Alternative fractional crystallization-based methods to produce high-purity aluminum

Abstract

The increasing demand of ultra-high purity aluminum for technological applications has triggered the improvement of purification methods during the recent decades. Some of the most relevant applications for ultra-high purity aluminum include electrolytic condensers, transistors, integrated circuit conductors, magnetic disks substrates and low-temperature superconducting magnets. The most commonly used industrial technique for refining high purity aluminum is the three-layer electrolytic process, reaching purification levels of up to 4N8 (99,998 wt.%). An alternative and less capital intensive method to achieve such purification level is fractional crystallization. While the three-layer electrolytic process remains almost unchanged among its proprietaries, the fractional solidification processes vary considerably on their techniques and setups used to achieve an efficient segregation of impurities from the base metal. The purpose of this article is to compare the cooled finger and zone melting, two existing fractional crystallization methods, available at the Institut für Metallurgische Prozesstechnik und Metallrecycling (IME) of the RWTH Aachen University. The purification effect caused by different types of convection from these two fractional crystallization techniques was experimentally shown and the common characteristics among fractional crystallization equipment; i.e. growth rate, thermal gradient and purification rate, were investigated. The competitive advantage of the cooled finger over zone melting was demonstrated. Although both techniques are capable of refining aluminum, the design of the cooled finger favors a better impurity segregation.