Investigation of properties change in the reacted molten salts after molten chlorides cyclic thermal treatment of toxic MSWI fly ash

Abstract

To realize the thermal detoxification of municipal solid waste incineration (MSWI) fly ash in a relatively mild environment, molten salts thermal treatment technology was proposed in our previous research, which showed good effects. To investigate the properties of molten salts (NaCl-CaCl₂) during cycling reusing, the change of the main components and the physical properties of the used molten salts were estimated. Results showed that the salts in fly ash would dissolve into molten salts. During this process, the concentration of K⁺, SO₄^2- kept increasing while Cl^- was decreased. The changing trend of Na⁺ and Ca²⁺ was dependent on the ratio of Ca/Na in raw fly ash. Ca(OH)₂ in fly ash would react with CaCl₂ to form CaClOH. Moreover, the introduction of the salt components on the thermal properties of molten salts were also studied. The melting point hardly changed by NaCl, CaSO₄, and SiO₂. Nevertheless, it was lowered to 431 °C with 15% CaCO₃ addition, while increased to 523 °C with 20% KCl. Besides, there were no significant influences on the viscosity, stability, and thermal diffusivity of molten salts. KCl had the greatest influence on the specific heat capacity of molten salt, with an increase of about 20%.

Keywords: Insoluble chlorides; MSWI fly ash; Molten salts cyclic thermal treatment; Reacted molten salts; Thermal properties.