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How Can I Reduce and Remove Zinc Dross

Current Q&As:
March 18, 2022We are facing a lot of drossing in our Zinc bath with the below condition.
Flux - Iron content 0.05%, Temperature of flux tank 40 degree Celsius and our dryer temperature 110 and using Hindustan Zinc with 99.99 purity. Also in bath chemist lead shows 0.05%.
Can some one please to suggest to reduce the Dross.
Employee - Pondicherry , India
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March 21, 2022
A. You didn't say how much dross. It might be that you are not making an unusual amount and it might be normal. Dross production is often reported as tonnes dross per tonne of steel galvanized.
Best to know if there is a problem before trying to solve it.

Geoff Crowley
Crithwood Ltd.

Bathgate, Scotland, UK

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Previous closely related Q&A's, oldest first:
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Q. Hello,
I'm having right now big problems with the dross in my galvanizing plant and I think someone here with more experience and acknowledge could help. First of all, I'm using high speed burners (heat the air that moves around the kettle with high speed) to control the temperature of the kettle, which is like 450 °C (working temperature). Sometimes the temperature rises to 460-468 °C and the burners stop and wait until temperature decreases.
I take out dross 2 times a month with a shovel, if anyone wants to see it could send a picture of it, but I'm not sure if it is really the appropriate tool or device to do this and I think it is not working because the production of Dross is excessive. I see the liquid zinc on the surface but when I introduce a steel round bar to measure the level It feels kind solid or clay inside and I have to push the bar to the reach the bottom. As a result of this clay or solid zinc, the useful level reduces a lot.
For the flux, I'm using a combination of water, ammonium chloride and zinc chloride. I believe that I'm introducing to the kettle a great amount of free iron particles that sticks to each material in the flux.
I need to remove the dross from the pot and the reduce the production of dross.
Engineer - Barranquilla, Colombia
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January 22, 2010
Sir: Galvanizing Consultant - Hot Springs, South Dakota, USA ^ January 23, 2010 A. There are two main systems of dross removal; ![]() Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK ![]() ^ |
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Q. We normally do drossing every 4 months. With an average of dross of 18 tons every 4 months.
1) Will there be any quality problem on galvanizing products?
2) Any problem on hot dip kettle?
Based on monthly, our total output is 1200 mt average and dross ratio is 4 ton every month. I'm thinking to do drossing weekly.
Can someone suggest the best way to do drossing?
Thanks/Regards
Jeet
Galvanizing - Klang, Selangor, Malaysia
^
November 30, 2012
A. 4 months is way way too long a gap for drossing.
Do it little and often.
Weekly or fortnightly is best.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

^
February 23, 2013
A.
1. Do you add lead to your kettle?
2. Have you checked your flux for iron content?
3. How often do you clean the water tank after pickling and before fluxing?
4. Do you check in the kettle twice a day if any dipped material has slipped in the kettle or not?
5. Have you checked for iron content in the zinc? It can be done by spectro.
See when you have excess of dross it sticks onto material that you are dipping. Secondly you are doing 1200 tonnes, so once in a month drossing is sufficient.
- Mumbai, India
^
July 17, 2013
Q. I am connected in a galvanizing firm producing 55% Al and 45 % zinc (galvalume)
How frequently should my dross removal be done? I'm operating at 21 tons/hr and producing 8000 tons/month on an average?
How to remove the dross? What's the procedure because we are doing the scooping once a month but the dross accumulation now in my kettle is very thick including the sides of the kettle approximately 60 mm thick from the wall?
- Trece Martirez city Cavite Philippines
^
July 20, 2013
A. Sir:
There are two types of dross using galvalume:
1)iron/zinc dross with about 6% iron and the rest zinc.
2)iron/aluminum dross and I do not know the composition.
The first type generally sinks to the bottom of the kettle whereas the second type floats on the zinc. Having a "dross" thickness of 6 mm on the sides of the kettle is likely problematic as the inside kettle walls may get too hot and burn a hole in the kettle walls. I have never worked with galvalume so it is likely I am not able to advise, although I have a kettle wall scraping device that works very nicely to remove dirt/ash/flux residue, etc., off the kettle walls. If the "dross" is not too tightly bonded to the kettle walls perhaps the device could work.
Regards,
Galvanizing Consultant - Hot Springs, South Dakota, USA
^
September 1, 2013
Q. I've read here discussing about dross pump, will it work in a galvalume kettle? It's true that there were accumulated scales in our snout and definitely it falls on the kettle bottom and accumulates forming a hard material like a bottom dross.
Arlene Canarias [returning]- Trece Martirez city Cavite Philippines
^
September 8, 2013
Q. Sir my question is that ammonium chloride sublimates at 388 °C and the furnace temp. is almost 468-480 °C, then how can it (ammonium chloride) reduces dross production if its sublimates at 388 °C? Exactly what reaction occurs between ammonium chloride and zinc, resulting in less production of zinc dross?
Priya Bhardwaj- HARIDWAR, U.P. INDIA
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How many zinc ingots will be consumed in galvanizing 605 MT and drossing?
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Dear Sir,
We have galvanized 605 mt of steel, what is the amount of zinc ingots I need to add after drossing to give me the same level of molten zinc in my kettle before drossing?
- Lagos Nigeria
^
July 2016
Hi Bola. I don't know much about galvanizing, but it seems to me that your equation has too many unknowns to be solvable.
Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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August 5, 2016
A. Hi, Mr Bola, - Vadodara, India ^ August 4, 2016 A. Yes, too many unknowns. ![]() Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK ![]() ^ August 4, 2016 A. What I summarise: I am sure that you are talking about Structures Galvanizing Plant. - Delhi. India ^ |
A. Sir
It is almost impossible to say. What material did you galvanize, what process did you adopt?
- Delhi, India
^
January 6, 2018
Q. Dear Sir, can you tell me after how many days DROSS is supposed to be removed from ZINC BATH?
SATISH KORI- JABEL ALI, DUBAI, UAE
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February 13, 2018
Q. Dear sir,
I'm operating a vertical Galvanizing kettle. Its depth is 4000 mm. We face excessive dross pimples on the Galvanized coating. We did dressing, using a dross grab, twice but still having same problem. We used potatoes to float unsettled dross. Finally, we started pumping compressed air -- contains almost 80 % nitrogen -- in the presence of Aluminum. It gave us excellent result.
My question is, is there any risk for kettle safety from compressed wet air?
Mohammed Farouk AbdallahBahra Cables - Jeddah, Saudi Arabia
^
March 19, 2018
A. I have used nitrogen in zinc bath; it reduces zinc consumption. Though many will argue that it will eat your kettle walls, I have not seen anything. Pimples can also be because of improper pickling of the material. Second you need to check your acid and flux for iron content; high iron content in flux tank will lead to eventually adding those free iron particles to your tank. This will result in high dross formation.
Vishal AgarwalINDANA STEEL PVT.LTD - Mumbai,Maharashtra,India
^
May 4, 2018
A. I would not put compressed air into zinc. While it certainly is 80% nitrogen, its 20% oxygen. That will react with the zinc consuming it, producing oxides that will flat to the surface, wasting zinc, messing up the surface.
Using pure nitrogen is different. We do this at same time as drossing and aluminium additions (just after) and find it helps even out the chemistry, and that results in less dross floating, more uniform aluminium distribution and nicer finish.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

^
"Galvalume: The BIEC Journey"
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Q.
I'm having right now big problems with the Bottom dross in my Galvalume ^ aluminum-zinc hot dip line and I think someone here with more experience and knowledge could help me. First of all, I'm using Induction pot (with 4 inductors) to control temperature of the kettle (pot), which is like 605 °C (±5°) working temperature).
This pot is the second pot on the line. The previous pot was emptied of melt after 3 years (100,000 ton of product). The bricks and refractories of the body were repaired. The inductors were replaced and melting of Galvalume ^ aluminum-zinc hot dip ingots was performed again. But the current pot, after 45,000 tons (1 year), contains a lot of bottom dross. Bottom dross focuses more on the walls, especially at the corners. During the last 4 months with using the drosser equipment, the bottom dross has been collected many times, but after a short time large volumes of drosses are replaced.
Current bottom dross is so much so that the equipment (especially Sink roll and stabilizer) sometimes do not fall into place. I need to remove bottom dross from pot, please guide me.
- Iran- Qazvin City
^
January 3, 2022
A. Captain Obvious here.
(1) You are having technical problems with 'Galvalume'.
(2) Galvalume is a licensed process and they provide technical support to their licensees.
(3) Galvalume has no licensed applicators in Iran per their website.

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.


^
January 2022
Thanks Tom. Indeed, Galvalume® is a registered trademark, and "BIEC International Inc. is the worldwide licensor of the technology and know-how associated with 55% Aluminum-Zinc alloy coated sheet steel (better known as GALVALUME)."
We want this site to be as helpful as possible, but don't want to contribute to dilution of trademarks nor to facilitate crowd-sourcing of trade secrets. It can sometimes be easy to discern them, and sometimes difficult :-)
Thanks for pointing out that an Aluminum-Zinc installation in Iran is probably not 'Galvalume'.
Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^
February 12, 2022
A. Dross always is a problem for strip galvanizing. There are several factors you can check as below,
1. Degreasing section: Please check degreasing result is ok or not.
2. Furnace section: Adjust O2 and H2 to reduce oxidation.
3. Pot section:
3.1 Charging alloy from pre-melting furnace instead of directly (if happened)
3.2 Control the pot temperature strictly. For example,600°, solubility of iron is 0.463%, 608°, solubility of iron is 0.500%. If pot temp. was changed from 608 to 600, then 0.037% Fe becomes dross, even temp. was increased from 608 again, dross still there.
3.3 Air knife system: First, better air knife design will reduce dross. Second, change normal air to Nitrogen can reduce dross also.
Integrated supplier of Galvanizing - Shanghai
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Ed. note: Please!
No abstract questions.
Huh?
Q. You are adding nitrogen in zinc kettle? In which form? Either it's in liquid form or in gas form?
Is there any effect of nitrogen on zinc coating or percentage of zinc usage?
How do we reduce zinc coating on material with using nitrogen?
Please give me a suggestion.
Employee - Maharashtra
^
December 20, 2021
A. You didn't explain your circumstances well, are you in continuous strip galvanizing, or batch? In batch galvanizing, Nitrogen is added to make the zinc melt homogenous. It doesn't save zinc.

Geoff Crowley
Crithwood Ltd.

Bathgate, Scotland, UK

^
December 31, 2021
Q. We have batch galvanizing plant.
We want to add nitrogen in kettle for quality work.
So we have already adding aluminium in kettle.
But for to reduce percentage zinc usage in product, we want to add nitrogen.
So in which form nitrogen added in kettle?
In liquid nitrogen form or pure nitrogen gas form?
And how much nitrogen added in kettle daily?
Please tell me sir
Employee - Maharashtra
^
January 1, 2022
Nitrogen isn't used to reduce zinc consumption, it is used to stir the zinc after drossing to achieve homogeneous mix. You might have added Ni, Al, Pb or perhaps some other alloys, but these can separate out a bit. Bubbling gaseous N2 through a steel pipe leading the gas to the bottom of the kettle will agitate the bath and mix it well.
But this doesn't reduce zinc consumption.
It is not done daily, but perhaps after each time you remove dross, which might typically be every week or 2 weeks. About 1-2 m3 of gas can be bubbled. It is not absorbed by the zinc, merely bubbles up and so stirring the zinc. The reason for using N2 is that it is inert to zinc, and unlike compressed air, will not oxidize the zinc as it rises. You could equally use Ar, but that costs more.

Geoff Crowley
Crithwood Ltd.

Bathgate, Scotland, UK

^