HCl turned blue/green then brown/yellow! What happened here?

[Syn]

So im quite new working with acids besides for testing purposes and i still have a lot to learn it seems, heres my latest test result bringing up some questions:

I put a small sample of pcb traces (gold over nickel over copper with tiny amounts of pcb left underneath) into diluted HCl - after 2 days of appearantly no reaction i added more Water and waited another day to find some white powder laying on the ground and the solution turning light cyan color!

Could it be from the pcb leftovers? Yes it must i thought but then..

.. i waited one more day and the color became more dense switching from a light blue to a more greenish hue. At this time the confusion rised and i added NaHCO3 and water to neutralize the solution. It took some minutes but the solution now turned yellow/brownish.
I filtered the solution and there was a yellow sludge (could it be NiCl2+Au?) left in the filter while the solution kept its color without change.
The base material is almost completely stripped of all Au, only parts with a defective Ni layer still hold gold (Au over/in Cu) - so that part of my little test worked..

BUT.
Where is the gold?
Is it inside the sludge or in the filtered solution?
What could the sludge be composed of?

Thanks for any help in advance!

 

[Syn]

I guess the biggest mistake i made was neutralization before filtering, gonna learn from that.

 

[nickvc]

The color change can be and probably is the metals dissolving , the blue green color is probably from the copper starting to dissolve.
Yes you shouldn’t have neutralized your solution but the good news is that any values should still be in your filter, you have made a mess but it can be recovered , you need to study more and experiment less until you know what to expect and what to do, it’s all here on the forum you just have to dedicate the time to studying.

 

[g_axelsson]

Why neutralize at all? I only do that when I treat waste liquids, never for anything with values.

Göran

 

[Syn]

@nickvc
Well i looked up the reaction but did not expect what happened here and i still don't. Copper should not react at all unless the solution was contaminated somehow?!

@g_axelsson
I did not forget about the stock pot but it takes time to plan all this, i am an collector at first and do not have a safe storage place yet for such stuff, i am not planning to go trough the whole processing, i will just see how it goes and may decide else

 

[nickvc]

Theoretically Hcl can’t dissolve copper but in real life given a little chloride or even oxygen it will, look up AP here on the forum, real life chemistry and theoretical are two different things :shock:

 

[Syn]

Which doesn't make it easier! okay back to reading for now :lol:

 

[nickvc]

Which doesn't make it easier! okay back to reading for now :lol:

Agreed but knowing what to expect and why will so yes back to reading and ask questions if you don’t or can’t understand any points raised in the threads.

 

[Syn]

Let me take a chance;

When i further diluted the acid, the reaction of HCL with water createad a perfect space to corrode the copper which resulted in the cyan/blue hue, the green color finally came out from the nickel overwhelming the copper reaction but it took its time as expected.

^
Correct path so far? (Hard to not dilute at one point with active metals in solution, gonna try to find a solution for this next, so far i only came up with not diluting but more filtering and reusing of solution)

But i am still trying to figure out what happened after i added the baking soda :?: (not that it matters for the recovery process, but it does matter for me somehow :arrow

 

[ChemGeek]

@Syn,
I don't expect you to get far with your approach.
Lets say, you have a pcb board or "fingers".
There is a plastic base and on it a layer of copper, then about 2-3 microns of nickel and then about 0.5 - 1 micron of gold.

You treat it with HCl (hydrochloric acid). If gold layer is perfect, reaction will not proceed at all.
Usually it is not, so it does proceed but you must understand that HCl alone is rather crap acid for dissolving Nickel or Copper. You need to add some oxidant. Air will do, but very slowly and incompletely, so forget it.
It is common to add hydrogen peroxide as oxidant in HCl based approach, but beware that too much of peroxide and you will start dissolving gold as well.
You should *not* neutralize anything before filtering off your gold bearing solid stuff (gold "foils" can be seen floating there).

 

[nickvc]

I have to disagree with your assessment of AP, it works for what most members need which is a simple fairly fail safe process that allows members to recover their gold for later refining.
Adding too much oxidant like peroxide causes more problems than it solves as it will dissolve gold freely and then cement it back out but in a form many would not recognize as gold whereas by using an air bubbler alone will free the foils as an obvious gold piece, time is rarely a problem for many as the quantities they work with are fairly small so a good recovery is far more important than speed so the standard bubbler and Hcl are an excellent choice allowing memebers to see their gold and to know they have recovered virtually all of it.

 

[Syn]

After the sludge dried i dissolved it in acetic acid and filtered it, and washed it twice with water and hcl.

The amount of waste created is way too high tho, next time i will be better prepared, thanks everyone.