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Currently, basic fluxing techniques remove impurities (Na, Ca, Mg, Li etc.) with poorly controlled injection of gases containing chlorine into the molten metal. Only a fraction of the injected gas reacts and the excess is reported as emissions of chlorides such as toxic HCl. The intention is to improve the technology to eliminate this waste (saving on the energy entailed in the chlorine production and aluminum loss as well as reducing pollution) by better dispersion of the injected gas throughout the metal. A combination of mathematical modeling and experimental measurements are being employed to determine ways to optimize fluxing operations. This work will be carried out in collaboration with Alcoa at their Technical Center near Pittsburgh, PA. Work at Berkeley will also determine how best to avoid splashing and spraying resulting from gas injection at high rates. In this way it is expected that the throughput of aluminum can be increased with resultant increase in productivity and reduction in heat losses per lb. of aluminum, saving additional energy. Reduced spraying is also expected to reduce particulate emissions from fluxing operations.
Previous studies of chlorine fluxing have clearly shown that the rate of removal of impurities during fluxing is dependent on the size, dispersion throughout the melt and residence time of the gas bubbles injected into molten aluminum. Consequently any fundamental study of fluxing, at either the laboratory or commercial scale, should entail measurement of these bubble parameters. No measurements have been reported on aluminum at the commercial scale and only a few laboratory investigations have been done. From 1995 to 1998 an investigation on chlorine fluxing of aluminum was carried out at Berkeley. Experimental data were collected and compared to a mathematical model that was concurrently developed. The model was based on an ideal bubble size distribution that is unlikely to represent the bubble distribution in an industrial fluxing unit. With this discrepancy in mind a capacitance probe was developed to detect bubbles and measure their size and velocity. The current project has refined the capacitance probe system for use in an industrial setting. Data gathered from industrial fluxing units can then be used as an aid in developing a mathematical model of chlorine fluxing in the aluminum industry.
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In the United States aluminum accounts for 99% of the beverage can market where over 65% of aluminum cans are recycled nationwide. An important step in recycling is adjustment of alloy composition (e.g., magnesium removal) by "chlorine fluxing".