Time dependent chlorination of CeO₂, La₂O₃ and Nd₂O₃ by ZrCl₄ dissolved in eutectic LiCl-KCl

Partial chlorination of CeO₂, La₂O₃, and Nd₂O₃ was achieved using ZrCl₄ dissolved in molten LiCl-KCl at 450 °C over a period of 4-5 h. ZrCl₄ was pre-volatilized into an argon stream to maintain a constant vapor pressure for equilibration with the salt. This approach was effective at maintaining the ZrCl₄ concentration at the solubility limit of 1.3 mol% in the molten salt throughout the duration of two of the chlorination experiments and within about 60% of the solubility limit in the third experiment. Based on the analysis of salt samples, fractional conversion after four hours of reaction for CeO₂, and five hours for La₂O₃, and Nd₂O₃ were calculated to be 0.97, 0.28, and 0.65 respectively. Assuming a shrinking core diffusion-limited model, the diffusivity of ZrCl₄ through the hypothesized ZrO₂ layer was calculated for the chlorination of CeO₂, La₂O₃, and Nd₂O₃ were 1.76 × 10^-10 cm²/sec, 7.85 × 10^-12 cm²/sec, and 7.42 × 10^-12 cm²/sec, respectively. Given the extremely low values calculated for diffusivity, it was concluded that the process is chemical reaction rate limited and that a complete reaction of each rare earth oxide could be achieved. A chemically controlled shrinking core model was shown to also fit the experimental data closely. (author)