Comparison between anticipated and measured mean residence time (MRT) along with analysis of the shape of residence time distribution (RTD) curves provides the most useful and valid information about mixing properties within a ball mill operating at industrial scale. In this work, the RTD measurements were carried out for a primary ball mill in closed circuit with hydrocyclones at two feed rates (i.e., 280 and 230 tons per hour (t/h)) by means of tracer technique. Also, perfect mixer, N-Mixer, and Weller models were studied to describe the RTD curves. It was found that the mixing condition inside the ball mill was very far from the perfect mixing circumstance. While, N-perfect mixer in series (N-Mixer model) showed better results compared to the large and small tanks in series (Weller model). According to the obtained results of the N-Mixer RTD model, when the ball mill throughput was enhanced from 230 t/h to 280 t/h, the MRT was decreased from 9.92 to 7.39 min, respectively. Moreover, relative variance of N-Mixer model diminished approximately 18% and particle cumulative passing from 75 m of ball mill discharge increased 9%. Finally, it was concluded that operating in over-filled condition and poor mixing circumstance of slurry were the main reasons of low grinding efficiency.