Flux bath: salt, iron, heat, pH
Weak flux is one of the largest single causes of zinc ash, and iron that rides through the flux into the kettle leaves as dross, about twenty times its own weight. Enter the weekly titration and see what the tank needs, what its iron is costing you, and which of the four flux conditions is off.
Dissolved iron raises the density, so a tank that is losing salt can read unchanged on the hydrometer while its iron climbs. Titrate to g/L every week.
Targets from the Zinc Consumption KPIs article (Revision 2.0, KPI 4); pH window and heated range from the HDG book, Chapter 5. Additions are concentration × volume: titrate again after mixing. Peroxide is the stoichiometric amount; plants dose above it.
Four conditions, all upstream of the zinc
The flux removes the last oxide film after pickling and leaves a salt film that protects the steel until it meets the zinc. A correctly concentrated flux leaves a continuous, dry, coherent film. A dilute one is patchy and dries poorly, so the work carries moisture and re-oxidised surface into the bath, and that becomes ash. Ash is a symptom; every control for it sits at the flux stage.
- Total salt. Double salt 400–450 g/L, the standard for batch plants with a drying oven. Triple salt about 310 g/L; it dries more readily and suits plants without a dryer. Double salt at 200–300 g/L is too weak and is consistently associated with high ash.
- Temperature. Heated, around 50 °C. A cold tank drains and dries poorly, so the film is still wet at the kettle.
- pH about 4. Below pH 3 acid attack and iron pick-up rise; above pH 5 the fluxing effect weakens.
- Dryness. Wet work flashes its water off at 450 °C, blowing flux and zinc off the surface as ash, and it is a safety exposure at the kettle.
Iron is the fifth number on the log sheet: below 2 g/L Fe, with action at about 4 g/L Fe. State the species: 4 g/L as Fe is about 9 g/L as FeCl₂. It is cleared with hydrogen peroxide, which oxidises ferrous to ferric chloride (2 FeCl₂ + H₂O₂ + 2 HCl → 2 FeCl₃ + 2 H₂O); ammonium hydroxide then precipitates Fe(OH)₃ for the filter press and returns NH₄Cl to the flux.
Run the whole chemical line on numbers
- Pickling Optimizer — every tank against the Kleingarn curve
- HCl Converter — titration to acid and iron
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Questions
What concentration should a galvanizing flux run at?
Report it as total salt in g/L. Double salt 400–450 g/L is the standard for batch plants with a drying oven; triple salt runs at about 310 g/L and suits plants without a dryer. Double salt at 200–300 g/L is too weak and is consistently associated with high ash.
Does diluting the flux reduce zinc ash?
No, the opposite. Weak flux is one of the largest single causes of high ash: a dilute film is patchy and dries poorly, so the work carries moisture and re-oxidised surface into the bath. Plants running dilute flux can generate as much as double the ash. Restoring the concentration reduces ash almost immediately.
How much iron is too much in the flux?
Aim below 2 g/L of iron, with action at about 4 g/L. State the species on the log sheet: 4 g/L as Fe is about 9 g/L as FeCl₂, and mixing the two is a common source of confusion.
Why does iron in the flux matter in the kettle?
Iron carried into the zinc bath leaves as bottom dross, an iron-zinc alloy that is about 95% zinc. Each kilogram of iron therefore takes about 20 kg of dross out of the bath with it.
Can I measure flux strength with a Baumé hydrometer?
Not on its own. Dissolved iron raises the density, so a tank that is losing salt can read unchanged on the hydrometer while its iron climbs. Titrate to g/L weekly and record the method on the log sheet.