A new modular process modelling tool has been developed to support the design and operation of metallurgical processes. A modelling concept was formulated and a flowsheeting software tool has been developed for the calculation of complex non-equilibrium phenomena. To test both the basic approach and software tool, the modelling and simulation of a LD converter process was undertaken. The validation of the simulation model shows good agreement of reported and calculated values for species concentration and temperature development in time. The aim of this approach is to achieve a high degree of accuracy in modelling and, at the same time, to correspond to the speed requirements of day to day design work. It is intended to extend the current research to obtain a tool not only for the design and simulation of established processes, but also for the computer-aided development of new process routes.