Abstract
High purity metals are nowadays increasingly in demand to serve in electronic, photovoltaic, and target materials industries. The zone refining process is the most common way to achieve high purity in the final step of metal purification. Zone length and crystal growth rate are the main parameters that control the zone refining process. To determine these values, information about temperature profiles in the molten zone is necessary due to its direct correlation with these values. As the determination of this profile is not practically achievable in the present, the novel approach of applying an infrared (IR) camera during the zone refining of 2N8 aluminum is the focus of the investigation in this work. The whole temperature profile of the region near the molten zone was recorded by IR camera during the entire running process. The zone length and the crystal growth rate at each thermographic image shooting moment were successfully extracted by thermographic analysis. Results showed that both factors varied significantly, which is in contrast to the assumption in literature about their stability while running under constant input power and heater movement velocity, though noticeable purification took place in all of these experiments. However, the impurity concentration during refinement fluctuated remarkably. This was well-demonstrated by the tendency of variation in crystal growth rate attained in this work. These results provide a better understanding of the mechanisms of zone refining with an inductive heater and contributes to the optimization of the process.
Highlights
Zone refining is the most common method used in the production of most high purity metals
In order to investigate the variation of zone length and crystal growth rate in the end section, experiments using a velocity of 1.2 mm/min were run over that moment and finished until the heater was located at the end of the crucible
An IR camera has been applied for the first time in zone refining of 2N8-pure aluminum with dopants Fe and Si in order to characterize and interpret the zone refining process
Summary
Zone refining is the most common method used in the production of most high purity metals (purity greater than 5 N). The purification takes place by redistributing the impurities at the freezing interface when one or a series of molten zone(s) move through a solid bar. Many experimental and theoretical investigations have been conducted in recent decades with focus on improving refining and/or production efficiencies [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The second is a focus on knowledge of the molten zone, such as zone shape, zone length variation, and the position of the molten zone, by taking heat transfer and heat balance into consideration [11,12,13,14,15]. The zone position and zone length variation are Materials 2018, 11, 2039; doi:10.3390/ma11102039 www.mdpi.com/journal/materials
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