Caustic soda solutions can freeze or crystallise in cold weather, blocking lines, damaging pumps and stopping dosing. The behaviour surprises many users, because the strongest solutions are not the ones that stay liquid at the lowest temperatures. Here is what you need to know.
Strength and freezing do not move together
For most salt solutions, more concentrated means a lower freezing point. Sodium hydroxide is different. Its solutions reach their lowest freezing point, roughly minus 28 °C, at a concentration of about 19% NaOH. Above that, freezing points rise again, and the common 50% solution starts to crystallise at around 12 °C, which is above normal room temperature in many cold climates.
What this means in practice
- 50% lye stored outdoors or in an unheated building in winter will crystallise. Tanks and lines need insulation and heat tracing.
- Diluting to a lower strength for winter dosing, around 20 to 25%, keeps the solution liquid at much lower temperatures. Check the exact freezing point for your chosen strength.
- Dilute cleaning solutions of a few percent behave more like water and freeze not far below 0 °C.
Practical tips for cold sites
- Store bulk lye indoors or in insulated, heated tanks.
- Heat-trace and insulate transfer lines and dosing lines.
- Keep the tank stirred or recirculated in cold spells so crystals do not settle.
- If lye has crystallised, warm it gently and slowly. Never use open flames, and make sure vents are open.
- Consider buying flakes instead of liquid. Solid flakes do not freeze, and you can dissolve them to the strength you need indoors.
Dissolving flakes safely
Dissolving flakes releases heat, which actually helps in winter, but it can also make the solution boil and spit if added too fast. Always add flakes slowly to cold water while stirring, and wear goggles, gloves and an apron.
Hot climates are different
In the Gulf the problem is the opposite: humidity, not cold. Muscat Chemical (Oman) supplies flakes 99% from ready stock in Barka for customers in Oman; their caustic soda page has the daily price, the safety data sheet and storage notes.