Consequential vs. attributional approach to the LCA of zinc recovery from spent pickling acids

Session
WE.3.C || Benefits of Retaining Materials and Their Quality in a Circular Economy – Case studies

Authors
Arguillarena, Andrea; Margallo, María; Urtiaga, Ane

Abstract
The LIFE2ACID project is demonstrating a sustainable technology for zinc recovery from spent pickling acids (SPAs). The innovative solution integrates membrane-based solvent extraction (MBSX) and electrowinning (EW) to achieve zinc recovery with the adequate quality to be used as secondary raw material or transformed into other products such as zinc oxide. After zinc extraction, SPAs retain iron chloride, a valuable compound for wastewater treatment. Despite the benefits of the valorization according to the circular economy concept, the MBSX and EW processes add an environmental impact that should be evaluated. In this work, we perform an life cycle assessment (LCA) of zinc recovery following the requirements of ISO 14040. The study has a cradle to grave approach, including the extraction of raw materials, waste treatment, and management. The life cycle inventory was collected from the pilot plant results, mass balances, and Sphera professional database for secondary data. CML 2001 was selected as impact assessment method including categories such as Abiotic Resources Depletion (elements and fossil) and Global Warming Potential. This LCA is accompanied by the analysis of the substitution of primary materials by secondary sources using consequential and attributional approaches. The results show that EW has a greater environmental impact than MBSX due to the energy demand of zinc electrodeposition. About the environmental credits, using the secondary zinc of the LIFE2ACID alternative under a consequential approach supposes a net gain of 33 and 21 MJ/kg by the electrometallurgical and pyrometallurgical routes. However, applying an attributional approach based on market mixes, the energy credits are reduced by 29-32%. This work highlights the influence of using a consequential or attributional approach on recycling credits. So, circular economy studies require a sensitivity analysis to have a complete overview of the environmental benefits of material recycling.