Impurity removal kinetics of hydrogen plasma arc melting

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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.