Process metallurgical systems are central and key enablers of the Circular Economy (CE). This contribution shows the simulation-based state-of-the-art approach to understanding the resource efficiency of very large-scale CE systems. Process simulation permits system-wide exergy analysis also linked to environmental foot-printing. It is shown that digital twins of large CE systems can be created, and their resource efficiencies quantified. This approach provides the basis for detailed estimation of financial expenditures as well as high-impact CE system innovation. Various examples will be shown that include (i) cadmium telluride (CdTe) photovoltaic technology life cycle, which brings several metal infrastructures into play, (ii) primary and secondary zinc processing, (iii) linking of metallurgical processing to cement production, (iv) design for recycling and the effects of product design on the CE, (v) aluminum recycling linking physical separation to specific alloy types production during remelting and refining and (v) magnet production from rare earth ore to magnet production and recycling. The results generally show that considerable work remains to optimize the CE system. Low exergy efficiencies resulting specifically from energy-intensive processes highlight areas with the greatest renewables-based improvement potential. This detail sheds light on the true performance of the CE and the inconvenient truth that it cannot be fully realized but only driven to its thermodynamic limits.