluminate mineral lattice. As a result, the Cd2+ ion was solidified in the sintered clay material. Comparative analysis of chloride and sulfate effects on cadmium volatilization and stabilization during sintering shows that chloride promotes the volatilization of Cd while sulfate promotes the dissolution of Cd in the material. Generation of stable cadmium silicaluminate (CdAl2Si2O8) is benefit to the solidification of Cd during sintering.