Erosion prediction of the solid propellent nozzle is vital for its design process. This erosion is caused by the impingement of agglomerated aluminum/aluminum oxide particles on the nozzle walls. Thus, a multi-phase numerical model is established based on the Eulerian–Lagrangian approach to model the aluminum particles burning inside the combustion chamber and simulate the mechanical erosion of the nozzle. The numerical model is validated against numerical and experimental results from the literature. Then it is simplified by eliminating the aluminum particles burning process as they do not reach the nozzle. The simplified model will be further used in modeling the agglomerates’ breakup and predicting the mechanical erosion for aluminum particles with lower surface tension. The results showed that applying the Reitz–Diwakar breakup model reduces the erosion rate by 6.2–24% depending on the injected droplets. In addition, it was found that a decrease in the erosion rate by 1–4.5% can be achieved by reducing the aluminum additive’s surface tension by 15%.