Electrochemistry Salt Water Electrolytic Cell

[Traveller11]

Hello

I was just reading a Brazilian patent held by a company named CETEM in Brazil for an electrowinning process. CETEM seems to be dedicated to removing cyanide and mercury worldwide from small gold mining operations and is working on alternatives to mercury amalgamation. This is one of them.

Essentially, the ore/ concentrate is finely ground and roasted or otherwise oxidized to oxidize sulphides in the ore/concentrate and cut back on current requirements in the cell. It is claimed that the raw ore can be placed in the cell but will take more electricity to work through the sulphides.

Large quantities of salt are mixed with water and placed into a plastic vessel. An anode and cathode (both of which can be graphite or titanium) are placed into the saltwater and DC current is applied. Gold is deposited on the cathode and a recovery rate of + 95% is claimed in a 2-4 hour period. The only requirement beyond this is that the concentrate/ore be somehow kept stirred; possibly with a motorized paddle or a guy with a gas mask and a paddle.

From my experience in water treatment, I can see this is basically a sodium hypochlorite generator. According to the patent, hypochlorite is further oxidized to perchlorate; an oxidizer with 4x the affinity for dissolving gold as cyanide has.

Other than the production of chlorine gas while in operation, this process seems better suited to the amateur as the handling of acids is not required.

Bob

 

[lazersteve]

Here's an old thread along the same lines:

Salt Cell

Steve

 

[Traveller11]

Steve

That was very interesting reading. I was especially impressed with the use of the fused silica cup to contain the anode. Would this cup stop silver chloride from passing through it? I ask this because I klnow that the placer gold I am attempting to recover is about 90% gold with the rest mostly silver.

Bob

 

[Wyndham]

I have been rereading the post on the salt cell and have a question on the titanium wire.Is it dissolved as the other wires are in the anode bath? There is a company called reactive metals out of AZ that has titanium wire as well as niobium wire. They sell this for jewelry but would be great for our uses. One other thing about the flower pot, I make pottery and it seem that what the anode cup is no more than a cup partially fired to about 1800 deg f any higher than that it starts to becomes fully fused and non porous.
Does the pore size other than too big have any effect on the reaction?
Ok on to the last part, if fine to micro fine placer gold in concentrate material were placed in a clay bowl wit a titanium plate on the bottom attached to the anode PS, would that surface area of the plate induce the current into the concentrate to possibly dissolve the gold into the solution?
I'm thinking about the beach gold that is so fine it's hard to recover with traditional means.
Wyndham

 

[Traveller11]

Wyndham
From what I can gather, the actual thing putting gold into solution is the compound perchlorate; a molecule made up of one chlorine atom shielded by four oxygen atoms. Reputedly, it has an affinity for dissolving gold 4x greater than cyanide. This unit is, essentially, a sodium hypochlorite (clorox) generator. The hypochlorite ion is further oxidized to perchlorate.
One design on this procedure I looked at had the placer material in a separate vessel with the electrolysis unit in another. The perchlorate laden electrolyte was pumped from the electrolysis chamber through the placer material and returned to the chamber where the gold it put into solution was deposited on the cathode. In other words, neither cathode or anode actually are in contact with the placer material or, for that matter, anywhere near them.
I wonder why the anode is placed in the clay/ fused silica pot; especially in the case of placer material. If the cathode was in the pot, the pot/fused silica cup would act as a filter and only allow gold laden electrolyte to reach the cathode. This, from all accounts, is the one great difficulty of this process; collecting the gold from the cathode. I should think it very handy to have a fused silca cup with only gold in it all ready to be fired.
Regards
Bob

 

[Traveller11]

Wyndham
I forgot to mention that the unit with the electrolysis chamber separate from the leaching vessel did not have the required stirring mechanism for the placer material mentioned in the original patent.
It was felt that, between the design of the placer material chamber and the flow of electrolyte introduced at the bottom of said chamber, enough mixing would occur to allow the electrolyte contact with all of the placer material.
Regards
Bob

 

[Traveller11]

Wyndham
Re: fine beach gold. What a coincidence, this is what I am working on, too. Let's keep in touch as we may be able to help each other.
Regards
Bob

 

[T3sl4]

Two glaring problems with this:

1. Perchlorate has no affinity for anything. It's completely inert in the body, aside from one notable exception: the body thinks it looks like iodide, so it fits into the machinery, but the machinery can't stick anything to it, unlike iodide. The result is hypothyroidism, because perchlorate displaces iodide in the machinery. The cure is the same as for radioactive iodine poisoning: take an iodide pill to force real iodide into the system good as new. Perchlorate is so unreactive, almost the only thing you can do (aside from concentrating it to >70% pure acid) is heat it to >400C where it finally decomposes.

2. Perchlorate is rather difficult to produce. To give you some idea, consider that a perchlorate cell smells weakly of ozone. There is some serious electrochemistry going on in that cell! It can only be produced with certain anodes: platinum and lead dioxide are the most important. Other metals, semimetals (like graphite) and compounds (like MMO) are unable to oxidize chlorate to perchlorate, either due to breakdown (graphite) or the potentials don't work out (i.e., oxygen gas is produced instead).

It seems to me, to heap ore into a salt cell, the intent is to dissolve gold, as the chloro complex, using hypochlorite or, better yet, if the pH is kept low, free chlorine. So you get an intermediate a lot like HCl/bleach makes. Then this is plated on the cathode, because...it's the cathode.

The downside to a salt cell is, chlorine evaporates rapidly at low pH, so you have to keep adding acid if you want to keep it on the chlorine side. With no additions, the pH gradually drifts up to the 9-10 range, which is quite basic, and I'm guessing gold isn't soluble there (aside from cyanide), even with the mess of hypochlorite present.

The overall result of salt electrolysis, assuming a suitably inert anode, is chlorate, then once all chloride is used up, perchlorate. Chlorate represents a loss, because it's not very reactive, so you'll need a very low pH (1-2) to keep it active as well (at low pH, chloric acid disproportionates to chlorine, which would be, HClO3 + 5 HCl = 3 H2O + 3 Cl2).

There is one thing. If you use a divided cell, you can keep the base around the cathode only (since the cathode is churning out hydrogen gas and hydroxide), which lets the anode bubble out pure chlorine gas. No need for extra acid. You need some way to carry gold ions to the cathode (otherwise, I suppose the barrier clogs with Au oxide?), which implies cyanide. Which is bad, and which eventually leaves the cathode cell (cyanide = anion = attracted to anode), meaning it becomes a consumable. No good.

Running such a cell sounds like a very irritating and corrosive prospect. You will absolutely need a fume hood and a few pounds of graphite.

Tim

 

[Traveller11]

Tim
Well, I was afraid it seemed to good to be true. However, the inventors of this process seem to think it a superior system. This may be only hype and wishful thinking, of course.
Rather than attempt to discuss things I don't fully comprehend, I will c/p the patent below for your perusal. It is registered with the Brazilian Patent Office under PI 0205482.
If the moderators of this forum prohibit c/p'ing from other sites, please delete this post and notify me that I may supply the link.

Here it is:

Electrolytic process for the opening of refractory gold concentrates and simultaneous return of the gold contained
"Electrolytic process for the opening of refractory gold concentrates and simultaneous return of the gold contained." This application for a patent (pi), an electrolytic process for the treatment of gravitational concentrate containing sulfides and / or synthetic mineral species, called refractory, and elemental gold encapsulated in the matrices of sulfides, which occurs in the first instance , the opening and dissolution of the sulfides contained (pyrite, chalcopyrite, pyrrhotite and arsenopyrite), resulting in the formation of sulfate ions and then with the course of electrolysis, the dissolution of gold and its alloys with simultaneous electrowinning of these elements in surface of graphite or titanium cathodes. The reaction system (Figure 1) consists of an electrolysis reactor (5), where electrodes are inserted, namely: one or more of graphite anodes and / or titanium (2), two or more cathodes of graphite or titanium (1) being provided with a mechanical stirrer (4), responsible for the suspension of concentrate solution of sodium chloride and connected to a continuous power source andlor DC pulse, or even rectified or pulsating current asymmetry ( 3), responsible for maintaining the levels and forms of power more appropriate. The dissolution of the sulfide concentrate occurs as the result of the reaction of the oxidant species generated by chlorine (heat-and hcio) from the dissolution of chlorine gas (Cl ~ 2 ~), which is generated as a result of oxidation of chloride ions (ci ^ - ^) on the surface of anodes. With electrolysis time, gold and its alloys, are dissolved by chlorate ions generated by oxidation of hypochlorite ions, produced constantly, and simultaneously by different routes. Moreover, the hydrogen (H ~ 2 ~) and the reduction of dissolved metal ions (AUCLI ~ 4 ~ -, AgCl 2 ~ 3 ~ ^ - ^, cu ^ ^ +2), resulting from oxidative processes, to their elemental states, on the surface of cathodes of graphite or titanium (2), constitute the main steps of the proposed procedures.

The wording may seem difficult to follow as this text is a translation from the original Portugese.
Regards
Bob

 

[T3sl4]

That sounds about like I described then.

I say it'll work, but maybe not as magically as you might've hoped. If the gold sticks to the cathode, seperation will be easy enough. If it's present as foam or flakes or something, it will probably wash off in the flow and you won't get anywhere. Maybe a mesh bag around the cathode would fix that, without the possible problems a full divided cell might have.

Ohh... for ores that aren't pure (i.e., containing other metals), you'll dissolve everything, with the possible exception of silver (since AgCl is quite insoluble; PbCl2 is soluble enough to allow lead to plate across, however). So you'll get a mixture of iron, copper, nickel, whatever else goes into solution. This might actually be an advantage, since the iron and copper crud might hold the values in place mechanically. Then you do the usual purification process on metal, instead of having to leach all that ore. In fact, just blasting it with nitric acid might suffice, which is quite handy because the gold falls off, leaving your electrode clean for more.

Tim

 

[Traveller11]

Tim

Thank you for your reply.

One of the reasons I was looking at this setup was the hope that it would be fairly selective in only bringing gold to the cathode. I can see now that it may not do that but, as you stated, a wash with nitric acid may remove all the unwanted metals; as long as there is not too much unwanted metal.

This leads me to another point in leaching or electrowinning that I do not understand. As the material I am working with is a beach placer, the majority of the base metals I will be dealing with are magnetite and hematite; both of which are oxidized forms of iron. There is also a small amount of pyrites; a sulphide of iron. I cannot determine, from following threads on this site, whether or not oxidized forms of iron (or sulphides) will be dissolved in leaching or electrowinning processes. If they do not dissolve, I am on my way to success. Can you help me with this?

As to recovering the gold from the cathode, earlier in this thread Lazersteve posted a link to another thread on this site titled "Salt Cell". A New Zealand fellow was using a salt electrolyte in his cell to dissolve and recover plated jewellery. He had a bowl with a fused silica cup in the centre of it; both the bowl and cup containing salt water. The cathode was placed in the large bowl and the anode, an alligator clip, was attached to the jewellery and placed inside the fused silica cup. He seemed to have great success with this, even claiming that he smelled no chlorine (?) from this process.

What I was wondering is, if the fused silica cup will let gold in solution go through it, would it be possible to place the cathode in the fused silica cup instead of the anode? This way, when the process is completed, I would have placer material in the big bowl and gold in the fused silica cup. What do you think?
Regards

Bob

 

[lazersteve]

Here's another thread with my anode from a test run of the salt cell a few years back:

Karat Gold Salt Cell

The cell worked, but eventually stalled out. I was using a fused silica crucible as my membrane.

Steve

 

[Traveller11]

Steve,

While I have your attention, can you tell me if oxidized forms of iron (ie. magnetite, hematite and iron pyrites oxidized by roasting) will be dissolved in either the acid/clorox process or this salt water cell?

Bob

 

[lazersteve]

Bob,

I don't see any reason why they should not, but they would also interfere with the dissolution of the gold if they are present in any quantity. For this reason, as much as possible they should be removed before going after the gold. I have used acetic acid and also citric acid to help remove excess iron oxides when they are masking gold from dissolving. Fluxing the ore with a collector for the values may also help if you are set up for furnace operations.

Several years back a guy I know told me that a metallurgist at a large mining operation told him that large amounts of iron in ore would contributed to a phenomenon know as 'pregnant robbing'. He was using the BDG (aka: DBC/dibutyl carbitol) route of extraction at the time.

Sorry I can't be of more help, but hopefully my comments will bring others forward with comments of their own.

Steve

 

[Traveller11]

Steve,
Sorry to keep asking you so many questions. One last one, though, and then I promise to give you a break.
To remove these iron oxides, the pyrites and any other base metals from my placer material, do you think your Hydrochloric Acid/Hydrogen Peroxide process would be successful?
Bob

 

[lazersteve]

Bob,

Straight HCl should work.

I would stay away from any wet combinations that dissolve gold along with the base metals just to keep everything in step. Acid peroxide (AP) will dissolve iron and micron gold, but will eventually become fouled and lose reactivity, leading to a big mucked up mess from the partially dissolved base metals and cemented values.

Like I said before, concentrate on selectively removing as much of the base metals first with the more 'mild' reagents, then go for the precious metals once the base metals are removed.

Steve

 

[T3sl4]

Incidentially, soaking rocks in acid generally leads to gunk, at least that was my experience. The gunk comes from the insoluble alumina, silica and clay type materials leftover, so you get a difficult-to-filter gel.

The easiest way to remove other stuff is a soak in acid. It may take a very long time to dissolve everything, as iron requires a low pH to dissolve, and it doesn't go quickly. Large grains may take a very long time indeed. The easiest way to leach, of course, is put everything in a column (or if you've got a lot to do, try an HDPE barrel with holes punched in the bottom) and let the acid soak through. Fresh acid, at the top, does the most, then it gradually gets more spent as it drains down. At the bottom, it's all used up (presumably). Add more, and the top is even cleaner, the middle is somewhat cleaner, and the bottom is starting to dissolve. Etc. Eventually, the top fraction is pretty darn clean, having been washed so many times, and the stuff on the bottom can go to the top of the next run, and so on.

Without a strong oxidizer, the gold won't dissolve during this step. So you put it in the cell next, and that should do it.

Steve: do you have any idea if the fused silica got clogged, possibly by gold as I suggested earlier? If so, it could be very difficult to clean out...

Tim

 

[Traveller11]

Tim
I've seen citric acid suggested many times on this site for dissolving iron into solution.
I checked citric acid out and it seems to come in many percentages; even a powdered form.
What percentage is recommended for pre-leach acid washing and can this be made from the powdered form?
Regards
Bob

 

[T3sl4]

Citric acid is neat stuff... it tastes lemony, and very sour!

Idunno how much. Heap some in and see?

Tim

 

[Barren Realms 007]

I think I have about 600 lbs of the citric acid in dry form being given to me in then next few days, it is food grade. I might be willing to part with some.