Reduction Kinetics and Mechanism of Rare Earth Oxide (Tm_2O_3 and Yb_2O_3) with Metallic Lanthanum

在高温真空条件下，研究了钐、铕、铥、镱氧化物与金属镧的还原反应及机理。各稀土氧化物与金属镧的还原反应随时间延长呈抛物线状进行。用反应活性度和时间两个参数可整理出各氧化物的还原曲线。对于每种稀土氧化物的还原，所获得的直线的斜率约为0.5。由Arrhenius图计算出氧化钐、氧化铕、氧化铥和氧化镱的还原反应表观活化能分别为96 kJ/mol、236 kJ/mol、154 kJ/mol和243 kJ/mol。采用基于瓦格纳理论的金属高温氧化电化学还原模型，研究了稀土氧化物与金属镧的还原机理。根据该模型，每种氧化物还原反应的速率控制步骤可以认为是镧离子或氧离子在还原产物氧化镧层中的转移过程，并研究了1193 ~ 1323 K温度范围内金属镧和钕在含LaF_3和NdF_3的LiF熔体中的溶解行为。在LiF-LaF_3二元熔体中，La的饱和溶解度随LaF_3浓度的增加而减小，Nd的饱和溶解度与NdF_3浓度无关，La在LiF-20mol%LaF_3二元熔体中的饱和溶解度远小于Nd在LiF-20mol%NdF_3二元熔体中的饱和溶解度。金属镧在熔体中主要与LiF发生置换反应生成LaF_2和LaF_3。另一方面，金属钕在溶解初期与熔体中的LiF发生置换反应生成NdF_3，并与熔体中的NdF_3发生歧化反应生成NdF_2。

Reduction reaction and mechanism of samarium, europium, thulium and ytterbium oxides with metallic lanthanum have been investigated under the conditions of high temperature vacuum atmosphere. Reduction reaction of each rare earth oxide with metallic lanthanum parabolically proceeded with prolonged time. Reduction curve for each oxide could be arranged by using both logarithms of reactivity degree and time. The slope of straight line obtained was about 0.5 for the reduction of each rare earth oxide. Apparant activation energies of the reduction reaction calculated from Arrhenius plots were 96 kJ/mol for samarium oxide, 236 kJ/mol for europium oxide, 154 kJ/mol for thulium oxide and 243 kJ/mol for ytterbium oxide, respectively. An electrochemical reduction model based on Wagner's theory being used for high temperature oxidation of metal was applied for the reduction mechanism of each rare earth oxide with metallic lanthanum. From this model, a rate-determing step for the reduction reaction of each oxide could be considered to be the transfer process of lanthanum ion or oxygen ion in lanthanum oxide layer of reduction product.Dissolution behaviors of metallic lanthanum and neodymium have also been examined in LiF melts containing LaF_3 and NdF_3 at the temperature range of 1193 K to 1323 K. Saturated solubility of lanthanum decreased with an increase in the LaF_3 concentration in LiF-LaF_3 binary melt, and saturated solubility of neodymium was not dependent on the NdF_3 concentration in LiF-NdF_3 binary melts.Saturated solubility of lanthanum in LiF-20mol%LaF_3 binary melt was extremely smaller than that of neodymium in LiF-20mol%NdF_3 binary melts. Metallic lanthanum predominantly reacts with LiF in melt by the displacement reaction to form LaF_2 and LaF_3. On the other hand, metallic neodymium forms NdF_3 by the displacement reaction with LiF in melt at the initial stage of dissolution, and also reacts with NdF_3 in melt by the disproportional reaction to form NdF_2.