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Particle classification

A process control mechanism to enable control of grinding efficiency, equipment wear and material quality.

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Details

The term "particle classification" can refer to a strategic process control mechanism, used in particle size reduction equipment, or to a class of grinding equipment known as an ACM (air classifier mill). The former significantly impacts the efficiency, economics, and product quality of mineral grinding operations. Instead of just grinding material down until everything is small enough, classification sorts the particles produced by a grinding mill into two main streams:

• Fines (or Overflow): Particles that have reached the desired size (or are finer) are sent to the next stage of processing (e.g. roller mill) or to use in the finished product for sale.
• Coarse (or Underflow/Return Load): Particles that are still too large are sent back to the grinding mill for further size reduction.

This creates a "closed-circuit grinding" system that optimizes efficiency and reduces energy consumption by preventing over-grinding and maximizing throughput. Without classification, a mill would continue to grind particles that are already fine enough, wasting energy, adding wear on internal mill components and grinding media, and even creating undesirable ultra-fine particles (that can be difficult to process in subsequent steps). The classifier is thus the "brains" of the operation, by quickly removing already-fine particles, it ensures that the mill's energy is focused on grinding only the coarser material, thus increasing throughput of the circuit.

Mineral processing plants often require a very specific particle size range for optimal recovery of valuable minerals in downstream processes (e.g., flotation, gravity separation, leaching). For ceramics, however, the focus is generally on achieving a target percentage of particles under a specific size, this directly impacts the product's performance and market value (e.g., specific grades of talc, calcium carbonate, silica, clay). Classification is key to meeting these quality standards.

How Does Particle Classification Work?

Classification primarily relies on differences in how particles behave in a fluid medium (air or water), based on their size, density, and shape. Common classification technologies include:

• Screens/Sieves (for coarser materials): While not typically called "classifiers" in the context of fine grinding, screens are the simplest form of sizing, physically separating particles larger or smaller than a specific aperture. They are common for the removal of oversize materials for recycling to the mill and to prepare a undersize feed for further particle size reduction in a more capable mill.
• Hydrocyclones: These are widely used in wet grinding circuits (e.g., with ball mills). A slurry (mixture of ground material and water) is fed tangentially into a conical vessel, creating a powerful vortex. Centrifugal force pushes coarser, denser particles to the outer wall, where they spiral downwards and exit through the "underflow" (returned to the mill). Finer, lighter particles migrate to the center and are carried upwards with the water, exiting through the "overflow" (to the next process).
• Air Classifiers (or Air Separators): Used in dry grinding circuits (common with Raymond, Trapezium, and Vertical Roller Mills, and jet mills). Ground material is fed into a chamber with a rising current of air. Finer particles are carried by the air current and exit with the airflow, while coarser, heavier particles fall back down for further grinding. Internal rotating vanes or rotors often enhance the separation efficiency by creating a centrifugal effect.
• Mechanical Classifiers (e.g., Spiral Classifiers, Rake Classifiers): These are older, less common in modern fine grinding, but they use mechanical means (like rotating spirals or reciprocating rakes) in a pool of water to separate particles based on their settling rates. Finer particles remain suspended and overflow, while coarser particles settle and are mechanically conveyed out.

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