According to the different ways of discharging ore, the ball mill can be divided into grid type ball mill and overflow type ball mill. Before selecting the type of ball mill, we need to know the differences between grid type ball mill and overflow type ball mill in structure, performance and application.
The discharging side of the grid type ball mill is equipped with a grid plate. There are many 7-20mm discharging holes in the grid. The material ground by the ball mill will pass through the grid plate, and the pulp lifting device is installed on the discharging side near the grid plate. It is a kind of radial rib bar, which divides the space between the grid plate and end cover into several fan chambers leading to the neck of the hollow shaft. When the ball mill rotates, the radial ribbed bar plays the role of lifting the pulp, lifting the pulp discharged from the holes in the grid plate to the neck of the hollow shaft, and then discharging it from the ball mill through the neck of the hollow shaft.
The structure of the overflow ball mill is basically the same as that of the grid type ball mill. The major difference is that there is no grid plate at the discharging end of the overflow ball mill. In addition, the bushing inner surface in the neck of the hollow shaft is equipped with anti-spiral blades, which can return the small steel balls overflows from the pulp and large coarse particles to the ball mill. The ore discharge of overflow type ball mill mainly depends on the pulp itself high over the lower edge of the hollow shaft bushing.
Compared with the overflow type ball mill, the ore discharge of grid type ball mill is low-level forced discharge. The pulp surface of the ore discharge port is low, the fine ore particles can be discharged in time, and the heavy mineral will not be concentrated in the ball mill, which greatly reducing the over-grinding phenomenon. In addition, the grid type ball mill has a large ball load, the size of the steel ball can be loaded and the small steel ball will not be discharged. Under the same conditions, the grid type ball mill has a higher grinding efficiency and larger production capacity, but its structure is more complex, the quality is higher and the ball mill price is higher.
The overflow type ball mill has a simple structure, easy to manage and maintain. But its discharge liquid level is higher, the material stays in the ball mill for a long time, and the production capacity is lower than the grid type ball mill with the same specification, which is easier to cause over-grinding.
In general, the productivity of the grid type of ball mill is 10%-25% higher than that of the overflow type ball mill, and the power consumed is 10%-20% higher than that of the overflow type ball mill. The grid type ball mill consumes a large amount of power, but its production efficiency is high. If calculated as “ton/kw x hour”, the efficiency index of grid type ball mill may be higher.
After mastering the difference between the grid type ball mill and the overflow ball mill, how should we choose the types of ball mill when purchasing them?
Usually, the concentrator chooses the grid type ball mill in the one-stage grinding and the overflow type ball mill in the two-stage grinding.
◆ Generally, the upper limit of particle size of the grid type ball mill is 0.2-0.3mm, so it is often used in the one-stage grinding, and formed the closed cycle with the spiral classifier. The grinding sizes of the overflow ball mill are generally less than 0.2mm, which is suitable for the concentrates regrinding. It can be used in the one-stage grinding or two-stage grinding to obtain the fine qualified products.
◆ As for the application range, the grid type ball mill is better if the material needs to be ground to the uniform coarse product (48-65 mesh). The overflow mill is better if the material needs to be ground to the fine products (150-200 mesh).
The grid type ball mill discharges the ore by relying on the grid plate, so its production capacity is higher than that of the overflow type ball mill. The overflow ball mill discharges the ore by the gravity flow with the slow speed, so its production capacity is relatively weak, and because the material stays in the ball mill for a long time, it is easy to cause the over-grinding phenomenon.
The steel ball inside the grid type ball mill is added in proportion. When the steel ball in the ball mill is lifted to a certain height, the relatively lower pulp has a larger falling height towards the steel ball. As a result, the steel ball has a stronger impact on the material, and the larger crushing effect on the material, so the grinding effect is better. while the grinding efficiency of the overflow type ball mill is relatively weak.
The overflow type ball mill has a relatively simple structure and easy in maintenance due to there is no grid plate configuration. The grid type ball mill has a better discharging effect because of the grid plate, but it also makes the structure of the grid type ball mill more complex, and increases the weight, also brings a certain degree of difficulty to the ball mill.
Under the same specification, the grid type ball mill has a larger weight and higher price than the overflow type ball mill.
The above five aspects are the basis that we can measure when choosing the grid type ball mill or the overflow type ball mill. No matter which type of ball mill you choose, you must know the production requirements of your own concentrator. At the same time, it is important to note that there are no same mines in the world of mine, even the same ore is also different. Therefore, before choosing the right type of ball mill, be sure to make a mineral processing test, and carry on the mine design and equipment selection according to the report results of mineral processing test.