In the present work, a new modelling approach has been put forward to study slag-metal reactions. Sulphur refining in a gas-stirred ladle has been taken as an example. A two-dimensional fluid dynamic model accounting for the steel, slag and argon phases has been incorporated with the thermodynamics of desulphurisation. Comparison of the results of the model calculation with plant data from Ovako Steel indicates that the present approach has great potential. It has opened up the possibility of developing process models based on fundamental equations.