Some time ago I had zinc in some of my melt. It came from range scrap, muriatic acid test was positive. I don't know how much but it was giving me hard time filling out my mold. So out of curiosity I got researching of ways to remove zinc at home relatively safely.
Sulfur doesn't sound nice, copper sulfide doesn't either. Besides it replaces zinc and Cu stays behind. It does dissolve in lead alloy to some degree and impacts mold fillout too. It also removes tin.
There is a process of lead desilverization with zinc where they intentionally add zinc to lead melt to extract the silver. Zinc preferentially alloys with silver and this alloy has higher melting point. This sounds promising as we could intentionally add silver by using silver containing solder in hopes to bring up both silver and zinc. But it has limit of applicability as zinc combines with silver only after zinc content in the melt is above 0.8%, meaning that this 0.8% of zinc needs to be removed later by other means.
Zinc vacuum vaporization that is industrially used for final zinc removal is out of reach for home setup.
I investigated molten salts on top of the melt hoping to find some salt compound that selectively reacts with zinc. There are some, but the temperatures needed go above 600C (1100F) and that seems too high for a steel pot that would be quickly corroded by molten salts. Besides there was chloride gas expected.
I investigated molten salt electrolysis at 400C (750F) where cathode would be solid zinc plate, anode would be the melt and molten salt would be the electrolyte. Seems promising but again some nasty gases are expected, and pot corrosion.
Caustics (Sodium hydroxide, NaOH) can be used to remove zinc, but it is not very active and leaves some sodium in the lead. I also don't fancy working with the most dangerous caustic when it is heated up to molten lead temps. Besides caustic also removes antimony.
I found that Zn reacts with phenols. C6H5OH + Zn → C6H6 + ZnO which is metane gas and zinc oxide. Where do you get phenols and how you introduce it into the melt? Well, I immediately thought that thermal pyrolysis of wood could be source of phenols, and it is. So what, cut a nugget off an aspen log, attach it to a steel paint stirrer, drown it into the melt and let it bubble while vigorously stirring with your power tool? Could be it, but there is lots of other **** in the products of pyrolysis that all tend to remain in the melt as impurities. Actually some of polymers like POM (plastic) are even cleaner than wood during pyrolysis, but I didn't really want to go there.
What really caught my eye was a process of solar hydrogen generation cycle that uses zinc oxide as a carrier of energy. It uses concentrated solar power to decompose ZnO at 1200C (2200F) into Zn and O2, after that Zn is delivered to reactor where it is mixed with superheated water steam to enable reaction Zn + H2O -> ZnO + H2.
https://en.wikipedia.org/wiki/Zinc%E...nc_oxide_cycle
I researched on steam reaction with metals and turns out that steam reacts with only metals higher than hydrogen on the reactivity series.
"Metals such as Mg, Al, Zn and Fe react with steam when red hot. Tin and lead react with superheated steam with great difficulty. Copper, Mercury, silver and gold do not react with water or steam at all.
Only metals which are above hydrogen in the reactivity series displace hydrogen from water or steam. The reactions of metals with water are redox reactions".
https://secondaryscience4all.files.w...-of-metals.png
By introducing steam into the melt we are doing redox reaction of water by using for that purpose the very catalysts we consider to be impurities of the melt. A redox reaction that acts as a flux. Brilliant! And it's selective.
Apparently this method of reacting zinc with steam is known in metallurgy already since at least 19th century, so its nothing new. Some industrial processes have even been that daring that they introduce liquid water into the bottom of the kettle in carefully controlled doses. It might be even possible at home, but only for the most daring.
So how could we introduce steam safely into the melt? It is very important that the steam is absolutely dry - which means superheated with a safe margin. Any water steam above 200C (400F) is dry. At 450C (842F) it is already suitable for reaction. Below that it reacts with zinc very slowly.
I had an idea that the simplest way to generate superheated steam at home is to take a pressure cooker, attach a small diameter copper tube (like brake line tubing) to the cooker (new entry, preserving safety and pressure control valve), bend it into a spiral heat exchanger and let the wet steam go through it. Then use gas burner to heat up the copper heat exchanger so that the steam temperature is raised to above 400C (750F). It becomes dry, pure H2O steam.
Now if the end of that copper tube is also bent into a spiral with a deadend and steam escape vents facing down, and this spiral is drowned into the melt, pressure cooker generated pressure is enough for this steam to displace inrushing lead and push the steam bubbles out into the melt. Stirring with plain steel paint stirrer above the bubbles creates enough reaction sites that over time all zinc should get reacted into ZnO and float on top of the melt as white powder. Hydrogen gas generated could be controllably burned without letting it escape into environment and causing potentially hazardous concentrations.
Steam seems to be the cleanest possible way to remove zinc and many other impurities. It removes Mg, Al, Fe, Ca, Carbon (char, soot), but does not react with tin, lead, antimony, copper.
The safety warning - we are intentionally introducing WATER into the bottom of the melt. If anything goes wrong with the steam superheater and wet vapor gets into the melt, the tinsel fairy might say hi in unforgettable manner. I think it is not hard to ensure water get superheated before entering the melt by having some of coil length submerged into the melt acting as heat exchanger before steam reaches the vents.
I haven't tried this process yet, but I'm really itching.
Thoughts? Has anyone here tried using steam before?