Electrochemistry question about cathode

[Traveller11]

This is an electrolytic setup I began looking at a while back and have finally decided to build. It is designed to extract very fine free and refractory gold from mining ores.



As the patent claims, a mixture of sodium hypochlorite and sodium chlorate is made at the anode. As the pH of this solution is between 7 and 9, a portion of the hypochlorite/chlorate will exist as the oxidizers hypochlorous/chloric acid. These oxidizers, along with the chlorides from the salt, will put gold into solution as a chloride. Further, this gold chloride will supposedly be attracted to the cathode, where the gold will be separated from the chloride, depositing at the cathode, and the chloride will be recycled as hypochlorite/chlorate.

The problem is this; the gold will fall right back into the ore and get re-mixed. Is it possible to place the cathode in a cathode bag or some kind of porous cup (fused silica?) or clay flower pot that will allow the passage of gold in solution into the cup or bag, but keep ground ore from entering?

Here is a little more info:

 

[Traveller11]

I realize there is a good chance the gold will adhere to the cathode but, there is also a good chance it will not. As can be read in this article below, from the 1890's, gold did not adhere to the cathode but "fell like a shower of fine spangles to the bottom".

http://www.miningandmetallurgy.com/metallurgy/electrolytic-precipitation-gold

 

[bswartzwelder]

It's been quite a while since I read the Shor patent information. But, I thought the porous cup was there to allow electric current to flow between the anode and cathode, but due to the small pore size, the gold would be trapped on the side of the cup where the anode is. The gold you are trying to deplate is actually the anode of the cell. If the anode is inside the cup, the gold deplates from the base metal and is trapped inside the cup. If you put the anode outside the cup, the gold deplates from the anode and is trapped outside the cup. Then, the gold is precipitated from the solution with SMB or some other precipitant. The porous cup allows for current flow, but keeps the gold in the solution wherever the anode is.

For small batches, the anode is inside the cup and the solution volume, and the amount of gold it can hold is small. If the anode is outside the cup, the solution is much larger and the therefore you can put a much larger amount of gold into solution.

Please keep us informed as to how well your system works.

 

[Traveller11]

Yes, I will keep you informed. The last piece for this unit arrived in the mail today; an 8" rectangular bar of graphite from Israel I purchased on Ebay for $18 (free shipping). That, along with an 8" piece of .5" titanium pipe from Latvia and a $16 (free shipping) battery charger from China, will complete all of the components of the cell. To stir it, I'm connecting a perforated piece of .25" plastic tubing to compressed air and placing it at the bottom of the cell.

I think you have misunderstood how this thing works. This cell will generate sodium hypochlorite which, if the pH of the cell is kept just above neutral, will exist as a 50/50 mixture of sodium hypochlorite/hypochlorous acid. The hypochlorous acid, in the presence of salt, will put gold into solution as gold chloride. However, there is no need to precipitate with SMB or ferrous sulphate as the gold chloride will migrate to the cathode. Once at the cathode, the gold is liberated from the chlorine and either plates the cathode or falls below it. The liberated chlorine is dissolved, combining with the water to make more hypochlorite/hypochlorous acid plus a small amount of hydrochloric acid. Although one might think this would make the solution acidic, it is already becoming basic by the production of a small amount of sodium hydroxide at the cathode during the manufacture of hypochlorite. It is hoped the two will cancel each other out, making salt, but, if necessary, the pH can be adjusted with minute additions of acetic acid.

 

[Palladium]

Here's an interesting read http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=53&t=1746

 

[Traveller11]

Well, I set everything up, just like in the diagram. A one gallon plastic ice cream bucket with salt dissolved in tap water, my Chinese battery charger (rated at 220 VAC input - only put out 7.8 VDC plugged into 110 VAC when I first got it but now puts out 15 VDC - no idea what happened there) with the positive connected to a titanium anode and the negative connected to a graphite cathode.

I plugged it in and nothing happened. No bubbles, nothing. I even tried adding the gold bearing clay, still nothing. I put the test leads of my voltmeter on the battery charger clamps and got only 1.0 VDC. Same 1.0 VDC when I placed the test leads further down the electrodes, just above the brine solution. THEN, I removed the electrodes from the brine and laid them on the bench, 3" apart from each other. Placing the test leads on the electrodes, I was able to get 15 VDC.

Any idea what happened? Do I need to add more salt to the solution to saturate it? What will happen to the conductivity of the brine as some of it converts to sodium hypochlorite/ sodium chlorate?

 

[FrugalRefiner]

Traveller,

When you touch your leads together, you remove all of the "resistance" to the flow of electricity, so you see your full reading. If you put your leads in a container of distilled water (with a space between them), you will have almost infinite resistance to the flow of electriciy, so you'll see a very low reading. When you add salt or other chemicals to the water, they dissolve into anions and cations that lower the resistance and allow more electricity to flow. If you place the leads far apart, the resistance is greater. If you put them close together, the resistance is less. Every cell acts as a variable resistor. The resistance can be adjusted by electrode spacing, electrode surface area, anion/cation concentrations, temperature, etc.

Hope that helps a little.
Dave

 

[niteliteone]

Well, I set everything up, just like in the diagram. A one gallon plastic ice cream bucket with salt dissolved in tap water, my Chinese battery charger (rated at 220 VAC input - only put out 7.8 VDC plugged into 110 VAC when I first got it but now puts out 15 VDC - no idea what happened there) with the positive connected to a titanium anode and the negative connected to a graphite cathode.

I plugged it in and nothing happened. No bubbles, nothing. I even tried adding the gold bearing clay, still nothing. I put the test leads of my voltmeter on the battery charger clamps and got only 1.0 VDC. Same 1.0 VDC when I placed the test leads further down the electrodes, just above the brine solution. THEN, I removed the electrodes from the brine and laid them on the bench, 3" apart from each other. Placing the test leads on the electrodes, I was able to get 15 VDC.

Any idea what happened? Do I need to add more salt to the solution to saturate it? What will happen to the conductivity of the brine as some of it converts to sodium hypochlorite/ sodium chlorate?

What was the current reading when the electrodes were in solution with power applied ???
It is possible that your voltage reading is a "phantom" reading that collapses when a load is applied ???

 

[pgms4me]

Traveller 11, Your description of what you did was clear. It appears to me your charger is not able to put out any current under even a small load. The 15 volt reading when no load shows that the charger can put out 15 volts,but when you put the clamps to your cell, the load of the cell drops the voltage down. You would see this if you put a current (amp) meter in series with one of the charger leads. Its probably not able to put out more than 1 amp or so,this drops the voltage down to about 1 volt. this may be because it wont work correctly at the reduced 110 input voltage or The circuitry of your charger, especially if it is a smart type charger,it may be confused because it does not see a battery as the load. You need to test the charger to see if it is working correctly before you proceed. An ammeter is a most necessary and valuable tool to monitor the current of any cell. Hope you get it going ok!

 

[Traveller11]

Thanks for the input everyone. I think I am beginning to see that my little $16 (free shipping) Chinese battery charger off Ebay was a little too good to be true. I think I'll bite the bullet and see if Canadian Tire has any good deals on battery chargers with a bit more "oomph" behind them.

As another member on this site is fond of saying, "The stingy man pays the most."

 

[Platdigger]

Some battery chargers because of their circuitry, will not allow power to flow unless actually hooked to a battery.

 

[Traveller11]

Some battery chargers because of their circuitry, will not allow power to flow unless actually hooked to a battery.

That's the interesting part. When I removed the electrodes from the brine and laid them on the bench (separated, of course), with the charger leads still connected to them, I was able to get 15 VDC by applying the voltmeter leads to them.

 

[niteliteone]

If you have a 12v battery connect that to your electrodes and connect the charger to the battery. That way if it is an electronic controlled charger it will see the battery and keep putting out power as long as the battery is in the circuit.

 

[Traveller11]

If you have a 12v battery connect that to your electrodes and connect the charger to the battery. That way if it is an electronic controlled charger it will see the battery and keep putting out power as long as the battery is in the circuit.

Okay, I'll give it a try.

 

[freechemist]

Well, I set everything up, just like in the diagram. A one gallon plastic ice cream bucket with salt dissolved in tap water, my Chinese battery charger (rated at 220 VAC input - only put out 7.8 VDC plugged into 110 VAC when I first got it but now puts out 15 VDC - no idea what happened there) with the positive connected to a titanium anode and the negative connected to a graphite cathode.

Hi Traveller11,

After having read the description of your electrolysis-setup, I'm coming to the conclusion, that the title of your posting should rather be "Question about anode". As an anode in an electrolysis-experiment, bare, i.e. untreated, titanium metal has a very high overpotential, until a significant current between the electrodes begins to flow. Said in other words, a very high voltage, up to 10 Volts and more has to be applied in your setup, to allow a current (measured in A/h; ampères per hour) to flow through the system. Titanium is a very reactive, non precious metal, whose exceptional corrosion resistance is caused by a very dense, firmly adherent and perfectly insulating oxide-cover (titanium dioxide, TiO₂) on it's surface. Thus a titanium sheet, connected as anode under constant high voltage and current for a prolonged time, can virtually be destroyed through growing of it's nearly impenetrable oxide-cover at the cost of it's metallic matrix. Titanium sheets, used as anodes in electrolysis (e.g. chlor-alkali-electrolysis) are always coated with an electrically conductive material, like metallic Pt (platinized Ti-anodes) or ruthenium-dioxide, RuO₂ (dimensionally stable Ti-anodes).

For not being stucked with your experiments, you can replace the titanium pipe with a graphite- electrode of appropriate design. Graphite was used in earlyer times as anodes in chlor-alkali-electrolysis, surviving anodic Cl₂-evolution for quite a long time.

What concerns cathodically deposited gold falling down into the ore and getting remixed, Palladium has given you a very useful link for solving these problems. An other solution would be, to place the cathode in a special compartment, separated through an anion-exchange-membrane, which can only be passed by anions (AuCl₄^-). Still another possibility consists in letting circulate constantly the clear, eventually filtrated electrolyte through a bed of strongly basic anion-exchange-resin, which holds back AuCl₄^- very selectively (up to at least 10% of the resin's weight).

Good luck and regards, freechemist

 

[Palladium]

Palladium has given you a very useful link for solving these problems.

I wondered if someone would see what i meant.

 

[Traveller11]

Well, I set everything up, just like in the diagram. A one gallon plastic ice cream bucket with salt dissolved in tap water, my Chinese battery charger (rated at 220 VAC input - only put out 7.8 VDC plugged into 110 VAC when I first got it but now puts out 15 VDC - no idea what happened there) with the positive connected to a titanium anode and the negative connected to a graphite cathode.

Hi Traveller11,

After having read the description of your electrolysis-setup, I'm coming to the conclusion, that the title of your posting should rather be "Question about anode". As an anode in an electrolysis-experiment, bare, i.e. untreated, titanium metal has a very high overpotential, until a significant current between the electrodes begins to flow. Said in other words, a very high voltage, up to 10 Volts and more has to be applied in your setup, to allow a current (measured in A/h; ampères per hour) to flow through the system. Titanium is a very reactive, non precious metal, whose exceptional corrosion resistance is caused by a very dense, firmly adherent and perfectly insulating oxide-cover (titanium dioxide, TiO₂) on it's surface. Thus a titanium sheet, connected as anode under constant high voltage and current for a prolonged time, can virtually be destroyed through growing of it's nearly impenetrable oxide-cover at the cost of it's metallic matrix. Titanium sheets, used as anodes in electrolysis (e.g. chlor-alkali-electrolysis) are always coated with an electrically conductive material, like metallic Pt (platinized Ti-anodes) or ruthenium-dioxide, RuO₂ (dimensionally stable Ti-anodes).

For not being stucked with your experiments, you can replace the titanium pipe with a graphite- electrode of appropriate design. Graphite was used in earlyer times as anodes in chlor-alkali-electrolysis, surviving anodic Cl₂-evolution for quite a long time.

What concerns cathodically deposited gold falling down into the ore and getting remixed, Palladium has given you a very useful link for solving these problems. An other solution would be, to place the cathode in a special compartment, separated through an anion-exchange-membrane, which can only be passed by anions (AuCl₄^-). Still another possibility consists in letting circulate constantly the clear, eventually filtrated electrolyte through a bed of strongly basic anion-exchange-resin, which holds back AuCl₄^- very selectively (up to at least 10% of the resin's weight).

Good luck and regards, freechemist

Sorry to be so long getting back to you. This is all very good advice I am getting here and I am grateful for all of it.

As I would like to stay with the original basic design of this setup, at least for now, do you know what could be used to make an "anion-exchange-membrane" to isolate the cathode and prevent the gold from re-mixing with the sand and clay?

On another thread, I was told there is a very good chance that the gold will plate out on the cathode and remain stuck to it, rather than simply falling below the graphite cathode. This might not be such a bad thing, as, in another very small cell, it might be possible to de-plate the gold from the graphite cathode.

 

[freechemist]

Hi Traveller11,

Not knowing your intentions, your equipment and a lot of other things of importance, I can give you only a few generalized answers. Please note, that they are based on some of my ideas only, I did never test in practice.

1.) Anion exchange membrane: These membranes are commercially available, like their congeners, cation exchange membranes. Anion exchange membranes are permeable only for anions (chloride, AuCl₄^-). That means, that in an anion-exchange-membrane compartmented electrolysis cell, only anions can pass the membrane, dividing the cell in two chambers (one with the anode, the other, on the other side of the membrane with the cathode). The positively charged base-metal-cations are rejected and remain in the anode-compartment. Thus only gold, as an anion AuCl₄^- can move to the cathode eventually, being reduced there.
By "googling" terms such as "anion exchange membranes", "electrodialysis", "Membrane electrolysis", "bipolar membranes", "membrane electrolysis cell" and the like, you will find much more information.

2.) Prevent remixing of gold with sand and clay: An electrolysis cell could easily be separated in two distinct compartments by inserting a tightly sealing frame, containing the separating membrane. The anode compartment holds the ore to be leached, whereas the solution in the cathode compartment becomes pregnant with gold slowly (hopefully!).

3.) Cation exchange membrane: Functions similar to an anion exchange membrane. Big difference: it lets pass only cations (e.g. base-metal-cations, H⁺), rejects anions. Could also be used, to separate an electrolysis cell in two compartments. Base-metal-cations, after having passed the membrane, can reach the cathode and are reduced there eventually. Gold, as AuCl₄^-, remains dissolved in the anode compartment, being concentrated slowly there.

Regards, freechemist

 

[Traveller11]

Hi Traveller11,

Not knowing your intentions, your equipment and a lot of other things of importance, I can give you only a few generalized answers. Please note, that they are based on some of my ideas only, I did never test in practice.

1.) Anion exchange membrane: These membranes are commercially available, like their congeners, cation exchange membranes. Anion exchange membranes are permeable only for anions (chloride, AuCl₄^-). That means, that in an anion-exchange-membrane compartmented electrolysis cell, only anions can pass the membrane, dividing the cell in two chambers (one with the anode, the other, on the other side of the membrane with the cathode). The positively charged base-metal-cations are rejected and remain in the anode-compartment. Thus only gold, as an anion AuCl₄^- can move to the cathode eventually, being reduced there.
By "googling" terms such as "anion exchange membranes", "electrodialysis", "Membrane electrolysis", "bipolar membranes", "membrane electrolysis cell" and the like, you will find much more information.

2.) Prevent remixing of gold with sand and clay: An electrolysis cell could easily be separated in two distinct compartments by inserting a tightly sealing frame, containing the separating membrane. The anode compartment holds the ore to be leached, whereas the solution in the cathode compartment becomes pregnant with gold slowly (hopefully!).

3.) Cation exchange membrane: Functions similar to an anion exchange membrane. Big difference: it lets pass only cations (e.g. base-metal-cations, H⁺), rejects anions. Could also be used, to separate an electrolysis cell in two compartments. Base-metal-cations, after having passed the membrane, can reach the cathode and are reduced there eventually. Gold, as AuCl₄^-, remains dissolved in the anode compartment, being concentrated slowly there.

Regards, freechemist

Hello freechemist

Thanks again for sharing your knowledge. I purchased a new battery charger from Walmart and found, as one member pointed out, it is necessary to have a battery connected to it to make it charge. I reported the results of my first attempt at electrolysis of NaCl and ore over on the Prospecting and Mining forum.

One question I do have. As you know, the electrolysis of NaCl brine first makes Cl2 at the anode and NaOH at the cathode, and then produces NaClO and H2 as these combine. Because of this, the chlor-alkali generator places a membrane between the anode and cathode sides, allowing the C-A generator to make pure Cl2 gas and NaOH. Would the anion exchange membrane act like the membrane in the C-A generator and prevent the combining of Cl2 and NaOH?

 

[butcher]

I have read old patents where they used cement, or asbestos or combination of these to make the membrane to separate the cells compartments, these membranes had a fairly short life span running industrially some around 5 months of run time, I am not sure how more modern membranes hold up.

Here is a website of one of our members that may be helpful
http://webpages.charter.net/dawill/tmoranwms/Chem_Chlorate.html