Abstarct: The type of mill shell liner, rotation speed and its amount of charge are the key factors that affect the charge behavior and consequently the comminution mechanism. In this thesis, the milling operations of ball mills in both industrial and semi-industrial scales is investigated using the discrete element method (DEM). This research has been done in three stages. At the first stage, seven different types of liners, which are different in terms of the number of lifters and their thickness, are designed and installed on the industrial mill shell with dimensions of 5.70 × 3.17 meters. Also, using the DEM method, their effects on shoulder, toe, impact and head charge points, as well as on "head height" and "impact zone length" (parameters introduced by the author) and in general on the comminution mechanism have been studied. At this stage, first an industrial-scale ball mill is simulated with a Smooth type liner, and then by changing the type of liner, i.e. Step, Osborn, Lorain, Ship-lap, Rib, Wave liners, six other independent simulations are performed. Considering that the higher the head height (thicker lifters) and the longer the impact zone length (appropriate number of lifters), the more comminution mechanism is in the form of impact and the better it is done, the results show that in smooth and wavy liners, due to their appearance, the charge head is not created properly and the balls slide on each other, as a result, the abbrasion mechanism in them is more than other liners. But in edged and angled liners, proper head height is created and the dominant mechanism is impact. At the second stage, in order to find the optimal number of lifters in the ball mills, a semi-industrial ball mill with dimensions of 2.0 × 1.11 m with cuboid lifters with dimensions of 2 m × 5 cm × 5 cm is simulated. The number of lifters used in the seven simulations related to the semi-industrial mill was: zero, one, two, four, eight, sixteen and thirty-two lifters, respectively. According to the results obtained from the second stage, and by examining the values of the charge head height and the impact zone length, the use of 16 to 32 lifters as a suitable number of lifters for ball mills is recommended. Field research on the ball mills in industry and the ball mills of three different plants showed that the plants use 20, 26 and 30 lifters in the ball mills, respectively. Meanwhile, for the semi-industrial mill, simulations were performed with 20, 26 and 30 lifters, and the values of charge head and impact zone length were obtained for this number of lifters, which confirms the accuracy of the result obtained in the second stage. It is worth mentioning that the first and second stages of the simulation were performed at two different mill speeds, i.e. at 70% and 80% of its critical speed (CS). In the third stage, an attempt was made to optimize the industrial scale ball mill using a combination of the results obtained from the previous two stages. Thus, with a slight change in the dimensions and thickness of each of the six types of liners in the first stage and with the average number of proposed lifters in the second stage (24 lifters), simulations of this stage were performed. According to the results of the third stage and increase of both the head height and the impact zone length for all six types of liners, it can be said that in Wave and Lorain liners, which had 8 and 6 thick lifters, respectively and the number of their lifters was far from the optimal range, by reducing the thickness of the lifters and increasing their number to 16, their performance can be greatly improved. In Rib and Ship-lap liners, which both have 12 lifters and the number of their lifters is close to the optimal range, by slightly reducing the thickness of the lifters and bringing their number to 16 to 24 lifters, their performance can be improved. But the Step liner with 19 lifters is in the optimal range from this perspective, and its performance can be improved by further angling its geometry. Osborn liner with 60 low-thickness lifters and having lifters much more than the optimal range does not have a good performance that by thickening the lifters and reducing their number to 32, its performance can be improved in a desirable way. In general, according to the results of the three stages, it can be said that liners that simultaneously produce both higher head height and longer impact zone length are recommended for use in industry. These liners should be edged, of medium thickness and with a number of lifters between 16 and 32 for industrial scale ball mills. Finally, in order to validate the simulation results, a laboratory scale mill with dimensions of 16.0 × 57.3 cm was simulated. Comparison of laboratory scale mill simulations with experimental results shows a good agreement that validates the DEM simulations and software used.