Steel-slag mass transfer is a key step in the removal of different components, such as sulphur or phosphorous, during the steelmaking process in the steel converter. As the reactions taking place are very fast, steel-slag mass transfer is an important stage in the overall process. Non-systematic data of the liquid-liquid mass transfer coefficients from real converters, or cold model set-ups, have been reported in the literature. This research work is a study of the steel-slag mass transfer coefficients in a cold model, as similar as possible to a real converter with respect to geometry and physical properties. The effect of the gas flow rate was studied in bottom and top-bottom combined blowing, comparing the efficiency of each blowing type. The results are also analysed developing a two-zone model, due to the existing mixing problems, thus determining the local variation of mass transfer coefficients in the converter. These values are important not only with the aim of modelling the steel-slag reactor, but moreover to analyse the effect of the method of adding reagents to the converter during blowing.