Dissolving stainless in dilute sulfuric acid

[sklatham69]

Anybody here know what would result if you put stainless steel in a dilute sulfuric acid solution? I'm thinking of processing some metallic materials from spent catalytic converters. Seems pretty easy from what I saw... but I'm new to chemistry. I'm an electric engineer by trade.

 

[kenvlach]

Please give the particular stainless alloy, temperature & concentration of the sulfuric acid, impurities in the sulfuric, and whether the sulfuric is air-agitated or otherwise contains oxygen or if oxygen-free.

If necessary, applying a protective anodic current will enable most stainless to resist the full range of composition of sulfuric acid solutions, 0-98%.

 

[moltenmetal]

If your plan is to go after the platinum and palladium in the converters, count on getting the monoliths out of their cases first. Anything which dissolves those metals will eat normal grades of stainless steels for breakfast.

 

[sklatham69]

Well, the plan is to use 10% or so sulfuric in an outdoor tank. The tank has a lid with a vent to take off fumes. I'm not sure what concentration is needed. I guess the idea is to dissolve the stainless and keep the platinum and other metals solid. Then put them in solution and then get them precipitate them out.

We're on step one right now. Just not sure if he's blowing smoke or not.

 

[sklatham69]

So, are you saying we could pass current through a solution and we place the stainless in the solution it won't dissolve?

That's pretty cool. So... if we used the mixture of acids that dissolve platinum and put current through the solution would the stainless stay solid?

 

[nbucska]

Wouldn't KCN be more practical to dissolve the Pt and leave
the SS behind ?



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[sklatham69]

KCN sounds very difficult to handle...

 

[kenvlach]

You can disssolve the SS in sulfuric given enough heat (try boiling) and enough time (maybe months, depending upon SS grade & thickness).

Something I looked up in a non-ferrous metallurgy book for another forum:
"The wet chemistry & pyrometallurgy of assaying and refining the platinum group metals involves strong acids, including aqua regia, and thermal decompositions giving off toxic gases such as SO₃. Just a hint: Nitric acid will dissolve only Pd and the less noble impurities, aqua regia will dissolve both Pd & Pt, while the residue after aqua regia leaching can be fused in molten anhydrous sodium bisulfate ca. 315 ^oC to form rhodium sulfate. Solubilize, filter, precipitate, dissolve, precipitate...themal decompose. Several such purifications give finely divided metal powders which can then be melted by electron beam in vacuum. So, skip the chemistry -- you might take it apart, but not be able to put it back together (in separate parts)."

http://www.finishing.com/439/83.shtml

Process flow diagram (simplified schematic) for refining precious metal ores:

http://www.implats.co.za/i/imp_flow_diagram.gif

Question: The catalytic converter shell is SS, but is the body (monolith) ceramic or SS?

 

[sklatham69]

The body is SS. That is the issue...

Thanks for the info. I'll update the thread as things progress.

 

[moltenmetal]

Why use acid to dissolve stainless steel and generate waste acid contaminated with iron, nickel and chromium?

Why not simply CUT, with an angle grinder/zip cutter, the case of each catalytic converter, open the cases with a prybar, take out the ceramic monoliths, SELL the scrap stainless steel, and then leach the precious metals off the monoliths?!

 

[sklatham69]

The insides are stainless steel... These are not ceramic monoliths.

 

[witchdoc]

Mechanical brushing may be an option. Automation of the process may be possible depending upon the grid structure.

A number of years back, we were involved in the process design for coating of an early generation of the metallic substrate. However, the process resulted in some coating being applied to the external surface of the can. We recovered this material with an automated brushing machine. It worked very well.

Grit blasting will also remove the PM.

 

[moltenmetal]

OK- I was confused by kenvlach's reference to the monolith/catalyst support as a "body". It's been a while since I've had one of these apart. So they've switched to stainless catalyst supports now? That sucks! Not only will this cause problems with the PM recovery, but they'd be a lot harder to punch through with a rod when they get plugged than the ceramic ones were!

If you've got all the time in the world, you might get away with room (outdoor) temperature dilute sulphuric acid, periodically replenished. Add a little chloride to help. Aerate it perhaps? Someone with more time might look up corrosion rates vs concentration and advise you- I'm used to looking at the problem the other way around!

I can't imagine an easy way to electrochemically assist that corrosion given your geometry, but you might be able to come up with something if you give it enough thought.

Have you considered room temperature hydrochloric or perhaps ferric chloride instead? Would probably get rid of the stainless faster, and doubt it would dissolve much of the PM content if you left it at low temperature.

 

[ballvalveguy]

We use Alloy 20 on our ball valves for sulphuric. It works from concentrated to dilute up to about 200F. It is not that much more expensive than 316 stainless steel.