Zinc Contamination Concerns

[RDub]

Hiya
Well I started casting bullets in the sixties, but after spending some time here, (excellent site!), I have realized that I have been doing some things incorrectly, specifically dealing with possible zinc contamination.

Let me start with what I 'used' to do with raw WW and other scrap lead metal, so please comment on the merits of this..
I would melt the metal in a medium sized cast iron pot, ( one that I can manage myself),.. Temperature unknown prior to my purchase of a Lyman casting thermometer a few years ago.. When ALL the metal is melted, and there is a generous amount of 'crap' on top, I would flux the pot at this point.
Now the reason I did this was my understanding that TIN, a very valuable and necessary part of a good casting alloy, would be apart of all the crap on top of the molten metal. Tin also floats to the top of molten lead if not fluxed, correct?
So, if I fluxed the mix, the tin would be mixed back in the alloy then all the 'crap' left on top would truly be crap and would then be skimmed off and discarded, leaving me with a shinny surfaced, ready to pour into ingots mix, PROVIDING, I do this under 700°F.

So my questions are;

1. In the above procedure, IF there was zinc present, and IF the metal was kept under 700°F would it rise to the surface and then be discarded, or would the fluxing procedure hopelessly contaminate the mix?

2. Once a batch of bullets or ingots are contaminated with zinc, (The above procedure, or bullet casting done ABOVE 700°F) can this contaminated metal be cleaned up, re-melted with zinc floating to the top?

3. To test for zinc contamination, a drop of pool acid (HCL) applied on a suspect sample of metal should do exactly what?

Thank you for any and all comments.

[Dale53]

1 - This is correct. Fluxing below 700 degrees will NOT pollute the metal with zinc as this is below the melting point of zinc (zinc being lighter than lead will be floating on top). Incidentally, I keep my first melt BELOW 700 degrees (right at 650 degrees).

2 - To the best of my knowledge, once the metal is contaminated there is no practical way to
remove the zinc. That batch is garbage.

3 - I have no answer for this...

Dale53

[Ricochet]

Look up processes for smelting and refining lead. The way zinc is removed from commercial lead is to melt it below the melting point of zinc and skim off the crystals that float to the top. Works with our lead pots, too.

[imashooter2]

Muriatic acid dropped on a zinc sample will react and bubble. Lead, much less so. Lead contaminated with a very low percentage of zinc? Not sure. I'm guessing it would be indistinguishable from the lead reaction.

[13Echo]

I wish Zinc were as easy to remove as just skimming. That can remvoe some, even a lot but it won't get it all. According to an article I just read removing the zinc is a bit more difficult as follows:

"Removal of zinc

The removal of the precious metals results in the contamination of the lead with zinc, which must then be removed. This can be done either by oxidation with gaseous chlorine or by vacuum distillation. This latter process involves melting the lead in a large kettle covered with a water cooled lid under vacuum. The zinc distills from the lead under the combined influence of temperature and reduced pressure and condenses on the underside of the cold lid."

I don't want to deal with chlorine and I suspect the temperatures required for vacuum kettle separation is going to be a bit high. It's best to avoid contamination in the first place.

Jerry Liles

[RDub]

Ok then, this nails it down a lot better.. Thank you for the replies. :)

[Ricochet]

You don't have to get it all. The Metals Handbook suggests some percentage of zinc above which it will cause casting difficulties. It is pretty small. I've looked it up and posted it before, but I don't remember and I'm too tired now. About to hit the sack.

[leftiye]

I melted and skimmed (linotype melts at about 450, and so I didn't go any hotter than necessary) a contaminated batch of Linotype recently - three times. I lost 55% of the metal by weight. The remainder is about stereotype hardness (BHN28) and casts well, but it still has some of the stuff coming out whenever I melt it.

Fluxing doesn't enter into it with this alloy. I just heat and skim as there isn't anything besides metal in there. I fluxed the original pot, after I heated it up to 700 plus degrees, and it recombined, but it produced crap so fast that ladle casting was impossible. The linotype was in letter form, obviously the printer had been printing with it!

[Bass Ackward]

I melted and skimmed (linotype melts at about 450, and so I didn't go any hotter than necessary) a contaminated batch of Linotype recently - three times. I lost 55% of the metal by weight. The remainder is about stereotype hardness (BHN28) and casts well, but it still has some of the stuff coming out whenever I melt it.

Fluxing doesn't enter into it with this alloy. I just heat and skim as there isn't anything besides metal in there. I fluxed the original pot, after I heated it up to 700 plus degrees, and it recombined, but it produced crap so fast that ladle casting was impossible. The linotype was in letter form, obviously the printer had been printing with it!


ouch!

People around here were scammed a few years back by guys using ingot molds shaped like lino bars / rods. The description was used or tin depleted lino to cover the dull appearance. They used mostly WW (I guess) and then heat treated it so the bars were hard enough to ring. They arranged one super "HUGE" buy for a single morning and were gone before guys discovered it was just WW. Problem was that they smelted anything and everything in that stuff and a little zinc just made it that much harder. :grin: So it was almost a total loss.

Too bad about your type metal. This is going to be the wave of the future now with the heavy recycling going on. Stuff ain't going to be separated, so learning how to get around it (not possible in every case) will be more common.

Steel molds to hold more heat, larger bullet diameters, hotter batches, and wider drive bands and grease grooves will be needed for even fair fill out.

[Bob Jones]

Going back to the original question, if there are zinc WW in the mix, yes they will rise to the top and can be skimmed out.

That's exactly how I smelt. I did about 2000 pounds last Fall and just dumped the lead into the big pot and heated until it just started to melt. An inspection would usually reveal one or two zinc WW completely intact floating on top and I skimmed them out. I was careful not to overheat the melt and it's been working fine casting this winter, no signs of zinc contamination at all. If you let it get too hot the zinc will melt and you'll have a mess, so it takes pretty constant attention to the process.

[RDub]

Ok, thanks again.. This helps a lot. :)