| 초록 |
Transition elements solute impurity such as V, Ti, Zr, Cr significantly affects aluminum (Al) electrical conductivity. Therefore, these impurities need to be removed prior to the manufacturing of electronic (EC) grade Al. In industry, boride treatment has been carried out to remove these elements. This involves the addition of boron (B) containing alloys (usually Al-B alloys) into Al melt in the casthouse to form transition metal borides which then let to settle at the bottom. This paper will describe a systematic study on the mechanism of formation of transition metal borides formation in Al melt during boride treatment. The study consisted of systematic thermodynamics and kinetics studies, combined with industrial trial. The thermodynamics prediction showed that metal diborides (MB2) are the most stable boride compounds of these impurities in the given working conditions. The ZrB2, TiB2 and VB2 phases are more stable compared to AlB2 and CrB2 hence do not dissolve readily. It is also shown that the relative stability of the boride phases is affected by the presence of other metal diborides. The laboratory study 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. It was also observed that the reaction was of the first order with respect to V in molten aluminum. At the early stage of the reaction, a VB2 ring was formed in which the thickness increased with time, depleting AlB12 and B from the melt. The process of V removal and the formation of VB2 were divided into two stages. Stage 1, where the kinetic of the reaction was controlled by the mass transfer of V in liquid phase; and Stage 2 where the VB2 layer provide barrier and that the reaction was controlled by the diffusion of B through the layer. The effect of stirring on the removal of V suggested that gas stirring was most effective due to the generation of uneven flow patterns in the melt. It also facilitates the breaking/disintegrating of the VB2 ring formed in the early stage of reaction, thus providing fresh effective interfacial area for chemical reaction. During the industrial trials, it was observed that the rate of V and Ti removal was faster compared to Zr and Cr during the boride treatment of smelter grade aluminum. Mass balance analysis revealed that 70 wt pct of V and Ti combined as borides in the first hour of the total 12 hours of boron treatment process. It was also observed that the source of boron (the type of borides AlB2 or AlB12) had an effect on the kinetics. An improved practice can be developed industry to get an optimum process by taking into account the type of boride sources, multiple stage boride addition and the was the stirring is applied. |
