zinc evaporation opinions.

I would like to hear opinions from other member on zinc burning off issue. As all probably know, it melts at 420 degrees and boils at 907 c. We use 30KW induction machine when smelting karat gold bars and we cook the molten metal around 1200 c for fifteen minutes before taking a pin sample and pouring off to ingot.At this temperature zinc must be evaporating from the molten metal and carried up the stack.However as it goes up it must be cooling down too and probably becomes zinc deposit in the stack or bag house. We still detect around 4-6 percent zinc in the pin sample. I have never seen complete eliminating the zinc from karat gold bar.

1. how come zinc does not completely evaporate based on its boiling temperature and our operating temperature.?

2. the refinery we recently started to work with indicates about 0.5% more melt loss on out well melted bars. it was never an issue with other refineries we worked with. they say it is due to excess zinc in the bar.?

3. we could increase the meting temperature of our machine to 1400c but that would really push the limits of out induction melter which we prefer not to do so.

any opinions?

1. Pure zinc boils at 907 C, but as an alloy it affects the boiling point and gets increasingly hard to separate out. The same effect is true for any liquid, physical chemistry, and can be seen for example in water. Add some salt to it and it's boiling point increases slightly but you need a good thermometer to notice. A more extreme example is to look at water mixed with sulfuric acid, it takes very high temperatures to boil off the last water.

2. Just thinking out loud here... Maybe the melt loss comes from oxidation of the zinc, melting in a graphite crucible would create a reducing atmosphere while using a clay crucible would allow the zinc access to air. The same argument goes with reducing or oxidizing flux. Maybe the two refineries uses different flux or no flux at all. That could affect the melt loss.

Göran

It may or may not be useful however I agree that Zinc is a massive pain in the butt. Personally I double refine anything dropped with Zinc, or alloyed with zinc before I even attempt to melt it.

It has this nasty habit of turning up like a bad penny.

Jon

Zinc vapours carry values. Silver is often recovered from zinc condensates. Zinc is usually distilled under vacuum, rather than evaporated, which lowers the boiling point. Google zinc vacuum distillation, silver crust, zinc refining. United States Patent US5232486