tin in AP

[Geo]

i need some opinions. tin in AP is a problem that anyone that uses the process knows first hand. ive been thinking about ways to remove the tin from AP after its saturated.there seems to be no good or efficient way to remove it. so after spending a few days (and nights) pondering on it, i came to this conclusion, and this is where i need help with.

we make stannous chloride to test for PM's.its a well known fact that stannous chloride has a short shelf life as it decomposes fairly rapidly. tin in AP is basically stannous chloride in the solution. if the solution was allowed to sit open to the air for a couple of weeks or more, would the stannous chloride in the solution decompose and leave the solution or just transition into some other troublesome compound.

i understand the best way is to avoid getting the tin in the AP to start with, but heres my particular problem. i have 500+ pounds of depopulated milspec boards that i sheared the components from. most of them contain a fair amount of gold in the form of pins that was sheared on the component side but are still 1-2 cm in length. i also have around 50 gallons of copper chloride solution from AP before i knew i could re-use the same solution.all of it was saturated with tin before i made a fresh batch. if the tin has decomposed, i would like to run these boards in 55 gallon drums with all the solder still in place.

 

[ericrm]

i dont know how much it will be for you...
when you electrolyse ap one of the first metal i was gottent was a dark gray/black spongy layer,only once that metal was removed from solution that i would have the reddish spongy layer.

my guess always have been that the more reactive metal get plated first .electrolysing cucl will make chlorine in your ap but maybe you can evaporate it or leave it on the side
this is just a guess but could be worth the try

 

[Geo]

thanks eric, ill keep that in mind. for right now, im not really wanting to remove the copper from solution (although thats exactly what i will do when im finished with it). what im trying to figure out is if the tin somehow miracle'd its way out of my solution or is it still there in some other form besides stannous chloride.

 

[maynman1751]

Hey Geo! Why don't you try a small test batch and see what results you get? John.

 

[ericrm]

Geo i didnt mean to remove copper but only the tin... as soon as you see the reddish sponge you remove your cathode and you would have ap with only copper in it.......
im also very interested in your post i hope someone know the answer

 

[philddreamer]

Eric, what do you use for an anode?

Phil

 

[ericrm]

it was graphite rod from zinc batterie,i also used the longer one sold tru ebay with the same result

 

[Marcel]

I guess I am not competent enough to provide you with a satisfying answer, but maybe one observation I made that could be of some use.
I love AP and I dont deal with nitric. Period.
So I have experimented several times with it and together with the values of standard electrode potentials
(http://en.wikipedia.org/wiki/Standard_electrode_potential_%28data_page%29), I worked out a way to eleminate the base metals and speed up my AP process ( I am not doing this as a business, rather experimenting)
So now I don´t use AP in one single step.
I did process some ultrafine high grade pins in a large bucket and it seemed to take forever to dissolve the basemetals.
Now I do it like this:
[*]100g high grade pins all mixed from various sources, so all kinds of basemetals included.
[*]500ml AP heated (around 50-70°C)
[*]a few drops of H2O2 (32%) added to start
[*]Wait until the solution is saturated and the reaction halts. (Solution gets blurry and a white sediment is noticed)
[*]Decant the solution, filter it.
[*]Now I got rid of SnCl and maybe PbCl and maybe FeCl ( It depends on the composition of the material and the amount that is in there)
[*]Next step I fill up with fresh, used AP, 10 Minutes have passed
[*]I add H2O2
[*]The color changes, first goldfoils come off imm.
[*]Once the reaction slows down and the solution gets dark I decant and filter it. I get Cu(I)Cl in my filter, maybe some other nearby bms as well.
[*]Next step I fill up with fresh, used AP, 20 Minutes have passed
[*]I add H2O2
[*]The color changes again, more goldfoils come off imm.
[*]Once the reaction slows down and the solution gets dark I decant and filter it. I get some AgCl in my filter.
... and so on
By doing so I work my way through, eleminating basemetals and AgCl step by step.
This due to the reactivity chart, that tells us, that an acid will always team up with the most reactive metal.
Now if you just dump all your material in one bucket and let it sit, it will take much longer for the process to get to the gold, because the remains of the first disolvings will hinder the reaction. It will therefore require much more acid or AP as well.
The nice part comes when I reach a mostly Cu+Ag level. It goes bang and the foils fall like snow in winter(dont forget my H202 32% and the heat added).
It is the 4th or 5th step. For eleminating copper it may take several steps, because of the mass of copper which is roughly 90% in goldplated pins.
But in exchange the AP works very fast and very precise.
I hope I didnt get something wrong here, if not correct me. After so muc Bla, my suggestion would be to just cover the material with AP and dissolve the lower basemetals such as tin. Decant the AP together with the sludge and then start the "real" AP. There is barely any risk to dissolve any gold, because we have not reached this element in the ladder of the reactivity chart, and with such little amount of AP, this solution will never be able to hold several metals at once.
Would that work, or is it just luck with the material that it has been perfect for me?

 

[Geo]

thanks Marcel, thats very interesting. i copied that so i can try on some high grade pins later. of course for what i want to try, i will be working with at least two drums of boards and AP (or rather copper chloride) so moving that much liquid several times would be more than im willing to do.i have medical nebulizer for the air supply, one for each barrel. i believe the label says 6 psi. i will be stuck with what i have when it is set up. the used AP solution was used to strip whole boards (depopulated with gold traces) but still had the solder in place. it was very saturated with tin. what im hoping is the tin has decomposed over the months.its been stored in buckets with lids in the dry but the lids had vents to keep from building fumes.i decanted one bucket and placed the liquid in plastic jugs. there was quite a bit of black precipitate in the bottom of the bucket that processed out over a gram of gold and some powder left over that wouldnt dissolve in hcl/Cl. i have 6 more buckets to do (they weren't completely full).im calling it my summer project. :lol:

 

[pesco]

Few remarks.
1) if you dilute the solution of SnCl₂
SnCl₂ + H₂O -> Sn(OH)Cl + HCl
Sn(OH)Cl is insoluble so should come down as grayish (I have seen colours from beige to nearly black) residue
As added benefit that extra HCl could do more digesting.

2) if you aerate the solution:

6 SnCl₂ + O₂ + 2 H₂O -> 2 SnCl₄ + 4 Sn(OH)Cl
same salt, Sn(OH)Cl is still insoluble
I don't

3) short lifespan of SnCl₂ is due to its readiness to oxidase to SnCl₄ in presence of oxygen. To prolong our tester lifespan it is advised to keep few fragments of metallic Sn in the vial/bottle. This is a hint. If:
SnCl₄ + Sn -> 2 SnCl₂
then it means that oxidised solution should be able to dissolve more Sn

4) interesting option sth I remember from old long time ago

SnCl₄ + H₂O -> SnO₂ + 4HCl

that would mean you could use solution of SnCl₄ to dissolve more base metals,
don't know details as never tried/tested/used this property


5) just for the sake of it :mrgreen:

SnCl₂ + Na₂CO₃ -> 2 NaCl + SnO + CO₂

SnO precipitates and it should filter nicely, it is not the dreaded (at least it shouldn't be - depends on conditions) hydrated stannous oxide everyone is so scared of :mrgreen:


Geo, run few tests, as usual it is impossible to predict exact results as they depend on what is in your brine :mrgreen:


Regards

 

[Geo]

Few remarks.
1) if you dilute the solution of SnCl₂
SnCl₂ + H₂O -> Sn(OH)Cl + HCl
Sn(OH)Cl is insoluble so should come down as grayish (I have seen colours from beige to nearly black) residue
As added benefit that extra HCl could do more digesting.

2) if you aerate the solution:

6 SnCl₂ + O₂ + 2 H₂O -> 2 SnCl₄ + 4 Sn(OH)Cl
same salt, Sn(OH)Cl is still insoluble
I don't
5) just for the sake of it :mrgreen:

SnCl₂ + Na₂CO₃ -> 2 NaCl + SnO + CO₂

SnO precipitates and it should filter nicely, it is not the dreaded (at least it shouldn't be - depends on conditions) hydrated stannous oxide everyone is so scared of :mrgreen:
Regards

so, its entirely possible that the black precipitate from the solution is mostly tin oxide. the solution was aerated up until i stored it and its still dark emerald green. i know there was a lot of lead chloride, but it cleared out as i was using it because i would add a little sulfuric acid in between batches.i know theres a lot of action left in it because i have used a little of the solution to make a kind of dirty AR to remove non-PM base metal from IC packages.when theres not going to be values to recover, it saves on fresh hcl.

 

[pesco]

Precisely.
You could use digesting powers of the solution to save on acid. When it stops dissolving anything then you could filter precipitate, test sample of it, test the liquid sample and process or get rid of it depending on its content.

I would myself save and play with both, precipitate and liquid, but I am not doing it for profit, I do it for pleasure :mrgreen:

 

[pesco]

i need some opinions. tin in AP is a problem that anyone that uses the process knows first hand. ive been thinking about ways to remove the tin from AP after its saturated.there seems to be no good or efficient way to remove it. so after spending a few days (and nights) pondering on it, i came to this conclusion, and this is where i need help with.

I never encountered Sn problems, but maybe that's because I never dropped pins etc. to AP and always used HCl on its own to dissolve base metals and AP or (HCl + bleach) to dissolve values for refining.

Just out of curiosity, why don't you drop values with Cu and then redissolve precipitate ?

 

[butcher]

Problem with tin in solution along with dissolved values as I see it:

Oxidized = missing electrons (metal dissolved into solution)
Reduced = gained back electrons (metal solution changed back to metal usually as powders)

The tin reduces the (oxidized dissolved gold in solution), so changing it back into a metal powder, problem is tin reduces the gold as colloids, (my guess is because tin does not dissolve well in the acid), these colloids will not precipitate they form charges that repel each other keeping this reduced gold in solution, repelling each other and floating around in solution,

and since this gold is already reduced in solution (gained electrons and changed back to elemental metal powders), it will not show up in the stannous chloride tests (which this test relies on the tin chloride solution to reduce (gain electrons) the oxidized (missing electrons) gold in the test)forming the colorful violet colloids in the test which we see as being a positive for gold in solution, now since the stannous chloride test will not work we cannot tell if we have gold in this solution or not.

Because of tin the solution does not filter well (this filtering problem gets worse the less acidic the solution is, the more dilute the solution is with water, or the colder the solution is), filtering is a problem slow or almost impossible.

Cementing values (using copper or another metal) from this solution also depends on the values being oxidized in solution and the elemental copper giving up its electrons (thus the copper is oxidized and dissolves into solution), and the gold gaining electrons from the copper (thus the gold being reduced back to elemental metal in the form of powders), here again these colloids are already reduced to metal in solution and so that cementing the values from solution will not work.

I do not know if oxidizing the tin in solution from SnCl2 to SnCl4 would be of any help to get the gold colloids to precipitate (I kind of doubt it would, but your guess is as good as mine on this) remember the gold is already elemental (reduced), would precipitating tin from solution do anything to break the gold colloids (I do not know but do doubt it will help).
Although I agree learning more about this would help us to get a better understanding of this problem, and research should be done to learn more.

OK so now we have this problem how can we deal with it?

colloids can be sometime broken with strong heat in a high acidic solution, (concentrating the solution then bringing it to a boil and adding a strong acid can help to break the colloids), if the situation is bad enough sometimes the best option is to concentrate to a syrup and neutralize the acidic syrup with NaOH, rinse the salts water that is formed from the neutralization process and dry the resulting powders and incinerate them (oxidizing the tin and base metals in these powders in the roasting process), then boil these powder in HCl to dissolve out the tin (adding just a little water before decanting these base metals dissolved in the chloride solution, rinsing the valuable powders in boiling water to remove more base metals, making these powders free from the tin and problem we had earlier with colloids from that tin.

Just a note: any time I have a solution that is trouble filtering I suspect tin in solution, and if I can filter some of the thicker colloids, these filters are neutralized, and rinsed, and sent to be incinerated with the next job of incineration,

The reason I neutralize, and rinse a chloride powder before I incinerate it is because gold or silver can be volatile as chlorides in a roast, and I do not want my values going up in the smoke, by neutralizing the chloride powders using a solution of NaOH, we form salt NaCl, which is water soluble and can be rinsed from our powders before we roast them, rinsing this salt is also important because if we had salt and gold in a roast the gold would form gold chlorides and some would go up in smoke, where rinsing out the salts prevents loss of values in our roasting process.
I also believe neutralizing also helps to oxidize or form oxides and hydroxides of these base metals, just assisting us in oxidizing the base metals in our incineration process.

 

[Palladium]

I hate tin!!!!

Here was what happened when i was still learning and didn't respect the tin.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=60&t=11596&hilit=i+got+robbed#p113446