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.

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.

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.

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

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.