Galvanising - Reduce & Recover Zinc Ash and Dross

A. An "ash box" is one name for a system of keeping the ash trapped at one end of the kettle, floating on the zinc. The hope of some is that this will allow some metallic particles to separate into the bulk of the zinc, allowing ash to be removed with lower Zn metal contained. It sometimes works. More useful though, is that it can concentrate the ash at one place so that it can be agitated in order to break down larger lumps, and free metallic pieces to re-join the bulk of the zinc. The ash box is sometimes called a "weir" (as in a dam that is open at the bottom. It is usually steel, and forms a barrier across the width of the kettle, and is perhaps 10-20cm deep.

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,

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.

Regards,

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.

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.

Regards,

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. 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.

Regards,

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.

Regards,

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.

Regards,

A. 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,

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.

Hi 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 :-)

Regards,

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?

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.

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.

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.

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.

Regards,

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%.

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.

Regards,

A. Sir:

I expect that your flux is poorly formulated. Likely your ACN is too low.

Regards,

A. 1,000 MT/month is 1,000,000 kg/month. Dross is normally 0.5% of production. Thus 1,000,000 Kg times 0.005 equals 5,000 kg dross/month. I think you wrote that you dross every two weeks at 2,500 kg per drossing and it recently doubled to 5,000 kg/drossing. My experience is that a sudden increase in dross usually comes from misaligned burners on the outside of the kettle or very excessive dross in the kettle due to using a clam rather than a scoop. Untreated the kettle is likely to burn through.

Regards,

I agree with Dr Cook. Any sudden increase in dross needs investigating. There's a chance that it has come from the kettle, and that's serious.
But it could also come from other sources, and while costly and inconvenient, they are less of an emergency. Sources could include higher Fe in solution entering the kettle, material loss (steel) into the kettle (broken wire?) etc.
If you're removing 5 tonnes dross, then you should increase the drossing frequency to weekly. As said above, use a scoop or spoon and not a clamshell. In my opinion clamshells are not good, they remove some but don't do what's really important - to ensure there's no piles of dross left up against a sidewall or endwall. Such piles of dross create local hotspots which accelerates kettle wear at that point. It does this by preventing convection currents in the liquid zinc from dispersing the heat input.
Be sure that you are removing dross and not excessive zinc. Once out of the kettle the dross should be vibrated, or cut with a shovel in order to squeeze out the zinc from the sandy crystalline dross. Keep the zinc in the kettle, get the dross out.

A. Geoff, I still do not have proof that iron in the flux causes dross. Iron +3 chloride is quite volatile-- melting at 282 °C and boiling at 315 °C and if hydrated with 6 waters it melts at 37 °C and boils at 282 °C. Thus the bag house for kettle smoke contains about 1% iron. I have a "wet" kettle client that had 9.5% iron +2 in his flux and he dumped it and put in new flux. His dross did not go down, however the use of new top flux decreased by 20%. There was a time that you said you had proof that iron in the flux causes dross and you said you would send to me. Where is it?

I just do not like all these galvanizers being fixated on iron in the flux. There are 6 or so parameters that are far more important in the flux.

In the old days I purified many, many fluxes for iron. The galvanizers said "O'boy the dross will go down." It never did. %GZU did go down and is far more important than dross.

Regards, Tom Cook

Tom,
I agree also that fixation of Fe in flux isn't good. There are many inter-related factors in running a galv plant. Fixation on flux (any parameter) isn't good either.
But although this thread has drifted a little from the problem of increased dross production (and I think we've together covered the potential sources), I believe there is a strong relationship between %Fe in flux and what you call GZU - total zinc consumption. We call it "zinc pickup" but actually that is an erroneous name, as it implies zinc on the job. We count total zinc consumption, expressed as percentage of tonnes steel galvanized.

Over the 10 year period of analysis the linear regression of that number, zinc pickup (GZU) and %Fe in the flux is ... GZU = 0.82x (%Fe in flux) + 5.19. with R2 being 0.88.
That R2 suggests a strong relationship, but not perfect. There are many reasons for variation, including my observation that GZU monthly is too short a period to get a meaningful measurement, and that nothing happens instantly in flux and kettle management, it takes time to see results.
As you have said though, there are many other factors and these include ratio (ammonium chloride to zinc chloride), strength (which we measure in % and you in density - Baumé), other contaminants such as Na, K, etc., temperature, drying of the steel, and that's before we get to the variations caused by differing metallurgical practices in the kettle (Ni, Al, Bi, Pb, Fe, Sn. etc.)

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

A. Amila: You need to make a "map" of the kettle with the vertical depth of the dross at various locations. To do this you need a 5/8 inch solid steel rod about 3 feet longer than the depth of the kettle. Grind the bottom end about as sharp as a pencil and about 4 inches from the bottom end make a right angle bend. Make another right angle bend in the rod at the top. These two horizontal rods should be in the same direction. Wrap a double wire around the rod at just the right distance from the bottom so that when the rod is pushed vertically down into the zinc the wire will be at the zinc air interface. To map the kettle push the rod slowly into the zinc (careful of zinc splashes) and rotate it on a vertical axis to "drill" through the dross, until you hit the kettle bottom. See that the double wire is at the zinc surface. Next select each end and the middle of the kettle and push the rod probe gently down and stop when you get to the dross. Record how many vertical inches of dross are at various locations in the kettle. Record the dross depths and report back to this website.

Now take the probe with the pencil point from the kettle bottom upward running the point directly on the kettle wall(S). If the point gets stuck and you cannot raise the probe unless you push the probe toward the kettle center, then the eroded cavity in the kettle wall is more than 3/4 inches deep.

Regards,

A. Geoff, Thank you for your formula. Thus if the Fe+2 in the flux is 1%, then the %GZU will rise to 6.01% with a probability of 88%? Did you try to curve fit with any other formulas?

Your formula is quite reasonable and is likely valid for your operation. It is nice to have someone to correspond with that thinks a lot like me.

Regards, Tom Cook

? Are your percentages of ash and dross relative to zinc consumed or production?

A. Hi Boyet. As a leading galvanizer, have you submitted your own figures to some site where they can be reviewed and incorporated into those tabulations of "industry practice"?

If not, we invite you to submit your figures here and we can try to assemble additional data to go along with them to arrive at standard practice. Unfortunately, to request other readers to offer you their data without you offering any of your own in return has unfortunately proven to usually be a waste of ink. But we do have a dozen or so threads here from which you may be able to glean at least a little info. Please try to summarize it and get back to us. Thanks, and good luck!

Regards,

A. Sir:
Average Dross/Production times 100% is about 0.5%
Average Ash/Production with MZR recovery is about 0.1%

A. Boyet Lucas: Many galvanizers record the zinc ingot produced only to keep an eye on the recovery process, then add the zinc back into the kettle, and do not show it as a zinc addition. There's then no need to attribute a value to it, except remember that it's a tonne of virgin zinc you didn't need to buy. Your zinc consumption is at a new (lower) level, but that zinc you recovered wasn't consumed. Nor is the splashed zinc around the kettle top. You don't do anything with that either do you? Just push it back in, it wasn't consumed.

A. Mohd Tarik: In my experience that steel doesn't produce more dross. It will have a different coating weight, and so consume a different amount of zinc, but not produce dross.

A. Wilfredo Lucas: Let's let Dr Cook answer too, but in my experience, you can have two different ash production levels. With o zinc recovery machine, you produce a lot of ash which contains zinc metallic particles, and if you just measure this ash, then you might expect an ash figure of about 1-2% of the tonnes galvanized. So galvanize 100t and produce 1-2 t ash. With ash recovery, you can expect to extract about 75% of the weight as zinc ingot to go back in the kettle. That means a new ash figure of about 0.5 to 1% of tonnes galvanized. Or, galvanize 100t steel, and make 0.5 to 1t ash.

A. Lucas, 0.1% is 0.001 (in fractional units). Thus multiply 1000 Mt by 0.001 which equals 1 metric ton of ash coming out of the MZR machine. To get this great result you must be correctly using clean quadraflux as shown in my article in the on-line library on this website.

adv.
Also the galvanizers getting such a good result are using my Nifty.

The skimmings are unworked.

Regards,

A. Hi Shankar. As you see, we appended your question to a long and interesting discussion about reducing dross and recovering the zinc from it. You may want to read the many comments and construct some specific questions about the suggestions. Good luck.

Luck & Regards,