The effectiveness of the grinding process depends on the interior components of the tumbling machinery used. The ball mill is a cylindrical machine used to break down the materials by the means of the grinding media in the form of steel balls. The interior part of the machine is lined to protect the shell of the ball mill and to raise the grinding media. One of the important concepts in the ball mill is the charge trajectory, that is the parabolic path that the mixture of the ore and grinding media (charge) takes while raising and lowering in the process of rotation. Understanding of these concepts is critical for solving the maintenance problems.
When the liners of the ball mill have worn beyond their operational life, the processing plants face expensive delays in their operations. The rapid wearing of the liners occurs due to the increased abrasive and impact loads as a result of the inadequate operating conditions of the machines. Factors such as hard ore, wrong speed of rotation of the ball mill and wrong sizes of the steel balls contribute to the increased forces applied to the lining of the ball mill (Wu et al., 2020).
The main cause of excessive wear due to incorrect lining wear patterns is incorrect trajectory of charging. For efficient operation of the liner, it should lift up the media causing it to cascade into the charge bed, thus enabling free fall on the “toe” of the charge bed, leading to energy transfer with maximal efficiency for ore breaking. If the trajectory is wrong, the steel balls will overshot the charge and fall directly on the liner. Such balls-liner interaction causes accumulation of kinetic energy and increases fatigue and profile deterioration rate significantly.
Trajectory correction is key to prolonging the service life of the liners and achieving effective grinding. An easy method is changing the mill’s rotating speed and charge filling degree. It can be done by changing the ratio between the actual speed of rotation of the mill and the critical speed. The shoulder angle (when the balls leave the charge bed) and the toe angle (when they hit the charge bed) will be affected. Lowering the rotational speed can solve the problem of balls jumping over the charge bed.
Physical adjustments and predictive maintenance play an important role in the correction of the trajectory errors. Optimization of the ratio between lifter spacing and height, along with physical adjustments in the face angle, makes sure that the grinding media will be properly elevated before falling off. Modern technologies utilize sophisticated software, for example, machine learning technology, which allows predicting wear profile of the liners (Pural et al., 2024). Modeling of the way that lifter wear affects movement of the charge enables to make proper adjustment of the mill speed in connection with the wear of liners, thus keeping the optimal trajectory.
To summarize, early wear of liners of the ball mill depends on the high-impact loads that emerge because of the improper settings of parameters. Misalignment of the trajectory, when media hit the liners rather than the ore layer, is the primary factor contributing to the wear of the liners. Adjustment of the operating speeds, lifter profile, and prediction models enable to correct the trajectory.
References
Fang, X., Wu, C., Liao, N., et al. (2024). Investigating the Influence of Medium Size and Ratio on Grinding Characteristics. Minerals, 14, 875. https://doi.org/10.3390/min14090875
Pural, Y. E., Ledezma, T., Hilden, M., et al. (2024). Application of Machine Learning for Generic Mill Liner Wear Prediction in Semi-Autogenous Grinding (SAG) Mills. Minerals, 14, 1200. https://doi.org/10.3390/min14121200
Wu, W., Che, H., & Hao, Q. (2020). Research on Non-Uniform Wear of Liner in SAG Mill. Processes, 8, 1543. https://doi.org/10.3390/pr8121543
