Air Classifying Mills

Grinding applications can pose particular challenges. Materials that are difficult to grind or require a narrow particle distribution require specialized milling solutions. The unique capabilities of the Prater air classifying mill (CLM) are designed to produce a finer grind and a narrower particle size distribution than standard mill designs. The Prater CLM mill is suitable for particle size ranges from 149 microns to a mean size of 5 to 7 microns.

Prater Air Classifying Mills (CLM) offers the combination of two-stage closed circuit grinding with internal air classification. Their unique capabilities out perform single-pass mills when handling difficult to grind products or those requiring a narrow particle size distribution. The Optional screen-less screen frame design reduces screen damage and wear while the secondary inlet provides additional air for an element of cooling.

Six sizes are available from the 3 HP CLM-18 pilot scale lab mill to the 250 HP CLM-101. All Air Classifying Mills come with standard safety features, including an automatic safety door interlock that prevents unintended access to internal rotating parts during operation.

Benefits of Prater CLM mills include:

• Final particle size range from 149 microns down to a mean size of 5 to 7 microns
• Durable, welded carbon steel or stainless steel construction
• Precision tolerances between rotor blade and grinding jaw or screen for uniform size-reduction
• Two-stage, closed circuit grinding for ultra-fine size reduction
• Interstage air classification for precise, on-line control and adjustment of particle size
• Large screen to horsepower ratio for greater capacity in each model
• Interchangeable screen & jaw combinations for more versatility
• Large access door and simplified interior for easy cleaning and maintenance
• Grinding blades, screens and jaws designed for easy replacement with minimal tools
• Outboard mounted spindle with high grade labyrinth seals for maximum bearing life
• Reversible rotor
• Standard-shaft grounding brush mitigates against internal electrostatic discharge
• Optional bearing monitoring package to compliment existing predictive maintenance programs
• Ten-Bar shock-pressure-resistant construction available
• Sanitary construction features available

Customers will appreciate the following benefits:

• Capability to grind challenging materials
• Narrow particle size distribution made easy
• Highly uniform particle distribution
• Proven lower ∆T, temperature rise of the material from input to output, versus competitors
• Enhanced product cooling from use of secondary pneumatic airflow
• Minimal and quick maintenance
• Optional screen-free design to ensure minimal wear and tear

Processing of hard-to-grind materials takes place via three successive steps in a Prater classifier mill.

• First Stage Grinding
• Classifying stage
• Second Stage Grinding

First Stage Grinding:   Convey air and metered product enter behind the rotor, where grinding blades impact and accelerate particles outward. Additional collisions take place against the jaws and screens until the particles are small enough to egress through the screen’s apertures.

Classifier:   Particles from first stage grinding are circulated outside the grinding chamber, where a secondary intake drafts pneumatic air to help fluidize and cool the particles. Material is pulled inward toward the classifying rotor, where size separation takes place. On-size particles pass through the rotor and are pneumatically conveyed to the next stage in the process while off-spec particles are rejected to second stage grinding. The air classifier rotor is independently controlled, allowing for precise control of particle size simply by adjusting the RPM using a Variable Frequency Drive, VFD.

Second Stage Grinding:  Rejected particles from the classifier re-enter the grinding chamber in front of the rotor. Grinding blades again impact and accelerate the particles outward, where size reduction is intensified by 360 degrees of aggressive grinding ring segments. Reground, second stage product combines with product from the first stage and then returns to the classifier.

The accompanying schematic illustrates how Prater Air-Classifying Mills, which are always integrated with a pneumatic conveying system, can be integrated into your total operating system:

Prater’s Air Classifying Mills operate by using the combination of two-stage closed circuit grinding/milling with an interstage air classifier all in one convenient unit. The internal air classifier continuously recirculates particles that are larger than desired back into the grinding zone ensuring an optimal final product particle size. When faced with materials that can be difficult to grind or that require a narrower particle distribution curve, the unique capabilities of the air classifying mill are often necessary. Large access doors on the air classifying mills allow for quick inspection or cleaning of the main rotor and classifier rotor.

Our air classifying mills are ideal for difficult to grind applications or narrow particle distribution curve requirements, and offers simple maintenance, uniform particle distribution, screen-less screen frame that prevents screen damage and wear and, lastly, a secondary inlet to provide additional air for an element of cooling. At Prater, safety is always first and all air classifying mills and fine grinders are equipped with an automatic door locking device to prevent accidental access to the inside while the rotor is operating. When using our Prater air classifying mills, three distinct steps are taken to process the material:

Initial Grinding Stage: Using the secondary inlet, additional air adds a cooling element to the material being processed.
Particle Rejection Stage: Particles that are larger than specified are rejected by the classifier and directed into the third stage.
Final Stage: These particles go into a separate part of the grinding rotor designed specifically for difficult-to-grind pieces. This material is then directed to the classifier, which allows only properly-sized product to exit the machine.

Prater stands ready to serve your needs. If your application requires a custom build, our staff will tailor a solution. Similarly, if you already own a Prater mill that requires parts, service, or an upgrade, please contact us for a free consultation.