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Material World
Process

Kroll process

Reducing titanium tetrachloride with magnesium in a sealed vessel — slow, batch-wise, and still the only economic route to titanium metal.

Titanium is the ninth most abundant element in the crust and one of the most expensive structural metals, and the Kroll process is the reason. Titanium's affinity for oxygen is so strong that it cannot be smelted from its oxide the way iron can — carbon will not take the oxygen away, and any attempt leaves a brittle carbide.

Instead the ore is converted to titanium tetrachloride, distilled to purity, and reduced with molten magnesium under argon. The result is a porous mass called titanium sponge, which must then be crushed, melted and cast before it is metal in any usable sense.

Processing

The process is a batch operation measured in days, in a sealed vessel, under an inert atmosphere, with the magnesium chloride by-product recycled by electrolysis back to magnesium and chlorine. Every one of those requirements adds cost that a continuous process would not have.

It has been the industry standard since the 1940s and continuous alternatives have been pursued throughout that time without displacing it. When a titanium component is described as expensive, most of the expense is here rather than in the machining.

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

  • Titanium — element · as sponge, which must then be crushed, melted and cast before it is usable metal
  • Zirconium — element · reduced from the tetrachloride with magnesium, by the same route and in the same kind of vessel as titanium
  • Hafnium — element · separated from zirconium first — the two are chemically almost identical, and only nuclear use makes the separation worth its cost — then reduced the same way

takes as input

  • Magnesium — element · the reducing agent, recovered afterwards by electrolysing the magnesium chloride by-product
  • Chlorine — element · converts the ore to a distillable tetrachloride, which is how the purification is done
  • Argon — element · the inert atmosphere, without which the titanium would take oxygen from the air
  • Rutile — mineral · converted to titanium tetrachloride, which is distilled to purity before reduction
  • Ilmenite — mineral · after the iron is stripped out to make a synthetic rutile substitute

succeeded

  • Metallothermic reduction — 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 Kroll process 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

  • Kroll process → takes as input (converts the ore to a distillable tetrachloride, which is how the purification is done) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation → takes as input (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Seawater
  • Kroll process → takes as input (the reducing agent, recovered afterwards by electrolysing the magnesium chloride by-product) → 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
  • Kroll process → takes as input (the inert atmosphere, without which the titanium would take oxygen from the air) → Argon → is produced by (drawn from an intermediate height in the column, between nitrogen and oxygen) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Kroll process → takes as input (converted to titanium tetrachloride, which is distilled to purity before reduction) → Rutile
  • Kroll process → takes as input (after the iron is stripped out to make a synthetic rutile substitute) → Ilmenite
  • Kroll process → takes as input (the reducing agent, recovered afterwards by electrolysing the magnesium chloride by-product) → 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

These are the most distinct paths back. Kroll process can be traced through others besides.

Downstream — what it becomes

  • Kroll process → produces (separated from zirconium first — the two are chemically almost identical, and only nuclear use makes the separation worth its cost — then reduced the same way) → Hafnium → is a component of (a fraction of a per cent, at the grain boundaries of the alloys that still have grain boundaries) → Nickel superalloy → is used in (cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely) → Turbine blade → is associated with (the jet engine created the superalloy industry, because no existing material survived the turbine inlet — and turbine temperature has risen about 500 °C since, almost entirely on the blade) → The wartime materials programmes complete chain
  • Kroll process → produces (as sponge, which must then be crushed, melted and cast before it is usable metal) → 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
  • Kroll process → produces (reduced from the tetrachloride with magnesium, by the same route and in the same kind of vessel as titanium) → Zirconium → is used as (fuel cladding, chosen because it lets neutrons through) → Nuclear fuel
  • Kroll process → produces (as sponge, which must then be crushed, melted and cast before it is usable metal) → Titanium → is a source for (from the residues of titanium and uranium processing and from bauxite tailings; no deposit is worked for scandium alone) → Scandium → is used as → Alloying
  • Kroll process → produces (separated from zirconium first — the two are chemically almost identical, and only nuclear use makes the separation worth its cost — then reduced the same way) → Hafnium → is used in (in the directionally solidified grades that still have grain boundaries, to strengthen them) → Turbine blade → is used in (and the capability is the casting yield and the coating rather than the alloy, whose composition is published — which is why jet engines are a three-company industry) → Aerospace manufacture
  • Kroll process → produces (as sponge, which must then be crushed, melted and cast before it is usable metal) → Titanium → is used as (as titanium dioxide, which consumes more titanium by tonnage than metal production does) → Pigment

These are the most distinct paths onward. Kroll process ends up in others besides.