# Induction furnace



## elfixx (Feb 10, 2011)

Can anyone tell me the difference between medium and high frequency induction furnace? For some reason high frequency unit tend to be cheaper than medium but seem more suitable for heat treating and welding than melting. Has anyone looked the different type of induction furnace curently offered on ebay by chinese manufacturer, what would you think of these?


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## godofwar (Feb 10, 2011)

Wikipedia

Operating frequencies range from utility frequency (50 or 60 Hz) to 400 kHz or higher, usually depending on the material being melted, the capacity (volume) of the furnace and the melting speed required. Generally, the smaller the volume of the melts, the higher the frequency of the furnace used; this is due to the skin depth which is a measure of the distance an alternating current can penetrate beneath the surface of a conductor. *For the same conductivity, the higher frequencies have a shallow skin depth - that is less penetration into the melt. Lower frequencies can generate stirring or turbulence in the metal.*


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## Lou (Feb 12, 2011)

You want low frequency for melting gold and such.


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## Harold_V (Feb 13, 2011)

Lou said:


> You want low frequency for melting gold and such.


It's relative. Starting weight and configuration, at least in second generation induction furnaces, will determine frequency. Said another way, higher frequency is required to melt small amounts, and even higher when there is no heel from which to start the melt. Melting gold powder would require a higher frequency than melting a 100 ounce ingot. 

Inductotherm published a chart that indicates ideal frequencies. 

Harold


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## Lou (Feb 13, 2011)

Indeed, but having done my share of shopping over the past few months for a new induction furnace, I've come to the realization that you cannot get a furnace that will melt your gold powder or platinum sponge and also melt the bulk material. Most of the IGBT driven units from Inductotherm are pretty expensive, I think their minimelt furnace for platinum/steel is about 16 kHz, far too low to work with powder.


Lou


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## HAuCl4 (Feb 14, 2011)

The secret to induction melting powders lies in heating the crucible. i.e. quartz crucible inside graphite or SiC conductive crucible/jacket, heated by induction.


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## Lou (Feb 14, 2011)

That's mainly true for gold and silver powder, not so much the PGMs. I don't know how long any graphite or SiC insert would last if oxygen got at it at 1800*C.


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## HAuCl4 (Feb 14, 2011)

I saw a diagram of Pt once melted in a vacuum tube with SiC insert and quartz mini-crucible inside, coil outside of it all. Let me see if I can find it.


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## Lou (Feb 14, 2011)

Oh I believe that, no questions about it. You can melt Pt resistively with Mo, W, or even C heating elements...catch is that they must be in a decent vacuum or in argon.


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## HAuCl4 (Feb 14, 2011)

I can't find the diagram at the moment, but I believe the small crucible was a zirconia boat, not quartz, and the insert wasn't anything high tech, just a piece to get the induction coil to heat something fast, and transfer the heat to the little boat. It was left to solidify inside the boat as one piece/ingot. All done inside a larger quartz tube. I can't remember if at a vacuum or an inert atmosphere as you say.


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## zachy (Dec 7, 2016)

Where you can get the quartz tube, I make a small induction furnace and I plan to put an Argos probe on the top of the crucible.


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## Lou (Dec 7, 2016)

It is always best to melt the platinum in air, preferably in lime or in magnesia crucibles (they purify the platinum and resist contamination), secondly with zirconia crucibles. But always in air.


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## zachy (Jan 14, 2017)

I want to melt platinum dust, but the induction does not affect the dust, and if I use gas torch, it becomes contaminated, if I use zirconium oxide crucible, it is contaminated, I do not know which crucible is better, so that my platinum is pure 99.99.

who knows?


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## ChemistGoldDigger (Jan 23, 2017)

Hi I'm new here, thought I would contribute. Good stuff is using Tesla coil induction system, don't even need to have a crucible as the stuff levitates so doesn't get contaminated and you can get it for a few queet on ebay. I attached images, google 'induction iron levitating coil ' for videos an stuff


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## g_axelsson (Jan 23, 2017)

ChemistGoldDigger said:


> Hi I'm new here, thought I would contribute. Good stuff is using Tesla coil induction system, don't even need to have a crucible as the stuff levitates so doesn't get contaminated and you can get it for a few queet on ebay. I attached images, google 'induction iron levitating coil ' for videos an stuff


If you want to contribute I suggest you first start to learn how things work in real life. You haven't even got the terminology correct. Tesla coils and induction melting is two totally different things.

This post doesn't add to this thread, it just dilutes it with a non working method.

I might sound a bit harsh, but this forum is dealing with reality, not sci-fi writing.

If I'm wrong you could easily prove it by showing a picture on your levitating melter. No, that picture isn't of melting gold, it is a piece of aluminium and comes from a youtube video... that is quite neat but nothing useful for melting pm powders.
https://www.youtube.com/watch?v=VydPQuLyEns

Göran


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## Yggdrasil (Jan 23, 2017)

Hi!
Welcome ChemistGoldDigger, this is an excellent place to learn, probably *the* place to learn refining of PMs.

The video is very neat and cool to watch but unless one can start with powders or at least drop it into a casting form or crucible in a controlled way, it is just that, cool and neat. 

What happens at the end of these videos? 
The molten lump just falls out, it doesn't seem to be shut off. Is it heated beyond its power capacity to keep up with the induced magnetic field to keep it levitated? 
If so this may be utterly useless for our purpose?

But back to the thread:
Lou wrote


> It is always best to melt the platinum in air, preferably in lime or in magnesia crucibles (they purify the platinum and resist contamination), secondly with zirconia crucibles. But always in air.


I guess vacuum may be unfortunate because of evaporation issues or something to that matter?
But does this mean that for Platinum open air is better than Argon or say plain Nitrogen (Air is after all appr 70 percent N2)?


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## g_axelsson (Jan 23, 2017)

The levitating induction melting is moot, since you can't start with powder, you need a solid slug. It falls when the power is switched off. The levitation is from the funnel-formed magnetic field from the induction loop and the induced current in the metal. The temperature has nothing to do with it, molten metal conducts current quite good.

Göran


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## Lou (Jan 23, 2017)

No, oxidizing conditions are best because PGMs form cermets under reducing conditions...

Pt aluminide, silicide, etc. Y, Zr, Mg, Ca oxides are more resistant to this than Al/Si which are more easily reduced. Obviously Pd has to be inert or vac melted but Pt should be cast in air.


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## Yggdrasil (Jan 23, 2017)

Thanks for the clarification Lou


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