An Optimal Reverse Material Supply Chain for U.S. Aluminum Scrap

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Only around half of U.S. end of life (EOL) aluminum scrap is recycled. We investigate the extent to which the EOL recycling rate is constrained by the ability to collect EOL scrap versus the ability to recycle scrap into new alloys given the compositional mismatch between available scrap and the new alloys demanded by industry. The compositional mismatch means that scrap must be "sweetened" during recycling with virgin metals (primary unalloyed aluminum and alloying elements) to satisfy the chemistry constraints of the new alloys. If the mismatch is too great, then recycling becomes unviable.We first review the reasons for the compositional mismatch by examining its three causes: contamination with tramp elements; mixing of different aluminum scrap during recycling; and shifting demand for new alloys. A linear optimization model is then used to analyze the current and potential domestic U.S. recycling rate at different levels of collection from end use scrap categories such as buildings. The model determines the minimum quantity of virgin metal needed to satisfy new alloy demand if different metal sources (including scrap and virgin metal) can be combined to make the new alloys just like they are in the recycling industry. Optimization constraints include not exceeding the use of available scrap, satisfying demand for new alloys, meeting the chemical composition limits of the new alloys, and an economic furnace constraint that limits the weight fraction of virgin metals used in a recycling furnace. The results show that even if all EOL aluminum scrap could be collected in the U.S. then, because of the compositional mismatch between scrap and new alloys, only around 70% could be recycled domestically.