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Material World
Process

Metallothermic reduction

Taking a metal out of its compound by offering a hungrier metal in exchange — the route to the elements carbon and electricity cannot reach.

Some oxides are too stable for carbon to take the oxygen away, and some metals form carbides instead of reducing when you try. For those, the answer is to offer a metal that wants the oxygen more: magnesium, aluminium, calcium or sodium, depending on what is being made and what can be afforded.

The reaction is a straightforward exchange and it is usually violently exothermic. The best-known instance is thermite — aluminium powder and iron oxide, which reaches around 2,500 °C and produces molten iron, and which is used to weld railway track in place because it needs no external power at all.

Processing

The choice of reducing metal is an economic argument dressed as a chemical one. Aluminium is cheapest and reduces most oxides; magnesium is used where aluminium would contaminate the product, as in the Kroll process for titanium and zirconium; calcium and sodium are reserved for halides and for the most stubborn oxides, and are handled with corresponding care.

The heat is both the mechanism and the problem. Enough of it makes the reaction self-sustaining once started, which is convenient; too much and the product melts into the crucible or the charge sprays out of it. Industrial practice is largely about moderating a reaction that would otherwise run away.

Separating the product from the slag is the other half of the job. The reduced metal and the reducing metal's oxide have to part cleanly, which they do when the temperature is high enough for both to be liquid and their densities differ enough for one to settle.

Economic significance

This is the expensive way to make a metal, and the elements made this way are produced in tonnes or hundreds of tonnes a year rather than in millions. That is not a failure of the method — it reflects what these metals are used for, which is small quantities in specific places.

Its largest use by tonnage is not making a metal at all but making a ferroalloy: ferrovanadium, ferroniobium and ferrotitanium are aluminothermic products, and they go straight into steel without the element ever being isolated.

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.

produces

  • Beryllium — element · magnesium reducing beryllium fluoride, and the reason beryllium is expensive before anyone accounts for how carefully it has to be handled
  • Boron — element · magnesium reducing boron oxide, which gives an impure amorphous boron — the crystalline element needs a different route entirely
  • Barium — element · aluminium reducing barium oxide under vacuum, the barium distilling off as vapour
  • Strontium — element · the same aluminothermic route as barium, and in similarly small quantity
  • Caesium — element · calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives
  • Rubidium — element · reduced from its chloride the same way, and usually as a by-product of the caesium it accompanies in pollucite
  • Titanium — element · the Kroll process is this reaction — magnesium reducing titanium tetrachloride — and this is the general case it belongs to

takes as input

  • Magnesium — element · the reducing metal where aluminium would contaminate the product, which is why titanium and zirconium go the magnesium route
  • Aluminium — element · the cheapest reducing metal and the commonest, and the aluminium in thermite
  • Calcium — element · reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job

was succeeded by

  • Kroll process — process · the Kroll process is the industrialised special case, for titanium and zirconium specifically

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Metallothermic reduction 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

  • Metallothermic reduction → takes as input (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
  • Metallothermic reduction → takes as input (the reducing metal where aluminium would contaminate the product, which is why titanium and zirconium go the magnesium route) → Magnesium → is produced by (from seawater and from salt-lake brine, precipitated as the hydroxide before reduction — the ocean is an effectively unlimited magnesium resource) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → Salt → is sourced from (by solar evaporation, which needs a dry sunny coast and is the cheapest route there is) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Metallothermic reduction → takes as input (reserved for halides and the most stubborn oxides, where the cheaper metals will not do the job) → Calcium → is produced by (from molten calcium chloride; there is no smelting route) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite
  • Metallothermic reduction → takes as input (the cheapest reducing metal and the commonest, and the aluminium in thermite) → Aluminium → is produced by (collects at the cathode while the carbon anodes are consumed) → Hall–Héroult process → takes as input (dissolved in molten cryolite and electrolysed) → Aluminium oxide → is sourced from (dissolved out with hot caustic soda in the Bayer process) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt
  • Metallothermic reduction → takes as input (the reducing metal where aluminium would contaminate the product, which is why titanium and zirconium go the magnesium route) → Magnesium → is produced by (from molten magnesium chloride, including the chloride returned by the Kroll process) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite
  • Metallothermic reduction → takes as input (the reducing metal where aluminium would contaminate the product, which is why titanium and zirconium go the magnesium route) → Magnesium → is produced by (from seawater and from salt-lake brine, precipitated as the hydroxide before reduction — the ocean is an effectively unlimited magnesium resource) → Brine evaporation → takes as input (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water

These are the most distinct paths back. Metallothermic reduction can be traced through others besides.

Downstream — what it becomes

  • Metallothermic reduction → produces (the Kroll process is this reaction — magnesium reducing titanium tetrachloride — and this is the general case it belongs to) → Titanium → is a component of (the other half of the strengthening phase) → Nickel superalloy → is associated with (the jet engine created the industry, because no existing material survived the turbine inlet) → The wartime materials programmes complete chain
  • Metallothermic reduction → produces (magnesium reducing boron oxide, which gives an impure amorphous boron — the crystalline element needs a different route entirely) → Boron → is a component of (about one per cent, and structurally essential — the compound is Nd₂Fe₁₄B, and without the boron it does not form) → Neodymium magnet → is used as (the strongest in commercial use, and the reason a motor, a hard drive and an earbud can be small) → Permanent magnets
  • Metallothermic reduction → produces (magnesium reducing beryllium fluoride, and the reason beryllium is expensive before anyone accounts for how carefully it has to be handled) → Beryllium → is used as (copper-beryllium alloys, for non-sparking and spring applications) → Alloying
  • Metallothermic reduction → produces (aluminium reducing barium oxide under vacuum, the barium distilling off as vapour) → Barium → is used as (barium sulfate for gastrointestinal X-ray contrast) → Medical imaging
  • Metallothermic reduction → produces (the same aluminothermic route as barium, and in similarly small quantity) → Strontium → is used as → Pigment
  • Metallothermic reduction → produces (calcium or barium reducing caesium chloride under vacuum — caesium is too reactive to survive most alternatives) → Caesium → is used as (as a photocathode, exploiting the lowest ionisation energy of any stable element) → Electrical conduction

These are the most distinct paths onward. Metallothermic reduction ends up in others besides.