- How Is Caustic Soda Made? The Short Answer
- What Is Caustic Soda?
- What Is Caustic Soda Made From?
- The Three Caustic Soda Production Methods
- Caustic Soda Production Process, Step by Step (Membrane Cell)
- From Cell Liquor to Flakes, Pearls and Liquid
- History of Caustic Soda Production
- Energy, Environment and Regulation in 2026
- Why the Production Route Matters When You Buy Caustic Soda
- Buy Membrane-Grade Caustic Soda from SUHA International
- Where Caustic Soda Goes After It Is Made
- Safety in Caustic Soda Production and Handling
- Frequently Asked Questions
- Sources and Further Reading
- Reach Out
- Top Products
- Contact Us
How Is Caustic Soda Made? The Short Answer
Technically reviewed and updated: 29 October 2026 · SUHA International Trading technical desk
Caustic soda (sodium hydroxide, NaOH) is made by passing a direct electric current through purified salt water (brine) in an electrolytic cell. This is called the chlor-alkali process. Sodium chloride and water split into sodium hydroxide, chlorine gas and hydrogen gas in fixed proportions:
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂Almost all new plants use membrane cells, which produce a clean, mercury-free liquor of about 32% NaOH. That liquor is evaporated to 50% liquid caustic soda, or concentrated further to an anhydrous melt and solidified into flakes or pearls of 98–99% NaOH.
- NaCl + H₂ORaw materials: salt, water and electricity
- ~1.5 tSalt per tonne of NaOH (1.46 t in theory)
- 0.89 t Cl₂Chlorine co-produced per tonne of NaOH
- ~32%NaOH in membrane-cell liquor before evaporation
- ≈2.5 MWhElectricity per tonne of NaOH, typical membrane plant
- 98–99%NaOH in finished flakes and pearls
The rest of this guide covers each stage in order: the raw materials, the three cell technologies, the membrane process step by step, how the liquor becomes flakes, pearls or 50% liquid, and the 2026 regulatory picture. It ends with what the production route means for anyone buying caustic soda in bulk.
What Is Caustic Soda?
Caustic soda is the trade name for sodium hydroxide (NaOH), a strong alkali. It is also called lye, but in industry "lye" can also mean potassium hydroxide, so the chemical name is safer on contracts. It is sold as white solid flakes, pearls (also called prills or beads), or as a colourless solution, usually 48–50%. It dissolves in water with strong heat release and corrodes skin, eyes, aluminium and zinc.
| Chemical formula | NaOH |
|---|---|
| Molar mass | 40.00 g/mol |
| CAS number | 1310-73-2 |
| HS code | 2815.11 (solid) · 2815.12 (aqueous solution) |
| UN number / class | UN 1823 (solid) · UN 1824 (solution) · Class 8, PG II |
| Melting point | 318 °C (604 °F) |
| Density | 2.13 g/cm³ (solid) · ≈1.53 g/cm³ (50% solution, 20 °C) |
| pH | ≈13 (1% solution) to 14 (1 M solution) |
What Is Caustic Soda Made From?
Caustic soda is made from salt, water and electricity. The salt supplies the sodium. The water supplies the hydroxide. The electricity drives the reaction, because salt and water do not react on their own.
| Input | Role in the process | Typical use per tonne NaOH |
|---|---|---|
| Sodium chloride (NaCl) | Rock salt, solar (sea) salt or solution-mined brine. Source of sodium and chloride ions. | 1.46 t in theory; ~1.5–1.6 t in practice |
| Water (H₂O) | Dissolves the salt into brine and is split at the cathode into hydrogen and hydroxide. | 0.45 t consumed by the reaction, plus process water |
| Electricity (DC) | Drives electrolysis. The largest single cost in caustic soda production. | ≈2.5 MWh (membrane, incl. auxiliaries) |
| Steam | Evaporates cell liquor from ~32% to 50% NaOH, and further for solids. | Varies with evaporator design |
| Treatment chemicals | Sodium carbonate and NaOH precipitate calcium and magnesium. Sulfate is removed by nanofiltration or barium salts. Chelating ion-exchange resin polishes the brine. Hydrochloric acid adjusts pH and is used in dechlorination. | Small quantities |
The Three Caustic Soda Production Methods
All industrial caustic soda production is electrolytic. What differs is how each cell keeps the chlorine made at the anode apart from the sodium hydroxide made at the cathode. That separation decides the product's purity, the energy bill and the environmental footprint.
Membrane Cell Process: the modern standard
Standard for all new plantsA cation-exchange membrane made of perfluorinated sulfonic/carboxylic polymer separates the anode and cathode compartments. Brands include Nafion™ (Chemours), Flemion™ (AGC) and Aciplex™ (Asahi Kasei). The membrane lets sodium ions through and blocks chloride ions, so the caustic side stays almost salt-free. Asahi Kasei started the first commercial membrane plant in Japan in 1975.
- Cell liquor: ≈30–33% NaOH, very low NaCl
- Finished 50% liquid: NaCl typically below 50 ppm
- Trade-off: needs ultra-pure brine; membranes are costly and are damaged by calcium, magnesium and other trace metals
Diaphragm Cell Process
Declining; asbestos types being phased outA porous diaphragm of asbestos, or of a newer non-asbestos polymer, separates the compartments. Brine seeps through it from the anode side to the cathode side. Because the diaphragm does not select ions, the cell liquor carries a lot of unreacted salt.
- Cell liquor: ≈10–12% NaOH mixed with ≈15% NaCl
- Finished 50% liquid: salt is crystallised out during evaporation, but around 1% NaCl usually remains
- Trade-off: tolerates less pure brine, but needs much more steam for evaporation and gives a lower-grade product
Mercury Cell Process (Castner–Kellner)
Phased out under the Minamata ConventionThe cathode is a flowing layer of mercury. Sodium forms a sodium–mercury amalgam, which is then reacted with water in a separate decomposer to give hydrogen and very pure 50% NaOH directly. Its purity was its strength. Its weakness was mercury release into air, water and product.
- Product: ≈50% NaOH straight from the decomposer, very low salt
- Trade-off: highest electricity use and mercury pollution
- Status: EU phase-out completed in December 2017; global phase-out date under the Minamata Convention was 2025
Membrane vs Diaphragm vs Mercury Cell: Comparison
| Feature | Membrane cell | Diaphragm cell | Mercury cell |
|---|---|---|---|
| Separator | Ion-exchange membrane | Porous diaphragm | Mercury cathode + decomposer |
| NaOH leaving the cell | ≈30–33% | ≈10–12% | ≈50% |
| NaCl in 50% product | < 50 ppm | ≈1% | Very low |
| Electricity at the cell, per t Cl₂ | ≈2.1–2.7 MWh | ≈2.5–3.0 MWh | ≈3.0–3.6 MWh |
| Steam for evaporation | Moderate | High | None for 50% |
| Main environmental issue | Energy use only | Asbestos (older plants) | Mercury emissions |
| Status in 2026 | Industry standard, growing | Declining | Phased out / closing |
Caustic Soda Production Process, Step by Step (Membrane Cell)
These are the stages in a modern membrane-cell chlor-alkali plant, from salt delivery to packed flakes or pearls.
-
Brine saturation
Salt is dissolved in water and recycled depleted brine to make a near-saturated brine of about 300–310 g/L NaCl.
-
Primary brine purification
Sodium carbonate precipitates calcium as calcium carbonate. Sodium hydroxide precipitates magnesium as magnesium hydroxide. Sulfate is controlled by nanofiltration, purging or barium salts. The solids are removed in clarifiers and fine filters.
-
Secondary purification (ion exchange)
Chelating ion-exchange resins polish the brine to below 20 ppb calcium plus magnesium and also cut strontium, barium, iron and other trace metals. At higher levels these metals precipitate inside the membrane, raise cell voltage and shorten membrane life.
-
Electrolysis
Brine enters the anode compartment, and dilute caustic soda circulates on the cathode side. Cells run at about 85–90 °C and roughly 3 V per cell. Many cells are stacked into one electrolyser. Anodes are titanium coated with ruthenium/iridium mixed metal oxides (DSA®-type), and cathodes are activated nickel.
Anode (oxidation): 2Cl⁻ → Cl₂ + 2e⁻
Cathode (reduction): 2H₂O + 2e⁻ → H₂ + 2OH⁻
Na⁺ ions cross the membrane and pair with OH⁻ to form NaOH at about 32%. -
Depleted brine recycling
Brine leaves the anode side at about 200–220 g/L NaCl. It is acidified and dechlorinated (vacuum and chemical), re-saturated with fresh salt and sent back to purification. This closed loop is why salt use stays close to the theoretical value.
-
Evaporation to 50% liquid
The ~32% liquor goes through multiple-effect evaporators to reach 48–50% NaOH. This is the main traded liquid grade.
-
Concentration to solid: flakes or pearls
For solids, 50% liquor is concentrated further in falling-film concentrators, typically heated by molten salt, until it becomes an almost water-free melt of about 98.5–99% NaOH at roughly 350–400 °C. The melt is then either:
- Flaked: picked up as a thin film on a water-cooled rotating drum and scraped off as caustic soda flakes, or
- Prilled: sprayed from the top of a prilling tower against rising cool air, where the droplets solidify into round pearls.
The solids are packed straight away in moisture-proof bags because NaOH absorbs water and carbon dioxide from the air.
-
Chlorine and hydrogen handling
Chlorine is cooled, dried with concentrated sulfuric acid, compressed and often liquefied for PVC, water disinfection and chemical synthesis. Hydrogen is cooled and used as plant fuel, burned with chlorine to make hydrochloric acid, or sold as a chemical or energy feedstock.
Mass Balance: What Goes In and What Comes Out
In (per 1 t NaOH, theoretical)
- Sodium chloride1.46 t
- Water consumed by reaction0.45 t
- Electricity (membrane, typical)≈2.5 MWh
Out
- Sodium hydroxide1.00 t
- Chlorine gas0.89 t
- Hydrogen gas0.025 t
Why it matters commercially: chlorine and caustic soda are made together in a fixed ratio. When PVC and chlorine demand is weak, producers cut run rates and caustic soda supply tightens, even if caustic demand has not changed. This chlorine–caustic link is one of the main drivers of caustic soda price cycles.
From Cell Liquor to Flakes, Pearls and Liquid
| Form | NaOH content | How it is finished | Typical buyer choice |
|---|---|---|---|
| Liquid caustic soda | 48–50% | Multiple-effect evaporation of cell liquor | Large consumers with tank storage; lowest cost per tonne of NaOH delivered nearby |
| Flakes | 98–99% | Anhydrous melt cooled on a flaking drum | Long-distance export, bagged handling, dissolving on site |
| Pearls (prills) | 99% | Melt sprayed in a prilling tower | Automated dosing, free-flowing, less dust |
Technical data sheets: Caustic soda flakes TDS (PDF) · Caustic soda pearls TDS (PDF) · Liquid caustic soda TDS (PDF) · Caustic soda MSDS (PDF)
History of Caustic Soda Production
Before electrolysis, caustic soda was made by causticisation, also called the lime-soda process. Sodium carbonate (soda ash) was boiled with calcium hydroxide (slaked lime):
Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃↓The soda ash itself first came from the Leblanc process (1791) and, from the 1860s, from the cleaner Solvay process. The Leblanc process made soda ash, not caustic soda directly, although the two are often confused.
| 1791 | Leblanc process for soda ash; caustic soda made from it by causticisation with lime |
|---|---|
| 1860s | Solvay ammonia-soda process starts replacing Leblanc for soda ash |
| 1890s | Industrial brine electrolysis begins: diaphragm cells (Germany, USA) and the Castner–Kellner mercury cell (UK) arrive in the same decade |
| 1900s–1970s | Diaphragm cells dominate North America; mercury cells dominate Europe and Japan |
| 1975 | First commercial membrane-cell plant (Asahi Kasei, Japan) |
| Dec 2017 | Mercury-cell production ends in the EU under the chlor-alkali BAT conclusions |
| 2025 | Phase-out date for mercury-cell chlor-alkali under the Minamata Convention (Annex B) |
Causticisation is still used occasionally where soda ash is cheap and there is no market for chlorine. For example, kraft pulp mills recausticise their own green liquor in a similar way. Almost all merchant caustic soda today is electrolytic.
Energy, Environment and Regulation in 2026
Energy is the biggest cost
Chlor-alkali is one of the most electricity-intensive processes in the chemical industry, so power price is the main factor in a plant's cash cost. Each region's production cost tracks its power price closely. Oxygen-depolarised cathodes (ODC) feed oxygen to the cathode so that no hydrogen forms, which lowers cell voltage. Developers report electricity savings of up to about 25–30% compared with standard membrane cells, but there is then no hydrogen co-product.
Mercury: phased out
The EU ended mercury-cell production in December 2017. The Minamata Convention on Mercury lists mercury-cell chlor-alkali in Annex B with a phase-out date of 2025. Parties could register time-limited exemptions, so a small number of plants may still be closing. Buyers who need certainty should ask for a mercury-free (membrane-process) declaration.
Asbestos diaphragms: being phased out in the USA
The US EPA's March 2024 chrysotile asbestos rule under TSCA banned asbestos imports for diaphragm use from May 2024. It requires the remaining asbestos-diaphragm chlor-alkali plants to stop using it on a staged timetable: first from 2029, with longer periods for companies converting several sites to membrane technology. Those plants represent about one-third of US chlor-alkali capacity. An industry challenge to the rule was argued before the US Fifth Circuit in June 2026.
Where caustic soda is made
China is the world's largest producer by a wide margin: about 50.4 million tonnes a year of capacity and around 42.6 million tonnes of output in 2025 (Mysteel). The other major producing regions are North America, Western Europe (around 8 million tonnes a year, per Euro Chlor), Northeast Asia, India and the Middle East.
Why the Production Route Matters When You Buy Caustic Soda
Two shipments can both say "caustic soda flakes 98%" and still behave differently in your process. The difference usually comes from how the product was made. Ask for these on the certificate of analysis (COA) before you sign:
- NaOH assay: 98% or 99% minimum for solids; 48–50% for liquid
- Sodium chloride (NaCl): the clearest sign of membrane vs diaphragm origin
- Sodium carbonate (Na₂CO₃): shows CO₂ pick-up during storage or packing
- Iron (Fe): critical for textiles, rayon and bleaching
- Sodium chlorate (NaClO₃): matters for food, pharma and electronics use
- Mercury (Hg): request a mercury-free / membrane-process statement
- Food-grade compliance: e.g. FCC / E524 specification if used in food
- Packing: bag liner and pallet or jumbo-bag spec to keep moisture out
Buy Membrane-Grade Caustic Soda from SUHA International
SUHA International supplies membrane-process caustic soda, including food grade, to importers, distributors and manufacturers worldwide. Shipments load from Türkiye and the UAE.
Every shipment comes with a COA, MSDS and full export documents. Send your grade, quantity, destination port and Incoterm, and our desk will reply with a firm offer.
Where Caustic Soda Goes After It Is Made
The largest single use is making other chemicals, organic and inorganic. Next come alumina refining, pulp and paper, soaps and detergents, textiles and water treatment.
- Chemical manufacturing: neutralisation, organic intermediates, sodium salts
- Alumina and mining: dissolving alumina from bauxite in the Bayer process
- Pulp and paper: kraft cooking liquor and bleaching
- Soaps and detergents: saponification of fats and oils
- Textiles: mercerising and scouring cotton
- Water treatment: pH control, metal precipitation, resin regeneration
- Oil refining: removing H₂S, mercaptans and acids
- Food processing: peeling, olive curing, cocoa and CIP cleaning (food grade only)
Safety in Caustic Soda Production and Handling
Sodium hydroxide causes severe skin burns and eye damage (GHS H314) and is corrosive to some metals (H290). Plants and users rely on the same core controls:
- PPE: chemical goggles plus face shield, alkali-resistant gloves (neoprene, nitrile or PVC), apron and boots. Use dust protection when handling solids.
- Materials: carbon steel, stainless steel, HDPE or suitable FRP. Never aluminium, zinc, tin or galvanised parts, because NaOH attacks them and releases flammable hydrogen.
- Temperature: 50% solution starts to crystallise at about 12 °C (54 °F), so outdoor tanks in cold climates need heating and insulation.
- Dilution: always add caustic to water slowly, never water to caustic, because of the violent heat release.
- First aid: rinse skin or eyes immediately with plenty of water for at least 15–20 minutes, remove contaminated clothing and get medical help. For eye contact, keep rinsing on the way to treatment.
Frequently Asked Questions
How is caustic soda made?
Caustic soda is made by electrolysing purified brine (sodium chloride dissolved in water) in a chlor-alkali cell. The electric current turns the salt and water into sodium hydroxide, chlorine gas and hydrogen gas (2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂). Modern plants use membrane cells, which give about 32% NaOH. This is then evaporated to 50% liquid or concentrated and solidified into 98–99% flakes or pearls.
What is caustic soda made from?
Caustic soda is made from three inputs: sodium chloride (common salt), water and electricity. In theory, about 1.46 tonnes of salt and 0.45 tonnes of water make 1 tonne of sodium hydroxide; in practice plants use around 1.5–1.6 tonnes of salt. Small amounts of sodium carbonate, hydrochloric acid and ion-exchange resin are used to purify the brine.
What is the chemical equation for making caustic soda?
The overall chlor-alkali reaction is 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. At the anode, chloride ions are oxidised to chlorine (2Cl⁻ → Cl₂ + 2e⁻). At the cathode, water is reduced to hydrogen and hydroxide ions (2H₂O + 2e⁻ → H₂ + 2OH⁻). Sodium ions cross the membrane and combine with the hydroxide ions to form NaOH.
Which process is used most to make caustic soda today?
The membrane cell process is the industry standard and the only technology chosen for new plants. It uses less electricity than diaphragm or mercury cells, contains no mercury or asbestos, and makes caustic soda with very low salt content. Diaphragm cells are still used in some older plants, mainly in North America, while mercury cells have been phased out under the Minamata Convention.
Why is chlorine produced together with caustic soda?
Electrolysis of brine always splits sodium chloride into both products at once, so chlorine and caustic soda are made in a fixed ratio of about 0.89 tonnes of chlorine per tonne of NaOH, plus 0.025 tonnes of hydrogen. Because of this link, demand for chlorine (mainly for PVC) affects how much caustic soda is available and strongly influences its price.
How are caustic soda flakes and pearls made?
Both start as 50% liquid caustic soda, which is concentrated in falling-film evaporators into an almost water-free melt of about 98.5–99% NaOH at roughly 350–400 °C. For flakes, the melt is cooled as a thin layer on a water-cooled rotating drum and scraped off. For pearls (prills), the melt is sprayed down a prilling tower, where the droplets solidify into small round beads.
What is the difference between membrane-grade and diaphragm-grade caustic soda?
Membrane-grade caustic soda is made in ion-exchange membrane cells, and its 50% solution typically contains less than 50 ppm sodium chloride. Diaphragm-grade caustic soda comes from cells with a porous separator, so the 50% product usually still contains around 1% salt. Membrane grade is preferred for food, pharmaceutical, rayon and other high-purity uses.
How much electricity does it take to make caustic soda?
A modern membrane-cell plant uses roughly 2.5 MWh of electricity per tonne of NaOH including auxiliaries, or about 2.1–2.7 MWh per tonne of chlorine at the cell. Diaphragm cells need more steam for evaporation, and mercury cells used the most electricity, about 3.0–3.6 MWh per tonne of chlorine. Electricity is the largest cost in caustic soda production.
Can caustic soda be made without electrolysis?
Yes. Caustic soda can be made by causticisation, the lime-soda process: sodium carbonate (soda ash) reacts with calcium hydroxide (slaked lime) to give sodium hydroxide and a calcium carbonate precipitate (Na₂CO₃ + Ca(OH)₂ → 2NaOH + CaCO₃). It was the main route in the 19th century and is still used in a few niche cases, but almost all commercial caustic soda today is made by electrolysis.
Is mercury still used to make caustic soda?
Mercury-cell production ended in the European Union in December 2017. The Minamata Convention on Mercury set 2025 as the global phase-out date for mercury-cell chlor-alkali plants, with limited exemptions possible. Buyers who need mercury-free product should ask the supplier for a membrane-process or mercury-free declaration with the certificate of analysis.
Which country produces the most caustic soda?
China is the largest caustic soda producer in the world, with about 50.4 million tonnes per year of capacity and around 42.6 million tonnes of output in 2025, according to Mysteel. Other major producing regions are North America, Western Europe, Northeast Asia, India and the Middle East.
Is caustic soda the same as sodium hydroxide and lye?
Yes. Caustic soda is the common industrial name for sodium hydroxide (NaOH, CAS 1310-73-2). "Lye" usually also means sodium hydroxide, but it can refer to potassium hydroxide (caustic potash), so the chemical name or CAS number should be used on purchase contracts and specifications.
What is the difference between caustic soda and baking soda?
They are different chemicals. Caustic soda is sodium hydroxide (NaOH), a strong and highly corrosive alkali with a pH of 13–14 in solution. Baking soda is sodium bicarbonate (NaHCO₃), a mild alkali with a pH of about 8.3 that is safe to eat in small amounts. They cannot be substituted for each other.
Sources and Further Reading
- Minamata Convention on Mercury, Annex B, Part I: mercury-cell chlor-alkali production phase-out date 2025 (minamataconvention.org)
- US EPA, "Asbestos Part 1; Chrysotile Asbestos; Regulation of Certain Conditions of Use under TSCA", Federal Register, 28 March 2024
- European Commission, Best Available Techniques (BAT) Reference Document for the Production of Chlor-alkali, 2014
- Euro Chlor, "How is caustic soda made?" (eurochlor.org)
- Mysteel, "China caustic soda capacity exceeds 50 mln t/y in 2025", 19 November 2025
- Covestro, Oxygen-depolarised cathode technology product sheet
Stoichiometric figures are calculated from molar masses (NaCl 58.44, NaOH 40.00, Cl₂ 70.90, H₂ 2.016 g/mol). Plant figures are typical ranges and vary by technology and site.