Engineering & Technology · Mining, Resource Recovery & Geoengineering Systems

Comminution, mineral separation and recovery principles

A finely crushed particle can still contain several minerals. Reducing its size and separating it are related but different tasks. Mineral processing connects the mineral’s physical occurrence to a useful product while accounting for the material, water and energy that leave through every stream.

Use Geotechnical & Ground Engineering, Process Balances & Design and Reliability, Safety & Maintenance for ground response, inventories and complete-system dependencies. Physics continuum materials and Chemistry separation science provide additional preparation. This chapter supplies process principles and evidence questions, not extraction, support, ventilation, chemical-treatment or reservoir-operating procedures.

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After reading, you can

Explain liberation and property-based separation, and calculate recovery and concentrate grade on a consistent dry-mass and target-element basis.

Particle size and mineral identity are different quantities

Comminution breaks material into smaller particles through crushing or grinding. It can expose or liberate grains of a desired mineral from surrounding material, often called gangue. Complete liberation at every size is not guaranteed. Excessively fine particles can become difficult to handle or separate, while energy use and wear also change.

Screening separates according to passage through openings; classification in a fluid depends on particle motion and its governing conditions. A small particle is not necessarily richer in the desired mineral. Characterise size distribution, mineral associations and the properties that the intended separation actually uses instead of treating size reduction as purification.

Separation mechanisms select different responses

Density-based methods use differences in how particles respond to gravity and the surrounding flow. Magnetic methods use magnetic response; electrostatic methods use electrical behavior. Flotation uses differences in surface interactions with liquid and bubbles. These are mechanisms, rather than interchangeable settings or universal rankings of effectiveness.

Mineral host, degree of liberation, particle size, feed variability and process-water chemistry can influence separation. A bulk assay alone may not show whether a target element follows the desired concentrate or another stream. Additional extraction or refining can be needed after concentration; the concentrated element may still be chemically bound in a mineral.

Follow a particle from its exposed surface to a product

Consider froth flotation as one representative property-based route. Wetting describes how water spreads over a solid surface. A hydrophilic surface is readily wetted; a hydrophobic surface is less readily wetted. When a suitable particle meets a bubble, thinning and rupture of the intervening water film can allow attachment. If the attached assembly remains supported and is carried upward, the particle can reach the froth, a surface layer of bubbles and liquid that is separated as a product. A particle that does not attach has no such bubble-carrying route. Its density alone does not decide this distinction.

Attachment is only one step: collision, persistence of contact, upward transport and retention in the collected froth must also occur. Particle size, surface condition and flow can change these steps. A water-wet particle can also enter froth with carried liquid, called entrainment. Thus “in the froth” does not by itself prove selective attachment, and “not attached” does not mean a particle must sink immediately. The following classroom packet excludes entrainment and detachment so that the chosen mechanism can be followed separately.

Liberation matters because a particle is transported as an object. A composite that contains both desired mineral and gangue may attach through an exposed floatable face and carry both materials onward. Conversely, a valuable mineral enclosed within a water-wet gangue surface may not obtain that attachment route. A bulk element assay therefore does not specify the exposed surface or the particle path. The separate mineral-host and flotation evidence supports this distinction; no particular mineral is assumed universally floatable.

For this invented packet, every particle has an opportunity to meet bubbles. The stated observations are that all liberated target-mineral particles attach and are collected, none of the liberated gangue does, and half the dry mass of the specified composite class attaches and is collected. The composites all have the same stated composition. These are supplied classroom observations, not efficiencies predicted from contact angle or proposed equipment settings.

A separate 100 kg dry-solids packet: supplied surface/collection observations, not plant data
Particle class	Dry mass	Target-mineral mass	Declared particle route
Liberated hypothetical target mineral	20 kg	20 kg	All attaches and is collected with froth
Uniform composites: half target mineral, half gangue by mass	20 kg	10 kg	Half the class mass attaches; the other half stays in the non-froth stream
Liberated gangue	60 kg	0 kg	No bubble attachment; all stays in the non-froth stream

Recovery and grade answer different questions

Supply 100 t/h of dry feed containing 1.0% of a target element: 1.0 t/h of that element. In an invented two-product separator, 4 t/h of concentrate has target grade 20%. It contains 0.8 t/h of target, so target recovery to concentrate is 0.8/1.0=80%.

Assuming no other material outputs or accumulation, tailings dry mass is 96 t/h and its target mass is 0.2 t/h. Tailings grade is 0.2/96≈0.2083%. Recovery measures the share of feed target captured; concentrate grade measures target concentration in that product. The figure tracks target mass separately from the much larger bulk streams.

Each case receives 100 t/h dry feed at 1% target grade, containing 1 t/h target. Case A concentrate 4 t/h at 20% captures 0.8 t/h, leaving 0.2 in 96 t/h tailings. Case B concentrate 8 t/h at 11.25% captures 0.9 t/h, leaving 0.1 in 92 t/h tailings. Two equal 500-unit bars each represent 1 t/h target; subdivisions are exact 80/20 and 90/10. Capture is not concentrate purity. In each bar, the left segment represents target in concentrate and the right segment target in tailings.

Compare two explicitly supplied separation outcomes

An alternative invented outcome produces 8 t/h of concentrate at 11.25% target grade. Target capture is 0.9 t/h, hence 90% recovery. Its 92 t/h tailings contains 0.1 t/h target, giving tailings grade about 0.1087%. This outcome recovers more target but sends twice as much bulk concentrate onward at a lower grade.

Neither result establishes the better process. The next stage may face different energy, water, impurity and transport requirements, while tailings management also changes. All grades are element mass fractions on dry solids; water has been excluded explicitly. Actual sampling, reconciled flow measurements and tests across feed variability are needed before predicting a plant’s performance.

Apply that separate particle packet on its own 100 kg basis. The hypothetical target mineral is defined to contain 50% target element by mass; gangue contains none. Feed target element is (20+10)×0.50=15 kg. Collected froth contains the 20 kg liberated-mineral class plus 10 kg composites, or 30 kg dry solids. Its target element is 20×0.50+10×0.50×0.50=12.5 kg. The non-froth stream contains 70 kg solids and 2.5 kg target, closing both balances: 30+70=100 kg and 12.5+2.5=15 kg.

Target recovery to this froth product is 12.5/15≈83.33%, while its element grade is 12.5/30≈41.67%. The composite particles carry 5 kg gangue into the froth along with 2.5 kg target. That physical connection explains why capturing more composite material can raise recovery while diluting the product. It does not alter the earlier two independent t/h examples or predict their mechanism.

If all 20 kg composites instead followed the collected route, froth would contain 40 kg solids and all 15 kg target: recovery 100%, grade 37.5%. If none followed it, froth would contain 20 kg solids and 10 kg target: recovery 66.67%, grade 50%. These changed, supplied particle outcomes show the trade-off; a real process still needs evidence for the attachment, water carryover, detachment and collection fractions across the feed distribution.

Try the reasoning

Why is 80% recovery different from 20% concentrate grade?
Show the answer and reasoning
The first divides captured target by feed target; the second divides captured target by concentrate bulk mass.
Where does the uncaptured 0.2 t/h of target go in the first model?
Show the answer and reasoning
To the tailings, because the supplied model has only concentrate and tailings outputs and no accumulation.
Does the 90% recovery outcome necessarily have lower total impacts?
Show the answer and reasoning
No. Its lower concentrate grade, larger onward mass and other process and waste conditions must be assessed over a common boundary.
In the separate particle packet, a changed surface observation shows that none of the composite class attaches, while the liberated classes behave as before. Rebuild the product account and explain the recovery/grade change.
Show the answer and reasoning
Only the 20 kg liberated target-mineral class reaches froth. It contains 10 kg target, giving 10/15≈66.67% recovery and 10/20=50% element grade. The 80 kg non-froth stream contains 5 kg target. Losing the composites lowers recovery but removes their gangue from the froth product. This conclusion uses the stated no-entrainment/no-detachment classroom route; it is not a universal process prediction.
A target-bearing composite is collected with froth. Does this prove that all minerals inside it separately attached to bubbles, or that its target element was chemically extracted?
Show the answer and reasoning
Neither follows. An exposed floatable part can carry the whole mechanically joined particle, including water-wet gangue; water entrainment can provide another route in a real process. Mineral concentration moves particles and can preserve chemical binding. Distinguishing those routes requires particle/surface and water-carryover evidence rather than only a bulk assay.

What to carry forward

Choose a mechanism suited to the mineral occurrence, then reconcile total mass, target mass, recovery, grade and downstream consequences.

References

Teaching resources and research supporting this lesson’s explanations and examples.