Milling Methods Explained — From Coarse Grinding to Micronization
Justin Klinger, Nov 23, 2020 7:00:00 AM
Even though most folks don’t think about it, in many manufacturing contexts, particle size and distribution can have a critical impact on everything from ingredient costs to final product quality.
Particle size reduction technologies also span a broad range, from the simplest methods of applying force, like jaw crushers, to advanced technologies that combine the use of gases, liquid slurries, and specialized milling media to achieve nanoparticles.
So how can you know which grinding method is right for your product or process? It’s not just a matter of matching material to mill — though that’s a good place to start.
Let’s take a look at a range of milling methods, from coarse crushing to wet grinding, to get a better idea of which techniques might be right for your materials, end uses, and particle size goals.
Primary Crushing & Coarse Crushing Methods
Jaw crushing is typically the most basic means of size reduction — in fact, mining and quarry operations typically have jaw crushers on site, where they are used to reduce large chunks of material to smaller chunks.
As the name suggests, this type of mill consists of two jaws, one fixed and one moving. The moving jaw compresses material against the fixed jaw using tear, shear, and impact forces to cause material to fracture. Jaw crushers are used on a range of industrial materials:
- Stone and gravel
- Ceramic and glass
- Construction aggregates
- Minerals and metal ores
The heavy-duty nature and simple action of jaw crushing lead to particle size ranges of 0.5mm and larger, depending on hardness and brittleness of the material. Jaw crushers are not suited for rubbery, sticky, or wet materials. If finer gauges are needed, additional grinding is called for using other milling methods.
Impact crushers also often serve in a primary crushing capacity. While their design is different from jaw crushers, they apply similar forces to materials inside their grinding chambers but with better control and smaller final particle sizes. Impact crushers tend to produce more cubical-shaped end results (think gravel) and fewer needle-like particles than jaw crushers.
Impact crushers are sometimes used as a second crushing step after jaw crushing. Materials suited for impact crushing include limestone, coal, gypsum, other minerals, and medium-hard, less-abrasive chemicals.
Milling Methods for Mid-Gauge to Ultra-Fine Particle Size Ranges
In a roller mill, material moves between pairs of counter-rotating cylinders, which exert crushing and shearing forces. The gap between rolls is adjusted according to raw feed size and goal particle size range. Material is typically gravity-fed, and the number of roll pairs and their increasingly small gap measurements determine the fineness of the end result.
Roller mills are best suited for friable materials that fracture under stress such as grains, plastics, limestone, and other minerals for goal size ranges between 100 microns and 2mm. Narrow particle size ranges can be achieved with expert operators, and roller milling produces less heat than some other methods.
Fibrous materials and pliable materials that deform or flatten are not suited for roller milling.
Hammer mills, true to their name, impart small hammers mounted on a rotor inside a milling chamber, using impact, shear, and centrifugal forces to shred and crush materials. Resulting particle sizes can range from millimeters down to the 100-micron range. Adjustments to hammer sizes, spacing, and rotor speeds can all impact the end result.
Unlike roller mills, hammer mills can reduce particle size on a broader range of materials, including tougher and more fibrous materials — but they are not well suited for sticky, very hard, or abrasive materials. Hammer milling generates higher temperatures, so it’s important to be aware of combustibility or low melt points, and apply technologies such as cryo-milling or inert gases when indicated.
Universal mills are well suited for materials ranging from chemical fertilizers to minerals to food and pharmaceutical products, and can achieve particle sizes in the range of micronization (tens of microns).
Universal mills’ grinding elements and rotor speeds are selected and adjusted to achieve the desired particle size, and cryo-milling can make this mill type an option for an even greater range of raw materials.
Universal milling is commonly used for food and pharma applications to achieve fine powder results.
Fine and Ultra-Fine Milling Technologies
Jet mills and fluidized bed jet mills use compressed air, gas, or superheated steam to cause particle collisions within a grinding chamber, resulting in particle sizes to 5 microns and even smaller — referred to as micronization, or nanonization, when the smallest particles are achieved.
Jet milling technology is indispensable for cosmetics, pharmaceuticals, dry lubricants, and minerals. The creation of extremely small particles results in much higher surface area, which can help products achieve desired results such as:
- Increased efficacy of active ingredients
- Lower dosages and reduced material requirements in pharmaceuticals or agricultural chemicals
- Precise color outcomes or sheen in pigments
- Exacting control over abrasives in polishes
Media Milling, Dry and Wet
Ball milling, or media milling, uses collision and shear forces to grind down material, whether wet or dry. A ball mill can operate at either high- or low-energy outputs, and material characteristics can span a wide range. Adjustable variables include ball types and sizes, speeds, material mass in slurry, and more.
While media milling can be applied to soft materials without a liquid slurry, wet grinding in a liquid slurry can achieve particle sizes down to the nanometer range with excellent control over size distribution. These micro- and nano-particle sizes are critical for transdermal, respiratory, and parenteral formulations in pharmaceuticals.
It’s important to get familiar with the range of milling methods available. Selecting the right approach, adjusting the variables, running the trials, and scaling up production require a reliable team of experts to guide you through the process.
You can further familiarize yourself with Milling Methods by downloading our Wet Grinding vs Dry Grinding Comparison Guide. Just click the link below to get your copy. And if you read through it and have unanswered questions, reach out. We’ll be glad to help.