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

本文研究了铝液在吹氩和机械搅拌条件下的脱氢动力学。探讨了真空吸脱气（VSD）技术在脱氢过程中应用的基本问题。结果总结如下：(1)被氧化膜覆盖的熔融金属自由表面的脱氢速率遵循以氢在金属侧的传质和氢在氧化膜中的传质为速率控制步骤的一阶速率方程。根据速率数据计算了金属侧传质系数k_H和氧化膜侧传质系数k_<OH，H>。(2) k_H与搅拌功率密度epsilon之间的关系与湍流传质理论的相关关系在定性上一致，该理论认为在熔融金属与固体氧化膜界面附近的液体侧存在阻尼湍流区。(3)吹至浴槽表面的Ar流量不影响脱氢速率，因此气侧传质不是速率控制步骤。(4)脱氢速率随温度的升高而增大，这可以用固体氧化膜中氢传质阻力的减小来解释。(5)多次用镁砂浸泡管进行的VSD实验均提高了脱氢速率。(6)假定由于Al与浸入管材料的反应导致氧化膜的特性变化，影响了存在于Al与浸入管界面的氧化膜中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.