Preventing Wetting Between Liquid Copper and Solid Steel: A Simple Extraction Technique
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COPPER contamination of end-of-life steel scrap has been identified as the main barrier to high-quality steel recycling.[1] Copper and steel are combined for applications in vehicles, appliances, and equipment. Conventional scrap preparation uses hammer shredding to fragment and compact these end-of-life products. Subsequent magnetic separation is not completely effective: copper wiring entwined with steel pieces and steel-encased electric motors remain, so significant quantities of copper follow into the steel melt.[2] Todate, a commercial extraction technique has not been developed, yet residual copper causes metallurgical problems—most notably surface hot shortness.[3] High-quality flat steel products have strict copper tolerances, so end-of-life steel scraps are typically recycled to make tolerant, lower-quality long products.[4] In the coming decades, new measures for copper control will be necessary to ensure that all end-of-life scraps are recycled into demanded products.[5]
KATRIN E. DAEHN, ANDRE´ CABRERA SERRENHO, and JULIAN ALLWOOD are with the Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK. Contact e-mail: [email protected] Manuscript submitted December 4, 2018. Article published online May 20, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
An extraction process could be developed to separate copper from steel. The possible separation routes were evaluated by their potential to remove copper, and their specific energy and material input requirements by Daehn et al.[6] This analysis shows that techniques such as sulfide slagging[7] and vacuum distillation[8] have received considerable experimental investigation, but are difficult to implement in practice. However, premelt techniques, such as improved physical scrap preparation, appear to be particularly viable. Trommel[9] or ballistic sorting systems[10] separate steel pieces by size or density to yield scrap with as low as 0.15 wt pct Cu. However, 20 to 30 pct of the steel scrap stream is diverted as copper-rich. To treat heavily contaminated steel scrap and recover pure copper, preferential melting could be relatively simple. Removing external copper before it is dilute within the steel melt requires less energy. The difference in melting points between copper (1083 C) and steel (1370 C to 1536 C)[11] is significant, and could be exploited to collect pure, liquid copper from solid steel. Melting is kinetically fast, and assuming the process is integrated into commercially available scrap heating systems (described by Manning and Fruehan[12]), this step would require little additional energy. Such a process would prevent additional copper from entering the steel cycle. Preferential melting to remove copper has not been developed because there are two main practical limitations: the irregular shape of end-of-life scrap, and the adherence of liquid copper to the surface of solid steel.
VOLUME 50B, AUGUST 2019—1637
Liquid copper could flow into the crevices of fragmented pieces, undermining separation, as d
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