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

Ore grinding machines can be classified in several ways according to their grinding media, mill structure, and discharge method. Different types of grinding mills have different working characteristics and are suitable for different grinding applications.

This article introduces the main types of ore grinding machines, their working principles, components, and the selection of grinding media.

 

Types of Ore Grinding Machines

Classification by Grinding Media

According to the type of grinding media used inside the mill, ore grinding machines can be divided into the following types:

• Ball mill: Uses steel balls as the grinding media.
• Rod mill: Uses steel rods as the grinding media.
• Gravel mill: Uses natural or artificial gravel as the grinding media.
• Autogenous (AG) mill: Uses the ore itself as the grinding media.
• Semi-autogenous (SAG) mill: Uses the ore itself together with a certain amount of steel balls as grinding media.

Among these types, ball mills and rod mills are widely used in mineral processing plants, while AG and SAG mills are commonly used for large-scale grinding operations.

 

Classification by Mill Shape and Discharge Method

According to the shape of the mill shell and the discharge method, grinding mills can also be classified as follows:

• Ball mills: Include short-cylinder, tubular, and conical designs, with either overflow or grate discharge.
• Rod mills: Mainly include cylindrical overflow-discharge mills and open-ended low-level discharge mills.
• AG and SAG mills: These are generally designed with a short cylindrical shell.

The choice of mill type depends on factors such as ore properties, feed size, desired product size, processing capacity, and downstream beneficiation requirements.

 

How Does a Grinding Mill Grind Ore?

When the grinding mill rotates at a certain speed, centrifugal force is generated, causing friction between the medium and the cylinder, and the medium rotates 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 Are Mill Specifications Expressed?

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.

 

Ore Grinding Machine: 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 System?

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 Types of Ball Mill Feeders Are Used?

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

 

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.

 

 

Ore Grinding Machine: Rod Mills

What Are The Working Characteristics of The Rod Mill?

A rod mill uses steel rods as the grinding media.

Its main working characteristic is that ore is crushed and ground primarily through the line-contact action between the rods and ore particles.

Compared with ball mills, rod mills generally provide a more selective grinding action. This can help reduce excessive fine production and produce a relatively uniform product size.

Rod mills are particularly suitable for applications where:

• The feed contains relatively coarse particles.
• A relatively uniform product size is required.
• Excessive over-grinding should be minimized.
• The product size is relatively coarse compared with fine-grinding applications.

As a general guideline, when the feed particle size is below approximately 30 mm, and the required product size is not finer than about 1 mm, a rod mill may offer advantages over a ball mill, depending on the specific ore and grinding conditions.

 

 

Grinding Media

 

How Many Types of Grinding Media Movement Are There?

The movement of grinding media inside a rotating mill can generally be divided into three main regimes:

1. Cascading

At relatively low mill speeds, the grinding media mainly roll and cascade down the surface of the grinding charge.

This movement emphasizes attrition and abrasion and is commonly associated with lower-speed operation.

2. Cataracting or Throwing

At an appropriate rotational speed, the grinding media are lifted higher before falling or being thrown toward the lower part of the mill.

This movement generates stronger impact forces, which are particularly important for coarse particle breakage.

3. Centrifugal Movement

At excessively high rotational speeds, the grinding media can remain attached to the inner wall of the mill and rotate together with the shell.

This condition is known as centrifuging. Because the grinding media no longer fall effectively, grinding efficiency decreases significantly.

 

 

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:

Medium Filling Rate (%)	Appropriate Mill Speed (%)
32～35	76～80
38～40	78～82
42～45	80～84

These values should be treated as reference ranges rather than universal operating parameters. Actual optimum conditions should be determined according to mill design, liner configuration, grinding media size, ore properties, and operating conditions.

 

How Is the Grinding Media Size Distribution Determined in a Ball Mill?

There are two common approaches for determining the grinding media size distribution in a ball mill.

Method 1: Determine Ball Size According to Feed Particle Size

The basic principle is:

The ball diameter should match the feed particle size, while the quantity of each ball size should correspond to the proportion and characteristics of the feed material.

Coarser and harder feed generally requires larger balls to provide sufficient impact force, while finer feed can be effectively ground with smaller balls.

Method 2: Use the Mill Diameter as a Reference

A simplified approach is to determine the proportion of different ball sizes based on the diameter of the ball mill.

Although this method is convenient, it is only a preliminary approach. For more accurate grinding-media grading, factors such as ore hardness, feed size distribution, target product size, mill speed, filling rate, and historical operating data should also be considered.

 

Conclusion

Ore grinding machines play a critical role in mineral processing because grinding determines the degree of liberation of valuable minerals from gangue. The major types of grinding mills include ball mills, rod mills, gravel mills, autogenous mills, and semi-autogenous mills. Ball mills use steel balls and are widely used for fine grinding, while rod mills use steel rods and can provide more selective grinding with less over-grinding in suitable applications.

We have provided a basic introduction to ball mills, rod mills, and grinding media above. Choosing the appropriate mill and optimizing grinding media can improve grinding efficiency, capacity, energy utilization, and product quality. For a specific ore-processing project, the optimum grinding equipment and operating parameters should be determined through ore testing, laboratory or pilot-scale grinding tests, and process simulations rather than relying on a single general rule. For more relevant mining knowledge, please follow our blog updates.