Understanding a mill’s operating conditions and how to adjust them ensures optimal results.

March 22, 2022

7 Min Read
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Bepex Pulvocron air-classifying mill utilizes high-speed impact and internal particle size classification to produce fine powders with a narrow size distribution. Image courtesy of Bepex International

Seth Urtel, product specialist, Bepex International

Designed for fine-grinding solids, an air-classifying mill is a high-speed impact mill with internal classification. Its unique design allows for adjustment in real time to change particle size or to correct particle size back to specifications as feed properties change or the internal components wear. 

Operation

While there are differences in design from one manufacturer to another, the mode of operation generally remains the same. The mill’s adjustable operating conditions, including tip speed, airflow, and classifier speed, provides control over resultant particle size, material temperature, and capacity. 

The feed material entering the grinding chamber is controlled by a feeder that meters the incoming product at a steady, controllable rate. Material is thrown to the periphery of the grinding chamber where it is impacted between rotating beaters and a stationary liner. This continuous impact produces several smaller particles from each feed particle. The internal classifier then sorts the ground particles, allowing smaller particles to leave the mill as finished product. The larger particles, as determined by the application, are returned either by gravity or process air back to the grinding chamber for further reduction.

Adjust Mill Speed and Feed Rate for Optimal Results

The mill speed refers to the tip speed of the rotating beaters, hammers, or grinding implements. Tip speed is one of the first areas of consideration when adjusting your operation. The tip speed controls the means of energy imparted into the particles to be ground. In any operation, it is critical to operate at the lowest required speed to achieve the required particle size.

When the mill speed is not adjustable, whether for mechanical or operational reasons, the feed rate can be adjusted to deliver similar results. Typically, lowering the solids feed rate to the mill will result in a finer grind, and increasing the feed rate will result in a coarser grind.

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Importance of Airflow for Uniform Particle Size

An airstream carries the material through the grinding and classifying areas of the mill and ultimately to the product collection device, typically a cyclone or baghouse. The volume, velocity, and temperature of the process air play an important role in the overall operation. The volume delivered will have a direct correlation with solids capacity. The velocity at which the air is traveling will directly impact the particle size. The process air may be conditioned (heated or cooled) to either simultaneously dry and mill wet cakes or pastes, or cool heat-sensitive materials that are prone to melting or degradation.

Adjusting Air Classification

The mill’s classifying section is ideally designed as a forced-vortex, and operates on the principle of air drag versus centrifugal force. In operation, the air sweeps through the grinding chamber and conveys the product to the classification section. The product stream is forced inward toward the discharge while the classifier imparts a rotational motion on the stream. The air drag force-centrifugal force phenomenon makes the separation.

For the larger particles, the centrifugal force overcomes the air drag. As a result, larger particles are thrown outward and recycled through an integral return or by gravity back to the grinding chamber. For the smaller particles, the air sweep overcomes the centrifugal force; thus, the smaller particles are carried out by the air to the cyclone or baghouse separators as final product.

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How to Adjust the Mill to Obtain Desired Particle Size Range

Many factors can affect the fineness of the desired grind. The key contributors include:

Beater Plate (Tip) Speed

The main determining factor of product size is the hammer/beater tip speed, controlled by the rotational speed of the beater plate. A higher mill speed creates more energy available to be put into the particles and this results in a finer grind. On the other hand, lowering the hammer tip speed imparts less energy on the particles and creates a coarser grind.

In an ideal setup the mill speed will be set as low as possible to obtain the desired particle size. This will allow maximum feed rate through the mill while maximizing available horsepower. The lower speed will also help reduce wear of beater plates and liners.

Classifier Speed

The speed of the classifier determines the “gate” at which particles of a certain size are accepted or rejected. This has its biggest impact on the top end fineness of the overall particle size distribution. Faster classifier rotation results in more large particles getting redirected for further grinding in the main grinding chamber. Ideally, the classifier is set to the top end of the particle specification without over-recycling and reducing overall throughput.

Air Flow

Air flow also plays an important role in an air-classifying mill, serving as the primary means of moving the material through the milling system. There will be a minimum airflow required to ensure proper conveying velocities throughout the system, as well as modulating temperatures. Increasing airflow pulls the particles through the grinding chamber faster, allowing fewer impacts between the hammers and liner, resulting in a larger particle size. A decrease in airflow will produce a finer grind since the material will pass through grinding chamber slower, allowing for more impact points thus milling the particles finer.

Feed Rate

Increasing feed rate will typically produce a coarser product. Decreasing feed rate will produce a finer product. This adjustment can be used only if classifier and mill motor amperages allow.

Clearance

The clearance, or space, between the rotating hammer tips and the stationary liner also plays an important role in final particle size. As the clearance gets larger (more space between the hammer tips and the liner), the particle size will become coarser. Over time, the clearance will grow due to component wear. As this happens, milling efficiency will drop. This may require an increase in hammer tip speeds, lower feed rate, or replacing the worn components.

Understanding the Cause and Effect of Adjustments

Each change made to a single variable may enact one or a series of changes to the process. When dialing in the optimal performance, it is critical to understand the role that each variable plays, and the resulting impact that its modification will have on the rest of the process. 

Though it’s posited here, it’s rarely as easy as saying, “The particle size is too large, so increase the mill speed and you’ll be all set.” Increasing the mill speed increases the amount of energy put into the particles, in turn increasing the heat generated on the product stream. Processing a heat-sensitive material at too high of a temperature can have a detrimental impact on the quality of the finished product.

Adjusting airflow has the greatest impact on the operation. It is a balancing act. Reducing airflow may produce the correct particle size. However, it may also overload the classifier, or increase material temperature. The result is an inefficient operation that delivers reduced throughput  and potentially increased component wear from abrasion.

It is important to work with a trusted and experienced manufacturer who understands each variable and the resulting impact on the process. Consult your equipment supplier for expert advice on how to adjust the machine for optimal results. The supplier will also provide the training necessary to overcome day-to-day challenges.

Adjustment Summary

Particle size too large

1. Increase mill speed.

2. Increase classifier speed.

3. Reduce air flow.

4. Decrease beater plate and liner clearance.

5. Install additional beater plates, if allowable.

Particle size too small

1. Reduce mill speed.

2. Decrease classifier speed.

3. Increase air flow.

4. Increase beater plate to liner clearance.

5. Remove beater plate(s), if multiple installed.

Summary

Air-classifying mills are key to producing fine particles with a narrow size distribution, as small as three to five microns. Adjustable operating conditions, including tip speed, air flow, and classifier speed, provide control over resultant particle size, material temperature, and capacity. Understanding operating conditions and how to adjust them will ensure reliable and consistent results.

Fine-Tuning Guide

Bepex provides a guide to fine tuning air-classifying mills. The guide addresses common issues including particle size being too large or small, excessive component wear, and early component failure. Visit www.bepex.com/better-milling

Seth Urtel is product specialist, Bepex International. Since 1897, Bepex has served the global food, chemical, and polymer markets by providing process development services and custom-designed industrial scale process systems and equipment. With an array of proprietary platform technologies, including thermal, size reduction, compaction/agglomeration, mixing and blending, and mechanical dewatering, Bepex custom designs each piece of process equipment to the exacting requirements of each process and customer, reducing time to market and increasing processing efficiency. For more information, visit www.bepex.com


Image1:

Air-classifying mills can grind solids down to as small as three microns, making them ideally suited for food, chemical, and mineral processing applications.
 

Image3:

A common air-classifying mill setup utilizing two beater plates and an internal classifier. 

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