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Technical Features

Know the Difference Between Impact and Attrition Milling to Meet Size Reduction Goals

Image courtesy of Gericke USA Rene_Meira_Medina_GERICKE.JPG
Rene Meira Medina, executive vice president, Gericke USA
Determine which type of size reduction method will meet your goals

I was touring a nutritional products manufacturing plant in South America when I watched two workers raise large chunks of frozen berries over their heads and slam them down onto the concrete floor. When the frozen pucks and chunks were finished sliding across the floor in every direction, the workers picked up the pieces and tossed them into a mixer. It certainly wasn't the most advanced size reduction system I'd ever seen. Even if we ignore the sanitary issues, this caveman-like, manual method creates a series of other problems downstream. For example, the randomized particle sizes and wide particle size distributions lead to wide variations in batch to batch quality and a fair amount of rejected product. The larger pieces may take too long to melt, preventing the mixer from achieving the required homogeneity, leaving powders and other ingredients unmixed, or they may even clog the discharge. Or, once discharged from the mixer, any remaining chunks may carry too much mass for the pneumatic conveying system to transfer the mixture downstream. Yet simply upgrading to an automated, mechanical size reduction system would not automatically solve all of these problems.

The properties of each material and the targeted particle size and shape are key factors in determining which type of size reduction method will meet the targeted goals most effectively and with the most efficiency. Aside from throwing agglomerated boulders onto the floor, the two primary methods we're working with in powder processing are impact milling and attrition milling. Often when impact milling is the right method for a given material, using attrition milling may result in negative outcomes such as particle degradation or excessively wide particle distributions, and vice versa.

Impact milling involves repeatedly bludgeoning lumps of material in violent collisions, until the particles reach the desired size and slip through a screen to discharge. One of the most commonly used types is called a hammer mill. Lumpy materials are typically fed into the hammer mill, where steel hammers or blades rotating around a central point impact the material flow over and over as agglomerates are broken down into smaller and smaller particle sizes. These milling machines are often specified for hearty materials and applications when moisture content and particle shape are of secondary importance to throughput, and when a fair amount of fine particles are acceptable. Animal feeds, metal waste chips, and glass, newsprint, and plastics for recycling are often processed on hammer mills.

However, the amount of heat hammer mills can generate, the amount of shear they apply to the materials, and their relatively high level of fine particle generation typically remove them from consideration for a variety of other applications. Further, these fines--coupled with the metal-to-metal contact between the rotor and screen--can pose a combustible dust explosion risk. For some examples, friable, sticky, oily, and fibrous materials may be more gently and effectively reduced in size by attrition milling than impact milling without compromising particle integrity. In attrition milling, an abrasive rubbing action presses the materials between two counter-rotating, metal discs. Materials enter from the infeed into the center of the discs and centrifugal force directs them outward and ultimately through the discs to discharge, ground into the desired particle size. As this method often operates at lower rates than impact mills, with cooler temperatures, and with more respect for particle sensitivity, attrition milling is commonly specified for spices, seeds, and other products where maintaining the unadulterated nature of the material is paramount.

It's certainly a challenge for a processor to know not only when to select attrition versus impact milling, but also to manage the investment in machinery to ensure access to the proper size reduction method for any material at any given time. Recent equipment advances, however, integrate both impact and attrition milling into a single lump breaking system for improved versatility. These deagglomerators use a low-speed, high-torque, rotating paddle assembly that cuts the material like an impact mill but without generating high heat or excessive fine particles.

At the same time, the material is also sheared through a screen via attrition milling but since the design concept eliminates any metal-to-metal contact, it's explosion-proof and safe from combustible dust events. Given the advances in size reduction machinery and the high value of on-spec materials, it's time we stop using the concrete floor as a lump breaker.

Rene Meira Medina is executive vice president of Gericke USA, Somerset, NJ. Founded in 1894, the company designs and manufactures a range of mixers, lump breakers, pneumatic conveying systems, and other powder processing equipment. For more information, call 855 888-0088 ext 805, email [email protected], or visit www.gerickegroup.com.

 

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