Optimization Analysis of Grinding Efficiency of Cement Ball Mill by Grinding Aid

The use of grinding aids or cement additives is growing, and there has been a marked acceleration in the past 20 years, mainly due to advances in cement additive technology and changes in the economic environment. The introduction of the carbon dioxide emissions trading system has further increased the application of cement additive technology because cement additive technology can reduce the clinker content in cement while maintaining the performance of cement, which can reduce the carbon dioxide emissions of tons of cement.

Obviously, the proper use of grinding aids is one of the many tools that process engineers can use to increase the efficiency of the grinding process. Since the working mechanism of the grinding aid has been discussed in detail in the previous article,1 this article will not focus on how the grinding aid directly improves the grinding efficiency, but focuses on the analysis of the working mechanism to optimize the mill cycle load. And grinding media to increase mill output.

Grinding mechanism of ball mill

The role of the grinding aid is mainly to reduce particle agglomeration, reduce paste ball; improve the powder selection efficiency of the classifier, the efficiency curve of the classifier is steeper, the bypass amount is less; and the material filling rate of the mill is reduced. It is important to reduce the material filling rate of the mill. This is because, under steady state, there is an optimum material filling rate compared to the ball loading to ensure maximum grinding efficiency (ie The optimum ball-to-batch ratio or the in-mill residence time of the material). It is generally accepted that approximately 85% of the material interstitial rate is the optimum value of 2 (material interstitial rate is the ratio of the material volume divided by the gap between the grinding media). However, under actual conditions, the optimal cyclic load often results in a ratio of material cavities exceeding 85%, usually between 110% and 120%, due to the total throughput of the mill material (fresh feed and return). Increasing and shrinking the size of the grinding media (especially 2, 3 bins) together lead to an increase in material caulking.

Mill cycle load optimization

For any closed-circuit ball mill system, there is an optimum cycle load corresponding to the maximum mill output and minimum power consumption (kWh/t). Increasing the cyclic load can reduce the amount of fine powder and reduce the effect of fine powder agglomeration in the mill. However, the increase of the cyclic load will increase the total throughput of the mill and the load of the classifier, thereby reducing the efficiency of the powder selection of the classifier and increasing the material caulking rate of the mill, reducing the mill's material filling rate. Grinding efficiency (when more than 85% of the best interstitial rate). Under a certain cyclic load, the optimum material interstitial rate must be determined through experiments, and the value depends on many parameters, including: mill diameter; mill internal wind speed; efficiency of the classifier; grinding medium size; grinding aid, The crushing performance of a warehouse.

The use of grinding aid can improve the material dispersion effect, reduce the interstitial rate of the material in the grinding, and thus improve the grinding efficiency. In addition, these grinding mechanisms of the grinding aids give the opportunity to optimize the mill cycle load after the application of the grinding aid. Simply put, the grinding aid allows the classifier and mill to increase the total throughput of the mill while maintaining the same efficiency, thereby optimizing the mill cycle load over a higher cycle load range.

The examples in the table outline a series of tests performed on a FLS mill using a third-generation classifier Sepax classifier and a second-stage small-grinding medium (15-30mm, 20mm). Under the first condition, the high material interstitial rate limits the optimal cyclic load, and forcing the cyclic load (the second condition) does not bring benefits. The third condition is the use of conventional grinding aids under the optimum cycle load available, under which the benefits of grinding aids are relatively small (up 6%). However, the use of grinding aids has improved a series of mill conditions, reduced the material filling rate of the mill, improved the powder selection efficiency of the classifier, and reduced the paste. Therefore, the benefits of using a high cycle load and applying a grinding aid (the fourth condition) are obtained. Therefore, the use of grinding aids and the re-determination of the optimal cycle load can significantly improve the efficiency and increase production by 15%.

Optimization of the second chamber grinding media

The optimal grinding media size and grading for ball mills has been extensively discussed for many years. One consistent conclusion is that reducing the size of the grinding media is beneficial to improve the efficiency of grinding fine particles. In a typical ball mill system, the grinding efficiency of the second bin can be improved by reducing the average size of the grinding media as long as the following prerequisites are met:

- A silo has sufficient crushing capacity - the goal is 95% of the particles are less than 1mm

- There is no excessive paste inside the mill

-There is not much material in Ercang (ie, the filling rate is not high)

Early research on the optimal size of the fracture medium was mainly carried out by Austin in the 1970s and 1980s. Obviously, once the size of the particles is less than 1 mm, a large grinding medium (such as more than 25 mm) can no longer provide the highest grinding efficiency. The chart below shows a summary of similar studies conducted by the previous Blue Circle Research Division. In cement grinding, once the size of the feed is reduced to less than 1 mm, a 15-25 mm grinding medium is the best choice for improving the grinding efficiency. Therefore, in the case where the first bin is in good condition, a small grinding medium can be assembled for the second bin to improve the grinding efficiency.

A rough estimate is that reducing the average size of the grinding media from 25mm to 15mm is expected to increase the grinding efficiency by 20%. However, the assumption is that all other important parameters remain unchanged. In reality, however, this does not exist because the use of smaller grinding media tends to result in a significant increase in the total number of grinding media (spheres), increasing resistance to material flow, resulting in material filling in the mill. The rate is increased (filling rate). If the interstitial rate exceeds the optimum value of 85%, the grinding efficiency will decrease. If the paste inside the mill is severe, the grinding efficiency will be further reduced. Previously, the "Blue Circle Cement Technology Center" also studied the effect of the average ball diameter (second bin) on the interstitial rate.

The results show that if the average ball diameter is reduced from 25mm to 15mm, the interstitial rate will increase by 10%-20%, resulting in a decrease in grinding efficiency of 5%-7%. Therefore, the net effect of using a small-sized medium is impaired. In addition, if coarse particles are present, the grinding efficiency with a smaller grinding medium will be further impaired if the paste is severe.

It is precisely because of these factors that our use of smaller grinding media does not necessarily lead to improvements in grinding efficiency and higher mill throughput and lower unit energy consumption. .

Fortunately, the use of a suitable grinding aid can reduce the interstitial rate, reduce particle agglomeration, and reduce the grinding paste. In this way, the use of grinding aids can greatly enhance the positive effect of smaller grinding media in Ercang.

summary

To optimize the grinding efficiency of the cement ball mill, careful consideration must be given to the many parameters involved, including the optimum cycle load and the use of smaller grinding media in the grinding zone (two warehouses). The use of suitable grinding aids or cement additives can also significantly improve the grinding process. However, from the perspective of the mechanisms involved, it is obvious that to maximize the benefits of each aspect, these aspects must be optimized at the same time.