Milling and grinding are common operations in the manufacture of foods, chemicals, materials and other products, and equally important at the laboratory scale for analyzing those products, quality control of large processes or preparing samples for analysis.
Grinding may seem low-tech, but after thousands of years of confronting particle size reduction problems, engineers have still not fully characterized the process mathematically. Engineers attempt to predict grinding and milling behavior through a patchwork of three grinding equations: Kick’s law (for particles larger than 50 mm in size), Bond’s law (about 50 mm down to 0.05 mm) and Rittinger’s law (below 0.05 mm). Today, academic institutions devote millions of research dollars to grinding and milling; one example is the Rutgers University (New Brunswick, N.J.) Dept. of Chemical Engineering’s Mixing in Fluids and Powders Group, headed by Prof. Fernando Muzzio.
Grinding and milling remain empirical sciences, says Stanley Goldberg, director at grinding machine distributor Glen Mills (Clifton, N.J.), who prefers the scientific term “comminution” to “size reduction.”
“Since it’s hard to predict mathematically how a grinding machine will behave, you have to prove it works on the actual material, with the customer’s own sample,” says Goldberg. Before customers purchase an instrument from Glen Mills, they typically put it through its paces on actual samples, either at their facility or at the vendor’s.
Patricia Jung, president of particle reduction instrument firm Retsch (Newtown, Pa.), has seen a lot of interest over the past several years from plastics, electronics, pharmaceutical and food companies, and in particular from the renewable energy and nanotechnology industries. Problems with human and pet food supplies have spurred the need for strict quality control measures that involve grinding meal for subsequent analysis. “Animal feed companies have to test every shipload, particularly when it comes from overseas,” Jung told Lab Manager.
The need for strict particle size control in biofuels and nanomaterials is a relatively recent trend as well, as both industries are in their infancy. Nevertheless, most industries are abandoning manual and other legacy grinding techniques in favor of higher-precision methods that deliver consistent, reproducible results, Jung says.
Pharmaceuticals is another sector where attention to particle sizing is evolving rapidly. Drug firms traditionally micronized active pharmaceutical ingredients to improve blending in pills, tablets or liquid formulations, and to alter how the drug is absorbed. Today, pharma is much more concerned with fine-tuning properties of drugs and other ingredients, to the point where pharmaceutical formulation and nanotechnology overlap. Pharmaceutical quality and development scientists increasingly use particle reduction of raw materials to create suspensions and even solutions.
“Do not disturb!”
Maintaining sample integrity is a recurrent theme in milling and grinding, particularly with mechanical milling. The “coffee grinder” approach to mechanical size reduction works with relatively inert materials such as stone, but introduces heat- and shear-related anomalies for foods, pharmaceuticals and many materials.
One way around this is through jet mills, which propel samples around a chamber at the speed of sound, reducing particle size continuously through high-speed collisions. Since jet mills use no moving parts or screens and generate little heat, they work exceptionally well with heat-sensitive materials.
Cryogenic grinders, also called freezer mills, process materials that are first rendered brittle by exposure to liquid nitrogen, then pulverized. Freezing samples before milling maintains chemical integrity while creating powders from virtually any material. Applications include biotechnology, materials, chemistry, geology, DNA extraction, plant research and pharmaceuticals. “The more brittle the material gets when frozen, the finer you can grind it,” observes Jung.
Cryogenic milling is possible with any grinding mechanism (e.g., high-speed rotor, impact ball and planetary ball mills) by employing a separate liquid nitrogen bath. Retsch claims to be the only vendor that offers a cryogenic mill that is directly connected to a liquid nitrogen reservoir, a factor that improves safety and convenience.
Smaller and smaller
Nanotechnology has played a huge role in industry’s appreciation for smaller particles. “As particles approach the 500 nm scale, their color, mechanical and electrical properties change dramatically,” observes Goldberg. “Classic size reduction equipment can’t achieve that level of fineness, at least not economically.” Nanotech and stringent demands for particle characterization have driven demand for bead mills that, according to Goldberg, economically achieve nm-sized particles with predictable size distributions.
The rise of nanotech drives two additional trends related to quality and analysis. Materials of construction for milling instruments have become “very critical,” Goldberg says, “because contamination from the instrument is unacceptable” in drugs and high-tech electronic or optical nanotechnology. Vendors are therefore moving toward inert product contact surfaces; for example, high-strength zirconia ceramics.
Similarly, nanotech has created an extraordinary need for analysis of very small particles and monitoring of the grinding process itself, particularly in regulated industries. “When the field first developed bead mills that could create nanoparticles, no one had instruments that could characterize them,” says Goldberg. Today, advanced grinders/millers enable capture of data, related to the milling process, which comprises the quality documentation that accompanies regulatory applications or data sheets.
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. A full-time freelance writer for more than 20 years, DePalma has written nearly 2,000 trade magazine articles on pharmaceuticals, biotechnology, materials and supporting industries. You can reach him at firstname.lastname@example.org.