Ball milling is a grinding technique for crushing material into finer particles and blending it thoroughly.
It is a versatile process that is used in various industries, ranging from pharmaceuticals to mining industries.
But how exactly is the ball milling process carried out? What are its types? And when do we use it?
This article provides a detailed guide on the ball milling process, its different types, advantages, applications, and a comprehensive comparison between wet and dry ball milling.
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What is Ball Milling?
Ball milling is the process of grinding a material into a fine powdered form. It consists of an enclosed cylindrical drum containing multiple balls made of ceramic, stainless steel, or flint. Rotating the drum results in the collision of balls with the workpiece material, breaking the material into fine powder.
The drum has an abrasion-resistant inner surface to prevent damage during the milling operation.
This process is generally used for grinding nanotubes, pyrotechnics, ceramics, etc.
Before feeding the material to the ball mill, it is crushed in large mechanical machines called crushers.
After that, the crushed material is delivered inside the ball mill drum at a uniform rate via an air suction system or a rotary screw conveyor.
The drum is tightly sealed and rotated around its central axis by a powerful motor.
This rotation causes the steel balls to rotate, cling to the inner drum surface, and fall after reaching a certain height when the gravitational force exceeds the centrifugal force.
These balls fall onto the crushed material, resulting in an impact force that breaks down the crushed material into finer particles.
Moreover, relatively different movements of the steel balls and the material cause them to collide and rub against each other, resulting in attrition, which also aids in grinding.
After thorough grinding, the material passes through a grater, which blocks all over-sized particles to ensure that they are adequately ground before passing through.
A classifier machine ensures that such particles are sent back to the ball milling drum for further grinding.
Apart from that, some industrial ball milling machines consist of different chambers with gradually decreasing ball sizes.
The ground material passes through a grater and enters successive chambers until the desired size of the ground particles is achieved.
This final output, received in a powdered form, is referred to as a fine grind.
Process Parameters That Affect Ball Milling Operation
The ability of a ball mill to grind different materials depends upon various factors.
Composition of Milling Balls
Milling balls are usually composed of stainless steel, hardened steel, or tungsten carbide.
For effective grinding, the composition material should be denser than the material to be milled.
Moreover, higher-density balls with a greater diameter have a higher impact force, which leads to better grinding.
Ball to Powder Ratio
The ball-to-powder ratio is the ratio of the weight of balls to that of the material to be milled.
Depending upon the application, this ratio can fall anywhere between 1:1 and 220:1.
Occupied Milling Drum Volume
Approximately half of the drum space is filled with the material and milling balls, so that sufficient space is available for them to move around freely.
Overfilling the drum can lead to inefficient milling while increasing the risk of wear on the milling balls and the drum's inner surface.
Ball milling involves the gradual breakdown of large chunks of material into fine powdered form.
This process requires a specific amount of time to achieve the desired results with no or minimal wear of the equipment.
A shorter milling time can lead to inefficient milling, whereas milling the material for a prolonged period can chip the milling balls and the inner drum surface, contaminating the milled material.
It takes approximately 2 to 4 hours for a typical ball milling process to be completed.
A ball mill consists of a rotating drum that exerts a centrifugal force on the milling balls, which then collide with the material resulting in the desired breakdown of the material.
The critical speed of a ball mill is the speed at which the centrifugal force equals the weight of the balls, overcoming the force of gravity.
Generally, optimal milling speed is considered to be around 70% of the critical speed of the ball mill.
A higher speed will force the balls against the walls of the drum without producing the desired grinding effect, while a lower speed will result in inefficient grinding.
Parts of a Ball Mill
A ball mill consists of various parts that function in synchronization to execute the milling process.
A ball mill consists of a cylindrical steel vessel called a milling drum which houses milling balls and the material to be milled.
The length of the drum is approximately 1.5 times its diameter.
The milling balls are usually 2 to 4 inches in diameter. They form about 20 to 30 percent of the drum volume.
They are commonly composed of stainless steel, tungsten carbide, or hardened steel.
The ends of the drums are connected to supports via bearings. Usually, large-size trunnion bearings are used, but some ball mills also use roller bearings.
It is important to lubricate the bearings regularly to ensure proper operation of the process.
The drum's inner surface, also known as armor, consists of individually fastened plates made of abrasion-resistant material like manganese alloy steel.
These plates are fastened to the drum body via studs and nuts.
The armor prevents the drum from abrasion caused by the sharp edges of the material being milled.
A 3-phase synchronous motor is connected to the ball mill drum for rotating it at the required RPM during operation.
This motor usually contains a variable speed drive which changes and regulates its RPM.
A gearbox links the motor with a large ring gear attached to the drum, thus transmitting motion at the required torque.
Inlet and discharge
The inlet section of a ball mill drum delivers the crushed material from the crusher to the drum, while the discharge section moves the finely ground material through a grater into a classifier.
Both these sections are cone-shaped and use an air suction system or a screw conveyor for moving the material.
Advantages of Ball Milling
Ball milling is a fast process that can grind several kilograms of crushed material into fine powdered form in a few hours.
Hence, it suits batch (intermittent) and continuous industrial processes.
This process applies to several materials, like metallic ores, polymers, cement, etc.
Moreover, materials with any degree of hardness can undergo this process, making it a popular grinding process in almost every industry.
The installation and running costs are low and feasible for medium-scale to large-scale industries.
Depending on the size and capability of the mill, the initial cost of the equipment can vary. However, the cost of equipment is comparatively cheaper than other grinding processes with similar capabilities.
Moreover, the operation of a grinding mill is simple, eliminating the need for a specialized operator, and the ease of maintenance further reduces the overall cost associated with ball milling.
Easy To Use
The process of ball milling is comparatively easier than other milling or grinding operations as it does not require continuous monitoring or complex job-holding techniques.
Once familiarized with the process, the operator can individually execute the process with no or minimal guidance.
Ball milling is an efficient grinding process that can be used for grinding particles of different sizes.
Reducing the ball diameter increases the surface area to volume ratio, which effectively increases the impact energy due to collision between the milling balls and the material.
This provides the ability to grind the material into extremely fine particles (nanoparticles) by using balls of different diameters.
Applications of Ball Milling
Ball milling is a versatile technique that can be used for grinding almost any material by using milling balls with a comparatively higher hardness index than the material to be milled.
This high material flexibility leads to its application in various industries.
The pharmaceutical industry employs small-scale ball milling for grinding powders or suspensions required for different syrups and tablets.
This industry is the largest user of the ball milling technique which employs this method for grinding metal ores and rocks to separate the debris from the mineral content.
Moreover, coal undergoes ball milling to enhance its efficiency for power generation.
The construction industry uses ball milling for grinding raw materials like sand and lime before they are processed into construction materials like bricks.
The ceramic industry uses ball-milled materials for manufacturing products like pots, vases, tiles, etc.
Apart from that, a small-size ball mill can also be used for smoothening and polishing rocks for decorative artifacts.
Various chemical solutions, like window cleaning solvents, bleaches, and color dyes, are manufactured from raw materials that have undergone ball milling.
Apart from that, ball mills are also used for grinding of nano-particles that are used in semiconductor devices.
Types of Ball Mills
The ball mills are classified based on different factors.
Based on Configuration
There are four types of ball mills based on their configuration.
Horizontal Ball Mills
This type is the most commonly used industrial ball mill. It comprises a horizontally oriented drum and is generally larger than other types.
Due to its large capacity and greater durability, it is the most commonly used ball mill configuration in large-scale industries.
Some common industries that use horizontal ball mills include the power generation industry, mining industry, ceramic industry, etc.
Vertical Ball Mills
These ball mills consist of a vertically oriented cylinder. This type is rarely used in the industry due to its limited capacity.
However, it is ideal for industries that require small or medium-scale processing due to its low cost.
Vibratory Ball Mills
These mills usually consist of a mass imbalance within the drum which causes vibration during drum rotation, which in turn causes more effective attrition.
Another configuration consists of a vertical cylinder intermittently moving up and down, causing vibration during rotation.
These ball mills are very expensive and have high running costs due to high electricity consumption.
Nonetheless, their high durability and efficiency make them ideal for material processing industries.
Reactive Ball Mills
This type includes the introduction of hydrogen gas which reacts with the material and enhances the attrition process.
These mills are similar to horizontal ball mills, but the additional requirement of hydrogen gas increases their running costs.
Although less common, they are also used in different industries like mining.
Based On Load Capacity/Size
There are two different types of ball mills based on the load capacity aspect.
Industrial Ball Mills
These ball mills have multiple drums interlinked to ensure thorough grinding and enable bulk production. This type can have any configuration or orientation.
Their high cost and greater space requirement make them more common in large-scale industries.
They are commonly used in factories or manufacturing plants, e.g., ceramic factories, cement plants, etc.
Planetary Ball Mills
This mill is a portable and lightweight modified version of vertical ball mills containing small milling balls.
Their portability and cheaper overall cost make them useful in research laboratories or small manufacturing plants.
Materials sciences laboratories often use these machines for teaching and experimentation purposes.
Based On the Discharge Mechanism
There are two types of ball mills when considering the discharge mechanism.
This type consists of at least one steel grater, which acts as a sieve and only lets finely powdered particles through the outlet section.
The remaining oversized particles are forced to undergo milling until they are adequately ground.
This configuration is commonly found in all horizontal ball mills. Other configurations like the vibratory and vertical mills also use similar mechanisms to filter large particles to grind them further.
Generally, this modification is found in all modern ball mills.
This ball mill type allows a natural overflow of finely powdered material through the outlet section when the material volume exceeds.
Its mechanism is analogous to how water overflows from a tub after filling it up.
This type does not use a suction system which cuts down the cost, but this makes this ball mill more time-consuming due to a slower rate of material transportation.
These mills are rarely used in the industry nowadays.
Wet Ball Milling Vs. Dry Ball Milling
Wet ball milling utilizes a slurry of water or anhydrous ethanol to move and carry the milling material in and out of the drum. In contrast, dry ball milling does not use any liquid medium.
|Parameter||Wet Ball Milling||Dry Ball Milling|
|Electricity consumption||Relatively lower||Relatively higher|
|Applicable Materials||Insoluble materials||All materials|
Difference between wet and dry ball milling processes
The power consumption in wet ball milling is lower due to more efficient grinding and blending even at lower RPMs.
Wet ball milling is suitable for insoluble materials like copper ores, iron ores, phosphate ores, etc., whereas dry ball milling is preferable for materials vulnerable to water contamination, like cement and marble.
Dry ball milling has a noisier operation, and the finely powdered material might leak from the drum inlet and exit ports and diffuse into the surrounding air, causing air pollution. This is not the case in wet ball milling.
Furthermore, the slurry in wet ball mills absorbs the heat dissipated during the milling process, ensuring that the heat is not released into the surrounding air.
Based on these factors, wet ball milling is comparatively more eco-friendly than dry ball milling.
However, the presence of a liquid medium results in various maintenance issues, increasing the overall cost of wet ball milling.
Frequently Asked Questions (FAQ)
How is a ball mill cleaned?
A ball mill is cleaned by pouring an alcohol-based cleaning solvent into the milling drum and rotating it for about 5 minutes. This step is repeated about three times, after which cotton or cellulose paper is used for drying and wiping the inner surface.
Can glass undergo ball milling?
Yes, glass can undergo ball milling, but the contamination of air with glass dust can cause static discharge. Therefore, it is important to follow the safety procedures when ball-milling glass.
What is the capacity of industrial ball mills?
Industrial ball mills usually have a capacity of 10 to 25 tons per hour.