Hot dip galvanizing: what is reasonable dross & ash material loss?

A. Consumption of zinc in a galvanizing plant are several:

To the job: The coating will consume perhaps 5% by weight of zinc (weight of item increases by 5%, this increase coming from zinc coating).

To dross: Dross is an alloy of zinc and iron formed free (unattached) in the zinc melt. It sinks by gravity to the bottom. The source of Fe to form dross, including the work, residual chemicals dripping off, and importantly, the kettle walls. The latter means it continues all the time, production or not. This material is a salable commodity. It is removed periodically by mechanical means. It could be 96% Zn by weight.

To ash: Molten Zinc in air will always oxidize and the oxides form on the surface. This happens during production and also at idle time. This is also a salable commodity and removed by skimming off the surface of the molten zinc.
This could be 70% metallic Zn, and much of the rest is Zn Oxide.

In "sleep mode", ash formation is reduced by placing covers on the kettle (also saves heat).

A. Sir:
For "Dry Kettle" galvanizing: Dross above 0.6% of production shows a problem. Dry ash above 0.8% of production (no ash box), above 0.4% of production (manual ash box) or above 0.2% of production (motorized ash box) is not acceptable.
The amount of zinc on the product can easily vary from 2% to 10% depending on the thickness of the steel (surface area) and the reactivity of the steel. Also the technology and skill of the workers is critical. Since you are doing thinner steels which are likely to be reactive (likely under the Sandlin curve), I expect the zinc on the product will be in the range 5% to 8%. You have other problems, which I will refer to later if you continue to ask questions.
Regards,

A. Dear Mr Sachidanandan
Your Consumption 5% is Net or Gross? The process is Dry or Wet?
Refer to the tips given by Dr, Cook: the second point, i.e., "technology & skill" -- if you put you can reduce the consumption by another 1% easily for structural steels.
Please write your actual problem instead of giving overall Zn consumption. The platform is extremely useful for gaining wisdom.
REGARDS,

A. Hi Ramesh. As you see, we appended your inquiry to an existing thread which answers it in good detail. If you have additional questions regarding your own particular situation please detail them. Thanks.

Regards,

A. Sir:

On the dross side-- Usually dross is 0.4% to 0.6% of production. I am working to lower this. I have evidence that NaCl in the flux causes more dross. Thus your use of NaOH to neutralize acid rinse is not acceptable.

On the ash (dry skims) side I now know ways to reduce this to 0.1% of production and I am currently writing this up.

A. Dear,
Ash formation is directly related to what process you have adopted for Galvanizing and quantum of material you Galvanize. And some what it related to Pre-Flux used. To get the right answer explain your process

A. Hello MR. aly elsokkary,

Neutralize your rinsing with NH4OH instead of NaOH...
You are requested to heat flux and observe the result.
Heated flux will reduce your flux consumption also.

It will reduce your Dross formation also.

A. Dear Mr. Aly Elsokkary,

You are requested to mention purpose of Ammonium Chloride in Zinc bath. Are you using WET process or to increase fluidity of zinc?
If you are using DRY process, then you're not required to add Ammonium Chloride in Zinc bath.
Please feel free for further information if required.

Rajesh

A. Dear All.

Ash formation also depends on top open surface of zinc bath.. more surface area open then the Ash formation would be more, also surrounding air circulation of plant cause more Ash formation.

Here in India we are considering maximum 1.0 % which include zinc ash and Zinc Dross (Hard Zinc )

A. Sir:

For a dry kettle, a dross production of 0.4% to 0.6% of production is good.

Regards,

A. There isn't enough information in the question to be definitive.
But, you should be able to get ash to 0.2 to 0.5% (defined as weight of ash divided by weight of steel after galvanizing)
And dross: 0.5 to 0.9%.
Where you get to in the range depends on your material type, whether you haver a drier or not, what temp your flux runs at, what ratio of ZnCl2 to NH4Cl, and density of flux solution. You gave %Fe in flux as 2g/l. Aim to get this under 1 g/l, as the more Fe in flux, the more ash.

You ask for some official reference document. I do not think you will find one!

You asked.. "are we getting more dross generated if we have more surface area of products with the same weight?"
Yes, you will.

There are several ways of expressing dross and ash production. As a % of zinc consumed and as a % of steel weight galvanized. The most common is as % of steel galvanized, weighed after galvanizing.

A reasonable dross production rate is 0.3-0.9% of the weight of steel galvanized. Your figure of 0.4-0.6% of weight of zinc consumed is very high, but possibly you meant as % of steel weight?

Yes, the amount of dross is proportional to the area of steel exposed to the liquid zinc. In many aspects of galvanizing compromises are made, and in such measurements the same applies. Many people relate matters to tonnes, and perhaps should be using m2, but as tonnes are so much easier data to gather they use that. So 1000t of solid will make less dross than 1000t of pipes, as the pipes have more m2.

Hi, Mr. Geoff, it was my mistake on the statement. It is actually dross rate at 0.4-0.6% of steel galvanised. So far our rate falls between 0.4-0.6% but in July we have 0.75% of dross removed from the kettle. We drossed with the same method as the month of April and May. After going through the details, found that the quantity in ton of hollow pipes dipped in July is 1.7 times of May and 2.4 times of April. Thus, it brings up the idea of dross formation related more to the area of goods galvanized. As you mentioned, generally people only relate the dross formation to total goods galvanised.
Anyway, Thank you for explaining and clearing my confusion.

A. Commonly used ratios for dross and ash are weights per weight of steel galvanized.
Ash: expect about 0.2-0.5% (as weight ash/weight steel and as %)
Dross: expect about 0.3-0.9%

The variation spread in these figures shows the typical range that general galvanizers experience. Over these % and you should start looking for problems to solve. Under these ranges, take a pat on the back, you're doing well.