Caustic soda: Production route
Membrane, diaphragm and the fading mercury cell
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.Almost all caustic soda is electrolytic and comes from chlor-alkali cells. Membrane cells give the purest product with the least energy; diaphragm cells give a salt-bearing grade that must be purified; mercury cells are close to extinct. A small volume, called chemical caustic, is made by reacting soda ash with lime (the old Le Blanc-era causticising route), mainly when caustic is expensive relative to soda ash (USGS).
Routes compared
| Route | Share of supply | Inputs per tonne | Typical scale | Yield | Co-products | Unit cost |
|---|---|---|---|---|---|---|
| Membrane-cell caustic | majority of world capacity, about 85% or more (indicative); 100% in India | about 1.5 t salt and 2.2-2.5 MWh per t caustic (the ECU's power divided by 1.13), plus steam | 100-1,000 kt/yr | high | 0.89 t chlorine and 0.025 t hydrogen per t caustic | cash cost of an ECU about $560 in NW Europe (Nov 2025); lower in China, US and Middle East |
| Diaphragm-cell caustic | minority, mostly US (indicative) | more steam and power than membrane (indicative) | large US Gulf sites | lower quality | chlorine, hydrogen | higher energy; closing (450,000 ECU closed Oct 2025, 520,000 ECU due Oct 2026 in the US) |
| Chemical caustic (soda ash plus lime) | small (USGS notes it as an alternative to electrolytic caustic) | about 1.3 t soda ash and 0.7 t lime per t NaOH (stoichiometric) | small | dilute product | calcium carbonate mud | competitive only when caustic is expensive versus soda ash |
How each route works
Membrane-cell causticSodium ions cross a cation-exchange membrane to the cathode, where they meet water to form about 32% caustic and hydrogen. Evaporators take it to 50%. The product is low in salt and chlorate, suitable for rayon, food and battery uses.
purest, lowest energy; tied to chlorine outlets
Argus Chlor-Alkali Outlook Nov 2025; EU BREF 2014 (indicative) Diaphragm-cell causticCell liquor of 10-12% caustic with about 15% salt is evaporated; salt crystallises out and is recycled. The final product holds about 1% salt.
older, cheaper to build originally; asbestos phase-out by 2029/2036
USGS MCS 2026; Argus Nov 2025 Chemical caustic (soda ash plus lime)Sodium carbonate reacts with slaked lime to give sodium hydroxide and calcium carbonate. It decouples caustic from chlorine but needs cheap soda ash.
no chlorine; extra CO2 from lime and soda ash
USGS MCS 2026 (Soda ash) Unit economics are indicative ranges from the sources named; costs move with feedstock and energy prices, plant size and region.