Silver refining with electrochemistry

[butcher]

Much of the silver if it is thicker in coating or contact points they can remain as pieces and flakes along with powders and salts of silver in the anode bag.

Silver sulfate is quite insoluble in water, temperature plays a minor role (so try not letting the cell overheat by trying to push too much current), the solubility of silver comes up to being quite soluble in strong or hot concentrated H2SO4 (the more free acid in the electrolyte the more the silver or its ions become soluble.

I find a raw Dc power supply works better than a filtered fancy lab supply or a computer supply which are basically switching power supplies switching high-frequency AC and then heavily filtering it for a very smooth DC output, a single diode battery charger has a pulsed DC coming on and off 60 times a second, this pulsed DC single diode rectified supply is helpful to the operation of the cell, a better option I have not tried yet is a supply that pulses a varying DC duty cycle with changing of currents with long positive polarity and with fast short negative polarity in the pulse stream.

The current density will work over a wide range or even cell voltages, much depending on the size of the cell and your operation.

When electrowinning copper they can use 8 to 10 times more power than they do electrorefining copper(2KW h/kg) for electrowinning, for copper refining cells power at around 0.25KW h/Kg.
Lower current where purity is a concern.

I am more focused on the recovery of silver and copper in these discussions, as old copper pipe bus bars and dirty motor contactor points are not a feedstock for refining, although the cathode copper from the cell can be used as an anode in a refining cell basically running it back into a copper cell with better electrolyte, I have no need for pure copper at this point I have enough electrical copper.

 

[FrugalRefiner]

Nice, i will google it, it looks intereresting! Could you maybe leave a link? I am finding different kind of books.

Hmmm... I downloaded it back in 2014. I'm not finding a link now either, so I'll just attach it here. It was provided by Google because the copyright has expired. One of the best books I've read.

Well, the pdf is too large to attach. I have a smaller version that I found that someone did OCR on. It has a lot of OCR errors in it, but I hope it will be of use to you. I started with the digital versions, but I felt it was good enough to buy, so I found an old library copy being sold online. I think it was less than $10.00

Dave

 

[kurtak]

Are you sure about that?? E° of Ag is 0.80 where the E° of Cu is 0.34...
In my opinion, copper will move with lower voltage than silver.

Correct - a copper cell is run at MUCH lower volt/amp then a silver cell

Kurt

 

[butcher]

What is this magic lamp?

Let us look at our copper cell.

In the cell voltage is not important we do not need to worry about it and just forget it while running the cell.
An electrolytic cell itself is a variable resistance to current flow, it will also vary in the voltage drop through the cell, depending on many different conditions the cell is operating under.

We will discuss voltage in setting up this discussion and the cell in our example here.

We are concerned about current flow (amperage) which is what does the work in our cell, our voltage is just along for the ride.

When we set the current through the cell, the voltage drop through the cell is determined by the resistance of the cell, so the voltage will change with the cell's function or its electrochemical or physical properties or conditions.
Factors or conditions which are variable in our cell such as metal ion type and concentration spacing anode to cathode, surface area, pH, temperature, passivation of anode or cathode, and many more variable often play a role.

We choose the current we wish to have through our cell or its circuits, we choose this by determining our cell's cathode area.

A common copper tank house may run a cell at 270A/m squared to 300A/m2 which would be about 25amps per square foot (144 square inches), or about 0.176A/sq in ( 176 milliamps per square inch of cathode area).

Now let's look at our simple power supply and our "magic tool" in our circuit the light bulb in series.

Acting as an indicator of the health and operation of our cell, as a current limiter regulating the amperage or current through the circuit, current drawn from our power supply and limiting and being a major factor in regulating the current within our in our copper cell or with any other type of cell.
With a little math and Ohms law, we can set up any cell and its parameters. here I will discuss a simple setup that can be expanded upon, I will try to cover some of the math and explain it the best I can.

Let us say we have a cathode area of our cell at 6 inches square, and we plan on running it at 176 milliamps per square inch (or at 25 amps per square foot of cathode area), so we need a cathode current through our cell of one amp. we will choose a power supply and a series lamp which will do its magic.

Different incandescent lamps in series in our circuit will give us different currents depending or relative to the wattage of the lamp (the filament resistance at that temperature...).

Our lamp is a load dependant resistor, with a low current the lamp is cool and runs as a lower resistance in the circuit, as the load resistance changes or draws more current the lamp filament will heat up glowing brighter, this heated filament becomes a higher resistance regulating and limiting current to the load our copper cell.
The lamps resistance in series with our cell also drops voltage (dependant on resistance...) to our cell or regulates the voltage our cell sees, as it will drop the remaining voltage from our power supply,

The lamp's filaments resistance varies with its temperature, this resistance change also changes the lamp's voltage drop. keeping the voltage of the cell in the proper range for optimum operation.

The lamp limits the current we choose by our choice of lamp we use in series with our cell.

Let's go with a nonautomatic simple car battery charger putting out 14.5 volts DC.
volts x amps = watts
to choose our lamp 14.5volt dc power supply x 1 amp current limit through circuit = we get 14.5 watts, we will choose a 15-watt lamp, a small lamp from our trucks indicator, or a courtesy lamp from the car, this in series with our battery charger will limit the current through our circuit (and our cell to 1 amp maximum.
The copper cells' natural variable resistance will drop some voltage to the lamp (just as the lamp drops the majority of the voltage of the circuit), thus the lamp will not glow full brightness during operation, the lamp's light output will vary with the cells resistance or its function or current...
when the copper cell is shorted and no longer is a resistance in the circuit the lamp protects our power supply or its fuses, the lamp takes full voltage and glows its brightest pulling the maximum value of current of one amp in this case, giving us a visible indication of operation and of a problem in our circuit.


In our circuit of 14.5 volts, the lamp will drop more or less 9/10 of the voltage (13 to 13.5 volts). the copper cell will drop the rest of the voltage 1/10 ( 0.5 to 1.5 voltage drop perfect for our small cell).
Here the lamp's current and the cell's current are set to 1 amp, the resistance of the lamp here would be around 13 ohms, and our copper cells resistance would be 1.5 ohms, as we plate copper out of solution.

A 25-watt lamp would work for a 2 amp copper cell (12 square inches of exposed cathode area).
35-watt lamp for a 3 amp regulator to your cell.
For a copper cell with a cathode area of about 30 square inches use a 65-watt headlamp from your pickup truck with your car battery or its battery charger, limiting the current through your cell and its circuit to 5 amps...

A lamp's resistance to the flow of current (temperature dependant on how hot the filament glows), a cold filament will be a very low resistance cold and allow more current through your cell (up to its limiting range).
As the light glows heating the filament the resistance climbs drastically limiting the current flow through it and through your copper cell.

You do not need a fancy regulated power supply, you can build one a fancy as you like if you are handy and can learn what you study, or use some simple DC source, with a little understanding of OHMs law we can build a regulated circuit perfect for our use of recovery or refining copper or our other metals.

 

[butcher]

How much copper will we deposit in an hour of operation with our simple copper cell with the magic lamp dropping the voltage of our 14.5-volt dc supply and regulating its current to 1 amp?

Well, we need some figures to start.
Faraday's constant:
one coulomb = one amp flowing for one second.

A charge of electrons carries 1.60 x 10 ^-19 coulombs.
One mole of electrons contains Avogadro's constant, or 6.02 x10 ²³ electrons.

This means that one mole of electrons must carry
(6.02x10 ²³ x ( 1.60 x 10 ^-19 = 96320 coulombs
for the farady constant here we will use i mole electron flow =96500 coulombs in our calculations.
As the faraday constant.
Ok
in our circuit, we have copper II sulfate Cu ²⁺ and we are going to reduce these copper ions to metal, writing out the formula for our reaction we see:

Cu²⁺ + 2e^---> Cu⁰(s)

Here we see we need to have 2 moles of electrons to reduce one mole of copper.
From our periodic chart of elements:
we see that a mole of copper = 63.5 grams of copper.
and we see from above:
one mole electrons = 96500 coulombs.


S in order to deposit one mole of copper (63.5 grams of metal), we need 2 moles of electrons (96500 coulombs for each mole of electrons).

Now understanding this we can calculate to see how much copper we deposit on our cathode with one amp of current running for one hour.

Cu²⁺ + 2e^---> Cu⁰(s)
2 moles of electrons = 2e^-= 2x96500C = 193000 coloumbs produce 63.5 grams of copper deposit.

Our example cell running at a current density of 1 amp of current flowing x 60 seconds x 60 minutes (one hour) gives us = 3600 coulombs of electron flow.

copper mass deposited on our cathode =
coulombs used/2 moles electrons (2x96500 coloumbs) x 1 mole of copper (63.5g),
or
3600/193000 x 63.5 grams = Cu solid =(0.18625) x 63.5g = 11.84 grams of copper.

14.5vDC supply <------- (lamp)-----+ Anode {CuSO4 ectrolyte } Cathode ------> negative supply

So with our 14.5-volt battery charger and the magic lamp a15 watt lamp in series with our cell, and a high copper content (silver plated) at our anode with our lamp regulating the cells current to one amp we will dissolve and deposit 11.8 grams of copper in an hour of operation (leaving us silver in our anode bag (sock).

 

[shadybear]

I have a headache

 

[Elemental]

It’s a great explanation on electro-chemistry and a simple way to build a cell. Thanks! @butcher posts are great for being accurate, concise, and straight to the heart of the topic.

 

[butcher]

The electrolyte of copper sulfate.

While running this copper cell to recover copper and silver, we want to keep the silver from going into solution as ions, or becoming a salt of silver as much as possible.

Ag2SO4 has pretty low solubility in water, or very dilute sulfuric acid solutions, the salts of silver II sulfate becomes much more soluble in concentrated solutions of sulfuric, with temperature playing a heavy role in both solubilities.
So we do not want to have a high concentration of sulfuric or free acid in the cell, we do not want to drive the cell with too much current which would cause heating of the cell, concentrate solution (splitting water into gases), and can begin to change pH...

Running our cell with as high of concentrations of dissolved copper ions as we can, and as free of other metal ions as much as we can, with as low silver content in solution as we can, and with using mostly a very high feedstock of fairly high purity of copper in our anode sock (mostly silver plated electrical bus bars, silver soldered copper pipe... While eliminating other base metals from the anode as possible in order to keep the electrolyte as free of other metals as much as possible, High in copper concentration.

Some metals like lead, their salts are also fairly insoluble as sulfates in water.

Other metals like iron will dissolve easier in dilute sulfuric but can passivate in concentrated acid.

When cleaning up the cell, recovering the electrolyte for reuse we will concentrate the solution after filtering for insoluble's as much as possible.
Evaporating the already fairly concentrated blue copper sulfate solution drives off the water, any free acid concentrates, this makes any silver sulfate even more soluble in the more concentrated acidic solution.
Where copper becomes less soluble in the concentrated solution, while this solution after concentrated and kept hot is liquid (keeping or dissolving more silver in a solution of a very concentrated solution of bright blue copper salts also dissolved in this solution.
Once we cool or remove the heat the copper salts will begin to form crystals growing as they push silver and other metal ions from their bonds, our dissolved silver, and the insoluble silver sulfate is washed from the crystals as they are jarred, stored under some of their own liquid and a few drops of fresh H2SO4, saving our blue copper sulfate for another project or cell or as a lab reagent as the pretty blue copper crystals are fairly pure.

Once cooled the copper crystals are more difficult to dissolve back into the water, for this reason, we have to learn to use heat and the facts of the salts solubility of different temperatures when working with these projects, it just makes things go much more smoothly.

Upon diluting the remaining highly concentrated acidic solution (we washed from our solid copper sulfate blue crystals) the white silver sulfate ( or yellow if less dilute and allowed to sit longer) silver salts precipitate in the diluted less concentrated acidic solution.

Recovering cathode copper, Blue copper sulfate crystals, and silver as metal with some silver salts, with as little waste generated as possible during the processes, leaving us with less waste to deal with.

 

[butcher]

Gold can also be recovered from a copper cell, very similar to how we can recover silver.

If we are dealing with 98% copper and attempting at recovering a tiny bit of gold silver or PGM's

It seems to me we should not think about being in the gold business.

But put our focus more on our copper business instead.

Besides, we are going to need tons of copper if we are all going to be riding around inside those tiny little electric "smart bicycles" with no mining of metals to make the needed batteries (we can make our own cells) for more pedal power up the hills.

 

[Swissgoldrefiner]

In the cell voltage is not important we do not need to worry about it and just forget it while running the cell.

I totally disagree with you about that. With voltage, you can setup almost which metal you remove and which you dont move.
The rest about the amp looks a mess too! In electronics, you never choose the amp, the best you do is chosing the max amp.
You decide the voltage and the resistance of a circuit and the amp comes from U=R*I
The resistance is depending of the concentration of the electrolyte solution, off curse the area of your anode and cathode and voltage.
With same area as you suggest, if you put the anode and cathode 10 cm from each other or 50 cm of each other you will not have same current.

How i do with my experiment? I use multimetre...i try to see the current during the process and i try to check the resistance between my anode and cathode.

 

[Swissgoldrefiner]

The lamp limits the current we choose by our choice of lamp we use in series with our cell.

This lamp methode is interessting methode, but cant be used in large scale. Our aim is to make it workable in large scale. Dont forget, if you use a lamp, you loose some part of the electricity, so you pay more for your refining.

 

[Swissgoldrefiner]

Ag2SO4 has pretty low solubility in water

Desagree again, at least wiki desagree: 8.3 g/L at 25°C.
What make this selective is the voltage and i am testing it. You never consider the E° of element...in my opinion, if you stay under 0.8 [V] you dont move any Silver into solution. The voltage that you need for copper is 0.3 [V].
What i will test is choosing as high as possible current with limited voltage around 0.3 [V].

Will let you know if it works.

 

[butcher]

If you understand the electricity and its reactivity in an electrolytic cell or basically any circuit works then you will understand my post.

If you do not understand it, you can do the research and experiment for yourself to see that what I stated are facts and how electricity works...

 

[butcher]

What I suggest will work with a large-scale copper recovery or refining operation, sure we may choose a different method of regulating current through the cell choosing a different DC source besides using an automotive lamp but the principles are the same.

I believe you misunderstand the wiki and the stated solubility of the silver salts. which have a low solubility compared to copper ions in solution, with the cell here as described, we are not running it at a potential to put silver into solution (or we are not forcing the electrons to move from the silvers atoms at the anode, we keep the cells parameters to keep as much silver or other metals out of solution as ions as possible) in our design here, we have set up the electrical conditions to pull electrons from the copper atoms (preferentially over the silver atoms), our goal here is to move copper ions (not the silver ions) so we set our parameters so that we do not dissolve or plate out silver as much as we possibly can with our lamp helping us with this trick.

Yes, we will get some silver in the cell electrolyte, we also recover that silver as I have explained above.

You cannot measure the internal resistance of the cell, measure its current, or measure its voltage directly with meters...

But we can use one or more of these types of meters to do some measurements and calculate the values of the cell using Ohms law, and see what is going on inside the cell by measuring the external circuit and calculating the unknown values within our cell.

 

[butcher]

The solubility of pentahydrate copper sulfate in water at 20 degrees centigrade is around 32g/100ml or 320 grams per liter.

The silver sulfates solubility at around 25 degrees centigrade is around 0.83 g/ml or 8.3g/liter.

Also, that with running the electrolytic cell at a lower potential as we have discussed, or not forcing electrons from the silver at the anode, we have put less silver into solutions as ions.

That, and with our highly concentrated copper electrolyte, we do not leave much room for silver ions.

With our copper (at the anode and with copper metal reduced at the cathode) in this solution, which tends to displace silver ions from solution (converting the ions to atoms or insoluble silver metal powder through a displacement type of chemical reaction (helping to lower silver ion content in our electrolyte solution.

Ag2SO4 (aq) + Cu (s) --> 2Ag (s) + CuSO4 (aq)

Our ideal would be for this to happen in our anode bag before the silver ions leave the anode area, where the copper atoms give up electrons to the silver ions as fast as they are formed in the anode compartment or anodes vacinity.

If you happen to be using city water, or have any chloride in the electrolyte solution that also will pull silver ions from the solution (although that is not our goal here in our discussion)...

Here we are discussing more of a metal recovery operation for these metals, but a refining type operation would be almost the same but with tighter parameters.

 

[olawlor]

... In electronics, you never choose the amp, the best you do is chosing the max amp.
You decide the voltage and the resistance of a circuit and the amp comes from U=R*I

I do electrolysis with a little lab power supply (to be specific, this one: https://www.amazon.com/gp/product/B077GVMP5X/ )
Like most lab power supplies, it runs in one of two modes:
Constant Current (CC): limits the current delivered (A)
Constant Voltage (CV): limits the voltage delivered (V)

Both values are displayed on the front panel at the same time, which is handy for debugging the cell. For example, a short circuit (due to a dendrite forming) will show up as CC mode, voltage nearly zero. An open circuit (due to the anode dissolving off, or a wire coming loose) will show up as CV mode, with amps zero.

For electrolytic metal purification (at least for copper, silver, and iron cells), I normally run the supply in constant current (CC) mode, based on the area of the cathode and anode metals. The cell voltage varies with the solution temperature and concentration, which will vary during a run. As butcher points out, the number of amp-hours through the cell is directly related to the amount of metal you deposit (and the current efficiency, which is normally high for precious metals).

With warm concentrated electrolyte and good cell geometry, I've seen the voltage run below 0.1 volt for copper, below E°, which surprised me! I think this indicates the metal-electrolyte-metal "battery" gives a zero net voltage when the cathode and anode are the same metal. I have needed much higher voltages when electrowinning, such as a few volts needed to recover iron from iron oxides, because then the magnetite-electrolyte-iron "battery" gives a net voltage we need to fight back to recover actual metal.

 

[Elemental]

https://archive.org/details/electroplatinga00wattgoog
Another link to Watt's book on Electroplating and Electrorefining.

 

[FrugalRefiner]

Yes! That's the version I tried to find. Thanks for providing the link.

Dave

 

[popslab]

Copper pipe silver soldered, silver contact points cut from bus bar, silver-plated copper bus bars, and bolts nuts...

The cell can be a large salad bowl.

The DC power supply can be a transformer and a bridge rectifier, (a capacitor could be used to smooth ripple but is not needed) fancier supplies you could add a voltage regulator and a couple of more capacitors these can be part of almost any electronic device or made from their scrap parts.
old computer power supply's can be wired or made to work.

A nonautomatic 12volt DC (14.5 volt DC) automotive battery charger (basically a step-down transformer (line voltage AC to about 15 volts AC), and a diode ( converting AC to DC current) works very well, making getting started easy.


Copper sulfate is highly concentrated as an electrolyte.
If you bag the silver/ copper in a sock with an anode (larger bar same material).
from the positive of your 12v battery charger's positive lead goes to a twelve-volt automotive headlamp the other side of that headlamp is jumpered to your anode the bagged material you wish to put into solution as ions, the cathode can be thin copper plate or foil (or stainless steel) connected to the negative lead of your DC power supply.

When the anode material is high in copper, the electrolyte is high copper most silver will stay in the anode bag, and fairly pure copper plates out.

After running large runs, the electrolyte can be crystallized through evaporation, you can recover the small amount or the rest of your silver from the bright blue crystal you are saving to reuse in the cell later or for some other purpose.

The lamp is useful to limit and regulate the current through the cell, it drops the voltage as it is a variable resistance being wired in series with the copper cell, the filament of the lamp changes resistance with heat or as the lamp glows.
The lamp is a visible indicator of the cell operation its current and voltage if it is working or there is a problem in the circuit.
The lamp protects your cell and the circuit from a shorted cell, (the shorted cell not being resistance to current any longer, would just ground out, or give full voltage to the lamp lighting it its brightest, the lamp limits the current of the cell or the load on your DC power supply...

copper sulfate can be bought (sewer pipe tree root killers).

Copper sulfate can be made from other copper salts or solutions, if you use copper waste solutions and you distill (safely understanding the dangers involved) you can convert copper nitrate into nitric acid and copper sulfate, or you can convert copper chloride into copper sulfate and hydrochloric acid, the very hot and concentrated copper sulfate remaining after the distilling process removed hot and is crystalized through cooling of the solution, the crystals reject impurities to the blue copper crystals if you add cut up pieces of copper to the solution you are distilling it helps drive the reaction forward if the copper is gold plated it is a bonus.

See my post on killing two birds with a stone for more ideas.

Thank you for tour answer i have been experimenting with this and did what swissgoldrefiner did and got the same results. I was ready to attempt with copper sulfate but really not sure on concentration of the electrolyte. Any suggestions?

 

[Yggdrasil]

Thank you for tour answer i have been experimenting with this and did what swissgoldrefiner did and got the same results. I was ready to attempt with copper sulfate but really not sure on concentration of the electrolyte. Any suggestions?

Read the book they refer to, it should be there.