• Publication Date: 12/01/2020
  • Author(s):
    Hendrickson, Willie
  • Organization(s):
    Aveka
  • Article Type: Technical Articles
  • Subjects: Size reduction

Willie Hendrickson, CEO | Aveka Group

Growing up on a dairy farm in southern Minnesota, feed milling was a simple process. The only parameter was the mesh size on the hammermill’s screen — fine mesh for oats and coarse mesh for corn. The livestock didn’t seem to mind if a few larger-than-average chunks occasionally passed through the mill. However, many milling processes aren’t so simple, such as applications that involve chemical reactions during grinding (or mechanochemistry), nanoscale size reduction, or grinding aids. In this column, I’ll review three factors that can complicate many milling processes, including multistep grinding, a material’s resistance to grinding, and milling inefficiency.

Multistep grinding

“Simple” size reduction, in which a single stream of large particles enters a mill and a single stream of smaller particles exits the mill, has been practiced for thousands of years. Many milling processes, however, require that a single material input stream be divided into two or more material output streams.

For example, a wheat kernel, or seed, is comprised of three primary parts: the bran (the seed’s outer layer), the germ (the seed’s oil storage and embryo), and the endosperm (the seed’s starch storage). Whole-wheat flour contains all three seed parts, which makes grinding relatively simple. White flour, on the other hand, contains only the seed’s endosperm, which complicates the grinding process. Producing consistent white flour involves multiple grinding steps combined with multiple classification steps. The initial grinding step breaks up the kernels into the seeds’ three component parts. A classification step then separates those parts into three fractions with minimal cross-contamination. Further steps then grind and classify the separated endosperm to the required particle size.

A single material stream can also require multiple grinding steps. Recycling old window glass to make glass for cell phones requires that the glass be ground into a fine powder. A jaw crusher first grinds the windows down to 1-inch chunks, which can be fed into a hammermill. The hammermill grinds the glass chunks into 250-micron particles, which are then fed into a jet mill to achieve a 30-micron final particle size.

Resistance to grinding

Simple grinding also can be complicated by a material’s resistance to the process. White flour particles from a mill, for example, are approximately 50 to 60 microns in diameter, while individual wheat starch granules are only 20 microns in diameter. Grinding the flour closer to a 20-micron granule size might generate higher yields during screening processes and offer some dough-making advantages to end users, yet ultrafine flour isn’t readily available on the market. This is partly because smaller flour particles would exhibit increased explosivity characteristics, but the primary reason is that the small starch granules are extraordinarily difficult to grind. Other materials also exhibit this resistance to size reduction.

When choosing a grinding method for a new material, you must first understand the material and the desired particle size distribution. For a simple-to-grind material, an impact mill such as a hammermill or pin mill can be a good starting point. If the material is soft or pliable, adding a grinding aid, such as a solvent or flow aid, or adjusting the material’s temperature can help. For some materials, adding liquid nitrogen or carbon dioxide can help embrittle the particles and enhance grindability.

Grinding any material depends on particle defects, such as cracks, crystals, or radiation, or on mechanically-induced defects. Microcracks or defects encourage crack propagation during grinding. Teflon scrap, for example, has few inherent defects and is very resistant to grinding. However, if you first expose the Teflon to gamma radiation, the radiation creates particle defects, and the material grinds very nicely. Teflon isn’t the only material with this characteristic. Many polymers, such as polycarbonate, nylon, and polyether ether ketone (PEEK), also have a few inherent defects and are resistant to defect formation and propagation during grinding. Grinding Teflon into very fine particles is impossible, however, even with gamma radiation. As the particle size is reduced, the formation of new defects decreases, and the material reaches a grinding plateau where further size reduction ceases under those process conditions.

Grinding inefficiency

Poor energy utilization is one of the largest barriers to simplifying particles size reduction, which allows efficient scaleup and the ability to predict material grading rates. The incandescent light bulb has given way to fluorescent lighting, which is now being replaced by LED lighting. The latest hybrid car that gets 35 mpg or better has trumped my first car’s 12 mpg. It’s no surprise that grinding has seen similar efforts to reduce energy use. Yet, nearly 150 years after Peter von Rittinger contributed the first law of comminution (or grinding), grinding efficiency still hovers in the 1 to 7 percent range of theoretical expectations.

The problem is physics. In the best-case scenario, every particle-­to-particle or particle-to-mill impact would result in both breakage and new defect formation. Not every impact in a mill results in particle breakage, however. Either the impact’s energy is below a minimum particle breakage amount or the particles deform rather than break. This wastes energy, increases milling time, and, in some cases, determines the final product’s profitability. 

PBE


For further reading

Find more information about mills and milling processes in previous “Milling Mentor” columns and articles listed under “Size reduction” in our article archive.


Willie Hendrickson (888-317-3700) is the founder and CEO of the Aveka Group. Prior to starting the company, he was a researcher and technical manager in particle processing at 3M. He has a PhD in organic chemistry from the University of Florida in Gainesville and is president of the International Fine Particles Research Institute, a particle technology consortium.

Aveka Group • Woodbury, MN
888-317-3700 • www.aveka.com

Copyright CSC Publishing Inc.

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