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Galvanizing flux bath has iron contamination

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Q. Good Day,
My name is Mutshidzi Mulelu, I am a Laboratory Manager. I am currently experiencing problems with my flux tanks.
I am getting Low Zinc and Ammonium Chloride content and low pH (less than 1) and High Fe Content of 4%.

Can low pH or a high Fe content have an effect on Zinc Chloride and Ammonium Content in the Tank?

Mutshidzi Mulelu
- Gauteng, So. Africa
August 24, 2022

A. Low pH and high iron indicates contamination of the flux tank by pickle acid. That dilution will reduce the apparent concentrations of ammonium chloride and zinc chloride.
If the flux has changed from brown to green, that helps confirm that its acid contamination.

Geoff Crowley
Crithwood Ltd.
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Bathgate, Scotland, UK
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Q. Dear sir,
I am from galvanizing industry and presently facing problem with my flux bath (zinc ammonium chloride), It has developed iron contamination. Can you suggest a method to remove iron from the bath? Is it possible to treat with Hydrogen peroxide or KMnO4?
Please guide me.

Dharampal Singh
Galvanizing job work - Dammam, Saudi Arabia

A. Sir: KMnO4 cannot be used. The residual Mn(+2) would contaminate the flux. The proper materials (both liquids) are: Hydrogen Peroxide and aqua ammonia (ammonium hydroxide). Needed information includes:
(1) volume of flux solution,
(2) % iron (Fe+2) in flux or grams/liter of Fe+2,
(3) density of flux at room temperature (either baumé or g/mL).

Do you have a filter press? Do you know how to use diatomacious earth with the filter press? Do you know about flocculating agents and have a source? Can you test the Fe+2 during purification so you will know when to stop adding the aqua ammonia and hydrogen peroxide? Do you pickle with HCl or H2SO4? How many rinses between acids and flux do you have? What flux do you use? (e.g., double salt, triple salt, quadraflux, etc.?) There are much better ways to deal with bad flux than to purify out the iron +2.

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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Q. Dear sir
Regarding flux we use Hydrochloric acid 20% in the pickle acid, followed by two water rinses changed every week. Please note we do not have water overflow system.

The flux develops iron in the course of time and we use the method of H2O2 +Liquid Ammonia for the iron removal treatment.

My question is what is the size of the iron particle in this flux solution?

During lab trials I found that We could actually filter these iron particles after H2O2 treatment by using "Whatman Filter Paper No. 42 but it takes time.

If we know exact size of particle in microns we can use relevant filter paper to filter out the iron particles formed and carried out in the filtrate after the settling process using the filter pump.

Sir please suggest me if this is OK. If so, please tell me about the size of iron particles so that we can arrange for the appropriate filter paper for the same.

Dharampal Salooja
Hot Dip Galvanizing Cable Trays - Jeddah, Saudi Arabia
February 10, 2014

A. Hi Dharampal. I am only the site curator, not an expert in the filtration of galvanizing flux for removal of iron particles ... but Whatman (GE Healthcare) No. 42 filter paper is their finest, and has a pore size of 2.5 µm. Can you either look at the used paper under a microscope or, better yet, analyze the filtrate from a couple of courser grades like No. 40 & 41 to get a feeling for what is going on and what the most practical pore size would be for filtration in production? Good luck.


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Iron content in triple salt flux

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Q. I read an article (the paragraph below) that iron doesn't matter in triple flux, however, I have read common posts stating that reducing iron content can reduce coating thickness and dross formation. In need of clarification.

"It is general belief that an increase in the iron content of the fluxing bath has an adverse effect on galvanizing process and results in an increased dross generation. while this consideration is indeed true for conventional type of fluxes where a remote chance exists for the formation of oxyacids (due to instability of fluxes at galvanizing temperature) as slated above, in the case of triple salt based fluxes, however, an increase in iron content of bath has no adverse effect. It rather helps in the fluxing action. Iron salts are converted to iron chloride and they sublime at the galvanizing temperature. These chlorides do not take part in dross formation and mostly either sublime or form iron oxychloride (FeO.HCl) and float on the surface as skimming. De-ironing of fluxing solution is therefore not only unnecessary but can only be carried out at a great loss of flux and wastage of reagents and manpower."

Would also like to ask if I were to treat my flux solution with ammonia for iron removal while the pH exceeds 6, after removing the sludge then only adding HCL to reduce the pH again back to 4.2. I have read somewhere else where it states if pH exceeds 5, the zinc chloride will be decomposed. This is my concern.

Kean Wong
- Selangor, Malaysia
January 31, 2018

A. I don't agree with that quoted paragraph.
Fe in the flux, in solution, will move to the kettle. It reacts with zinc to form dross.
The difference between triple salt, double salt (and quadraflux for that matter) is only the ratio of the two components, zinc chloride and ammonium chloride. Usually stated as a molecular ratio. In practice, the ratio is rarely an integer value, and if you think you are running double salt, the ratio might wander from 1.5 to 2.5, and if you are testing the flux, the result might indicate additions of only one of the components to adjust the ratio. But the main supply of the flux will be the target ratio (double, triple, etc.)
To think that the chemical reactions suddenly alter at triple salt compared with double is a bit misleading.
Iron oxychloride is stated as being "FeO·HCl". I think that's a typing error, and that it's FeOCl. I have never seen purple ash on a galvanizing bath (as FeOCl is). The formation of this is often Fe2O3 + FeCl3 => 3 FeOCl. There's certainly some FeCl3 around, from pickling and carried over. But Fe2O3? Where did that come from? By the time the steel has been pickled, all the Fe2O3 should be gone, and is now FeCl3.
I don't think you can remove Fe from flux with only ammonia. That would raise the pH, and perhaps precipitate some dissolved things, but don't you need to oxidise the Fe also? Most removal of Fe from flux results in iron hydroxide as a solid which can be removed by settling or filtering.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
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February 6, 2019

A. Kean/Geoff:

45 Years ago I removed the iron (+2) from many galvanizing fluxes. Not a single galvanizer said his dross production was reduced. All of these galvanizers said their % GZU was lowered.

25 years ago I went to Troyes (pronounced Twa) in France. I installed a motorized ash machine directly on the kettle to recover zinc from the ash. It reduced the ash by 75%. The company was happy except they noticed the dross was increased by 10%. They asked me to come back and explain why the dross increased. The newly formed dross was directly under the motorized ash machine. I went back to France and went through the economics showing that reducing ash by 75% was far more economical even with a 10% increase in dross.

I have a USA wet kettle client (does not have space for an acid rinse tank) who had 9.5% iron (+2) in his flux. When he dumped his flux tank and got new flux, his dross did not change from 1.1% of production. He did notice that he consumes 20% less new top flux and his dry skims are reduced by 33%.

Recently the dynamic angle tests were applied to two fluxes. The first a very clean quadraflux the second to a flux having 1.5% iron and 4,500 ppm (e.g. 0.45%) NaCl (sodium chloride). The tests were video taped and the quadraflux gave excellent results, whereas the 1.5% iron and 0.45% NaCl flux gave very excessive white smoke and very excessive ash. Also the iron contaminated flux showed many small bare spots in the 20 second dynamic flux test.

The idea as expressed in the quoted paragraph presented by Kean, that FeCl2 in the flux is GOOD is WRONG. I am interested to know where that paragraph came from.

Because most all drosses are about 0.4% to 0.6% of production for dry kettles, I do not think iron +2 in the flux is all that important in terms of dross production. I do however know that iron +2 and other impurities in the flux is VERY important in terms of skims/ash production. For lower smoke a good flux is also very important. A good flux gives skims (unworked) that can give 85% zinc recovery in an MZR machine. A properly operated quadraflux can provide 0.10% ash/production after passing through an MZR machine.

I do not think that Fe+2 or Fe+3 chlorides react with molten zinc to form dross. If however, in an ash box or MZR machine the atmosphere is reducing (chemical term) enough then perhaps these iron IONS can convert to metallic iron then it certainly could produce dross.

I cannot imagine where FeOCl is coming from and where oxy acids are coming from?


Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA
February 7, 2019

Q. Dear sir, what will be impact of Fe available in the flux solution on ash and dross generation. What is the calculation to know the exact impact of Fe, suppose if we are maintaining the Fe 0.05 and 0.1 then what will be the difference on ash dross generation.

Harjinder singh
- Indore- Madhyapradesh India
November 21, 2021

Please see also --

Topic 41209 "Reduce Iron (Fe) in Galvanizing flux"

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