All Glossary

Particle size reduction

Ceramic materials that we receive as powders have to be crushed and dried, and then ground into fine powders. Many types of equipment are used for this.

Key phrases linking here: particle size reduction, vertical roller mills, vertical roller mill, air classifier, hammer mills, hammer mill, trapezium, raymond - Learn more

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Pulverization of ceramic materials, especially clays, into a powder is termed "particle size reduction". Various technologies are used, these are the most common.

Hammer Mills: Material is fed into a grinding chamber where a rapidly rotating rotor, fitted with multiple free-swinging or fixed hammers, repeatedly strike the material. Compared to alternatives, the design is simple, the initial cost is low, energy consumption is high and wear is low (other than the hammers). Because hammermills grind clay through impact alone, they produce a significant amount of oversize particles (and undersize), so the device must be paired with a screen to pass undersize and return oversize. Throughput of screens drops dramatically beyond 42 mesh, thus limiting the effectiveness of the hammermill as the sole grinding device in a process needing finer particle sizes.

Ball Mills use a combination of impact and attrition (abrasion) for grinding. A horizontal rotating cylindrical shell is partially filled with grinding media (typically steel balls, but can also be ceramic, flint pebbles, etc.) and the material to be ground. As the mill rotates, the balls are lifted by the mill's rotation and then cascade or tumble down, impacting and grinding the material between the balls and the mill lining, and also through the abrasive action of the balls rubbing against each other and the material. Can be operated in wet or dry modes to produce very fine to ultra-fine powders (e.g., from 325 mesh down to sub-micron. Have long been used for grinding ball clay and other industrial minerals to finenesses in the 200-325 mesh range. They are almost always operated in a closed circuit with an air classifier to remove the fine particles and return the coarser material to the grinding zone. They are of simple construction and considered highly reliable. However, they are noisy, energy-intensive, heavy and require a large footprint and have high wear rates on grinding media and liners.

Raymond Mills operate on the principle of grinding by compression and impact. Material is fed into the grinding chamber, where it is caught by rotating "shovels" or "ploughs" and thrown between grinding rollers and a stationary grinding ring. The rollers, which are suspended from a rotating "spider" or "plum rack," swing outwards due to centrifugal force and press against the grinding ring, crushing the material (thus these are sometimes called "Pendulum Mills". An air classifier separates fine particles, with coarser material returning for further grinding. These can be used as the second stage after a hammer mill.

Trapezium Mills are an evolution of the Raymond Mill, incorporating several design improvements. It also uses rollers and a grinding ring for compression grinding, but with a unique "trapezium" shaped working surface for the rollers and ring. This design is intended to optimize the material flow and prolong grinding time, increasing efficiency.

Vertical Roller Mills (VRM) employ compression and attrition on a rotating grinding table. Material is fed onto the grinding table, and heavy rollers (which can be conical, cylindrical, or other shapes) press down on the material bed. The combination of the rotating table and the pressure from the rollers grinds the material. Hot air often flows through the mill simultaneously, providing a drying function. An internal classifier separates the fine product from coarser particles, which are returned to the grinding table. These represent state-of-the-art, both because they integrate drying, grinding and classification in one device and because they are capable of very high capacities. They thus represent considerable investment.

Air Classifier Mills (ACM) combine mechanical impact (of high-speed rotating elements like hammers, pins, or blades) with a dynamic air classifier (integrated within the mill's housing). An internal fan or airflow system carries ground material particles upwards to a rotating classifier wheel. This wheel creates a centrifugal force that opposes the drag force of the air. Finer particles are pulled through the classifier wheel and exit the mill as product, while coarser particles are rejected and fall back into the grinding zone for further impact grinding. ACMs are not typically the primary mill for raw, lumpy clay, but if the clay is already pre-ground by a hammermill, an ACM can be used for the final fine grinding and precise classification to 200 mesh. ACMs can handle hard materials and fine to ultra-fine grinding (e.g. down to 1000 mesh). They are known for achieving a narrow particle size distribution. Generally suited for smaller scale operations, specialized or high-value materials.

Jet Mills are a separate category used for high-value materials where ultra-fine grinding, high purity, and precise particle size distribution are critical (not typical in traditional ceramics). Material is introduced into a grinding chamber where high-pressure gas streams are injected through nozzles, creating a turbulent, high-energy vortex. Particles accelerate rapidly within this vortex and collide with each other (particle-on-particle impact) and sometimes against the mill walls. The intense collisions cause the particles to fracture into smaller pieces. A built-in classification zone (often a centrifugal classifier or the mill's inherent aerodynamics) allows only particles of the desired fineness to exit, with coarser particles recirculating for further grinding.

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