Mineral liberation
- Also known as
- liberation, liberation size
Mineral liberation is the degree to which grains of a valuable mineral have been freed from the surrounding gangue by crushing and grinding. Only liberated or mostly exposed mineral can be separated efficiently by flotation and other surface-based processes.
What is mineral liberation?
In most ores, the valuable minerals occur as small grains intergrown with gangue. Crushing and grinding, together called comminution, break the ore into particles. Some particles end up as pure valuable mineral, some as pure gangue, and many as mixtures, called middlings or locked particles. Liberation describes how much of the valuable mineral ends up in free or well-exposed particles.
Why liberation matters
Separation processes act on particles, not on individual mineral grains:
- In froth flotation, a particle floats only if enough of its surface is the hydrophobic valuable mineral. Locked particles either fail to float, reducing metallurgical recovery, or float and carry gangue with them, reducing concentrate grade.
- In leaching, solution must reach the mineral, so exposure matters even when full liberation is not needed.
Poor liberation therefore puts a ceiling on the grade-recovery curve that no reagent can fully overcome.
Choosing a grind size
Grinding finer improves liberation, but it has costs:
- Energy. Comminution is typically the largest energy consumer in a mineral processing plant, and energy use rises steeply as grind size falls.
- Fines and slimes. Very fine particles float poorly, are easily entrained, and are harder to dewater.
Plants therefore choose a grind that balances liberation against cost and fines, and often regrind a rougher concentrate to liberate more mineral before cleaning, rather than grinding all the ore finely.
How liberation is measured
Automated mineralogy systems, which combine electron microscopy with mineral identification, map the minerals in thousands of polished particles and report how liberated each mineral is at each size. These measurements guide grind-size decisions and help diagnose poor recovery.
Frequently asked questions
Middlings, or locked particles, contain both valuable mineral and gangue. They are a main cause of lost recovery and diluted concentrates.
Because energy costs rise steeply with finer grinding, and very fine particles are harder to float, more prone to entrainment and harder to dewater.
Sources
- Wills' Mineral Processing Technology, 8th edition, B. A. Wills and J. A. Finch, Butterworth-Heinemann, 2016
- SME Mineral Processing and Extractive Metallurgy Handbook, Society for Mining, Metallurgy and Exploration, 2019