Copper reduction represents one of the steps in the process of fire refining of blister copper. Process of copper reduction with natural gas as performed in the anode furnace was modeled. Numerical analysis was conducted using the IPSA (Inter- Phase-Slip Algorithm) module of PHOENICS package and standard turbulent model k - ε. The model takes into account the liquid phase - slag and phase of natural gas which is supplied through the tuyere from the bottom of the charge. Calculations were made for two different diameters of tuyeres for supply of reducer. Based on the calculations and analysis, it was found out that the gas flow which causes strong movement in the bath guarantees homogeneous composition of the liquid copper in each process stage. Application of a tuyere with a larger diameter results in a greater intensity of the reduction process and better use of the reducer.