undissolved pins, undissolved flys, brownnish green liquid.

[martymcfly]

Hi,

I have found in my work area a beaker with some ceramic cpus that i must have started well over a year ago but not got back to. the liquid is greenish brown and has undissolved pins in it and some sludge, It also has some unlucky dead flys floating on top. Im unsure if ive just run out of nitric during the process or if my acids have gone off from sitting so long.

The reason i think they may have gone off is i would have thought anything organic would have been dissolved completely and they have been sitting open for over a year maybe 2. If thats not the case i can maybe just add a few more drops of nitric to finish off where i left last time.

Oh, Solution also tested negative with stannous which further confused me, unless all the gold had precipitated out onto the undissolved pins.

Any ideas on this?

thanks

 

[4metals]

You didn't say what acids you used other than nitric. If nitric is all you used, the gold wouldn't have dissolved into a nitric solution, just the base metals would dissolve.

If you did use aqua regia, it is entirely possible the gold dissolved in the beginning but the base metals in the pieces in acid dissolved as well and cemented out the gold.

You need to give details as to what acids you used and quantities and how much scrap was put in there to start.

What amazes me is if you had this uncovered beaker sitting there for however long why everything in your work area hasn't rusted away!

 

[martymcfly]

It was in my fume hood in my shed where everything is pretty rusty anyway.

It is aqua regia and i usually add my nitric in increments but this has been sitting so long i cant remember what exactly is in it, i just swirled it around and saw a few ceramic cpus and some undissolved pins.

Ive actually gone and added a few drops of nitric and added a little heat to see if anymore dissolves by the morning but im still thinking maybe my hcl has gone off, if thats even possible.

 

[4metals]

Years ago I did huge digestions of hybrid circuit thick film ceramics. We added aqua regia to drums of the stuff in a large titanium reactor. After reacting until the reaction died, we decanted all of the spent acid, which had no values in it, (we always tested to be sure by AA in the lab) and added more acid to whatever sludge and undissolved was left.

This technique eliminated a filtration process on 500 gallons of acid with tin in it so would have filtered unbelievably slow. The second digestion always had the gold dissolved and the tin did not present much filtration issues after the first dissolve.

Essentially it is possible you did the same thing on a small scale over a long time frame.

 

[Geo]

I think that the fact that you still have undissolved metals means that both acids has been depleted. HCl will dissolve iron over a long period of time. Some gold dissolved in the initial dissolution and cemented back out on the undissolved metal so your sludge contains fine cemented gold powder. I would suggest that you add fresh HCl to what you have and heat. This will put as much of the oxide into solution as the acid can hold. Repeat this step until no more of the sludge will dissolve. Proceed with the AR dissolution from here.

 

[4metals]

If he has dissolved a good portion of base metals and cemented out the gold what is the benefit in re-solvating everything? I would test, to be sure, decant as much as possible, and then start with fresh acid.

 

[Barren Realms 007]

I'd have to agree with 4metals on this one.

If you are only dealing with a small amount of solution, say 100ml-1000ml Geo is right as well.

That point just depends on what you are use to working with and what you feel safe working with.

If you have checked and the solution is barren of PM's then either siphon the solution off being careful not to stir the sediment then re-constitue the chip's with HCL and proceed with your HCL. Or pour off the existing barren HCL solution and let it set for a couple of days and then siphon this off leaving any sediment and wash the sediment back into your chip's and proceed with dissolving the material again.

 

[Geo]

I apologize for not making myself clear. I was talking about the solids only, the solution is spent and can be sent to waste disposal. Sorry about that.

 

[martymcfly]

Thanks guys, all input is appreciated. I will decant it tomorrow and restart the a/r process on the remaining materials.