Industrial size reduction generally falls into two basic categories. Particles are either crushed or milled. While crushing breaks down larger chunks, milling reduces the material to a low micron range and, in some cases, can even achieve sub-micron reductions. Attrition milling is used for a wide range of applications, including chemical processing, to reduce particles into a narrower range.
What is an Attrition Mill?
Attrition mills mechanically reduce solid particles through the intense acceleration of particles against one another in a curved or flat grooved surface called a stator. These mills use a high-speed rotor to create centrifugal forces that facilitate the necessary particle interactions. Rotors' designs can vary, where some include multiple pockets with blades on a vertical shaft that accelerate particles against a curved stator. Others are designed as flat discs with grooves that abrade the particles against a parallel and opposed stator offset by an adjustable gap. With both types, the variance in movement creates particles with greater surface area than other types of mills. Attrition mills work best for medium-sized particle ranges, generally producing end products that are 100 mesh, sometimes finer.
How Attrition Milling Works
Attrition mills are equipped with a stationary milling disc on the outside and a rotating milling disc on the mill’s interior. The rotating disc can be adjusted to widen or narrow the gap between the discs, with a narrower gap producing smaller particles and a wider gap producing bigger ones. The material then enters the gap through the outer disc’s midpoint, which has a wider gap where the material is introduced. The centrifugal force provided by the rotating disc then moves the material outward, grinding it between the discs.
The finely ground product then leaves the grinding zone, after which it’s released into a product container. The throughput of attrition mills varies according to the particle size required for the material being reduced, providing higher throughput for coarser end products. The gap size, number of teeth on the discs, and rotor’s rotational speed all determine the fineness of the end product. Using ambient air instead of a cooling gas also keeps operating costs lower.
Materials Attrition Mills Best Handle
Attrition mills are well suited to process brittle or fibrous materials that need to be within a comparatively narrow particle size range.
Attrition mills are used in the production of:
- Blended materials, including face powders and certain insecticides.
- Fibrous materials like cellulose, cork, and wood chips.
- Foods made from cereals, grains, and peanuts.
- Spices like cinnamon, paprika, and pepper.
- Minerals, metals, and pigments.
Attrition milling is also more economical when grinding softer or heat-sensitive materials, as the process doesn’t require conditioned air, liquid nitrogen, or other cooling gas.
Comparable Effectiveness of Attrition Milling
Attrition mills work better than impact mills, such as hammermills or pin mills, or screen classifying cutters for reducing materials like grains, metal powders, minerals, seeds, spices, and tobacco, along with fibrous materials like plastics and waxes that don’t break down well using other types of impact milling. They also offer greater efficiency than vibratory ball and conventional tumbler mills for hard materials like carbides, ceramics, and hardened metals.
Additionally, attrition milling offers an advantage over other milling processes for applications that involve blending and mixing certain metallic powders, such as tungsten carbide or cobalt, as they produce finer particle sizes and have shorter milling times. However, tumbler ball mills are usually employed for higher production runs, as attrition mills have lower output. Reducing metal via attrition milling also works well in inert atmospheres, such as in mechanical or solid-state alloying processes.
Types of & Uses for Attrition Mills
There are numerous types of reducing equipment that utilizes attrition milling processes for a variety of purposes.
These include:
- Buhrstone mills: The oldest of attrition mills, Buhrstone mills are still used today for wet milling corn and whole-grain flour.
- Planetary mills: As a subcategory of ball mills, planetary mills are used primarily for reducing sample material in laboratories, though they are also used for mechanical alloying.
- Rotormills: Also referred to as “long gap mills” due to the lengthy annular gap between its inlet and discharge, rotormills are used to manufacture abrasive products and for less brittle materials.
- Sand mills: These are used primarily to manufacture coatings, inks, and paints.
- Tower mills: Often referred to as regrind, stirred, or vertical mills, tower mills are used in a variety of mining and ore processing applications.
Rotormills by Prater Industries
Purchased in 2021 by Prater Industries, International Process Equipment Company (IPEC) manufactures rotormills that utilize attrition milling techniques.
Prater’s Rotormills:
- Allow high rates of fine milling production.
- Combine several operations to eliminate the need for multiple handling systems, including those used for surface coating and de-agglomeration of materials.
- Ability to dry AND grind in one step.
- Use particle-to-particle collisions in its attrition milling process to grind abrasive and friable materials that aren’t easily reduced by fine grinders or classifying mills.
Using a high level of turbulence, Prater’s Rotormill pulverizes particles via action generated by a high-speed internal spinning rotor. The machine’s heavy-duty bearings enhance stability, while the bearings’ placement outside the grinding chamber prevents product contamination. Within the mill, small pockets of turbulent air move at high velocity to produce collisions between particles breaking down the material. A continuous flow of air keeps the mill cool, while adjustable grinding, specialty plates, and variability in airflow allow the mill to process a diverse array of materials.
