Study of Aluminum Degasification with Impeller-Injector Assisted by Physical Modeling

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Study of Aluminum Degasification with Impeller-Injector Assisted by Physical Modeling M. Hernández-Hernández1, E. A. Ramos-Gómez1, M. A. Ramírez-Argáez1 1 Facultad de Química, UNAM, Departamento de Ingeniería Metalúrgica. Edificio “D” Circuito de los Institutos s/n, Col. Cd. Universitaria, C.P. 04510 México D.F., México. ABSTRACT A full-scale water physical model of a degassing unit is built and used to evaluate the performance of several impeller designs. Four impeller designs are tested: a) one smooth not commercial impeller for reference purposes, b) a commercial design by FOSECO®, called standard impeller in this work, c) a commercial design by FOSECO® with notches, and d) a new design proposed in this work. Since the physical model is easy and safe to operate, a full experimental design is performed to evaluate the effect of the most important process variables, such as impeller rotating speed, gas flow rate, impeller design and the point of gas injection (a conventional gas injection through the shaft and a novel method of injecting gas through the bottom of the ladle) on the kinetics of oxygen desorption of water which is similar to dehydrogenation of liquid aluminum. The new design of impeller proposed in this work shows the best performance in degassing of all impellers tested in this study. It is found that the rotor speed and its design are the most significant variables affecting degassing kinetics, and therefore the analysis of the existing commercial impeller designs may be useful to optimize the fluid dynamics of the process, which in turn would increase efficiency and productivity of the process. Finally, the novel gas injection method through the bottom, proposed by our own group, presents slightly faster degassing kinetics than the conventional injection of purge gas in the conventional way through the impeller. INTRODUCTION Aluminum alloys are used in several applications in automotive, aerospace and food technology industries, among others. The versatility of those materials is due to a combination of low density and high mechanical resistance. However, molten aluminum is susceptible to dissolve hydrogen from atmospheric humidity. Hydrogen solubility increases proportionally with an increase in temperature but decreases during aluminum solidification, resulting in parts with porosities, which are detrimental to the mechanical properties of the manufactured castings [1-3]. The actual industrial technology for liquid aluminum dehydrogenation consists in purging gases into the liquid aluminum alloy during a vigorous agitation by a graphite impeller rotating at highspeed velocities. The gas is conventionally injected through the rotor shaft. Due to the shear of the rotor, the gas is dispersed in the form of small bubbles in order to increase the degassing efficiency. Several variables can be identified in this process: gas flow rate, geometric impeller design, impeller rotating speed, and gas injection position, among others. Optimal degassing parameters can be obtained by performing experiments vary