Role of CaCl2 and MgCl2 addition in the vaporization of water-insoluble cesium from incineration ash during thermal treatment

We studied the vaporization mechanisms of water-insoluble Cs from Cs-doped ash by using CaCl2 and MgCl2 additives during thermal treatment. The ash sample was mixed with different amounts of CaCl2 or MgCl2 and treated at 900-1200 °C in a lab-scale horizontal furnace. Dependence of the water-insoluble Cs vaporization on O2 content in the reactant gas and reaction time was also examined. Thermal treatment of pollucite ore was employed to further verify the vaporization mechanisms of Cs. The results indicate that vaporization of Cs with the addition of the two additives could probably be attributed to the formation and subsequent decomposition of Cs-bearing compounds: CsCaCl3 from the CaCl2 addition and Cs2MgCl4/Cs3MgCl5 from the MgCl2 addition. CaCl2 was more successful in vaporizing Cs than MgCl2, since the lower decomposition rate of CaCl2 facilitated the formation of CsCaCl3 and promoted the vaporization of Cs. Reducing O2 content in the reactant gas increased the vaporization of Cs for both additives by preventing their decomposition. The high content of CaCl2 delayed the vaporization of Cs, and the delay was compensated by increasing the temperature and/or the time of the reaction.