SEPARATION OF CHEMICALLY-BOUND COPPER IN STEEL SCRAP FOR RECYCLING

Jin, Hyunsoo

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SEPARATION OF CHEMICALLY-BOUND COPPER IN STEEL SCRAP FOR RECYCLING

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Demand for recycled steel keeps increasing due to greenhouse gas regulations and the advantage of lower energy consumption. However, recycled steel includes various impurities, and some elements compromise the steel grade, such as copper and tin. Copper is a toxic element in steel because that can induce hot shortness and poor mechanical properties. However, conventional sorting systems can not completely remove copper debris from scraps. Therefore, various methods have been conducted to overcome this problem, but the diluting process is the most viable. Dilution is dominant where low impurity iron-bearing products, such as pig iron, DRI, and HBI, are mixed with high copper post-consumer scrap to lower its negative impacts. In the future, however, the composition of steel scrap will show more copper and tin because more electric appliances, electric vehicles, and other WEEE need to be reprocessed. Therefore, the dilution method will face some processing as well as cost limitations. Since low-impurity iron sources are expensive, scrap will need to be reused. For this reason, we need to prepare alternative ways to solve this problem. Theoretically, various methods for copper separation have been proved, but the experimental results have some disagreement. There are several ways to group the copper removal methods, such as processing media and copper location. In this study, chemically bound interstitial copper is selected for the starting sample copper location, and the fate of copper has been evaluated in these samples. Based on the sample conditions and previous results, non-ferrous bath extraction, slag extraction, and brief hydrometallurgical removal were selected for this research. These methods are considered due to their favorable economic aspects, such as by-products generated, non-vacuum conditions, and lower processing temperatures (<1000°C) to explore the viable opportunities for replacement of the dilution method. The copper concentration of current field scarps is not enough to predict future compositions of steel scrap with high copper concentrations; therefore, synthetic samples (4wt.%) are used for the experiments in this work. Thermodynamically, copper and iron do not have strong interactions, and the synthesized sample followed the theoretical model. SEM-EDS, XRF, ICP-OES, Spark-OES, and XRD were used to check for copper concentrations and characterizations. The copper diffusion rate in the steel matrix is the most significant factor for extracting the interstitial free lattice copper. Therefore, temperature is one dominant factor because diffusion is temperature dependent. The temperature range is set from 750 °C to 1000 °C based on the Curie point for steel (770 °C). Based on the previous studies, 1 to 30 minutes has been used as the initial treatment time. The previous literature shows aluminum bath extraction is the most viable for surface copper. However, for interstitial copper, aluminum forms an intermetallic compound with steel and grows faster than copper diffusion rate and detaches from the steel. Consequently, this steel debris poisons the by-product (Al-Cu alloy precursor). Therefore, tin bath extraction is selected to replace the aluminum bath to solve the intermetallic formation problem. But this method can only be used for simple shaped samples because tin can also induce the hot shortness in steel. Although tin is very deleterious for steel, the tin bath allows a lower operating temperature (500 °C) and a sellable by-product (Cu-Sn alloy precursor). Furthermore, sulfur containing slag cleaning method and hydrometallurgy based acid and base treatments are investigated to check their feasibility. This study selected and conducted several promising methods to replace dilution, which is dominant in solving copper impurity problems. However, the overall results are not feasible for application at present. The previous studies are focused on the pretreatment methods that can remove surface attached copper. However, in the future homogenous high copper scrap needs better strategies to overcome the problems.

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