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Author: Subject: Metal melting propane furnace construction
axehandle
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[*] posted on 5-5-2004 at 12:07
Metal melting propane furnace construction


<b>I just had one of those once-a-year ideas.</b>

Imagine dental casting cement (phosphate bonded cement). It's retains most of its strength when dehydrated and the shrinkage is only about 0.5% -- and it's good for casting gold, which means it can easily handle a propane flame.

Then image mixing it with perlite (a volcanic mineral made up of mostly air), the second-to-none-except-vermiculite-or vaccuum insulator.

Guess who's going to buy a 25kg sack of phosphate bonded cement tomorrow?

I'll use this thread for documenting my new furnace construction process.

[Edited on 5-12-2004 by Polverone]




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[*] posted on 5-5-2004 at 22:09


According to a random webpage, expanded perlite softens at 871-1093°C.

Introducing closed pore minerals or air bubbles into the cement sounds like a good way to make your kiln explode when you fire it.

Whats wrong with ordinary firebrick?
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[*] posted on 6-5-2004 at 01:23


Quote:
Originally posted by Marvin
According to a random webpage, expanded perlite softens at 871-1093°C.


Pearlite seems like a good choice, I'm sure the phosphate cement matrix will hold the soft beads in place and they'll reharden.

As for the vermiculite, I don't know about the insulation properties of the stuff, but it'll absorb water like nobody's business..

In masonry walls pearlite was used, cheap like borsht and repelled water like the anus of a duck.




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[*] posted on 6-5-2004 at 05:57
Explode!?


Quote:

Introducing closed pore minerals or air bubbles into the cement sounds like a good way to make your kiln explode when you fire it.

That's funny, my previous furnace has been operated for months without exploding.... besides, the cement is permeable to gasses when dehydrated. It's used for casting jewelry and dental pieces.

Ooooooooh..... I really should get a car.... I just carried a 23kg floppy sack of phosphate bonded cement home..... my back.... my arms.... and wait! I haven't eaten today! Perhaps I should do that.

Edit1: Then it's off to IKEA to buy that lidded bucket thingy.... I feel like I'm going to faint.

Edit2:
Quote:

Whats wrong with ordinary firebrick?

Nothing, but I want a circular furnace intersection. Btw, some firebricks actually contain perlite, I read it somewhere yesterday.

[Edited on 2004-5-6 by axehandle]

[Edited on 2004-5-6 by axehandle]




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[*] posted on 6-5-2004 at 08:35


What does your old furnace look like? If you use brick I assume you just stacked bricks into a box. If so, what did you use to insulate the joints?

I built a cheapo box for $12 from the brick shop, and surrounded it on all sides with quarter inch steel plate. I was contemplating making a new one that would heat better. This sounds interesting. keep us updated.




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[*] posted on 6-5-2004 at 09:53
My old furnace


It looked like this:


The outer shell is a mild steel waste bin. The refractory was a mix I developed myself incorporating ideas from many others. Basically it's fireproof cement, bentonite and perlite. The burner is stuck in through a tangentially aligned steel pipe at the bottom of the inside hollow cylinder.

The new one will use almost the same design, but it will be equipped with a hinged lid, it will be larger (much larger), and I will use a different refractory composition. I'm going to use this investment cement: http://www.srs-ltd.co.uk/invest.html
together with perlite, the perlite being packed together maximally. Mostly air, so to speak.

I'll use the same home-built burner, it's documented here: http://species8472.dyndns.org/burner/burner.html




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[*] posted on 6-5-2004 at 10:26


what is wrong with the present refractory?
perlite + fire proof cement which itself could withstand atleast 1200 C?

/rickard
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[*] posted on 6-5-2004 at 10:32


Quote:

what is wrong with the present refractory?
perlite + fire proof cement which itself could withstand atleast 1200 C?


It takes a week to cure and dry, and is expensive (fire proof cement is actually quite expensive in large quantities). Also, it develops cracks after months of (ab)use. Finally, it's a horrible mess to blend by hand.

The new refractory will only need 90 minutes to cure, and then it just has to be dehydrated. And I believe it can withstand temperatures up to 2000C, someone told me something about casting platinum using it (as a mold).

Edit: NO, i stand corrected. It can only take 1400 or so... but that's enough for my purposes.

Edit2: To give you all an impression of the size of the soon-to-be furnace (I'm right now experimenting with mixing the cement with water-soaked perlite), I present this picture. I am about 1.75m tall and quite sturdy, weighing in at about 82kg. It sure is one big bucket!



[Edited on 2004-5-6 by axehandle]

[Edited on 2004-5-6 by axehandle]




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[*] posted on 6-5-2004 at 14:23
Vibrator.


To cast this, I'll need a vibrator that can shake the bucket when it weighs about 30kg. Any ideas? I'm in need of some help, my creativeness quota for today is exceeded....



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[*] posted on 6-5-2004 at 14:32


Take a large motor and put something on the turn shaft that is unevenly weighted, it will vibrate at high speed, experiment with different speeds and weight dispersions for the best results. Heavily wound tape works well for fine tuning.



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[*] posted on 6-5-2004 at 15:48


Quote:

Take a large motor and put something on the turn shaft that is unevenly weighted, it will vibrate at high speed, experiment with different speeds and weight dispersions for the best results. Heavily wound tape works well for fine tuning.

Thank you. That will be easy.




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[*] posted on 7-5-2004 at 02:28
Some success.


The hard thing is finding the right water ratio, since soaked perlite oversaturates the cement mix with water. But I think I'm on the right track with batch #5:


Edit: I'm off for the weekend now. No updates until Monday...

[Edited on 2004-5-7 by axehandle]




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thumbdown.gif posted on 9-5-2004 at 13:44
OK, conclusions.


1) The dehydrated mix of phosphate cement and perlite was the best refractory material I've ever held in my hand. A block 15mm thick could be put on a 2500W hotplate at full blast for 40 minutes and still feel only lukewarm at it's top.

2) The same material breaks when a fly lands on it, or when the tide shifts.

Conclusion: It can only be used if protected by a layer of another material preserving its structural integrity. Such as, e.g., a fireclay chimney pipe in a metal bucket, refractory cast into the "outer shell".

But I'm not going to bother. I'll use the standard: Portland cement, bentonite, perlite and water. Perhaps a little bit of sand as well.

Lesson learned: Bright ideas are often not.




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thumbup.gif posted on 12-5-2004 at 06:48
OK


The last needed part for the new furnace is in my hands: A 15 litre thin, tall bucket (actually a plastic trashbin) to be used as the inside mold.

Expect a Sturmangriff of pictures tonight.

Step 1) Here is the bentonite, mixed with water. Actually it's a water suspension of kitty litter (mostly bentonite). It looks like someone has had a severe case of some stomach illness....


Step 2) The outer shell and its lid. Here is a panorama image of the bucket with its now hinged lid:


And here is a closeup of the lid. Those 18 angle irons will hold the lids refractory block in place. To cast it, I'll simply surround the circle of irons with a plastic strip and pour (with the lid horisontal, upside down). There's already a PP pipe in the middle to form the top vent.


Another closeup of the lid, hinges temporarily removed so I can cast the refractory for both furnace and lid simultaneously:


Here it is, the whole bucket with its lid closed:


I have to take a break now, my back is hurting like hell (yes, I have a very bad back).

[Edited on 2004-5-12 by axehandle]




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[*] posted on 12-5-2004 at 15:12
Phew!


All done. But a word of advise: don't mix concrete by hand:


This is really gonna hurt tomorrow....

Edit: It's dripping pus and blood.... seems I got a really nasty alkaline burn from the calcium hydroxide in the cement....

[Edited on 2004-5-13 by axehandle]




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[*] posted on 12-5-2004 at 18:19
caustic burns


Your total dedication to your projects is astounding! You have replaced vulture as my idol.

But seriously, I'm sorry to hear of your caustic burns. Take good care to allow healing as well as possible.

When a young man I was helping to poor a concrete slab. I was wearing rubber boots but the wet concrete worked its way in between my legs, just below the knees (in front), and the boots. As a result of the abrasion/caustic I had permanent scars that didn't disappear for decades.
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[*] posted on 12-5-2004 at 18:23


Thank you. BromicAcid suggested vinegar, but since I didn't have any I mixed a VERY dilute HCl (aq) solution and dipped my hands in that. That, followed by thorough cleaning and applying an acidic hand lotion have helped, it seems. The pus has almost stopped coming and it doesn't hurt as much anymore.



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[*] posted on 12-5-2004 at 22:40


while working on with my quest to find out more and more about thermit welding... I found out that the crucibles used to withstand the heat of the reactions of Fe3O4+Al (apparently this combination burns at 3 times the temp that Fe203+Al burns at) used Magnesite(MgCO3). (shelled by regular steel)

I also read in an old Metallurgy book from 1914, that dolomite (MgCa(CO3)2) was used in furnaces to retain heat required to smelt steel.

My geology friend(GF) says that neither is especialy difficult to find/extract if you know what to look for, however you may need to find a way to bind the dolomite particles together) I dont know why I didnt ask my GF this before about refractory materials... duh.




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[*] posted on 12-5-2004 at 22:46
bucket of doom


on another note... What is that bucket made out of? rather... thats not galvanized is it?



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[*] posted on 13-5-2004 at 02:14


It's galvanized alright... but the heat will never be in contact with the galv. steel, there's refractory between them ---- I know about the toxic fumes.



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[*] posted on 13-5-2004 at 07:45


heh thats what I said about my 2nd furnace. Zinc Oxide would fume off the sides over a long period of time.

What do you think about using Magnesite? I will try to do this *adds another project to his warboard*




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[*] posted on 13-5-2004 at 09:18


My old furnace was galvanized too. No zinc oxide there.... my refractory is pretty good.

I'm wondering how dangerous it really is, after all it's used in white paint. It's probably the risk of silicosis that's the danger, which could be averted by wearing breating protection...

Or, one could simply sand off the zinc, or remove it with HCl.




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[*] posted on 13-5-2004 at 09:48


Remove it with NaOH. This will take the zinc away but wont attack the iron so bad as HCl does. If you want to use HCl, use it in diluted form - 5% to 10%.
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[*] posted on 24-5-2004 at 09:14
ho, ho, ho...


My hand looks as if it will be healed in about a week. I'm testing my (presumably failed) refractory mix now. There was a bit of a.... eeeehhh... backdraft..... so I'll have to cut my beard a bit short. The fuel/air explosion took some of it away, which was annoying but not painful.



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[*] posted on 24-5-2004 at 13:10
I'll be damned.


The new furnace seems to hold together. The refractory hasn't cracked yet (I'm drying it out at full blast). If only I knew what the evil fumes coming from it are. They irritate my eyes but don't seem to affect the lungs. It's probably water and perlite dust, perhaps with some ZnO added in for comic relief....

We'll see, I'm beginning to feel hope again...

Please note that I wouldn't advice anyone to repeat my experiment. My lungs are my own, I'm only risking my own health here.

If the whole thing works, I'll get a propane refill tomorrow or the next day, then it's FINALLY casting time. If I could only find a decent crucible (i.e. cast iron, not thinwalled stainless)....

Edit: Out of propane. The refractory seems fine. I'll have to let it cool down slowly, the propane didn't last long enough for all those pesky water moleculles to be driven out. More to come.

On crucibles, I have a request for comments. You see, I have this old 80mm ID, 90mm OD cast iron old sewage pipe which I found at a construction site. What if I cut off a piece, say 250mm long, and construct this:


It should work, but I feel I've missed something really obvious?

[Edited on 2004-5-24 by axehandle]




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