Mineral Liberation Size: Why Finer Grinding Is Not Always Better

MINERAL PROCESSING KNOWLEDGE · CRUSHING AND GRINDING

Understand mineral liberation, grind-size selection, overgrinding, slimes and the testwork used to link crushing and grinding with downstream separation.

Technical overview. Actual process selection depends on representative ore samples, mineralogy, testwork and site conditions.

The purpose of crushing and grinding is not simply to make ore particles as small as possible. It is to expose valuable minerals from the surrounding gangue so that a downstream process can separate them. The size at which a mineral becomes sufficiently liberated is often called the liberation size, but it is not one fixed number for an entire deposit. It depends on mineral texture, grain size, intergrowth, breakage behaviour and the separation method that follows.

Technical note: Grinding finer can improve liberation in some ores, but it also consumes energy and can create slimes. The right target is the coarsest practical size that gives the required separation response, established through mineralogical and metallurgical testwork.

1. What does mineral liberation mean?

An ore particle may contain valuable mineral, gangue or both. A particle that consists mainly of the valuable mineral is more liberated than a composite particle in which the valuable mineral remains attached to gangue. Liberation affects how efficiently gravity, magnetic separation and flotation can separate the valuable mineral from unwanted material.

Liberation is commonly evaluated across size fractions using microscopy, image analysis or automated mineralogy. A single bulk assay cannot show whether valuable minerals are exposed, locked or distributed among different particle sizes.

2. Why the required grind size varies

• Grain size: coarse mineral grains may be liberated at a relatively coarse grind, while fine intergrowth may require finer grinding.
• Texture: disseminated, vein-hosted, laminated and complex intergrowth textures break differently.
• Mineral association: the valuable mineral may occur with several gangue minerals that have different hardness and breakage behaviour.
• Separation method: gravity, magnetic separation and flotation respond differently to particle size, shape and surface condition.
• Product requirement: concentrate grade, recovery and downstream specifications influence the acceptable degree of liberation.

3. What happens when ore is under-ground?

If the ore is not ground sufficiently, valuable minerals can remain locked with gangue. These composite particles may report to tailings or lower the concentrate grade. In flotation, locked particles may not present enough exposed valuable-mineral surface. In magnetic separation, a particle containing both magnetic and non-magnetic minerals may report unpredictably depending on its composition and size.

4. What happens when ore is over-ground?

Overgrinding can create an unnecessarily large proportion of fine particles and slimes. Fine particles may have low collision probability in some separation processes, can be entrained with water in flotation froths, and may complicate thickening and filtration. Excessive grinding also consumes additional power and can increase wear and operating cost without delivering a proportional improvement in recovery or concentrate quality.

This does not mean fine grinding is always harmful. Some ores genuinely require fine or ultrafine liberation. The point is that a finer grind should be justified by measured improvement in separation performance, not by a general assumption that smaller is always better.

5. How to establish a practical grinding target

1. Collect representative samples that reflect ore types and variability.
2. Characterize mineralogy, texture, hardness and particle-size distribution.
3. Conduct comminution tests appropriate to the project stage.
4. Prepare several grind-size products for size-by-size mineralogical and metallurgical testing.
5. Measure liberation, concentrate grade, recovery, mass pull and downstream dewatering behaviour.
6. Compare energy, throughput and wear implications across the tested conditions.
7. Select a target range and define how the plant will monitor it during operation.

6. Why closed-circuit grinding and classification matter

A grinding circuit commonly works with classification. A classifier or hydrocyclone separates material by size or settling behaviour; coarse particles return for further grinding while sufficiently fine material advances to the next stage. This arrangement helps control the product-size distribution and can prevent unnecessary treatment of material that is already fine enough. The actual circuit design depends on ore properties, throughput, water conditions and required product size.

7. A simple conceptual circuit

1. Primary crushing reduces run-of-mine ore to a size suitable for the grinding circuit.
2. Grinding creates new fracture surfaces and progressively liberates minerals.
3. Classification separates coarse and fine fractions.
4. Coarse material returns to grinding if required.
5. The classified product feeds gravity, magnetic separation, flotation or another downstream process.
6. Testwork and plant surveys are used to adjust the grinding target and circulating load.

8. What data should accompany a ball mill enquiry?

• Ore type and mineralogy.
• Maximum feed size and full feed-size distribution.
• Required throughput in t/h or t/day.
• Target product size or the downstream separation requirement.
• Available Bond work index or other comminution test results, if available.
• Wet or dry grinding duty, water conditions and site power supply.
• Any existing circuit data or operating constraints.

The Jaw Crusher and Ball Mill pages show examples of equipment in the current catalogue. Actual model selection must consider the material, feed size, required throughput, grind target and circuit configuration.

Conclusion

Liberation is a mineralogical and process question, not just a mill setting. A useful grinding target balances liberation, recovery, concentrate quality, energy use and downstream separation behaviour. The best answer comes from representative testwork and a mass balance across the relevant size fractions.

For mineral-specific examples, see Iron Ore Beneficiation and Copper Ore Processing.