Ask an Expert – Preventing material build up during milling

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Ask an Expert: Avoiding material build up and screen blinding during milling
Q: The heat generated from our milling operations causes material to build up and harden on the mill parts and causes screen blinding. How can we prevent these problems?
Gregory J. Shemanski, Custom Processing Services, says:
There are several different cooling methods that can help you avoid material build up and screen blinding during milling operations. In an impact or mechanical grinding milling system, the traditional cooling method is achieved by cooling the airstream with liquid nitrogen. This method can be quite expensive, but if you need blanketing to prevent oxidization, embrittling, or an explosion this could be a viable method. Another method is to use cooled or refrigerated air if the heat pickup (temperature increase) during milling isn’t so great. You can even recirculate the air if the heat pickup is lower than the ambient temperature.
In both liquid-nitrogen and refrigerated-air cooling, the process also dehumidifies the air, benefitting any material that’s sticky or has hygroscopic tendencies. You may have to consider reducing your material’s feedrate into the mill so your work input (horsepower) is lower than your cooling capability. This method is less economical, but sometimes it’s the only way to handle heat-sensitive materials in a mechanical grinding application.
Some materials, including precipitated silica, requires a grinding aid or flow additive to prevent buildup in a grinding unit. Be sure to consult your formula chemist or applications manager to determine which flow agents can be added without detrimental effect. Keep in mind that additive percentages can range from 0.1 to 5 percent or greater, depending on your application. Another method to combat material buildup is to introduce a much higher amount of additional ambient airflow through the system, helping to keep the material cool.
You may also want to consider using a classifier mill because this equipment doesn’t use a screen, which can be prone to screen blinding and material buildup. A classifier or universal mill uses air to carry the material through the milling and classifying process. When the material reaches the desired particle size, it passes through the classifier wheel.
There have also been some ingenious mill designs in which the mill’s grinding chamber is clad with a jacket so cooled liquids can be passed through the jacket to keep the mill cool. By jacketing the grinding chamber you can increase the work input, which results in a higher throughput rate. However, if you’re grinding an abrasive material and your mill is lined with ceramic or urethane tiles, it’s more likely that the lining will act as an insulator and keep the heat in.
When grinding coarse, heat-sensitive materials, adding dry ice to the mill can be effective. Consult with your mill manufacturer to make sure the unit is constructed of an appropriate material, such as stainless steel, which can withstand cold temperatures.
Fine material grinding in a mechanical system can create a very high work input per unit of product. If you would like to avoid this problem, another method is to use nonmechanical grinding, such as an air jet mill. For producing even finer powder materials with particles less than approximately 250 microns, consider using a jet mill.
Compressed air, which is the motive force for accelerating particles in particle-on-particle collisions in a jet mill, can be cooled to temperatures lower than the material’s softening point. For example, a typical plant air compressor should be able to produce cooled air through its after-cooler at temperatures of no more than 5°C to 10°C (41°F to 50°F) above ambient temperatures, which are well below typical material melting temperatures. This cooled airflow through a jet mill keeps the entire system and all the surfaces at or near the compressed air’s initial temperature.
If you have materials that are especially prone to reagglomerating or plating out (when a material layer coats parts of the mill’s grinding area) on the inside of the mechanical units, a fluidized-bed jet mill works well.
Powder and Bulk Engineering – PBE-News – June 2015