Reaction mechanisms underpinning the removal of Cs from simulated Cs-contaminated ash during thermal treatment with NaCl or KCl

•Vaporization of Cs in ash during thermal treatment with NaCl or KCl was investigated.•Interactions between Cs and Cl-bearing solid species were responsible for Cs vaporization.•An N2 atmosphere facilitated the vaporization of Cs relative to an air atmosphere. The vaporization mechanism of cesium (Cs) in simulated Cs-contaminated ash during thermal treatment with the addition of NaCl or KCl was systemically examined in a laboratory-scale horizontal electrical-heating furnace. A thermodynamic equilibrium calculation was performed to predict the distribution of Cs and Cl in the solid and gas phases under different experimental conditions. The results indicated that Cs vaporized following heat treatment of the simulated ash with the addition of NaCl or KCl. The vaporization ratio of Cs increased as the reaction temperature. As the content of NaCl or KCl was increased, the vaporization ratio also increased at > 1000 °C; whereas, it initially increased but then decreased at 900 °C. Both NaCl and KCl underwent direct vaporization and decomposition via chemical reactions with silicate/aluminosilicates in the ash, thus releasing HCl/Cl2. The interactions between Cs and solid Cl-bearing species (e.g. NaCl, Na2AlCl6, and KCl) formed CsCl and were responsible for the vaporization of Cs. Nevertheless, the release of HCl/Cl2 from the decomposition of NaCl and KCl had an insignificant effect on the vaporization of Cs. Compared to an air atmosphere, N2 facilitated the vaporization of Cs because the vaporization and/or decomposition of NaCl, Na2AlCl6, and KCl were delayed under the N2 atmosphere, which in turn enhanced the chlorination of Cs. By comparing the effect of adding NaCl, KCl, CaCl2, or MgCl2 on the vaporization of Cs, alkali chlorides (NaCl and KCl) were found to exhibit the best performance.