Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

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fluxing to maintain desired lead alloy (1 reply)

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PhilN
6 days ago
PhilN 6 days ago

I am a bullet caster and am being told by the very experienced smelters/casters that if I don't flux and stir the melt that as I scrape off the dross I am actually removing the tin and antimony possibly to the point of ending up with pure lead.  Being in the electronics industry for 40+ years I am very familiar with using flux to remove surface oxides and to ensure a clean solder joint.  I understand using flux to help remove any oxides from the melt makes sense, but by not fluxing actually removing tin and antimony from the melt I don't buy it.  The experts say to use wax, oil or pine sawdust and stir it into the melt to end up with simply black ash on top of the melt.  I like to keep my melt at about 700 °F for best operation.  I try to stir all the stuff floating on the top into the pot to reduce the amount of alloy that I would scrape off with the dross.  In our wave solder machines we daily scrape off the dross and frequently send a melt sample out for analysis.  To date we have not gotten results suggesting our alloy needed adjusting.

Am I fighting a loosing battle with the supposed experts and better served to be educated and keep my mouth shut?

Please enlighten me.  Thank you.

PhilN

J
Jorge
2 days ago
Jorge 2 days ago
1 like by David

It is important to know the metal quality. When lead is melted, it will become covered with a thick layer, which is removed, so as constantly to expose a fresh surface. This layer may content oxides of tin and antimony, which are more readily oxidized than the lead. In general, arsenic, antimony, zinc, iron and tin can be separated from their respective alloys with lead, as arseniate or antimoniate of lead, in the first two cases, or as oxides in the case of the remaining metals by melting the alloy in an oxidizing atmosphere.

The presence of antimony is desirable in hard lead (antimonial lead), for the production of type metal, metal for bearings, shot and bullets. Lead used for rolling into sheet should, on the contrary, be free from arsenic and antimony. In the preparation of these alloys, the hard lead forms the base to which, when melted, are added the other components. In starting with the metals, when part of the lead is melted, antimony is added, this followed by the rest of lead, and finally tin es added. Another process of operating is to melt the lead and add the tin, heat the alloy to 600 oC, and pour into it the antimony melted in a separate crucible.

Lead and tin form eutectic mixtures, and since lead forms neither a chemical compound nor a solid solution with either of the two metals, except to a small extent with tin, the crystals separating upon cooling are the same as those of the tin and antimony alloys. Owing to the differences in density of the lead crystals and the solid solutions of tin-antimony, the alloy shows a tendency to segregation. Therefore, the alloy is composed of lead and solid solutions of tin-antimony.

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