Am I dealing with the excess of nitric acid?

[Hombressino]

Hi guys,
as a couple of times before I dissolved known material in Nitric acid, filtered, denoxed, dissolved in AR, got a gold drop well under my expectations and... sice that point I started to make a series of steps which screwed it up. Let me explain the whole story, followed by a humble plea to experienced community members for help.

Material: 853 g of old pins from 70's, fully gold plated, mostly de-soldered from PCB (Sn+Pb 60:40 solder). Half of them springy = bronze base, second half copper base. Plus few grams of different type - magnetic ones which escaped my attention during sorting. Expected yield: 8-10 g of Au per kg.
After thorough wash with hot water I dissolved pins in two batches in 50% nitric with a little splash of sulphuric acid. A lot of metastanic blue-white gunk on the bottom of the beaker which was expected as there is a lot of tin i the springy ones plus in solder. I filtered the most of the metastannic out (through the series of polypropylene mesh filters). After a thorough wash I dissolved remaining solid part in AR. The solid part contained small residues of undissolved pins so I decided to use more AR to let it all dissolved. I noticed I had to use more nitric than usually (for AR I got used to use 65% HNO3 p.a.). Then, I continued with denox by sulfamic acid (70°C, adding small amounts of sulfamic acid crystals until there's no fizzing). Cooled by couple of ice cubes. Then filtered through medium-speed paper filter. I let the filtered solution left for 1 day and on return I noticed a thin sediment at the bottom of the flask (probably metastannic) so I decided to filter it again through slow-speed paper filter. This time there was no sediment and the solution was emerald green. Then I added 100ml of distilled water and added small amount of HCl to make the solution more acidic and dropped the gold with SMB. It seemed OK except of the amount: I let it settle overnight but I saw only the bottom of the beaker is covered by the brown powder. I tested the solution with stannous - strong positive test. So, I added more SMB and noticed something strange - a brown foam on the surface which quickly disappeared in the swirl, followed by brown fume. I took a short video of this:
Since this point I started to make ... mistakes.
Based on observations I thought I had not done the denox enough. I warmed the solution to 70°C and added sulfamic acid again. As I saw a lot of fizzing I thought my suspection was right. As the second clue I put a sheet of copper to the solution and saw a reaction. A dark-brown coating on the copper strengthened my conviction I didn't do the first denox right. After the second denox I cooled the solution overnight and tried to drop it with SMB again. There was no brown powder at all at the bottom this time. I added more SMB and probably overshoot the SMB (strong sulphuric smell), but still, no signs of drop, and again - the same behavior as on the video above. Well, I decided to drive-off the excess nitric (as I thought I still have excess nitric in solution) by slowly evaporating the solution. It went down from 900 ml to 350 ml. When it cooled it crystalized - see attachment. I diluted it with distilled water to 450 ml and added a splash of HCl. It became liquid again. I added SMB - no brown fumes this time but there is still that brown foam disappearing in the swirl as can be seen on the video. Stannous test is still strongly positive - see attachment.
I'm puzzled.
"When in doubts, cement it out!" - is this a right advice to apply now? How do I proceed so that I don't make things worse?

Thank you very much in advance for any help.

 

[Yggdrasil]

Hi guys,
as a couple of times before I dissolved known material in Nitric acid, filtered, denoxed, dissolved in AR, got a gold drop well under my expectations and... sice that point I started to make a series of steps which screwed it up. Let me explain the whole story, followed by a humble plea to experienced community members for help.

Material: 853 g of old pins from 70's, fully gold plated, mostly de-soldered from PCB (Sn+Pb 60:40 solder). Half of them springy = bronze base, second half copper base. Plus few grams of different type - magnetic ones which escaped my attention during sorting. Expected yield: 8-10 g of Au per kg.
After thorough wash with hot water I dissolved pins in two batches in 50% nitric with a little splash of sulphuric acid. A lot of metastanic blue-white gunk on the bottom of the beaker which was expected as there is a lot of tin i the springy ones plus in solder. I filtered the most of the metastannic out (through the series of polypropylene mesh filters). After a thorough wash I dissolved remaining solid part in AR. The solid part contained small residues of undissolved pins so I decided to use more AR to let it all dissolved. I noticed I had to use more nitric than usually (for AR I got used to use 65% HNO3 p.a.). Then, I continued with denox by sulfamic acid (70°C, adding small amounts of sulfamic acid crystals until there's no fizzing). Cooled by couple of ice cubes. Then filtered through medium-speed paper filter. I let the filtered solution left for 1 day and on return I noticed a thin sediment at the bottom of the flask (probably metastannic) so I decided to filter it again through slow-speed paper filter. This time there was no sediment and the solution was emerald green. Then I added 100ml of distilled water and added small amount of HCl to make the solution more acidic and dropped the gold with SMB. It seemed OK except of the amount: I let it settle overnight but I saw only the bottom of the beaker is covered by the brown powder. I tested the solution with stannous - strong positive test. So, I added more SMB and noticed something strange - a brown foam on the surface which quickly disappeared in the swirl, followed by brown fume. I took a short video of this:
Since this point I started to make ... mistakes.
Based on observations I thought I had not done the denox enough. I warmed the solution to 70°C and added sulfamic acid again. As I saw a lot of fizzing I thought my suspection was right. As the second clue I put a sheet of copper to the solution and saw a reaction. A dark-brown coating on the copper strengthened my conviction I didn't do the first denox right. After the second denox I cooled the solution overnight and tried to drop it with SMB again. There was no brown powder at all at the bottom this time. I added more SMB and probably overshoot the SMB (strong sulphuric smell), but still, no signs of drop, and again - the same behavior as on the video above. Well, I decided to drive-off the excess nitric (as I thought I still have excess nitric in solution) by slowly evaporating the solution. It went down from 900 ml to 350 ml. When it cooled it crystalized - see attachment. I diluted it with distilled water to 450 ml and added a splash of HCl. It became liquid again. I added SMB - no brown fumes this time but there is still that brown foam disappearing in the swirl as can be seen on the video. Stannous test is still strongly positive - see attachment.
I'm puzzled.
"When in doubts, cement it out!" - is this a right advice to apply now? How do I proceed so that I don't make things worse?

Thank you very much in advance for any help.

These three droplets are your Stannous test?
If so, they do not look right to me.
There might be too much of everything in there, giving a so called false positive,
so I would use some proper and thick Copper and cement out everything that will cement out and restart from scratch.

 

[Hombressino]

Hi, yes, it's a Stannous in various stages of my "tries". Unfortunatelly I can see now it doesn't appear the same on a photo than in real. I can see a distinct black-to-dark violet tint.
Edit: I have to take the photo again, there's not the last "droplet". I probably uploaded an old photo.
Nonetheless I agree with the conclusion. May I ask for details how to cement it out on copper? I know it sounds dumb but I've never did that before.

Thanks!

 

[Yggdrasil]

Hi, yes, it's a Stannous in various stages of my "tries". Unfortunatelly I can see now it doesn't appear the same on a photo than in real. I can see a distinct black-to-dark violet tint.
Nonetheless I agree with the conclusion. May I ask for details how to cement it out on copper? I know it sounds dumb but I've never did that before.

Thanks!

Hang a thick Copper piece in the solution and stir it or bubble air through it for a day or two.

 

[orvi]

Hi guys,
as a couple of times before I dissolved known material in Nitric acid, filtered, denoxed, dissolved in AR, got a gold drop well under my expectations and... sice that point I started to make a series of steps which screwed it up. Let me explain the whole story, followed by a humble plea to experienced community members for help.

Material: 853 g of old pins from 70's, fully gold plated, mostly de-soldered from PCB (Sn+Pb 60:40 solder). Half of them springy = bronze base, second half copper base. Plus few grams of different type - magnetic ones which escaped my attention during sorting. Expected yield: 8-10 g of Au per kg.
After thorough wash with hot water I dissolved pins in two batches in 50% nitric with a little splash of sulphuric acid. A lot of metastanic blue-white gunk on the bottom of the beaker which was expected as there is a lot of tin i the springy ones plus in solder. I filtered the most of the metastannic out (through the series of polypropylene mesh filters). After a thorough wash I dissolved remaining solid part in AR. The solid part contained small residues of undissolved pins so I decided to use more AR to let it all dissolved. I noticed I had to use more nitric than usually (for AR I got used to use 65% HNO3 p.a.). Then, I continued with denox by sulfamic acid (70°C, adding small amounts of sulfamic acid crystals until there's no fizzing). Cooled by couple of ice cubes. Then filtered through medium-speed paper filter. I let the filtered solution left for 1 day and on return I noticed a thin sediment at the bottom of the flask (probably metastannic) so I decided to filter it again through slow-speed paper filter. This time there was no sediment and the solution was emerald green. Then I added 100ml of distilled water and added small amount of HCl to make the solution more acidic and dropped the gold with SMB. It seemed OK except of the amount: I let it settle overnight but I saw only the bottom of the beaker is covered by the brown powder. I tested the solution with stannous - strong positive test. So, I added more SMB and noticed something strange - a brown foam on the surface which quickly disappeared in the swirl, followed by brown fume. I took a short video of this:
Since this point I started to make ... mistakes.
Based on observations I thought I had not done the denox enough. I warmed the solution to 70°C and added sulfamic acid again. As I saw a lot of fizzing I thought my suspection was right. As the second clue I put a sheet of copper to the solution and saw a reaction. A dark-brown coating on the copper strengthened my conviction I didn't do the first denox right. After the second denox I cooled the solution overnight and tried to drop it with SMB again. There was no brown powder at all at the bottom this time. I added more SMB and probably overshoot the SMB (strong sulphuric smell), but still, no signs of drop, and again - the same behavior as on the video above. Well, I decided to drive-off the excess nitric (as I thought I still have excess nitric in solution) by slowly evaporating the solution. It went down from 900 ml to 350 ml. When it cooled it crystalized - see attachment. I diluted it with distilled water to 450 ml and added a splash of HCl. It became liquid again. I added SMB - no brown fumes this time but there is still that brown foam disappearing in the swirl as can be seen on the video. Stannous test is still strongly positive - see attachment.
I'm puzzled.
"When in doubts, cement it out!" - is this a right advice to apply now? How do I proceed so that I don't make things worse?

Thank you very much in advance for any help.

Sulfamic start to work properly only over 85-90 °C. What you experienced is typical example of still active AR. You added quite a bit more than you should. Best de-noxing agent is more material. If you have some pins in hand, add them to the solution followed by healthy bit of HCL (100+ ml to make the liquid strongly acidic). Heat the solution with pins to nearly boiling and observe reaction. You wil clearly see gold "bleeding" from the pins, and then redissolving again. Then, it will take more permanent brown colouration and eventually solution will go full brown, gold will pack with some stirring and sink to the bottom. Let it simmer for some time, stannous check and then you are good to pour the barren liquid off and treat the residue with HCL and nitric - this time carefully.

Second option is to heat the solution hot, add HCl to recreate strongly acidic conditions and prepare a slurry of SMB in water (enough water that after swirl in the beaker it can be poured). Stir the solution vigorously and simultaneously while you are stirring SLOWLY pour the slurry in. It will have a tendency to erupt and react very vigorously, but it can be done relatively OK. Replenish the HCL as HCL is spent when SMB reacts with solution (SMB is a base). Eventually you will kill all of the nitric and gold will drop permanently. Very important points are - it must be a slurry (otherwise active reagent just fizz away from the surface - best is to bubble SO2 gas but...), it must be hot (75+°C is OK) and you need to do it slowly.

Hang a thick Copper piece in the solution and stir it or bubble air through it for a day or two.

No air bubbler is needed as he has only gold in solution. Sticking copper bar/thick sheet in and heating near to the boiling point will precipitate all the gold in less than hour - he does not have that terrible ammount in the beaker and this will be quicker. Doing it with open beaker to let the NOx fumes escape, not redissolve-oxygenate and recreate nitric or NOCl. Choose taller beaker. Best pair with sulfamic acid, as NOx gasses evolved will be immediatelly killed and transformed to nitrogen gas (colour of the outgas is good lead - if hitting brown tint, add more sulfamic). Simmer for few dozen minutes with occasional stirring to pack the gold, then let it cool. Do not left the solution over copper too long as it will be eaten away to form CuCl unnecessarily. If the solution is just brown, nearly translucent brown colour (assuming you done copper material), it does not have oxidation potential to dissolve gold from solid form back to liquid - brown means there is Cu2+ alongside Cu+. Negative stannous, temperature below 30 °C and you are good to pour/filter the baren liquid. If there werent PGMs and only gold, you can go directly to waste treatment, no stockpot needed for this solution.
Then refine obtained gold powder as usual.

 

[Hombressino]

Hi Orvi,
at first, a big thanks for the very thorough explanation and for providing such a huge amount of detail.

You added quite a bit more than you should.

Yes, and I recall now when it happened: it was those magnetic pins which refused to dissolve and I stubbornly tried to make them so, adding more and more nitric to AR.
Secondly, thank you for partially restoring my self-confidence that I might have learned something while reading GFR, Hoke and many other sources, so that I correctly guessed the cause of my problems. (The fact that later I convinced myself that I was wrong is another matter ).
As I've been provided with 3 options, I will divide the solution to 3 equal parts and try all three - as a part of my education
I will gradually send the results here.

Once again, thank you very much.

 

[orvi]

Hi Orvi,
at first, a big thanks for the very thorough explanation and for providing such a huge amount of detail.

Yes, and I recall now when it happened: it was those magnetic pins which refused to dissolve and I stubbornly tried to make them so, adding more and more nitric to AR.
Secondly, thank you for partially restoring my self-confidence that I might have learned something while reading GFR, Hoke and many other sources, so that I correctly guessed the cause of my problems. (The fact that later I convinced myself that I was wrong is another matter ).
As I've been provided with 3 options, I will divide the solution to 3 equal parts and try all three - as a part of my education
I will gradually send the results here.

Once again, thank you very much.

You are welcome.

Learning is much easier if you have some theoretical background and it looks like you start to have some good thoughts about stuff what happens in your beakers

Just be careful with that SMB slurry denoxing. It isn´t recommended here, because it can be unpredictable when cold - you can literally shovel the SMB in and not much happens, and after a while all contents of the beaker suddently erupt out Due to this, it must be done hot. For sure, this is the cheapest de-noxing option aside of more material added. Fill the beaker only to 1/3 with gold bearing solution and don´t expect clean gold product at the end It usually shoots like 95+% Au after rinsing with HCL.

And as a good measure with all reactions with expected gas evolution - if it is clear that "appropriate" ammount of gas was not released after addition, wait and do not add another load of reagent. This can be very dangerous due to phenomenon called "induction period". You basically oversaturate the reaction with reagent (assuming it wasn´t enough based on absent reaction observation), and when reaction suddently kicks in, it go out of control

In this reaction, SO2 is active reagent (SMB reacts with free HCL and liberate SO2) which reduce nitric acid to NO and NO2 gasses - which obviously want to escape out. If your liquid is saturated with ferrous metals, they have tendency to complex NO and NO2 to form nitrito and nitrosocomplexes. These solutions are very tedious to de-nox by standard means (sulfamic), and best in these scenarios is combination of sulfamic and copper plate - which simultaneously cement the gold.

If some of the above mentioned method struggle to deliver results, acidify with HCL and use copper plate with heating. This is definite method which will always work if gold is in solution.

 

[Hombressino]

Time to re-open this thread I had to pause my refining hobby for some (various reasons) time so I wasn't able to return to the topic above until a two weeks ago. Good news: Thanks to Orvi's advice I recovered my gold from the solution via the "SMB slurry denoxing" process described above.
I've been taking some pictures and videos throughout the process so as promised I will share my experience here, in following posts. Not everything went smoothly, mostly because of my inexperience and sometimes impatience, as you will see below
I must say thanks to Orvi once again for the process and all the hints provided

 

[Hombressino]

I prepared a slurry SMB. In attached video you see the first try, which turned out to be little too sparse, so after the first pouring I began to make It denser.
View attachment VID_20231029_155822.mp4

 

[Hombressino]

The first pouring. I think a lot of SMB reaction took place on the surface, nonetheless - judging by brown fumes - it did what it had to do, at least partially.

 

[Hombressino]

One of the subsequent pourings during which I almost experienced an "eruption"

 

[Hombressino]

So, did everything go according to plan? Not really. Yeah, finally I probably overshot the SMB. I created a quarry - a beautiful crystals that didn't make me happy. But a brown powder is here for sure as well!

 

[Hombressino]

Luckily, after couple of thorough washes in warm water I got rid of it. The dropped gold was finally better than I expected.

EDIT: after the first water wash I saved the rinse water, just for instance... and after 48 hours it turned out to be a good idea: part of the gold was in form of a super-fine powder which won't settle in one day. The rinse water was of a dirty yellow color, which I mistakenly thought was pollution from a large amount of sulfur. It finally settled so I recovered some extra gold.

 

[Hombressino]

2nd time in AR, filtering, SMB drop, water washes, rinse in hot HNO³ (I have better results with nitric than hydrochloric acid so I got used to using nitric), water washes, ... I finally got there. The yield was slightly beyond what I expected sooooooo... Success))

 

[Martijn]

2nd time in AR, filtering, SMB drop, water washes, rinse in hot HNO³ (I have better results with nitric than hydrochloric acid so I got used to using nitric), water washes, ... I finally got there. The yield was slightly beyond what I expected sooooooo... Success))

Beautiful colored powder!