# Caustic soda: Production route

Membrane, diaphragm and the fading mercury cell

Source page: https://www.materialinsights.org/chemicals/caustic_soda/routes/

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 caustic**

Sodium 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 caustic**

Cell 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.

## Other sections

- [Summary](https://www.materialinsights.org/chemicals/caustic_soda/index.md)
- [Overview](https://www.materialinsights.org/chemicals/caustic_soda/overview/index.md)
- [Correlation](https://www.materialinsights.org/chemicals/caustic_soda/correlation/index.md)
- [Production](https://www.materialinsights.org/chemicals/caustic_soda/production/index.md)
- [Feedstocks](https://www.materialinsights.org/chemicals/caustic_soda/feedstocks/index.md)
- [Cycles](https://www.materialinsights.org/chemicals/caustic_soda/cycles/index.md)
- [Trading](https://www.materialinsights.org/chemicals/caustic_soda/trading/index.md)
- [Movement](https://www.materialinsights.org/chemicals/caustic_soda/movement/index.md)
- [Regulation](https://www.materialinsights.org/chemicals/caustic_soda/regulation/index.md)
- [Characteristics](https://www.materialinsights.org/chemicals/caustic_soda/characteristics/index.md)
- [Emissions](https://www.materialinsights.org/chemicals/caustic_soda/emissions/index.md)
- [Relationships](https://www.materialinsights.org/chemicals/caustic_soda/links/index.md)