How caustic soda is made
Caustic soda is a leftover that a chlorine plant must sell, because it cannot make one without the other.
- stations
- 5
- things upstream
- 2
- raw sources
- 2
- step from the ground
- 1
The short version
Caustic soda is made at the same time as chlorine, by passing electric current through salt brine in a membrane cell. Sodium ions cross the membrane and combine with hydroxide to form a weak sodium hydroxide solution. Evaporators boil off water until the liquid is strong enough to sell, and a plant can also flake or pearl it as a solid.
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Make and purify brine
Salt is dissolved in water and the brine is treated to remove calcium, magnesium and other contaminants. Impurities would clog and ruin the cell membrane.
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Electrolyze it in membrane cells
Current splits the brine. Chlorine bubbles off at one electrode and hydrogen at the other, while sodium ions slip through the membrane into the cathode chamber and pair with hydroxide ions to form caustic soda.
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Draw off the weak lye
The caustic liquor leaving the cells is a weak solution. Some is recirculated through the cell and the rest moves on to be concentrated.
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Evaporate it stronger
Evaporators boil water off the liquor until it reaches the standard 50% strength. This step uses a lot of steam, which is why cell design aims for the strongest liquor possible to start with.
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Ship as liquid, flake or pearls
Most caustic soda leaves the plant as a 50% liquid in tanker trucks and rail cars. Some is dried to solid flakes or pearls and bagged.
Worth repeating
- The chloralkali process makes two moles of sodium hydroxide for every mole of chlorine, so the two products have to find buyers in the same ratio.
- Caustic soda is used to make wood pulp and paper, textiles, soap and detergents, and drain cleaner.
Back to the ground
Follow every input upstream and caustic soda starts as these 2 raw sources.
Where it goes next
Caustic soda feeds 16 lines directly and reaches 291 things in total.
Further down the line
Keep walking
Sources
Cite this page: "How caustic soda is made." Assembly Line, 2026-11-03. https://assemblyline.org/make/caustic-soda