black ppt from nitric after trying copper cementation

[apkmetal]

Need some help here. Have a unknown, medium density material from a dental office. Material is gray colored like cemented silver but does not have much weight. I took 10 gms of material and covered with 15ml of distilled water. Added 3ml conc. HNO3 and let react slowly. Started bubbling up thick, grey bubbles. Let die down and added another 4ml HNO3 and let react some more. Let sit over night and next day had a gray sludge on the bottom with a light brown solution above. Heated the solution slowly and added 2ml conc. HNO3 and heated to just below boiling for about 1 hour. Removed from heat and let sit overnight again.

Next day, I filtered off the sludge, washed with distilled water and diluted down the solution with distilled water to about 100ml. Solution was a light brown color. Added a sheet of clean copper and immediately it started forming a black ppt on the copper. It continued for over two hours, and continued to drop a considerable amount of black ppt to the bottom. The copper was forming bubble at the water surface, so I assume the ph was still too acid. I withdrew a 1ml aliquot of the solution and dropped in a small amount of deionized NaCl. It immediately turned white indicating with Ag, Hg or Pb. Have not tested the white ppt yet. I am letting the copper sit in the solution overnight to see how much of this black ppt drops?

Solution was slowly turning from brown to a faint green while the black ppt was dropping. Given that only Ag, Hg, Pd and Pt could be in solution from pure HNO3 solution, I am puzzled what the black ppt could be. It is not real fine and settles quickly.

Any ideas, what I might have here? I would not expect other base metals to drop with copper, but maybe I missed something.

 

[Barren Realms 007]

Did you test your solution with stannous or DMG?

 

[apkmetal]

Have not tried the DMG yet and I was under the impression stannous does not work on a solution with free HNO3 in it, or I might be thinking of AR that was not been denoxxed. If stannous can work on this nitric solution, then I will try it in the morning.

 

[apkmetal]

Ran a DMG and Stannous chloride test on the filtrate today after filtering off the black ppt. The DMG had no reaction and the stannous initially showed no reaction, but later after looking at the Q-tips I had used and placed by the heater, the Q-tip was a medium purple color. I am scratching my head on that reaction, because I only did a pure nitric digestion and had no other chlorides or oxidizers that could have dissolved gold. I doubt there is gold in the solution, but this makes me wonder?

I then checked a small amount of the filtrate with some salt and immediately got a white ppt. Added about 8 gms of salt to the brown filtrate and got lots of white ppt. Most of it dropped to the bottom but some of it stayed in a suspension, like a moderate gel had formed. Was not a gel, but the particles were floating on the surface of the solution and then very slowly tried to settle. After about 1 hour the top cleared about 1/2" down and below that was like a thick layer going all the way to the bottom. The white particles were stuck in the solution, but if I tapped on the beaker, they would start to settle. My first thought is that there is both silver and some tin in solution.

In the morning I will redissolve the black ppt in nitric/distilled water and try to cement it again with copper to see if it changes color. I will filter off the white ppt and check to see if it is Ag, Pb or Hg.

These results so far have me puzzled, as I do not know what the black ppt is forming on the clean copper? Any suggestions would be appreciated.

 

[kurtak]

the stannous initially showed no reaction, but later after looking at the Q-tips I had used and placed by the heater, the Q-tip was a medium purple color.

It sounds to me like you are dealing with the silver amalgam used for dental fillings - if so - You Need A Retort To Do This Properly :!:

The reason you did not get a color in the initial stannous test (you did get a reaction - it was a "white" precipitate) but then showed a color after sitting for awhile was that the white precipitate (AgCl &/or HgCl) photo changed to give you the purple color (they both photo change when exposed to light)

Hg like Ag is below copper in the reactivity series - so the black on your copper could be the Hg forming a passive layer on the copper &/or its cementing the Ag & Hg as an amalgam at an atomic/colloid level thereby reporting black rather then the expected gray --- the bottom line here being - you will not effect a parting of the silver from the Hg buy trying to cement the silver from a nitrate solution that is a Ag/Hg amalgam

Like wise - adding NaCl &/or HCl to a Ag/Hg nitrate solution is going to precipitate both AgCl & HgCl

You need a retort set up to deal with amalgams - other wise all you are doing is changing elemental mercury to chemical compounds & producing Hg containing waste without effective ways to part the Ag from the Hg

My advice - return it to where it came from & don't deal with it at all

Kurt

 

[kurtak]

And by the way - a stannous test will work in a PM containing solution that has free nitric - it will still show the PM color indicator - the color change will just go away after a short time due to the free nitric re-dissolving the PM's

Kurt

 

[sri steve]

Hi, I deal in dental waste from dental practises and laboratories and over here in the UK, the governing body (GDC) set rules on how waste is controlled.

Any tooth extraction which contain precious metals comes under the category of low risk clinical waste, laboratory waste is defined as scrap metal as this kind of waste contains no organics.

Amalgams which does contain a high amount of Ag and mercury is in the category high risk clinical waste and can only disposed of by certified companies, all the dental practises have amalgam sludge containers to store for collection.

Over here if any dental practise was found to be, selling this kind of waste would be closed down and prosecuted.

This kind of practise puts you at risk and others of mercury poisoning; I don’t have to tell you that exposure to this very toxic chemical can lead to very serious illness.

My advice report it to the authorities where you obtained it from, to protect yourself and others.

 

[apkmetal]

Hello Siri,

I understand your concerns and just want to to know I have plenty of experience dealing with mercury for many years. I understand about methyl mercury, vaporized mercury from dropped mercury thermometers and all the other sources of possible mercury exposure. I even have a professional mercury retort which I used in years past, but these days it is not worth my time anymore. As I told Kurt, I will just return it to the dental office and let them send it off to a proper amalgam recycler.I had told them I did not want their amalgams, so it was not something I expected. I received a bunch of different items and this was just thrown into the box along with the other items at the last moment. So maybe in their haste they gave it to me by accident. I am not going to make a big deal of it here. I will just return it and that will be the end of it.

 

[apkmetal]

Hello Kurt, just noticed my reply to you for some reason did not post and ended up in the vapor. I want to thank you for your help and I did not get a chance to get the white ppt tested for Ag, Hg or Pb today. The material I got it from was fairly low in density, so I really did not expect Hg, but now it make sense based on how my material behaved.

I will just return the material back to them as I had originally told them I did not want any amalgams to deal with. I could if I wanted to, but I don't like to mess with mercury anymore.

On another note, I received from them a silver recovery cartridge that they used for their photo fixer. The 3.5 gallon pail was still sealed, but it had a large crack going up the side which had drained out all the fluid. I removed the top and inside there was still some untouched steel wool , but for the most part everything else inside had badly rusted from exposure to air. I want to try either Harold's technique of borax, soda ash and iron or Manuel's process using potassium nitrate to reduce the silver sulphide. I am only wondering if the iron oxide is going to create any problems? I am drying the material right now to getting it ready for torch work or smelting.

In addition, I appreciate the info on the stannous testing. I have read multiple different posts and sources for using stannous chloride to test for metals, but none of them talk about all the little things to do and watch out for. I have to make my own "cheat sheets" for everything I work with to get all the info in one place. I have to rely on Hoke , Ammens and the forum to get all my answers.

 

[Harold_V]

I am only wondering if the iron oxide is going to create any problems?

If processed by furnace, it should be absorbed by the flux. How much flux is required may be a concern.

Might be a good idea to run a small amount in a melting dish, using a torch, to see how it behaves. If the slag isn't fluid when it's hot, and shiny after it has cooled, you may need more.

Be advised, this process is hard on crucible life, especially when soda ash is introduced.

You can limit the frothing one experiences by using borax glass or anhydrous borax.

Harold

 

[kurtak]

I was going to answer the iron oxide question but see Harold beat me to it 8) (thanks Harold)

If you have a furnace then I would go with the flux/iron method - you need a cone mold - or a V mold (made from a piece of angle iron with end caps welded on the ends)

The only other thing I would ad is that if there is still elemental iron in it (from steel wool) let it run in the furnace for awhile to be sure that iron gets dissolved & goes into the slag & then ad a piece of rebar to insure the conversion of silver sulfide to silver is made complete --- you know when the conversion is complete when it stops eroding (dissolving) the rebar --- if you ad the rebar after your mix/load is molten be sure it is preheated - don't put it in cold as that could cause a shock explosion

Also be sure to mix the flux with the material

Kurt

 

[apkmetal]

Thanks Harold and Kurtak! I was thinking along the same lines as Harold about doing a small torch test first and then moving onto the furnace if it shows good results. I will weld up a large angle iron cone mold, as I have needed a larger one for a while. I have some 3/8" thick 6" angle sitting around which would make a nice sized mold.

Yes, I know about the frothing of soda ash and the mess it can make. Will follow your advice about the anhydrous borax and steel and leave the soda ash out for now. Will let you know how it turns out.

 

[kurtak]

Thanks Harold and Kurtak! I was thinking along the same lines as Harold about doing a small torch test first and then moving onto the furnace if it shows good results. I will weld up a large angle iron cone mold, as I have needed a larger one for a while. I have some 3/8" thick 6" angle sitting around which would make a nice sized mold.

Yes, I know about the frothing of soda ash and the mess it can make. Will follow your advice about the anhydrous borax and steel and leave the soda ash out for now. Will let you know how it turns out.

Do NOT leave the soda ash out of your flux mix - it preforms 2 important tasks in the smelt (1) it makes the molten flux more fluid which in turn allows the silver to settle to the bottom of the crucible so you get less small ball metal(silver) hanging up in the slag (2) its reacts with the sulphur & iron to complete the conversion of silver sulfide to elemental silver & then the iron sulfide slags off in the flux

using anhydrous borax (instead of 20 mule team) will reduce foaming (not eliminate it) so leave room in the crucible - fill crucible about 1/3 (or a little more but less then half) to start & allow reaction to complete - when the foaming dies down add some more of our silver sulfide that has been premixed with your flux & wait for the reaction to die down again --- you should be able to make 2 or 3 additions before you run out of room to allow for foaming & worry about boil over

Its one of those thing you just have to do in order to get a feel for it as far as how much to add &/or how many additions you can make - just be sure the reaction is complete before making each addition

Soda ash is hard on crucibles also but in this case it is needed for the reaction to complete the conversion

Kurt

 

[apkmetal]

Well I finally finished doing the smelt on the black silver sulfide(?) material and it did not work out as expected. I did 5 total smelts on the material. One using the potassium nitrate process from Manuel and 4 different tries doing it with the process from Harold. I ended up with about 15 lbs of a black sludge washed out from the bottom of the 3 gallon pail . Used a heater and heat lamp to dry out the sludge over 5 days. In the end, had 5 lbs. of rust brown material that was more like red lava rock. Took 50gms of the ground up material and mixed with 25gms of a 2:1 borax/ soda ash flux. Put in my crucible with a large iron nail and heated to 1100 C for 30 minutes. Poured out into a preheated cone mold and let cool. Turned over and had zero metals showing. Did a fresh batch and this time doubled the amount of flux. Same results, no metal even after making sure as much iron metal dissolved as possible from the iron nail and made sure the melt was well mixed with a graphite rod. I even tried it in one of my muffle furnaces at 1150 C for 1 hour soak time and still ended up with no visible silver or other metals. Using the KNO3 process was no better. I got one tiny bead today after the last smelt , but it was only .05% Ag yield based on starting weight. I think the flux and smelting processes were fine, the material was just vacant of any decent silver.

What has me confused is why the dark , black sludge turned rust brown and lost all the black color just from drying? The heat was never more than 40 C based on a non-contact thermometer. Based on how much black silver sulfide I thought I had originally washed out, I would have felt I should be seeing at least 60-80% Ag yields. Was disappointed it did not turn out, but my only thought is that this unit was used as a final fixer polish unit to remove any last traces of silver, and that is what I ended up with.....traces! I learned a lot from processing this, so the next time I will be prepared for it. Just realized that somethings are not always the way they seem at first look. Thanks to everyone for their help here on helping me work on this project. Time to move on to something else!