Process design of molten salt distillation separation of ZrCl4 and HfCl4 for nuclear purposes

Abstract

Zirconium and hafnium are important raw materials for the nuclear industry, but they are chemically similar and often coexist in natural minerals. Molten salt distillation separation technology is a representative technique for the preparation of nuclear-grade ZrCl₄ and HfCl₄. An investigation was conducted into the process design to produce nuclear-grade ZrCl₄ and HfCl₄ through the molten salt distillation separation technology. By processing the vapor pressure data of ZrCl₄ and HfCl₄ in KAlCl₄ molten salt from the literature, the relative volatility (α) of ZrCl₄ and HfCl₄ was found to be 1.18 for a mole fraction of 0.103 of ZrCl₄ and HfCl₄ between 623–773 K. The phase equilibrium relationship for the distillation process was established, through the material balance calculations for the distillation column. The rectifying, stripping, and q-line equations were formulated. Using the Fenske-Gilliland method and the step-by-step plate calculation method, the theoretical number of plates (N_T) required for the target separation effect (the mass percent of HfCl₄ in ZrCl₄ < 100 ppm, the mass percent of ZrCl₄ content in HfCl₄ < 0.5 wt%) was determined when the actual reflux ratio (R) was 1.5 ∼ 3.0 times the minimum reflux ratio (R_min), ranging from 93 ∼ 142 and 99 ∼ 123, respectively. The O’Connell correlation was employed to estimate the overall column efficiency as 0.422, and based on the results of the step-by-step plate calculation method, the actual number of plates (N) was further calculated to be 234 ∼ 291. Through this research, the design and calculation of key parameters for the molten salt distillation separation of ZrCl₄ and HfCl₄ were successfully achieved.