Management of Impurities in Cast House with Particular Reference to Ni and V

It is apparent that the concentrations of impurities in raw materials (particularly petroleum coke) are increasing with significant implications for the ability of cast houses to meet customer chemical specifications. A large fraction of the metal content (Ni, V) of the coke and alumina (Fe, Si, Ga, Zn) reports to the Al in the reduction cells. In some smelters the concentration of these impurities is beginning to exceed customer cast product specifications. These impurities can have detrimental effects in certain alloys. In order to formulate control strategies, this paper reviews process options including the classical melt treatment processes of salt fluxing, degassing and filtration for removing impurities in the cast shop. The mechanism of Al boride treatment to remove titanium, chromium, vanadium is examined to get a picture of the possibility of speeding up the process. Those processes used for producing super purity Al are also examined along with refining methods in other non-ferrous metals for potential application to Ni and V control. An investigation into potential for a melt nickel removal process is recapped. We conclude that currently cast house cannot provide a complete solution to the issue of Ni and V control.