My Iron-Melting Furnace

[greentwin]

I made the exterior form in sections, which turned out to be not really necessary.
I cut and glued the sonotube to get the diameter I needed for the exterior tube.

The sonotube has to be waterproofed, else it will absorb the water from the refractory, and collapse.

 

[greentwin]

I decided to cast this "hot face" (hot face being the dense refractory that lines the interior of the furnace) inverted, and monolithically, which I have never seen anyone else do.

The refractory was mixed similar to concrete, and then rammed into the space between the formers in 4" layers, ramming with a 1/2" dowel rod to be sure the space was fully filled. The difference between concrete and refractory is that concrete will expolde if used as a furnace refractory, and you only use a tiny amount of water when mixing refractory, which makes it very difficult to mix.
Adding more than the recommended amount of water to refractory will weaken it, so don't do that.

The hot face turned out well, and while I was at it, I cast a few plinths for the crucible to sit on.

Many use ceramic blanket only when building a furnace.
I wanted the durability of a hot face, and so I used cast refractory (Mizzou), but tried to keep it as light as possible.
This hot face was a good compromise on mass and durability, and Mizzou is known to work well at iron temperatures, with a high resistance to iron slag.

There are followingers in some casting circles who insiste that refractory MUST be vibrated into place.
I disagree with that, and did not use a vibrator.
As you can see, there were only minor defects in this hot face. I was careful to ram each 4" layer with a 1/2" wood dowl rod, to drive out any major air bubbles.
I have seen major defects with folks using vibrated refractory, and I did not want any settling of the refractory aggregate, which can happen if you over-vibrate it.

 

[greentwin]

I cast the tuyere on this furnace, and am not entirely pleased with how this turned out, although it does function well.

The fact is that the tuywere and burner tube (if your burner is operating correctly) operate almost cool to the touch, and so there is really no need for high temperature refractory extending out from the furnace.

If I had to do over, I fould have used soft fire bricks at the tuyere, with the flat part of the bricks resting on the furnace base.
You can drill soft fire bricks with a hole saw, and I would have made a jig to be able to drill the bricks at an angle.

Note that the burner tube should be supported independently of the tuyere.
My tuywere broke away from the furnace, but I patched it without problems.

Also note that the end of the burner tube should not extend into the furnace, else it will overheat and start melting off.

 

[greentwin]

I had a couple of false starts on the lid, and originally cast a flat one.
A flat lid can be problematic, although many commercial furnaces to use that design.

All refractory will crack over time, and that is not a problem, since you can easily patch refractory with high temperature patching compound.
But if the lid refractory cracks, you could potentiall have if fall into the furnace, which would be very bad if you had a crucible full of molten iron in the furnace.

I ended up using Mizzou to cast a domed lid.
The forms were a bit tricky to make, and I was not able to ram the top of the refractory, but the bubbles on the top of the lid refractory are cosmetic, and don't affect the performance of the furnace.

I have since discovered "plastic refractory", which is a refractory that comes in a solid block, and is flexible like stiff putty, and can be formed.
Plastic refractory would make furnace lid construction simple, since you would only need one form, and could just pack an inch of plastic refractory onto it.

This lid turned out to be very functional, and withstands iron temperatures well.
If the refractory cracks, the lid will be self-supporting due to the arch.

I use a short chimney, since that seems to help keep some heat in the furnace, and directs the vented hot gasses upward and away from the casting area.

This lid is 1" thick Mizzou refractory.

 

[greentwin]

I could have used two layers of 1" ceramic blanket on top of the lid, but I wanted some sort of insulating hard surface over the ceramic blanket, to prevent the ceramic fibers from fraying and becoming airboard.
My solution was to cut insulating fire bricks (2,600 F rated) on an angle, and then use a 1" layer of ceramic blanket under them.

This approach actually works quite well, although you could also use a sheet metal cap over two layers of 1" ceramic blanket.

As I mentioned early on, don't inhale any refractory dust or fibers when you cut them.

My angular cuts on the insulating fire bricks were not exact, but I got it done, and made this lid cover in about an hour.

 

[greentwin]

I initially thought I would use a lightweight sheetmetal cover over the base frame, and thus all the angular support members.
I ended up using a steel plate to cover it all, and could have dispensed with much of the framework under the plate.

I used metal wheels, having learned with the first furnace that any plastic or rubber within about 10 feet of an open iron furnace will begin to deteriorate relatively quickly.

 

[greentwin]

I was torn between using two layers of ceramic blanket on the outside of the hot face, or using 2,600 F insulating fire brick.
I went with both; ie: one layer of insulating fire brick, and then two layers of ceramic blanket wrapped around the brick.

I used angle grinder and 4.5" disk to dress the inside of the bricks to give them a round shape for a good fit against the outside of the hot face.
Wear a good commercial dust mask if you do this.

The aluminum pan was discarded, and I used the steel baseplate shown in the previous post.

I used one layer of insulating fire bricks under the hot face, with some ceramic blanket strips.
I wanted the hot face to be fairly rigidly supported, and using a 1" layer of ceramic blanket under it woud cause the blanket to compress, and cause the hot face to settle and perhaps rock around.

The idea behind the soft fire bricks was to provide some level of rigid support behind the hot face.
The stainless steel band that I installed at the top of the hot face was a mistake, and caused the hot face to have excessive cracking (which was repaired; more on that later).
Don't put a band at the top of the hot face.

I also originally intended the lid to notch down into the insulating fire bricks, and that did not work.
The insulating fire bricks should come up flush with the top of the hot face (more on that later also).

 

[greentwin]

This is the lid lifter design I came up with for this furnace.
It allows the lid to rock slightly so that it will seat completely on the top of the furnace without any gaps.

I learned to never support a furnace lid from the refractory (on my first furnace build), and so this lid and vertical support shaft are supported independent of the furnace body.
This arrangement also makes it easy to disassemble the furnace to transport it, and makes it easy to dissable the furnace to work on it, since this is a modular design, and the various components are independent, and not adhered to each other.

The vertical lift shaft sits on a ball bearing, made from a caster wheel.

My latest lid design is a better one than this, and it is just a wrist joint to lean the lid up and back, with a bearing under it for horizontal rotation.
This lid lifter works well, so I will keep it, but the wrist-lift-pivot is a much better design.

 

[greentwin]

It was finally time to fire up the furnace.
I think I dried the hot face for about an hour with low propane heat.

The valve tree was a temporary arrangement that I made so that I could quickly determine what the best fuel flow was.
I could change the fuel flow in increments of 1 gal/hr.
At the time I built this furnace, I was still unsure exactly what fuel flow rate was best for melting iron.

You can see in the first photo how modular the furnace is.

The lid with the large opening was a temporary arrangement, and was replaced with the domed lid with chimney.

 

[greentwin]

I could not find anywhere online an exact concensus about what fuel and combustion air flows produced the hottest furnace interior.
I set up the new furnace with a temporary lid with extra large opening, and ran the burner at night, to better observe how hot it was getting.

I used a PVC valve to control the combustion air flow from the Toro leaf blower, and a valve tree to adjust the diesel fuel flow.

By trial and error, I discovered that a fuel flow of between 2.5 and 2.7 gallons per hour produced the highest furnace temperature, judging temperature by the intensity of the red glow inside the furnace.

The combustion air is adjusted during startup to give about 4 inches of yellow flame out the lid opening, to produce a reducing flame, which creates an slightly rich burn inside the furnace, thus minimizing oxidation of the iron.

Reducing the combustion air flow to the point where no flames come out the lid opening creates an oxidizing burn, which tends to oxidize the iron and create excessive slag on top of the melt.

This was a big deal for me to figure out the correct fuel/air flow rate that would minimize melt times for iron.
I had incorrectly assumed that more fuel would produce a hotter furnace interior, but that turned out to be a false assumption.

What is happening is that the interior of any given furnace can only completely combust a fixed amount of fuel using the opimum amount of combustion air, based on the area of the interior of the furnace.
Using any more or any less fuel and appropriate amount of combustion air will cause the temperature inside the furnace to drop.

The idea is to reach pour temperature, which is about 2,400-2,500 F, in as short a period of time as possible.
Typical time to pour with this furnace, using a #10 crucible, is about 1 hour.

I don't have an immersion iron pyrometer to measure the iron temperature, but I can generally tell when I reach pour temperature, because small sparks begin to fly out of the melt.


View attachment TEST-NEW-FURNACE-01.mp4