How to Choose the Ball Loading and Loading Ratio For a Ball Mill? How can you determine if the ball loading ratio is reasonable?
Ball mill is one of the commonly used pieces of equipment in mineral processing. For a ball mill, the amount of loading is crucial as it affects not only the grinding efficiency and productivity, but also the service life of the ball mill. Therefore, it is essential to determine the correct amount of ball loading and the loading ratio of the ball mill during its usage.
Selection of Ball Loading
When a ball mill operates, its efficiency generally depends on the sum of work done by each ball (number of impacts and impact force). Therefore, it is necessary to ensure that each steel ball operates efficiently, and the steel balls moving along different trajectories should not collide as much as possible.
When the speed of the ball mill is fixed and the loading capacity is 30%, the movement of the ball is mainly sliding, but the inclination begins to appear. When the loading capacity exceeds 40%, the movement of the ball is mainly inclined. When the loading capacity reaches 50% of the total capacity of the cylinder, the surface of the ball group is just on the diameter of the cylinder, which causes the balls to tilt. Tilting is needed for ball mill production, so it is more appropriate to load balls between one third and one half, and the most ideal loading ratio is close to one half.
According to the actual results of laboratory tests, the output is the highest when the filling quantity of steel balls in the cylinder is 40%, while the filling rate of steel balls in production is about 35%. This means that keeping the volume of steel balls lower than that of the horizontal center line is preferable.
Selection of Loading Ratio
To determine the appropriate ball loading quantity, it is necessary to choose the appropriate ball loading ratio, which takes into account factors such as the size of steel ball, ball diameter series, and the proportion of balls of various specifications. Rational gradation of steel balls is the key to meeting crushing requirements, which directly affects the grinding efficiency and ultimately affects the output of ball mills.
The main function of steel balls in a ball mill is to break the material by impact and also play a role in grinding. To determine the gradation of steel balls, factors such as the size of the ball mill, internal structure of the ball mill, and product fineness requirements should be considered, along with the characteristics of the grinding materials (grindability, particle size, etc.). Generally, the following principles should be followed:
- The steel ball must have enough impact force, and its diameter should be large if the material hardness is also large.
- On the premise of ensuring enough impact force, the diameter of the abrasive body should be reduced as far as possible, and the number of steel balls should be increased to improve the number of impact times on materials and thus improve grinding efficiency.
- The steel ball should have the ability to control the material flow rate to ensure that the material has enough residence time in the mill to be fully crushed.
When the ball mill is used for pulverizing material, the highest ball milling ratio is achieved with the following steel ball sizes:
In production, the two-stage ratio and the multi-stage ratio are more common. Generally, the multi-stage ratio follows the principle of “less at both ends and more in the middle”. The two-stage distribution method uses steel balls with two different sizes, with larger diameters to carry out the gradation.
The theoretical basis is that the voids between big balls are filled by small balls to increase the packing density of steel balls. The big balls break the material by impact, improving the impact ability and impact times of the mill, which accords with the functional characteristics of the grinding body. Meanwhile, the small balls can fill the gaps between the big balls, increase the accumulation density of the grinding body, and control the material flow rate. Additionally, the coarse particles in the gap can be squeezed out and placed in the impact of the big ball.
Generally, coarse ore particles are attacked by large steel balls, and fine ore particles need to be milled by small steel balls. If there are too many large balls, the particles will be under-ground, while too many small balls will increase the surface area and cause over-grinding. Insufficient ball strike force due to too few big balls can also lead to under-grinding. Therefore, it is important to determine the size of the maximum and minimum balls correctly to achieve reasonable ball loading and improve grinding efficiency.
Comparison of Two-Stage Ratio and Multi-Stage Ratio:
The effectiveness of the two types of ball matching varies greatly depending on the material. The multi-stage ball matching is more sensitive to changes in the material, resulting in greater fluctuations in the output of the ball mill. On the other hand, the two-stage ball matching is less sensitive to material changes, resulting in relatively stable output. In other words, when the material has better grindability, the output of the multi-stage ball is more than that of the two-stage ball. Conversely, when the material has worse grindability, the output of the multi-stage ball decreases more than that of the two-stage ball. Therefore, the multi-stage ball is suitable for materials with smaller particle size and better grindability, while the two-stage ball is suitable for materials with larger particle size and worse grindability.
The two-stage ball method requires the determination of the following parameters:
(1) The determination of the diameter of the large ball, which depends on the size of the ball mill, the particle size, and grindability of the grinding material. Generally, the diameter of the second-level ball in the multi-stage ball match is used as the standard. For instance, if the maximum diameter of a ball mill is 100 mm in the multi-stage ball distribution, steel balls with a diameter of 90 mm should be selected when two-stage ball distribution is carried out.
(2) The determination of the diameter of the small balls, which depends on the size of the gap between the large balls, i.e., the diameter of the large ball. Under normal circumstances, the diameter of the small ball is larger than the diameter of the large ball by 20%-30%.
(3) The ratio of large to small balls, which should be ensured that the addition of small balls does not affect the filling rate of large balls. Ordinary balls occupy 3%-5% of the weight of the large ball.
How can you determine if the ball loading ratio is reasonable?
In actual production, the reasonableness of the ball loading ratio can be assessed in the following ways:
- Keep the feed quantity and overflow fineness of the classifier constant while reducing the amount of returned sand. A decrease in the amount of returned sand indicates that the grinding efficiency of the ball mill has increased, allowing for an increase in feed quantity.
- Analyze the screening results of the returned sand from the classifier. If the proportion of ore particles close to the overflow size increases, this indicates that there are not enough small balls; on the other hand, if the proportion of coarse grains increases, it suggests that there are not enough large balls.