Isothermal and non-isothermal sublimation kinetics of zirconium tetrachloride (ZrCl₄) for producing nuclear grade Zr

Sublimation of ZrCl₄ is important for the production of nuclear grade metallic Zr in Kroll's process. The sublimation kinetics of ZrCl ₄ was investigated by thermogravimetric analysis under both isothermal and non-isothermal conditions. The sublimation rate of ZrCl ₄ increased with increasing temperature under isothermal conditions. ZrCl₄ sublimation was confirmed to be a zero-order process under isothermal conditions, whereas it was first-order kinetics under non-isothermal conditions. The activation energy of ZrCl₄ sublimation under isothermal conditions was 21.7 kJ mol^-1. The activation energy for non-isothermal sublimation was 101.4 kJ mol^-1 and 108.1 kJ mol ^-1 with the Kissinger method and Flynn-Wall-Ozawa method, respectively. These non-isothermal activation energies were very close to the heat of sublimation (103.3 kJ mol^-1). Sublimation occurs by two elementary steps: surface reaction and desorption. Therefore, the overall activation energy of ZrCl₄ sublimation is 104.8 (±3.4) kJ mol^-1. The activation energy of the surface reaction and desorption steps are proposed to be 83.1 kJ mol^-1 and 21.7 kJ mol^-1, respectively.