The ball mill is a grinding machine which is often used for cement grinding and raw meal preparation in cement plants.
The grinding media is the most important part of a ball mill. When facing different production needs, we must properly choose the type, loading capacity, grading, and filling rate of grinding media to make the ball mill achieve the optimal production efficiency.
Grinding Media Types
Grinding media with different shapes and sizes achieve different grinding performances. There are three types of grinding media that can be used in ball mills of cement plants.
1. Steel Balls
Steel balls are one of the most widely used grinding media in cement ball mills, their diameters can range from 20mm to 120mm according to different grinding requirements. In a cement ball mill, generally, φ50-100mm steel balls are used in the coarse grinding chamber, φ20-50mm steel balls are used in the fine grinding chamber.
2. Short Cylinders
Short cylinders are the grinding media having the cylinder shape. They can be used in the fine grinding chambers of ball mills in both open and closed circuit grinding systems.
The specification of short cylinders is expressed by the number of mm of diameter multiplied by length. The commonly used specifications are φ 15mm × 20mm, φ 18mm × 22mm, φ 20mm × 25mm, and φ 25mm × 30mm.
3. Steel Bars
Steel bars are round steel rods cut into the size. The steel bars used in ball mills are usually with diameters between 40mm and 90mm. The length of steel bars should be 50-100 mm shorter than that of the mill chamber.
Grinding Media Material
The grinding media in the ball mill is required to have great wear resistance and impact resistance. The material to make the grinding media must be hard, wear-resistant and not easy to crack. Burr and crack are not allowed on the surface of grinding media, and the out of roundness of steel ball shall not exceed 2% of its diameter.
In recent years, the materials of grinding media used in cement ball mills are as follows:
1. High Chromium Cast Iron
High chromium cast iron is a kind of alloy white cast iron with high chromium content. It has good wear resistance, heat resistance, corrosion resistance and good toughness. The wear resistance of high chromium cast iron ball is 8-12 times of that of common carbon steel ball.
2. Low Chromium Cast Iron
Low chromium cast iron contains a small amount of chromium, which allows the white cast iron to obtain martensite metallography. The toughness of low chromium cast iron is slightly worse than that of high chromium cast iron, but it has good wear resistance. It is suitable for making small balls, short cylinders, and lining plate in fine grinding bin.
3. Forged Bearing Steel
Forged bearing steel can be used to make steel balls of various diameters. Its carbon content is about 1.0%, chromium content is about 1.5%, and other elements are conventional content. The loss efficiency of forged bearing steel ball is higher than that of high chromium cast iron ball in practical use. However, due to its low alloy element content, it still has a broad market.
Grinding Media Filling Rate
The filling rate of grinding media refers to the percentage of the volume of grinding media loaded into the ball mill to the effective volume of the ball mill. It can reflect the amount of grinding media loading in the mill.
The filling rate of grinding media has a great impact on the grinding efficiency of the ball mill. When the conditions of ball mill are the same and the speed is constant, If the filling rate of the grinding media is too low, the sliding of the grinding medium will be increased and the impact surface of the material will be reduced; if the filling rate is too high, the grinding media will lose its normal dropping track, which will not only reduce the grinding efficiency, but also increase the power consumption. Therefore, we should choose the most suitable filling rate through experience accumulation in production practice.
Grinding Media Loading Amount
The loading amount refers to the total weight of the grinding media loaded to the ball mill. The loading amount is related to the filling rate of grinding media.
When the filling rate is determined, the loading amount of grinding media can be calculated. Due to the positive proportion between the loading amount and the filling rate, the output of the mill can be improved by properly increasing the loading amount of the grinding media in both open-flow grinding and circular-flow grinding under certain conditions.
Grinding Media Gradation
In a ball mill, the gradation of its grinding media directly affects its output, production quality, and grinding media wear rate. When the ball mill is running, the material is both being impacted and ground by the grinding media. Obviously, in unit time, the more contact points between grinding media and material, the easier grinding will be completed. When the loading amount of the grinding media is fixed, the contact point between the material and the grinding body can be increased by reducing the size of the grinding media. But at the same time, the grinding media must be large enough to break the large pieces of material.
The task of the ball mill is not only to ensure the large pieces of material can be broken but also the material can be ground to certain fineness. Therefore, under the condition that the length of each grinding chamber and the particle size of the feed material remain unchanged, only by selecting the proper size and gradation of the grinding media can it be completed.
There are many factors that may affect the gradation of the grinding media, such as material properties, product fineness requirements, mill liner shape, circle flow grinding or open grinding, etc. A reasonable gradation of the grinding media is relative and temporary. The most suitable grading scheme should be based on the specific situation of the unit, and the statistics, analysis, measurement, and summary should be carried out continuously through long-term production practice, so as to achieve continuous optimization.
Parameters of steel ball commonly used in cement ball mill：
|Diameter (mm)||Mass of 1 ball (kg)||Number in 1 ton||Mass of 1 m3 (kg)||Surface area of 1 ball (mm2)||Surface area of 1 ton of balls (m2)|