Solving multifunctionality in the carbon footprint assessment of primary metals production: Comparison of different approaches

Multifunctionality is a recurring issue in the life cycle assessment (LCA) of primary metals production, as one metal is often co-produced with other metals. In this context, this study aims at i) reviewing the approaches actually implemented in the literature for solving multifunctionality in the context of the LCA of primary metals co-production; ii) comparing the currently existing multifunctionality-solving approaches as defined by the ISO hierarchy, considering the carbon footprint assessment (in a life-cycle perspective) of copper (Cu), zinc (Zn), lead (Pb) and silver (Ag) co-production through the implementation of on-site "mine-to-metal" technologies at pilot scale in a Spanish mine; iii) discussing the implications associated with the use of these different approaches in the context of primary metals co-production and the extent to which they may affect the assessment of the environmental performances of metals. Overall, the allocation approach stands for the first option for solving multifunctionality in the LCA of metals co-production based on the review of 27 studies; despite the ISO hierarchy recommends the use of allocation as last resort. Based on a representative set of primary data drawn from on-site trials at a pilot scale, the potential climate change impacts induced by the co-production of 1 ton Cu and 6 tons Zn cathodes along with 2.65 tons Pb and 0.045 ton Ag precipitates are calculated to amount to 90,324 kg CO2-eq in a life cycle perspective. Depending on the multifunctionality-solving approach, the apportionment of these impacts to each co-product shows significant discrepancies. The implementation of partial subdivision combined with market prices allocation attributes most (80%) of the impacts to Zn and Ag. The use of allocation approaches attribute the largest share of impacts to Zn (62%) and Pb (27%) when considering a mass criterion; while the impacts are primarily attributed to Ag (respectively 48% and 45%) and Zn (respectively 30% and 33%) when considering economic criteria (respectively market prices and production costs). In terms of representativeness, the use of partial subdivision shall be considered as a more representative approach for solving multifunctionality than the mere application of allocation. Moreover, the results obtained through the application of economic allocation are considered more consistent than those obtained by mass allocation in this case study; while the use of production costs may be seen as an alternative for overcoming the limits associated with market prices. Finally, the use of system expansion shows that the recovery of Zn, Pb and Ag in addition to Cu potentially avoids 25% of the total climate change impacts. The approach implemented to solve multifunctionality largely influences the estimated impacts of each co-produced metal, subsequently with potential large implications for the footprints of products and sectors using these metals. The way multifunctionality is solved shall therefore be transparently reported in the scope of LCA studies, with privileging subdivision whenever possible.