Ball mill vs Rod mill, how to choose?

Ball mill and rod mill are the widely used equipment in mineral process plant. They have similarities in appearance and working principle, but also have much differences in the structure, performance and usage. In this article we analyze their similarities and differences from 7 aspects, it will help to choose ball mill and rod mill.

From the analysis of working principle, either ball mill or rod mill, when the cylinder rotates, the grinding medium (ball or rod) inside the drum will be lifted to a certain height under the action of friction and centrifugal force, and then thrown down according to a certain linear speed, so that materials will be crushed by the impact, grinding and extrusion.
ball mill working principle

The working principle is similar, but there are differences as follows

1. Appearance & structure

The cylinder shape ratio is different. For the rod mill, the ratio of cylinder length to diameter is generally 1.5-2.0, also the inner surface is vertical flat liners on both sides covers. For the ball mill, the ratio of cylinder length to diameter is smaller, and in most cases the ratio is only slightly more than 1.

In addition, the rotation speed of rod mill is lower than the rotation speed of ball mill with the same model, so that the steel rods inside work in a cascading state.
deya ball mill

rod mill

2. Grinding medium

Rod mill commonly uses steel rods with diameter of 50-100mm as grinding medium, while ball mill uses steel balls as grinding medium, and the difference of grinding medium is the main difference between them.

The steel balls of ball mill are in point contact with each other, while the steel rods of rod mill are in line contact with each other, so the working characteristics of the two have obvious differences.
grinding balls

steel rods

3. Discharge methods

Among the ball mills, the commonly used ones are grate type ball mill and overflow type ball mills, they are named from the different structure of the discharge section. While rod mills do not use grate plate to discharge minerals, there are overflow and open type for discharging. Also for the rod mill, the diameter of the hollow shaft at the discharge end is generally larger than that of the same size of ball mill.
ball mill discharge

ball mill inside

4. Grinding medium filling rate

Medium filling rate refers to the percentage of mill volume occupied by the grinding medium. For different grinding methods, mill structures, operating conditions and medium shapes, the media filling rate has a suitable range, either too high or too low will affect the grinding effect. Usually the filling rate of ball mill is 40%-50%, rod mill is 35%-45%, in addition, the filling rate of autogenous mill is 25%-40%, and the filling rate of gravel mill is about 43%.
ball mill filling rate

5. Operation stability

There is no inertial impact in the operation of the ball mill, which ensures the normal and efficient operation of the equipment, it reduces the downtime of the equipment and improves the production efficiency as well.

6. Applications

When process minerals of tungsten-tin and other rare metals by gravity method separation or magnetic separation, rod mill is often used to prevent over-grinding. In the two-stage grinding mineral process plant, rod mill is generally used as the first stage grinding equipment, because it has larger production capacity and higher efficiency. When dealing with soft or less hard ores, rod mill can be used instead of short head cone crusher or sand making machine for tertiary crushing, it simplifies the configuration of the plant, also reduce dust during working.

The ball mill is not suitable for some rare metal process plant by gravity method due to the over-grinding phenomenon. It is not good for the sand making plant, either.

7. Performance

The process of rod mill is characterized by rougher products, but uniform particle size, containing less coarse particles and mud, less over-grinding, and the particle size characteristics of rod mill are related to the conditions of raw materials.

The performance of ball mill is characterized by high production capacity, strong adaptability to the material, high fineness of the material, fine size of the discharge, easy to fine grinding, significant energy saving, but its over-grinding phenomenon is serious.
ball mill

Summary

Understanding the difference between ball mill and rod mill also lays the foundation for the purchase of equipment. Generally speaking, users can choose according to their raw materials, finished product size and efficiency, production methods, etc.

1. According to the raw material processing requirements.

If the user needs to process the raw material for tungsten tin ore, rod mill is a better choice, and if the user needs to grind the material for aluminum ash, lime, slag, molybdenum ore, etc., the ball mill has a stronger processing capacity.

2. According to the production mode

In the gravity or magnetic separation mineral plant, in order to prevent the over-grinding phenomenon, rod mill is better than ball mill. While in a cement plant, ball mill is better than rod mill.

3. According to the size of the finished product

In coarse grinding, when the required product size is 1-3mm, the processing capacity of rod mill is larger than that of ball mill of the same specification, when used in fine grinding, like product size less than 0.5mm, ball mill has better grinding efficiency comparing same model of rod mill. That is, the product of ball mill tends to be more fine, while the product of rod mill is coarser but more uniform.

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