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How do foundries prevent lower boiling point metals such as zinc from boiling away when alloyed in a furnace with higher boiling point metals such as aluminum?

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When alloys are made by mixing molten metals (actually an alloy only need contain one metal and at least one other compound, metal or not) the metals only need to be heated to their melting point, not all the way to their boiling point. In the example you've given, the melting point of aluminum is $\pu{660^oC}$, which is $\pu{247^oC}$ below the boiling point of zinc, so the volatilization of zinc is negligible under these conditions.

However, the issue you bring up does present problems in other cases. For example this article states the following:

One difficulty in making alloys is that metals have different melting points. Thus copper melts at $\pu{1,083^oC}$, while zinc melts at $\pu{419^oC}$ and boils at $\pu{907^oC}$. So, in making brass, if we just put pieces of copper and zinc in a crucible and heated them above $\pu{1,083^oC}$, both the metals would certainly melt. But at that high temperature the liquid zinc would also boil away and the vapour would oxidize in the air. The method adopted in this case is to heat first the metal having the higher melting point, namely the copper. When this is molten, the solid zinc is added and is quickly dissolved in the liquid copper before very much zinc has boiled away. Even so, in the making of brass, allowance has to be made for unavoidable zinc loss which amounts to about one part in twenty of the zinc. Consequently, in weighing out the metals previous to alloying, an extra quantity of zinc has to be added.

Summary, TL;DR:
In your example of aluminum and zinc, each metal melts well below either of their boiling points so that loss via volatilization is not a problem. There are cases however, such as alloying copper and zinc, where the boiling point of one metal is lower than the melting point of the other. One way to minimize (but not eliminate) the loss of the more volatile metal is to quickly dissolve it in the high-melting metal and then cool the solution. Although this does not eliminate losses due to volatilization, it can greatly reduce the problem. And actually, since alloys are frequently composed of predominantly one metal, it is not uncommon to dissolve the lesser components into the primary component as a matter of practice anyway.

I hope the example I gave addresses your question. Please don't hesitate to ask for any clarifications in the comments below.

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  • $\begingroup$ I assume you then have to process the (e.g.) brass alloy melt relatively quickly, to avoid continuous further loss of zinc? $\endgroup$ – hBy2Py Jun 13 '17 at 18:09
  • $\begingroup$ @hBy2Py , yes, I'm actually impressed that they keep the losses of Zn so low (~5%, see above). I assume the addition of Zn would be about the last step of the process before cooling it down. There is probably some decrease in the vapor pressure of Zn due to solvation, but don't quote me on that. $\endgroup$ – airhuff Jun 13 '17 at 18:56
  • $\begingroup$ <nod>, I also would assume the Zn vapor pressure is lower in Cu solution... but it's by no means certain! $\endgroup$ – hBy2Py Jun 13 '17 at 18:58
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In the real world zinc losses from brass are kept very minor by keeping temperature below the boiling point of Zn and fluxes added to make a covering of slag. When a founder screws-up and over heats a furnace it makes a " zinc boil" , it produces an exceptionally dense white "fog" of zinc oxide. I will admit to having done it once. In steel the Zn, Pb, etc. are deliberately vaporized (and collected in bag houses) to clean the steel.

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  • $\begingroup$ This reminds me of recycling , at one time ( and maybe today) ; Die cast scrape was melted on one furnace hearth . Liquid would run down to a lower furnace through an air trap ( like a plumbers "P" trap ). The second furnace is enclosed and hotter, above the boiling point for zinc. Relatively pure zine is distilled and condensed . On about a monthly basis the residual from the second furnace is drained; it was lead, zinc , copper, etc. and could be used at a brass/bronze smelter. $\endgroup$ – blacksmith37 Jun 17 '17 at 20:08

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