Electrochemistry Salt Water Electrolytic Cell

[T3sl4]

Dayum... you could make a whole city pucker with that!

Tim

 

[Traveller11]

I am reviving this thread because, after careful study, I think I have finally figured out what the designer of this process had in mind.

I think, and correct me if I'm wrong, this process involves an electrolytic sodium hypochlorite generator, a leaching vat and an electrowinning unit; all rolled into one.

I recognized the sodium hypochlorite generator part because I operate a small water system here and we have been looking at purchasing a NaClO generator to cut down on our disinfection costs. This setup will not produce chlorine gas. The chlorine gas generator is similar but has a two part cell with a membrane separating the anode and the cathode side, generating Cl2 on one side and sodium hydroxide (NaOH) on the other with hydrogen vented. In the electrolytic cell described in the gold recovery process, without a membrane, chlorine gas exists only briefly at the anode and is instantly converted into sodium hypochlorite.

So, in this unit, sodium hypochlorite is generated and, I'm guessing, puts gold into solution and then, through electrowinning, deposits it at the cathode. It was further improved in design by having the salt brine and the cathode and anode in a separate container from the ore. As sodium hypochlorite was generated by electrolysis, it was pumped to the vessel containing the ore and flowed through it, from top to bottom, and returned to the electrolysis vessel where, presumably, gold leached from the ore is deposited.

The burning question in my mind is, will this process put base metals into solution, and, if it does, will they be deposited at the cathode along with the gold?

http://www.artisanalmining.org/casm/sites/artisanalmining.org/files/publication/Veiga_Replacing_Hg_in_ASM%20Operations.pdf

Pges 48 & 49

 

[Traveller11]

Okay, I realize now I was wrong again. Further study has cleared things up.

I assumed this setup was making sodium hypochlorite but the literature kept referring to the production of sodium chlorate. I became concerned, as our water system is seriously entertaining purchasing a sodium hypochlorite generator to make on demand bleach, from salt, for disinfecting drinking water. Sodium chlorate is not a good thing to have in drinking water and the Maximum Acceptable Concentration in drinking water, set by Health Canada, is 1.0 ppm.

It turns out that the electrolytic process for making both, from salt brine, is identical. The only difference is the temperature of the electrolyte. Below 50° Celsius, it will make sodium hypochlorite. At 50-70° Celsius, the cell will make sodium chlorate.

According to what I have read, this higher temperature is achieved by applying higher elecrical amperage to the electrodes. I had always wondered why sodium hypochlorite generators only made a concentration of .8% sodium hypochlorite bleach (or less, as opposed to household bleach at 3-6%) and it seems to be because it would take higher current to do this and it is likely the higher current would increase the cell temperature to the point of making sodium chlorate. Or, it could just be that .8% is the most economical level to make bleach at and it is no problem to just increase the dosage going into the water, rather than wasting current trying to make a higher concentration.

Anyways, it looks like the agent at work in this cell is sodium chlorate (NaClO-3), a very powerful oxidizer.

 

[Traveller11]

Okay, I realize now I was wrong again. Further study has cleared things up.

I assumed this setup was making sodium hypochlorite but the literature kept referring to the production of sodium chlorate. I became concerned, as our water system is seriously entertaining purchasing a sodium hypochlorite generator to make on demand bleach, from salt, for disinfecting drinking water. Sodium chlorate is not a good thing to have in drinking water and the Maximum Acceptable Concentration in drinking water, set by Health Canada, is 1.0 ppm.

It turns out that the electrolytic process for making both, from salt brine, is identical. The only difference is the temperature of the electrolyte. Below 50° Celsius, it will make sodium hypochlorite. At 50-70° Celsius, the cell will make sodium chlorate.

Heating ordinary household bleach will form salt and sodium chlorate. Also, as sodium hypochlorite decomposes over time, about 90% of it will make sodium chlorate.

3 NaClO = 2 NaCl + NaClO-3

According to what I have read, this higher temperature is achieved by applying higher electrical amperage to the electrodes. I had always wondered why sodium hypochlorite generators only made a concentration of .8% sodium hypochlorite bleach (or less, as opposed to household bleach at 3-6%) and it seems to be because it would take higher current to do this and it is likely the higher current would increase the cell temperature to the point of making sodium chlorate. Or, it could just be that .8% is the most economical level to make bleach at and it is no problem to just increase the dosage going into the water, rather than wasting current trying to make a higher concentration.

Anyways, it looks like the agent at work in this cell is sodium chlorate (NaClO-3), a very powerful oxidizer.

 

[butcher]

Traveller11,
T3sl4 Tim can help you understand these cells better, he has expierience in this field.

I like the discussion here.

 

[Traveller11]

Well, I've kind of ignored the salt water electrolytic cell as my life has been very busy this last while. However, before I die of old age (LOL) I am determined to build this thing and try it out.

Looking back at the information available on it, I noticed a couple of things. The first is that they recommend using a plastic tub as the container for the cell. I think this provides us with an important clue as to the compound being generated by the cell. As I stated earlier, the electrolysis of brine will make one of two things (or both) depending on the temperature of the brine. This temperature is directly controlled by the amount of amperage applied to the electrodes.

Below 50° Celsius (122° F.), this cell will produce sodium hypochlorite. Between 50-70° Celsius (122-158° F.), it will produce sodium chlorate. However, I don't imagine the change occurs abruptly. At 40-50° C., there is likely a mix of NaClO and NaClO-3 being produced. It also seems unlikely that anyone would be running this process in a plastic tub with temperatures approaching 70° C. (158° F.). Plus, as this is designed for artisanal miners without access to large amounts of electricity and processing large amounts of ore at once, I would tend to think this cell is meant to operate at a lower temperature and produce a greater volume of sodium hypochlorite.

While they say to use a graphite rod as the cathode in this process, they do not mention what to use as the anode. From another thread on here, I learned that a stainless steel anode will break down quickly in this application. Could a graphite rod be used as the anode or would a titanium rod be better? Where would a person acquire suitable graphite rods for this purpose?

Because the cathode and anode are immersed in the ore being treated, any gold collecting at the cathode will fall right back into the ore. I've read about "cathode bags". If the cathode was contained in one of these, would it collect the precipitated gold inside it, isolating it from the ore? Would it be simpler just to suspend a plastic cup in the brine, just above the ore, and place the cathode in it?

 

[Geo]

Lazersteve sells graphite electrodes on his website. you could place the cathode in a porous container like a clay flower pot.

 

[Traveller11]

Thanks Geo. Do you think the graphite would not break down if I used it as an anode?

 

[Geo]

if it does, it will be very little. from what i understand about it, the electrodes are unaffected by chlorine. i followed a very old thread involving saturated salt electrolyte cell and tried a few experiments. i tried to dissolve the base metal from pins while leaving the gold plating whole. it dissolved the whole mass including the piece of stainless steel i was using as an anode. i used the graphite electrode from a 6v lantern battery and placed the pins in a porous cup placing the graphite electrode on top of the pins to try and contain the gold solution as it dissolved. i used a piece of copper as the cathode hoping to selectively remove copper. again, the pins dissolved but the cup did not hold the gold as there was gold throughout the electrolyte and the cathode eroded badly. the only bright spot was the graphite electrode did not show any wear.

 

[Traveller11]

Sounds good. I'll check out Steve's site and order some graphite rods.

 

[Traveller11]

I checked out Steve's online store and he doesn't seem to sell graphite rods. I did find graphite rods on Ebay, though. Think 1/2" would be thick enough?

 

[jimdoc]

Here is a website that may help;
http://www.graphitestore.com/index.asp

 

[Marcel]

I did try the salt cell a few months ago and wheter my setup was correct or not, it appeared to me that this cell does nothing else but:
- create Cu(II)Cl
- dissolve unlying copper layer, so that gold will fall off.

I measured the amperage from the start and it was very low when starting, so I added more salt to the solution, which increased the current (because the electrical resistance of the water was reduced by the salt). Then I got some light blueish solution which turned out to be Cu(I)Cl and Cu(II)Cl. So all this salt cell experience led me to another experiment:
Take your AP solution and add anode and cathode to it and the whole AP process will run much faster.
Solution is electrically conductive, so there is no need to interconnect each part you put in the solution.
Of course the process is different from dissolving gold, it is the good all AP process just with a turbo. In the electrochemistry it is all about energy. If you add energy - in which form ever, heat or electric energy, this energy will speed things up.
I somehow doubt that the salt cell will dissolve gold in a timely manner and very efficient. There is chlorine in there from the salt, there is copper in there from the material. All you are doing here is just an altered (very slow at the start) AP process. That is the truth about the salt cell to me.
Skip the the salt and start with AP+electricity right from the start and you may find a good process in there.
Salt cell is a myth to me.

 

[Traveller11]

First off, this process was not designed for recovering plated gold from copper based metals. It was designed to recover gold from ores and placer materials. It is a variation of the +100 year old "Plattner's Process" for extracting gold with chlorine. The description below, from an 1898 paper, tells you why the copper in your material made your cell unsuccessful:

"The most scientific and perfect mode of gold extraction (when the conditions are favourable) is lixiviation by means of chlorine, potassium cyanide, or other aurous solvent, for by this means as much as 98 per cent of the gold contained in suitable ores can be converted into its mineral salt, and being dissolved in water, re-deposited in metallic form for smelting; but lode stuff containing much lime would not be suitable for chlorination, or the presence of a considerable proportion of such a metal as copper, particularly in metallic form, would be fatal to success, while cyanide of potassium will also attack metals other than gold, and hence discount the effect of this solvent."

Plattner's Process involves introducing chlorine gas, under pressure, into a chamber containing ground ore and water. Once dissolved in water, chlorine becomes hypochlorous acid and hydrochloric acid. It is the hypochlorous acid that puts the gold into solution.

In the electrolytic salt cell, sodium hypochlorite is produced, along with some sodium chlorate. Adding sodium hypochlorite to water also gives us hypochlorous acid, along with sodium hydroxide.

It is felt by some that it would be simpler to just use Clorox (sodium hypochlorite bleach) and create chlorine gas by adding hydrochloric acid to it. It is not quite as simple as that. If you have 6% Clorox bleach, it means you have 6% sodium hypochlorite in 94% water. The obvious question is: Why doesn't the 6% sodium hypochlorite combine with the 94% water to form hypochlorous acid? The reason is simple; during the manufacture of Clorox, sodium hydroxide is added to raise the ph of the bleach up over 12. If the ph of Clorox falls below 11.86 (say by dilution with water) the sodium hypochlorite decomposes rapidly. However, sodium hypochlorite produced in an electrolyic cell does not have this extra amount of sodium hydroxide added to it.

Because it requires an acidic solution to dissolve gold in this process, a fair amount of hydrochloric acid must be added to the Clorox to not only convert it to hypochlorous acid but to also bring the ph down into an acidic range. In Plattner's Process and the saltwater electrolytic cell, if we assume a ph of 7 in the water being used, the process itself creates enough acid (hypochlorous acid from sodium hypochlorite and chloric acid from sodium chlorate) to bring the solution into the correct acidic range to dissolve gold.

 

[Traveller11]

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

If iron requires such a low ph to dissolve, wouldn't it be simpler to keep the ph of the chlorine solution higher and just not dissolve the iron at all?

 

[Traveller11]

A further variation on Plattner's Process, source unknown:


"Electrolytic Precipitation of Gold"








0

Share on Tumblr

Email
Share






In order to perform the electrolytic precipitation of gold, from the filtration vessel the gold chloride solution was conducted into the outer or battery jar of an electrolytic cell. The electrode of the outer cell was connected with the negative pole of a dynamo and the electrode on the inner cell was connected with positive pole or terminal of the dynamo. The gold solution was sent into the jar near the bottom and slowly circulated upwards, and at the same time a current of electricity was passes through the cell. When the reaction was produced, the gold chloride was decomposed and felt like a shower of fine spangles to the bottom, while the liberated chlorine passed into the inner cell where it was absorbed by the water circulating and formed a chlorine solution.

The receiver was charged with chlorine solution generated by the electrolysis of salt. The unit comprised a battery, a conductor from the electrode in the large battery jar to the negative pole of the electrical generator, the conductor from the electrode in the porous cell to the positive pole of the generator, a reservoir containing a saturated solution of sodium chloride, which passed to the battery through a pipe and a reservoir containing water connected by a pipe with a porous cell.

In order to treat the ore more effectually with the chlorine solution it was advantageous to expel the air from the chlorinator. For this purpose the chlorinator was provided with a valve, so that the air contained in the chlorinator passed out as the chlorine solution passed in. the valve was closed immediately the air was expelled. The chlorinator after disconnected was slowly revolved by means of a pulley and strap from an engine, or in other suitable way until the gold was dissolved as a gold chloride. The time required for treatment in the chlorinator varied from one to two hours according to the characteristics of the gold ore treated. The ore and solution were discharged into a suitable filtration vessel placed beneath the chlorinator. The vessel was a shallow vat constructed of oak or other material, the lower part was made cone-shaped and of the same capacity of the chlorinator.

The vat was closed by a cover bolted down. In the center of the cover there was hopper-shaped inlet for receiving the ore and solution from the chlorinator. A perforated diaphragm covered with asbestos cloth, over which was advantageously placed a layer of other suitable filtration media such as ground asbestos, which was fixed from one to two inches below the top of the vat. The gold chloride was washed out of the ore by a stream of water from a tank. The water entered the vat through a pipe at the lowest part and percolated upwards through the ore until the gold content in the solution was extremely low. It was important to take samples of the solution to determine the presence of gold. A sliding door in the bottom of the vat was opened and the residue from the ore as discharged by means of a large outlet opened into a truck placed underneath.

The gold chloride and water descended through a pipe into a receiver and was conducted from the receiver into the outer or battery jar of the electrolytic cell. The gold solution flowed into and entered the jar at the bottom and slowly circulated upwards and at the same time a current of electricity was passé through the cell to reduce and precipitate gold from the solution into a perfectly pure state, upon the bottom of the jar, from where it was removed. The chlorine was liberated at the same time at the electrode in the inner or porous cell and in contact with the water circulating and formed a chlorine solution, which was sent to receiver vessel."

 

[Traveller11]

What is really interesting about the last bit of +100 year old material I posted (previous post to this one) is that it proves the gold is not put into solution by electrolysis. Rather, a "chlorine solution" (sodium hypochlorite/sodium chlorate) is created by electrolysis and piped to a leaching barrel where the gold is actually put into solution. What is unique here is that the gold in solution, or auric chloride, is filtered and piped back to the unit where saltwater is undergoing electrolysis. There, it is divided into gold and chlorine gas; the gold dropping to the bottom of the electrolysis chamber and the chlorine going into solution again to be piped back to the leaching chamber.

In other words, it is a chlorine leaching solution generator, leaching vat, electrowinning unit and leach recycling unit all in one. And, once the operator is finished, a liberal dose of sodium thiosulphate will neutralize everything in the chambers.

I've spent the last couple of days trying to find a design of a streamlined unit similar to what is described in the last post, with no luck. In this newer design, rather than the chlorine solution being made in one unit in a batch and piped to the leaching unit, only to be returned to the electrolysis unit once leaching is finished, this design called for a continuous loop. The ground ore was placed in a tub of saltwater. The electrolysis unit, also full of saltwater, was located beside this tub. A plastic pipe led from the electrolysis unit to the bottom of the tub, entering it there through a bulkhead fitting. A pump was located on this pipe to bring solution to the tub or "leaching vat". A motorized paddle is immersed in the leaching tub and turned slowly to keep the chlorine solution in contact with all of the ore. The water overflows through a port near the top of the leaching tub and returns by gravity to the electrolysis unit, passing through a filter on the way.

The electricity is turned on and sodium hypochlorite/chlorate is produced in the electrolysis unit. After a period of time the pump is turned on, sending hypochlorite/chlorate (in reality, hypochlorous and chloric acids at this point) to the leaching tub and bringing fresh saltwater (and eventually auric chloride) back to the electrolysis unit where the gold from the auric chloride is deposited on the cathode and the chlorine is transformed into hypochlorite/chlorate. The cycle is continued for up to four hours or until no more gold is seen depositing at the cathode.

 

[butcher]

Will this help? http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Industrial_Electrolysis_Processes

 

[Geo]

Butcher, you may have answered a question i had on another thread. thank you.

 

[Traveller11]

Will this help? http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Industrial_Electrolysis_Processes

The Chlor-Alkali cell is identical to the salt water electrolytic cell except for one thing; there is a membrane separating the anode side of the cell from the cathode side. This membrane stops the chlorine produced at the anode from reacting with the sodium hydroxide produced at the cathode, thus producing chlorine gas on demand from salt.

Our water system tried to find a mini version of the Chlor-Alkali cell to make chlorine gas for disinfecting drinking water. This is much safer (and cheaper) than shipping and storing pressurized bottles of chlorine gas. Plus, salt is easier to store and does not decompose, as sodium hypochlorite solutions do. Unfortunately, the only Chlor-Alkali cells we could find were very expensive and produced chlorine gas in amounts far beyond our needs.