Converting from wet galvanizing to dry galvanizing

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, drier, 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. Such 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.

A. Hi Muktar. In electroplating, which is reasonably analogous to galvanizing, Larry Durney suggests in "Trouble in Your Tank" that the no. 1 starting point, the no. 1 weapon you have, is making sure everything is "letter of the law right". Because 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 truly and actually understand. Thus if we let anything be an uncontrolled variable instead of being carefully controlled (and Geoff suggested a long list of such things), cause & effect become essentially indeterminable. So to reduce ash & dross, perhaps step No. 1 isn't to change something, but rather to NOT allow anything to change until you have a reliable and well-documented baseline from which you can very confidently see cause & effect. Just a thought :-)

Luck & Regards,

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.

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 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,

A. Atul,
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,

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,

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. 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,