Chlorine: Production route
Why mercury cells vanished and membranes took over
What this tab covers How it is made: the main production routes or, for product families, the main product types and technologies, compared on share of supply, inputs per tonne, plant scale, yield, co-products and unit cost.All commercial chlorine comes from electrolysis of sodium chloride brine (a little comes from potassium chloride, which yields caustic potash). Three cell types exist. Membrane cells now dominate because they use the least energy and make high-purity caustic. Diaphragm cells survive mainly in the US and use asbestos or polymer diaphragms. Mercury cells are almost gone: banned in the EU since December 2017 and phased out under the Minamata Convention; only one operated in the US in 2025 (USGS). Small volumes are recovered by oxidising HCl (Deacon-type and electrolytic HCl recycling, used at isocyanate plants).
Routes compared
| Route | Share of supply | Inputs per tonne | Typical scale | Yield | Co-products | Unit cost |
|---|---|---|---|---|---|---|
| Membrane-cell electrolysis | majority of world capacity, about 85% or more (indicative); 100% in India and nearly all of China's coastal capacity | 1.65-1.75 t salt and about 2.5-2.8 MWh electricity per t Cl2, plus steam for caustic evaporation | typical plant 100-500 kt/yr chlorine; largest sites above 1 Mt/yr | about 95-97% current efficiency | 1.13 t caustic soda (100% basis) and 0.028 t hydrogen per t Cl2 | electricity of 2.7 MWh at $60/MWh is about $160 per t Cl2 or $150 per ECU before salt, steam and fixed costs (indicative); Argus put cash cost at about $560 per ECU in NW Europe in Nov 2025 |
| Diaphragm-cell electrolysis | a minority of world capacity, concentrated in the US Gulf (indicative) | about 2.9-3.3 MWh electricity plus more steam per t Cl2 (indicative) | large legacy US Gulf complexes | lower caustic quality (about 1% salt) | caustic soda and hydrogen | higher energy cost than membrane; plants are closing (450,000 ECU of US diaphragm closed in October 2025, 520,000 ECU scheduled for October 2026) |
| Mercury-cell electrolysis | residual; banned in the EU since end-2017 and being closed worldwide | about 3.3-3.6 MWh per t Cl2 (indicative) | small, old plants | high-purity caustic | caustic soda, hydrogen; mercury-contaminated waste | highest energy cost |
| HCl recycling (oxidation or electrolysis of by-product HCl) | small, a few per cent of chlorine supply (indicative) | about 1.03 t HCl per t Cl2; electrolysis about 1.0-1.1 MWh per t Cl2 with oxygen-depolarised cathodes (indicative) | 100-200 kt/yr units at MDI sites | high | water | not published |
How each route works
Membrane-cell electrolysisPurified brine flows into the anode side of a cell divided by a cation-exchange membrane. Chlorine forms at the anode; sodium ions cross the membrane and meet water at the cathode, giving about 32% caustic soda and hydrogen. Caustic is evaporated to 50%.
lowest energy and purest caustic; needs very clean brine and membrane replacement every 3-5 years
EU BREF Chlor-alkali 2014 (indicative); Argus Chlor-Alkali Outlook Nov 2025 Diaphragm-cell electrolysisA porous diaphragm, historically asbestos, separates anode and cathode. Brine seeps through, so the cell liquor is about 10-12% caustic with salt, which must be evaporated and salt removed. Energy use is higher.
tolerates less pure brine; asbestos diaphragms must go by 2029 for most US plants and by 2036 for all under EPA rules
USGS MCS 2026 (Asbestos); Argus Chlor-Alkali Outlook Nov 2025 Mercury-cell electrolysisSodium forms an amalgam with a flowing mercury cathode, which is decomposed with water in a separate vessel to give 50% caustic directly. Mercury losses to air, water and product were the main legacy pollution of the industry.
pure 50% caustic without evaporation; unacceptable mercury risk
USGS MCS 2026 (Mercury); EU BREF 2014 (indicative) HCl recycling (oxidation or electrolysis of by-product HCl)Isocyanate and polycarbonate plants make by-product HCl. It can be oxidised back to chlorine over a catalyst (Sumitomo, Covestro processes) or electrolysed with oxygen-depolarised cathodes, closing the chlorine loop inside the site.
avoids co-producing caustic; only where large HCl streams exist
EU BREF 2014 (indicative) Unit economics are indicative ranges from the sources named; costs move with feedstock and energy prices, plant size and region.