Preventing wetting between liquid copper and solid steel: a simple extraction technique

Daehn, Kathrin E.; Gonzalez Cabrera Honorio Serrenho, Andre; Allwood, Julian

Copper contamination of end-of-life steel scrap is the main barrier to high-quality recycling. Preferential melting of copper from solid steel scrap is a potential extraction technique, which could be integrated into conventional scrap re-melting with little additional energy. However, previous investigations show removal of liquid copper is limited by its adherence to solid scrap. Preventing wetting between liquid copper and steel is essential to enable separation. The carbon content of steel, initial surface oxidation and applied coatings effect wetting behavior, but have not been systematically studied. In this study, the individual and combined effects of these parameters on wetting behavior in an inert gaseous environment are observed with a heating microscope. Carbon content appears to be the most significant factor: blistering of the oxide scale on medium carbon steels causes liquid copper to flow rapidly between the oxide and steel substrate. Liquid copper exhibited a stable droplet on low carbon steel, regardless of the initial level of oxidation. The tested coatings did not consistently improve non-wetting behavior, but impaired the connection between the scale and steel substrate. This study confirms the potential of the preferential melting technique, but further investigation is needed to determine the most robust process conditions to handle diverse, fragmented scrap at an industrial scale.