A mortar and pestle is the simplest and oldest version of a laboratory mill. Though low tech, mortars and pestles still find use in many science labs today. However, depending on the application, samples may have physical properties—for example, hard, soft, wet, dry, brittle, fibrous, elastic, or pasty—that require a specialized mill. Fortunately, for labs with special requirements, a wide range of lab mill options specifically suited to the samples in question are available.
Matching mills to samples
For hard, dry, or brittle samples, a variety of lab mills are available that use impact, pressure, and friction to reduce particle sizes [see sidebar]. Jaw crushers, which crush samples under high pressure between a fixed and a movable jaw, may be used as the first step in the process of crushing hard materials like glass and coal. These devices are commonly used in mining, metallurgy, geology, and the glass industry. Analytical mills are also commonly used to perform impact grinding on hard, brittle samples such as frozen food and dried vegetation.
For soft, elastic, or fibrous materials, particle sizes are reduced through cutting and shearing effects [see sidebar]. Mortar grinders are considered universal grinders, as they can be applied to inorganic and organic samples, either dry or in suspension. They are commonly used for homogenizing pastes and creams on a laboratory scale and are thus used prominently in the chemical/pharmaceutical industry. Bead mills are ideal for cell and tissue culture preparation. Ceramic beads work best for soft animal or plant tissues; meanwhile, metal beads should be used on harder tissues, and glass beads are the best option for processing microorganisms.
Preparing samples for grinding
Although choosing the right grinder for your sample type is crucial, getting the best grind sometimes requires a sample preparation step. Elastic or fibrous samples that do not break easily, such as rubber or plant materials, benefit from being made brittle with liquid nitrogen or dry ice prior to grinding. Cryogenic grinding is particularly useful when the application is extraction of nucleic acids, proteins, or other biochemicals, as freezing the sample turns it into a powder without altering the biochemical of interest. Wet samples can stick to mill components and block them from functioning; drying these samples prior to grinding can vastly improve the results.
These days, the world of laboratory mills extends far beyond the humble mortar and pestle. Understanding the physical properties of your samples, what you need to get out of them, and how to prepare them will help you narrow down the right lab mill for your application.
Methods of Particle Size Reduction
On its website, Retsch outlines the following mechanisms by which common laboratory mills and grinders work. Learn more at www.retsch.com/applications/knowledge-base/particle-size-reduction
For hard, dry, brittle materials:
Pressure: Force is applied between two solid surfaces. Examples: jaw crushers, toggle crushers.
Impact: Force at a solid surface is achieved by opposing particle acceleration. Examples: mixer mills, planetary mills, impact mills, jet impact mills, drum mills.
Friction: Force results from vertical pressure of one surface and simultaneous movement of another surface. Examples: mortar grinders, disc mills, hand mortars, rod mills.
For soft, elastic, fibrous materials:
Shearing: Force is between two or more solid surfaces moving in opposing directions. Examples: rotor beater mills, cross beater mills, ultracentrifugal mills.
Cutting: Force is between two or more sharp-edged surfaces, at least one of which is moving. Examples: shredders, cutting mills, knife mills.
For additional resources on mills and grinders, including useful articles and a lis of manufacturers, visit www.labmanager.com/mills-grinders