Caustic soda (sodium hydroxide, NaOH) is the standard strong alkali of industry. Its main applications are refining bauxite into alumina, pulping and bleaching in the paper industry, the manufacture of organic and inorganic chemicals, soaps and detergents, water treatment and textile processing. It is sold as a 50% solution, called liquid caustic or lye, and as solid flakes and pearls.
Caustic soda applications by sector
The European Commission's reference document on the chlor-alkali industry lists the main areas of application as chemicals, metallurgy and alumina, pulp and paper, textiles, soaps and surfactants, water treatment and consumer products. The table sets out what the alkali does in each.
| Sector | What caustic soda does | Form usually bought |
|---|---|---|
| Alumina | Dissolves the aluminium minerals out of bauxite | 50% liquid, by ship |
| Pulp and paper | Cooks wood chips, extracts lignin during bleaching | 50% liquid |
| Chemicals | Neutralises acids, supplies sodium, drives reactions | 50% liquid |
| Soaps and detergents | Turns fats and fatty acids into soap; makes surfactants | Liquid, flakes or pearls |
| Water treatment | Raises pH, precipitates metals, regenerates resins | Liquid or pearls |
| Textiles | Mercerises cotton, scours fabric, makes viscose | Liquid or flakes |
| Food and cleaning | Cleans process plant, peels fruit and vegetables, refines oils | Pearls or liquid |
Alumina
In the Bayer process, ground bauxite is digested in hot caustic soda solution under pressure. The aluminium minerals dissolve as sodium aluminate and the iron-rich residue, red mud, is left behind. Alumina hydrate is then crystallised out and the caustic liquor is recycled. The liquor is reused many times, but some caustic is lost with the residue and has to be replaced, and refineries buy that make-up in shiploads. The European Commission's document names Australia, for its alumina industry, as one of the main importing areas. See alumina and our guide to US Gulf caustic soda and Bayer alumina.
Pulp and paper
In the kraft process, wood chips are cooked in a liquor of sodium hydroxide and sodium sulphide, which dissolves the lignin and frees the cellulose fibres. Caustic soda is used again in the bleach plant to extract lignin between bleaching stages, and in de-inking recycled paper.
Chemicals
The chemical industry uses caustic soda wherever an acid has to be neutralised or a sodium salt is wanted. Examples are epoxy resin intermediates, propylene oxide, sodium hypochlorite bleach, sodium silicate, sodium phosphates and dyes. Petroleum refiners use it to wash acidic sulphur compounds out of fuels.
Soaps and detergents
Soap is made by saponification, the reaction of fats and oils with caustic soda to give the sodium salt of the fatty acids and glycerine. Caustic soda also neutralises the sulphonic acids used in synthetic detergents and is the active ingredient in drain and oven cleaners.
Water treatment
Waterworks and effluent plants dose caustic soda to raise pH, to reduce the corrosiveness of water to pipes, and to precipitate dissolved metals as hydroxides. It regenerates ion-exchange resins in demineralisation plants.
Textiles
Mercerising treats cotton yarn or fabric with strong caustic soda solution, which swells the fibre and improves its lustre, strength and dye uptake. Caustic soda is also used to scour raw cotton and to dissolve cellulose in the viscose process.
No sector percentages are given here. Published breakdowns differ by region and by year, and the detailed ones sit behind paid market reports.
Where caustic soda comes from
Caustic soda is made by passing electricity through salt brine, the chlor-alkali process. The electrolysis yields chlorine, caustic soda and hydrogen together and in fixed proportion: 1.128 tonnes of caustic soda, counted as 100% NaOH, for each tonne of chlorine. A producer cannot make one without the other. Chlorine is hard to ship and goes mostly into PVC on site, so caustic soda supply follows chlorine demand, and its price can move a long way when the two markets are out of step.
The three commercial forms
Liquid caustic soda, 50% (lye). The cell liquor is concentrated by evaporation to a 50% solution. This is how most caustic soda is sold. It moves in parcel tankers, rail cars, road tankers and ISO tanks, and it can be stored for long periods. It needs heated lines in cold weather. Quantities are often invoiced in dry metric tonnes, meaning the contained NaOH, so check whether an offer is per dry or per liquid tonne.
Flakes. The 50% solution is evaporated until almost no water is left, and the melt is solidified on a cooled drum and scraped off as thin flakes. Sodium hydroxide melts at 318°C, so this takes a good deal of energy, and flakes cost more per tonne of NaOH than liquid. Their advantage is that they travel in bags in an ordinary container to buyers with no tank storage.
Pearls. The melt is sprayed in a prilling tower to form small beads, also called prills, micropearls or beads. Pearls flow freely, dust less than flakes, dissolve evenly and are easier to dose, so they are preferred for cleaning products, water treatment and laboratory use.
Solid caustic soda is hygroscopic. It absorbs water and carbon dioxide from the air, cakes, and slowly turns to sodium carbonate at the surface. It is packed in 25 kg bags with a sealed polyethylene liner, or in lined jumbo bags, and kept closed. It is a corrosive substance under dangerous goods rules in every form.
Caustic soda flakes specification
The table below is the published sheet of a European producer, PCC Group, for its caustic soda flakes. It is a high-purity grade and is given as an example of what a sheet covers.
| Property | Limit |
|---|---|
| Appearance | White or almost white hygroscopic flakes |
| Sodium hydroxide (NaOH) | 98% min |
| Sodium chloride (NaCl) | 0.02% max |
| Sodium carbonate (Na2CO3) | 0.2% max |
| Iron, as Fe2O3 | 15 mg/kg max |
Three points help when comparing this with other offers.
- Sodium hydroxide. Flakes are sold as 98% or 99%. The balance is mostly water and carbonate.
- Sodium chloride. This shows how the product was made. Membrane-cell caustic is very low in salt, as in the sheet above. Diaphragm-cell caustic carries much more, which some uses cannot accept.
- Sodium carbonate. Carbonate rises when the product has been exposed to air, so a high result on arrival points to damaged packing or old stock.
Other producers publish wider limits for carbonate and chloride on standard industrial flakes, and some state iron as Fe instead of Fe2O3. Compare sheets line by line and ask for the test method. Our guide to reading a spec sheet and certificate of analysis explains guaranteed and typical values.
Choosing a caustic soda pearls or flakes supplier
Ask for the producer's name and plant, the cell technology, the specification with guaranteed limits, the bag type and liner, the pallet and container loading, the dangerous goods documents, and the date of manufacture. A seller who cannot name the plant is reselling without control of the product.
How caustic soda is traded
Liquid caustic soda trades in tanker parcels, FOB or CFR, priced per dry metric tonne. Flakes and pearls trade in container loads of bagged product, mostly from Asia, the Middle East and Europe. Contracts state the form, the strength, the specification limits, the packing and the Incoterm.
CBIL brokers caustic soda between a named producer or seller and a named buyer, against an independent inspector's certificate, from SGS, Intertek or a similar company, issued at loading.
Request a caustic soda quote with the form, strength, quantity, packing and destination port.