Effective distribution coefficient (ke) plays an important role in zone melting purification but has never been systematically calculated as an important parameter. A novel analysis method for impurity separation separating from Al using determination of impurities (Si, Fe, Cu, Zn) content and calculation of ke has been investigated. Experimental results show that the impurity separation effect was significantly improved due to the ke closely approximating the equilibrium distribution coefficient (k0) via optimization of purification times and speed of the melting zone. When the times of purification were increased from 15 to 20 times, the ke values of Si, Fe, Cu, and Zn at the sample (No. 3) were 0.603, 0.702, 0.364, and 0.502, respectively. Correspondingly, the removal rate of impurities of Si, Fe, Cu, and Zn increased by 6.25%, 1%, 10.49%, and 27.58%, respectively. The effect on the ke of Cu and Fe is more significant at the sample (No. 3) when the melting zone speed is changed. The removal rates of Fe and Cu in the sample are 99.0% and 94.75% when the melting zone speed was 1/3 mm/min, and, correspondingly, the removal rates of Si and Zn were 81.7% and 88.51%, respectively. The Al content at sample position No. 3 is more than 999994.59 ppm when the sample was purified 15 times at a speed of 0.5 mm/min, which meets the standard of 5N high purity aluminum.

When classifying impurities in commercial pure antimony (Sb, 99.8%), arsenic (As) and lead (Pb) should be brought to the forefront consideration. Due to the known difficulty of As removal through zone refining, it is meaningful to investigate its separation tendency through alternative methodologies such as vacuum distillation, promoted by the large difference of their vapor pressures. Here, a series of vacuum distillation trials with different process parameters were at first conducted with the aim of As removal. Pb, as an always accompanying impurity, seemed to be able to be significantly separated from Sb, so that its content in the refined phase could be reduced too, e.g. from 1200 ppm to less than 30 ppm. The reduction of As, however, is highly dependent on the distillation ratio of Sb and hence limited just to 450 ppm. The biggest obstacle here was the simultaneous evaporation of Sb and As when using high temperature and low pressure. In order to suppress the evaporation of As more intensely in vacuum distillation or selectively capture As in zone refining process, the additives – aluminum (Al) and zinc (Zn) – were studied and selected by using the respective phase diagrams as well as thermochemical Software FactSage and then individually added to Sb as alloying elements with the aim of intermetallic formation with As. The addition of Al led to a considerable reduction of As in vacuum distillation as well as while zone refining process. During vacuum distillation, 67% less As was obtained in the condensate in comparison to the trial without additive. Meanwhile, a huge As concentration gradient appeared in the residual Sb. During zone refining process, As concentration in the whole bar was considerably reduced from 456 ppm to below 150 ppm after only one zone pass, due to the enrichment of Al at the end of the bar in accompanying with As in form of an intermetallic compound. The addition of Zn, on the contrary, did not convince as an effective improvement in purification of Sb. In order to achieve a higher efficiency of As removal from Sb, the authors at the first priority suggest an addition of Al directly into the zone refining process. If specifically a vacuum distillation process is preferred, a multi-stage condenser, equipped with controlled temperatures, attendant with the addition of Al in the charge material, can deliver effective results as well.

In this paper, we proposed a modified zone vacuum melting process for producing high-purity aluminum. By modifying the effective redistribution coefficients, axial segregation of impurities are investigated. The impurities distribution along the ingots are obtained with different conditions, such as pass number, zone travel rate, initial impurity concentration. Based on the analysis, optimization of the vacuum purification process design is proposed. The samples with aluminum content of more than 5 N (99999.2 ppm) were obtained, and the optimum purification efficiency is significant when the zone melting rate is 1 mm/min, especially in the middle of the sample (No.3, No.4), which the removal rate of Fe, Zn and Cu is more than 70%, and the removal rates are 73.63%, 82.13% and 75.75%, respectively. The samples meet the requirements of commercial high-purity aluminum standard.

Zone refining, as the currently most common industrial process to attain ultrapure metals, is influenced by a variety of factors. One of these parameters, the so-called “zone length”, affects not only the ultimate concentration distribution of impurities, but also the rate at which this distribution is approached. This important parameter has however neither been investigated experimentally, nor ever varied for the purpose of optimization. This lack of detections may be due to the difficult temperature measurement of a moving molten area in a vacuum system, of which the zone refining methodology is comprised. Up to now, numerical simulation as a combination of complex mathematical calculations, as well as many assumptions has been the only way to reveal it. This paper aims to propose an experimental method to accurately measure the molten zone length and to extract helpful information on the thermal gradient, temperature profile and real growth rate in the zone refining of an exemplary metal, in this case aluminum. This thermographic method is based on the measurement of the molten surface temperature via an infrared camera, as well as further data analysis through the mathematical software MATLAB. The obtained results show great correlation with the visual observations of zone length and provide helpful information to determine the thermal gradient and real growth rate during the whole process. The investigations in this paper approved the application of an infrared camera for this purpose as a promising technique to automatically control the zone length during a zone refining process.