Impurity removal kinetics of hydrogen plasma arc melting

研究发现，即使在常压下，氢等离子体电弧熔炼（HPAM）对杂质的去除也有很强的效果。本研究旨在阐明HPAM过程中杂质消除的机理。主要研究结果如下：热力学计算结果与实验结果吻合。这表明，在氩等离子体气体中加入氢可以使系统比氩等离子体熔化更快地达到类平衡状态。发现杂质的去除主要取决于杂质与母材之间的蒸汽压差，尽管Sb、P、O等几种杂质以氢化物的形式被去除。杂质去除遵循第一定律反应，分为两步，不同的去除速率。在较高浓度范围内（步骤1），随着等离子体气体中氢气浓度的增加，去除率显著增加。加氢也降低了从步骤1过渡到步骤2.4的杂质浓度。步骤1和步骤2的杂质消除分别受到气相和金属相边界层扩散的限制。热力学和数学模拟表明，氢的加入使等离子体电弧的性质和性能发生了剧烈的变化，并使熔融金属表面附近的温度升高。最后，发现HPAM作为一种新的精炼方法具有显著的性能，并将其应用于几种金属的提纯。

It has been found that hydrogen plasma arc melting (HPAM) has a strong effect on impurity removal even under an atmospheric pressure. The aim of this study is to clarify the mechanism of impurity elimination during HPAM.The main results are follows.1. The thermodynamic calculations agree with the experimental data on the impurity elimination. This indicates that hydrogen addition to the argon plasma gas allows the system to achieve the equilibrium like state much faster than argon plasma melting.2. It is found that the impurity removal mainly depends on the vapor pressure difference between the impurity and the base metal, although several impurities such as Sb, P and O are eliminated in the form of hydrides.3. Impurity removal obeys the first law reaction and consists of 2 steps with different elimination rates. In the higher concentration range (step 1), elimination rate remarkably increase with the hydrogen concentration of the plasma gas. Hydrogen addition also lowers the impurity concentration at the transition from step 1 to step 2.4. Impurity elimination of step 1 and 2 are found limited by diffusion through boundary layers in gas phase and metal phase, respectively.5. Thermodynamic and mathematical simulations clarify that hydrogen addition gives drastic change of the properties and performance of the plasma arc and increases the temperature in the vicinity of the molten metal surface.6.Finally, HPAM is found to possess an remarkable capability as a new refining method, which is confirmed by applying it to the purification of several metals.