Summary of Learning on Ball-Grading for Power Plant Mills

During the production process, the grinding media—steel balls—in a ball mill continuously wear down and are consumed, leading to changes in ball diameter and size distribution. Based on the patterns of these changes, regular replenishment of worn-out balls is a critical step for maintaining stable grinding efficiency and product quality. Therefore, investigating the wear behavior of steel ball diameters and the evolution of ball size distributions within the mill, as well as developing effective ball-replenishment strategies, is of great significance for enhancing grinding efficiency.

Based on various data regarding wear-induced mass loss and diameter reduction of steel balls at different grades, the following summary is presented:

1. Radial wear of the ball diameter

The radial wear values vary with the diameter of steel balls at different grades. Although high-chromium cast-iron balls exhibit high hardness and low wear, the radial wear of steel balls still shows considerable variation. Specifically, the radial wear increases as the ball size increases, following a regular increasing trend. Furthermore, it can be observed that larger balls experience greater radial wear than smaller ones; however, when assessed in terms of weight-based wear rate, smaller balls exhibit a higher wear rate than larger ones.

During the production process, the radial wear of steel balls exhibits a pattern where newly manufactured balls experience significant initial wear followed by reduced wear in later stages, with consistent wear levels across all grades of steel balls.

2. Changes in Ball Diameter and Gradation

During the production process, steel balls undergo continuous wear and consumption, necessitating regular replenishment. Following the conventional periodic method of replenishing large-diameter steel balls, ball replenishment is carried out at different intervals for the mill, allowing determination of the changes in ball size distribution and gradation. Here, 500 kg of steel balls is used as the baseline for replenishment, and the calculation formula for the replenishment cycle is as follows.

T = ΔG/K

In the equation: t — ball-replenishment cycle, h;

　　　ΔG—replenishment ball weight, kg;

　　　K—The hourly ball consumption of this mill is 0.846 kg/h based on the test results.

Substituting the known data into the above equation yields a ball-replenishment cycle of 591 hours.

Based on the production trial test data, the weight loss values of steel balls at various grades, and the test run duration, the post-weight-loss weights and diameters of the steel balls can be calculated for different cycles. After replenishing the ball consumption at different cycles,

Over time, noticeable changes occur in the ball diameter and the weight distribution of the grinding media. With the grinding-media charge remaining essentially constant, the diameters of the steel balls in each size class gradually decrease, the weight proportions of the size classes shift, and the average ball diameter progressively declines. As the steel balls wear down and their diameters shrink—often by nearly one size class—the imbalance in the weight distribution becomes even more pronounced, with a marked reduction in the 80 mm size class.

Changes in the weight distribution of the grinding-media charge and a reduction in the average ball diameter are the primary reasons for the gradual decline in mill grinding efficiency during operation. Among these factors, an imbalance in the weight distribution of the grinding media has a far greater impact on mill performance than a decrease in the average ball diameter. While it is now difficult to restore the average ball diameter to its original value, adjustments to the weight distribution of the grinding media—by appropriately supplementing the media with balls of different sizes—remain feasible. This approach provides a scientific basis for determining the optimal replenishment strategy in response to changes in the media charge within the mill, thereby helping to stabilize the weight distribution, enhance grinding efficiency, and create favorable conditions for sustained mill performance.

3. Replenishment of ball consumption

Replenishing ball consumption is not only intended to maintain the grinding media charge and average ball diameter within the mill; more importantly, it serves to adjust the changes in the weight proportions of the grinding-media gradation, thereby stabilizing the mill’s grinding efficiency. Periodic replenishment with the largest-sized steel balls in the gradation can no longer meet the requirement for a stable weight proportion of the grinding-media gradation. Based on the patterns of gradation changes observed during production, the ball-replenishment method should be improved to achieve the goal of maintaining a stable gradation weight ratio.

Initial appropriate supplementation with large grinding balls has only a minor impact on the average ball diameter and does not significantly affect the mill’s grinding efficiency; moreover, since the weight proportions in the ball-size distribution are adjusted, this practice is beneficial for maintaining stable grinding efficiency in the medium term. In production, we have conducted multiple trials of supplementing 90-mm balls based on the mill’s operating conditions, and practical experience has demonstrated that such supplementation does not have any significant effect on the mill’s grinding efficiency.

4. Summary

During the production process, the wear of grinding-media balls inside the mill and the resulting changes in ball diameter and gradation constitute a highly complex phenomenon. The ball diameter calculated from the measured ball weights obtained during production trials provides a relatively accurate quantitative representation of the radial wear of the grinding media.

Given the varying radial wear rates of steel balls at different size classes, the resulting changes in ball size distribution and gradation within the mill become even more complex. Therefore, it is necessary to conduct analytical studies on how to replenish ball consumption in order to meet the requirements for adjusting the weight-based gradation ratio.

In summary, based on the patterns of radial wear and gradation changes in the grinding media, making appropriate adjustments to the ball replenishment specifications to maintain a stable weight ratio of the media gradation within the mill and minimizing large fluctuations is a crucial measure for preventing the gradual decline in grinding efficiency during operation and for further enhancing that efficiency.