Zone-melting for efficient impurities removal and high-purity aluminum recovery from 7000 series computer numerical control machining scraps

This study investigates zone melting (ZM) as an innovative method for recycling 7000 series aluminum alloy scraps, a byproduct of computer numerical control (CNC) machining in smartphone production. Traditional fluxing methods are ineffective at removing Zn, a key alloying element. Vacuum atmospheric ZM utilizes the evaporation of Zn and Mg impurities and solidification segregation to concentrate elemental impurities within the melt, facilitating their efficient removal. Pre-treated aluminum scrap with high impurity content (>2.27 wt.%) was melted to form an ingot. Subsequent ZM under vacuum conditions significantly increased the aluminum content to high-purity levels (99.5 wt.%), equivalent to 1000 series aluminum alloys. Optimization of ZM parameters, such as atmosphere, zone travel speed, and the number of passes, further enhanced the homogeneity of aluminum distribution and overall purity. Thermal conductivity measurements on zone-melted samples confirmed the correlation between increased Al concentration and higher thermal conductivity, an indirect indicator of impurity levels. A sample with 99.5 wt.% Al exhibited a thermal conductivity of 155 W/m-K, confirming its high purity and the efficiency of ZM for refining 7000 series alloys. These findings demonstrate the potential of ZM for efficient aluminum scrap recycling, promoting sustainable resource management and contributing to the development of a circular economy in the electronics industry.