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Thread: Melting aluminium
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6th August 2009, 02:17 AM #16
hydrogen explosion
I have worked at the smelter down the road a bit from here and during inductions they talked about the possibility of explosions... cue video of said explosions... filmed under controlled conditions, two possible types of explosion, one steam and one hydrogen, from what I remember what they worry about is a sealed can/container of something ending up in a molten bath of Alu in a furnace. the sealed container can be anything that has moisture in it... baked beans, vegemite, soup, etc etc, rust is another thing they dont like.
If the can of stuff gets chucked in and stays on the surface of the molten metal no real drama, just melts and gets burnt up steam given off, however if the can of stuff sinks it becomes a bomb due to xpansion of steam, 1700 times seems to ring a bell, it remains a steam explosion if all the steam can get away to the atmosphere, it does this by emptying a furnace of the metal, 1 to 2 tonne of molten alu, woulndt want to be standing at an open furnace door when that happens, if all the steam can't get away then u have a hydrogen explosion and thats a whole lot more serious.
I dont know the chemistry or the physics of this reaction but based on the demo in the vid and the small amount of molten metal required, the second explosion was far more violent and I wouldnt want to be anywhere near if a can of stuff got chucked in to a furnace.
Peter
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6th August 2009 02:17 AM # ADSGoogle Adsense Advertisement
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6th August 2009, 03:17 AM #17SENIOR MEMBER
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damn thats cheap, I paid $4kg for some aluminium plate not long ago, 25mm thick. 70 odd kg. from the recycling yard.
I was thinking atleast double that for ingots.
I haven't had time for quotes but ill get some tommorow been flat out with my business.
How does it seperate a liquid into hydrogen, I would think any explosion is a rapid expansion of gases.
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6th August 2009, 11:04 PM #18
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7th August 2009, 12:25 AM #19
Alu explosion
Like I previously said I'm not sure on the chemistry/physics of it others will know more than me, however from what I think I understand, the liquid (in the can of stuff) first changes to water vapour/steam and yes rapid expansion of gas, if the steam has not reached the air but is still trapped in the molten bath of alu due to the depth of alu, then chemical reaction thingy happens and the hydrogen component of the steam is free to ignite and burn and probably aided by the now free oxygen.
The chemists will be able to explain it
Peter.
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7th August 2009, 01:00 AM #20
Those who want to read up on the chemistry can look at this paper http://www1.eere.energy.gov/hydrogen...r_hydrogen.pdf
briefly, molten aluminium can dissociate water into hydrogen (the oxegen bonds with the Allie to form aluminium oxide) and the process has been seriously examined as a means of generating hydrogenregards from Alberta, Canada
ian
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7th August 2009, 11:30 AM #21SENIOR MEMBER
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Cool paper.
Note that the this paper uses an alloy of aluminium and lithium or aluminium and gallium to produce hydrogen, not the aluminium silicon alloys that are being made in smelters. I would say the danger with water in a smelter is that aluminium is smelted by running an electric current thorough it, and this would definately split water into hydrogen and oxygen. This process is easy enough to do at home with a battery charger at room temp. However, at the casting stage there is no longer a current running through the aluminium and the potential for it to disociate water into Hydrogen and Oxygen is nil.
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7th August 2009, 12:21 PM #22
splitting water is easier that many would think.....it is nothing more than school boy science (boy scout science actualy).......if you have a hot enough surface and spray water onto it just right it will bypass or rapidly trasit thru the vapour stage and split into hydrogen and oxygen.......this is one of the problems with oil fires that have a substantial metal mass involved in them.....the steel typicaly will get and retain enough heat to split the water sprayed onto the fire and superfuel it.
I remember the process being detailed in a book on fire I read as a kid....the practical use is high temp incineration.
on a slightly different note
some large aluminium facilities completly ban glass bottles from the sites......what I understand is that the problem is that they can find their way into the recycling bins and then into the furnaces.....and that they may contain water and may cause an explosion in the furnace.
anyway back to the original point.
yep its a bigger casting job than it first seems..........in the past ( in the foundry age)... casting metal was a realy attracive way of doing lits of stuff.....thats why arround a certain time lots of stuff was cast.......these days there are so many other options......casting is only done where there are no more economical and practical methods.
seriouly it would be easier to fabricate it out of rolled products and machine it.
cheersAny thing with sharp teeth eats meat.
Most powertools have sharp teeth.
People are made of meat.
Abrasives can be just as dangerous as a blade.....and 10 times more painfull.
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7th August 2009, 02:11 PM #23seriouly it would be easier to fabricate it out of rolled products and machine it.
1/2" or 5/8" steel plate of that size would probably cost a few hundred, and machining finished fabticated item would surely not cost that much.
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7th August 2009, 05:11 PM #24Scattered woodworker
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Hi Guys,
I did a bit of engineering work at Alcoa for 3 years.
This is the process at Alcoa Point Henry......
Smelting the bauxite into aluminium is done by electrical current.
It is then syphoned from the smelting pot, taken to the pot room furnaces, (they hold 50 tonne molten aluminium), alloys are then added to the molten aluminium. The alloyed aluminium is cast into 8 tonne ingots for rolling or into 250 kg sow's for others to remelt and add further allows as the used in miltary and aircraft industries.
All aluminium cans are banned from Alcoa sites. Plastic bottles are ok.
The aluminium can was the most common type of explosion in furnaces before cans where banned.
That's partly why all of Alcoa's can recycling is done at Yennora in NSW. The have a specific process that can elimate liquid entrapments.
The worst explosion comes from the water being trapped during the casting of the 8 tonne ingots. This is a water cooled "drop cast process".
Water becomes trapped and the steam becomes superheated, temps excede 900 degrees and boom.
I hope this helps.
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7th August 2009, 05:50 PM #25
A few years back I caught a couple of kids stealing lead flashing from one of my sites.
They were cutting it up and inserting it into alluminium cans before crushing the cans to get weighed at the recycler.
Kids will always come up with a scam.
I wonder what the lead does to the whole smelting process??
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7th August 2009, 08:53 PM #26Of course truth is stranger than fiction.
Fiction has to make sense. - Mark Twain
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8th August 2009, 01:08 AM #27
Very interesting, I'm learning something,
The process Boyne Smelter... alumina is loaded into the potline cells and smelted by passing electrical current thru the pot, in goes white powder out comes molten alu, it is then transferred to the casthouse where it is cast into either molds for ingots or billets in a vertical drop casting method, both these casting methods use water for cooling so possibly only a steam explosion could occour here but on the potline a steam and or hydrogen explosion could occour because of the electrical current. Does this sound right?
Was it 1.2volts dc or 12 volts dc anyway low volts but very high current must be a high resistance process. The magnetic field around the busbars would pull your foot towards it because of the steel in your boots, it would drag tools across the tray of the ute as u drove past, it would sound any vehicle dash alrms in the utes, it would stall some vehicles so u needed a run up to get over the busbars in some places.
Peter
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9th August 2009, 11:57 PM #28
Brendan
this is not the forum to debate the physical chemistry of the reaction of water and molten aluminium.
Surfice it to say the reaction occurs easily with pure aluminium and water, but in the "normal" world the reaction does not occur because the aluminium is covered by a layer of Aluminium Oxide. Molten Al doesn't have this protective layer so can readily split water. From what I remember of my chemestry, the addition of lithium or gallium to Al, as described in the paper, is most probably about preventing the formation of a protective layer of Al oxide.
BTW This is not an experiment to try at homeregards from Alberta, Canada
ian
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10th August 2009, 12:16 AM #29SENIOR MEMBER
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I bought a flat sheet of alum 1200x1200 x 12mm thick for a base ,just screw some angle to stiffen up and you have it ?? Cheers bob
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11th August 2009, 08:45 PM #30Senior Member
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So did I mate ! Machining Al can be a big problem if it is too soft . It gums up your cutters and smears al around stoping the clean cutting action .
Melting scrap could result in an unknown machinability . It could be good or it could be bad.
Grades like 6061 are good to machine with carbide inserts and HSS tooling . Giving nice surface finish .
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