Vaporization Behavior of Cs, K, and Na in Cs-Containing Incineration Bottom Ash during Thermal Treatment with CaCl2 and CaO

The vaporization behaviors of the alkali metals Cs, K, and Na were investigated at 900, 1000, and 1100 degrees C in a lab-scale electrical-heating horizontal furnace using a Cs-doped ash with the addition of CaCl2 and/or CaO. Knudsen effusion mass spectrometry was employed to measure the vaporization of the alkali metals in the Cs-doped ash with CaO under a high vacuum. Molecular beam mass spectrometry was used online to measure their vaporization from the Cs-doped ash with either CaCl2 or a mixture of CaCl2 and CaO. The addition of CaO caused some vaporization of these elements, which was probably due to the replacement of the Cs+, K+, and Na+ cations in aluminosilicates with Ca2+ cations during the thermal treatment. The vaporization propensity of the three elements followed the sequence of Cs > K > Na. The vaporization of Cs, K, and Na were observed during a thermal treatment with CaCl2. An increase in the content of CaCl2 or the reaction temperature facilitated the vaporization of Cs, K, and Na. O-2 and H2O in the reactant gas showed an inhibiting effect on the vaporization of Cs, K, and Na through accelerating the release of Cl from the decomposition of CaCl2. A synergistic effect was observed between the addition of CaCl2 and CaO on the vaporization of Cs, K, and Na because they delayed the release of Cl, which provided a longer contact time between the three metals and the gaseous Cl. Moreover, when the mixture of CaCl2 and CaO was used, the CaO produced unstable Cs, K, and Na that readily reacted with gaseous Cl, enhancing the vaporization of the alkali metals during thermal treatment. At 1100 degrees C, 93% of the Cs was vaporized from the Cs-doped ash with 5% CaCl2 and 20% CaO while the vaporization ratio of K and Na was 69% and 63%, respectively.