Metal finishing Q&As since 1989

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Galvanizer in Cyprus is Suffering Excess Dross

Dear Sir/s

I operate a small bath for hot-dip galvanizing, for one year now, and I have the following question among many other questions.

Whenever I add zinc into the bath, I notice an immediate increase in the dross level. Approximately an increase of 80-100 Kg of dross for every 1000 Kg of new zinc I add.

Is this logical to happen or there is something I should do or something I shouldn't do whenever I add zinc, to avoid the additional dross formation.

At present I just drop the new zinc in the bath until it fills up.

Thanks and waiting for some help!

George Yiallouros
TechnoPlan Ltd - Lemesos, Cyprus
2001

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Some work practices that help control dross are:

1.Specify when ordering, that zinc bundles must be delivered in a covered van, shrink wrapped, or otherwise kept covered to prevent precipitation from wetting bundles.

2.Bring bundles into the pot area at least 24 hours before anticipated use, so zinc can stabilize to the ambient temperature.

3.If you see, or can feel moisture on slab surface, or there is any bubbling, or spitting, from the molten metal pool, as slabs are added, you need to also warm bundles to 200 deg.F., minimum, to dry them, before charging them into the pot. This sometimes can occur just from bringing bundles from a cold warehouse into a relatively warm, humid shop, where moisture in the atmosphere condenses out, and tightness of fit between slabs in a bundle, keeps it from evaporating very easily.

4. Check your slab type for chemistry impurities allowed. You may find that paying slightly more for the next higher purity grade will significantly reduce the dross. It may be worth getting one lot as a trial, and starting with a clean pot, (like after pot was emptied for a heater refit, etc.).

5. Avoid sources of shop induced contamination, like charging bundles with the steel bands still on, or any metals like copper, left intermixed with scrap, if you reuse spills, etc.

W. Carl Erickson
- Rome, New York
2001