KU Leuven and RWTH Aachen University are collaborating on the HARARE project to study the hydrogen reduction of molten copper slag. This presentation showcases KU Leuven's CFD simulation work aiming to understand the hydrogen conversion mechanism at the gas-slag interface, enabling control of selective reduction conditions for copper and iron. A multiphase flow CFD model tracks the interface via VOF and calculates the local transient temperature with consideration of reaction heat, convection, and radiation. The H2-O2 interaction mechanism simulates gas species evolution with six species and seven chemical reactions. Results show that gas injection causes a heterogeneous temperature distribution, influencing hydrogen conversion at the interface. Under present experimental conditions, molecular hydrogen dominates copper slag reduction instead of atomic hydrogen. This work provides a numerical tool for studying hydrogen conversion in high-temperature slag reactors and insight into the design of copper slag hydrogen reduction. Experimental research at IEM,