Air Classifying Mill Operation Process

The air classifying mill is an indispensable industrial pulverizer for the bulk solids processing industry. The overall design consists of a vertical, two-stage, closed-circuit grinding chamber and an interstage air classifying as a unit.

The air classifying reduces the size of the material by impact with the high-speed grinding rotor. The screen frame is precisely mounted around the grinding rotor. It contains a screen and serrated jaws that provide additional shear and impact to accelerate the particles. An independently controlled internal air classifying separates out-of-spec particles and recirculates them back into the grinding chamber for further size reduction. As a result, air classifying mills are superior to single-pass impact mills for applications with difficult to grind products or where a narrower particle distribution is required.

l Operating Procedure

l Benefits

Operating Procedure

Our air classifying mills operate in an induced air stream provided by a vacuum conveying system. This air is essential for product transport through the grinding process and for the acceleration and dispersion of the particles used for classification. During operation, the material is processed in three stages: first stage grinding, classification/re-circulation and second stage grinding.

During the first stage of grinding, material and air enter the primary side of the grinding chamber where the particles are impacted by the rotor and accelerated outward, striking the screen and jaws. The screen and jaws contribute to the further size reduction process by causing rapid deceleration, shearing and rebound into the rotor path. This process continues until the particles are small enough to pass through the screen with a high probability.

The next step is sorting and recirculation. Once out of the grinding chamber, the particles are rapidly fluidized by secondary air introduced tangentially from the bottom of the mill. As the particles accelerate toward the classifying wheel, their resistance in the air stream encounters a reverse centrifugal force determined by the RPM of the wheel. This centrifugal force classifies the particles according to size, where larger particles with greater resistance are repelled back into the grinding chamber, while smaller particles with less resistance leave the machine.

The final step of the process is the second stage of grinding. Rejected particles re-enter the secondary side of the grinding chamber, which consists entirely of serrated grinding plates that provide more aggressive particle cutting. As with primary grinding, the particles are impacted and accelerated by the rotor, then sheared and decelerated on the grinding discs. From here, air conveys these particles back to the primary grinding chamber where they are again discharged through the screen.

Benefits

Some materials in the graded mill may be less friable and therefore more difficult to grind. In other cases, downstream processing may require a narrower particle distribution. The main advantage of air classifying mills is the ability to grind non-friable materials or to meet narrow particle distribution requirements. Unlike other designs, air classifying mills can meet these requirements without extensive disassembly and reconfiguration. Precise adjustment is as simple as the electronic speed control of the air classifying wheel.

Our Air Classifying Mill features a convenient one-piece housing design with large access doors for easy cleaning, quick inspection and simple maintenance. Its horizontal shaft orientation and outboard mounted bearing assembly provide a clean interior and simple disassembly compared to the vertical shaft design. The mill runs in air purge for lower operating temperatures and lower temperature rise.

As with any grinding plant or equipment, safety is of paramount importance. To prevent any possibility of industrial accidents, our air classifying mills are equipped with an automatic door locking device that is activated while the rotor is running. They also include a premium shaft grounding brush and grounding system to prevent electrostatic discharge in the grinding chamber. Customers also have the option of a pre-engineered bearing condition monitoring system to enhance their existing predictive maintenance practices.

conclusion

We are ready to meet your needs. If your application requires a custom build, our staff will customize a solution for you. Likewise, if you already have an air classifying mill that needs parts, service or upgrades, contact us for a free consultation.