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Converting from wet galvanizing to dry galvanizing
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Q. Sir,
We are hot dip galvanizers and currently using wet galvanizing process. Now we need to go for dry galvanizing process, i.e., we will remove blanket of flux over the zinc bath. I would like to know how much zinc I will lose due to increased oxidation either in absolute quantity or as fraction of zinc consumption. Percentage of zinc oxidation may vary from plant to plant so in any case MINIMUM how much zinc I will lose to oxidation. It will be better if someone tells me zinc oxidation will be minimum ____% of gross zinc consumption.
regards,
ERW pipes - Murbad, Maharashtra, India
++++++A. Atul,
Dr. Thomas H. Cook
For general galvanizers (not automatic pipes) in a wet kettle the ash is usually about 5% to 8% of the zinc used. For general galvanizers in a dry kettle the ash is usually about 10% to 15% of the zinc used. Put a different way, for general galvanizers using a dry kettle based on production, dry ash is about 0.8% (of production) for no ash box, 0.4% for a manual ash box, and 0.2% for a motorized ash box. Pipes galvanizers should produce less dry ash. For automatic pipes a very much better flux solution is required for dry kettle than for a wet kettle. Also the method of pushing the pipes into the zinc is extremely important in a dry kettle (the pipes cannot be allowed to push back up during bending). The best push down method that I know is a "progessive ventillated screw."
Regards,
- Hot Springs, South Dakota
++++++
Q. Sir,
Currently we have wet galvanizing process and remove inactive blanket flux daily. Quantity of removed flux is about 1% of total zinc consumed and it contains around 25% zinc. What should be idle zinc content of this. Now when I will move to dry galvanizing process so instead of removing flux we shall be skimming and removing dry ash,i.e., oxidised zinc. Please let me know what will be/should be its zinc content, metallic and oxide.
regards,
ERW pipes - Maharashtra, India
++++++A. Atul,
Dr. Thomas H. Cook
If you need to remove inactive flux blanket daily, then something is wrong. Have you tried to add ammonium chloride (as large lumps) to reactivate your flux blanket? While in India I found some commercial flux containing about 20% sodium chloride and potassium chloride. This is not suitable for most types of galvanizing (including pipes). At another location in India they made their own flux using zinc chloride and fertilizer grade ammonium chloride (which upon testing was quite pure). They got excellent results and a %GZU of 5.3% (no credits for dross or ash)."Wet" "spent" top flux is normally about 50% zinc (and is used to make flux or fertilizer). The METALLIC zinc content of "dry" ash is normally above 90%. In fact, an 80% recovery is common in a retort machine marketed in Europe. I strongly recommend that you have your new, incoming flux fully tested.
Regards,
- Hot Springs, South Dakota
++++++
Q. Sir,
Frankly speaking we remove spent flux daily because it is a practice in our plant rather than necessity. Sometimes when we realise that previous day zinc consumption is more or quantity of material available for galvanizing is not very much, then we skip waste/spent/inactive flux removal. In fact we do not know weather we should remove it daily or not. Sometimes when we do not remove it we get flux sticking on galvanized pipes and extraction unit rolls or pipes getting bend due to thick blanket flux cover. Our method of removing flux is that first we add ammonium chloride in powder form (not in lump form) mix it well then we add saw dust, mix it, and then remove it. Normally process is repeated till flux removing trey is full. When black spots occurs we add pre prepared flux or ammonium chloride at the required area of bath. Frankly speaking we do not know weather we should add flux or ammonium chloride to stop black spots.
Another important point is that area of bath from where pipe is withdrawn is naked and not covered by flux. Dry ash continue to form there which is periodically removed and mixed with flux cover on remaining area of zinc bath. when we remove inactive flux it is the mix of flux and dry ash. We have to do this as we do not have pulveriser to remove zinc particles from dry ash, so we mix it with blanket flux and try to melt zinc particles by adding ammonium chloride then we add saw dust and remove the so called inactive flux. Please comment on this.
ERW pipes - Maharashtra, India
++++++A. Atul
Dr. Thomas H. Cook
With use, top flux becomes rich in zinc chloride (butter of zinc) and lacking in ammonium chloride. "Spent" top flux is about equal molar in zinc chloride and ammonium chloride (mono salt). Most galvanizers use triple salt (one mole zinc chloride and three moles ammonium chloride) to make up top flux and then add ammonium chloride in small amounts during production to keep the top flux active. In time impurities (e.g. iron from the flux solution) contaminate the top flux and removal is required. Using the top flux for a long time gives more dross. The sawdust makes bubbles and increases the depth of the top flux. Other agents have been used.
A company makes a device that recovers about 70% metallic zinc from dry ash. The recovered zinc has about the same composition as the kettle zinc except it is 10 times more concentrated in iron (e.g. 0.4% as compared to 0.04%). This gives slightly more dross but is not a problem. This day I telephoned US rep. and recommended they become sponsor of finishing.com.
If you are able to convert from "wet" to "dry" kettle your dry ash should about double and your dross should become about one-third of what it is now.
Pipe galvanizing is about 20 times more difficult as compared with general job-shop galvanizing. You do need expert help.
What is your percent gross zinc usage (no credits for dross or ash; e.g. %GZU = (zinc consumed/production)times 100%?
Regards,
- Hot Springs, South Dakota
++++++
Q. Zinc used varies according to size, for 15 mm(1/2") it is 7% for
50 mm(2") it is 5% and for 150 mm(6") it is 2.7%. It includes all coating, dust, dross etc.
When dry ash is mixed with blanket flux and ammonium chloride is added with saw dust as I explained previously, can we recover some zinc. Another thing is that does aqueous ammonia, when added to spent inactive flux, give better results then ammonium chloride. I am under impression that when ammonium chloride is added, ammonia escapes and HCl remains in bath which converts bath zinc to zinc chloride thus causes the loss of zinc. So adding aqueous ammonia is better. Please comment.
ERW pipes - Maharashtra, India
++++++A. Atul,
Dr. Thomas H. Cook
You are right, in pipes galv. dia. of pipe (actually wall thickness of pipe) is very important because of the variance of surface area. I remember in Southern India that zinc usage (overall) was about 5.3%. This plant was the first one with a withdrawal angle of 14 degrees that I worked with (the previous plants having a withdrawal angle of 11 degrees to be able to use the "walking beam"). This steeper angle gave a better result. Some equipment now have an angle of 25 degrees and then a lay down to 11 degrees for blowing. The results are said to be good.
I have no experience with aqua ammonia adding to dry ash. Indeed adding ammonium chloride on the zinc surface converts some zinc to zinc chloride. However some ash (actually small zinc particles do remelt into the bath. The literature says an 80% recovery is possible with "fresh" dry ash whereas only a 40% recovery can be done with "cold" and old dry ash.
I have taken coarse zinc blowings and mixed a small amount of ammonium chloride in a 1 inch diameter test tube and heated strongly with a propane burner and recovered 90% of the powder to ingot. I am confident that the dry ash recovery device available in Europe could also handle the coarse zinc blowings. If India has high import duty for zinc, then recovery of zinc from dry ash and coarse zinc blowings is very important economically.
Steam accumulators near the blow head can give much better internal pipes blowings.
Regards,
- Hot Springs, South Dakota
++++++
Q. Dear Dr Cook
While reading your valuable suggestions made to Mr Atul, I take the opportunity to interact with you!
Dr my first submission is that, which flux is most appropriate for steel wire galvanizing,out of- Mono, Di, & Tri ZINC- AMMONIUM-CHLORIDES.The ordinary stoichiometric mixing practiced by some suppliers may not facilitate the controlled formation of Fe-Zn intermetallic layer.I request you to kindly enlighten me on the characteristics of all the three salts.
My second submission to you is that, you suggested U K retort M/C,for distillation. In case the high quality Indian clay retorts availability is in your memory please also advise
REGARDS,
- BHILAI, Chhattisgarh, INDIA
++++++A. Dear Dr. Maitra:
Dr. Thomas H. Cook
My experience with single strand wire galvanizing is very limited. My article: "Composition, Testing, and Control of Hot Dip Galvanizing Flux," is available from Metal Finishing (the journal published by Elsevier). Online it is metalfinishing.com I think you can pull it off the internet directly. I think it was a "first choice" for the editor that month in 2004. Otherwise you can get it from interlibrary loan for xerox charges. In Pakistan they used straight ammonium chloride for wire flux (likely due to low price and poor quality of commercial fluxes found in much of India).
Your second submission is about the dry ash recovery machine. It is NOT distillation but simply a melting machine. It looks rather like a small cement mixer. In general galvanizing the usual recovery for fresh dry ash is about 70% with this machine.
Regards,
- Hot Springs, South Dakota
++++++
Q. Dear Dr Cook
I feel privileged in getting replies from you, I will try to read the referred documents & may seek your advice again.
Regarding my next point I would like to inform you that we generate bottom sludge in our kettle,and instead of selling it we preferred to destabilize the Fe -Zn intermetallic to recover Zn-4 grade,metal out of it .The recovery in this endeavour was only 50%. To achieve higher recovery we may have to go for Zn distillation & require retort, that's why I requested to you to inform me the sources.
WITH KIND REGARDS
Dr P K Maitra
- BHILAI, Chhattisgarh, INDIA
++++++A. Dear Dr. Maitra,
Dr. Thomas H. Cook
I think that some zinc producers have distillation equipment to produce very pure zinc. I suggest that you contact them.
Regards,
- Hot Springs, South Dakota
May 5, 2011
Q. Hello Dr. Cook & others
I just have some doubts with what you have posted before
in this thread and other threads about norms on zinc ash and dross consumption.
We are (wet) galvanizing steel transmission towers and our zinc consumption (after equ) is 4.12% (our target 4%) [Kindly comment on our current consumption and whether target is realistic]. The zinc consumption (before equ) is 4.43 % (104.3 MT). The split up (as a percentage of total production) is
Dross 0.38% (8.75 MT)
Ash 0.48% (11.58 MT) (your suggestion was 0.8%)
Pick up 3.56% (84.30 MT) (we calculated it by subtracting dross & ash from total zinc consumed. Is it a correct way?)
The above figures are well within the limits you have suggested as norms. We don't use ash box.
But while calculating the same as a percentage of zinc consumption, the figures are well above the norms u have stated. Why it is so?
Dross 8.36%
Ash 11.07% (your suggestion was 5% to 8%)
Thanks in advance
- Ahmedabad, Gujarat, India
January 19, 2012
Q. Sir,
We have a general galvanizing unit producing tower /communication steel items with the zinc bath 7.5 x1.0 x1.2 size of zinc kettle.
We maintain flux with double salt Zinc ammonium chloride and maintain 18 -22 Be. the Iron content does not go more than 0.22 % and have continuous filtration unit.
The flux heating is not enough and we are looking into that.Our ash formation is quite high and almost 22%.
Can you suggest any methods to reduce the same.
Hoping to hear from you,
regards
work here as senior Manager - Puducherry, India
January 29, 2012A. The Ash formation basically depends on
Umesh Dalela
1. Type of Galvanizing Process - Dry or Wet. Normally in Structural Galvanizing we use Dry process with pre fluxing. And Ash formation is slightly higher. SKIM BATH ONLY THE AREA TO BE USED; NOT FULL BATH. REST AREA KEEP IT COVER WITH ASH.
2. In wet bath process add Zinc Chloride approx. 60% in flux; you will get less Ash.
3. Keep the Zinc Bath temperature as low as possible even during production- you will get less Ash.
4. Use of Aluminium also helps in low Ash formation.
- Mumbai, Maharashtra, India
February 2, 2012A. Sir:
Dr. Thomas H. Cook
For this size kettle to be profitable, about 10,000,000 pounds of steel should be galvanized each year. Thus calculating at 7%GZU this would mean a consumption of 700,000 pounds of zinc. For a manual ash box the usual ash is 0.4% of production which would be: 10,000,000*0.004=40,000 pounds of dry ash/year.
You are calculating dry ash based on zinc consumption which means: 700,000*0.20 = 140,000 pounds of ash/year. Clearly this extra 100,000 pounds of ash each year is a major problem. You are producing 3.5 times more dry ash than is normal.
Regards,
- Hot Springs, South Dakota
March 22, 2012
Q. Why do we only use a combination of ammonium chloride and zinc chloride in flux.
krunal patil- nagpur, India
March 23, 2012
Hi, Krunal.
I am not a galvanizing expert and cannot answer your question. But as the website operator I can advise that you will get much more enthusiastic response from the readers if you will describe your situation, and why you are asking :-)
When people don't know whether you are a student researching a science project, or a vendor of ammonium chloride who will bend their ear about why they don't need zinc chloride, or a galvanizer who has found a single salt flux workable so far ... they tend to not say as much as you'd like. Good luck.
Regards,
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey
March 26, 2012
A. Dear Raghavan Sir & Dr. Thomas H Cook,
While Raghavan Sir has given data of Ash formation, he has forget to mention the thickness of sheet used to make tube which is galvanized. He has also forget to write if the tube is heavily rusted. The rejection after galvanizing is above 20 % (fabrication error, material defect, improper pickling due to heavy rust as well as galvanizing defect).
Q. My question to Cook Sir is: do the above factors lead to ash formation as told by Raghavan sir or the furnace heating is also to be blamed?
- Vadodara, Gujarat, India
March 29, 2012A. Mr. Singh:
Dr. Thomas H. Cook
A rejection rate of 20% and high dry ash formation is quite characteristic of very BAD FLUX. Likely the flux has very high iron (Fe+2) and or high sodium chloride, potassium chloride, calcium chloride, and magnesium chloride. Also the concentration of the flux (Baume') is likely very high.
Regards,
- Hot Springs, South Dakota
April 2, 2012
Q. Dear Dr. Cook,
I have one simple question.
Why consumption of Zn is calculated in MT / hour or MT / year for a dry zinc kettle.
As per me, it should be in Total surface area immersed / dipping and so on...
(In our case 5 MT can have different surface area in each dipping).
Can we calculate on same base?
- Ahmedabad, India
April 3, 2012A. Mr. Shah:
Dr. Thomas H. Cook
I have just returned from a two week trip to three galvanizing plants putting old and new technology into these plants. Over inhibited stopped pickling and under inhibited caused very bad fumes in the plant, and short acid lifetimes. Bad inhibitors cause a whole range of problems. There is an easy and effective test for inhibitors and I have published it.
In my previous response in this thread to the question of 20% reject and excessive ash I reported likely bad flux. It is equally possible that something is wrong in the pickling (over inhibited acid causing no pickling or the acid is "spent" in which case testing the free acid content and the iron and zinc in the acid is required, or perhaps the acid is too cold). There are possibly as many as 10 reasons why there are so many rejects and such high ash. Keep in mind that finishing.com is a website which can be useful, however real on-site WORK is REQUIRED to identify and solve the major problems that are submitted to this site.
Getting back to your question--The large galvanizing plant had a %GZU of 5.5%. Because of the very heavy steel (low surface area of the steel), I am sure that the %GZU is now between 3.0% and 3.5%. If you are a general galvanizer the simple way is to measure %GZU (which is based on steel weight rather than surface area of the steel).
Again %GZU = Total Zinc Consumed/Steel Galvanized times 100%
(No credits for dross or ash).
Regards,
- Hot Springs, South Dakota
April 25, 2012
Q. Dear Mr. Cook,
We are using different types of steel which contain different % of Si (0.22% in one and 0.04% in second). Should we care in immersion time of both materials in Zn kettle to avoid extra coating?
Also I have heard that immediate dipping after fluxing & drying in oven is preferable rather than any delay. Is it so? We have 10 to 15 mins delay due to change of fixture after drying. Is that the reason of "Splashing" in kettle? Should we change the sequence?
Regards,
- Ahmedabad, India
April 25, 2012A. Sir:
Dr. Thomas H. Cook
If you get a copy of the Sandlin curve, your question will likely be answered. Phosphorus is even more important than silicon.
I assume that you use HCl and the European fluxing system? If so your baume' is likely high (18 to 25) and your ACN is low (likely 0.6 to 0.9). Also the temperature of the flux solution is likely low (likely around 40 to 50C). Under these conditions even a drying oven is not effective. You likely also lack a good wetting agent.
From the other Finishing.com thread, you will notice that for an HCl plant you have up to one hour after fluxing before galvanizing is required. However if the pH is high and if the product is oven dried (corrosion is faster at a higher temperature) then zinc spatter and black (bare) spots are very likely. Also with the European fluxing system the cook-off time in the kettle is longer giving thicker zinc coatings on the product. I very much like the USA system of fluxing which avoids all these problems. It is a pleasure to answer your questions as best I can on the finishing.com website.
Regards,
- Hot Springs, South Dakota
May 3, 2012
Q. Dr. Cook,
We are using fluxing, oven drying and galvanizing in our angles galvanizing plant, doing 2000 tons angles per month. Since our plant has been procured from Europe, I believe we are following the 'European Fluxing' model.
It is the first time, while going through your article, that there is a 'USA fluxing' model that is different from the European model.
Request to kindly highlight differences and merits/demerits in both the processes.
With regards,
- Bhiwadi, Rajasthan, India
May 4, 2012A. Sir:
Dr. Thomas H. Cook
I have already made these comments on this website several times. I am pleased that you understand the likely advantages of the USA fluxing system. In South Africa we converted a flux from 30 deg. Baume,' ACN 0.4, temp. 25 C to a flux at 13 deg. Baume,' ACN 1.4, temp 71 C and reduced total galvanizing costs by $9,000,000/year (35,000 MT/year). And this was when zinc was cheap (1985). Also production increased 20% with no additional labor. The advantages of USA fluxing are quite clear.
Regards,
- Hot Springs, South Dakota
First of two simultaneous responses -- May 4, 2012A. I'm confused by this European vs USA fluxing method (and the references to 1985).
The fluxing methods of European galvanizers are not very similar now to those in Eu of 1985.
In about 30 years it would be hoped that both Eu and USA methods might have improved. I think both probably have.
What differences are there today (if any)?
The biggest difference I'm aware of between these places is acid type. Sulphuric in USA vs (predominantly) Hydrochloric in Eu.
I suspect with so much cross ownership of galvanizing companies and suppliers between Eu and USA, that there are few other differences.
Geoff Crowley
galvanizing &
powder coating shop
Glasgow, Scotland
Second of two simultaneous responses -- May 5, 2012A. Dear Dr. Cook,
MADHU RANJAN
Thanks for the information. How do we proceed in this matter of changing over to USA system from European system? Kindly advise.
Regards,
- Bhiwadi, Rajasthan, India
May 12, 2012A. Geoff,
Dr. Thomas H. Cook
In the USA I think HCl and H2SO4 are about equally used. Also I think the European and North American systems of flux formulation and usage have not materially changed in the last 30 years. Europeans think they are correct and Americans think they are correct. The advantages of the North American methods are compelling. The NA system requires more control.
M.R.
I cannot in good faith describe in detail what North American exact flux may or may not suit you well. The formulation, testing, and maintaining of good flux REQUIRES "in house" testing and means to correct problems as they occur.
Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA
- Hot Springs, South Dakota
May 15, 2012A. Of the galvanizing plants I know that have been operating for say 20-30 years in the UK, I think most have changed their flux management practices twice.
25 years ago very few had prefluxing systems with a tank of water dissolved flux.
30 years ago many had flux blanket systems.
Today flux control is much more precise, as the margins in the business require it to save on zinc consumption.
Most use double or triple salt ratio ammonium chloride / zinc chloride. An increasing number are using potassium chloride.
Its rare to use sulphuric acid in Europe.
Geoff Crowley
galvanizing &
powder coating shop
Glasgow, Scotland
May 17, 2012A. Geoff:
Dr. Thomas H. Cook
As you know the "ACN" = ammonium chloride/zinc chloride. I invented "ACN" about 1985 in South Africa. ACN's for various fluxes are: Double = 0.8; Triple 1.17; and quadraflux = 1.57.
Recently three new plants have been put into the USA using European technology and all are using double salt with high baume' and a drying oven.
Within the last 15 years (here on the ranch) I mixed up a single flux solution with an ACN of exactly 1.00. I sent portions of this flux solution to 13 "certified" testing labs some of which cost me $300 for testing. NONE of these labs came even close to the correct answer. The other ions dissolved like iron, sulfate, etc. were also incorrectly reported. From these data it is clear that testing and control of flux is greatly lacking. I also sent two samples of this flux to two galvanizers that I taught how to test and they got results that were "spot on."
I later published my results in the journal METAL FINISHING which could likely be obtained on the internet or via inter library loan from a large library. The name of this article was something like: TESTING AND CONTROL OF HOT DIP GALVANIZING FLUX.
Shorter kettle "cook out" times in the kettle, much less ash, elimination of icicles on the steel, little or no kettle spatter, and other advantages are characteristic of quadraflux (if properly formulated and maintained). I have been doing this for 40 years. I have tested thousands of fluxes and have found many bad impurities.
Regarding potassium chloride use in a hot dip galvanizing flux: My experience is that it reduces smoke at the kettle, however it increases cook out times giving thicker zinc coatings on the product. It also appears to cause salt and pepper like black (bare) spots in the corners of lightweight product.
Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA
- Hot Springs, South Dakota