# strange gray sludge



## armymp1 (May 25, 2017)

I'm wondering if anyone can help me out... I put a gold plated PCB in nitric acid to flake the gold off but ended up with gold flakes in a gray sludge. The PCB is an unknown metal plated with gold. After filtering the liquid through a coffee filter I was left with the sludge as well. Any ideas what is may be or how to get the gold flakes out of the sludge?


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## g_axelsson (May 25, 2017)

Any solder on the circuit board? Then it is metastannic acid.

Göran


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## kurtak (May 25, 2017)

What kind of PCB - I ask because without a picture of the CBs I cant say for sure - but - if the CB had solder on them - then most likely the gray sludge is stannic tin

Nitric acid does not "dissolve" tin - however it (nitric) does react with the tin turning it to a white paste (sludge) known as stannic tin

the "white" stannic tin will appear as other colors (gray, purple, black, yellow, green) as a result of other metals being dissolved by the nitric & then being "reduced" to colloids of those metals by the stannic tin

It is for this reason that we "always" FIRST remove the tin with HCl from CBs &/or its components BEFORE treating them with nitric --- other wise you end up with the mess of the stannic tin sludge you now have

Or we cut the gold plated part of the CB off the board so that only the gold plated part (with no solder) is treated in the nitric

You DO NOT want to dissolve the gold foils along with the stannic tin - that will result in at least some gold lost due to gold colloids formed by the stannic tin &/or the fact it is next to imposable to filter/wash all the dissolved gold out of the stannic tin

In order to now get rid of the stannic tin you need to start with washing out as much as possible the nitric trapped in the sludge - then you need to roast/incinerate the sludge (with the gold foils) to convert the stannic tin to tin oxide which can then be dissolved in HCl --- you may have to repeat this process (wash acid out - roast/incinerate - treat with HCl) two or more times to get rid of all the tin

During roasting "toxic" fumes are going to be produced so it is important that this is done with some kind of GOOD fume control (fume hood &/or fan to draw fume away) --- DO NOT count on a respirator mask protecting you - even a good cartridge respirator does NOT filter out all toxic fumes

Here is a link to info I posted about dealing with stannic tin - please read the whole thread 

:arrow: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=37&t=15324&p=155859&hilit=tin+oxide#p155859

Also - could you please provide some pictures of you sludge mess :!: 

Kurt


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## kernels (May 25, 2017)

Awesome answer Kurt


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## g_axelsson (May 25, 2017)

One small detail...


kurtak said:


> the "white" stannic tin will appear as other colors (gray, purple, black, yellow, green) as a result of other metals being dissolved by the nitric & then being "reduced" to colloids of those metals by the stannic tin


Stannic tin is tin(IV) and doesn't have the power to reduce other metals.

Stannous tin is tin(II) and does have the power to reduce other metals, most notably precious metals as gold, palladium and platinum.

In a redox reaction something is reduced and something is oxidized. Ignoring all the other bits and pieces in the stannous test we get...
3 Sn(II) + 2 Au(III) => 3 Sn(IV) + 2 Au(0)
3*2 + 2*3 => 3*4 + 2*0
Au(0) is metallic gold.

The tin(II) is used up in this reaction, stannous that has gone bad is also Sn(II) that has been oxidized to Sn(IV) and lost it's power to reduce. So metastannic acid shouldn't have any possibility to reduce any gold back into colloids, but it can easily trap gold chloride in it's matrix and mechanically keep it from being washed out.

Incineration into SnO2 makes it possible to dissolve the gold and then wash it from the solids. There is no need to fully dissolve the tin, just to put it in a manageable form. SnO2 is also stannic, Sn(IV), so it doesn't affect the gold in solution.

Best way to deal with it is just as you write, to remove the tin first. Then the problem will never appear.

Disclaimer : After spending an hour digging around in science literature about tin, so high above my head I can't see the top, I'm not so sure I'm right, but this is based on my meager understanding on tin chemistry.
One chapter called "Fundamentals in Tin Chemistry" was 267 pages long and mentioned the name stannous once. :lol: 

Best and most basic text I found so far was a four page "Tin chemistry summary"


> B-Stannic Acid, or Metastannic Acid. The product of the
> action of nitric acid upon tin is a hydrated stannic oxide like the foregoing
> substance, but is not identical with it. It is not easily soluble
> in alkalies. By boiling it with caustic soda, however, and then extracting
> ...



Göran


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## armymp1 (May 25, 2017)

Thanks for all the answers these are the pcb's I am working with


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## armymp1 (May 25, 2017)

After I took those photos I dropped all in HCL. The reaction was almost instantaneous. The acid began to boil and turned dark gray to almost black in color. Fumes were thick like smoke. I poured an equal amount of water to dilute the acid, and slow the reaction since the acid was boiling so vigorously it was about to boil over. The acid calmed to a simmer and the bubbles receded leaving a thick yellow to white residue on the side of the glass bowl I was using. I'm still not sure what the metal core of the PCB is comprised of. There isn't a single drop of any sodder anywhere on the surface. So I'm assuming the core might possibly be tin?


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## Topher_osAUrus (May 25, 2017)

You dropped the whole thing into HCl?
...looks like the board is aluminum?
The way you describe the reaction is making me think yes.


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## armymp1 (May 25, 2017)

Yes I dropped 5 or 6 pieces into HCL after the reaction started boiling into poured an equal volume of water to slow it down. Any idea what the yellow to white residue on the edge of the bowl might be?


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## glorycloud (May 25, 2017)

My friend, you have a lot to learn about the refining process.
You may want to consider reading here and studying before
you proceed any further. 

Not knowing what a PCB is and putting aluminum in HCL makes me wonder
what else you don't understand. You will get your self hurt. For example,
I don't see you wearing nitrile gloves or chemical resistant gloves. 

Not trying to be harsh but you will hurt yourself and others.


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## armymp1 (May 25, 2017)

I have many different pcb's I'm processing I'm not aware of what they all are made of as all are different. As far as putting aluminum in HCL I'm aware of the risk with hydrogen gas released by the reaction I added an equal volume of water to the HCL to slow the reaction and reduce the gas created. I also have gloves but I was only taking photos of the pcb's at the time not processing them. Though you are correct I am relatively new to this and still learning. This forum has answered many of my questions just by reading through the various threads.


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## rickbb (May 25, 2017)

Step 1. Remove the gold plated parts from everything else. No metal backing, brackets, de-solder and remove all components. (Dropping it whole into acid will only make a mess, as you have learned.)

Step 2. Soak the now stripped boards in HCL with some stirring, until all remaining solder is gone.

Step 3. Learn about the "AP" process.

Recovering gold from PCB's is one of, (if not the), easiest process you could learn here. The more you strip and clean and other wise isolate the gold part the easier it is and you will yield higher purity gold.


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## kurtak (May 26, 2017)

armymp1 said:


> Yes I dropped 5 or 6 pieces into HCL after the reaction started boiling into poured an equal volume of water to slow it down. Any idea what the yellow to white residue on the edge of the bowl might be?






> I put a gold plated PCB in nitric acid to flake the gold off but ended up with gold flakes in a gray sludge.



armymp1

I see a pattern here - one that is a "common" mistake among people wanting to get into the recovery of gold from electronic circuit boards (including me when I "first" started)

That mistake is --- "I can just throw it ALL in some acid & the acid will dissolve everything except the gold leaving me with nothing but the gold to filter out

Sounds easy enough - BUT - it doesn't work that way :!: - as you are finding out :!: 

First of all (as you are finding out) different acids react different with different metals 

For example - nitric acid will react to dissolve "some" metals - but wont react "at all" with other metals - & then again it will react with other metals, but not actually dissolve them & in stead change there chemical composition to a complexed salt of the metal (a state of oxidation that is in a solid form)

Example - if those CBs in the picture ( CB on top of the aluminum blocks) are the same ones you "first" put in nitric - & then put in HCl - you learned that the nitric dissolved copper - did nothing to the aluminum - & turned the tin to the sludge you ended up with 

On the other hand - if you had put them (CBs) in the HCL first - it would have dissolved both the tin & the aluminum - but not the copper - AND - if you did not dissolve ALL of the aluminum in the HCl - the aluminum would cause the tin to cement (or precipitate) back out of HCl solution as a fine powder of tin metal --- that is due to where on the reactivity series different metals are & we use that (reactivity series) to recover metals from solutions by using one metal to displace another metal from a solution

The point being - you cant just throw stuff (CBs) into some acid & expect to have nothing but gold left --- doing so will result in nothing other then a BIG mess - &/or you asking - "where did my gold go"

You need to learn what acids are the right acid to target "particular" metals - & as well the right order in which to target the different metals

There is a book available here on the forum (for FREE) by C.M. Hokes that does an excellent job of explaining this along with small experiments you can do to become familiar with acids, metals & reactions

You will find the book for down load at the "bottom" of FrugalRefiners post here
:arrow: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=19798#p201558

Now then - just for the sake of discussion - lets say there was an acid you could just throw the boards in that would dissolve everything & leave you with nothing but the gold - you would NOT want to do that anyway because ------------

1) you would use up a HUGE amount of acid - so much acid that the cost of the acid would be greater then the gold you would recover 

2) AND - you would also produce a HUGE amount of waste --- TOXIC waste that you can not just dump down the drain - "toxic" waste that FIRST needs to be properly treated to dispose of it in a safe manner

That is why we FIRST "harvest" the gold bearing "components" from the CBs before doing any chemical work with them - that way we keep our chemical waste to a minimum - which in turn makes treating the waste for safe disposal much more manageable

Bottom line - there is a LOT for you to learn BEFORE you actually start playing with the chemicals - the good news is you have come to the right place to learn about this - GRF is the BEST source of info you will find anywhere in the world :!: 

So I suggest - for now - put the chemical aside - start doing some research "here" on the forum & when you have questions ask them & we will be glad to help answer them - & in the mean time keep collecting & saving CBs

You may find that the chemical part of this is not for you - but all is not lost because the CBs them selves a worth good money & there is "always" a market for them

And as a final note - the gold plating you see on the boards is not all of the gold CBs have to recover - there is more gold that you do not see - such as bonding wires inside the IC chips

Kurt


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## kurtak (May 26, 2017)

Goran

You just like to pick on me - a - mate :wink: :lol: :mrgreen: 

Just kidding my friend :!: 

Good post - thanks for the "details" :G 

Kurt


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## g_axelsson (Oct 25, 2019)

kurtak said:


> Goran
> 
> You just like to pick on me - a - mate :wink: :lol: :mrgreen:
> 
> ...


Hey Kurt! You have met me in real life now, so you know I'll always fight you whenever I get an opportunity.
I don't know how we both survived all our fights at Jon's place...

Adhering to the rules of this forum, I hope everything is well with you my friend... but you know what I really mean!

:wink: :lol: :mrgreen: 

Good post about making a mess.

Göran


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