The closed-circuit grinding system formed by a ball mill and a separator is a type of widely used cement milling system in cement plants and grinding stations.

There are many factors that may affect the ball mill’s working efficiency and product quality during the operation. In this article, we will discuss the measures that can improve the ball mill’s performance.

## Improve the Feed Material Fineness

The particle size of the feed material is an important process parameter that restricts the grinding efficiency of the ball mill. Due to the different physical and chemical properties and microhardness of the materials (the grindability of materials in raw meals decreases in clinkers), the clinker discharged from the cement kiln must be pretreated to reduce its particle size so as to increase the output and reduce the power consumption of the ball mill.

The equation below shows the relationship between ball mill grinding efficiency and material fineness:

**K _{d} = G_{2}/G_{1} = (d_{1}/d_{2}) X (1)**

**Note*

*K _{d}: the relative productivity of the mill;*

*G _{1} & G_{2}: respectively represent the mill output (T / h) when the feed particle size is d_{1} and d_{2};*

*X: the index, which is related to material characteristics, product particle size and grinding conditions.*

Now, taking the general production condition X = 0.20 as an example, the relative productivity K_{d} of the mill with different feed particle sizes is calculated, as shown in table 1.

From table 1 we can learn that if the particle size of the feed material is reduced from 25 mm to less than 2 mm, the mill output can be increased by at least 60%, which is relatively consistent with the actual production.

There are two methods for clinker pretreatment: pre-crushing and pre-grinding. 1) The pre-crushing uses a crusher to crush the clinker before grinding, which can reduce the diameter of clinker particle to 5 ~ 8mm. 2) The pre-grinding adds a roller press to the cement grinding system. In this system, the clinker is extruded circularly, dispersed and separated, and becomes powder with diameter less than 2 mm;

## Gradation of Grinding Media

The gradation of grinding media is also an important factor in improving the efficiency of ball mills. A reasonable gradation can only be calculated after analyzing the performance of the mill, the property of the feed material, and equipment layout in the closed-circuit grinding system.

The size of the grinding media is calculated based on the grinding capacity of the mill and the size of the feed material. Because of the complex movement of the grinding media and the material in the mill, and because the actual production situation of each cement plant is different, it is difficult to determine a universally applicable grading rule. Only through long-term production practice can we get the appropriate gradation scheme.

The gradation of grinding media is constantly changing in the process of mill operation, and the wear law of different size of grinding media is also different. Therefore, the supplementary of grinding media can only keep the loading capacity relatively balanced, but can not keep the gradation consistent.

The stable grinding process largely depends on the material of grinding medium. Different materials of grinding media lead to different wear consumption. If the hardness and wear resistance of the grinding media are poor, it is easy to deform and crack during the operation, which not only affects the grinding efficiency and blocks the grate gap, but also makes the partition device difficult to discharge material, and finally leads to the deterioration of the mill operation. Therefore, improving the quality of the grinding media is an effective way to ensure the long-term stable operation of the mill, otherwise, no matter how reasonable the grading scheme is, it is difficult to ensure that the expected grinding effect can always be achieved.

Once the grinding media and other equipment is properly selected for the grinding system, then the gradation can be determined according to the particle size of the feed material. However, no matter how reasonable the grading scheme is, it is always relative.

The maximum grinding media diameter is determined by the average maximum particle size D95 expressed by the sieve aperture of 95% material passing through.

The average grinding media diameter is determined by the average particle size D80 expressed by the sieve aperture of 80% material passing through.

## The Diameter of Grinding Balls

The ball bearing height of ball mills can be different due to different specifications, diameters, rotating speeds and liner forms of the ball mills. And the potential energy produced by different height of the ball is completely different. Therefore, the reasonable grinding ball diameter should not only match with the mill specifications, but also adapt to the liner form of the mill.

Large size mills with lifting liner bring grinding balls to higher heights and generate stronger impact force, so the diameter of grinding balls can be smaller. The ball diameter should be different according to the aging degree of the inner liner: new liners bring grinding balls to higher height, so the ball size can be smaller.

It can be seen from the experiment that when a grinding ball with a diameter of 70 mm falls freely from the height of 40 cm, its potential energy can completely crush a clinker particle with a diameter of 25 mm. Therefore, the minimum ball diameter should be selected on the premise of sufficient impact energy to increase the number of grinding balls, increase the impact times of balls on materials, and improve the grinding efficiency.

Max Material Particle Size (mm) | Max Grinding Ball Diameter (mm) |

>30 | >100 |

25~30 | 90~100 |

15~20 | 70~80 |

10~15 | 60~70 |

5~10 | 50~60 |

3~5 | 40~50 |

1~3 | 30~40 |

0.5~1 | 20~30 |

0.3~0.5 | 16~20 |

0.2~0.3 | 12~16 |

0.1~0.2 | 10~12 |

*table 2*

Avg Material Particle Size (mm) | Avg Grinding Ball Diameter (mm) |

27.0~38.0 | 89 |

13.0~19.0 | 79 |

6.7~9.5 | 57 |

4.7~6.7 | 49 |

2.4~3.3 | 40 |

1.2~1.7 | 31 |

0.6~0.8 | 25 |

0.3~0.42 | 20 |

0.15~0.2 | 16 |

0.075~0.1 | 12.5 |

*table 3*

Table 2 and table 3 show the relationship between the material particle size and the grinding ball diameter for reference only. When determining the ball diameter, it is necessary to adjust it according to the cement plant’s own situation.