Ore Grinding Machines: Ball Mill, Rod Mil & Grinding Media

Ore grinding machines can be divided into many types according to different classification methods, each with significant characteristics. The following is a general introduction to the types of ore-grinding machines and their characteristics.

According to the different media of the mill, the grinding machines are divided into:

• If the medium is steel balls, it is called a ball mill;
• If the medium is steel rods, it is called a rod mill;
• If the medium is gravel, it is called a gravel mill;
• It is called an autogenous mill if its ore is used as the medium.
• Adding an appropriate amount of steel balls to the autogenous grinding machine forms a semi-autogenous grinding machine.

According to the shape of the casing and the discharge method, the grinding machines are divided into:

• The ball mill is divided into short cylindrical, tubular, and conical overflow discharge and grid discharge.
• The rod mill is further divided into cylindrical overflow discharge and open low-level discharge. Autogenous grinding or semi-autogenous grinding machines are mainly short-cylindrical.

What is the grinding process of the ore by the grinding mill?

When the grinding mill rotates at a certain speed, centrifugal force is generated, causing friction between the medium and the cylinder, and rotating with the cylinder. When it reaches a certain height and its gravity is greater than the centrifugal force, it will be ejected from the cylinder and fall, thus crushing the ore. At the same time, due to the sliding phenomenon inside the medium, the ore is also ground. Under the impact and grinding force of the medium, the ore is crushed.

How do you express the specifications of a mill?

The expression is diameter x length, expressed as D x L. For example, a 2700 x 2100 mm ball mill has a diameter of 2700 mm and a length of 2100 mm.

Ball Mills

What are the types and characteristics of ball mills?

The ball mills commonly used in beneficiation plants are grate type and overflow type. The only difference between them is that the overflow type does not have a fan-shaped grate plate device in the discharge part. The slurry of the overflow-type ball mill is automatically discharged through the discharge port, while the grate type has a forced discharge function because of the grate plate installed, thereby increasing the processing capacity and reducing the over-crushing of the ore.

What are the components of a ball mill?

It is mainly composed of six parts: the cylinder, the feeding part, the discharging part, the transmission part, the bearing part, and the lubrication system.

What are the components of the feeding part?

The feeding part is composed of the end cover of the hollow shaft neck, the feeder, and the inner sleeve of the shaft neck.

What are the types of feeders?

The feeders used in the ball mill of the ore dressing plant are drum type, volute type, and combined feeder.

What are the components of the discharging part? (Grid type)

The discharging part is composed of the end cover of the hollow shaft neck, the fan-shaped grid plate, the center liner, and the inner sleeve of the shaft neck.

Rod Mills

What are the working characteristics of the rod mill?

Its working characteristics are: it crushes the ore by the crushing and grinding action generated by the “line contact” of the steel rod. Therefore, it has a selective crushing effect, reduces the over-crushing of the ore, and the product particle size is uniform.

Grinding Media

How many types of grinding media movement trajectories are there in the mill?

There are three types of media movement trajectories in the mill:

• Cascading (when running at low speed);
• Throwing (when running at normal speed);
• Centrifugal operation (the media and the cylinder movement trajectory are consistent).

What is the movement law of the grinding media in the mill?

The movement law of the media in the mill mainly includes the following four points:

1. When the mill is running at a certain speed, the media performs regular cyclical motion under the action of centrifugal force and gravity;
2. The movement trajectory of the grinding media in the cylinder consists of upward movement in a circular arc trajectory and downward movement in a parabolic trajectory;
3. The rising height of each layer of media is different. The height of the media from the outermost layer to the innermost layer decreases successively;
4. The rotation period of each layer of media is different, and the closer to the inner layer, the shorter the rotation period.

How to choose grinding media?

• Grinding media shape
  According to the purpose of grinding and the test results, spherical and P2 rod-shaped media have the best grinding effect; the collision of spherical media during grinding is point contact, so the grinding efficiency is high and it is suitable for fine grinding, but compared with rod mills, the over-crushing phenomenon is serious. Generally, when the feed particle size is below 30mm and the product particle size is not less than 1mm, the rod mill is more efficient.
• Grinding media material
  When selecting media materials, the specific gravity, hardness, wear resistance, price, processing, and manufacturing conditions of the material should be considered comprehensively. At present, steel or iron materials are generally used. In production, steel is often used for large balls, iron is often used for small balls, steel is suitable for coarse grinding, and iron can be used for fine grinding. In recent years, rare earth medium manganese ductile iron balls have been successfully used to replace steel balls.
• Grinding media loading size
  Its size is determined according to the properties of the ore. The harder the ore is and the coarser the particle size is, the larger the media should be added to produce a greater impact grinding effect. Conversely, smaller media can be used to enhance the grinding effect.
• Relationship between medium filling coefficient and mill speed
  That is, the filling rate, which refers to the percentage of the grinding medium in the mill volume. Its relationship with the mill speed is as follows:

What are the methods for the loading ratio of ball mill grinding media size?

There are two main methods for the loading ratio of ball mill grinding media size:

1. Load balls according to the total feed particle size of the mill. The principle is the ball diameter is adapted to the feed particle size, and the ball quantity is adapted to the feed particle content.
2. Simple method: The ratio of various balls is proportional to the ball mill diameter.

The above is a basic introduction to ball mills, rod mills, and grinding media. For more relevant information, please follow our blog updates.