Extract Silver from Alloy

[Enthalpy]

Hello everybody!

Pure silver is too soft and often alloyed to make objects. For instance sterling silver often contains 92.5% Ag, plus Cu, possibly Ni and others. Recycling may need to separate Ag from cheaper alloy elements.

I propose to distill Ag away from Cu, Ni and others. The 1atm boiling points spread nicely:
  Ag 2162°C – Cu 2927°C – Ni 2913°C
Reduced pressure would make at least the temperature compatible with ceramics like MgO, ZrO₂ and maybe Al₂O₃. Distillation would take far less energy than electrorefining. One step, without a distillation tower, seems to suffice.

Marc Schaefer, aka Enthalpy

[John Cuthber]

Good luck making it cheaper than electrorefining.

In theory the voltage required for electrorefining is near zero; the electrolyte is reusable  and the process happens near ambient temperature.

Also, there's not much demand for pure silver. YOu can reuse the alloy as it is.

What you need is a cheap simple way to turn pure silver into the alloy.

[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

 

Hi JC, thanks for your interest, I'll come back!

[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 for instance to cast titanium impellers for turbochargers
01336015 at hal.archives-ouvertes.fr
pdfs.semanticscholar.org
and one company melts 500kg
01333975 at hal.archives-ouvertes.fr

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

[Enthalpy]

On 11/16/2019 at 4:47 PM, John Cuthber said:

Good luck making it cheaper than electrorefining.

In theory the voltage required for electrorefining is near zero; the electrolyte is reusable  and the process happens near ambient temperature.

For copper, electrorefining needs a minimum of 0.5V to start (so the theories are oversimplified), and industries operate around 2V for decent speed. Monovalent ions consume then 0.2MJ/mol of electricity, an expensive energy that makes a significant fraction of copper cost, less so for silver.

The evaporation of silver costs 0.3MJ/mol of much cheaper energy: heat.

I found quickly the compared costs of energy for households, not industries. In €/MWh, including VAT.

150   Electricity (in France! Germany rather 300!)
100   Heating fuel
 86   Natural gas
 43   Logs
 35   Wood chips
 xx   Sunheat

And a price for natural gas "at city gate", it's 4usd/1000cuft or variable 6usd/1000cuft for "industrial price"
eia.gov and eia.gov
1000cuft contain 1156mol whose lower heating power is 242+394-75=561kJ/mol so 6usd buy 649MJ=0.18MWh.
Electricity in industrial amount costs rather 0.08€/kWh:
statista.com

 80   Electricity, industry amount
 30   Natural gas, industry amount

The metal condensation heat is available at a lesser temperature than is needed to evaporate it. A kind of heat pump would save much heat but isn't trivial to build at such temperatures.

After JC's comment, I also suggested the separation of Zn from CuZn, where distillation advantageously leaves all element in metallic form. Different aspect.

Edited November 21, 2019 by Enthalpy

[John Cuthber]

"for copper electrorefining is 0.0 V. In practice, overvoltages at the anode and cathode and the resistance in the electrolyte and electrical system result in the need for an applied voltage of approximately 0.3 V. "

From
http://doccopper.tripod.com/copper/er.html

 

Or 0.15 to 0.3V from

http://www.ct.ufrgs.br/ntcm/graduacao/ENG06631/5-b_copper.pdf

I recognise those are for copper, but silver's not that different. 

So the energy use is roughly  a tenth of what you said.
0.02MJ / mole

Now a MW Hr of (domestic) electricity is 3.6GJ and costs- as you say about 150 Euros

So that's 4 cents per MJ

So the electricity cost is 0.02 MJ/ miol times 4 cents per mol which is 0.08c per mole

Roughly 10 moles to the Kg

so the electrical cost is about  1 cent per kilo or about 10 Euros per tonne. Today's price is about 500 euros per kg or roughly 500,000 per tonne.

Essentially, unless you can recover 90% or so of the energy that you put into evaporating the metal, it's not worth it

Electro refining at about 0.02 to 0.05 MJ / mole is going to be cheaper than boiling at 0.3 MJ/ mole, even when you allow for gas being roughly a third the price of electricity (on a J per $ basis)..

On 11/16/2019 at 3:47 PM, John Cuthber said:

Good luck making it cheaper than electrorefining.

Running it in a stream of argon means you also spend energy heating argon.