Air classification mills are used in the agricultural, chemical, food, mineral, pharmaceutical and other industries that depend on processing bulk materials.  Alfalfa, bran, charcoal, clay, flour, gelatine, iron oxide, limestone, phenolic resin, powder coating, salt, sugar, wheat and zinc oxide are just some of the many materials that air classification mills process. While other material-handling machinery can be used for these products and materials, air classification mills have unique characteristics that make them better for certain applications.  

As they don’t generate heat during grinding operations, for example, air classification mills are especially useful for heat-sensitive materials. This is largely due to the air flowing through the machine, which keeps the material cool while being ground. In addition, due to their ability to achieve a very narrow distribution of particle sizes, air classification mills work particularly well for applications requiring a specific and narrow range of particle sizes. As per other material processing equipment, the key to maximizing performance for an air classification mill is keeping the machine well-maintained.

Maintaining Air Classification Mills to Maximize Performance

To keep an air classification mill’s performance optimal, it’s important to keep track of signs of wear, which might include damaged components, leaky seals, poorly fitting parts or worn bearings. Any damaged parts ought to be replaced immediately to prevent a knock-on effect that could lead to equipment failure. To prevent this from happening and the unplanned downtime that would result, regular inspections are necessary.

Just like any other machine with moving parts, an air classification mill also requires basic maintenance. Keeping parts sufficiently lubricated negates the grating sounds that often occur when unlubricated metal moves against metal, which often presages imminent mechanical problems. Lubrication of moving parts should be done regularly, though it should also diligently follow the manufacturer’s instructions, as over-lubrication can lead to product contamination.

Cleaning is another basic maintenance operation, which helps prevent contaminants from getting into the final product. Just a like vacuum cleaner requires regular cleaning, especially its interior, and so does an air classification mill. However, using abrasive cleaning products can cause damage, so milder cleaning solutions or detergents should be used. After cleaning, the air classification mill should be given a thorough rinsing to remove any residue left by cleaning agents to prevent contaminating bulk materials from being handled.

Proper airflow is essential in air classification. Mills’ air intakes and exhaust vents should thus be inspected frequently to ensure no blockages develop. Checking for dust and other debris will help keep these vital elements of the mill clear so that air can move unhindered through the machine. Vents should also be regularly cleared to prevent overheating that can result when airflow is blocked. Using these general guidelines, operators can easily monitor an air classification mill to ensure it performs as it should.

Best Practices for Optimizing Performance

Systematic monitoring of an air classification mill’s performance helps operators spot abnormalities or issues that may lead to bigger problems. Tracking operating parameters like airflow, grinding speed and temperature against the machine’s recommended specifications will help ensure trouble-free operation. To keep air classification mills running optimally and to achieve the correct particle sizing and classification, it’s important to consider best practices for their operation.

Operators should consider the following guidelines: 

  • Airflow: Lower airflows normally result in finer grinding with lower throughputs, while higher airflows usually result in coarser grinding with higher throughputs.
  • Feeding: Uniform feeding is desirable, as it provides for efficient operation, higher capacities and a smooth, uniform draw in amperage, while feeding that isn’t uniform achieves the opposite; to increase throughput in certain cases, a product needs to be pre-grinded.  
  • Mill shaft speed: When it comes to the pulley on the mill shaft, a smaller pulley causes the mill to run faster, producing a finer product, while coarser products are made with a larger pulley.
  • Motor speed: For air classification, mills that feature a VFD (Variable Frequency Drive) on their motor can produce coarser products with lower frequencies that slow the mill’s feed; in contrast, adjusting the VFD for higher frequencies causes the mill to move faster, producing a finer product. 
  • Screen: The difference in the size of a screen’s apertures affects particle sizes, with smaller holes producing a finer product and larger holes producing a coarser one.
  • Shutdown: Time delays will occur before an air classification mill shuts down to ensure all products are clear before it shuts down completely, so this should always be taken into account when shutting down the machine before any maintenance.
  • Start-up: There will always be a time delay between when the mill starts and when it reaches full operating speed, so operators should delay introducing the product until the mill has reached full operating speed.

Following these best practices will lead to lower mechanical stress on the air classification mill, which in turn will lead to less maintenance.

Maintenance Tips

Generally, air classification mills are designed to operate with a minimum of maintenance. However, routine inspections should be used to identify any problems, especially those that might be due to broken or worn components. Such issues can lead to real problems, resulting in downtime and suspended production if not dealt with timeously. Operators of air classification mills should pay special attention to any unusual vibrations or other uncharacteristic noises. These sounds may indicate a problem with drive belts, rotor blades, screen assemblies or other important parts.

  • Lockout procedures: No inspections or maintenance should be done on an air classification mill until rotors come to a full stop and the machine has been properly disconnected from the power supply.
  • Routine inspections: Both screening and rotating equipment should be regularly inspected, with preventive maintenance work done expeditiously to prevent unplanned downtime.
  • Rotor blade checks should be carried out more often if the air classification mill handles abrasive materials; patterns of wear vary depending on operating conditions, though visual inspection should suffice to show when changing blades is necessary.
  • Screen checks should be done often to ensure the particle size matches the required size for the final product; screens should be cleaned and maintained to ensure maximum throughput, while they should be replaced should they show too much wear.
  • Bearings: The bearing assembly is normally connected to the air classification mill’s housing by bolts and washers; before replacing and reassembling, bearings should be checked to ensure they’re free of burrs, dirt, grease or other contaminants.
  • Bearings should be replaced annually or once they reach a recommended number of operating hours, though this depends upon the manufacturer’s guidance.
  • Belts: The tension of the air classification mill’s belts should also be regularly checked, as it affects how much energy the belts transmit; belts should be re-tensioned when necessary, with the lowest tension being that at which the belts can readily operate when handling a full load.

Additionally, replacing worn or damaged blades with new blades requires that the blade directly opposite the one being replaced should also be replaced to ensure the rotor remains balanced. Blades that should be inspected regularly include the grinder rotor blade, classifier top fan blade and classifier rotor blade. Also, if one blade is found to be cracked, it’s a good idea to use a magnetic particle tester on the other blades to reveal any unseen cracking.

Scheduled Maintenance

Above all, it’s imperative to follow the manufacturer’s instructions when conducting maintenance on an air classification mill. For more than basic maintenance, it’s often more cost-effective to use professionals from the manufacturer to conduct regular inspections and perform scheduled maintenance. Many manufacturers provide preventive maintenance services for air classification mills. These service agreements differ from manufacturer to manufacturer, so what’s serviced should be ascertained before signing any contract.

Scheduled maintenance agreements should include services like: 

  • Delivering quick and factual technical support
  • Detailed preventive maintenance agreements that show services to be rendered
  • Documentation of technical issues
  • Enquiries on availability and prices of components
  • Equipment rebuilds
  • Handling returns
  • Information on the air classification mill’s warranty
  • Operating manuals for products
  • Providing spare parts
  • Replacement component ordering
  • Servicing and repair in the field
  • Shipping enquiries
  • Tracking a history of parts ordered

In addition to scheduled maintenance, it’s also a good idea to keep an inventory of the most essential components on hand to decrease the amount of unplanned downtime. Manufacturers who engage in partnerships with equipment manufacturers for inspections and maintenance experience lower operating costs, less downtime and more efficient operations.

Classification Mills Made by Prater Industries 

Prater Industries both makes and provides maintenance services to its air classification mills, as well as installation. As an industry leader in the manufacture of material handling equipment, Prater takes great pride in our air classification mill that features closed circuit grinding in two stages within a single efficient machine. For more information about our preventive maintenance program for our air classification mills and other material processing equipment, contact Prater today.