Chlorine fluxing for removal of magnesium from molten aluminum: Part II. Mathematical model
- PDF / 324,134 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 12 Downloads / 231 Views
ON
Part I of this two-part article described laboratory experiments designed to examine the kinetics of removal of magnesium from molten aluminum by ‘‘chlorine fluxing.’’ Those experiments entailed the measurement of the size of gas bubbles in aluminum melts, as well as the residence times of the bubbles in the melt, by acoustic techniques. Semiquantitative information was obtained on the emissions of chlorine and aluminum chloride from the melt, and analysis of melt samples for magnesium yielded more precise data on the progress of reaction. The experimental data were consistent with the following explanation of the removal of magnesium from a batch of liquid Al-Mg. The chlorine content of a bubble of the chlorine-argon fluxing gas typically used in industry is rapidly converted to gaseous aluminum chloride as the bubble enters the melt from a submerged nozzle. Thereafter, the aluminum chloride reacts with magnesium in the melt to yield solid or liquid magnesium chloride. Below the melting point of magnesium chloride (985 K), there appears to be some impediment to the gas-liquid reaction by solid magnesium chloride, so that emissions of aluminum chloride and residual chlorine become significant. This was thought to be an impractical regime for industrial operations, and much of the experimental investigation was concentrated on temperatures above 985 K. In this higher-temperature range, reaction of the aluminum chloride proceeded efficiently (i.e., chloride/chlorine emissions were small), provided the magnesium content was above a critical value. Above this critical value, the rate of magnesium removal was constant and was determined simply by the rate of supply of chlorine QIAN FU, Graduate Student, and JAMES W. EVANS, Chancellor’s Professor, are with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted September 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
to the reactor. As the magnesium content fell below the critical value, chloride/chlorine emissions increased and the magnesium removal reaction became first-order in magnesium content. The insensitivity of the rate of reaction to temperature at these lower magnesium contents suggested that, under these conditions, magnesium transport to the bubble surface is rate determining (rather than chemical kinetics at that surface). The purpose of the present article is to present a mathematical model for the demagging of aluminum alloys that provides a quantitative interpretation of the results of Part I of this article. The predictions of the model for large-scale industrial melts are then described. II.
DEVELOPMENT OF THE MATHEMATICAL MODEL
Figure 1 is a schematic diagram of the system described by the model. Gas bubbles are rising through the aluminum-magnesium melt with a velocity of UB. The submergence of the bubble is h. The principal assumptions of the model are as follows. (1) Chlorine is converted to aluminum chloride at the gas injection point in a time that is short compare
Data Loading...
