Thermodynamic analysis of transition metal borides formation in aluminium melt

Smelter grade aluminium can be used as an electrical conductor application if the level of transition metal impurities is controlled precisely. Bauxite and petroleum coke are the main sources of such impurities in aluminium. Most of the impurities have significant effect on aluminium properties. Efforts have been done to supply relatively pure coke to aluminium smelters but still a list of impurities is found in the potline metal. The effect of transition metal impurities on electrical conductivity could be minimised if they are out of solution from aluminium and present in some solid phase. Industrially, boron treatment has been employed to combine transition metals into their borides. However, solution thermodynamic and reaction mechanism of borides formation are not well understood and further investigations are required. In this article a detailed thermodynamic analysis has been carried out to investigate the formation of borides. It is shown that diborides (MB2) are the most thermodynamically stable boride compounds of these impurities in the given working conditions. It is also shown that diborides of Zr, Ti, and V are the most thermodynamically stable phases at 650-900 degrees Celsius as compared with Cr and Al diborides. Thermodynamic stability of different phases is also affected by the presence of other diborides phases.