Degassing of static melts by insoluble purge gases
- PDF / 510,533 Bytes
- 6 Pages / 583.28 x 777.28 pts Page_size
- 103 Downloads / 144 Views
THE demand for high quality semifinished alloy products throughout the metallurgical industry often requires the removal of dissolved gaseous impurities from the melt prior to casting. The most notable of these gases are hydrogen and oxygen, which are common impurities in aluminum, copper, and steel processing. Although vacuum degassing techniques do find limited application, the most widely used method for degassing consists of flushing the melt with insoluble purge gases to scavenge the unwanted dissolved gases. The kinetics of this type of process has been considered by Pehlke and Bement] who studied the removal of dissolved hydrogen from molten aluminum using argon as a purge gas. In their analysis of the experiments, an overall mass transfer coefficient which depended on the purge gas bubble size was used to correlate the observed degassing rates. The geometrical conditions of the introduction of the purge gas were not considered, and as a consequence the mass transfer coefficient must be adjusted when Pehlke and Bement's model is applied to experimental conditions which are different from those used in their initial experiments. In a later study of the deoxidation of liquid copper by carbon monoxide, Andreini, et al z used a more refined experimental technique for determining the mass transfer coefficient, but they still did not account for the melt geometry in a systematic way. A model which accounts for the effects of both reaction kinetics and diffusion in the melt has been developed. It is presented here as it applies in particular to the degassing of aluminum melts. The model may be applied to other processes by suitable alteration of the kinetic expressions.
J. A. DANTZIG and D. E. TYLER are Senior Research Scientist and Chief, respectively, Process Metallurgy Section, Olin Metals Research Laboratories, New Haven, CT 06511. J. A. CLUMPNER is Manager, Metal Services, Consolidated Aluminum Corporation, St. Louis, MO 63178. Manuscript submitted September 20, 1978.
FORMULATION OF THE MODEL In aluminum batch fluxing operations, purge gases are introduced at or near the bottom of the melt via fluxing tubes or porous plugs. A column of bubbles rises from the point of introduction and hydrogen is removed from the melt by the bubble column in a three-step process. First, dissolved hydrogen which is present in the melt in atomic form diffuses through the melt to the purge gas bubble column. Once they reach the surface of a purge gas bubble, hydrogen atoms react to form the molecular species, which is then adsorbed onto the bubble surface, and desorbed into the interior of the bubble. Finally, the bubble leaves the melt through the upper surface, removing hydrogen with it. In common practices, gas is introduced into the melt in only a few locations, typically several meters apart. The bulk of the melt is undisturbed by the rising bubbles. Within the column of bubbles, however, there is strong agitation of the melt caused by the rising gas bubbles. Circulations in the melt caused by the bubble columns are small i
Data Loading...
