Review of Impurity Removal Methods in Steel Scrap Recycling

Impurities, such as copper and tin, have been identified as barriers limiting the use of recycled steel scrap for producing certain grades of steel product. Their accumulation during the whole recycling process could induce detrimental effects on the surface quality of downstream steel products, resulting from the formation and penetration of Cu-enriched liquid phase into grain boundaries, which would appear at 0.1wt% Cu content or above during hot working. In order to eliminate such detrimental effects, impurity removal methods have been researched considering the physical separation and chemical treatment at different stages of steel scrap recycling. For physical separation, apart from regular shredding and magnetic separation, common types of sensor-based sorting technology are discussed referring to the feasibility and efficiency. For chemical treatment, different technologies are introduced and classified according to the existent state of steel scrap during the removing process. As a result, in this review, possibilities and limitations of these methods have been presented and evaluated based on their removal mechanism to weigh their applicability. It becomes evident that sensor-based sortation would be feasible for impurity removal if specific physical or chemical characteristics can be identified and applied to distinguish impurities from Fe shreds. Considering the chemical treatment, most of the reviewed technologies were explored with laboratory scale, demonstrating limited industrial applicability due to the involving either high temperature or vacuum or complex process.