Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-04-21T02:33:06+00:00
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How to recover Silver from Jewelry (22 replies)

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I am chemist and I want to recover silver from jewelry rubbish by electrowinning. Does somebody have some information or book where I can find detailed information about it?

OberstGruppen
1 year ago
OberstGruppen 1 year ago

First off, you do realize that electrowining is a second step, following dissolving the precious metals with either cyanide or a strong acid (or mixture of acids: aqua regia if gold is present also).
You might want to look at this article: Electrowinning Precious Metals http://is.gd/htndaK

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I am recovering Silver dissolving the precious metals with HNO3 followed by copper cementation. Then, I attack the rubbish with aqua regia to dissolve gold followed by sodium metabisulfite precipitation. So the electroplating systems I want to develop are:

To reduce silver nitrate in the presence of copper (II) nitrate and ferric nitrate.
To reduce gold in the presence of ferric and copper (II) chloride.

Ace Levy
1 year ago
Ace Levy 1 year ago

You can EW the silver from the nitric acid solution by controlling the applied potential. Silver is easier to reduce than copper or iron but you must be careful to let the current density decrease as the silver concentration decreases. Try using copper metal to cement out the gold. Then re-dissolve the gold in HCl/H2O2, filer out any insols and re-precipitate the gold by raising the pH. You may need to add more peroxide to reduce the gold. The copper chloride is not reduced by the peroxide.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Do you have further information about technical details of the EW of silver from the nitric acid solution by controlling the applied potential? I will start working on that because the quality of the obtained gold is enough by now.

Ace Levy
1 year ago
Ace Levy 1 year ago

Use insoluble anodes either stainless steel or graphite. You can determine the correct potential by experiment if you have a rectifier that lets you vary the voltage independently. Set up your EW cell then start at zero volts and slowly increase the voltage until significant current begins to flow. You may want to turn down the voltage a bit as long as you still have an acceptable level of current. Remember the current will decrease as the silver concentration decreases. If you have a very high copper to silver ratio in your leach solution, you may not be able to plate out all the silver before some copper plates out. You can use the Nernst Equation to determine the maximum allowable copper silver ratio.

Gruppen
1 year ago
Gruppen 1 year ago

You are correct, but in my long experience you will never get all the silver to plate out and end up losing silver or using Cl- to drop out the final silver as AgCl. I believe you would save time by first using distilled HNO3 to dissolve the HNO3 soluble metals from shot prepared by pouring the molten crude metal into DI water. This will leave residual gold and a few other HNO3 insoluble metals which you should thoroughly rinse with DI water to remove Ag+ ions (Pb+ ions too if Pb was present in the crude metal) and then use the AR to attack and dissolve the gold, etc. (insoluble will be primarily Rh, etc.).

I recommend using SO2 to drop the gold out of the AR solution after you destroy the excess nitrates rather than introduce metallic ions that could contaminate the gold. I would then drop the silver as AgCl (and lead as PbCl2 if it was present) from the solution of metals soluble in HNO3 with salt water or waste acid left over from the AR attack (if Pt or Pd, etc. are not present in the post Au drop solution), then rinse the AgCl to remove copper, etc. then convert the AgCl back into metal to melt and cast into anodes for final refining in a Thum cell to produce pure silver crystals.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I have 2 more questions:

Connected with the excess of nitrates in the AR, how do you destroy it?
I have read in lot of books that using a silver cathode is the best option to recover silver. Is it a good idea? Besides I would like to know the physical properties of the electroplated silver. Is it going to be a powder I will collect in the bottom of the flask or the silver cathode is going to grow up with the recovered silver?

Ace Levy
1 year ago
Ace Levy 1 year ago

You can destroy nitric acid / nitrates by fuming it off using HCl. Add excess HCl and boil down the solution to thick syrup, add more HCl and retreat until no more oxides of nitrogen come off. You can use a silver cathode if you like but is expensive as you are tying up some of your product in inventory. You can probably improve the quality of your deposited silver with leveling agents if you want a continuous adherent film. Silver tends to form dendrites when electrodeposited, this not usually a problem because the large crystals wash and filter readily.

Bill Rico
1 year ago
Bill Rico 1 year ago

Nov 2004 issue of the South African Institute of Mining & Metallurgy had a very relevant article. Small-scale recovery of noble metals from jewelry wastes. I think this will help you out.

Obergruppenfuhrer
1 year ago

Do you keep the silver chloride in solution in concentrated hydrochloric acid and then electro-win the silver? What do you use as anode?

Ace Levy
1 year ago
Ace Levy 1 year ago

I would not try to electro-win the silver from HCl solution. The chloride will attack your anode, chlorine gas will be evolved and the electrolyte will become contaminated. It is much easier and cleaner just to add zinc metal to the AgCl under dilute HCl. If the AgCl is well compacted, the electrons from the zinc flow through the silver metal to reduce the AgCl at the interface.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

What do you think about transforming AgCl to Ag2O adding NaOH. Then I was thinking to dilute the oxide with diluted HNO3 to obtain a solution of AgNO3. I have to precipitate the AgCl from a large volume of acid solution. I want to precipitate it and re-dissolve to filter insoluble matter which co-precipitates with AgCl. The purpose of this process is to reduce the volume and then work at small scale with acid solution. I hope you can understand it is a homemade lab.

Obergruppenfuhrer
1 year ago

I have done the following process before and work like a charm.

Use an overhead agitator for this process and the plastic containers must be as twice as big as the volume of your solutions.

Dissolve your silver alloy with a minimum of nitric acid as possible.
Decant your solution into another container. The solid at the bottom will be your gold.
In the solution containing the silver add sodium carbonate until you get a pH around 5 to 6. You can use pH paper indicator if you like.
Add caustic soda until your pH is around 13 then add some more excess. You will get a black precipitate of silver oxide (Ag2O). Keep agitating for about 1 hour.
Add carefully glucose or dextrose, wait for the reaction to start and make sure you have some available water next to it in case the reaction is becoming too violent.(believe me it does)
Slowly keep adding the glucose or dextrose until there is no more reaction.
Now your silver oxide is reduced to a fine silver metal. Let the particles to settle (use flocculent) and then wash with plenty water then dry the solids.
Smelt with some borax you get 99.9% silver. The copper is not co-precipitated.

I have also successfully reduced the silver oxide with ordinary sugar but it takes a little longer.

Please use the right PPE to do this process. Caustic soda is very dangerous particularly if a drop comes into your eyes. 

Ace Levy
1 year ago
Ace Levy 1 year ago

Sorry. I think we were talking past each other. Your process looks fine and it sounds like the reaction we used in Organic Chemistry as a test for reducing sugars. Have you ever produced a silver mirror using this process?

Obergruppenfuhrer
1 year ago

You can use the same process I described above with your silver chloride. In that case you add caustic soda until the pH is stable to about 13 then add slowly the glucose (in powder form) to reduce the silver oxide to silver metal. Be careful the reaction is quite violent when the glucose is added use a tall plastic container. What is happening the sodium combines with the chloride of the silver chloride forming an unstable silver hydroxide that turns into a black silver oxide. The glucose or dextrose reduces the silver oxide to silver metal. Yes it is very similar to the silver mirror making but beside in the chemistry school I never been interested to produce mirror. On my first attempt to clean up a highly silvery material from an AARL cathode I managed to produce 5 kg of silver by the process I described above and the gold was recovered from the primary nitric attack. The mine I consulted to get the separation has kept using that process successfully since then. I was even amazed how well it works.

Ace Levy
1 year ago
Ace Levy 1 year ago

Have you ever tried dissolving the AgCl in aqueous ammonia then adding the reducing sugar?

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I tried. But you need a large amount of NH3. Once it is dissolved it works.

Ace Levy
1 year ago
Ace Levy 1 year ago

Maybe you don't need as much ammonia as you think. You do not have to dissolve all of the AgCl before you add the reducing sugar. As the silver ammine is consumed more will form and drive the overall reaction to the right. In fact having a limited amount of the silver ammine will help to keep the reaction from getting out of control. As mentioned that was a problem.

Obergruppenfuhrer
1 year ago

Why are you using ammonia? On the method I suggested above you make the whole thing alkaline with caustic soda to pH 13-14. Ammonia is use to make mirror and has a more gentile action on the silver precipitation that is why it is used. I'm not sure why you are trying to complicate your life with the smelly ammonia (By the way it is NH4OH and not NH3 which is a gas)?

I had a very nasty beaker explosion when I tried to dissolve silver oxide with ammonia it has formed a silver azide AgN3 that exploded on its own and I end up in hospital with eyes injuries.

Ace Levy
1 year ago
Ace Levy 1 year ago

I know that ammonia is a gas, but so is HCl and it is their aqueous solutions that we use. I was not talking about dissolving silver oxide but the chloride. Precipitating the Silver from nitric acid solution with chloride, filtering and washing and then dissolving it in ammonia and reducing it with glucose is a process you might want to try.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

I did it at the beginning of the project. 1 year ago. I perfectly understood your process.

Obergruppenfuhrer
1 year ago

If you read the method I wrote above: Silver chloride and caustic soda turn silver into oxide and the glucose reduce it into silver metal. That method even can also bypass the silver chloride process. Complicating the process with ammonia is a waste of time and chemicals. But never mind be free to experiment as you wish.

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