■ Design Principles of Grinding Mill Liners
Under the conditions of power, type of grinder, ore quantity, size, mineral and filling of grinding body, the structure design of grinding mill liners is able to minimize the impact wear of grinding body on liners and extend service life as much as possible. Moreover, the direct impact of grinding mill on liners can also be avoided. Therefore, AG mill and SAG mill are very important for toe calculation and finite element analysis of grinding mill body. Besides, the design structure should also consider the ease of demolition.
■ Main Factors of Grinding Mill Liners
Various variables in the actual operating experience, including the height and spacing of the lifting bar ,the inclination of the liners (increasing the angle of the strip surface) and the movement track in the working surface of the lifting strip are all concerned so as to change the impact point of the material and the medium in the grinding mill; Reasonable clearance of lifting strip can not only maximize the material lifting rate, but avoid the phenomenon of clamping to improve the performance of the grinding mill.
◆ Influence of the inclination of the liners
Included plane angle of the lifting bar is an important factor affecting the power of the grinding mill. That is to say, the inclination angle of the liners corresponding to the peak value of the grinding mill power increases with the increase of rotating velocity. When low liners speed with a small inclination angle is selected, the change of the inclination angle of the liners has little influence on the power. During the grinding process, the inclination of the liners is gradually increased due to wear. The impact energy and the number of impacts jointly determine medium grinding properties, so the impact energy distribution can serve as an indicator of material fracture performance. While the inclination of the liners and the number of low energy impacts increase, the number of high energy impacts and the highest impact energy are reduced.
◆ The influence of liners height
The variation of liners height will result in the variation of the effective diameter of the ball mill, thus exerting a great influence on the power of the grinding mill. The power calculation formula of the ball mill is as follows: Power = mgxω, where: m represents the effective mass of the material (ie, the mass of the material without generating centrifugal motion); g represents the acceleration of gravity; x represents the distance between the center of gravity of material and the center line of grinding mill barrel; ω represents the rotational angular velocity of the grinding mill. As the height of the liners increases, the effective mass of the material remains unchanged, while the lifting effect of the liners on the material is enhanced, and the height of the material lifting is increased, that is increased. Therefore, the power of grinding mill increases with the increase of liner height, and the impact frequency and corresponding energy level of material in grinding mill increase accordingly. Moreover, the dropping and falling height of material increases with the increase of liners height, thus leading to a corresponding increase of the impact energy.
◆ The influence of liners row number
The mill power increases with the increase in the numbers of liners. When the number of liners increases to a certain value, the grinding mill power will keep a stable value without continually increasing. Nevertheless, the number of lifting bars of grinding mill is more than that when power reaches stability in actual production. From this point of view, it is reasonable to neglect the influence of the number of lifting bars on power calculation. The less the rows of the liners, the less the number of high-energy shocks. The grinding action of the grinding mill is mainly caused by the impact of the medium falling, moreover, only when the collision energy reaches a certain value can it be effectively applied to the crushing of the material. So the reduction in the number of shocks (especially those with high energy) means a decrease in the probability of breaking up the medium. This shows that the grinding mill with more liner rows has better crushing performance.
From the two aspects of the net power and impact energy distribution of the grinding mill, the influence of the design parameters of the liner plate on the grinding effect of the ball mill is simulated and analyzed, and the conclusions can be drawn as follows:
✔ Increase the number of liner rows, to a certain extent, increase the power of the ball mill.
✔ Different speeds can generate different inclination of the liners of the grinding mill power and the power consumption decreases with the increase of the height of the liners. The higher the speed, the more obvious this trend.
✔As the dip angle decreases, the magnitude of the resultant force of the individual media in the ball mill increases, such as the probability of breakage of the material increases.
✔ The proportion of high energy impact increases with the increase of the number and height of liner, and the impact energy level corresponding to the numbers of the same impact increases with the increase of the height of liners; with the increase of the inclination angle of liners, the number of low energy impact increases, while the number of high energy impact times and the maximum impact energy decreases. Therefore, if the particle size and hardness of the abrasive material are relatively large, the liners with smaller inclination angle and higher height should be selected, and the number of liners should be increased appropriately; otherwise, the liners with larger inclination angle and lower height should be selected.
■ Discrete Element Method of Liners
Based on experience of traditional design，the number of lifting bars of SAG mill is twice the foot diameter of grinding mill. The height and plane angle of lifting bars are determined, and then DEM analysis is used.
The calculation of discrete element method DEM is basically divided into two segments:
◆ the appropriate contact model is used to solve the force exerting on the particles and the resultant force of the particles is obtained;
◆ According to Newton second law coupled with the central difference method, the new displacement of the medium under the action of unbalanced force is determined. The EDEM software regards the medium as a rigid body, regardless of its deformation; the rigid medium at the contact point is allowed to overlap, and the overlap，which is related to the contact force, is much smaller than the diameter of the medium. Mill Trajectory design is professional liners design software. The movement track of steel ball in cylinder is simulated to judge whether the liners design is reasonable or not by inputting grinding mill and liner parameters.
