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Material World
Process · Changes the form, not the material

Froth flotation

Separating a mineral from crushed rock by making it stick to air bubbles — the step that turns a rock containing half a per cent of copper into a concentrate worth smelting.

Froth flotation is how nearly every sulfide ore in the world is concentrated, and it works on a difference so slight it is hard to believe it can be industrialised: whether a mineral surface prefers water or air.

The ore is ground to a powder fine enough to free the individual grains, mixed into a slurry, and dosed with a reagent that attaches to the target mineral and leaves the rest alone. Air is blown through. The treated grains stick to the bubbles, rise, and are skimmed off the top as a froth; everything else sinks. A rock that was half a per cent copper leaves as a concentrate that is twenty-five per cent copper, and the eighty tonnes of waste that never had to be smelted are the whole economics of the industry.

Processing

The reagents are what make it selective. A collector — typically a xanthate for sulfides — adsorbs onto the target mineral and presents a water-repelling tail to the slurry, so that grain will now attach to a bubble. A frother stabilises the bubbles long enough to be skimmed. Depressants and activators then tune which minerals respond, and it is this that allows a single ore to be separated into a copper concentrate and a separate molybdenum one, or a lead concentrate and a zinc one, by floating them in sequence rather than together.

The grind is the constraint nobody escapes. A grain still locked inside a particle of waste rock cannot be floated no matter what reagent is used, so the ore must be ground until the minerals are physically free — and grinding is the largest single consumer of energy in mining, accounting for a significant fraction of total industrial electricity use worldwide.

What comes out is a concentrate of the same mineral that went in. Flotation moves a mineral; it does not transform it, which is why the chemistry of extraction begins at the next step.

History

The principle was noticed in the 1860s and made to work at scale at Broken Hill in Australia in the first years of the twentieth century, on tailings that had been discarded as unworkable. That is the pattern the process has repeated ever since: it makes previously worthless material into ore.

Its consequence is the reason copper grades have fallen for a century without supply failing. Deposits that would have been ignored in 1900 are routine today, because flotation moved the boundary of what counts as ore — and that boundary, rather than the amount of metal in the crust, is what the word means.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

  • Chalcopyrite — mineral · the archetypal flotation feed — porphyry copper ore runs well under one per cent copper and leaves as a concentrate of twenty-five or more
  • Galena — mineral · floated ahead of sphalerite, the zinc mineral being held back with a depressant and taken in a second stage
  • Sphalerite — mineral · reactivated with copper sulfate after the lead has been floated off, which is how one ore yields two clean concentrates
  • Molybdenite — mineral · naturally water-repellent, so it floats almost without a collector — and is separated from copper concentrate rather than from the ore
  • Pentlandite — mineral · with the copper sulfides and the platinum-group minerals that accompany it, all of which report to the same concentrate
  • Scheelite — mineral · with a fatty acid collector rather than a xanthate, calcium minerals needing a different chemistry from sulfides
  • Fluorite — mineral · by the same fatty acid route as scheelite, and with the same difficulty separating it from the calcite alongside it
  • Apatite — mineral · the step that makes phosphate rock into fertiliser feed by removing the carbonate and silica around it
  • Water — compound · the medium the whole separation happens in — a slurry of ground ore in water, with air blown through it
  • Battery graphite — material · read from the other end — flotation is how flake graphite is concentrated before any of the anode processing begins

was succeeded by

  • Roasting — process · the concentrate goes to the furnace; flotation moved the mineral, and roasting is where its chemistry is finally changed
  • Smelting — process · or straight to the smelter, where the concentrate is reduced without a separate roasting stage — the porphyry route, which delivers a concentrate at around twenty-five per cent copper from ore at less than one
  • Solvent extraction and electrowinning — process · the alternative to the smelter for what flotation cannot concentrate — a parallel route rather than a later stage

is used in

  • Mining and quarrying — industry · the process that made low-grade disseminated sulfide deposits economic, and therefore made the modern copper industry

Sources

  • Material World
    Our own writing

Where it comes from, and what it becomes

Follow Froth flotation back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Froth flotation → takes as input (read from the other end — flotation is how flake graphite is concentrated before any of the anode processing begins) → Battery graphite → is produced by (which is where synthetic graphite comes from, and the furnace runs at around 3,000 °C for weeks to convert disordered carbon into ordered sheets) → Acheson process → takes as input (sand and coke, heated to 2,500 °C, which is how silicon carbide has been made since 1893) → Sand → is sourced from (the ultimate source of most of it — granite weathers, the feldspar becomes clay and washes out, and the quartz survives as grains) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
  • Froth flotation → takes as input (the medium the whole separation happens in — a slurry of ground ore in water, with air blown through it) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Froth flotation → takes as input (the archetypal flotation feed — porphyry copper ore runs well under one per cent copper and leaves as a concentrate of twenty-five or more) → Chalcopyrite
  • Froth flotation → takes as input (by the same fatty acid route as scheelite, and with the same difficulty separating it from the calcite alongside it) → Fluorite
  • Froth flotation → takes as input (the step that makes phosphate rock into fertiliser feed by removing the carbonate and silica around it) → Apatite
  • Froth flotation → takes as input (floated ahead of sphalerite, the zinc mineral being held back with a depressant and taken in a second stage) → Galena

These are the most distinct paths back. Froth flotation can be traced through others besides.

Downstream — what it becomes

  • Froth flotation → is used in (the process that made low-grade disseminated sulfide deposits economic, and therefore made the modern copper industry) → Mining and quarrying → is associated with (steam pumping allowed mines below the water table, which is what made deep coal possible) → Industrial Revolution complete chain