[knowncovvert]

I am trying to purify an alloy of sterling silver (containing copper, nickel and silver) into as pure silver as possible. I am currently planning on dissolving the alloy in nitric acid, then precipitating out the silver using sodium chloride.

However I am aware that some of the silver chloride can form AgCl2^- and remain dissolved in solution. What would be the best way to get the last little bit of silver out of this solution? I was thinking either electrorefining or thermal decomposition?

Any help would be much appreciated. Thanks ~

[Enthalpy]

Electrorefining is efficient: 99.999% pure copper is made that way, which separates silver
http://en.wikipedia.org/wiki/Copper_extraction_techniques#Electrorefining
as a by-product of copper mining.

But it has drawbacks...
- Amount of electricity
- Size of electrolysis cells. Duration.
- It must require know-how.

Could you distill the alloy under low pressure?
The standard method to refine magnesium uses that http://en.wikipedia.org/wiki/Pidgeon_process
Your elements spread nicely: Bp (1 atm): Ag 2162 °C - Cu 2927 °C - Ni 2913 °C.
Reduced pressure would make the temperature compatible with ceramics like ZrO₂ and maybe Al₂O₃.
Distillation would take far less energy and time than electrorefining.

[NobleMetalWorks]

I am trying to purify an alloy of sterling silver (containing copper, nickel and silver) into as pure silver as possible. I am currently planning on dissolving the alloy in nitric acid, then precipitating out the silver using sodium chloride.

However I am aware that some of the silver chloride can form AgCl2^- and remain dissolved in solution. What would be the best way to get the last little bit of silver out of this solution? I was thinking either electrorefining or thermal decomposition?

Any help would be much appreciated. Thanks ~

Why not cement the silver nitrate on copper, directly converting it to elemental silver?  Then you could melt into an anode bar and electrolytically refine it to high purity?  Precipitating AgCl will end up being about the same purity as cementing silver on copper, but with extra steps to refine to high purity.

If you plan on processing your sterling silver using the silver chloride method, you could also use HCl to precipitate AgCl, I believe you would convert all the silver nitrate to silver chloride in this case.  Reduction can be done by using Kyro syrup or other methods, and once in elemental form you could melt into an anode to refine in an electrolytic cell to high purity.

What is your intent, why are you refining this silver?  Is this for a hobby?  Or are you wanting the silver for another process?  Knowing a little more about your intended purpose might be good in giving you the correct information for your intended purpose.

Scott

[Enthalpy]

Sn-Pb alloy was and is a solder for electronic boards, copper tubing and more. Pb too could be distilled away from Sn for recycling, as the 1atm boiling points spread nicely:
  Pb 1649°C - Sn 2602°C
Here too, reduced pressure would make the temperature compatible with ceramics like MgO, ZrO₂ and maybe Al₂O₃.

Marc Schaefer, aka Enthalpy

[Enthalpy]

From Wiki and interpolating:

K for   K for   K for   K for   K for   K for
  1Pa    10Pa   100Pa    1kPa   10kPa  100kPa
====================================================
  610     670     750     852     990    1185    Zn
  882     997    1097    1412    1660    2027    Pb
 1283    1413    1575    1782    2055            Ag
 1509    1661    1850    2089    2404            Cu
 1497    1657    1855    2107    2438            Sn
 1783    1950    2154    2410    2741            Ni
====================================================

Zn is easily evaporated from brass CuZn. At 1356K to melt Cu, Zn has roughly 750kPa and Cu 0.1Pa, clear case with one crucible. Even a few per-cent Pb in brass (664Pa) separate easily from both, optionally in two steps for Pb-Cu.

Pb is easily evaporated from Sn63 Pb37. At 1660K for 10kPa Pb, Sn has 10Pa. Leaving a bit over 0.1% impurity in each takes two steps, so crucibles suffice.

Ag could be recovered from Sn95.9 Ag3.8 Cu0.7 solder where it makes half the value. At 1782K that give 1kPa Ag vapour pressure, Sn has 43Pa and Cu 44Pa. The pressure ratio 4/100 is also the initial composition ratio, making few steps inefficient. A distillation column is better.

Ag could be recovered from Sn61 Pb37 Ag2 solder. At 1660K for 10kPa Pb, Ag has 257Pa so Pb would separate first with very few stages, but then the separation of Ag from Sn needs a distillation column anyway.

Cu and Ni can be separated by a distillation column or several crucible steps. This needs high temperatures.

Cu and Sn shouldn't be separated that way.

With Pb, Ag, Cu, Sn, Ni more noble than Mg and Al, the ceramics MgO and Al₂O₃ have chances to resist the molten alloys and possibly molten Zn. Suggested operating temperatures in air are 2500K for MgO, 1800-2100K for Al₂O₃, with big variations.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Levitation melting lifts the melt from the crucible to avoid pollution at high temperature. Patented a century ago, it's often a demonstrator or a research tool, but one team melts 0.5kg
https://hal.archives-ouvertes.fr/hal-01336015/document
https://pdfs.semanticscholar.org/7bb9/c15bc4bcfbaa9ddd56ada3a96e24003dace8.pdf
for instance to cast titanium impellers for turbochargers, and one company melts 500kg
https://hal.archives-ouvertes.fr/hal-01333975/document

Most designs are very crude: no magnetic material, hence coils of small section, cooling fluid parallel to the current, wires too wide for the frequency. An expert magnetic designer should improve that.

The 0.5kg team claims with citation that an axisymmetric field can't levitate metal at its centre, which has to hold by capillarity. To my understanding, the outlet at the centre prohibits coils there, and this is what reduces the force.

A temperature not limited by the crucible would let evaporate less volatile metals. Density would prevent boiling at depth: 10mm of 10000kg/m³ melt add already 1kPa. Possibly metal would evaporate from the lower faces too because the electromagnetic pressure needs a Kelvin effect depth to build up, but any layer of the volatile metal condensed in the magnetic field would evaporate quickly.

So I suppose distillation accepts coils up to the centre in a simpler apparatus, shallow and wide. Once the evaporation is finished, the melt can levitate and cool in a lower frequency induction before landing.

Many small melts, down to individual drops, could be better than one big to accelerate the evaporation and save electricity. Very small melts could evaporate more quietly, without boiling.

Marc Schaefer, aka Enthalpy

[Enthalpy]

An ascending gas jet can levitate an alloy drop to evaporate the more volatile metals without any polluting contact.

If for instance the drops are 10mm³ small on a 10mm×10mm pattern, then 1m² can process 0.5-1kg at once. A robot would place and possibly pick the samples.

Hot argon is one natural choice to levitate and heat the droplets. It would carry away the vapour of the more volatile metal. The nozzles must resist the temperature but don't risk to dissolve in the melt.

The heat source can be cheaper than electricity. Maybe the condensation heat coud be recycled, but being available at a lower temperature than evaporation needs, it would take some heat pump equivalent, which isn't trivial at these temperatures.

Smooth evaporation seems preferable to boiling. The carrier gas pressure shall realize that.

Marc Schaefer, aka Enthalpy