Jen Lepore, Sep 27, 2024 2:45:00 PM
Understanding the right grinding and milling techniques is key in manufacturing, as particle size impacts ingredient costs and product quality. From basic jaw crushers to advanced nanoparticle technologies, choosing the right method depends on your material, end use, and particle size goals. Here's a look at various milling methods, from coarse crushing to wet grinding, to guide your decision.
Primary and Coarse Crushing: Breaking Down the Basics
Jaw Crushing
Jaw crushing is one of the most fundamental methods of size reduction. Mining and quarry operations often have jaw crushers on site, where they break down large chunks of material into smaller, more manageable pieces.
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
- Recyclables
- 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 ideal for handling rubbery, sticky, or wet materials. For achieving finer particle sizes, additional grinding with alternative milling techniques, such as ball milling or jet milling is required.
Impact Crushing
Impact crushers also often serve in a primary crushing capacity. Although their design differs from jaw crushers, they use similar forces to process materials within their grinding chambers. However, they offer better control and achieve 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 often used as a second crushing step following jaw crushing. They are effective for grinding materials such as:
- Limestone
- Coal
- Gypsum
- Various Minerals
- Medium-hard, less - abrasive chemicals
Methods for Achieving Mid-Gauge to Ultra-Fine Particle Sizes
Roller Mills
In a roller mill, material passes between pairs of counter-rotating cylinders that apply crushing and shearing forces. The gap between the rolls is adjusted based on the raw feed size and the desired 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. With expert operators, milling machine techniques such as roller milling can achieve narrow particle size ranges. Additionally, roller milling generates less heat compared to some other methods.
Fibrous materials and pliable materials that deform or flatten are not suited for roller milling.
Hammer Mills
Hammer mills, as their name suggests, use small hammers mounted on a rotor within a milling chamber. These hammers apply 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 handle a wider range of materials, including tougher and more fibrous ones. However, they are not ideal for sticky, extremely hard, or abrasive materials. Hammer milling can generate higher temperatures, so it's crucial to consider combustibility or low melting points. In such cases, technologies like cryo-milling or the use of inert gases may be necessary.
Universal Mills
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 use adjustable grinding elements and rotor speeds to achieve the desired particle size. Additionally, incorporating cryo-milling technology can expand the range of raw materials that can be processed with this type of mill.
Universal milling is commonly used for food and pharma applications to achieve fine powder results.
Advanced Milling Technologies for Fine and Ultra-Fine Particle Sizes
Jet Milling
Jet mills and fluidized bed jet mills use compressed air, gas, or superheated steam to induce particle collisions within a grinding chamber, achieving particle sizes down to 5 microns or even smaller. This process, known as micronization or nanonization, is used to create extremely fine particles.
Jet milling technology is crucial for the processing of materials in 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, also known as media milling, utilizes collision and shear forces to grind materials, whether wet or dry. A ball mill can operate at various energy levels, accommodating a wide range of material characteristics. Adjustable parameters include:
- ball types and sizes
- speeds
- material mass in slurry
- and more.
When media milling is used for soft materials without a liquid slurry, it's effective, but wet grinding in a liquid slurry takes it a step further by achieving particle sizes as small as the nanometer range. This process offers excellent control over size distribution, making it ideal for the micro- and nano-particle sizes critical in pharmaceutical applications like transdermal, respiratory, and parenteral formulations.
It’s important to get familiar with the range of milling methods available. Choosing the right approach, fine-tuning variables, conducting trials, and scaling up production all depend on having a dependable team of experts to guide you through every step of the process.
You can further familiarize yourself with Milling Methods by downloading our Wet Grinding vs Dry Grinding Comparison Guide. Click the link below to get your copy. If you have any questions after reading it, don’t hesitate to reach out—we’re here to help!
