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ElectrolysisIndustrial scale

Chlor-alkali process

Electrolysis of brine (NaCl) into chlorine (Cl₂), caustic soda (NaOH) and hydrogen (H₂) in a single process. Cornerstone of mineral chemistry — world production ~85 Mt Cl₂/year and ~80 Mt NaOH/year.

Decomposition driven by electric current

Key reaction

2 NaCl + 2 H₂O → Cl₂ + 2 NaOH + H₂ (anode : 2 Cl⁻ → Cl₂ + 2 e⁻ ; cathode : 2 H₂O + 2 e⁻ → H₂ + 2 OH⁻)

Operating conditions

Temperature
80-90°C
Pressure
1bar
Catalyst
Anode DSA (Ti/RuO₂-IrO₂) ; cathode Ni-Ru-O ; membrane Nafion™
Phase
liquid + gas

How it works

Schema coming soon

How it works

The chlor-alkali process turns an abundant cheap raw material (salt) into three fundamental chemicals whose markets are independent: chlorine (PVC, disinfection, organic intermediates), caustic soda (soaps, alumina, paper, textiles) and hydrogen (refining, chemistry, energy). It's a massive co-production — for every tonne of Cl₂, you get ~1.12 t NaOH and ~28 kg H₂. This fixed stoichiometry is both the strength and weakness of the process: if Cl₂ demand collapses, NaOH still gets produced without buyers (and vice versa). There are three industrial technologies. (1) Mercury cell (Castner-Kellner, 1892): liquid Hg cathode forms a Na-Hg amalgam, decomposed separately with water to give 50 % NaOH + H₂. Phasing out (banned in the EU since 2017) due to Hg emissions. (2) Diaphragm cell (1888): separates anode and cathode by a porous asbestos membrane. Produces dilute NaOH (~12 %) contaminated with residual NaCl. (3) Membrane cell (since 1975): dominant today (~75 % of global capacity). Perfluorinated cation-exchange membrane (Nafion™ type) only lets Na⁺ through. Directly yields very pure 30-35 % NaOH, with no Hg or asbestos. In a modern membrane cell, saturated brine (~300 g/L NaCl) is fed to the anode side (titanium coated with RuO₂-IrO₂, dimensionally stable anode DSA) where Cl⁻ is oxidized to Cl₂. Na⁺ cations migrate through the membrane to the cathode compartment (nickel cathode coated with Ni-Ru-O). On the cathode side, water is reduced to H₂ + OH⁻, which combine with incoming Na⁺ to form NaOH. Typical voltage is 3 V, Faradic efficiency 95-97 %, consumption 2100-2400 kWh/t Cl₂ (modern membrane cell). World production 2023: ~85 Mt Cl₂/year, ~80 Mt NaOH/year. Chlorine demand is driven by PVC (~38 %) and chlorinated organics (isocyanates, solvents); caustic demand is driven by Bayer alumina, paper (declining), detergents and organic chemistry.

Key components

The role of each main part, and the elements / compounds it involves.

  • Brine purification system

    Removes Ca²⁺, Mg²⁺ and SO₄²⁻ that would poison the membrane.

    Ca/Mg precipitation with soda + Na₂CO₃ (forms insoluble CaCO₃ and Mg(OH)₂), filtration, then ion-exchange resin treatment (chelating type Lewatit™ TP260) that drops residual Ca²⁺ below 20 ppb. The Nafion membrane needs ultra-pure brine or it clogs — even 1 ppm Ca²⁺ can degrade performance within weeks.

    Ca²⁺ < 20 ppb · résine chélatante · Na₂CO₃ + NaOH

    See also :naclnaohna2co3

  • Dimensionally stable anode (DSA)

    Oxidizes Cl⁻ to Cl₂ without being consumed. Key innovation that replaced graphite anodes.

    Titanium plate or mesh coated with mixed RuO₂-IrO₂-TiO₂ oxides (~3 g/m² Ru). Patented by Henri Beer (1965) and commercialized by De Nora. Very low Cl₂/Cl⁻ overpotential (~50 mV) — this is what enables 3 V operation instead of 4 V with graphite. Lifetime 8-12 years, ~1500 kg Cl₂ produced per gram of Ru.

    Ti / RuO₂-IrO₂-TiO₂ · ~3 g/m² Ru · 8-12 ans · η ~50 mV

  • Cation exchange membrane

    Separates anode and cathode compartments, allowing only Na⁺ to pass.

    Multilayer perfluorinated membrane (Nafion™ DuPont, Flemion™ AGC) with sulfonate groups (-SO₃⁻) on the anode side and carboxylate (-COO⁻) on the cathode side. Thickness 100-200 µm, Na⁺ selectivity > 95 %. Cost ~€1500/m²; lifetime 4-6 years. This is the innovation that retired mercury and asbestos cells.

    Perfluorée bicouche · 100-200 µm · sélectivité Na⁺ > 95 % · 4-6 ans

  • Activated nickel cathode

    Reduces water to H₂ + OH⁻ with low overpotential.

    Pure nickel mesh or perforated sheet (resists 32 % NaOH), coated with a Ni-Ru or Ni-Mo catalytic deposit that lowers HER overpotential by ~100 mV. Recovered H₂ is purified (removes traces of O₂, Cl₂) and either valorized (refining, chemistry) or burnt in an auxiliary boiler — depending on local H₂ market value.

    Ni pur · revêtement Ni-Ru / Ni-Mo · η_HER ~100 mV abaissée

    See also :h2ni

  • Cl₂ compression and liquefaction

    Dries, compresses and liquefies Cl₂ for storage and transport.

    Concentrated H₂SO₄ dryer (wet Cl₂ is highly corrosive), then multi-stage compressor (Monel or Hastelloy C-276 casing) to 8-15 bar. Liquefaction by cooling to −34 °C. Liquefied Cl₂ is stored in pressurized rail cars or piped locally (PVC plants typically locate next door to the chlor-alkali unit to avoid Cl₂ transport).

    Sécheur H₂SO₄ · 8-15 bar · liquéfaction −34 °C · transport wagon ou pipeline

    See also :h2so4

Physical and chemical principles

The fundamental laws that make this process possible — and the constraints they impose.

  • Anode/cathode electrochemical coupling

    Electrolysis enforces strict product separation: Cl₂ must NEVER meet NaOH (would form NaOCl + NaCl) or H₂ (explosive mix). The membrane provides this separation. Theoretical decomposition potential is 2.19 V; overpotentials and ohmic drop bring industrial voltage to ~3 V — every mV saved translates into millions of euros at the scale of a 200 kt/year Cl₂ plant.

    E°(2 Cl⁻ → Cl₂) = +1,36 V ; E°(2 H₂O → H₂ + 2 OH⁻) = −0,83 V
    Applies to components :membrane-nafionanode-dsa

  • Stoichiometric co-production

    1 mole Cl₂ ↔ 2 moles NaOH ↔ 1 mole H₂: stoichiometry is fixed by electrons. To decouple markets, some plants add a downstream step: Cl₂ + H₂ → 2 HCl (hydrochloric acid synthesis) or Cl₂ + 2 NaOH → NaOCl + NaCl + H₂O (bleach) — consuming seasonal excess without altering the global balance.

Compounds involved

Input

Output

World production

85 Mt/yr
2023

Main applications

  • PVC (polyvinyl chloride)38 %
  • Chlorinated organic chemistry (isocyanates, solvents)22 %
  • Alumina (Bayer) and paper (NaOH)18 %
  • Water disinfection and treatment8 %
  • Detergents, textiles, soaps14 %

Residual mercury and decarbonized power

The industry has nearly eliminated mercury cells (still ~5 % of global capacity in 2023, mostly in India and Russia), but historic-site decommissioning leaves 2-3 kt of Hg still in industrial circulation, strictly framed by the Minamata Convention (2013). On the energy side, the process consumes ~2 % of global industrial electricity — its decarbonization comes from low-carbon power (already done in Norway, Sweden) and from cathode innovations (oxygen-depolarized cathodes that cut electricity by ~30 % by replacing H₂ production with its immediate consumption).
  • Cellule à cathode à dépolarisation O₂ (ODC) — −30 % d'électricité
  • Membranes perfluorées plus fines (efficacité +5 %)
  • Anodes DSA de génération 4 (η < 30 mV)
  • Couplage avec PV/éolien (demand response sur les usines flexibles)

Similar or competing processes

Related industrial processes — alternative chemistry, alternative technology.

  • bayer

    Major NaOH downstream user — the aluminium industry consumes ~10 % of global NaOH.

  • solvay

    Partial competitor for caustic — Solvay yields Na₂CO₃, but 1 t Na₂CO₃ can often replace 0.75 t NaOH depending on the use.

History and discovery

Discovery year1892
First industrial deployment1894
Hamilton Castner · Karl Kellner· Royaume-Uni / Autriche
Sources
  • Ullmann's Encyclopedia of Industrial Chemistry — Chlorine
  • Euro Chlor — Chlorine Industry Review (annual)
  • World Chlorine Council — Capacity Database
  • O'Brien, T. — Handbook of Chlor-Alkali Technology
Processes