Removal of Alkali Metals from Aluminum

Abstract

The alkali metals Na, and to a lesser extent Li and Ca, are found as undesirable impurities in aluminum. These must be removed to extremely low levels for acceptable product quality. The traditional way to refine the metal is to flux with chlorine and an inert gas. This process is poorly understood, however, and excessive chlorine consumption and chloride emissions to the atmosphere may occur. This paper presents the results of an extensive experimental program conducted to characterize and optimize the process of fluxing with chlorine. One of the unique features of the program is the utilization of an on-line emissions monitor to control the fluxing process. A 25,000 pound melting furnace in the casting complex at the Reynolds Metals Company Corporate Research Center was used in these trials. The furnace is tilting with porous plugs in the bottom for fluxing. In-line gas fluxing was also done outside the furnace with an efficient spinning degasser. The experimental results obtained in this study, together with a theoretical analysis, have allowed Reynolds to construct a detailed model of the alkali metal removal process. This understanding allows one to significantly reduce chlorine consumption and chloride emissions during the metal refining process.