Silver Cell for Dummies.

[amesametrita]

Part 1. Uniqueness

Why silver cell is unique compared to conventional electrorefining cells like copper or gold ones?
Every other cell operator aim for dense and well adhered cathode deposit.
They make starting cathode as a thin foil of desired metal.
Anode dissolves, some metals remain dissolved, some metals drop to bottom as slimes and desired metal deposits on cathode and removed as fat cathode later.
For some reason early silver cell inventors refused to follow "the proper" way.
They wanted slimes in anode bags.
But the main difference - they wanted a loose cathode deposit.
So the cell should has a flaw or defect.
Cathodic polarization is the way to have a loose cathode deposit.
Everybody fight against cathodic polarization as parasite power consumption but silver cells need it for a reason.

 

[amesametrita]

Part 2. Conductivity of electrolyte

There are two types of electrical conductivity.
Metallic conductivity and electrolytic conductivity.


Quote:
A fundamental difference exists between the two kinds of conduction. In both cases the current produces heating and magnetic effects; but in "metalic" conductors the electricity passes through without the accompaniment of any ponderable quantity of matter, whereas in electrolytic conductors the movementof the electricity is always associated with the movement of matter.


Metal ions move from anode to cathode, NO₃ ions move from cathode to anode.

The major metal in our cell dissolves on anode and deposits on cathode. So it can move and move and move. But we have a finite quantity of NO₃ in solution, at some point in time (quite early) NO₃ movement should cease.

From the other side metals less noble than Ag can't deposit on cathode due to not enough overvoltage. So they concentrate in vicinity of cathode. We can count H⁺ ion as metal here.

The solution becomes non-uniform. Non-homogeneous.

 

[amesametrita]

Part 3. Concentration gradient
Attraction force between oppositely charged particles is inversely proportional to the square of the distance.
This means that ions in electrolyte don't attract directly to cathode or anode.
It's a kind of a chain.
Closest to the cathode Ag⁺ deposits, leaving the area with excess of NO₃^- ions, and this area attracts another Ag⁺ which is further from cathode. This Ag⁺ moves and makes a new negatively charged area and so on.
What is going on if ions can't gas out or deposit on cathode or anode?
Let's investigate the anode side.
NO₃⁺ build up near anode, making negatively charged boundary layer. The rest NO₃⁺ in solution stop moving to anode because they don't want to move in to direction of negatively charged layer.
The same time on anode Ag oxidize to Ag⁺ and is more than happy to jump to that negatively charged layer.
The problem is that lately Ag⁺ doesn't want to leave this layer. The layer attracts.
Until enough Ag⁺ jump from anode and make the layer electroneutral.
Looks like we have higher concentration of both Ag⁺ and NO₃^- near anode.
In case of too high concentration of AgNO₃ in the solution it will take a lot of energy by friction forces to penetrate this boundary layer.

You can't run silver cell effectively with a huge concentration of Ag
Normal concentration is up to 150g/l
Less concentration = more effective electroplating/electrorefining.
Moebius used in his cell only 1g/l of Ag concentration
Just 1g/l

Update: Transport number of AgNO₃ is very slightly dependent on concentration. For parts of water to 1 part of AgNO₃:
1:2.48 = 0.532
1:5.18 = 0.505
1:14.5 = 0.475
1:49.44 = 0.474
1:104.6 = 0.474
1:247.3 = 0.476

 

[amesametrita]

Part 4. Transport
What is the majority of energy applied is coming to?
Net energy balance for Ag on the anode and the cathode is zero.
We spend energy to do the "work" of moving matter from anode to cathode through electrolyte.
If we decrease the travel distance we will have better efficiency.
This is why Moebius cells are more energy effective.
Anode to cathode spacing is less than in horizontal cells.
If we decrease anode to cathode spacing we have to remove deposit more frequently to combat short-circuit.
Moreover the cell becomes unstable.
When anode dissolves the effective distance increases.
In case of 10mm of difference if we have 100mm spacing it is only 10%, in case of 30mm spacing the difference is 30%
Industry adopts about 50mm (2 inches) spacing as a good trade-off.

There were some early works in "mechanically assisted" transport.
You can simply pump electrolyte in anode bags and suck it near cathodes.
Drawback is mechanically pushing slimes through anode bags.

As I know the only one company (located in Australia) achieved some good results in mechanically aided transport. And mainly for copper.

 

[amesametrita]

Part 5. Ohm law.

If the concentration of AgNO₃ is too low the resistance of the cell is too high.
The Ohm law says: I=U/R
The current (the amperage) is the measurement of how much work we do.
We need faster refining so we need more amperes.
Resistance is high.
To boost up the amperage we can increase voltage.
But to some extent.
If cathodic potential is high, other metals besides silver start to deposit on the cathode.
Why is resistance is high?
Because the average distance between Ag⁺ ions out of anode boundary layer is to big.
And electric forces for big distances are weak.
We don't want to introduce more Ag.
What if we introduce some other positive ions?
They will take part in the chain of moving Ag⁺ and let them move faster.
Of course that ions will move to cathode as well.
But if they are less noble then Ag and with right cathodic potential they will not deposit.
With agitation they will move away from cathode and will help Ag again.
Agitation is essential here.
Vertical cells use electrolyte recirculation and/or mechanical stirrers.
Horizontals depend on Archimedes law.
Anyway agitation in horizontal cells is weaker so, again less effective work.

Any suspects for the role of friendly positive ions?
Let's investigate 3 of them.
Cu⁺, H⁺, K⁺

 

[johnny309]

Yes ...the OHM law...is I=U/R,voltage has the simbol "U"...and this is international.
So...how to grow more crystal in less time?
Let's think.....mmmm......I know.... :lol:


.

To charge a Pb battery you will do an electrolisys procedure......

To have many Amps per hour you will need a greater surface....
So...the rule is the same here ...in silver cell.....do not increase the voltage(it will give you dirty crystals)....just increase the size of the cell,the anode and the cathode.

 

[amesametrita]

Part 6. Parasitic reaction

Unfortunately at the cathode we have not only one reaction:

Ag⁺ + e = Ag

We have also:

NO₃^- + 2H⁺ + e = NO₂ + H₂O
NO₃^- + 4H⁺ + 3e = NO + 2H₂O
2NO₃^- + 10H⁺ +8e = N₂O + 5H₂O

And they become more active with the elevation of H⁺ concentration near the cathode.
They literally destroy our electrolyte for no good.

 

[amesametrita]

Part 7A. Hydrogen as a transport's friend.

We can introduce H⁺ as HNO₃ to boost up amperage.
But we increase parasitic reaction rate (Part 6)
Also we will redissolve some already refined silver.
Moreover H⁺ is very mobile and will run from solution to concentrate near the cathode.
Amperage boost that you see on an amperemeter after adding HNO₃ is nothing productive but electron consumption by parasite reaction.
More harm then help.

Part 7B. Copper as a transport's friend.

Naturally copper builds up in the solution during refining.
And becomes the main contaminant.
But is it a good idea to intentionally add it to fresh electrolyte?
Some people start without and wait, but their initial cathode deposit is too dense because initial current is too low.
Some people add it and intentionally short their electrolyte life.

Part 7C. Potassium as a transport's friend.

Fortunately K⁺ ionic radius is 2 times bigger then Cu⁺
Means it moves to the cathode slower.
And minor agitation keeps it distributed in solution.
It moves almost the same speed as Ag⁺
And will never be co-deposited on the cathode!
Ideal transport's friend.

Update: For voltage drop 1volt per 10mm of electrolyte @18deg Celsius the ionic speeds:

K⁺ = 0.00669mm/sec
H⁺ = 0.03415mm/sec
NO3^- = 0.00640mm/sec

 

[amesametrita]

Part 8. pH

We need to have acidic electrolyte.
Different sources recommend pH between 2 and 3.

If we have too acidic electrolyte palladium starts to dissolve at anodes and to deposit on cathodes.
The rule of thumb - if suspect palladium then increase pH

The problem again in parasitic reaction (Part 6)
Slowly pH of electrolyte increases.
If it increases over pH 4 then hydroxides of base metals start to precipitate contaminating the refined material.

One solution is to periodically add some HNO₃.
The problem is that HNO₃ is a strong acid.
And it dissociates very fast.
H⁺ run to cathode and intensify parasitic reaction.
What if we add a weak acid?
Like tartaric acid...
It will dissociate slower, will do it in a volume and maintain acidity longer.

Or HNO₃ dosing pump in recirculation contour.

 

[amesametrita]

Part 9. Cathode contaminants

There is one quite good modern work in public domain about contaminants by Slovak researchers:
http://www.scribd.com/doc/46524126/Refining-Electrolysis-of-the-Secondary-Silver-Alloys
The problem of this study is that they use small electrode gap, high current and no agitation.
So figures slightly differ from industry averages.

Let's investigate WHY contaminants deposit on the cathode.

Mechanical drag?
We all know that crystals reject impurities when build up.
This property of crystals is used in many purification technologies (like AgNO₃ purification by evaporation and crystallization).

Open the table of standard potentials:
Au³⁺ / Au + 1.68, Pt²⁺ / Pt + 1.2, Pd²⁺ / Pd + 0.987, Ag⁺ / Ag + 0.799, Cu²⁺ / Cu + 0.337, H⁺ / H 0, Pb²⁺ / Pb - 0.126, Sn²⁺ / Sn - 0.136, Ni²⁺ / Ni - 0.25, Fe²⁺ / Fe - 0.44, Zn²⁺ / Zn - 0.763

Looks like only Ag and more noble metals should deposit...
Sure?
The trap here is that all contaminants exist in oxidation state (II).
And we expect that they reduce to neutral metals.
In the real world the reaction is two step.
And Cu(II)⁺ reduces to Cu⁺ as easy as Ag⁺ to Ag
If there are a lot of Ag⁺ around then Cu⁺ loses electron by displacement reaction.
Have you noticed fine silver powder floating sometimes?
This is outcome of such reactions.
If there is not enough Ag⁺ around, the second part Cu⁺ -> Cu can occur.
As we already discussed copper ions are more mobile than silver ones.
And they take part in concentration polarisation more readily.

Conclusion: We almost don't care about how much copper we have in solution.
We have to care how much silver we have in the cathode boundary layer.
And this controlled by:
1) Copper/Silver concentration ratio.
2) Current density.
3) Agitation.
You see, there is no such thing like maximum copper content or minimum silver content.
I tell you again, Moebius run his cells at 1g/l of Ag and 40g/l of Cu.

Less Cu/Ag ratio, you can use more current and less agitation.
More Cu/Ag ratio, then reduce current and agitate.

 

[amesametrita]

Part 10A. Noble contaminants
Your anode bag is not ideal.
Some colloidal gold and PGMs will penetrate it.
There are some technologies against it like solid bottom of the bag, sucking solution from the bag to make counterflow etc.
They all work for Moebius cells only.

Moreover, if you have high anodic potential some PGMs including platinum will go into solution. If solution is quite acidic they will definitely reach the cathode and deposit. If pH is high, they will form oxides and drop inside the anode bag after leaving the anode.

So, if you expect PGMs then rise pH and decrease voltage.

Part 10B. Base metal contaminants
If the pH is too high base metals will start dropping as hydroxides.
So you have to keep pH low.
Without PGMs the solution is simple, just acidify the electrolyte.
But what if you have PGMs the same time?
There should be a trade-off.
For example you keep pH high, and hydroxides contaminate refined silver.
The good point is that they are not alloyed to crystals.
So after harvesting wash crystals with well diluted HNO₃.
Hydroxides redissolve very fast leaving pure crystals.
This very cheap operation considerably increases the purity.

Clarification: pH high means 3. pH low means 2. Below 1.7 and above 3.3 is prohibited territory where a lot of parasitic reactions start taking place.

 

[Drimacus]

I want to maximize the life of the nitric acid electrolyte, so will be using silver anodes of at least 99% purity. It would also be desirable to minimize electrolyte decomposition by parasitic reaction as discussed in part 6. My question is, will reducing the current/amperage help to minimize the formation of nitrogen oxides?
Also, in the Moebius patent US532209A of 1895 the electrolyte used is a strong acidied solution of potassium or sodium nitrates. Why this and why has it fallen out of favor? Just curious.

 

[amesametrita]

Part 11. Tellurium and Selenium

The only contaminant I'm afraid of in a silver cell is Tellurium.
Even at concentration of 20 milligrams per litre it makes a mess.
You can see the appearance of deposited crystals in the famous post of KADRIVER:
http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=50&t=16132

With 0.2% of tellurium in the electrolyte the process completely halts.
Anodes can contain tellurium in as Ag₂Te.
During anodic dissolution following reactions take place:
Ag₂Te - 2e = Te + 2Ag⁺
Ag₂Te + 3H₂O - 6e = TeO₃(II)^- + 2Ag⁺ + 6H⁺
Ag₂Te + 4H₂O - 8e = TeO₄(II)^- + 2Ag⁺ + 8H⁺

H⁺ run to the cathode and you spot unusual intensity of NO_x forming from parasite reaction and grey sponge deposit.

Insoluble AgHTeO₃ and Ag₂TeO₃ drop down from the electrolyte and contaminate crystals.

Selenium also forms insoluble Ag₂SeO₄ from electrolyte. The good point here that it doesn't hurt the main process.
And selenium fully burns out during following crystal melting.

 

[amesametrita]

I want to maximize the life of the nitric acid electrolyte, so will be using silver anodes of at least 99% purity. It would also be desirable to minimize electrolyte decomposition by parasitic reaction as discussed in part 6. My question is, will reducing the current/amperage help to minimize the formation of nitrogen oxides?

It will help.

 

[amesametrita]

Part 12. Feedstock

If your feedstock is 900+ppt of Ag you can make anodes directly from it.
I don't like the industry standard which is Min 750ppt Ag, Max 200ppt Au and PGMs, Max 75ppt of base metals.

If your feedstock is more dirty, you have to upgrade it first.
There are two methods.

1) You can run a separate silver cell using the spent electrolyte from the main one.
Run it at lower current and voltage until 1g/l of Ag or until you see it works very slow.

2) Nitric digestion and cementation on copper.

You don't have to pre-process the whole amount of the metal you have.
Make enough "almost pure" silver to alloy 900+ppt anodes.

It's a good idea to flux out tin, lead, arsenic and antimony at this point.

Nothing about electrochemistry here. Just rules of thumb for maximum profit.

 

[amesametrita]

Part 13. Electrolyte reconditioning

I can't see any reason for electrolyte reconditioning when dealing with low grade feedstock.
It's just economically unfeasible.
And even more economically unfeasible is nitric digestion and cementation of the whole feedstock.

But for the brave souls I can share the recipe we used long time ago.
So far I think it's the best one.
And I don't recommend it.

Initially you need silver oxide.
Even contaminated one.
The easiest way to produce it is to dissolve sterling silver in HNO₃.
Then to add 1% NaOH to the solution and increase pH to 9
Settle, decant, you have the powder.

Let's regenerate our electrolyte.

First step - Add our dirty Ag₂O, stir well.
Aim to pH5.5
If you stop before pH5.0 almost nothing will drop.
If you overshot 5.8, a lot of contaminants forming.
Centrifuging or press-filtering will give a way cleaner electrolyte.
Why it's better to stop before 5.8?
To keep Ag in solution.
Anyway at pH5.5 about one third of precipitate is silver.
You have to part it out later.

Next stop is pH 6.0-6.2
You will have more precipitates.
If you stop below 5.9 lead and nickel will not drop down.
If you overshot 6.3 a lot of silver will drop.
Stir, stir, stir.
No chance to filter the second precipitate.
Settle the solution for 5 hours, then decant refreshed electrolyte.
Add HNO₃, Ag.
Ready.

Keep the WET precipitate from the second step, don't try to part it.
Next electrolyte reconditioning you use it in the first step to increase pH
It will save you Ag₂O


Let's assume we have 90/10 sterling silver.
Initially somehow we produce 37.5g of cemented silver.
And take 100g of sterling
Dissolving in HNO₃ (10*4.15ml + (90+37.5)*1.22ml = 197ml)
Dilute to 1.5 liter
Ag 85g/l Cu 5g/l
Add KNO₃ or NaNO₃ or Cu(NO₃)₂ to boost up amperage
Critical amount <25g/l Ag
Till 25g/l Ag we can remove 60 g/l
And introduce 60/3.4=17.6 g/l Cu
To introduce 17.6 g/l we can refine 176g/l of sterling in the cell
176*1.5l = 264g - refined
60*1.5 = 90g - depleted
264*0.9+90=327.6 harvested
Plus we have 37.5g Ag in solution which we cement with only 37.5/3.4=11g Cu
And start over.

Total:
327.6g of pure Ag using 197ml of HNO₃ and 11g Cu

Normal method even not counting electrolyte (price is divided by ~10 refinings):
327.6g pure Ag -> 364g sterling
To dissolve 327.6*1.22+32.7*4.15 = 535.3ml HNO₃
To cement 327.6/3.4=96.3g Cu
Counting electrolyte is even bigger

 

[amesametrita]

I'm not going to copy it to the Part 14 here.

 

[Lou]

Silver oxide method is eerily similar to one Metalor uses...

 

[amesametrita]

Because there are no secrets in electrochemistry.

 

[amesametrita]

Part 15. Purity

Nitric digestion and lazy copper cementation usually achieve 990ppt of silver average.
"Proper" cementation consistently gives you 998+
Horizontal cell usually output 999-999.5ppt from 900+ feedstock
Modern vertical cells achieve 999.9 from 900+ feedstock

What if we need more?
You have to re-electro-refine.
But rules change.
Everything counts.
For example you distil HNO₃ twice.
Air-filtered working rooms.

How the electrolyte is produced?

You take 999.9 as a feedstock.
Dissolve in fresh double distilled HNO₃.
Boil out till Ag concentration of 1200-1300g/l
Then cool down.
Then collect crystals of AgNO₃ with minor contamination.
Then melt AgNO₃ in controlled atmosphere at 300 degrees Celsius. (Melting point around 208C)
Base metal nitrates decompose to insoluble oxides or insoluble alkali salts.
Silver nitrate stays intact.
Then pour molten mass to chemically pure water.
Then let it settle and decant the solution of AgNO₃ with just some extremely small traces of contamination.

Silver cell is used very seldom.
Usually it's refined in conventional cells with low current, without anode bags and with priming cathodes made from extra pure silver.

All that madness repeated 2 times produces 999999ppm silver.
It's 1 gram of impurities per 1000 kg of silver.