In order to attain a highly efficient dephosphorization, multi-phase slags consisting of liquid and solid phases have been used for the hot-metal treatment processes.However, the viscosity of a multi-phase slag dramatically increases with the fraction of coexisting solid phase and transits to a Bingham fluid, which may cause difficulties in discharging the slag after the refining. In this study, in order to understand flow behaviors, the simulated multi-phase slag discharged from a dam was observed by a high-speed video system. The criterion for the free jet to wall flow was obtained. The travelling time of the simulated slag increased dramatically with the solid fraction in the Bingham fluid region. A three-dimensional smoothed-particle hydrodynamics simulation program for Bingham fluid was developed and its calculations were compared with the experimental observations. The simulation replicated the observed flow in the cold model experiment.