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The main function of the steel ball in the ball mill is to impact crushing the material and also play a certain grinding effect. Therefore, the purpose of grading steel balls is to meet the requirements of these two aspects. The quality of the crushing effect directly affects the grinding efficiency, and ultimately affects the output of the ball mill. Whether the crushing requirement can be achieved depends on whether the grading of the steel ball is reasonable, mainly including the size of the steel ball, the number of ball diameters, and the ball of various specifications. Proportion and so on. In addition to factors such as the size of the ball mill, the internal structure of the ball mill, and the fineness requirements of the product, the parameters of the milled material (wearability, particle size, etc.) should be considered.
In order for the material to be effectively crushed, several principles must be followed when determining the grading:
First, the steel ball should have sufficient impact force to make the ball mill ball have enough energy to crush the particulate material, which is directly related to the maximum ball diameter of the steel ball.
Secondly, the steel ball must have enough impact times on the material, which is related to the ball filling rate and the average ball diameter. When the loading amount is constant, under the premise of ensuring sufficient impact force, the diameter of the grinding body is reduced as much as possible, and the number of steel balls is increased to increase the number of impacts on the material to improve the pulverizing efficiency.
Finally, the material has sufficient residence time in the mill to ensure that the material is fully comminuted, which requires the steel ball to have a certain ability to control the flow rate of the material.
The so-called two-stage ball-matching method uses a steel ball of two different sizes and different diameters to perform grading. The theoretical basis is that the gap between the large balls is filled by the small balls to fully increase the bulk density of the steel balls. In this way, on the one hand, the impact capability and the number of impacts of the mill can be improved, which is in accordance with the functional characteristics of the abrasive body. On the other hand, the higher bulk density enables the material to obtain a certain grinding effect. In the two-stage ball, the role of the big ball is mainly to impact crushing the material. The function of the ball is to fill the gap between the large balls, increase the bulk density of the grinding body, to control the material flow rate, and increase the grinding capacity; The energy transfer function transmits the impact energy of the large ball to the material; the third is to discharge the coarse particle material in the gap and place it in the impact zone of the large ball.
The two-stage ball method needs to determine the following parameters:
(1) Determination of the diameter of the large ball. It depends on the size of the ball mill, the size of the milled material and the wearability. Generally, the second-order ball diameter in the multi-level ball is taken as the standard. For example, if a billiard mill has a maximum ball diameter of 100 mm in a multi-stage ball, a steel ball with a diameter of 90 mm should be selected for the two-stage ball.
(2) Determination of the diameter of the small ball. It depends on the size of the gap between the big balls, that is, on the diameter of the big ball. Usually, the diameter of the small ball is preferably 20%-30% of the diameter of the large ball.
(3) The ratio of big and small balls. In principle, it should be ensured that the incorporation of the pellet does not affect the filling rate of the large sphere. Generally, the small ball accounts for 3%-5% of the weight of the big ball.
In the multi-stage ball, the impact force on the steel ball, the number of impacts, and the ability to control the material flow rate are mainly based on the average ball diameter, that is, affected by a variety of ball sizes. In the two-stage ball, the impact force and the number of impacts of the steel ball are determined by the diameter of the large ball, and the ability to control the flow velocity of the material is mainly determined by the diameter and loading of the ball, and is greatly affected by the diameter of the large ball. The contradiction between the impact force, the number of impacts and the ability to control the material flow rate is alleviated. In contrast, the two-stage ball-matching method is relatively simple, and it is easy to consider comprehensively when determining the grading parameters.
As the hardness and particle size of the material increase (can be reflected by the different content of the mixed material), the contradiction between the impact force of the steel ball and the material flow rate control ability in the multi-stage ball is more prominent, so the output decline is more obvious. . In the two-stage ball, there is no such contradiction, because it can meet the requirements of the impact force and flow rate control ability by the big and small balls respectively, so the output change is not obvious, which shows the superiority of the two-stage ball. .
The use of two-stage ball can also reduce the number of clearances. It only needs to be replenished according to the ball mill's output, current, fineness and ball consumption ratio of the grinding material. Except for special cases, it usually does not stop the clearance and improves the ball mill operation rate.
in conclusion
When the material has small particle size and good wearability, multi-stage ball should be used, because the number of impacts of the steel ball is increased at this time. Conversely, when the material has large particle size and high strength, it is the key to improve the impact force of the steel ball. Therefore, the use of two-stage ball can show obvious superiority.
The effect of the two ball types is greatly affected by the material change. The multi-stage ball is sensitive to the material changes, and the ball mill output fluctuates greatly. The two-stage ball is less sensitive to changes in the material and the yield is relatively stable. That is to say, when the material is more wearable, the multi-stage ball production increases more than the two-stage ball. Conversely, the multi-level ball production declines to a greater extent than the two-level ball. Therefore, the multi-stage ball is suitable for materials with small particle size and good wearability, and the two-stage ball is suitable for materials with large grain size and poor wearability.
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