Chemical Using Zinc to deal with Tin Oxide from dirty Copper Chloride etches- experiment writeup

[haveagojoe]

I use Copper Chloride etching on a wide range of source material, including plated connectors, pins (incuding magnetic ones) and all kinds of other plated or flashed escrap. This means my collected foils are often quite heavily contaminated with other metals.

Also I tend to aggitate my etches quite aggressively by shaking, so my foils are often broken down into finely divided black powder. Black powder also forms when gold is dissolved and cemented back out, for example when hydrogen peroxide is added (though I don't do this any more).

Tin oxide is a particularly annoying contaminant to find among my foils- a white precipitant which looks similar to Copper (I) Chloride but will not redissolve with the addition of fresh HCl. It's difficut to separate from the foils, and furthermore it can hydrolyze to form the dreaded metastannic acid, which in water is a jelly-like substance which can trap gold and is very challenging to deal with. Ideally foils should be as free of other metals as possible - particularly tin- prior to dissolving, in order to ensure best purity of the refined gold powder and to avoid complications.

Other contaminants can include Iron and Nickel oxides - which appear as fine black or brown powders (mud), which can be mixed with (and mistaken for) finely divided gold.

The common approach to dealing with tin oxide has been to roast the foils, in order to convert it to soluble tin compounds, and wash again in HCl prior to refining. I have never really been convinced by the effectiveness of this approach and it's a bit of a pain to do.

I came across an alternative approach: to convert Tin Oxide to soluble Tin (ii) Chloride by reduction using Zinc. In a post-etch cleanup of the foils with fresh HCl this allows Tin to be separated from the foils by filtration.

SnO2+Zn+2HCl→SnCl2+ZnO+H2↑

Zinc Oxide is also soluble in HCl, forming Zinc Chloride, so would be in liquid phase and would pass through the filter.

ZnO+2HCl→ZnCl2+H2O

Using Zinc would also have the advantage of reducing Nickel oxide and Iron oxides to soluble Nickel and Iron chlorides:

Fe2O3+Zn+6HCl→2FeCl2+ZnCl2+3H2O

NiO+Zn+2HCl→NiCl2+ZnO+H2↑


I performed an experiment, dissolving the zinc casings of two AA batteries in about 300ml of fresh 10% HCL in a beaker which contained my foils and black powder, and a layer of white tin oxide contaminant.

Zinc powder would be ideal for this approach but it's expensive, and I found that the zinc battery casings were quite adequate and even though I did not cut them into small pieces they dissolved completely within about 5 minutes. The resulting hydrogen produced quite a head of froth as my HCl is store-bought muriatic acid which contains a foaming agent- I was able to knock it down with a spray bottle.

After stirring for a futher 10 minutes the Tin oxide appeared to have been successfully converted to soluble Tin Chloride as no white mud was visible. The solution was very dark brown, almost black, loaded with the suspended black powder - which, since Iron oxides and Nickel oxide should also have been reduced to soluble chlorides, should now consist only of finely divided gold.

I let the solution settle for about an hour, but it was very slow to settle and remained almost black. So I decanted the solution off the layer of settled foils and black powder, through a coffee filter. The tin chloride can still hydrolyze to form metastannic acid so I wanted to separate it from the bulk of the foils as soon as possible. A lot of the suspended particles were too fine to be caught by the filter; the filtrate remained dark so I left it to settle further.

The gold foils had turned pink at this stage- evidently some copper was present had cemented onto them. I added fresh HCl to the beaker of foils, which quickly appeared to redissolve the copper, restoring their normal gold colour.

The filtration will take a long time, and both the filtrate and the beaker of foils will likely take a day or two to settle, but at this stage I am quite confident that this approach has successfully dealt with the Tin oxide, as well as Nickel and Iron oxides. If it proceeds as expected, I intend to go straight to dissolving the foils with fresh HCl and pool chlorine tablets, without the need for roasting.

I will update this thread as the experiment concludes.

 

[orvi]

I never personally dealt with tin oxy-chlorides in my whole refining career while using AR as my primary method. Though I cannot say for sure if this also applies for AP - where tin can be present in lower (II) oxidation state, rather than Sn(IV) as it is in AR solutions.

But my hints:

1. Metastannic acid "pain" is glorified in my opinion. Yes, it is slow to filter with regular methods, but with Buchner, piece of plastic mesh and real filter paper (not coffee filters) - not that problematic anymore. Also, you can advantageously use some filtering aid such as Celite or bentonite.
When I compare metastannic from say nitric leaches with say crushed MLCCs leached disintegrated ceramics - I would rather pick to filter metastannic suspension 10x than once dealing with disintegrated cerams.
Not saying it isn´t beneficial to look for alternatives, but good old filtration may be the best option.

2. If you maintain acidic enviroment, there should not be any nickel or iron hydroxides/oxides/oxohydroxides. When enviroment of leach is deprived of acid - which provide stabilizing counterions to dissolved metal - only possible option is to take the anion from water So top it up with some HCl and you will be good to go.

3. If you want to produce zinc powder, buy zinc oxide. Prepare strong NaOH solution (say 20 % will be OK) and saturate it with zinc oxide - add comfortable excess so it settle to the bottom. Then, electrolyze this solution using nickel electrodes (you can try quality stainless steel like 316, but I always used nickel). Zinc would grow as spongy fluffy deposit, which barely hold together - but if you don´t knock it off, it holds even if 1,5cm thick. When there is nice layer of the zinc, remove the electrode and crumble the deposit into container with water. You need to wash it relatively quickly because when disconnected, zinc would be eaten back up by hydroxide.
Easily scaleable, just need some fiddling with current and voltage - when you reduce hydrogen, lower the voltage until it just stop bubbling (as with nearly all electrolysis from solutions). The prepared zinc is very active - I would sometimes say too active.

4. From my experience, hydrolysis of tin chloride is acid-dependant. Much much more, than time-dependant. If you want to avoid hydrolysis, wash with some dilute HCl - at least 3M HCl (10 %) was nearly always effective to practically stop the hydrolysis.
I found this when I prepared stannous chloride solution from old, partially hydrolyzed SnCl.2H2O which I have in the lab. When done with water or say weak 1-2 % HCl, cloudiness was very evident and persistent. When I dissolved these white powdery decomposed crystals in say 3M HCl, I obtained slightly milky solution which cleared quickly (particles settled nicely).

Another good source of high zinc alloy are wheelweights - at least here in EU, where we have strict regulations and lead was phased out due to enviromental pollution reasons.

I am looking forward for more good experimental work
As it is said frequently: Difference between messing around and doing science is properly writing up everything

 

[haveagojoe]

Thanks Orvi!

Metastannic acid "pain" is glorified in my opinion.

I agree, I was being a little hyperbolic. I only encountered it once and I basically just ignored it, but I think tin carried through into my melt.

there should not be any nickel or iron hydroxides/oxides/oxohydroxides

Yes, you're right, they should stay in solution. I am just surprised at the amount of black powder and couldn't trust that it's all gold! Perhaps I should kick my habit of shaking up my etches, it breaks the foils up so small they take forever to settle.

produce zinc powder, ....
The prepared zinc is very active - I would sometimes say too active.

Yes, after seeing the speed which the battery casings dissolved, I think I will not need to use zinc powder, I think it could produce quite a violent reaction.

hydrolysis of tin chloride is acid-dependant. Much much more, than time-dependant.

Yes I think that must be true too. The one time I encountered it was when I stored foils in plain water for several weeks. But nowadays I can only get 10% HCl, so if I use the spray bottle it gets diluted even further, maybe to a point where hydrolysis could start if I left it too long.