Electrochemistry question about cathode

[Traveller11]

I think those cells were in Chlor-Alkali generators, weren't they? These were designed to only allow current to pass through the membrane, and their products were chlorine at the anode and sodium hydroxide at the cathode.

 

[butcher]

Current is external to a cell it would not pass through membranes,electron flow is external, ion flow is internal, in a cell it is ions that would flow in an electrolyte (thus ions that pass through the membranes), yes the chlorate cell and a chlorine/NaOH generator are similar, and the principles of how the cell works also similar, although the conditions the cells are run at are different.

 

[Traveller11]

Current is external to a cell it would not pass through membranes,electron flow is external, ion flow is internal, in a cell it is ions that would flow in an electrolyte (thus ions that pass through the membranes), yes the chlorate cell and a chlorine/NaOH generator are similar, and the principles of how the cell works also similar, although the conditions the cells are run at are different.

A sodium hypochlorite generator, as used to manufacture bleach to disinfect drinking water, is identical to a chlor-alkali generator used to generate chlorine gas (also employed to disinfect drinking water) in every respect except the membrane separating the anode and the cathode in the chlor-alkali generator.

 

[butcher]

Water in electrolysis can be separated differently depending on conditions.
H2O can be written different ways
H2O, HHO, HOH
with electrolysis of water we can separate the hydrogen and oxygen. Hydrogen H+ (cation) and the Oxygen O- (anion), here the hydrogen H+(cation) would migrate to the cathode and form Hydrogen gas H2 (g), and the oxygen O- (anion) migrating to the anode area oxidizing to oxygen gas.
2H2O --> O2(g) + 2H2(g)

Another way we can separate water is to separate the hydrogen H+ from the Hydroxide OH-
Example is in the electrolysis of water in a salt solution.
2NaCl + 2H2O --> Cl2(g) + H2(g) + 2NaOH

In electrolysis the conditions; like temperature, PH, and current, and also conditions like if the cell is separated into compartments like with a membrane or salt bridge (two compartment cell linked together with a bridge of ionic salts) Etcetera, can make a big difference in what products are produced and by what reactions occur in the cell or gases that leave or are remixed in the cell, the oxidation states of the products, or reduction states, whether acids or alkalis form...


In electrolysis of brine NaCl Salts we can separate the sodium and chloride ions, the bit of water in the cell can also separate.
With water we can generate a Hydrogen gas (H+) from electrolysis of water (at the cathode), and leaving Hydroxide (OH-) from the water at the cathode area.
2NaCl + 2H2O --> Cl2 (g) + H2(g) + 2NaOH

Separating the brine salt ions by electrolysis The sodium Na+ (cation) wants to form at the cathode, the chloride Cl- (anion) at the anode, where it can be oxidized to form chlorine gas Cl2 (g), Hydrochloric acid HCl, Hypochlorous acid HCLO, or chlorates CLO3, which can depend on several factors, whether the chloride ion oxidized at the anode to form leaves or stays in solution to mix with water, How well the chlorine gas oxidized at the anode stays in solution or mix with water, the PH of that water (which will be different depending whether a membrane is used in the cell or not, and the effectiveness of the membrane).

A chlorine gas Cl2 (g) and sodium hydroxide NaOH, cell or generator will use a membrane
with no membrane in the cell our product can be sodium hypochlorite NaClO and sodium chlorate, much depending on conditions the cell is run (how the chloride Ions are oxidized to chlorine and how that chlorine stays in solution the pH. Involved or that forms by the reactions, and temperature can play a roll in the oxidation state at the anode area.

To complicate this further whether the anode (or cathode) is a metal or inert, and whether there are metals that would react with the products formed in the cells vicinity of the anode or cathode areas, the ability of these metals to be oxidized or reduced, or form ions in the electrolytic solution...

 

[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

Hello freechemist

The experiment is going well, though slowly. I have been in contact with a fellow at a university here in our province who was involved with the CETEM company when this electrolytic process was first being worked on. I was surprised to learn from him that the process was originally designed to remove mercury contaminants from the tailings left behind by small placer miners in South America. While it worked exceptionally well at recovering mercury, it was discovered that the tailings also contained great amounts of ultra fine gold that had not amalgamated with the mercury, and this process was recovering this gold, as well.

I asked him whether the gold stayed on the cathode or if it fell below the cathode. His answer was that some of the gold deposited, as a sludge, on the cathode while some of it stayed in solution as AuCl4. Also, according to him, his recovery was just over 75% (after examining his tailings) which seems at odds with the claimed 95% recovery rate. He did say, though, that he had employed a magnetic stirrer to keep the ore agitated and that it had stirred his ore very vigorously; maybe too much so, according to him. He felt that some of the gold may have been abraded from the cathode and returned to the ore, while some gold stayed in solution as AuCl4.

This leads me to a rather obvious question. If the gold is oxidized by the HClO and goes into solution as AuCl4, then migrates to the cathode, liberating Cl2 and Au, and the liberated Cl2 instantly combines with H2 and O2 to make more HClO, what stops the cathodically deposited Au from going into solution again as soon as it is oxidized by the freshly made HClO?

You mentioned circulating filtered electrolyte through a strongly basic anion exchange resin, as this resin would selectively hold back AuCl4. How well would the electrolyte have to be filtered? Would it matter if ultrafine suspended clay particles remained in the electrolyte as it passed through the resin (I'm assuming them to be in the form of beads)? Also, as this process requires the pH of the solution to be at a pH of 6-8, in order to allow the NaClO to exist as the oxidizer HClO, would passing the electrolyte through a strongly basic resin cause the pH of the electrolyte to rise?

 

[butcher]

A little to think about.
A discussion of another type of gold cell first.
In a cell if the anode is a metal it can be oxidized, even metals that normally would not oxidize normally in a solution can be forced to oxidize by the external current applied from the battery or power supply, example gold will not dissolve in a brine salt water slurry, but if gold metal is placed is in this brine electrolyte solution and is connected as the anode it can be forced to dissolve (forced into oxidation) of course here in this example the concentrated brine NaCl and water is also split by the external current, here sodium Na+(Cation) moves to the cathode area , and the chloride Cl-(anion) moves to the anode area (where the chloride ion can be changed into chlorine or chlorinating compounds, which with gold can form AuCl3 in solution as part of the electrolyte solution), at the cathode area Hydrogen H+ (Cation) (split from the water can evolve as gas H2) leaving the cathode are basic (OH-) from the water, the AuCl3 in this brine solution electrolyte the gold ions will be reduced at the cathode as the gold ions gains electrons from the electron rich cathode, and these gold ions convert back to elemental gold metal how well this gold will plate out to the cathode can depend on several factors (purity of electrolyte) cathode material ...


For this gold to be oxidized and go into solution as an electrolyte it must be compatible with the electrolyte or its chemistry, and for the gold ions to stay in this electron solution long enough to get over to the cathode to be reduced by contact with the cathode back to metal and plate out to the cathode (or fall off into a cathode bag), the gold ion cannot be reduced back to metal by the electrolytes conditions…

A little more discussion of the splitting of the water involved, hydrogen (from the water) the current can play a big factor on what gases are formed or how they react in the cell whether they stay in solution and recombine or leave the cell as gas, whether the hydrogen would remain in the cell or not (voltage and the electrolyte resistance plays a role here), if hydrogen leaves the cell its electrolyte will become more and more basic with hydroxides as the reaction proceeds.

Chlorine and whether it forms a gas and leaves the cell or mixes back into solution and the conditions of the solution in the anode area play a big role in what products are formed, how this chloride and what oxidation state it forms into, whether the electrolyte mixes or is separated by a salt bridge or membrane (gas, acidic, basic oxidation state...) all play a big role in what happens in the cell.


Now lets look again at the other two cells we have discussed, with, and without a membrane.

Note the cell without the membrane the electrolyte and free Anions and Cations are free to mix back together

The cell with the salt bridge or membrane these Anions and Cations can be separated., and kept separated

This can make major differences in the products formed or the reactions involved and with the ore in these electrode areas of the different circumstances of these cells.

These membranes can be chosen to allow only positive or negative ions to flow through them (size of the ions can also play a role here), and the membrane can help to keep these ions separated from each other, so that with a membrane we can keep ions on one side or the other, also basically making different electrolyte or conditions of the electrolyte in the two sides of this membrane, example in the cell with the membrane we can generate chlorine on one side and produce hydroxide on the other side, keeping hydroxide on one side of the cell where this membrane does not let it mix with the chlorine, or chlorides being oxidized to chlorine at the anode.

Or we can allow them to mix and form hypochlorites or chlorates, like we would in the cell without this ionic membrane.

Now you have a much more complicated setup trying to get gold out of ore.

A big player can be the ore itself and its chemistry, the other metals in the ore and not only these metal Cations, but you can also have other anions (sulfides oxides hydroxides... that the ore is composed of, when you have ore in an electrolyte, these can also be oxidized in this soup of Anions and Cations, or can oxidize and reduce other ions in solution basically complicate the electrolyte or the cells chemistry by the reaction of the ore and its chemical and electro-chemical reactions in the cell.

Factors of cells construction and operation, can greatly effect the cells reactions involved, membrane or no membrane, the current and pH of the electrolyte or the pH it starts forming as the running of the cell progresses, how these electrolytes are allowed to mix or are separated, how the gases are formed and if they are forced out of solution or the cell is run to where they mix back into solution, all play a big factor in what the electrode areas become, what chemicals are formed in this electrolyte solution or separate solutions, and whether the Chlorine gas leaves the solution or forms HOCl HCl NaClO NaClO3 NaOH, all paying a factor in what reactions and products are formed in solution, example chlorine leaves as gas or is recombined into solution and there is enough generated which can combine with gold to form AUCl3, or if the solution becomes acidic or basic in these electrode areas, where in this mix the gold in the ore is (if the gold is free to dissolve into solution or in the case of sulfide locked up in the sulfide bond) and if it can be oxidized by the products formed from the reactions in the cell...

Unless the gold was in contact with the anode electrode it would not be oxidized at the anode, (most all of the gold ore in the tank is not), but this gold could be dissolved into solution as part of the electrolyte as gold chloride by the products formed in the electrolyte by the oxidation of the chloride to chlorine and its compounds dissolving the gold up into this dirty electrolyte solution (most of this would be in the anode area or compartment of the cell) as the cathode area would be more of a chemistry to reduce the gold, or these gold ions would reduce at the cathode.

if the gold was not captured as it reduced it could continually stay in the cell being oxidized and reduced and re-oxidized over and over again (other metal in this ore could also).

Also the ore make up in this electrolyte can play a big role here, as stated if there were more reactive metals in the ore (base metals) which could be oxidized much easier that the gold not leaving enough oxidation power of the cell to oxidize the gold, so the more concentrated the ore and the more pure this gold ore was, the state of the ore roasted to remove sulfide...


The more you think about it the more complicated it becomes.

I tried to put these random thoughts together as best I could, I do not know if they will make any sense once I printed them out.

Regardless even if you put gold into the solution, how well it will plate out to the anode would depend on the electrolyte, in this dirty electrolyte of ore it would not be pure and probably would have a very hard time plating to the cathode, although if in solution would reduce at the cathode back to metal most likely as a powder, bagging the cathode you could catch it as it falls from the cathode with the other metal Cations which are reduced at the cathode.

 

[freechemist]

Hi Traveller11,

You ask: 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?

The primary anodic oxidation product in the electrolysis of aqueous concentrated NaCl-solution is gaseous Cl₂. The primary cathodic reduction product is gaseous hydrogen, H₂, from reduction of H⁺-ions, originating from water, which can dissociate into H⁺-cations and OH^--anions.

Dissociation of water: H₂O <==> H⁺ + OH^-
Reduction of hydrogen-cations: 2 H⁺ + 2e^- ==> H₂

In an undivided (not compartmended) cell, Cl₂ and OH^- combine to hypochlorite, OCl^-, Cl^- and water:

Formation of hypochlorite: Cl₂ + 2 OH^- ==> OCl^- + Cl^- + H₂O

Introduction of an anion-exchange membrane to separate the cell into two compartments will not prevent the combining of Cl₂ and OH^-. The internal electrical current-flow (through the solution) will be established by anions, moving from the cathode through the membrane to the anode, these anions being OH^- and/or Cl^-. The only way to prevent formation of hypochlorite is, in my opinion, to replace the NaCl-electrolyte with an equally concentrated HCl-electrolyte. This will not only prevent OCl^--formation, but also precipitation of insoluble base-metal-hydroxides.

Your second question: This leads me to a rather obvious question. If the gold is oxidized by the HClO and goes into solution as AuCl4, then migrates to the cathode, liberating Cl2 and Au, and the liberated Cl2 instantly combines with H2 and O2 to make more HClO, what stops the cathodically deposited Au from going into solution again as soon as it is oxidized by the freshly made HClO?

The cathode never oxidizes anything, that means, there is no Cl₂, nor O₂ formed. It reduces cations, like H⁺, dissolved base-metal-cations, and/or (hypothetical) Au³⁺-cations from [AuCl4]^-. What is set free, are equivalent amounts of anions, namely chloride ions, Cl^-.

Reduction of [AuCl₄]^-: [AuCl₄]^- + 3 e^- => Au(metal) + 4 Cl^-

Further questions: You mentioned circulating filtered electrolyte through a strongly basic anion exchange resin, as this resin would selectively hold back AuCl4. How well would the electrolyte have to be filtered? Would it matter if ultrafine suspended clay particles remained in the electrolyte as it passed through the resin (I'm assuming them to be in the form of beads)? Also, as this process requires the pH of the solution to be at a pH of 6-8, in order to allow the NaClO to exist as the oxidizer HClO, would passing the electrolyte through a strongly basic resin cause the pH of the electrolyte to rise?

After my experiences with ion-exchange-columns, it is important to work with well filtered solutions, to prevent clogging. However, a lot can be done by pumping the solution through the column from the downside to the upside, thus holding the filling beads somehow in suspension.
If the absorbing resin-beads are loaded with a neutral anion, like chloride, the pH of the electrolyte will not be changed.