Antimony—how hard could it be?

[Bill Kahn]

As folks on this list may know, I am building up a small collection of 1” x 2” x 6” bars of elements. Safe elements—no arsenic. (But yes, lead) I have a dozen now on the mantle. Each is wonderful in some special way.

I am now working on Antimony. Melts at a reasonable temperature—under 1200F. So, with a small hobby kiln should be easy. Not.

First picture shows the nice shiny lump I bought. Sort of crystalline (and heavy) like Bismuth.

First problem is that when hot (like near to melting) it is reactive with air. Oxidizes? Rusts? Burns? Whatever—I am not a chemist. Forms a brown crumbly material. The second picture shows the outside oxide layer of the original lump, the inside had, indeed, melted down. I learned that if I keep the kiln closed the whole time there is not too much oxide formed. Maybe I can put some carbon or such into the kiln to suck up the oxygen before the antimony does?

I do my melting in a small steel “bread pan” I welded up. See third picture. Has work well for a half dozen elements so far. I coat the inside with the release agent LB771 Loctite Nickel Anti-Sieze.

Second problem (so far) is that it seems like hot antimony corodes steel. Even though I have the LB771 coating. Ate a hold right through the steel. See fourth picture. Something about the liquid Antimony embrittlement or dissolving the steel? Anyway, I plan to weld up the hole. Smooth out the side. And try again with thicker LB771 coating.

Have not yet started to try to machine the Antinomy—but, given how the crystals look, I fear this is going to be my terrible Bismuth experience all over again. Tiny tiny bismuth powder sprayed everywhere—requiring a hood to be draped over the mill, breathing mask, and an hour of careful clean-up. Also, sacrificial aluminum rails to prevent edge shattering. We will see.

-Bill

 

[homebrewed]

I think it's unlikely you will be able to keep antimony from alloying with your steel vessel. Find yourself a glazed ceramic one -- it probably doesn't need to be a formal crucible -- at a garage or estate sale (or perhaps a ceramic flower pot?). Pyrex MIGHT work but I'd want ceramic, it would have less of a tendency to shatter due to thermal stress.

A blanket gas like argon or nitrogen would prevent oxidation, but, if you've got it, CO2 would likely work. You might be able to make a CO2 generator by stuffing some dry ice in a jar fitted with a gas line. A pressure blow-off valve might be a good idea if you go that route -- if the line plugged you would be in trouble! You also might need to wrap the jar with a heating blanket to encourage the dry ice to sublime fast enough. Of course, if you've got a MIG or TIG you likely have what you need right there.

 

[whitmore]

As folks on this list may know, I am building up a small collection of 1” x 2” x 6” bars of elements. Safe elements—no arsenic. (But yes, lead) I have a dozen now on the mantle. Each is wonderful in some special way.

I am now working on Antimony. Melts at a reasonable temperature—under 1200F. So, with a small hobby kiln should be easy. Not.

First picture shows the nice shiny lump I bought. Sort of crystalline (and heavy) like Bismuth.

First problem is that when hot (like near to melting) it is reactive with air. Oxidizes?...

Second problem (so far) is that it seems like hot antimony corodes steel.

The second problem, first: yes, like dissolves like, so a liquid metal in a metal container doesn't always work out.
The corrosion probllem is more serious; usually, you add a flux that makes a floating slag layer to get around
that, and some reducing agent (coke, for iron/steel) must be added to scavenge oxygen, hopefully before it gets
to the antimony. If you use granules of carbon, there will be some CO formation (ventilate well).

This description of processing may help: Antimony Smelting

 

[benmychree]

Be advised that molten antimony in the presence of oxygen evolves a quite toxic fume, purple smoke, it should not be melted without a protective respirator specifically rated for the element. I bought a quantity of antimony in the chrystaline form to make a batch of babbit metal; I melted it in a gas fired crucible furnace, and added a quantity of lead, which causes the alloy to melt at a lower temperature, the melting was done in a graphite crucible; after the master alloy was created, it was remelted in an steel pot, more lead added along with a quantity of tin to make the desired babbit alloy.

 

[pdentrem]

Brittle and not really machinable. Just like the Bismuth experience you went through.

 

[homebrewed]

Be advised that molten antimony in the presence of oxygen evolves a quite toxic fume, purple smoke, it should not be melted without a protective respirator specifically rated for the element. I bought a quantity of antimony in the chrystaline form to make a batch of babbit metal; I melted it in a gas fired crucible furnace, and added a quantity of lead, which causes the alloy to melt at a lower temperature, the melting was done in a graphite crucible; after the master alloy was created, it was remelted in an steel pot, more lead added along with a quantity of tin to make the desired babbit alloy.

Good practical metallurgical info! Purple smoke, what's not to like about that....except for its toxic nature....

Out of curiosity I did a little more investigation into the properties of antimony. It's a strange one in a number of ways. Like tin, it can form a gaseous hydride called stibine -- and that's one way to test for it, similar to what is done to test for arsenic (look up "arsenic mirror"). While it is classified as a metal, some of the antimony compounds act like it's an acid-forming element and some act like it is a base-forming element (much like tin). The technical term for this is that some of its compounds are amphoteric.

In combination with certain other elements like indium it switches from being a metal to a semiconductor, and a very cutting-edge one at that. It seems odd to combine two metals and end up with a semiconductor. Antimony has a number of different crystal forms (called allotropes). One is very unstable and will actually explode under some conditions, like being ground in a mortar and pestle. I might do the same if I was mistreated in that way! The process of melting antimony won't produce this form so you don't need to worry about that.

So once you get your block of antimony you will have a hunk of a very unusual element. It could be "fun" to get there.

 

[Bill Kahn]

...evolves a quite toxic fume, purple smoke, ...

Well, I have let the magic purple smoke out of many electronic devices, have never let it out of a molten metal.

Wow, thank you very much for this warning/heads-up.

My plan now is to find a ceramic butter dish, upside down, as my antimony mold. And to limit its exposure to oxygen by 1) keeping the kiln closed 2) wrapping and crimping the butter dish in stainless foil. And 3) to run the furnace outside so the magic purple poison smoke that does form dilutes immediately. If needed, the plan after that it will be to try putting carbon into the stainless foil pouch to scavenge the oxygen that does get in and then after that to attach a hose to the kiln and continuously flood with nitrogen to displace any air (i.e. 21% O2) that does drift in. (any reason for me to flood the kiln with argon or CO2 instead of nitrogen?) I am hoping the antimony trioxide that does form is on the outside and is easy to band saw away as I will not be able to have access to the molt, to skim the dross (being in a closed kiln and crimped closed in foil.)

Thank you very much to all here who have warned me about the smoke, the need for using ceramic, and the terrible milling experience again still in front of me. Forewarned and forearmed and all.

If I ever get my bar of antimony made, I’ll post a picture here—another grey bar to joint the 12 I already have (plus the pretty copper one).

-Bill

 

[homebrewed]

If you've got it available, N2 will work just fine. Argon will too, of course. I don't think hot antimony will reduce CO2 to CO (by grabbing one of its oxygen atoms) so CO2 probably will be OK. Caveat: the comment on CO2 is "informed speculation" on my part so I COULD be wrong!

As mentioned in an earlier post, elemental carbon might burn incompletely & form CO, so even if antimony didn't make that nice purple smoke you'd want to do the operation outside or under a fume hood, if you've got one.