A possible alternative/improvement to graphite electrodes in arc furnaces

[JWR Electric]

My next experiment will be to create an electrode from a 50/50 ratio of ultra fine silicon carbide and pure graphite powder bonded with veegum and a pinch of zirconium silicate to increase strength and decrease shrinkage. My theory is that said electrode will still have enough electrical conductivity to strike an arc and get the tip of said electrode hot enough during the initial few seconds to electrically activate the silicon carbide. And since silicon carbide has a massively high thermal breakdown temperature (about 2700 celsius compared to 800 celsius for grapite), theoretically this should result in an electrode that lasts a lot longer and doesn't get the metal melt nearly as contaminated with carbon as with a pure carbon electrode.

 

Actually, I might start with a graphite electrode and then surround it with said new SiC/C mixture. I will have to see how conductive said mixture is initially.

 

I also have a couple more theories. First, what if I replace the silicon carbide with a 40/60 mixture of zirconium silicate and graphite? will the intense heat cause the two components to fuse and create the super refractory zirconium carbide? It seems I found my answer in this article: https://www.osti.gov/biblio/4569725-carbonization-zirconium-silicate-plasma-arcs

 

Second, what if I introduce some tungsten carbide powder into the carbide/carbon mixture? Will this massively increase the conductivity of the electrode? My only worry is that the tungsten carbide could rapidly transfer into the metal melt. However, I suppose trickling some argon into the furnace will fix that.

[Phil K.]

Just use a Carbon Arc gouging rod. They come in several sizes up to 5/8 on an inch, maybe bigger. Work great for electric arc furnace if you have the power to back it up.

 

[JWR Electric]

Yea I power my furnace with my 240v 47A welder. The problem is my electrodes get eaten up rapidly both in DCEN and DCEP, even with CO2 or Argon atmospheres, which leads me to believe the metal pool is breaking down and absorbing my electrodes. Also, I make large electrodes by packing 7 of them tightly into a hexagonal pattern, filling the gaps with bonded graphite, and coating the outside with bonded zirconium silicate.

Also Im flat broke so Im left with only a few electrodes, but I have a LOT of silicon carbide and veegum, as well as some tungsten and zirconium silicate.

[LeeJustice]

3 hours ago, JWR Electric said:

Second, what if I introduce some tungsten carbide powder into the carbide/carbon mixture? Will this massively increase the conductivity of the electrode?

I found this regarding what you ask:

https://www.thomasnet.com/articles/metals-metal-products/tungsten-vs-tungsten-carbide/

[JWR Electric]

16 minutes ago, LeeJustice said:

I found this regarding what you ask:

https://www.thomasnet.com/articles/metals-metal-products/tungsten-vs-tungsten-carbide/

Thank you for the info. However, I already know about the properties of tungsten and tungsten carbide. What I don't know however is whether adding a small amount of tungsten carbide to the electrode mixture will have any noticeable increase on initial conductivity. I would imagine it does since the tungsten carbide essentially meshes with the graphite and forms a matrix of sorts. However, Im going to do a quick test to see if my logic holds true.

[CGun]

How much metal are you trying to melt? My electrodes (3/8" gouging electrode) last probably an hour or each on DCEN. I have a bottom limit switch on my furnace that bumps the carriage when it hits the bottom of travel (electrode too short) and stops the furnace until I change the electrode.  Mine is a computer controlled furnace and I am running it at about 120A most times and am trying to melt up to maybe a few pounds of metal at a time, not much more. Large furnaces figure about 200Wh/pound of metal, but that is economy of scale when you are melting 300 tons at a go in a $275,000 crucible lined with magnesium-Carbon bricks and 1.6GJ or power. I figure If I can melt 4lbs in 45 minutes to an hour at about 4,000-5,000W I am doing pretty good. Water cooling the bottom anode helps a lot, but I need to add another radiator. The one I have on my furnace was the right size when all I was doing was cooling the electrode armature, but with the bottom electrode in contact with the molten pool, it takes a good bit more cooling now. It is always a balance of trying to dump as much heat into the pool as possible but also needing to actively cool the components, which sucks heat from the pool. 

[JWR Electric]

I tried to melt about 4 pounds of steel, but that's a significantly longer amount of time than I expected. I'm a welder, so I'm used to steel melting instantly when I'm running beads and fusing joints. Its starting to sound like an arc furnace really isn't worth it with the amount of power available to me considering I don't have to finances to afford fancy electronics to control the rod automatically. And no way xx xxxx am I gong to sit there for an hour manually moving an electrode up and down. I think I'll just have to stick with using my propane furnace. 0_o

So that being the case, I'm just going to fall back on my backup plan and create a new thread with ideas on improving propane furnaces, since that seems much more accessible to me currently.
 

Thanks everyone for the help!

That said however, I'll still try to figure out if its possible to develop this hybrid electrode. It seems promising so far. 

Edited October 28, 2021 by Mod30
Remove excessive quote. Language

[CGun]

I wouldn't count it out just yet. The arc furnace I built LOOKS like it should be complicated, but it is basically just a little $8 microcontroller that I programmed to turn a $22 stepper motor and rod on a homebuilt carriage. The rod moves an electrode holder up and down. The coolant system pumps water through the rod holder (plumbing parts soldered together), through the bottom anode (about $15 of plumbing parts) through a fan driven radiator ($10 eBay) and back into a holding tank ($20 of PVC and hose barbs). Unless you are using a constant voltage source (MIG welder or SMPS, in which case current fluctuates), for electric arcs, shorter arc=lower voltage/higher amperage, long arc=high voltage/low amperage. Since most DC stick welders are constant current, the only thing that varies much is voltage. The trick is to get an arc length that maximizes the Wattage (Voltage x Current). so it is usually somewhere in the middle. Then you program the microcontroller and essentially tell it: "if the voltage is too low, back the rod up x number of steps, if the voltage is too high, turn the rod down x number of steps, if it is in the optimum range don't do anything." Then the microcontroller does this for you hundreds to thousands of times a second through a stepper motor controller ($19 on amazon). Because of this it will start the arc for you since it starts out at open circuit voltage (about 80V for my setup) and then dips to nearly 0V when it makes contact with the metal in the crucible (arc start) then backs up to the proper voltage, drawing the arc out. I have found for steel and arc about 1/2"-3/4" is about optimal as it is stable and isn't super loud but also dumps heat into the pool pretty quickly. As the metal melts, the voltage changes and the microcontroller automatically adjusts for that, maintaining or restarting the arc as necessary.

Mine has a lot of other functions I wanted on it, like the upper and lower limit switches (so the arm doesn't bottom out at the top or bottom of travel), the arc length and step size control knobs, the screen that gives you a real time readout, and the pre-heat function. But none of those are necessary to get the thing to start and maintain an arc automatically. If you already had a welder, I would estimate you could build the rest of it for about $100-$150.

Also, surrounding your crucible with a kaowool/high temp insulating refractory will go a long way to reduce the time it takes to melt anything given less heat loss.

don't give up on it! Once you get it running it is so exciting and then you just get to watch it do its thing and melt your metal for you. 

The other thing you might look into would be an induction furnace. That is a much more complicated (read fancy, expensive electronics) build, but ultimately more efficient and less contamination to the steel. Also much faster if you have a big enough one.