Sugar mill evaporators build up scale every season. Calcium salts, silica and organic matter from the juice deposit on the heating tubes, cutting heat transfer and forcing more steam use. A caustic boil-out, usually followed by an acid wash, is the standard way to clean them. This guide explains the basics.
Why evaporators scale
As cane juice is concentrated, dissolved salts such as calcium sulphate, calcium oxalate and silica exceed their solubility and crystallise on hot surfaces. Organic matter binds the scale together. The later effects, where the juice is most concentrated, usually scale fastest.
How a caustic boil-out works
Hot caustic soda solution, often combined with soda ash, softens and loosens the scale. It converts calcium sulphate into calcium carbonate, which is easier to remove with acid afterwards, and it breaks down the organic binder. A following acid wash, with the right corrosion inhibitor, then dissolves the converted scale.
Typical sequence
- Drain and rinse the evaporator bodies with water.
- Caustic boil: fill with caustic solution, commonly a few percent NaOH, often with soda ash, and boil for several hours as set by your mill's procedure.
- Drain and rinse until the rinse water is close to neutral.
- Acid wash with an inhibited acid if your scale requires it, then rinse again.
- Inspect the tubes and mechanically clean any that are still blocked.
Concentrations, times and temperatures vary with scale type, so analyse a scale sample and follow your mill's established method.
Safety during boil-out
- Hot caustic causes severe burns. Wear face shields, chemical suits, gloves and boots.
- Never add acid while caustic is still in the vessel.
- Treat vessel entry as confined-space work, with isolation, ventilation and gas testing.
- Neutralise spent solutions before disposal according to local rules.
Planning caustic stock
Boil-outs use large quantities of caustic soda in a short time, so order well before the off-season cleaning starts. Store bags dry and sealed. For sugar and food companies with operations in Oman, a published daily price for caustic soda flakes is available on Muscat Chemical's caustic soda page, with delivery within Oman.
