Synergistic Mechanisms of CaCl2 and CaO on the Vaporization of Cs from Cs-Doped Ash during Thermal Treatment

This study aimed to clarify the roles of CaCl2, CaO, and their mixture in the vaporization of Cs from Cs-doped ash during thermal treatment. In particular, potential mechanisms of the synergistic effect of the addition of a mixture of CaCl2 and CaO on Cs vaporization were investigated. Vaporization experiments were carried out in a lab-scale horizontal furnace at 900, 1000, and 1100 °C. The results indicated that adding a mixture of CaCl2 and CaO produced a synergistic effect on Cs vaporization when the reaction temperature was above 1000 °C and the vaporization ratio was noticeably increased in comparison to that when adding CaCl2 or CaO alone. The formation of wadalite (Ca6Al5Si2O16Cl3) and/or igumnovite [Ca3Al2(SiO4)2Cl4], derived from chemical reactions among CaCl2, CaO, and aluminosilicates in the Cs-doped ash, delayed the release of Cl during thermal treatment, thus extending the contact time of Cs and gaseous Cl. Furthermore, CaO destabilized the aluminosilicate structure, resulting in a higher volatility and reactivity of Cs, and thus, a reaction readily occurred between activated Cs and gaseous Cl released from the decomposition of wadalite and/or igumnovite.