Removal of Vanadium from Molten Aluminum-Part I. Analysis of VB₂ Formation

Aluminum has been used as an alternative to copper for electrical conductor applications. However, the presence of certain impurities in aluminum, such as V, Ti, Zr, Cr, and Fe, has detrimental effect on its electrical conductivity. These impurities can be removed by the addition of an Al-B master alloy containing AlB₂ or AlB₁₂ phase, known as boron treatment. The detailed mechanism of borides formation, however, is not well understood. In the current study, a systematic investigation of vanadium diborides (VB₂) formation in the Al-V-B alloys was carried out. The study comprised thermodynamic assessment and experimental investigation on the Al-V-B system under typical industrial processing conditions. It was predicted from thermodynamic analysis that VB₂ are stable borides of V and do not dissolve readily in the temperatures ranging from 948 K to 1173 K (675 °C to 900 °C). The experimental investigation showed that the mechanism of VB₂ formation and V removal in an Al-V-B system is complex as it involves a number of steps such as chemical reaction, mass transfers in bulk liquid and inside the VB₂ ring, and diffusions of B and V through the VB₂ layer. Comparative analysis of thermodynamic and experimental results showed that the aluminum alloys were far from equilibrium and that the overall process was limited by the kinetics of B diffusion through the VB₂ ring.