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HOW COULD I REDUCE AND REMOVE ZINC DROSS

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January 20, 2010

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.

Nelson Visbal
Engineer - Barranquilla, Colombia

simultaneous January 22, 2010

Sir:

I presume you have thermocouples IN the molten zinc to control temperature. I also assume you have an electronic controller to provide burner control. 468 °C (874 °F) is NOT normal and with an iron kettle will cause the iron in the kettle and the hot zinc to form much dross. Your temperature control system may not be correct. Keep in mind that it takes about 10 minutes for heat to travel through 50 mm of kettle wall thickness.

Send a photo of your dross shovel. Also what are your kettle measurements (length, width, depth)? Dross in most cases (except spin galvanizing) is usually less than 0.6% of production. The clay or solid feeling with a rod IS DROSS. Likely you are burning up your kettle.

What proportions of ammonium chloride and zinc chloride are you using. From what you have already said, the high zinc temperature is likely by far your biggest problem. A diagram of your kettle furnace would be useful. Likely you may have a problem with design. I have a published article regarding kettle furnace design that may be of use to you.

I think the best dross bucket/scoop/shovel design is in the shape of a capital "T" with a counter weight on one side of the top of the "T" with the actual bucket/scoop/shovel at the bottom of the "T". The crane is connected to both tops of the "T". I like 3/8 inch holes on 1.5 inch centers. The actual methods in drossing also are very important.

Regards,

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


January 23, 2010

There are two main systems of dross removal;
Firstly, a shovel (sometimes called a spoon) which is a dredge like shovel on a long steel handle, and typically about 500-800 mm wide mouth, and about square on the bottom, with high sides on three sides except the mouth side.
This is lowered to the dross level and moved along the length of the kettle to scoop up dross. Lifted very slowly to the surface, and then out, the dross is vibrated or worked with a spade to allow free zinc to drain off.
Do this repeatedly in both directions until you can't get any more. Make up the level of zinc with fresh new zinc.

Next is the dross grab. Crane lifted and with two jaws, this device is similar to those used to empty a cargo ship of a grain or granular cargo. As with the shovel, keep going until you can't get any more out.

Its important to get the dross in the corners.

There is often one or two pieces of steel that had accidentally dropped in, so get these out too. You won't get all the dross out without removing them.

Drossing frequency should depend on tonnage galvanized, but it's often more convenient to do it on a timed basis, perhaps each second weekend or so.

Dross is formed from the reaction between iron and zinc. To reduce its production requires reducing the amount of iron available to react. It's free iron that reacts to form dross, not the steel you're galvanizing.
So: what's the Fe % in your flux? What flux strength? What ratio of zinc chloride to ammonium chloride? Temp? any drying? What material type you galvanizing?

Your 460-468° sounds a bit high in temp. The higher you go the more you'll form dross from dissolving your kettle. Which also shortens kettle life.
450° sounds OK, can't you control it to about +/- 4 °C ?

Geoff Crowley
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland




November 26, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

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

Param 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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland


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.

Nitesh Agarwal
- 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?

Arlene Canarias
- 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,

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



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
- Manila, 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



How many zinc ingots will be consumed in galvanizing 605 MT and drossing?

July 29, 2016

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?

bola jinadu
- Lagos Nigeria


July 2016

thumbs up signHi Bola. I don't know much about galvanizing, but it seems to me that your equation has too many unknowns to be solvable.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


simultaneous August 5, 2016

A. Hi, Mr Bola,

You need 21 Mt zinc @3.5% and 3 Mt for addition after drossing.

Rajesh

Rajesh Patel
- Vadodara, India


August 4, 2016

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
Geoff Crowley
galvanizing & powder coating shop
Glasgow, Scotland



August 4, 2016

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.

U. C. Dalela
- Delhi. India


December 21, 2016

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

Umesh Dalela
- Delhi, India

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