Module 2 · Mining and concentrating

Crushing, grinding and flotation

Level 2 · Working knowledge 11 min read 4 questions

The step that turns under one per cent copper into about thirty per cent, using more electricity than anything else on the site and a chemistry trick about a hundred years old.

Ball mills in the grinding circuit of a copper concentrator in northern Chile — Grinding is the single largest consumer of electricity on the site, and the fineness is set by the ore.Jose Luis Stephens · Enhanced licence · Adobe Stock 564278551

Chalcopyrite is 34% copper. The ore is under 1% copper. The concentrator's job is to separate the mineral grains from everything else, and it does that in two stages: break the rock until the grains are free, then float the grains away from the rest.

Comminution: breaking rock, and paying for it

Comminution is crushing and grinding. Rock goes through crushers and then into mills - typically a semi-autogenous grinding mill followed by ball mills - until the particles are fine enough that individual mineral grains are physically separate from the gangue around them. That condition is called liberation, and the required fineness is set by the size of the mineral grains in that particular ore.

NoteComminution is the largest single electricity consumer at a typical copper mine, and most of the energy put into it becomes heat and noise rather than new surface area. Grinding finer than liberation requires wastes energy; grinding coarser loses copper to tailings. The whole plant is tuned around that trade-off. The general treatment is at comminution and flotation.

Flotation: making one mineral hate water

Ground ore is mixed with water into a slurry and fed into flotation cells, where air is blown through it. Reagents called collectors attach selectively to the surface of the sulfide mineral grains and make them water-repellent, so that when a bubble touches one it sticks. The bubbles rise, carrying the sulfide grains into a froth at the surface, which overflows and is collected. The gangue stays in the water and goes to tailings.

Slurry preparationGround ore plus water, with pH adjusted by lime to control which minerals respond.Conditioned slurry

Rougher flotationA first, permissive pass that recovers as much copper mineral as possible even at low grade.Rougher concentrate

Regrind and cleaningThe rougher concentrate is reground and floated again, more selectively, to raise its grade.Cleaner concentrate

ScavengingThe rougher tailings get one more pass to catch what was missed.Scavenger concentrate, and final tailings

Thickening and filtrationWater is removed from the concentrate for shipping, and recovered for reuse.Concentrate, roughly 25-30% Cu

Selectivity is the art. Depressants are added to keep unwanted sulfides - pyrite especially - out of the froth, and where a deposit carries molybdenite, a second flotation circuit separates molybdenum from the copper concentrate and sells it separately. That is one of the by-product credits from lesson 3 appearing physically.

What leaves the plant, and what stays

Copper content through the early chain

Stage	Typical copper content	Physical form
Ore fed to the plant	Under 1%	Crushed rock
Concentrate	About 25-30%	Damp grey powder
Blister copper	About 98%	Molten, then cast
Refined cathode	99.99%	Plates

Indicative; site chain page figures · commodityeducation.com, Copper to the Grid
Concentrate grade is a commercial choice, not a maximum. Pushing it higher generally costs copper recovery, and the smelter's terms decide which trade-off pays.

Read the numbers correctlyConcentrate is not copper and should never be counted as copper. A tonne of concentrate contains roughly a quarter to a third of a tonne of copper, and the rest is iron, sulfur and residual rock. Trade statistics that mix concentrate tonnage with metal tonnage are wrong by a factor of three or four.

Tailings

Everything that did not float is tailings: finely ground rock in water, stored behind an engineered dam more or less permanently. Because so little of the rock becomes metal, tailings facilities are among the largest structures the industry builds, and their failure is the industry's most serious accident category. Sulfide-bearing tailings also generate acid when exposed to air and water, which is a management obligation lasting long after the mine closes. See tailings.

What to carry forward

Grinding to liberation is the largest electricity consumer at a copper mine, and the fineness is set by the ore.
Flotation makes sulfide grains water-repellent so bubbles can carry them out of the slurry.
Concentrate at 25-30% copper is not copper; counting it as metal overstates by three to four times.

Terms introduced here

Comminution: Crushing and grinding.
Liberation: The particle size at which mineral grains are physically separate from the gangue.
Collector / depressant: Reagents that make a mineral float / stop it floating.
Concentrate: The flotation product, roughly 25-30% copper.

Go deeper

Comminution and flotationon this site
Tailingson this site
Molybdeniteon this site
Cerro Verdeon this site
Chilean copper concentrate, tracedon this site

Check yourself

1. Why is a copper ore ground to a specific fineness rather than as fine as possible?

2. Flotation separates copper sulfides from gangue because collectors:

3. A trade report lists 'copper concentrate exports, 1 million tonnes'. The contained copper is roughly:

4. Molybdenum from a copper mine is produced by: