Reduce Zinc Ash / Zinc Waste; Convert from Wet Galvanizing to Dry

A. In my experience the zinc numbers you mention vary with plant layout, pretreatment chemistry, product type and volume. But typical numbers for average plants are: Ash 2-5 kg/tonne, dross 3-9 kg/tonne, and zinc consumption between 4 and 6 % (40-60 kg)

Your post title asks if double salt is the problem. You mention there being no dryer.
Dipping wet steel increases ash production (which of course also increases zinc consumption), and increases energy consumption too in the kettle. So some form of drying is always an advantage.
Without a dryer, triple salt will allow the steel to dry more easily, as the lower concentration of zinc chloride, (which is somewhat hygroscopic) allows better drying, even where no dryer exists.

In my experience the concentration of the flux in many plants, sometimes justified as a cost saving, is actually increasing zinc consumption. For double salt the total amount of salt in flux should be about 450g/Litre. High Fe concentration in flux makes it harder to dry the steel, and also increases dross production in the kettle.
Many use density as a pseudo check of flux concentration. That works provided Fe% is not very high, as it to contributes to density.
So what to do? Pay a lot of attention to your flux, it's the most critical of the pretreatment stages, and unlike acid pickling, its effects are not visual.
Find a way to dry the steel, but not at the expense of production, for example by hanging over the kettle.
Check the flux %Fe, as well as concentration of each of the two components, ammonium chloride and zinc chloride. Check the ratio too. You might consider moving that ration to triple salt, but watch for lowered fluxing action at the kettle, and also watch for oxidation of the steel between flux and zinc stages.

Suppliers of flux often provide chemical analysis methods, but they are not difficult to source, and the chemistry is not so difficult, its high school level laboratory work using simple chemical analysis techniques.

? Before any recommendation, are you use you are using a double salt, molecular formula of ZnCl2.2NH4Cl or your supplier tells you it is a double salt, zinc chloride plus ammonium chloride, mixed?

A. Atul,
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 "progressive ventilated screw."
Regards,

A. Atul,
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,

A. Atul
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 to 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,

A. Atul,
You are right, in pipe galv. the 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 pipe blowings.
Regards,

A. Dear Dr. Maitra:
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,

Ed. update: Metal Finishing magazine, its website, and the company no longer exist. Elsevier bought them and abruptly shut them down in 2012. Don't follow expired URLs, hackers seek them.
There are several on-line paywall services behind which you can find that article -- just put the article name in google. You can view it on line, with interspersed ads, at scribd.com

A. Dear Dr. Maitra,
I think that some zinc producers have distillation equipment to produce very pure zinc. I suggest that you contact them.
Regards,

A. The Ash formation basically depends on
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.

A. Sir:

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,

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).

A. Mr. Singh:

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 (baumé) is likely very high.

Regards,

A. Mr. Shah:

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 & 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 & 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,

Ed. note: We thank Dr. Cook for his generous yeoman work in steering readers towards general approaches to solving their problems, and acknowledge that work is required; sharing general ideas often falls far short of actually solving the problems  🙂
Expert help is obtained by retaining an expert to take things beyond the truisms that will fit into forum responses.

A. Sir:

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 baumé 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 50 °C). 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,

A. Sir:

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. baumé,' ACN 0.4, temp. 25 C to a flux at 13 deg. baumé,' 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,

A. Geoff,

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

A. 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.

It's rare to use sulfuric acid in Europe.

A. Geoff:

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 baumé 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, sulphate, 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,

A. Marcus,

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.

Without an ash box expect ash to be about 0.8% of production. With a manual ash box expect about 0.4% ash to production. With a motor driven ash box expect about 0.2% of production. Use a thin steel sheet cover to reduce smoke. If you still use lead in the zinc (Prime Western), then the guy working the ash box may be at risk for lead in his blood or urine. If you use nickel in the zinc there may be a nickel exposure to the guy working the ash box. If ash is more than 0.8% to 0.2% of production for the above conditions then there is something wrong with the flux or how you use it. For example, a contaminated, high baumé flux, with low ACNV and high temperature will give very excessive ash. Using this type of flux results in two layers of flux on the product: (1)The outer layer of flux dried to a shell, and (2)The inner layer of wet mushy flux which causes violent explosions upon entry to the molten zinc. This flux and its use is a very bad "hybrid" of North American and European fluxing systems.

I have seen an excellent motor driven ash box at a galvanizer. Also there was a guy making commercial automatic ash boxes, but this site is not for advertising^*. I think this guy is now concentrating on immersion heaters.
Regards,

* A small clarification: although 'comment spam', i.e., steering readers to specific vendors is discouraged both in professional journals and this forum, advertising is welcome here in the same way as it is in the journals: advertising should be both paid for and marked as such; we certainly don't discourage vendors of automatic ash boxes or any other products to advertise here     -- the Editor

A. It is true.
A light sprinkling (a cupful or less) sprinkled finely helps to get some of the metallic zinc to get wet and re-enter the molten zinc in the tank.
It creates a lot of fume though.

? Hi Andrew. "Pressure"? Maybe that was an auto-correct and you meant "measure"? More words please ... at least as many words in the question as you want in the answer  🙂

Thanks!

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Regarding galvanizing ash-- I have one client who has 0.1% of production ash. He uses an MZR machine to recover zinc at 85% from unworked ash. His production is 500,000 pounds/day.

I expect you are using very concentrated (high baumé), and likely contaminated flux. Contamination could come from your flux or ammonium chloride supplier or from "hard water." Perhaps your flux formulation is wrong.

Regards,

A. I've found that in general around Europe and UK, that between 0.1 and 0.3% (or production tonnes) is the typical weight of ash produced.
But MZR can confuse the statistics. Is ash measured before the metallic zinc is extracted or after? We measure both in order to know what's happening at each point of the process. What's happening at the kettle, and what's happening in the MZR plant. Both are important to know.
But post-MZR ash must surely be much lower than pre-MZR, so comparing an MZR plant with a non MZR plant who reports post MZR ash figures would lead to misunderstanding.

A. Your ash at 1.24% of the galvanized weight processes is very high. You could use a target of between 0.2-0.5%
Your dross (hard zinc some call it) at 0.33% of galvanized weight is good (target 0.5-0.9%)
Another number to summarize these is to state the weight of zinc consumed to galvanize the 223.9 tonnes you state as production. From what you've said, it sounds like it might be about 6%. (A little high).

A. Good performance in HDG isn't usually a simple question of one or two aspects. It's usually a combination of many things, including: Pickle performance, rinsing, dryer, Fe control in flux, dipping rate, withdrawal rate, additions to zinc, types and quantities; steel chemistry, shapes, allowances for drainage of zinc; bath practices, dross performance and maybe more that these.

A general question of "How can I reduce zinc consumption?" is beyond the scope of a forum such as this, but this forum is good for answering specific questions. There are many questions and perhaps some observations of the factory practices that can help pinpoint the issues to concentrate on in improving performance.

Ed. note: Well put, Geoff. Broad subjects require broad, wide-ranging, treatments ... and simply can't be answered in the few paragraphs commensurate with a public forum. Books, training courses, and/or hands-on instruction are required for broad questions.

A. Hi Muktar. My primary experience is in electroplating, but it is reasonably analogous to galvanizing. Larry Durney suggests in "Trouble in Your Tank" ⇦ this on Amazon [affil link] that the starting point, the No. 1 weapon you have, is making sure everything is "letter of the law right".

Not only are things complicated & interrelated as Geoffrey notes but, despite metal finishing being a science, there are numerous things at the molecular level that we don't actually truly understand. Thus when we let anything be an uncontrolled variable instead of being carefully controlled (and Geoff suggested a long list of such things), cause & effect become indeterminable.

So to reduce ash & dross, step No. 1 probably isn't to look at what is the first thing to change, but rather to not allow anything to vary or change until you have a reliable and well-documented baseline from which you can then confidently adjust one element at a time. For example, if Geoff's "allowance for drainage" is varying, you can't learn from your other efforts. Just a thought  🙂

Luck & Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Mukhtar,
I missed your latest question -- I was away helping a customer.

To add Al to a zinc kettle the most common is to use an alloy of about 5% Al. 95% Zn. Al will not melt at the normal temp of a galv kettle, but will slowly dissolve. Using alloy with similar melting point to the kettle helps a lot. It does dissolve slowly and the limits of solution are higher than where problems in general galvanizing becomes a problem. Trying to add "pure" Al usually results in quality problems.

For Pb, the MP of Pb is much lower, and it melts easily, but does not dissolve readily, to just over 1%. The rest melts and due to the much higher density, it sinks where it sits on the bottom of the kettle. The purity isn't so important as what the impurities are? Watch out for Fe, Sn, Cu. As not much Pb is required, usually most scrap Pb will do, or you can buy new shiny ingots of high grade.