Sciencemadness Discussion Board

High temperature Retort is made from ... ?

metalresearcher - 16-9-2014 at 09:08

For high-temp experiments (e.g. making Phosphorus) I want to use a retort. Three years ago I tried with mild steel tubes heated in a propane flame to 1200C which burned out with a leak in it and releasing the newly formed Na or K vapor which burned again.
The temperature itself is not an issue. With propane+air or even natural gas+air I can reach 1500C.
So I though about stainless steel (up to 1200 C) but still that burns out in a few times. I have heard about Chrome steel which withstands until 1200C for more times.
When going to 1500C more exotic metals / alloys are required (Mo, W ?) or even Rh, Ir but these I cannot afford .....
Ceramics such as quartz or Al2O3 will be attacked by molten salts (Ca3(PO4)2 + SiO2 + C for making P or Na2CO3 + C for making Na which vapor also attacks quartz).
The only possibility is using narrow graphite-clay crucibles of which the bottom is heated till 1500C and the top should be sealed which is another issue (glueing with softened glass ?).

How do they do it in industry ? Possible beyond reach of amateurs ?



[Edited on 2014-9-16 by metalresearcher]

halogen - 16-9-2014 at 09:48

Glassy carbon, industrially, is made by very slowly heating a phenolic (thermostting) polymer in a vacuum. The volatile components of hydrogen and oxygen are driven out by the extreme heat, leaving a shrunken object. You might, however, try to make an inferior product by using inferior methods. More reasonably, I believe they extract chucks of carbon out of the ground, jet for instance can be used in jewelry, you might look for larger specimins of a finer grade like anthractite or coal, and carve out your apparatus from it.

careysub - 16-9-2014 at 10:39

Quote: Originally posted by metalresearcher  
...
So I though about stainless steel (up to 1200 C) but still that burns out in a few times. I have heard about Chrome steel which withstands until 1200C for more times.
When going to 1500C more exotic metals / alloys are required (Mo, W ?) or even Rh, Ir but these I cannot afford .....
Ceramics such as quartz or Al2O3 will be attacked by molten salts (Ca3(PO4)2 + SiO2 + C for making P or Na2CO3 + C for making Na which vapor also attacks quartz). ...
[Edited on 2014-9-16 by metalresearcher]


I think it is a mistake to plan on pushing high temperature limits AND do it with highly corrosive, difficult to process, materials. Optimizing for one or the other is probably easier (i.e. less expensive).

Nickel alloys are used for molten sodium hydroxide and carbonate. Examples are Alloy 600 Inconel nickel-chromium, and Alloy 400 Monel nickel-copper.

http://www.nickelinstitute.org/~/Media/Files/TechnicalLitera...

They are good up to around 1000C.

One foot of 3/4" OD Alloy 400 tubing form MacMaster-Carr costs $45, $54 for Alloy 600.

How about machined graphite crucibles?
http://www.graphitestore.com/items_list.asp/action/prod/prd_...

And then there are the magnesia crucibles from Ozark Technical Ceramics, smaller ones aren't terribly expensive. These go to 2200C, and handle many very corrosive materials:
http://ozarktech.com/otc-product/mgo-crucibles/

You can probably contact them to ask specific questions about resistance to molten salts.

Dan Vizine - 5-10-2014 at 11:42

Industrial preparation of P relies on ceramic materials to lead the product vapors away from the reactor which is heated by a huge sets of carbon electrodes which draw an arc between them. The reactor walls are not directly heated from the outside as you want to do. External heating puts incredibly high requirements on the material of construction because it has to simultaneously:

a) be a reasonable conductor of heat

b) withstand the corrosive conditions within (at temperature)

Most ceramics can't satisfy (a) very well and no metal will satify (b) for too long. The ceramics that do (a) really well are expensive (SiC, hafnia, beryllia, and a few others). Since any metal you choose won't last long, it has to be a ceramic. Since the ceramics that have high thermal conductivity are all expensive, realistically speaking, the choice has to be a reasonably priced ceramic. You just have to tolerate the heating inefficiency. I'm afraid that you are probably going to have to resign yourself to periodic repair/rebuild episodes. Industry does with their high temperature equipment.

WGTR - 5-10-2014 at 15:45

You could try doing the reaction under a vacuum.

I haven't gotten around to trying this again (and posting pictures), but last time I performed the metaphosphate/silica/aluminum reaction (as shown in the stickied phosphorous thread) it was in a borosilicate test tube. I'm not sure how good the vacuum was, but I was pulling it down with a Fisher two stage rotary vane pump.

The advantage of using a vacuum is that the phosphorous is distilled out at a much lower temperature than would otherwise be needed. The heat required, however, was still high enough that the borosilicate tube was collapsing in places.

If the reaction tube is contained within a larger containment vessel that is also under a vacuum, this prevents the reaction vessel from collapsing. It also keeps oxygen away from the hot reaction tube. The outer vessel could be submerged in water to keep it cool. Heating, obviously, would have to be electric.

This is a setup that I intend to try at some point, but haven't yet.

The products of my small test tube experiment, however, condensed in the cooler parts of the tube. Letting a little oxygen into the cooled tube gave a brilliant flash of light, and then an unhealthy glow of vapors as the gradual oxidation commenced. It was a fun experiment, but just a curiosity.

Dan Vizine - 8-10-2014 at 10:42

So, you're saying that the reason phosphorus furnaces are run so hot is because of volatility issues more than chemistry? I can't reconcile that with the low bp of phosphorus, it's below 300 C.

Maybe its just a mass transport issue? It seems reasonable that a gentle argon purge might accomplish that. It's simpler than vacuum.

WGTR - 9-10-2014 at 08:43

Quote: Originally posted by Dan Vizine  
So, you're saying that the reason phosphorus furnaces are run so hot is because of volatility issues more than chemistry? I can't reconcile that with the low bp of phosphorus, it's below 300 C.

Maybe its just a mass transport issue? It seems reasonable that a gentle argon purge might accomplish that. It's simpler than vacuum.


Well, in the case of the industrial method, carbon is used to reduce a phosphate.

https://www.sciencemadness.org/whisper/viewthread.php?tid=65...

From reports that I have seen on SM, this requires "hell fire" temperatures to work. I haven't tried it, personally.

This reaction here is the one that I prefer:

6NaPO3 + 10Al + 3Si02 = 3Na2Si03 + 5Al203 + 3P2

Polverone initially posted it here:

https://www.sciencemadness.org/whisper/viewthread.php?tid=65...

It's a lower temperature reaction than the former one, but I couldn't seem to get it to work in a test tube until I performed it under vacuum. I'm not sure that I understand why (and I am not well-educated), however, there is a detail that might make a difference.

When there is no gas inside the reaction tube, convection losses are greatly diminished, leaving losses by radiation to predominate, as well as a little from evaporation of the phosphorous. If there is any appreciable thermal resistance in the reactants, perhaps the heat is better distributed in the mixture when under a vacuum.

One advantage of using a vacuum, is there is no gas to carry heat up the tube. Glass is a relatively horrible conductor of heat by itself. The phosphorous boils out of the reaction, and condenses on the tube a couple of inches higher, where the tube can be kept cool.

If the reaction tube is electrically heated in a vacuum chamber, then other materials can be used for the reactor that would ordinarily be oxidized by air. Also, if the outer chamber is polished (or silvered, like a dewar), then radiation losses aren't too high.

The last time I tried making phosphorous, I didn't have my small kiln. I was heating things with a propane torch. If I can find the time I'll try making a small batch again. The kiln is temperature controlled, so I can see exactly when things start happening.