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Material World
Process

Chlor-alkali electrolysis

Electrolysing brine into chlorine, sodium hydroxide and hydrogen — three products in fixed proportion whether or not demand agrees.

Passing a current through sodium chloride solution splits it: chlorine at the anode, hydrogen and sodium hydroxide at the cathode. The chemistry is simple and the consequence is not, because the products arrive in a ratio fixed by stoichiometry rather than by what anyone wants.

Roughly a tonne of caustic soda accompanies every tonne of chlorine, and demand for the two moves independently. That coupling has shaped the chemical industry for a century: when PVC demand rises the market floods with caustic soda, and the price of each is driven as much by the other's demand as by its own.

Uses

All industrial chlorine and most sodium hydroxide. Chlorine goes overwhelmingly into PVC and the chlorinated organic chemicals most pharmaceutical synthesis passes through; caustic soda into pulp and paper, alumina refining, soap and a wide range of chemical manufacture.

The cell technology has changed for environmental reasons rather than economic ones. Mercury cells gave the purest caustic and lost mercury continuously to the environment; diaphragm cells used asbestos. Membrane cells, which use an ion-selective polymer and neither, are now standard, and the Minamata Convention has closed the mercury route in most of the world.

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

  • Halite — mineral · as brine; the salt is the feedstock for both products at once
  • Water — compound · as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode
  • Salt — compound · the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on

produces

  • Sodium hydroxide — compound · at the cathode, in fixed proportion to the chlorine whether demand agrees or not
  • Chlorine — element · at the anode
  • Hydrogen — element · the third product, often burned for process heat rather than sold

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Chlor-alkali electrolysis 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

  • 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 → 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
  • Chlor-alkali electrolysis → takes as input (as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → 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
  • Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • 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 sourced from (mined as rock salt from bedded deposits, or dissolved in place and pumped up as brine — which is what the chemical industry wants anyway) → Evaporite → is sourced from (what is left when a body of seawater evaporates faster than it is replenished — the salts come out in order of solubility, which is why the sequence is readable) → 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
  • 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 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
  • 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 sourced from (mined as rock salt from bedded deposits, or dissolved in place and pumped up as brine — which is what the chemical industry wants anyway) → Evaporite → is composed of (precipitated early, when the water has reduced to roughly a fifth of its volume) → Gypsum

These are the most distinct paths back. Chlor-alkali electrolysis can be traced through others besides.

Downstream — what it becomes

  • Chlor-alkali electrolysis → produces (at the anode) → Chlorine → is an input to (converts the ore to a distillable tetrachloride, which is how the purification is done) → 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
  • Chlor-alkali electrolysis → produces (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Sodium hydroxide → is an input to (hot concentrated caustic soda dissolves the aluminium hydroxides) → Bayer process → produces (crystallised from the liquor and calcined, ready for the smelter) → Aluminium oxide → is an input to (dissolved in molten cryolite and electrolysed) → Hall–Héroult process → produces (collects at the cathode while the carbon anodes are consumed) → Aluminium
  • Chlor-alkali electrolysis → produces (the third product, often burned for process heat rather than sold) → Hydrogen → is used as (via ammonia synthesis, which consumes about half of world production) → Fertiliser
  • Chlor-alkali electrolysis → produces (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Sodium hydroxide → is used as (in the Bayer process and in metal cleaning before coating) → Corrosion protection
  • Chlor-alkali electrolysis → produces (at the anode) → Chlorine → is used as (chlor-alkali production underpins much of industrial chemistry) → Catalysis
  • Chlor-alkali electrolysis → produces (at the cathode, in fixed proportion to the chlorine whether demand agrees or not) → Sodium hydroxide → is an input to (hot concentrated caustic soda dissolves the aluminium hydroxides) → Bayer process → produces (crystallised from the liquor and calcined, ready for the smelter) → Aluminium oxide → is a component of (alumina is the most-used engineering ceramic) → Technical ceramic → is used as (alumina and silicon carbide are the dominant manufactured abrasives) → Abrasive

These are the most distinct paths onward. Chlor-alkali electrolysis ends up in others besides.