Kinetic Studies on Dehydrogenation of Molten Aluminum by Vacuum Suction Degassing Method

在本研究中，熔融铝在吹氩和机械搅拌条件下的脱氢动力学方面进行了研究。探讨了真空吸气法（VSD）在脱氢中应用的基本问题。结果总结如下。(1)从被氧化膜覆盖的熔融金属的自由表面的脱氢速率遵循一级速率方程，其中氢的金属侧传质和氢在氧化膜中的传质被认为是速率控制步骤。由速率数据估算了金属侧传质系数k_H和氧化膜中的传质系数k_<OH，H>。(2)k_H与搅拌功率密度ε之间的关系定性地与湍流传质理论所得到的关联式相一致，在该理论中，在熔融金属与固体氧化物膜之间的界面附近的液体侧考虑了阻尼湍流区。(3)吹到浴表面的Ar的流速不影响脱氢速率，因此气体侧传质不是速率控制步骤。(4)脱氢速率随温度的升高而增加，这一事实是由固体氧化物膜中氢传质阻力的降低来解释的。(5)在几个采用氧化镁浸渍管的VSD实验中，脱氢速率得到了提高。(6)据推测，由于Al与浸没管材料之间的反应而导致的氧化膜的特性变化影响了存在于熔融Al与浸没管之间的界面处的氧化膜中的H+的传质。

In the present study, the kinetic aspects of dehydrogenation of molten aluminum under the conditions of Ar blowing and mechanical stirring are investigated. The fundamental aspects of application of Vacuum Suction Degassing (VSD) method to the dehydrogenation are also investigated. The results are summarized as follows. (1) The rate of dehydrogenation from the free surface of the molten metal covered by oxide film follows a first order rate equation in which the metal side mass transfer of hydrogen and the mass transfer of hydrogen in the oxide film are considered as the rate controlling steps. The metal side mass transfer coefficient, k_H, and the mass transfer coefficient in the oxide film, k_<OH,H>, are evaluated from the rate data. (2) The relation between k_H and the stirring power density, epsilon, agrees qualitatively with a correlation obtained from the turbulent mass transfer theory in which a zone of damped turbulence is considered in the liquid side in the vicinity of the interface between the molten metal and the solid oxide film. (3) The flow rate of Ar blown to the bath surface does not affect the dehydrogenation rate, and hence the gas side mass transfer is not a rate controlling step. (4) The dehydrogenation rate increases with increasing temperature, which fact is explained by the decrease in terms of the decrease in the hydrogen mass transfer resistance in the solid oxide film. (5) The dehydrogenation rate is enhanced in several VSD experiments with magnesia immersion tube. (6) It is supposed that a characteristic change in the oxide film due to the reaction between Al and the material of the immersion tube affects the mass transfer of H+ in the oxide film which exists at the interface between the molten Al and the immersion tube.