Last reviewed 1 Oct 2026 · 8 min read
What is size reduction?
Size reduction (comminution) is the breaking of solid materials into smaller pieces by mechanical forces. In food and agricultural processing it is used for milling grain into flour, grinding spices, crushing sugarcane and oilseeds, chopping fodder, making feed, and producing powders.
Objectives
- Increase the surface area for faster drying, extraction, cooking and chemical reaction.
- Make material suitable for mixing or further processing (flour for dough, feed ingredients).
- Release valuable components (starch from the grain, juice from cane, oil from seeds).
- Obtain a product of required size (fine flour, grits, coarse meal).
- Facilitate handling and transport; improve palatability and digestibility (for animal feed).
Mechanisms (types of forces)
| Force | Action | Example |
|---|---|---|
| Compression (crushing) | Squeezing between two surfaces | Jaw crusher, roller mill (hard, brittle material) |
| Impact | Sudden blow | Hammer mill |
| Shear (cutting) | Slicing | Knife cutters, chaff cutters |
| Attrition (rubbing) | Rubbing between surfaces | Burr (stone) mill, disc mill |
| Combination | Mixed forces | Most grinders |
The mechanism is chosen for the material: hard, brittle — compression or impact; fibrous — shear or cutting; soft or sticky — attrition or cutting; heat-sensitive (spices, oilseeds) — cool grinding (cryogenic, or slow-speed mills).
Energy requirement — the laws of comminution
The energy needed depends on the material, the initial and final size, and the mill efficiency. Three classical laws state that the energy to produce a small size change is a power of the particle size :
| Law | Energy | Applies best to | |
|---|---|---|---|
| Kick's law | 1 | — energy depends on the reduction ratio | Coarse crushing (large particles) |
| Bond's law | 1.5 | with in µm; = work index (kWh/t) | Intermediate grinding — the most widely used |
| Rittinger's law | 2 | — energy is proportional to the new surface created | Fine grinding (fine powders) |
where = initial size and = final size (commonly the sieve size passing 80 % of the feed and product).
- The work index is the energy (kWh per short or metric ton) needed to reduce the material from a very large size to a size 80 % passing 100 µm. It differs among materials.
- Efficiency: only about 0.1–2 % of the energy input actually creates new surface; most becomes heat, sound and friction.
A grain is ground from a feed of (80 % passing) to a product of (80 % passing). The work index is kWh/t.
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For a 2 t/h mill the power draw is about kW; with a motor and mill efficiency of 80 % the motor power is about 12 kW.