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

本文研究了铝液在吹Ar和机械搅拌条件下脱氢的动力学特性。对真空吸气脱气(VSD)法在脱氢中的应用进行了初步探讨。研究结果总结如下。(1)被氧化膜覆盖的金属熔体从自由面脱氢的速率服从一级速率方程，其中氢的金属侧传质和氢在氧化膜中的传质是速率控制步骤。根据速率数据求出了金属侧传质系数k_H和氧化膜中的传质系数k_H，H&gt；(2)k_H与搅拌功率密度epsilon之间的关系与湍流传质理论所得到的关联式定性一致。(3)吹入熔池表面的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加，这是由于固体氧化膜中氢的传质阻力减小所致。(5)在多次采用氧化镁浸泡管的VSD实验中，脱氢速率得到了提高。(6)认为Al与浸没管材料之间的反应引起的氧化膜的特性变化影响了存在于Al熔体与浸没管界面处的氧化膜中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.