Melting powdered metals 8

[BurningKrome]

I'm a biotechnologist, however I am learning metallurgy as a hobby. I have a question I hope the professionals can help me understand.

I've been working with the easy metals (pewter, tin, silver). I melt the metals for casting in a ceramic crucible with a handheld butane torch, using borax as flux.

I ordered some powdered tin with the expectation that it would melt like sugar due to the large surface area from the small particle size (80um). To my surprise, I find it nearly impossible to melt completely. I've basically destroyed one crucible due to all the metal powder residue impregnated on the surface.

Can anyone explain why powdered tin melts so poorly compared to pellets? Is it the oxidation on the particle surface?

 

[SwinnyGG]

Powders are hard to melt because the primary heat transfer from the crucible to the material is conduction.

 

[swall]

You need to find a safer hobby. Open flame heating of metal powders can lead to an explosion. Stick with melting of ingot form of material.

 

[blacksmith37]

Pewter is tin. Why not melt tin in a SS sauce pan on the stove ? ( I have messed up tin coated copper pans that way.

 

[BurningKrome]

Hi Blacksmith37,

Good suggestion, Actually, I've done that as well, but I find the crucible easier to pour small molds. Mostly, I was just curious about the physics behind why powder doesn't melt as well as pellets (my chemistry background).

I've stuffed the powder into a tool chest, probably never to be seen again...but I'm still curious

 

[EdStainless]

in addition to poor heat transfer there is a little pit of oxide on the surface of all of the particles, this makes melting very difficult.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

You are using a torch to heat up a ceramic crucible, hoping the crucible to melt Tin? I think the bigger problem was the ceramic crucible and the torch. it is hard to believe you cannot melt Tin powder if you place your crucible in a furnace with a low temperature, say 400C. Melting point of Tin is 232C, nano powder has even lower melting point at 177C. However, if Tin is oxidized, as ED pointed out, it will be very difficult to melt. the melting point of SnO is over 1000C.

 

[MikeHalloran]

Another star for swall.

Some people say that powder/dust is the fourth state of matter, not without reason.
Metal powders in particular are dangerous in a variety of ways.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Did you use flux when melting the powder? Flux is used to dissolve surface oxides and to prevent further oxidation. Powdered metals have a lot of surface area and often contain a significant percentage by weight of oxides for this reason. The surface oxide layer will prevent particles from coalescing together even though the metal inside is molten. The same thing can be observed if you try to melt aluminum foil (which many still call tin foil).

 

[berkshire]

Mike Halloran.
As a teenager I used to make model rocket motors from Zinc powder and Sulphur,
We were warned not to get the dust in the air when assembling these.
B.E.

You are judged not by what you know, but by what you can do.

 

[tbuelna]

You probably purchased a low cost tin powder. And the common method used to produce this type of tin powder is air atomization of the molten metal. This process leaves an oxide film on the tin particle surface. As others noted, this oxide film has a much higher melting temperature than the tin metal itself. So the oxide film on many of the particles is not reduced at the bulk temperature existing in the metal charge in your crucible.

 

[BurningKrome]

Hello tbuelna,

Great answer. Thanks for that! Question, is there a low-hazard way to chemically de-oxidize the metal? I'm not thinking about the Tin (since it's so cheap), but in this case thinking about bronze powder.

Thanks!

 

[Compositepro]

That is what flux is for. Flux dissolves surface oxides and protects the metal from further oxidation. The metal should sink and coalesce under the flux floating on top. Another option would be to use a controlled atmosphere furnace with a reducing atmosphere. Hydrogen gas will reduce oxides. It is also explosive so this is not a very useful hobbiest technique.

 

[EdStainless]

You can buy small cylinders of hydrogen, but even then it isn't to be done casually.
You would put the powder in an enclosed container (steel box) with gas inlet and outlet lines (usually SS).
You would start heating with inert (Ar) gas flow, say to 250F. Then once it is well purged (no oxygen) switch to hydrogen and have a burner on the vent line to flare it off. Then ramp the temp up to just above the melting point.
Though you may need to go higher to get enough reducing capacity to actually reduce the oxides.
Stick with flux.

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P.E. Metallurgy, Plymouth Tube

 

[BurningKrome]

Hi EdStainless,

Thanks for the interesting post. I'm very interested in learning about metallurgical techniques, from an academic standpoint, although - youre right - thats far beyond my needs.

Although, I used borax as flux initially (and tried quite a lot ultimately) and it was not enough to allow the tin powder to melt. However, the torch only gets to 710c, which is a far sight shy of the 1000c needed fore oxidized tin.

I'll be sticking with tin pellets in the future

 

[EdStainless]

Someday you might try this, melt pure tin and suck it up into a glass tube (pyrex works).
Then you seal the ends of the tube after it solidifies.
Now you can zone melt it inside the glass, heating a small portion at the bottom and slowly working your way up.
If you are careful the result will be a single crystal rod of Sn.
After breaking the glass away the rod is interesting. If you bend or stretch it you will clearly see the slip planes as the Sn yields. And the whine that it makes when bent is interesting.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Ed, could that give you more chance an oriented columnar polycrystal? No need a "spiral" kind of mold at the beginning of solidification to get a single crystal,maybe anything special with Sn?!

 

[EdStainless]

You don't have any control over the orientation, but it is easy to get a single crystal.
What gets me is the sound that makes when it yields.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

Well, you've got your tin powder now and it would be a shame to waste it!

Try pressing it with a hydraulic press inside a cylindrical depression or crude piston. That will get rid of most of the air and increase the thermal conductivity a lot. Then you can cover the resulting pellet or hockey puck with a covering of flux.

I've never had any luck torch-melting stuff, and never had the luxury of an oxy-acetylene torch at home- too cheap to buy bottles. Torch melting is really meant only for silver and gold, which can withstand the oxidation. Instead I built a little reverberatory furnace- basically a box made of insulating firebricks stacked on edge, held together with wire and wrapped with a batt of mineral fibre (Roxul Flexibatt- the crap we insulate houses with here). The floor is a 12x12x2.5" hard refractory firebrick tile, and the lid is two firebricks set side by side and wired together around the outside, with a wire handle and a hole in the middle as a vent cut with a holesaw (insulating firebrick is easy to work with). The burner is a piece of 3/4" stainless pipe, stuck on the end of a little squirrel cage blower, with the wall tapped for a Swagelok fitting through which I add propane from a barbeque tank and regulator (and long hose, so I can keep the propane bottle far away from where I'm working) with a needle valve to regulate the flow. This crude burner pipe is just shoved through a hole cut through one of the firebricks. You light it by rolling 1/4 section of newspaper into a cylinder and lighting that, inserting it into the firebox and then you start the blower, gradually adding propane. When the wall is glowing red (takes about 2 minutes) it will stay lit on its own. I use fireclay fire assay crucibles because they're cheap and durable, and can have brass up to pouring temperature in about 15 minutes (I have a thermocouple in the firebox so I can monitor the temperature- it's the highest tech part of this apparatus), plus home-made crucible tongs etc. The bricks don't like splashes of flux so you need to be careful with that, but otherwise they last quite a long time and insulate very well- much better than the crappy refractory concrete or plaster cylinders you see people using on the internet, which suck heat like pigs and take forever to heat up. Obviously a reverberatory furnace like this is a tool which needs to be used outdoors only...I wheel it out onto the stoop of my shop whenever I need to use it.

I melt scrap aluminum and brasses/bronzes exclusively. And I use either home-made greensand (20 parts sand, 3 parts ground kitty litter plus just enough moisture applied with a spray bottle and mixed by hand, until a clump in your hand can be broken cleanly), or Petrobond oil sand (much easier to work with, but much more expensive).

 

[berkshire]

One thing you can do with your powdered tin is to mix it into a tinning flux like JOHNSON'S TIN-EZY POWDER with PURE TIN this is a purpose made flux for working with Tin.
As Composite pro has said , this will deoxidize your tin and keep it together while it melts, Sometimes to get tin powder to melt, a little solid tin bar is needed to get the melt started. Remember heat the retort, do not put the flame directly onto the powder.
B.E.

You are judged not by what you know, but by what you can do.