In the paper results of numerical analysis of KCl and KF contamination particles present in liquid aluminium assimilation to the slag are presented. Authors analyzed contamination particle movement in the slag model, which base on buoyant, capillary, viscosity, Newton and repulsion forces, interfacial tensions at the interface of phases and surface energy during the particle movement through phases boundary. On the base of the mathematical model computer program was written to make simulations under different conditions. In the paper results of particle position in the slag for different particle radiuses: 1, 5, 10, 20 µm, and constant slag viscosity including velocity evolution are presented. Another approach was used to indicate influence of slag viscosity on particle and slag penetration depth. During computations selected slag viscosities of 0.0012, 0.0015, 0.0018 [kg / m·s] were taken into account. Different comparisons were made for chosen particle sizes. Each examination takes into account impact of contamination particle type. Results clearly show that for larger particles slag penetration depth is greater and viscosity of the slag has an impact on velocity evolution during assimilation process.