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.

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

Hammer Mills primarily use impact to break down materials. Material is fed into a grinding chamber where a rapidly rotating rotor, fitted with multiple free-swinging or fixed hammers, repeatedly strikes the material. The material is crushed against breaker plates or a screen until it is small enough to pass through the screen openings, which determines the final particle size. Compared to alternatives, the design is simple, the initial cost is low, and wear is low (other than the hammers). Produces powders having a wide particle size distribution, not suitalbe for materials having moisture. Energy consumption is high.

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. Widely used for large-scale, continuous grinding processes, with very high capacities. 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.

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. It often includes features like bevel gear overall drive and internal thin oil lubrication.

Vertical Roller Mills employ a different grinding mechanism, primarily using compression and attrition on a rotating grinding table. Material is fed onto a rotating 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.

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