08
2011
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03
Ball Mill Steel Ball Size Distribution Adjustment
Author:
The grinding media gradation is influenced by factors such as material properties, feed and discharge particle size, product type, grinding method, and mill configuration, and must be determined through multiple adjustments. Under normal operating conditions, a sudden mill shutdown should be observed: in the coarse grinding chamber, about half of each steel ball should be exposed above the material surface, while in the fine grinding chamber, the grinding media should be covered by a thin layer of material.
1. For a typical ball mill, the maximum ball diameter in the first chamber is 90 mm; when the feed particle size is large, it is best not to exceed 100 mm.
The selection of the average particle diameter can be based on the following considerations:
1. For materials with large particle size and high hardness, the average ball diameter should be increased; conversely, it should be decreased.
2. If the product fineness is coarsened, the feed rate increases, the ball-to-material ratio in the mill decreases, and the buffering effect strengthens, thereby compromising the coarse grinding performance; in this case, the ball diameter should be increased.
3. Under the same breakaway angle, the larger the mill diameter, the greater the impact height of the grinding balls, allowing for a corresponding reduction in ball diameter.
4. With a high relative rotational speed of the mill, the centrifugal force on the grinding balls is substantial, resulting in greater lifting height; therefore, the average ball diameter should be smaller.
5. In a closed-circuit mill, the recirculating material increases the amount of material within the mill and reduces the ball-to-material ratio; therefore, the ball diameter should be increased. However, since the recirculating material also reduces the particle size of the feed, the average ball diameter should be decreased. Consequently, the average ball diameter in a closed-circuit mill is essentially the same as that in an open-circuit mill.
6. A double-layer partition plate is used, ensuring that the discharge from the first chamber is not obstructed by the material and grinding media in the downstream chamber, thereby enabling smooth discharge and resulting in a smaller average ball diameter compared with a single-layer partition device.
II. Determination of Gradation
The ball-size distribution is influenced by factors such as material properties, the particle size of feed and discharge materials, product type, grinding method, and mill configuration, and must be determined through multiple adjustments. Under normal operating conditions, a sudden mill shutdown should be observed: in the coarse-grinding chamber, about half of each steel ball should be exposed above the material surface, while in the fine-grinding chamber, the grinding media should be covered by a thin layer of material. No more than four gradation levels should be used per chamber. On the basis of meeting the required average ball diameter, it is advisable to adopt a gradation with smaller balls at both ends and larger balls in the middle, and then make further adjustments according to the grinding performance and the desired product fineness.
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