Grinding machines, ranging in various shapes and sizes, reduce particle size during the processing of diverse bulk materials essential for manufacturing numerous products. Particle size reduction and classification of materials are commonly used when handling bulk materials in chemical processing, metalworking, mineral processing, mining, pharmaceutical and many other industries. To achieve the right particle size range for a material, selecting the best mills or grinding machines for the application is important.

Within the various grinding machines, fine grinding mills – often referred to as fine grinders – and air classifying mills pose two typical means for breaking down materials. While fine grinders provide a finely ground product, air classifying mills can reduce particle size even smaller, while also providing uniform particle size distribution to the material. When manufacturers have a choice between incorporating a fine grinder or air classifying mill within their processing system, it’s important to choose the grinding machines that best suit the application.

Types of Fine Grinding Machines & Air Classification

Fine grinding machines are used for a variety of processes by multiple industries, including agricultural, food and beverage, mining, pharmaceutical and other sectors. The use of mechanical grinding methods for reducing bulk materials can alter reactive processes, decreasing particle size to increase the surface area of particles. This in turn can reduce the temperature or improve the speed at which reactions occur when putting material through specific processes downstream.  

Fine grinding machines are normally classified into the following categories:

  • Attrition mills: These grinding machines press materials between metal disks that rotate in opposing directions, with material fed into the disks’ center and then flung outward via centrifugal force through the disks and to the discharge point once particles reach the desired size; attrition mills generally have lower throughput rates, generating lower temperatures to enable processing of more heat-sensitive materials like seeds or spices.
  • Ball mills: These types of mills use spherical media of differing sizes for blending and grinding; machines use both attrition and impact to reduce particles into extraordinarily fine sizes within a hollow, rotating cylindrical shell; ball mills are often used for separating minerals from ores, producing powdered products, mixing processes and changing properties of materials.
  • Impact mills: Collisions against a solid surface allow impact mills to reduce bulk materials through impact force that then discharges material through a screen at the bottom of the machine once particles reach the desired size; common types of impact grinding machines include hammer mills and pin mills.
  • Jet mills: Used widely for exceptionally fine grinding, machines employing this concept reduce material through impacting and grinding particles with the aid of compressed gas – usually steam or air – to cause collisions with both the mill’s interior surfaces and other particles; unlike air classification mills, they don’t put bulk materials through a classifying process on their own.
  • Roller mills: Using cylindric rollers against flat plates or in pairs spinning in opposite directions, roller mills compress bulk materials through crushing and grinding; machines using this particle reduction technique are used with grain, gravel, mineral ores, plastic resins and other materials.  

Essentially, air classifying mills combine an air classifier with a mechanical impact mill for fine grinding. Machines that utilize moving air to control particle size distribution tend to more accurately size particles, while also putting the material through multiple stages of classification. An air classifying mill is comprised of blades or hammers on a rotor assembly, a chamber featuring stationary pins or grinding plates in which material is ground and an air classifier to regulate the size of particles.

Air classifying mills as such are impact grinding machines that also classify particles with the help of an independently controlled air classifier wheel. Material is fed through an inlet into the grinding chamber, where a motor-driven rotor spins at high speed to generate centrifugal force. The rotating blades or hammers grind the material with the help of immobile pins attached to the sides of the chamber. As this grinding action occurs, the air classifier separates particles, directing them to specific outlets based on their size to achieve the desired particle size distribution for a material.  

Grinding Machines: Fine Grinder vs. Air Classifying Mills

On the surface, it may seem like air classifying mills are superior to mills used for fine grinding. Machines like fine grinders do, however, still have an important place in the bulk handling and processing sector. While air classifying mills can achieve very narrow particle size distribution, in certain chemical processing and pharmaceutical applications a wider particle size distribution is desirable to enhance specific material properties.

In many cases, air classification is an essential part of the process and directly affects product quality. For less friable materials that require grinding, machines that incorporate air classification are indeed better. Air classification is also necessary for materials that are difficult to grind or products that require narrow particle size distributions.  

Advantages of using an air classifying mill include:

  • Allowing manufacturers to break down, classify and separate materials with a single grinding machine, reducing the need for additional processing and equipment.
  • Capabilities to handle and process a wide array of materials, including those that are abrasive and heat-sensitive.
  • Easy to clean, inspect and otherwise maintain, lowering downtime.
  • Efficient and accurate reduction of particle size.
  • Generating powders with narrow particle size distribution.
  • High-speed rotation and air vortex with the mill’s grinding chamber make the process more efficient than other grinding machines.

The applications for air classifying mills are many and varied, though they’re particularly important in the pharmaceutical, food processing, chemical processing and cosmetics sectors. The pharmaceutical industry, for example, often requires fine powders to improve the bioavailability of medications in capsules, tablets and other forms.  

Spices, starch, sugar and other food ingredients are often processed into fine powders with air classifying mills to enhance flavor and improve processing. Air classification is also used for makeup and skincare products made from powders, that often require greater consistency and uniformity in particle size distribution. The chemical processing industry also uses air classifiers to produce catalysts, dyes, pigments and other products for the same reasons.

Integrating Prater Grinding Machines 

Prater Industries makes a variety of grinding machines that both include and exclude air classifiers. With an in-depth knowledge of material handling and processing systems, along with an intimate understanding of Prater equipment, our engineers have the knowledge and experience to develop efficient systems to process and handle bulk materials. Their skill and expertise will ensure your processing system will properly control particle size, while also warranting its efficient operation.  

Prater’s system integration services include: 

  • Complete project management services to oversee system development.
  • Expertise in auxiliary equipment within a system to ensure everything works together correctly.
  • In-house process controls team that manages projects to keep work on time and within budget.
  • Precise documentation of the system, using everything from Gantt scheduling charts to complete layouts of systems.
  • Provision of Prater-made sifting, milling, metering and grinding machines.

Prater’s systems integration expertise is considered among the best in the industry. We invite you to consult with us about any installation of our grinding machines into an existing system or to help you design a whole new system. To learn more about the products and services Prater provides and our ability to develop material processing systems from scratch, contact us today.