Galvanizing Kettle Dust Collector - Bags Get Oily & Wet

A. Hi Aaron,
No dust belongs in people's lungs, regardless of its constitution ... so dust masks are required regardless of what the bags have collected. As for other PPE, sorry I haven't seen the operation of bag changing that you are describing.
Luck & Regards,


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

A. The dust you are collecting is hygroscopic, so unless you prevent it, it will absorb moisture from wherever it can.
Ways to prevent it:
Do not let steam get into your extraction system. (if a moveable collection canopy for example, don't park it over wet tanks).
Do not let it suck moist air or even cool air when not extracting smoke from your process.
Have the whole filter lagged and trace heated.
Watch for dewpoint, do not allow the filter to go below dewpoint temp.
If filter is outside in rain, ensure there are no leaks allowing rain in. Remember it's under negative pressure, so will suck in any water at those leaks.
Don't dip wet steel. That steam goes straight into the filter and combining with the dust, forms mud which blocks the bags.

A. Part of the dust is ammonium chloride, but there's zinc oxide and other stuff there too, perhaps even residues of burnt oils, depending on how clean your steel is before immersion.
A good bag filter will come with a self cleaning system. This is often by using a shot of compressed air into the bags to flex them and break off the accumulated dust, provided the dust was kept dry. It will fall off into a hopper under the compartment of bags, for later removal.

If the bags get wet though, this won't really work, and washing them sometimes works, but the bags deteriorate, and often a single wash is all you will get before the material disintegrates beyond further use.

The actual humidity and temperatures need to be studied throughout the system to ensure that you don't get below dew point. This might require heating somewhere in your system, but that fact that you have a humid climate doesn't prevent engineering a solution. The humidity is high where I live too, (though temperatures far lower!), but the system can be made to work.

A. It doesn't matter how you keep the dust dry, just that you DO keep it dry.
I have no experience using a desiccant, but if you work out that this is a cost effective option, then try it.

Its more likely that heat, enough to keep the dust and gasses above dew-point, is going to work.
Good insulation of all ducting and the filter and its dust collection hoppers will help a lot, by retaining any heat applied. If you use electric trace heating, it can be thermostatically controlled to minimise energy use.

Do as much as you can to prevent sucking cold moisture laden air into the filter. What happens when there is no fume evolution at the zinc? (during a dipping cycle there's only a few minutes when fume is produced).
There's no requirement for extraction during all other periods, so what do you do? If you leave the filter running on normal extraction volume, then change that. Either fit a switch and turn it off, or an inverter and have it run at low revolutions until next part of the cycle that needs extraction. This way you will eliminate a lot of the moisture from entering.

A. Sir:

I have a USA client that 20 years ago asked me why his baghouse dust was wet. I tested his flux and it had an ACN of 0.8 (double salt). I advised him to increase his ACN by adding ammonium chloride. He raised the ACN to 1.6 (quadraflux). His baghouse dust immediately became dry. This galvanizer died from lung cancer from smoking and now his son-in-law is managing the company. I recently tested his flux and the ACN was 1.2. I recommended that he raise the ACN to 1.4 which has now been done (to lower the %GZU). At no time in the last 20 years has the baghouse dust become wet. My experience is that above an ACN of about 1 the baghouse dust remains dry. In order to use triple salt or quadraflux there are some requirements.

Regards,

A. Sir:

I invented ACN in the mid 1980's and it is the ratio ammonium chloride/zinc chloride and is unitless and independent of concentration.

You say your baumé is around 21 to 24 (I assume measured at room temperature). If we assume a baumé of 23 and allow 3 degrees baumé for the ammonium chloride dissolved in the flux (a good approximation) then my formula:

grams zinc chloride/L = 4.2 * baumé ^1.22

Then: grams zinc chloride/L = 4.2 * 20^1.22
Equals: 162 grams zinc chloride/L

Using the ACN formula this would be 162g/L times 1.24
Equals: 200 grams ammonium chloride/L

Does this agree with the lab results?

About 2003 I published the article: "Composition, Testing, and Control of Hot Dip Galvanizing Flux" in the journal METAL FINISHING. Table 4 in that article shows the test results for 13 USA certified labs testing a flux that I made up here on the ranch. I used exactly the same masses of zinc chloride and ammonium chloride (ACN=1.00). Results for ACN from the 13 labs ranged from 0.18 to 1.51. Two labs of the 13 labs got usable ACN's but only because they got both the zinc chloride and ammonium chloride concentrations wrong.

For your information I do not use ACN at all anymore. Rather I use ACNV in which the volatile components (only ammonium chloride in this case) is the only numerator and the denominator is the sum of the non-volatile components which include zinc chloride and iron(+2) chloride. Thus for ACNV you would have to know your iron(+2) and convert it to iron chloride. I have not "looked it up" in the Chemistry/Physics Handbook but I rather suspect that iron (2) chloride is also hygroscopic as is zinc chloride which is in your baghouse dust. Have your baghouse dust tested for chloride anion.

Even if the lab results are correct, your flux composition does NOT correspond to a European or USA type usable flux. Your ACN is too high to use a drying oven (without burning the flux) and your baumé is too high to use hot flux which would give a double layer on the product (dry outer layer and a wet mushy under layer) which would produce very excessive spatter and horrendous amounts of ash. This "highbred" flux is a mess; It is like trying to use the best of European and USA technologies, but the result is quite bad indeed.

Regards,

A. Sir:

If the 6.4 g/L Fe+2 concentration is correct (best is by dichromate titration) then FeCl2 concentration is 14.5 g/L. This is NOT a high value and corresponds to about 0.5% Fe+2. By using the lab FeCl2, my best estimate is that ACNV is about 0.1 unit lower than ACN. From knowing your FeCl2 (if it is correct) your ZnCl2 is likely about 150 grams/liter, because FeCl2 contributes to the baumé.' There is no way that I can guess at your ammonium chloride concentration. I presume your lab used a specific ion electrode or a Kjdahl distillation. Both methods are difficult to get a good result.

I do have in-plant methods to get an accurate ACNV.

What temperature is you flux solution? How long do you leave product in the flux tank? After withdrawal from the flux solution and drying for a few minutes is the flux fully dried or do you have a dry outer flux shell with a mushy wet under-coating? What is the pH of your flux? How is it measured? What type of flux do you buy? (double, triple, or quadraflux) Do you periodically add ammonium chloride to your flux? Is your fluxed steel black? white? clear? or what color? Does it turn orange, grey, or black if you do not galvanize it quickly?

A. Re Fe concentration in flux: There's a high correlation between higher Fe conc and dross formation. Higher Fe, more dross produced = higher zinc consumption.

Re condensation off operating hours:
Don't let the filter and steelwork get as low temp as dew-point. Use insulation, and if necessary, electric trace heating, thermostat controlled to keep above dewpoint.

A. Leong and Geoff,

Permanganate solution is such a strong oxidizer that it degrades about 5%/month, whereas dichromate can be used as a primary standard with very long term stability.

9.8 g/L Fe+2 is about 0.86% Fe+2 and 35 years ago would have disturbed me. Now there are many, many ways to compensate for this medium level of contamination. Without knowing these ways to compensate for your iron (+2) you will get about 10% thicker zinc on the steel.

97.9 g/L Zn+2 corresponds to 204 g/L zinc chloride (ZnCl2 = (MWZnCl2/MWZn)*97.9g/L). In my previous post the estimated ZnCl2 is 150 g/L from the baumé.

ACN is a weight/weight ratio, NOT a mole ratio.

If the lab was correct for NH4Cl then your ACN is about 0.8 (121/150), and your ACNV is about 0.7. These are quite reasonable numbers because you do not add any separate make-up ammonium chloride. In other words on "day-one" your make up of new triple salt flux will have an ACN of 1.17, but as time goes by, the refluxing of the steel rejects (or previously galvanized racks) cause zinc to dissolve in the flux giving lower and lower ACN and ACNV. With ACN 0.8 or ACNV 0.7 this flux would be very hygroscopic and collect water from the air.

pH meters do NOT work in flux solution.

Clear color after fluxing is typical of low ACNV and so is the steel turning yellow.

I am currently at my client's plant in another state in the USA. He has no problems with moisture in his bag house, even though it is raining about half the time just now. Tomorrow we do ACNV in-plant testing. Much of the equipment is from Walmart.

My considerable experiences is that Fe(+3)hydroxide (orange solid) does cause more dross. It is also my considerable experience that iron(+2) chloride (green solution) does NOT cause dross (in a dry kettle), but does give thicker zinc coatings on the steel. From about 1972 until about 1980 I purified (removed Fe+2) from many, many in-plant, operating fluxes. In many cases the galvanizers said they were looking forward to less dross. It NEVER happened. Zinc coatings on the steel were decreased by about 10% to 25%.

I am here, in my client's plant, working on a direct replacement for nickel in the zinc and it is working very, very well.

Regards,