Crucible failure nightmare

[markymark]

So today I was melting some gold in a forced-air propane furnace that I built myself. During the melt, a hole formed through the graphite crucible and most of the gold spilled to the bottom of the furnace. I didn't realize this until the pour. During the pour, the rest of the gold spilled out the hole onto my driveway. :evil:

After everything cooled down, I gathered every piece of matter in and around the area using a fine toothed brush. There were quite a few tiny gold pebbles scattered around.

The furnace had a firebrick floor topped with a thin layer of inswool that was covered in satanite. I removed this the inswool-satanite layer in order to recover dropped pieces of gold.

I started off with 1072 grams of mostly 14k scrap. So far, by manually picking through the mess, I was able to gather 1059 grams of it back. The rest of it is in a large mixture of crushed slag, satanite, rock, pieces of inswool,dirt, graphite, and other debris.

I'm thinking that I should try to recover gold from the debris mixture by crushing everything to a fine grain and then panning it out. As for how to crush and pan the mixture, I'm not really sure. Do any of you have any ideas about how I should go about doing this? The satanite breaks apart really easily. I'm thinking I should put it into a metal box and pound everything with a hammer. Then, I can put it into my rock tumbler along with some .45 caliber lead balls, and let it tumble for a few days. After that, use a gold pan. There are YouTube videos on how to pan out the gold.

Now my questions are about melt loss. I'm concerned that some of my gold has gone to oblivion. From my experience in melting my own gold, I've never had much melt loss, never more than .10 %. Is it possible that a significant amount of my losses are attributable to the vaporization of impurities that were in the scrap, such as zinc? I removed all the removable stones and clasp springs. There's no way that there were 12 grams of junk in my lot. I'm not sure if there's much gold in my debris mixture.

It doesn't get much hotter than 2500 degrees F inside my crucible, but I'm sure it gets way hotter on the floor of the furnace, and this is where most of the gold had spilled. It probably gets well above 3000 degrees there. The gold was sitting down there for quite a while. Perhaps I won't have much melt loss when I re-melt and take the gold to my refiner. (There's always a melt loss at the refiner. I'm wondering if that's beacuse they heat it up hotter than I do.)

 

[Palladium]

Wow! Don't feel bad you're not the first one this has happened to. I just chiseled a few lbs of silver out of my furnace a couple of weeks ago. The lessons we learn. :mrgreen:

My question is why are you melting it if you have a refiner? Are you trying to take a sample to assay? Are we talking Karat or gold filled? It has to be gold filled or that would be a few thousand dollars that hit the floor, ouch! Normally i would say just chemically run it and the trash will be left over for later recovery, but being 14k and it's probably in blobs that have silver so it will need to be melted and inquarted so it needs to be melted again. Once melted the slag can be scrapped off the top and then you can ball mill that to recover any trapped values in the slag making the overall recovery from your mess easier. Or........ You can just send it to me! :twisted:

What kind of crucibles are your using?

 

[kurtak]

Then, I can put it into my rock tumbler along with some .45 caliber lead balls, and let it tumble for a few days.

Do not do that --- you will end up with lead smeared on the surface of any gold in the mix as well as bits of lead coming off the balls contaminating the whole lot

go to your local scrap yard &/or if there is a local electrical motor repair shop & see if they have any LARGE scrap bearings laying around that you can buy cheap as scrap

cut the outside race of the bearing with an angle grinder so you can break out the balls

or order some milling balls from Legend --- they have milling balls from 1/4in to 2in which they sell by the pound (ether hard steel alloy - or stainless steel)

Legend - milling balls :arrow: http://www.lmine.com/mm5/merchant.mvc? --- Edit; - I see the link takes you to their home page - so type "balls" in their search box

picture is example of scrap bearing --- knock the bearing out of the housing (big hammer) then cut outside race with angle grinder - balls fall out

Kurt

 

[4metals]

Did you use a crucible rest on the bottom of the furnace? A common problem causing crucible failure in a gas furnace is either no crucible rest or one that is too short. A crucible rest allows the flame to swirl beneath the crucible and not hit the crucible wall as it shoots out of the gas jet. That hot spot created by the forced flame hitting the crucible wall is usually the site of the crucible failure.

I realize this is not going to help you with your metal recovery issue, but it may help prevent another.

 

[markymark]

My question is why are you melting it if you have a refiner? Are you trying to take a sample to assay? Are we talking Karat or gold filled? It has to be gold filled or that would be a few thousand dollars that hit the floor, ouch!

It wasn't gold-filled. It was 14k, the real deal. Just imagine the look on my face when I was pouring and nothing went into the mold. I was like "Where did my 25 grand go??!!!" :x

But anyway, I got everything re-melted and ready to take to my refiner tomorrow. The reason I started melting is so that I can take samples and know what I've got, rather than selling it blind.

What kind of crucibles are your using?

The one that failed was one of those 2kg crucibles that come with Hardin electric furnaces (Hardin furnaces are junk btw.)
For the re-melt, I used an 8kg capacity clay-graphite crucible that I ordered from ebay-China. With this crucible, a lot gold got stuck in the pot. I had to scrape it out. I guess I'll throw that stuff into the ball-mill along with the rest of my mixture. And I suppose that when this crucible reaches the end of its life, I can crush and ball-mill the entire crucible itself.

Here's the link to see it.

http://www.ebay.com/itm/271890530879?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Do not do that --- you will end up with lead smeared on the surface of any gold in the mix as well as bits of lead coming off the balls contaminating the whole lot

Thanks for the tip! On eBay there is a plethora of steel ball bearings selling for very cheap. I can get a set of three 1" balls for just $5, shipped. What size balls should I choose? The cylinder of my rock-tumbler is 4.5" in diameter and 9" in length.

Did you use a crucible rest on the bottom of the furnace?

The crucible that failed hung suspended from the lid, so it wasn't touching the floor. But thanks for the tip. When I re-melted today, I used a crucible on the floor with no rest.


Another question I have is this. With the large crucibles like my 8kg one from China, what's a safe way of extracting them out of the furnace? Currently, I've been using two Triplett TT-200 LongLocker pliers to pull out. Doing it this way requires me to stand almost directly over the furnace, and my furnace gets way too hot to do this. Today, I didn't get burned, but my hands and forehead feel like I've been bathing in the sun all day. I could have easily gotten burned though. (Yes, I was wearing kiln gloves. )

The reason I built my furnace so that it gets super super hot is so that I can do copper castings. The next time I get some free time, I am going to build a vacuum-chambered pouring device. That is, I'll melt the copper in my gas-furnace, with the bath covered in crushed graphite, then do what I need to do to de-oxidize, i.e. plunging lithium. Then, I'll move the crucible into the pouring device, cover, suck out all the air, introduce nitrogen, and finally pour dry into a mold coated with aluminum oxide. But this is story for another day. I'll be back on here to talk about all the details when that time comes.

 

[GotTheBug]

Just a .02 worth on crucibles and dishes from China, don't. I think it's definitely worth the extra little money to buy better quality tools, especially considering the added learning curve you are undergoing right now. My preference for crucibles is Jet's Tools out in California (818.509.1314). Any failures I've had with their products was solely my fault, and the people are excellent to deal with. He sent me one of the chinese crucibles to test, and it ended up with a needle hole right in the side, like it had a gap when it was manufactured. Again, just my .02.
Paul.

 

[Palladium]

Salamander crucibles period for me.

 

[4metals]

With this crucible, a lot gold got stuck in the pot. I had to scrape it out.

One word here, flux! Here is a good thread from the library for you to read about fluxing and recovering gold from your slags. http://goldrefiningforum.com/phpBB3/viewtopic.php?f=85&t=22135

Tongs for crucibles can be had from these guys. http://smallfoundrysupply.com/store/index.php?cPath=32_68

And buy a good crucible! 25 grand in the crucible deserves something you can count on!

 

[nickvc]

And buy a good crucible! 25 grand in the crucible deserves something you can count on!

:roll: :shock: :lol:

 

[Lou]

What 4metals said^

Get a silicon carbide crucible.

Clay graphite is prone to such failures as the graphite burns out of the clay matrix.

Lou

 

[Anonymous]

Just a .02 worth on crucibles and dishes from China, don't. Paul.

Hi Paul

Have to disagree on the little melting crucibles from China if I may. Cheap, effective, and never had an issue with them yet. For larger stuff I would probably agree with you but for stuff that will melt up to 2oz of gold they are fine.



Jon

 

[etack]

To melt my silver I use a http://www.Backyardmetalcasting.com kit and 90g one-shot crucibleshttp://www.Lmine.com. I remove it from the furnace it a tool made after the http://www.foundry101.com tool. My handle is longer and the part the curves around the crucible is made from bar stock so it can adjust and it gives it a better grip.

I cam do 80+TOz at a time in it. I've had some failures and it was always to putting the crucible too close to the motion of the flame.


Eric

 

[kurtak]

Did you use a crucible rest on the bottom of the furnace? A common problem causing crucible failure in a gas furnace is either no crucible rest or one that is too short. A crucible rest allows the flame to swirl beneath the crucible and not hit the crucible wall as it shoots out of the gas jet. That hot spot created by the forced flame hitting the crucible wall is usually the site of the crucible failure.

4metals - OK - I have done quit a bit of smelting in the last couple years & have been doing it without a crucible rest & so far have not had a problem with crucible failure --- so guess I have been lucky so far probably because in part I use Morgan supper A salamanders

I have always known that a rest/stand should be used but can't seem to find them anywhere - they don't list them on the Morgan web site nor does Legend Mining or Action Mining list them & I have looked on Ebay & other foundry supply sites (guess maybe I need to do some actual calling - I would think these companies have them - just not listing them on the web sites - or I am blind)

Anyway - this next week I am planning on doing some major smelting in my big furnace with a (new) #20 super A salamander - it will be an all day project with multiple charges so is going to be a lot of metal ($$$) & I don't want to take a chance on crucible failure - so were do I get a rest/stand?

Or - can I make my own using the fire brick used for lining wood burning stoves (pictured) by cutting it round - & if so should it also have a hole cut out of the center?

Another option would be to cut the bottom off (2 - 3 inches) of another crucible & set it up side down on the furnace floor --- I have a #40 budget salamander that has already been used quit a lot for incinerating IC chip in (so has already seen quit a bit of heating) as well as a #30 budget salamander that has chips (& possible cracks) (it came as a freebee with my furnace) so I don't mind sacrificing ether of these by cutting the bottom off to make a stand/rest (& save a few $)

Last question - is it possible that with the salamander A crucible that a stand/rest is actually not needed? --- due to the fact that the flame is not making a direct hit on the side wall (due the taper of the up side down A shape) compared to a crucible with straighter walls? (& at least in part why I have not experienced crucible failure)

Kurt

 

[Justincase]

Did you use a crucible rest on the bottom of the furnace? A common problem causing crucible failure in a gas furnace is either no crucible rest or one that is too short. A crucible rest allows the flame to swirl beneath the crucible and not hit the crucible wall as it shoots out of the gas jet. That hot spot created by the forced flame hitting the crucible wall is usually the site of the crucible failure.

4metals - OK - I have done quit a bit of smelting in the last couple years & have been doing it without a crucible rest & so far have not had a problem with crucible failure --- so guess I have been lucky so far probably because in part I use Morgan supper A salamanders

I have always known that a rest/stand should be used but can't seem to find them anywhere - they don't list them on the Morgan web site nor does Legend Mining or Action Mining list them & I have looked on Ebay & other foundry supply sites (guess maybe I need to do some actual calling - I would think these companies have them - just not listing them on the web sites - or I am blind)

Anyway - this next week I am planning on doing some major smelting in my big furnace with a (new) #20 super A salamander - it will be an all day project with multiple charges so is going to be a lot of metal ($$$) & I don't want to take a chance on crucible failure - so were do I get a rest/stand?

Or - can I make my own using the fire brick used for lining wood burning stoves (pictured) by cutting it round - & if so should it also have a hole cut out of the center?

Another option would be to cut the bottom off (2 - 3 inches) of another crucible & set it up side down on the furnace floor --- I have a #40 budget salamander that has already been used quit a lot for incinerating IC chip in (so has already seen quit a bit of heating) as well as a #30 budget salamander that has chips (& possible cracks) (it came as a freebee with my furnace) so I don't mind sacrificing ether of these by cutting the bottom off to make a stand/rest (& save a few $)

Last question - is it possible that with the salamander A crucible that a stand/rest is actually not needed? --- due to the fact that the flame is not making a direct hit on the side wall (due the taper of the up side down A shape) compared to a crucible with straighter walls? (& at least in part why I have not experienced crucible failure)

Kurt

I just have bag of refractory cement and cast my own as needed.
You can make it any size or height you need.
In the bottom of my furnace I have a little dimple in the center.This matches a small nob I cast on the bottom of the piece to keep it held in the center..
It is important to have a raised floor in your furnace as it give's the jet of hot gasses form that they maintain up and around the outside wall.
This way it heat's the refractory material properly ,it is this refractory material slowly heating up and retaining energy that give's us the high temperatures we need in our furnace.
The little addition will save fuel and time.

 

[Palladium]

I don't have a whole in the bottom of my little furnace and have always just sit the crucible on the floor. I to use the salamander crucibles. The only time i have had a problem with failure is when the flame contacts the side of the crucible. The side that the flame contacts will physically wear away from the inside out where the flame contacts the side quicker than it will elsewhere in the crucible. It's a hot spot. I've seen the same thing happen in the commercial furnaces i use to design and build. I use to sit my crucible on a inverted pyroceram dish. With a piece of refractory under it it's just more efficient as far as i can tell. You can use refractory to make one or you can find an insulating fire brick or bricks and use them. Make sure it's insulating and not just a fire brick. I use a different furnace design for silver now so no more crucibles! Which by the way silver seems to be getting harder and harder to find from certain sources right now.

 

[4metals]

In addition to hot spots on the crucible, which will shorten any crucibles life, a crucible rest raises the crucible up so the heat that surrounds the bottom of the crucible is as hot as the top of the crucible.

Using a crucible on the floor of the furnace results in the flame being above the bottom of the crucible and can cause, and I have seen this with gas melters, the bottom of the crucible to not melt completely while the top is molten. This can lead to losses as the bar is not always homogeneous so it is not sampleable with a high degree of accuracy available from a completely molten and well stirred charge in a furnace.

I don't think I have ever bought a crucible rest, I have always made them from a cardboard ring which acts as a form that conveniently burns up on the first melt and refractory cement. I always make about a half dozen so they can air dry completely, which isn't always quickly. I prefer to use a crucible rest that is about 1" in diameter less than the base diameter of the crucible I am using, so I make a few different sizes. Usually one for a #16 crucible and one for a # 8 will suffice.

 

[4metals]

Another option would be to cut the bottom off (2 - 3 inches) of another crucible & set it up side down on the furnace floor -

Usually crucibles have a glazed coating that can get sticky at melt temperatures. Using an old crucible bottom would likely cause the rest to stick to the crucible which makes for an unpleasant surprise when you lift the crucible to pour.

I always keep a small container of bone ash near the melter which is great to soak up and prevent any flux spills from making everything stick together. A caution, label the bone ash and note it is a refractory material never to be used in flux preparation. I have seen it mistaken for soda ash and it made a God awful mess of a melt. Label it, big letters!

 

[Barren Realms 007]

Another option would be to cut the bottom off (2 - 3 inches) of another crucible & set it up side down on the furnace floor -

Usually crucibles have a glazed coating that can get sticky at melt temperatures. Using an old crucible bottom would likely cause the rest to stick to the crucible which makes for an unpleasant surprise when you lift the crucible to pour.

I always keep a small container of bone ash near the melter which is great to soak up and prevent any flux spills from making everything stick together. A caution, label the bone ash and note it is a refractory material never to be used in flux preparation. I have seen it mistaken for soda ash and it made a God awful mess of a melt. Label it, big letters!

I'm sorry but the mental picture of someone making that mistake is just so funny even though I know it isn't when it is done.

 

[kurtak]

Just wanted to say thanks for the input guys :!:

Kurt

 

[upcyclist]

The reason I built my furnace so that it gets super super hot is so that I can do copper castings. The next time I get some free time, I am going to build a vacuum-chambered pouring device. That is, I'll melt the copper in my gas-furnace, with the bath covered in crushed graphite, then do what I need to do to de-oxidize, i.e. plunging lithium. Then, I'll move the crucible into the pouring device, cover, suck out all the air, introduce nitrogen, and finally pour dry into a mold coated with aluminum oxide. But this is story for another day. I'll be back on here to talk about all the details when that time comes.

Can you at least turn down your airflow so it doesn't get to crucible-wrecking temps for just gold?

Personally, I just use an acetylene/air torch, but I'm primarily a jewelry caster, so I'm measuring gold or silver only in the tens of grams (and almost always less than 2 ozt.). But the upside of the hand torch is that I can move the flame around and heat evenly, thus avoiding hotspots and temp differentials. But that won't fly for the kilogram amounts you're dealing with.