Melting gold using resistive heating.

[Amol Gupta]

So I came across this video where individuals have used step down transformers to achieve high current flow at low voltages.



They are able to melt metal using such high currents.

I was wondering if I could use the same principle and connect the two ends of the wire to a graphite crucible which contains my metal, my intuition is it should be hot enough to melt gold without any significant metal loss.

Any thought on the idea would be appreciated.

Thanks.

 

[Amol Gupta]

I know people use induction heating furnaces which work on the principle of induced current but the machines themselves cost a lot, this seems to be a good cheap alternative.

 

[Yggdrasil]

I know people use induction heating furnaces which work on the principle of induced current but the machines themselves cost a lot, this seems to be a good cheap alternative.

As long as the Graphite are protected from most of the Oxygen it should be ok.
I guess.

 

[Amol Gupta]

As long as the Graphite are protected from most of the Oxygen it should be ok.
I guess.

I guess I'll have to sacrifice a few crucibles to check that out.
Speaking of oxidation, the induction furnaces I was talking about, the manufacturers state the machine works in vaccume but also they use argon gas to fill the chamber, never understood why argon being inert may help reduce oxidation in that case I did understand the purpose of vaccume

 

[Yggdrasil]

I guess I'll have to sacrifice a few crucibles to check that out.
Speaking of oxidation, the induction furnaces I was talking about, the manufacturers state the machine works in vaccume but also they use argon gas to fill the chamber, never understood why argon being inert may help reduce oxidation in that case I did understand the purpose of vaccume

Argon do not react with anything no matter how hot.
Most of the gases in air do.
Vacuum works too but needs powerful vacuum pumps.

 

[Amol Gupta]

Argon do not react with anything no matter how hot.
Most of the gases in air do.
Vacuum works too but needs powerful vacuum pumps.

I'm not sure if it's just me being stupid but I have seen manufacturers state the induction furnaces is a vaccum induction melting machine but I have seen argon gas tanks attached to such machines.

 

[Yggdrasil]

I'm not sure if it's just me being stupid but I have seen manufacturers state the induction furnaces is a vaccum induction melting machine but I have seen argon gas tanks attached to such machines.

There are many types.
Some have vacuum and pressure.
Another thing is that if you introduce low pressure Argon you do not need the deep vacuum needed without.
Depending on application of course.

 

[Amol Gupta]

There are many types.
Some have vacuum and pressure.
Another thing is that if you introduce low pressure Argon you do not need the deep vacuum needed without.
Depending on application of course.

Oh ok, it always bugged me how can we state the machine as vaccum operated while still introducing argon, thanks for that.

Frankly I'm pretty excited on testing out melting some metal in my graphite crucible using high current either from a transformer or a high current power supply.

 

[goldshark]

Oh ok, it always bugged me how can we state the machine as vaccum operated while still introducing argon, thanks for that.

Frankly I'm pretty excited on testing out melting some metal in my graphite crucible using high current either from a transformer or a high current power supply.

I don't know if you do much welding, and if so, have you heard of Carbon arc gouging. and the Graphite rods used. For others who do not know of this type of welding accessory, Carbon arc gouging consists of a Copper coated graphite rod. The rod is held by a typical electrode stick holder, but has the additional feature of an air port, with thumb operated on/off. This type of welding is not joining pieces of metal, but is used instead to remove welds, and cut steel. An arc is struck with the electrode, combined with the air, creating an impressive, albeit noisy, show of sparks from the metal being blown away. OK, fast forward to how this would work for melting metals. Obtain Carbon arc rods, place one in the grounding clamp, the other in the electrode holder. Strike an arc before introducing to metals in crucible. Immerse slowly, maintaining the optimum arc distance. Steel foundries use this technique. They have a jig containing the rods, set immersion speed controllers, etc.. In all, a big investment. The rods I use are typically 1/4 - 5/16 inch in diameter. I use a miller 250, which is pushing the maximum duty cycle of the welder, for this size rod. I think it runs at 24 Volts @ 250 amps. Perhaps this could be a small scale way of melting metals, that has not been explored on this forum yet. The one thing I do know, plasma obtained from this technique is very hot. Perhaps leading to excess PM losses, hence why this technique is not used.

 

[Yggdrasil]

I don't know if you do much welding, and if so, have you heard of Carbon arc gouging. and the Graphite rods used. For others who do not know of this type of welding accessory, Carbon arc gouging consists of a Copper coated graphite rod. The rod is held by a typical electrode stick holder, but has the additional feature of an air port, with thumb operated on/off. This type of welding is not joining pieces of metal, but is used instead to remove welds, and cut steel. An arc is struck with the electrode, combined with the air, creating an impressive, albeit noisy, show of sparks from the metal being blown away. OK, fast forward to how this would work for melting metals. Obtain Carbon arc rods, place one in the grounding clamp, the other in the electrode holder. Strike an arc before introducing to metals in crucible. Immerse slowly, maintaining the optimum arc distance. Steel foundries use this technique. They have a jig containing the rods, set immersion speed controllers, etc.. In all, a big investment. The rods I use are typically 1/4 - 5/16 inch in diameter. I use a miller 250, which is pushing the maximum duty cycle of the welder, for this size rod. I think it runs at 24 Volts @ 250 amps. Perhaps this could be a small scale way of melting metals, that has not been explored on this forum yet. The one thing I do know, plasma obtained from this technique is very hot. Perhaps leading to excess PM losses, hence why this technique is not used.

He wants to use the resistive heating of the crucible only.
No arc.
Just clamp conductors on the crucible and heat it direcly.

 

[Amol Gupta]

I don't know if you do much welding, and if so, have you heard of Carbon arc gouging. and the Graphite rods used. For others who do not know of this type of welding accessory, Carbon arc gouging consists of a Copper coated graphite rod. The rod is held by a typical electrode stick holder, but has the additional feature of an air port, with thumb operated on/off. This type of welding is not joining pieces of metal, but is used instead to remove welds, and cut steel. An arc is struck with the electrode, combined with the air, creating an impressive, albeit noisy, show of sparks from the metal being blown away. OK, fast forward to how this would work for melting metals. Obtain Carbon arc rods, place one in the grounding clamp, the other in the electrode holder. Strike an arc before introducing to metals in crucible. Immerse slowly, maintaining the optimum arc distance. Steel foundries use this technique. They have a jig containing the rods, set immersion speed controllers, etc.. In all, a big investment. The rods I use are typically 1/4 - 5/16 inch in diameter. I use a miller 250, which is pushing the maximum duty cycle of the welder, for this size rod. I think it runs at 24 Volts @ 250 amps. Perhaps this could be a small scale way of melting metals, that has not been explored on this forum yet. The one thing I do know, plasma obtained from this technique is very hot. Perhaps leading to excess PM losses, hence why this technique is not used.

You have stated the reason for not using arc furnaces, the heat coming from the arcs is hot enough to vaporize precious metals, the whole premise of such resistive heating is to reduce the losses in melting.

 

[haveagojoe]

I think to reach the temperature needed to melt gold in a resistive crucible would require enormously high current, it would be very inefficient and pretty dangerous. Induction is the way to go, there are quite cheap kits available these days from China or it would even be possible to build a custom one, I'd say 3kW would be ideal for small amounts. I think the trick used with induction is to melt a small amount of gold powder first and then add more to it when it's up to melting temperature.

 

[Yggdrasil]

I think to reach the temperature needed to melt gold in a resistive crucible would require enormously high current, it would be very inefficient and pretty dangerous. Induction is the way to go, there are quite cheap kits available these days from China or it would even be possible to build a custom one, I'd say 3kW would be ideal for small amounts. I think the trick used with induction is to melt a small amount of gold powder first and then add more to it when it's up to melting temperature.

There is not much need for induction with Gold.
Any electric or gas driven furnace can do that.
And yes you need very high Amps to heat it enough.

 

[Amol Gupta]

I think to reach the temperature needed to melt gold in a resistive crucible would require enormously high current, it would be very inefficient and pretty dangerous. Induction is the way to go, there are quite cheap kits available these days from China or it would even be possible to build a custom one, I'd say 3kW would be ideal for small amounts. I think the trick used with induction is to melt a small amount of gold powder first and then add more to it when it's up to melting temperature

Fiddling around with 3kw power is more dangerous than applying small alternating current for 700 Amps at 2V.
This is an experiment I wish to conduct I'm guessing I will be able to melt close to 100gm gold using this technique.

 

[haveagojoe]

Fiddling around with 3kw power is more dangerous than applying small alternating current for 700 Amps at 2V.
This is an experiment I wish to conduct I'm guessing I will be able to melt close to 100gm gold using this technique.

3kw is the input. It puts about 50A through the coil so about 60V.
700A x 2V = 1400W so still quite a lot of power.
Mr Electron on Youtube has done it your way to melt aluminum here, it seems dangerous to me and even for a very small amount of aluminium it did not melt it very well. I would be very surprised if you can melt 100g of gold, maybe I'm wrong.

 

[Amol Gupta]

3kw is the input. It puts about 50A through the coil so about 60V.
700A x 2V = 1400W so still quite a lot of power.
Mr Electron on Youtube has done it your way to melt aluminum here, it seems dangerous to me and even for a very small amount of aluminium it did not melt it very well. I would be very surprised if you can melt 100g of gold, maybe I'm wrong.

Thanks a lot for the reference so I did go through the video, the current running across the circuit is close to 100A at approx 2.5V which boils down to close to 250-300W which is good enough for melting alluminium, the circuit has fairly high resistance, if one can lower that we will have more current output(the infrastructure he has built is capable of outputting higher current, close to 500 Amp.).
The game here is to lower the resistance of the circuit given the heating from resistive losses grows exponentially(P=i^2*r) with current or increase the output voltage coming out of the transformer to make more current flow from the circuit.

 

[haveagojoe]

The game here is to lower the resistance of the circuit given the heating from resistive losses grows exponentially(P=i^2*r) with current or increase the output voltage coming out of the transformer to make more current flow from the circuit.

I don't think that's right, surely you need higher resistance and/or higher current for more heat, not higher voltage?
P=VI
V=IR
The reason we use high voltage and low current for overhead powerlines is because high voltage doesn't cause heating of the cables but high current does. The point of a step-down transformer is to lower the voltage and increase the available current. If we consider an ordinary electric heater, it has a long thin wire coil, to give high resistance in order to produce heat. In the video he uses a small crucible- if he used a larger one, the current would have more available material to pass through so the resistance would be lower. It's all about the cross-sectional area which the current passes through, hence why cables are rated by cross sectional area which allows the user to calculate the maximum current it can handle at a given voltage.
To produce more heat the setup would need a bigger transformer with more turns on the primary or fewer turns on the secondary- probably both, although there's already only 2 turns on the secondary. Alternatively it could be done with multiple transformers in parallel, but it would place a heavy draw on the supply which would probably trip the MCB. It would also need very fat cables, both around the secondary and from the transformer going to the crucible otherwise they could just melt.
I still think it's a dangerous experiment to mess around with- there is a common misconception that if the voltage is low then it's safe to touch and won't cause electric shock, but in fact even at low voltage a current above 10A is enough to cause serious electric burns, both externally and throughout internal tissue and organs. If your body has lower resistance than the crucible, you will become the heating element.

 

[Yggdrasil]

I don't think that's right, surely you need higher resistance and/or higher current for more heat, not higher voltage?
P=VI
V=IR
The reason we use high voltage and low current for overhead powerlines is because high voltage doesn't cause heating of the cables but high current does. The point of a step-down transformer is to lower the voltage and increase the available current. If we consider an ordinary electric heater, it has a long thin wire coil, to give high resistance in order to produce heat. In the video he uses a small crucible- if he used a larger one, the current would have more available material to pass through so the resistance would be lower. It's all about the cross-sectional area which the current passes through, hence why cables are rated by cross sectional area which allows the user to calculate the maximum current it can handle at a given voltage.
To produce more heat the setup would need a bigger transformer with more turns on the primary or fewer turns on the secondary- probably both, although there's already only 2 turns on the secondary. Alternatively it could be done with multiple transformers in parallel, but it would place a heavy draw on the supply which would probably trip the MCB. It would also need very fat cables, both around the secondary and from the transformer going to the crucible otherwise they could just melt.
I still think it's a dangerous experiment to mess around with- there is a common misconception that if the voltage is low then it's safe to touch and won't cause electric shock, but in fact even at low voltage a current above 10A is enough to cause serious electric burns, both externally and throughout internal tissue and organs. If your body has lower resistance than the crucible, you will become the heating element.

You two are talking past each other in a way.
First V are not used in these equations. The symbol U are used.
So U=RxI
At the same time P= UxI which in turn means
P= RxI²

If you as usual have an output limited by the transformer the increase i either Voltage or Current will drop the other.
So theoretically speaking if you have a transformer with 100W stepped down to say 0.01V
And this means 10 000A.
It will have a lot of capacity to heat but needs practically no insulation since the potential is so low it cant jump any gaps, you need direct contact.
But get that through your body and you are in deep ouch.
On the other hand if you reverse this and increase the Voltage to 10 000 you will need good insulation and proper separation since the potential is so high it will jump quite big gaps.
But 10mA is safe to the body in almost all settings.

The Power is the same but the application is vastly different.

For heating this way just look into resistive welding where you add a High Current over a short distance.

The best way to get this properly done in my view is to have a conductor under and a clamp in the upper middle part.

Or even better use a ordinary electric furnace.

 

[Amol Gupta]

I don't think that's right, surely you need higher resistance and/or higher current for more heat, not higher voltage?
P=VI
V=IR
The reason we use high voltage and low current for overhead powerlines is because high voltage doesn't cause heating of the cables but high current does. The point of a step-down transformer is to lower the voltage and increase the available current. If we consider an ordinary electric heater, it has a long thin wire coil, to give high resistance in order to produce heat. In the video he uses a small crucible- if he used a larger one, the current would have more available material to pass through so the resistance would be lower. It's all about the cross-sectional area which the current passes through, hence why cables are rated by cross sectional area which allows the user to calculate the maximum current it can handle at a given voltage.
To produce more heat the setup would need a bigger transformer with more turns on the primary or fewer turns on the secondary- probably both, although there's already only 2 turns on the secondary. Alternatively it could be done with multiple transformers in parallel, but it would place a heavy draw on the supply which would probably trip the MCB. It would also need very fat cables, both around the secondary and from the transformer going to the crucible otherwise they could just melt.
I still think it's a dangerous experiment to mess around with- there is a common misconception that if the voltage is low then it's safe to touch and won't cause electric shock, but in fact even at low voltage a current above 10A is enough to cause serious electric burns, both externally and throughout internal tissue and organs. If your body has lower resistance than the crucible, you will become the heating element.

You are exactly right with respect to power lines it is the current that heats the wires and not voltages.

So here what I think of the video.
1. The transformer used in the video is a 1KVa transformer.
2. The voltage coming out of the transformer solely depends on the number to turns on the primary and the secondary irrespective of load attached.

I the video the most amount of power the creator was able to produce was 2.5V * 500 Amps = 1Kw(Approx) while melting a small metal part. His system(transformer primarily) is able to transform such amount of power.

Now the voltage at the secondary is fixed at 2.5V(assuming he has not changed the winding) but when melting the alluminium he is only able to get 100A to flow from the circuit which is a power of 250W, but his system is capable of pumping a current of 500Amps @ 2.5V, the resistance of the crucible along with all the wiring is close to 0.025 ohms, if he reduces this resistance he will be able to increase the current flowing through the circuit.
The other way to increase output power is increase the voltage coming out of the secondary by changing the number of turns if he increases the output voltage to say 4V he has a resistance of 0.025 ohms as before he would be able to get close to 160 Amps running through the circuit which is now a power of 640W, something his system(of 1kw) is capable of doing.

 

[haveagojoe]

even better use a ordinary electric furnace.

I agree. The main issue with doing it this way is that, assuming the crucible is made of pure graphite, there's no way it could possibly produce enough resistance. Graphite is a pretty good conductor.
I have no idea how the guy in the video managed to melt aluminium, perhaps he faked it. I did some rough calculations: based on a crucible with a 2cmx2cmx2cm cube-shaped interior giving a volume of 8cm^3 (which would fit 100g of gold with about 7mm height left at the top) the resistance from one side to the other, or from top to bottom, would be in the order of less than a milliohm, when empty. To achieve even 0.5ohm you'd need walls thinner than paper. And it gets even worse when you fill it with gold, a highly efficient conductor of current!

Perhaps if it were some kind of highly resistive graphite clay...
Or, as you say, simply use a long thin heating coil wrapped many times around a non-conductive crucible, with lots of insulation. Trying to reinvent the wheel is fun but alas there's always a reason why it hasn't been done before.

@Amol Gupta :
Here are some of the calculations I started on but I abandoned them when I started calculating the resistance of the crucible and realized it wouldn't work. Maybe you will find something useful here for your calculations since I spent time on it:

Using the UK supply for example purposes- we have 240V supply and for the sake of argument we can assume it's fused at 13A as per normal (it's possible to get up to 16A if you wire in directly but that's illegal unless you're an electrician).

Example:

Supply is 240V and can give max 13A

Assume a starting temperature of 20degrees Celcius
Gold melts at 1064 degrees Celcius
So our gold will need to be heated by 1044 degrees to reach melting point ("deltaT")

Gold has a specific heat capacity of 129 J/kg ("c")
Gold has a latent heat of fusion of 64500J/mol ("L")

We want to heat and melt 100g
so the mass of our gold =0.1kg ("m")

Gold has an atomic mass of 197g/mol
so the molar mass of our gold is 100/197 = 0.5076mol ("Moles")

Gold has a density of 19320kg/m^3
so the volume of our 100g of gold = 0.1 / 19320
= 5.18 x 10^6 m^3
= 5.18cm3 ("Vol")

Calculating the energy requirement:

Heating the gold to melting point:

Energy required = Mass x Specific heat capacity x Change in temperature
Q(heating) = m x c x deltaT
= 0.1 x 129 x (1064 - 20)
= 13442J

Melting the gold:

Energy required = Molar mass x Latent heat of fusion
Q(melting) = Moles x L
= 0.5076 x 64500
=32740.2J

Total energy requirement = Q(heating) + Q(melting)
=46182.2J

Resistance of crucible:
Abandoned