How to Achieve Consistent Particle Size and Dispersion with Your Mill or Grinder

Angelo DePalma is a freelance writer living in Newton, New Jersey. You can reach him at [email protected].

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Consistency is the lifeblood of laboratory work. A consistent process or operation is predictable, well-controlled, and amenable to troubleshooting when things go awry. More products than one may realize depend on the consistent pulverizing, micronizing, or particle size reduction of raw materials, followed by their uniform dispersion into the product matrix.

According to Andreas Stummer, business line manager for dispersion at BYK-Gardner (Columbia, MD), sample viscosity is one of the leading factors influencing consistency in particle size reduction and dispersion. "How the material moves through the mill, and ultimately how easily it disperses within the formulated product, is critical to obtaining consistent milling results. Other factors include milling bead size, bead filling ratio, bead material, type of bead mill, and dwell time within the mill." BYK's core business covers a multitude of grinding and dispersion applications, with specialization in paints, pigments, and inks. The company sells horizontal and vertical bead mills, basket mills, plus instruments for measuring temperature, color, and other characteristics of both raw materials and final products. BYK also maintains a state-of-the-art testing lab in Wallingford, CT where it provides demonstrations and trial runs for potential customers.

Temperature factors into product quality due to the significant mechanical energy applied during all stages of milling. In keeping with the notion of consistency, milling should occur at a uniform temperature consistent with efficient particle size reduction, and which will not harm the product. Biological samples, particularly animal tissues, can be susceptible to heat damage. "We use several ways to control temperature," Stummer says. "Our basket mills, for example, are double-wall jacketed to allow water cooling."

Milling time is another factor. Users should take care to time each step involving sample contact. Milling or grinding typically involves several steps. Pigments typically exist as agglomerates, which consist of heterogeneous particles held together through chemical, mechanical, and weak attractive forces. These are first wetted, then broken down into simpler aggregates, through simple shear forces (e.g. a milling disk), before serious particle size reduction can occur. Milling reduces these to primary particles— the product—which themselves are susceptible to aggregation. Once primary particles form, additives are required to keep them suspended. "It's important to remember that the goal is particle size reduction, not particle destruction," Stummer says.

Other factors to consider during the initial predispersion step are the blade and milling vessel geometries. Sample size should also be appropriate for the vessel's volume. Operators have several work-arounds for suboptimal combinations of the above; for example, the ability to use additives to improve wetting and dispersion, and to inhibit flocculation.

Many options exist for grinding high-purity chemical or biological samples for extraction. Michael Steinert, product marketing manager for biosciences at Cole-Parmer (Vernon Hills, IL), advises purchasers to confirm that samples are compatible with whatever mill you are considering. "For smaller samples, a simple benchtop mill will do the trick or even a small hand operated solution. For a larger capacity of material, a jar mill will usually provide the most efficient solution." Among its many offerings, Cole-Parmer sells jar mills, which employ hard media to pulverize samples. As the jar rotates, particles are reduced to fine powder.

"It should not be difficult to find a jar mill compatible with your samples, but you need to make sure that the grinding media and jar are compatible," Steinert adds. "If you are using stainless steel balls you should use stainless steel jars, which come in a large number of sizes. The jar mill works not just for large quantities, but smaller batches as well, as the jar does not need to be filled to capacity to operate. "

For uniform particle sizes, Steinert suggests using a sieve shaker with appropriate sieve sizes. "This ensures the most consistency and also that you are only using the particles called for in your application. However, with whatever option you choose, make sure it meets the criteria in terms of particle size, accuracy, chemical/hardness compatibility, and batch size."

"Many options exist for achieving consistent particle size reduction," says Stummer. "It's of course possible for things to go wrong, but many of those options will lead to success."