Comminution represents one of the most important operations in mineral processing due to the high energy cost and tool wear. This paper presents a new population balance model (PBM) of ball mills that understands the ball mill process as a hybrid of a perfectly mixed mill and piston flow mill. Usually, PBM for grinding is related to a perfectly mixed mill. In this case, the piston flow was introduced for a more realistic process. The ball mill modelling process is described as the point where the feed entering the distribution size is coarse, and where there is an overflow and discharge of the mill, the distribution size is fine and equivalent to the product distribution size. In this work, the evolution of the size of particles along the mill piston flow process was studied. The relationship between the particle size and position in the length of the mill was established. The equation of the balance population model was formulated, and the parameters were determined for a tungsten ore.