Tin Monster rears its ugly head.

[gold4mike]

I hope we ultimately figure out the best way to proceed with this situation because I sometimes get the same result.

I have put all those filters, including the foils and goo, into a bucket to save until I can incinerate them all together when the weather warms up.

 

[Sucho]

Guys,

i never trying to filter this tin gel ( H2SnO3 , or SnO2. xH20, it depends on literature, in older you will find metastannic acid or stannic acid, in new literature i found its hydrated tin dioxide, which can have same summar formula as metastannic acid, but different structural constitution and charge arrangement - different orbitals in bond)

the thing i do when this happens ( it happens almost everytime - Cu/Sn/Zn and other alloys with Sn are used widely), i only let it settle for a day or two. it is never more than 1,5 litre of glue in my 10k beakers i use for settling after 1 kilo batch of pins

then i siphon copper nitrate solution till i reach tin gel surface

after this i only pour some HCl to it. Most of copper nitrate and other less abundant nitrates are away in siphoned solution, rest contains mostly hydrated tin dioxide and some residual nitrates.

when you add HCl to this mixture, aqua regia is produced and gold foils are dissolved in a while leaving greenish gelly suspension ( suspended tin dioxide in "Aqua Regia"- CuCl2, HAuCl4 and other chlorides)

when you heat this suspension for some time ( 1-3 hours ) to cca 80 degrees celzius, it will change to crystalclear dark green solution

now it is possible to filter it without problems


little problem comes in final step

Sn is in oxidation state +4 in chloride form. if it is in ox. state +2 - gold reduced to colloidal particles -this wasnt prooved

solution contains in decrease Sn +4 , Cu+2 and some Au+3 in chloride forms


precipitation is quite risky, but i used only SMB as reducing agent

it has to be used almost no excess of SMB at very low pH ( because SMB can also reduce Sn +4 to Sn+2 and looses are in a game) so you have to know how much gold is inside

butyl diglyme/ dibutyl carbitol L/L extraction is also possible, but this solvent has high uptake also for Sn +4 :lol:

i would try different precipitants ( oxalates , nitrites, hydrazine, FeSo4 etc) and find out which precipitant gives most rigid, quantitative reduction of Au+3 to Au0 without reducing of Sn+4 to Sn+2 with a subsequent formation of colloidal Au particles

 

[g_axelsson]

Göran,

“Hey, this is fun, discussing and arguing with you. I hope you feel that way too.”

Hey it is better than fun, I do not feel like we are arguing, I feel you seen something I did not, I also feel we are just debating a point so that we both learn, or at least here I am learning.

You are so correct, I'm sorry I expressed my self in that way, sometimes the finer nuances of English is lost on me or I'm thinking on a nearby Swedish word that means almost the same thing. I think the expression argumentation is closer to discussion and was what I tried to say.

I was more focused on the copper salt, I did not even think that much about the Tin salt that much of being an oxide or nitrate of tin, I was not as concerned whether it would form nitric acid as I was of converting it to a sulfate and to remove it from the gold.

It is not arguing it is learning and sharing Ideas and what we are learning.

Exactly and that is what makes this forum so great. 8)

... I cut out a lot of relevant things that I agree with fully ...

But in dilute cold nitric acid we actually form the nitrate salt of tin Sn(NO3)2 (tin nitrate salt) when we slowly dissolve the tin.
(this may mix with some meta stannic acid of this I am unsure):

I've read that even in cold solutions the tin nitrate is unstable and easily breaks down to metastannic acid.

... even more detailed description of what happens with tin in nitric acid ...

Göran, I do enjoy these discussions and like to (argue) discuss the science with you I seem to learn a lot in our discussions.

Thanks for showing me the error of my ways of my thinking (thinking has always got me into trouble) :lol:

I enjoy it too. In over 5000 posts I found one that you made an error in, it wasn't easy to find, I've read a lot of your postings and many times I learn something new. I've learned a lot out of our discussion too, I had to do some deep research on tin chemistry just to be able to put forward my arguments for my point of view... did I get it right this time? :mrgreen:

I think we are on the same page now... but we still have left the question of what to do with a metastannic mess, if there is any general solution.
I think that the best way to treat it (from a purely theoretical point of view) is incineration that turns it into cassiterite (SnO₂), grind it to a fine powder and then treat it with hydrochloric acid.

I had prepared this response but Sucho posted an interesting description before I was done writing. I'll just add my answer here too.

Sucho, could this explain why your aqua regia is able to dissolve the tin?
According to

"If the metastannic acid is treated for a long time with concentrated hydrochloric acid, the Sn5O5 group is finally broken down, and the tin goes into solution in the form of ordinary alpha-stannic chloride:
Sn5O5(OH)10+20 HCl=15 H2O + 5SnCl4

I guess that this is what happens when you treat it with heat for a longer time.
Would it be possible to cement the gold onto copper from this mixture?
That could be one way to avoid the risk of creating new metastannic acid.

Göran

 

[srlaulis]

This thread makes my head hurt. So much to learn :shock:

Yeah, but it's a pleasurable pain. It's called knowledge. There is so much to learn on this forum, and I try my best to read all the threads I can, especially for the task that I'm working on.

Kevin

Yeah I read every post throughout each day. Comprehending this one is a little more challenging. I too love it though.

 

[Geo]

i was told you cant reduce tin oxide by calcining (incineration) like you can with other oxides. even though the thread i started really kind of petered out, it was generally accepted that the only way to deal with it was to incinerate and BOIL in hcl. i was told by Harold to grind or mill and screen and grind again and boil in hcl until there was no color change in the solution. i may not understand the chemistry but it has always worked for me.

 

[Sucho]

Goran ! your post hit me like lightning and opened my mind !

Tin chemistry is quite tricky due to its amphoterous character, complexicity, ox. states etc

If the metastannic acid is treated for a long time with concentrated hydrochloric acid, the Sn5O5 group is finally broken down, and the tin goes into solution in the form of ordinary alpha-stannic chloride:
Sn5O5(OH)10+20 HCl=15 H2O + 5SnCl4

this equation changed my point of view...

according to reaction between Sn and nitric acid

Kunkel likewise recorded the fact that to dissolve tin the nitric acid employed must be cold, or calx of tin would be precipitated. The explanation of these observations is that whilst tin dissolves slowly, in very dilute nitric acid to produce stannous nitrate, the stannous nitrate first formed, when hot and more concentrated acid is employed, is very unstable and quickly decomposes, yielding the form of hydrated stannic oxide known as β-stannic acid. Probably α-stannic acid is first produced from stannic nitrate, which then passes into the β-form.

β-stannic acid is also called β-metastannic acid or hydrated stannic oxide
α-stannic acid has different constitutions of hydroxyle groups and oxygen in space, also secondary structure, it is soluble in strong acids / alkalis

for an explanation what happens when you let your tin gel on air

Gelatinous, precipitated β-stannic acid has the empirical composition SnO2.4H2O, when air-dried SnO2.2H2O, and when dried in a vacuum, SnO2.H2O; These formulae do not, however, convey a just idea of the nature of the β-acid, which is gained from a study of its salts and other derivatives.

in my opinion, the most probable structural nature of this compound is a net structure or a ring with oxygens between Sn 4+ and hydroxyles above and under the plane of this twisted ring

Sodium β-stannate, prepared by the action of cold sodium hydroxide solution on β-stannic acid, is a sparingly soluble crystalline powder, having the composition Na2Sn5O11.4H2O. Similarly the potassium salt is K2Sn5O11.4H2O. Thus the molecule of β-stannic acid appears to contain five tin atoms; and the air-dried acid becomes H2Sn5O11.9H2O instead of SnO2.2H2O, whilst the acid dried in a vacuum is H2Sn5O11.4H2O instead of simply SnO2.H2O. Alternative formulae are Sn5O5(OH)10.5H2O and Sn5O5(OH)10 respectively, which suggest that β-stannic acid may possibly contain a ten-membered ring of alternated tin and oxygen atoms. At least an analogy is suggested between β-stannic acid and the polymerised silicic acids.

as i find out, it is impossible to dissolve dryed "tin gel" in HCl ( read higher - β-stannic acid looses some water molecules- this changes its structure - properties )

it is important to let your "tin gel" wet, without changing pH ( to avoid structure changes)- that means let it sit and siphon solution above

Goran, it is not "my aqua regia", it is only reaction of strong acid - HCl with residual nitrates (mostly Cu nitrate ) with a production of nitric acid and subsequent production of nitrosyl chloride and gaseous chlorine in situ

to conclusion, my opinion is, that due to highly hydrated status and large structure which contains at least 5 tin atoms it is more unstable- that means more reactive friendly for reaction with HCl. after drying restructuralisation occurs with a loss of water molecules, this structure is more stable - unable to dissolve

it is possible to cement gold on copper from this mixture, but i dont like this method due to production of very fine particles ( few times i got colloidal particles with a transmitance in purple spectra- it can be find what was particle size )

 

[glondor]

WOW so much to study. Thanks for the input everyone, It looks like we are narrowing down the scope if the issue. Now I must read....

 

[g_axelsson]

I think I soon join the group with heads that hurt. :lol:

I'm not a chemist and I have very little practical experience of chemistry. I'm a physicist, so I have to read a lot to keep up in this discussion.

Half a year ago I thought that the name metastannic acid was a stupid name, it was only tin oxide, but today I don't feel that way any more. It is so much more than only tin oxide, it contains H⁺ in it's formula so to call it an acid is justifiable, although it is a very weak acid. Tin chemistry is really complex. 8)

It seems to be a very good advice to keep the metastannic acid wet if you are going to treat it without incineration. I also suspect that time could play a role here. Any material that is kept for a longer time at a high enough temperature is going through recrystallization and smaller crystals disappears as bigger ones grows. In a saturated water solution this is something that happens as long as the water is liquid and not frozen. This is a physical process driven by surface energy of the crystals. There will always be harder to dissolve or chemically react with larger crystals than smaller ones.
How much it will differ between an old solution or a fresh one of metastannic acid is something to be found out through some experiments.

I haven't had time to read any more about tin chemistry so I don't have so much to add to this discussion right now. If I find out something new I'll add it later.

I also want to run some experiments, but that has to wait until warmer weather.

Göran

 

[butcher]

I think it would be fun to try some test tube expieriments, with tin, it seems the more I learn about it the more complicated it gets, and seeing some reactions may help in understanding.

 

[lazersteve]

Glondor,

What was your starting material (eg: clean pins or pins with other components and wires attached).

I'm assuming you used diluted nitric acid to get the mess you showed in the first photo?

Steve

 

[glondor]

Hey Steve, these were clean pins from some type of railroad connector. That is all the info I was given.

 

[lazersteve]

Odd how there was that much tin with no solder or wires present. Maybe the base metal of the parts contained an alloy of tin?

I've had great success with diluted nitric acid on a wide variety of plated pins and connectors and never once seen the size of mess that you had. I have run into trace amounts of tin gel, but never on the scale you are seeing. Most of the time it's due to a few stray wires or soldered ends on the scrap material and is easily managed. I try to sort these all out before bulk processing and treat them with diluted HCl first to remove the solders and other odd exposed metal parts, but some get through on occasion. The treated parts then get several water rinses before moving to the HNO3. My batch sizes are on par or greater than the lot you ran.


Steve

 

[glondor]

Yes the volume of tin was large, however, this was a 5 kilo batch, and I guesstimate that the pins were about 20%-30% tin. Maybe railroad connectors is the key. I don't know what they looked like tho. Here are some of the pins.>>>>>

 

[Geo]

notice the curved end? its meant to hold tension against the other connection pad.regular brass or even copper does not have the springiness to hold the tension and would bend out of shape.the pins are made of phosphor bronze. from wiki:
Phosphor bronze is used for springs, bolts and various other items used in situations where resistance to fatigue, wear and chemical corrosion are required (e.g., a ship's propellers in a marine environment). The alloy is also used in some dental bridges.
Grades A, C and E – C51000, 52100, 50700 are commonly used nonferrous spring alloys. The combination of good physical properties, fair electrical conductivity and moderate cost make Phosphor Bronze round, square, flat and special shaped wire desirable for many springs and electrical contacts and a wide variety of wire forms where cost of properties does not prescribe Beryllium copper. [1]
Phosphor Bronze (94.8% copper, 5% tin, 0.2% phosphorus) is also used in cryogenics. In this case the combination of a fair electrical conductivity, and a low thermal conductivity allows to make electrical connection to devices at ultra low temperature without adding an excessive heat load.

http://en.wikipedia.org/wiki/Phosphor_bronze

 

[g_axelsson]

That is really nice pins... I have some myself that I got from some process equipment from 1975, but I haven't refined them yet.
When I got them I thought it would be a good test to refine them via the AP process, but I made a real mess out of it , I've got a lot gold foils mixed in with a paste of copper chloride. Your post shows me that I probably have a lot of tin mixed in that mess too. Before I finished that batch life interrupted for a couple of years. That mess will be the last bit of my early gold refining trials I have to clean up before I'm ready to start with new materials. I got some more of those pins and this time I'm planning to refine them in a sulfuric acid cell.

Thanks for showing me that I have more to deal with than only copper chloride.

Göran

 

[lazersteve]

I feel like a warm dilute sulfuric acid pretreatment would have at least reduced the tin encountered and likely even removed all of the phosphor bronze base alloy. I save my pretreatment liquids and re-use them until they become exhausted.

My typical batch size is greater than 5 kg, but I always process a 100 g sample of any newly encountered scrap. This has an added advantage of giving me a yield baseline for the larger batch so I can quantitatively know what's still tied up in if the yields fall short for whatever reason.

My outlook on processing is that an ounce of prevention is worth a pound of cure. If I have to take an extra step to save myself many additional steps, I'll gladly take the extra step. Chemicals are cheap compared to my time. This is also a good example of where to choosing the right 'chemical tool' in your processes makes all the difference in the world as to the outcome. Finally it speaks to the importance of knowing your scrap and doing acquaintance tests on small samples to find the right tool for a job. I meticulously sort the scrap and work up a recovery cycle on a small scale for each type of scrap I encounter. I note this in my lab book for future reference when dealing with these and similar types of the scrap. If you had discovered the alloy on a scale of say 100 grams, it would have been apparent what you were up against and you could have corrected your approach to solve the tin issue before it became so large. Remember there is no one size fits all recovery/refining process that works for all types of scrap, contrary to what many may believe and/or advertise.

Please don't take any of my comments as derogatory, to the contrary learning this type of information, and having an intimate understanding of each type of scrap you process will put you head and shoulders above the competition.


Steve

 

[solar_plasma]

the first thread I just had to copy and store to analog display (paper), too much to think of in 5 minutes.....thank you guys! This forum is brillant!

 

[glondor]

For sure the teachings here are top notch. So many ways to attack the problem from pretreat to end of stream solutions. It certainly is wise to pretreat if time and space allow, and it is something I will put to good use in the future. The end of stream issues seem to be very complex, which just screams out "get rid of the tin first"

A question for Harold. (All the rest of this is moot if the answer is yes)
Would your incineration method as a first step eliminate all of this issue?
IE pre-treated via heat with torch or furnace to a certain temp, for a certain time, then process as usual? Would this simple step mitigate all of the rest of these issues by providing a filterable result that will not hold back gold?
If so, are you laughing at us for not listening? : )

There is a solution to every issue, and since it seems that no 2 materials are alike, all have issues. Good notes, lots of bookmarks and for sure some copy/paste files are a must.

I did not report on the final result by mistake, the pins yielded a little over 2 g per pound and I am confident we achieved 99+% recovery using the incineration/hcl boil method of the metastanic acid . The "bones" tested barren after a good boil in A/R. All in all, I would consider them good pins to do, yield wise even though the base metal weight was quite high. The foils were substantial enough in thickness to be pushed almost into what I consider "high grade pins"

Thanks to all for contributing to this thread, I like pushing tin to the forefront as every time I do, new ideas and avenues, methods and chemistry come to the surface.

I wonder if we could make a category called "Tin and how to kill it" It should be right up there with Gold, Silver, Plat and Pal. As the biggest and baddest nuisance metal.

All in favor say aye! Cheers Mike

 

[lazersteve]

Mike,

I'm not Harold, but I can tell you that incineration will not solve the 'tin monster' if the tin is in the alloy. Incineration as a first step will remove grime and coatings from scrap, but it won't eliminate tin from the equation. Incineration as an intermediate step after tin is converted to metastannic acid is a viable solution followed by an HCl treatment, but you already knew that.

Bottom line for alloys containing tin metal you must choose a reagent that either leaves the tin unreacted, or one that dissolves it completely without converting it to metastannic acid. Nitric acid and formulas which involve nitric acid are not effective when tin is part of the alloy, unless you want to deal with metastannic acid downstream.

Depending on the alloy of tin HCl or Sulfuric acid are good choices for dealing with the pesky metal upstream of your recovery process.

Steve