Hot Dip Galvanizing: Reduce and Remove Zinc Dross

A. 4 months is way way too long a gap for drossing.

Do it little and often.

Weekly or fortnightly is best.

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.

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,

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,

A. Hi, Mr Bola, You need 21 Mt zinc @3.5% and 3 Mt for addition after drossing. Rajesh Rajesh Patel - Vadodara, India A. Yes, too many unknowns. This is like asking "What fuel economy will I get in an unknown car driving up an undefined hill at an unknown speed?" What type of steel to be galvanized? What flux type will you use? How long will the immersion be? What temp will the zinc be? Will you dry before galv? What is the surface condition of the steel? and other questions too. Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK A. What I summarise: 1. You want to take out Dross with minimum free Zinc 2. How the Dross can be minimised. 3. Frequency of dross taking out from Zinc Kettle. My say is: 1. Your spoon whatever shape, whatever size must be well perforated with 10 mm dia drill. Hit the spoon vigorously on the Zinc bath as soon as it comes out. Then after, transfer it to the again perforated moulds. 2. Keep your Zinc bath Temperature Max. 450, preferably 445 °C. 3. After the addition of 50 MT Zinc plan for Drossing. I am sure that you are talking about Structures Galvanizing Plant. In normal circumstances the Zinc consumption remains 4.5% of steel Galvanized means you have to add say 28 MT of Zinc. Normally Dross formation will be approx. 10% of Zinc added to produce the above. By the way Dross formation depends on so many things: % of Iron in Flux. Zinc bath Temp. and the quality of material Galvanized. Umesh Dalela - Delhi. India

A. Sir
It is almost impossible to say. What material did you galvanize, what process did you adopt?

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.

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.

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.

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,

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.

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.

A. 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.

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.

A. You are unlikely to find "documentary evidence" for answers to this question. But the answer is "it depends".

It depends on tonnage galvanized, time, temperature operated at, and type of steel galvanized. Generally though, a time based frequency of not more than 3 weeks, and a tonnes based frequency of 1000 tonnes. In other words, every three weeks or every 1000 tonnes, whichever comes first.

A. Dear Friend.

Drossing cycle will be based on your Production cycle.
It's better to do drossing every week or maximum by 10 days for better efficiency.

A. Several things occur to me about your description of your setup. (thanks for a good description, it helps those trying to help you to understand)

You said you have an air knife between flux and dryer. Would this not just remove the flux from the wire? So that the wire enters the dryer with almost no flux? And so with no flux on rusting starts, and Fe2O3 formed would help make more dross in the zinc.

Then you said the flux density is 1.10. That sounds a bit low. Density is a measure of the strength of the flux (and easy and quick way to estimate strength), but a more normal SG would be in the range 1.19 to 1.22. 60 °C for flux is OK, but with that air knife removing it, why bother fluxing? The job that flux does is at the kettle. To reduce water in the flux, the wire is best dried (as you do), but you want the dissolved NH4Cl / ZnCl2 to remain on the wire to do its job at the immersion into zinc.

You mentioned having a ceramic kettle to avoid having to change the kettle every 2 years. Depending on your production level (you said 1500 t/ month), then a steel kettle could last 8-10 years. Something is wrong if you're only expecting 2 years.

15 t of dross a month is high (from 1500 t wire). For your plant size I'd expect more like 0.75%, not 1.0%

What is your flux %Fe?

You said the dryer runs at 200 °C. That is too high. 120 °C is about the max. The reason is that as the water evaporates leaving crystalline flux behind on the steel (normally, perhaps not if your air knife removed it!), then if you heat the wire too hot, then the water of crystalisation boils off exploding the flux into powder, none of it still on the steel.
But you didn't say where that temp is measured? Is that air temp in the drier at 200C or metal temp? It can be OK to have air temp in the drier if the residence time of the product is so low that the flux doesn't get burned off as the steel doesn't get hot enough.