Chlorine fluxing for removal of magnesium from molten aluminum: Part II. Mathematical model

Abstract

This second part of a two-part article presents a mathematical model for the “chlorine fluxing” of aluminum alloys, in particular, for the “demagging” of Al-Mg alloys such as those resulting from the recycling of used beverage cans. The model is based on the experimental results described in Part I and, in conformity with those results, assumes that neither the reaction kinetics at the melt-bubble interface, nor mass transfer on the gas side of that interface, are rate determining. With the introduction of one correction factor (applied to the surface renewal model for mass transfer on the melt side of the melt-gas interface), the model fitted the experimental data well, once measured values for the bubble size and rise velocity were introduced. The model was then used to predict the progress of demagging operations on an industrial scale. Computed results for these larger melts suggest that gross emissions of chlorine/chlorides are avoidable in bringing the magnesium content down to a critical value (which depends on operating characteristics such as bubble size). A multiple-step strategy is suggested when a batch of alloy is to be brought to yet-lower magnesium levels. In that strtegy, the chlorine content of the injected gas is reduced as the processing of the batch proceeds. The predicted effects of other operating changes (deeper nozzle submergence, broad bubble size distribution, etc.) are reported.