Thermodynamics and kinetics analyses of ZrB2 formation in molten aluminium alloys

Smelter grade aluminium can be used as a source for electrical conductor grade aluminium after the transition metal impurities such as zirconium (Zr), vanadium (V), titanium (Ti) and chromium (Cr) have been removed. Zirconium (Zr), in particular, has a significant effect on the electrical conductivity of aluminium. In practice, the transition metal impurities are removed by adding boron-containing substances into the melt in the casthouse. This step is called boron treatment. The work presented in this paper, which focuses on the thermodynamics and kinetics of Zr removal from molten Al-1 wt-% Zr-0.23 wt-%B alloy, is part of a broader systematic study on the removal of V, Ti, Cr and Zr from Al melt through boron treatment carried out by the authors. The thermodynamic analyses of Zr removal through the formation of ZrB2 were carried out in the temperature range of 675-900 degrees C using the thermochemical package FactSage. It was predicted that ZrB2 is stable compared to Al-borides (AlB12, AlB2) hence would form during boron treatment of molten Al-Zr-B alloys. Al-Zr-B alloys were reacted at 750 +/- 10 degrees C for 60 minutes, and the change in the chemistry and microstructure were tracked and analysed at particular reaction times. The results showed that the reaction between Zr and AlB12/B was fast as revealed by the formation of boride ring at the early minutes of reaction. The presence of black phase (AlB12), i.e. the original source of B, after holding the melt for 60 minutes advocated that the reaction between Zr and AlB12/B was incomplete, hence still not reached the equilibrium state. The kinetics data suggested a higher reaction rate at the early minutes (2 minutes) of reaction compared to at a later stage (2-60 minutes). Nevertheless, a simple single-stage liquid mass transfer controlled kinetic model can be used to describe the overall process kinetic. The analysis of integrated rate law versus reaction time revealed that the mass transfer coefficient (km) of Zr in molten alloy is 9.5 x 10(-4) m s(-1), which is within a typical range (10(-3) to 10(-4) m s(-1)) observed in other metallurgical solid-liquid reactions. This study suggests that the overall kinetics of reaction was predominantly controlled by the mass transfer of Zr through the liquid aluminium phase.