you can use nitric acid,

aqua regia or fuming sulfuric acid. These are very corrosive substances and should be used

only when required. Chromic acid, commercially available as Chromerge® cleaning solution,

dissolved in concentrated sulfuring acid is also a very powerful cleaner. However these are highly toxic to the environment and pose a severe

waste disposal problem. So my recommendation is to use nitric acid and u can sonicate the PTFE in ultrasonic bath fo 25-30 minute.

You can try Piranha solution.

Dear Luke,

If I were in your place, I would not try to find a solution for cleaning them. I suggest you to use new ones.

I was growing Fe-sulfides in teflon liners which turned them black after every use... found no other good solution as to thin the walls on the lathe after every change of the recipe. 1 mm seems to work for me, and I recycled them up to 10 times (depends on the original thickness of the walls).

Thank you all for your valuable input. 

Actually, Akrema Aki, I am one of few people here who still use piranha solution, and do so extremely carefully. Have you actually used this for the cleaning of PTFE, or is this just  a general solution? I, personally believe that we should be less reluctant to use piranha sol'n,  but unless there is a good case...

Janez, I was thinking of a similar thing. I had tried to physically remove layers using abrasive paper, but got nowhere.

...I think I have an idea, and I will keep you all posted on how it goes...

Maryam, thank you for your idea. I was thinking of this also.

However, I would think that all of the labs who are experienced in this kind of synthesis would not be replacing very expensive PTFE liners every, say, three to five syntheses. This would make such a project financially untenable. Do you have experience in hydrothermal zeolite synthesis? If so, I would like to hear more of how you have dealt with the issue.

you can use nitric acid,

aqua regia or fuming sulfuric acid. These are very corrosive substances and should be used

only when required. Chromic acid, commercially available as Chromerge® cleaning solution,

dissolved in concentrated sulfuring acid is also a very powerful cleaner. However these are highly toxic to the environment and pose a severe

waste disposal problem. So my recommendation is to use nitric acid and u can sonicate the PTFE in ultrasonic bath fo 25-30 minute.

A bit of a long shot, and probably a little crazy on it's face, but could the stubbornness of the contamination have anything to do with static build up? Using abrasive materials (presumably dry sand paper), could lead to more build up.

If this is the case, probably some anti-static tools and/or blowing ionized air (anti static device) maybe the solution... an expensive fix though...

Hello there!

Did you ever come up with a solution to removing the contamination from the PTFE liner? I have just started my PhD, using autoclaves to hydrothermally synthesis minerals, including zeolites, and I too am struggling to remove the contamination. I currently use diluted nitric acid, which helps a little bit, but does not remove all of the contamination.

Hi Victoria.

I've learned a little bit about this after working with the guys at JH Institute, Prague. They wash their PTFE liners with HF. This is pretty much the only acid that will remove silica properly. Don't worry about the colour - the liner will always go brown/grey over time - the key is to remove traces of previous syntheses.

Hope this helps. All the best.

Thank you very much Luke, this helps a lot

Hi Victoria, 

The color change comes from other ions diffusing into the teflon. If you heat between 1-4M nitric in the autoclave around 180C it will remove the ions (and color!) sometimes it takes a couple treatments but the lining should return to white or mostly white. Afterwards heat water in the autoclaves until the pH returns to ~7 after heating and you're ready to go again. 

Hope this helps. 

Dear Luke,

Autoclaves are perfectly adapted to zeolite synthesis and your will of cleaning the liners is more than respectable. BUT:

1. Piranha is for organic pollutants (ozonide, molozonide, etc). So it won't remove any metallic pollutant, except by electrochemical reaction if the potential with some manganese is appropriate.

2. Aqua regia is ideal for all metallic pollution but should be used under fume cupboard as the reactive agent produced is a gaseous species, dissolved in the liquid phase.

Check it but it should be ok to use it in a closed vessel at room temperature (no risk of explosion) for long time. This way you would be exonerated of of the onerous and dangerous heating phase if you let it overnight closed under a fume cupboard. After that, you rinse it with deionised water several times (3 times with one hour resident time, high temperature) and it should be ok.

3. Sulfochromic or chromic reagents WERE efficient BUT ARE forbidden in European Union (yeah, Brexit has not happened yet and, more likely, shall never happen for real) for cleaning lab use (to the best of my knowledge) and are really toxic for human. So please don't use them ;-) , even in Newcastle (I survived in Man-CHAS-TA =) ! )

You have the greetings from a bloody French chemist and let us know if you solved your issue

Most of the times in the cases that the PTFE liners have became discolored (grey) is because the linear has adsorbed metal anions.

As to remove the residues metals, except of the Aqua Regia which does very good job (personally I use it a lot), another acid is 2 - 3% of EDTA (Ethylenediamine tetracetic acid) which does also effective work. Prepare the solution in a quite large pyrex beaker and soak the liner for overnight.

In my case, I use a concentrated sodium hydroxide and heated it up for 2-3 hours at 110-120 oC, depend on the contaminated compound. You can search the reference about Ksp of the metal hydroxide which is the final product after cleaning.

I use Hydrofluoric acid (HF) solution diluted in water (HF: H2O = 1:1). Just add HF solution in PTFE liners, and soak it for 2 hrs or more. But please be careful, because HF is very dangerous solution.

I have used conc.HNO3 to clean the PTFE of hydrothermall autoclave.Put conc HNO3 Overnight or add 1-2 ml 0f con HNO3 in the vessel tightly seal it and keep it in the oven for 2-3 hours.Dont add excess Conc HNO3 IT MAY LEAD TO EXPLOSION.This technique will work.