Problems with Thiosulfate and goldleaching of e-waste

[future gold refiner]

I have want to develop a harmless leaching solution based on thiosulfate.
(...maybe some of you remember "thio based solution with e-waste @ large scale" post)

after leaching my card´s and board should look like "CARD 1" picture but 2/3 of my leached material looks like
"CARD 2" and "CARD 3" -----> any idea why this happends ???
...by the way the same phenomenon at golden fingers !!





It is a closed 60 L glass reaktor with 5 kg board´s and cards´s and approximately 35 L thiosulfate with ammonia and copper sulfate.
The solution is heated / the pH is controlled by a pH-meter and a dosing pump / the solution is pumped (38 L /min) / and oxygen comes from a aquarium pump

I tried several batches (5 kg) at different temperatures from 1 to 4 days. - same result !!!
after 3 days (@ 35°C) the gold content in solution drops down because all of the thiosulfate is destroyed ( determined by AAS analysis ).

at 35 °C most of the card look like picture 1 after 24 hours but then nothing happends
at the next 48 hours the gold content rises but only a little little little bit ( determined by AAS analysis )

at 55 °C ( my maximum ) some cards look like picture 1 after 3 hours but then -> same effect !

the copper content rises all over the time ( determined by AAS analysis ) -> and hinders the leaching but this explains not why it leaches partially.

I place a 486 CPU in my solution for visual controll and thy are completely gold free [I thought this layers are thicker then "card-gold-layers" ???]

please HELLLLP me :-D -> ask me for more informations ( I stopped writing because the length of the post ^^)

(did anyone have experience with iron as catlyst ??)
(excuse my english i am german)

 

[g_axelsson]

I think you are experiencing some kind of galvanic protection. Some metal that is electrically connected with the golden finger works as a sacrificial anode. (http://en.wikipedia.org/wiki/Cathodic_protection)

Try cutting the copper layer between the finger and the rest of the card and see if the protective effect disappear.
If you selectively removes components and cuts wires you could probably find which component or metal that is the culprit.

/Göran

 

[Lino1406]

1. It is irrational to soak all the board, recommended to
separate the fingers and soak them separately - that would
augment extraction (also may reduce anodic pprotection)
2. The gold plating which remains is simply of higher quality

 

[future gold refiner]

....i tried some test´s with an organic gold leaching system and the same problem appears - what does suggest to me that it is really a kind of galvanic protection. ( ... said better it confirms me in the faith. )

...now i am play with the thought to crush all the circuit board´s. ( to cut the circuit path´s)
maybe the finger´s are connected among themselves through the circuit path in the complete board´s ( what g_axelsson and Lino1406 suggested)

...did anyone leaches complete PCB´s wiht cyanide and had the same problems ?? ( can´t test with cyanide )

....it is not possible / practical to cut off all the fingers from tonns (!) of PCB´s.

 

[future gold refiner]

what speaks also for a galvanic protection are these photos. (fresh out of my reactor )

- because the organic solution attacks only gold but look at these strange ramifications at the solder connections !!





...it seems they protect my gold to dissolve and go into my solution. -> and become to my profit :lol:

 

[future gold refiner]

I cut completely unleached fingers from the card´s an placed them in the same solution where they placed befor for 65 h
---->and look the effect is not loger present

after an other 20 h they are completely gold free, that is the verification (hope the is the right word) that it is/was galvanic protection.