Comparison of allocation methods in the life cycle assessment of nickel products

Ni is essential for stainless steel and electric vehicle batteries, but its pyrometallurgical process creates allocation challenges in life cycle assessments due to by- and co-production. This study addresses the complexity of multi-output allocation in the Ni-Cu-S symbiotic metallurgical system. By comparatively analysing avoiding allocation (substitution method) and single-criterion allocation methods such as thermodynamic, mass, and economic allocations, this study highlights their applicability and limitations. Based on this analysis, a hybrid allocation method with a double-hierarchy allocation structure is proposed to effectively address the limitations of traditional methods in multi-output systems. The results indicate that in the Ni production symbiotic system, physical relationship allocation (thermodynamic) is the most scientifically robust method but requires substantial fundamental material data. The effectiveness of the substitution method in handling products in different states depends on the accuracy of the inventory. Mass and economic allocation methods, while simple to implement, exhibit significant numerical deviations, such as mass allocation, yielding an S factor of 81.55%. The hybrid allocation method achieves the allocation of S (4.86%), Cu (27.56%), and Ni (67.58%), accurately reflecting the material transformation relationships among these three elements in the system. Compared with single-criterion allocation methods, hybrid allocation achieves a superior balance between data accuracy and operational feasibility, demonstrating methodological robustness and engineering applicability. This methodology is expected to be applicable to other polymetallic symbiotic systems, providing a key tool for lifecycle management in the metallurgical industry.