#rodmill

As we all know, the cost of grinding operations accounts for a considerable proportion of the cost of the entire mineral processing plant. Since the grinding investment is huge and the energy consumption is extremely high, it’s widely agreed that “Without a mill, there exist no mineral processing”, from which we can see through clearly the importance of grinding operation. Therefore, the dressing plants are also very cautious in the selection of grinding equipment, in order to yield twice the result with half the effort.

At present, the most widely used grinding equipment in the dressing plant is ball mill and rod mill. As far as the working principle is concerned, both ball mills and rod mills are the same. The grinding medium (steel ball or steel rod) installed in the cylinder is lifted to a certain height with the rotation of the cylinder under the action of friction and centrifugal force, and then thrown at a specific line speed to impact, grind and squeeze the materials in the cylinder to achieve grinding purpose.

But for many people, ball mill and rod mill are just like a pair of "twin". These two types of mills belong to the long cylindrical grinding equipment, so it’s difficult to distinguish them merely from the shape. But when we explore them in depth, we will find that there is a big difference between them. It is the differences in details that determine the purchasing tendency of grinding equipment. In general, we can identify the most suitable type of grinding equipment for the plant based on its equipment construction, grinding media and discharging methods, grinding media filling ratio, equipment performance and scope of application.

1. Equipment Construction of Ball Mill and Rod Mill

On the surface, many people are difficult to distinguish ball mill and rod mill, but in fact, these two mills have differences in the proportion of the cylinder shape. Typically, the barrel length to diameter ratio of the rod mill is maintained between 1.5 ~2.0, and the inner surface of the liner on the end cap is in a vertical plane. The ratio of the barrel length to the diameter of the ball mill is relatively small, usually equal to 1 or slightly greater than 1. In addition, the rotation speed of the rod mill cylinder is generally lower than that of the ball mill of the same specification, so the grinding media in the rod mill cylinder is usually in a sloping state.

2. Grinding Media of Ball Mill and Rod Mill

According to the equipment name, the rod mill usually uses steel bars with a length of more than 500mm as the grinding media, while the ball mill uses steel balls with a diameter of less than 100mm as the grinding media. The difference in grinding media is also the most obvious difference between these two types of grinding equipment.

3. Discharging Methods of Ball Mill and Rod Mill

Different discharging methods are adapted to different grinding processes. For the ball mill, we can choose the overflow type discharging ball mill and grid type discharging ball mill according to the different production requirements. The rod mill has no grid plate, only overflow type and open type, and the diameter of the hollow shaft of the discharge end is larger than that of the same size ball mill. The slurry level of grid type ball mill is lower, and the qualified products can be discharged in time, but sometimes the grid plate is easily blocked and hard to be repaired; and the overflow type ball mill is well managed and easy to maintain, but the discharging slurry level is high and the slurry stays longer in the mill so the capacity per unit volume is small. Choosing a reasonable discharging method will have a certain improvement effect on the grinding efficiency of the mill.

4. Grinding Media Filling Ration of Ball Mill and Rod Mill

The grinding media filling rate refers to ratio of the grinding media’s volume to the grinding equipment's volume. The difference in grinding method, equipment construction, operating conditions and medium conditions naturally leads to the need to maintain a suitable range of media filling rate. Too high or too low filling rate will affect the grinding efficiency. According to practices, the filling rate of the ball mill is usually 40%-50%, and the rod mill is 35%-45%.

5. Equipment Performance of Ball Mill and Rod Mill

Since the grinding medium of the rod mill is in line contact with the ore, it has a certain selective grinding effect, only has the grinding force, the grinding product has a more uniform particle size, less pulverized ore particles, and the processing capacity of the rod mill and grinding effect is greatly affected by the ore particle size. In rough grinding, when the output size is 1-3mm, the processing capacity of the rod mill is greater than that of the ball mill of the same specification; when used for fine grinding, the particle size of the product is less than 0.5mm, the grinding effect of the rod mill is far inferior to the ball mill with same specification. The ball mill is in point contact with the material, and has both impact and grinding power, and the product granularity is finer. Moreover, ball mill has strong adaptability to materials, high production capacity, easy grinding, and remarkable energy saving, but its disadvantage is that the over-grinding phenomenon is serious.

6. Application Scope of Ball Mill and Rod Mill

In the gravity separation or magnetic selection of tungsten-tin ore and other rare metal ores, in order to prevent over-grinding and dust pollution, the grinding of materials is often carried out by a rod mill. In the second-stage grinding process, the rod mill is usually used as the first stage of open grinding, with large production capacity and high efficiency. However, when dealing with low hardness and brittle materials, the rod mill can completely replace the Cone Crusher for fine crushing. The ball mill is prone to over-grinding due to the fine grinding function, so it’s not suitable for the gravity separation process in metal beneficiation, but it is widely used in other mineral separation processes.