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Galvanising Plant - Zinc Ash and Zinc Dross

A discussion started in 2005 & continuing through 2017 -- add your Q to bring it back to the Hot Topics page.


Q. Dear sir:

I am interested in knowing if there is a relationship between the production of my galvanizing plant(Ton) and the production of Dross and Ash, under normal operation of the plant (controlled temperatures on the kettles 450 deg C).

Should a galvanizing plant have these values under a specific percentage?
What are these percentages?

Thanks for your help.

Juan Carlos Sanin
General Galvanizer - Medellin, Colombia


A. In Italy we see these numbers as correct:
ash < 12% zinc consumption
dross < 13% zinc consumption
optimum when ash+dross< 20% zinc consumption

greater numbers are generated by problems in flux-dryer or pickling operations

Kindly regards,

Federico Priuli
- Verona, ITALY, EU

To minimize searching and offer multiple viewpoints, we combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition.

January 12, 2008

Q. We have a galvanising plant which has been operating very successfully.

We would like to know what should be the percentage of Zinc Ash and Zinc Dross, of total Zinc consumed in the process.

What could be the reasons for high ash and dross generation?

Chandra Prakash
Plant Manager - Sharjah, UAE

January 17, 2008

A. Sir:
Thirty years ago I related ash, and dross to zinc consumed. In those days ash above 13% and dross above 10% (both related to zinc consumed) was considered excessive. Thus 77% of the zinc was on the product.
In more recent years I related ash, and dross to product galvanized. Ash above 0.8% (no ash box), ash above 0.4% (manual ash box), and ash above 0.2% (motorized ash box)in relation to production is considered excessive. Dross above 0.6% in relation to production is considered excessive.
Excessive ash is usually a result of bad flux, and excessive dross is usually a result of poor kettle furnace design or poor kettle maintenance.
Ash is now often reclaimed to zinc with a machine that looks like a small cement mixer/heater. A recovery of 70% to 80% is common for a "dry" kettle galvanizer.


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

January 19, 2008

A. Iron (Fe) in the flux (possibly due to poor or no rinsing after acid pickling) is a prime cause of dross formation. Fe concentrations should be kept below 1% in the flux.

Very wet steel entering the zinc is another prime cause of ash formation.

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

January 22, 2008

In the early days, I purified many, many flux solutions. This involved removing the iron +2 using hydrogen peroxide and ammonium hydroxide, by testing first and then using formulas and methods that work. In plants using sulfuric acid the sulfate was removed from the flux using barium chloride dihydrate (anhydrous powder barium chloride is too dangerous). These were all "dry" kettle galvanizers. Many of these galvanizers were looking forward to less dross after iron +2 removal from their flux. It NEVER happened. The dross formation was unchanged after iron+2 removal from the flux solution. Their %GZU was decreased by 10% to 30% (mostly thinner zinc on the product and less dry ash). I suspect that iron in the flux is converted to small "dross islands" in the zinc surface on the galvanized steel.
I have no experience with iron+2 removal from a "wet" kettle galvanizer's flux solution. I suspect that it WOULD reduce dross because the top flux would collect iron from the flux and then form dross at the top-flux/zinc interface. Some "wet" kettle galvanizers change out their top flux often and I suspect that they have found that this reduces dross formation.
In recent times we have found other methods to improve galvanizing, to the point, where many galvanizers use very, very bad flux and still they get excellent results. It is amazing how new technology changes "the rules of the game."

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

Faults in Hot-Dip Galvanizing

Hot Dip Galvanizing

May 2, 2008

Q. For molten bath temperatures in the range 475° C are required. I am investigating the best suited type of heater for this duty. I suppose that electric induction heaters are a recent phenomenon; but I doubt its utility in the areas where power shut-downs are frequent. Does ruling out this option not leave behind any other than furnace oil (dirty but potentially cheapest fuel) heaters. I look forward to hear from the experts. Thanks a lot in advance.

Rajesh Kulkarni
- Pune, Maharashtra State, India

March 30, 2011

Q. Hello.
Sir, I want to know what should be percentage and actual percentage of Zinc Ash & Zinc Dross during manufacturing of Galvanized Steel Pipes and input/output ratio of Zinc for this process.

DR Dhundhara
- Jaipur

August 3, 2011

Q. How to purify zinc dross to get pure zinc?

Ganesh Gaikwad
student - Thane, Maharashtra, India

September 17, 2011

Q. In a galvanising line with 120 MT Zinc bath, is dross generation of 5 MT per month, on an average production of 4500 MT high or acceptable ?

Shishir Desai
Product engineer - Mumbai, India

September 20, 2011

A. Sir:

Your dross to production percent is:

%Dross/Production = 100% times 5/4500

%Dross/Production = 0.11%

This is four times lower than ANY I have ever calculated.

I suspect that you have a new kettle and have not had enough production yet (likely only 3 months or less of production) to experience normal dross levels. Another possible answer is that you are not getting the dross out of the kettle (e.g., using a clam), thus allowing the dross to accumulate.


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

June 23, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. How to reduce ash formation in galvanizing.

abhijit palaskar
- dubai, uae

June 23, 2013

A. Hi Abhijit. We appended your inquiry to a thread which offers good hints. We also have several other threads on the subject if you are patient with the search engine at the top of the page. If you have a highly specific question on the subject, feel free to post detailed data and people can try to help you. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

July 27, 2013

Q. Hi
Thanks for your helpful comments
We have some problems in our dry jobbing galvanizing line, too. Some problems are usual, but the high amount of ash, produced in our line, is not usual and I'm looking for a solution to reduce it.
We have a steel kettle (6*1.5*1.2) and use Hydrochloric acid, and Ammonium chloride for flux -- about 0.01 Al and 20 Cm Pb at bottom, and no ash box.
We are galvanizing usual low carbon steel, and in little part, the cast iron.
My Question is how much ash production is normal in our line ?
Is there any way to reduce it ? or, is there any alloying metals or additional materials to use for reduce ash production ?
I heard Cd, Mg, Sn and Sb can effect on ash production, is that true ? and how much we should use to inhibit ash production?

Ali Kouchaki Nejad
- Karaj, Tehran, Iran

July 30, 2013

A. Sir:

Cd and Sb (antimony) are extremely toxic. Magnesium causes much white rust. Tin cracks the steel if over 0.4%. Cast iron is known for excess dross. Usual dross is less than 0.6% of production. Ash with no ash box is 0.8% of production, with ash box ash is 0.4% of production and with motorized ash box is 0.2% of production. Bad flux (e.g. straight ammonium chloride) causes much smoke and ash.


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

August 3, 2013

Q. Dr. Thomas H. Cook , thanks for your answer.
You say about using "ash box", I search about it, but I find just some general explanation : (A manual ash box is about 16 inches square on top and about 18 inches square on the bottom (or a rectangle of similar square inches). This semi-pyramid shape keeps the ash from plugging up the ash box. (The sides get bigger as you go down.) Both top and bottom are open. The manual ash box is mounted about 1/3 in the zinc and about 2/3 out of the zinc. The total height of the ash box is about 18 inches, etc.)
But, How does it work?!
Could you please send some picture or video about working ash box ?
We use ammonium chloride in direct, as melting flux, and you hint that using directly, increase ash and smoke production. Is there any other flux to use with less ash production?
Thanks a lot.

Ali Kouchaki Nejad [returning]
- Karaj, Tehran, Iran

August 6, 2013

A. Sir:

To use an ash box, simply use the ash shovel with 1/4 inch holes and skim off the ash and put it into the ash box. From time to time poke the ash in the box downward and it will remelt. This is a dirty, smoky job for the worker and the zinc should not have lead or nickel in it. Normally this cuts ash in half. A motorized box does this automatically and zinc recovery is better. A thin steel cover keeps much smoke down. For kettles with not enough surface area for an ash box there is a commercial ash recovery machine (looks like a small cement truck) costing about $80,000 which pays out in one year for a galvanizer doing 30,000,000 pounds galvanizing/year.


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

August 13, 2013

Q. Thanks a lot for your answer.
In general, we use Ammonium chloride in direct, as flux, and as we know, it makes lots of smoke, dross and ash.
We have not big problem with smoke and dross. In fact, the ash production is our Big problem and I have to reduce it.
What you offer to use as flux (liquation) in direct for "driving out" stage (and less ash production)?

Ali Kouchaki Nejad
- Karaj, Tehran, Iran

August 28, 2013

Q. Is there any other material to use?
Thanks for your helpful answers.


Ali Kouchaki Nejad [returning]
- Karaj, Tehran, Iran

April 21, 2014

Q. We are a recycling company with more than 10 year on our field. Our goal is to supply galvanizers with quality zinc, and selling the low quality zinc in other applications. In the past days we have received a complaint regarding the quality of our zinc (we have spectrometer and analyze each melt) and the zinc we supply for galvanizers is 99.70% Zn Min and .15% to .30% Fe Max. Our customer is telling us our zinc is generating excessive dross and ash in their process (I have supplied the same zinc to other galvanizers who haven't had any problems). They are wire galvanizers and have increased their production in the past months, as I know they have an induction heater in the pot and they check the concentrations on the flux but don't review the iron content on it. My question is the following, which could be the causes generating the excessive dross and ash?

Enrique Garcia
- Monterrey, NL, Mexico

April 2014

Hi Enrique. Dr. Cook asserts that excessive ash is most often the result of poor flux, and excessive dross is the result of poor kettle design and maintenance. Mr. Crowley added that Iron in the flux, perhaps from poor rinsing after pickling, and very wet steel, are other causes of excess ash. These sound like they may be related to your customer's increased production rates? Please try your best to frame your questions in terms of the answers that were already provided if you can :-)

Thanks and Regards,

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

May 14, 2014

A. Dear Mr. Enrique Garcia

Your Iron content is too high for a galvanizing operation, Iron solubility at 450 °C is 0.035 %; excessive iron will combine with zinc (1 gram iron with 24 grams of zinc) and create dross.

I guess you are doing pyrometallurgical reduction, but galvanizers require 99.995% purity for zinc, which cannot be achieved by your method.

Ozge Saracoglu
- Doha, Qatar

May 22, 2014

Q. I will change the flux bath to below composition :
NH4Cl = 172 gr/lit
ZnCl2 = 225 gr/lit
pH = 3.9 -4.2
B.N (@15 °C)=25
IRON content=1.19 gr/lit
volume of bath = 23 M3
dry galvanizing system
WHAT IS THE DROSS/production CHANGES in variation of bath temperature?

at kab
- tehran, iran

Control of ash generation

May 12, 2015
-- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear sir,

We are facing the problem of ash generation in kettle. Our ash generation is 1.16 %. It is very high.
We are using 30 baume double salt flux for galvanizing, and we are galvanizing tower lines (5 mm to 20 mm thickness) and cable trays (2 mm) material. Our flux temperature is 30 °C.

How are we to solve this problem?

Please give your expert advice to me and help us to solve the problem.

kiran gaikwad
student - satara,maharastra,india

June 2015

thumbs up signHi Kiran. We appended your inquiry to one of several earlier threads on the subject. After noting some of the good advice that has been offered, you may wish to post some more specific questions about that advice. Best of luck! For the record, "expert advice" can be offered only by paid consultants with on-site access to the full situation, but hopefully this forum chatter will be of some help :-)


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

June 13, 2015

A. Some more info would help to answer this.
What is the flux Fe %?
Do you have capacity to heat flux to higher temp?
What is flux full analysis?
Do you rinse before flux? once or twice? Clean water or dirty?
Do you have a drier between flux and zinc?
When you state zinc consumption, what terms are you using? Is that ash weight as a percentage of zinc consumed? or wt ash as % of steel coated? Or what?

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

Hot dip galvanizing: The ash box

July 14, 2015

Q. Good morning!
We run a galvanizing plant and are reviewing some processes, in order to reduce zinc consumption. We do not flux neither rinse the material and also we haven't a ash box.
On this post there is some explanation on what it is and how to use a ash box, but there are some things that are not clear.

It is said that the ash will be put on the ash box, and from time to time the ash should be poked downward to remelt. In my understanding, the ash has a different chemical composition than pure zinc, so I suppose what will remelt is the zinc that gets removed together on the skimming, not the ash itself?

Also, considering that the zinc is what remelts and the ash stay on the box, eventually it will be full. When that happens, and we poke the ash little bit more it will float back (through the bottom opening) outside the box? I suppose it should not happen, and when the ash box is full we should clean it. But how is it done? I mean, if we just pull the box up the ash will fall trough the bottom opening on the box (which is larger than the top one). Or I'm wrong, and the ash will be stuck in the box (and that's why the bottom opening is larger, so we can poke and force the ash out)?

Thanks for the help!

Lucas Corsaletti
- Londrina, Parana, Brazil

July 18, 2015

A. You could probably reduce zinc consumption a lot by rinsing and fluxing.
Without this there must be high zinc consumption, increased bare spots, and other quality issues.

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

July 24, 2015

Q. Hello Geoff!

Thanks for your answer!
We are working on setting up the fluxing process, but due to space and layout constraints the rinsing will have to wait a little bit =(

But another major issue that we have is that the ash removed contains a lot of "pure" zinc, which get removed together with the ash while skimming. Then we sift the ash and throw the (now solidified) zinc back on the kettle. This process takes a considerable amount of time and raises a lot of ash in the air, preventing us from continuing the work until it settles. We only do that at the end of day, but still it is a process that I believe could be completely removed with an ash box, because there will be little to no solidified zinc on the ash (because it will drain back in the kettle, right?). Besides, an ash box is just a box, easy to build and setup, and we could start benefiting from it really quickly.

Lucas Corsaletti [returning]
- Londrina, Parana, Brazil

July 28, 2015

A. Do some calculations before you do a lot of work removing metallic zinc from ash.
There's labour cost, heat, flux maybe, and tools.
The ash you end up with doing this work will get a lower price from those that buy such ash.
If you don't remove the metallic zinc they pay more for the ash.
Check the difference. It's just math to see if its worth the cost of the work to get it out and end up with a lower value ash.

You can never get all the metallics this way, perhaps get it to a point where 20% of the weight of the ash is returned to the kettle.
If you use an ash processing machine, then you might get 70% of the initial ash weight back as metallic zinc for putting back in the kettle.

There's no right and wrong in this, you have to work out what works best in your own situation. In different countries labour costs are different, ash sales prices different, gas (heat) costs are different.

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

May 3, 2016

Q. Dear Sir,

I want an assistance on the followings;
1.What amount is the zinc content of zinc ash after galvanization?Are there any difference between zinc ash and zinc dross?Please let me know if there are any elements apart from zinc in the ash withe their respective percentage amount.
2.If there are any figures about the percentage of lead,and similar elements in their respective ashes after certain processing like galvanization

Mengistu Wondimu
Ethiopian metals nindustry development institute - Addis Ababa, Ethiopia

May 5, 2016

A. The question is too vague.

The metallic content of ash is highly variable from 80% to 10%, depending on what is done with it, how the plant is operated and so on.

It would be easier to answer with more information on what you really want to know.

Ash and dross are very different. The only common thing about them is that they contain some zinc.
Dross is iron/zinc alloy crystals, and the chemical analysis varies from one plant to the next.

The content of residual elements in both is also highly variable.

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

May 14, 2016

A. Sir:

Skimmings or ash, in my experience 40 years ago were 95% to 99% metallic zinc. More recently with contaminated fluxes the ash does contain various flux contaminations including: aluminum, nickel, calcium, copper, magnesium, manganese (from the steel), potassium, and sodium. In the USA many galvanizers are lead free and using special high-grade zinc. In the "old" days with 1% lead in the zinc, heavily "worked" ash can be as high as 5% in lead. Aluminum in the ash is usually 100 times more than in the molten zinc and may be present as chloride or oxide. Uses for ash include micronutrient fertilizer and micronutrients for farm animals, depending on impurities.

Galvanizers' dross is usually between 2% and 4% iron. Small amounts of chromium and nickel are sometimes present. Uses for dross are brass and other alloys.


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

April 1, 2017

Q. Sir I'm plant in-charge in panipat india. My group works in steel Structure line and galvanisation. Our monthly production in GI is 550 mt and approx. 30 mt zinc consumed per month.

Dross generated is 2.5 mt to 3 mt, white ash 3 Mt and black ash 2.5 mt. So, sir, please guide me whether its quantity is satisfactory or not satisfactory?

Jitendra Gupta
Employ - Panipat india

April 6, 2017

A. Your Dross formation is almost OK, it is approx. 10%. It can be further reduced.
Ash again 3 MT + 2.5 MT. It is tremendously high. It needs immediate attention. It can be brought down to total 7%.

- Delhi, India

July 10, 2017

Q. Sir,
I have a galvanizing plant and our problem is we are not able to get good finishing. In galvanizing of cable trays, in perforated cable trays zinc is getting stuck in the gap; and zinc usage is more than what it should be.
Can you please suggest what should I do to make the finishing better and avoid zinc wastage?
Thank you sir.

Maya Nair
galvanizers - Vadodara Gujarat India

July 2017

A. Hi Maya. It sounds to me like you are saying that the galvanizing is bridging a gap that you feel it is not supposed to be bridging. Photos would probably help.


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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