How Can I Reduce and Remove Zinc Dross

Sir:

I presume you have thermocouples IN the molten zinc to control temperature. I also assume you have an electronic controller to provide burner control. 468 °C (874 °F) is NOT normal and with an iron kettle will cause the iron in the kettle and the hot zinc to form much dross. Your temperature control system may not be correct. Keep in mind that it takes about 10 minutes for heat to travel through 50 mm of kettle wall thickness.

Send a photo of your dross shovel. Also what are your kettle measurements (length, width, depth)? Dross in most cases (except spin galvanizing) is usually less than 0.6% of production. The clay or solid feeling with a rod IS DROSS. Likely you are burning up your kettle.

What proportions of ammonium chloride and zinc chloride are you using. From what you have already said, the high zinc temperature is likely by far your biggest problem. A diagram of your kettle furnace would be useful. Likely you may have a problem with design. I have a published article regarding kettle furnace design that may be of use to you.

I think the best dross bucket/scoop/shovel design is in the shape of a capital "T" with a counter weight on one side of the top of the "T" with the actual bucket/scoop/shovel at the bottom of the "T". The crane is connected to both tops of the "T". I like 3/8 inch holes on 1.5 inch centers. The actual methods in drossing also are very important.

Regards,

There are two main systems of dross removal;
Firstly, a shovel (sometimes called a spoon) which is a dredge like shovel on a long steel handle, and typically about 500-800 mm wide mouth, and about square on the bottom, with high sides on three sides except the mouth side.
This is lowered to the dross level and moved along the length of the kettle to scoop up dross. Lifted very slowly to the surface, and then out, the dross is vibrated or worked with a spade to allow free zinc to drain off.
Do this repeatedly in both directions until you can't get any more. Make up the level of zinc with fresh new zinc.

Next is the dross grab. Crane lifted and with two jaws, this device is similar to those used to empty a cargo ship of a grain or granular cargo. As with the shovel, keep going until you can't get any more out.

Its important to get the dross in the corners.

There is often one or two pieces of steel that had accidentally dropped in, so get these out too. You won't get all the dross out without removing them.

Drossing frequency should depend on tonnage galvanized, but it's often more convenient to do it on a timed basis, perhaps each second weekend or so.

Dross is formed from the reaction between iron and zinc. To reduce its production requires reducing the amount of iron available to react. It's free iron that reacts to form dross, not the steel you're galvanizing.
So: what's the Fe % in your flux? What flux strength? What ratio of zinc chloride to ammonium chloride? Temp? any drying? What material type you galvanizing?

Your 460-468° sounds a bit high in temp. The higher you go the more you'll form dross from dissolving your kettle. Which also shortens kettle life.
450° sounds OK, can't you control it to about ± 4 °C ?

A. Hi, Mr Bola,

You need 21 Mt zinc @3.5% and 3 Mt for addition after drossing.

Rajesh

A. Yes, too many unknowns.
This is like asking "What fuel economy will I get in an unknown car driving up an undefined hill at an unknown speed?"

What type of steel to be galvanized? What flux type will you use? How long will the immersion be? What temp will the zinc be? Will you dry before galv? What is the surface condition of the steel? and other questions too.

A. What I summarise:
1. You want to take out Dross with minimum free Zinc
2. How the Dross can be minimised.
3. Frequency of dross taking out from Zinc Kettle.
My say is:
1. Your spoon whatever shape, whatever size must be well perforated with 10 mm dia drill. Hit the spoon vigorously on the Zinc bath as soon as it comes out. Then after, transfer it to the again perforated moulds.
2. Keep your Zinc bath Temperature Max. 450, preferably 445 °C.
3. After the addition of 50 MT Zinc plan for Drossing.

I am sure that you are talking about Structures Galvanizing Plant.
In normal circumstances the Zinc consumption remains 4.5% of steel Galvanized means you have to add say 28 MT of Zinc.
Normally Dross formation will be approx. 10% of Zinc added to produce the above. By the way Dross formation depends on so many things: % of Iron in Flux. Zinc bath Temp. and the quality of material Galvanized.