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Author: Subject: Thoughts On Anodes
jpsmith123
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[*] posted on 30-12-2005 at 00:59
Thoughts On Anodes


For quite a few months, I've been researching the question of anodes for small scale but serious production of chlorates and perchlorates.

In case anyone is interested, here are some of my conclusions so far:

(The following materials, as anodes, will make either chlorate or perchlorate).

Platinum

Extremely expensive, though highly efficient, and requires some process control for maximum longevity.

IMHO, the combination of high cost with non-negligible rate of erosion rules out platinum as an option for *serious*
production at home; at least as far as "batch" processes (i.e., processes where the proportions of ionic species change dramatically) are concerned.

If you want to make a few lbs. of ClO4 just to say you did it, Pt is probably the easiest way to go, IMO.

***************************************

Lead Dioxide

Generally almost as efficient as Pt, and rather cheap, but labor intensive and somewhat problematic to coat a substrate with.

It is also mechanically fragile, has a non-negligible erosion rate, and it and its precursors are toxic compounds.

If I were going to use it, I suppose I would use it with a conducting substrate, and, rather than the hassle of spinning the substrate during the plating, I would use a surfactant. According to Wouter's page, Triton-X (which seems fairly cheap and readily available) won't work, but according to some patents I've seen, it supposedly does work.

As far as I'm concerned, the biggest drawback is the toxicity. I was actually preparing to use it, but I decided against it
as I just don't want the hassle of dealing with toxic lead compounds, especially the soluble precursors.

*****************************************

MnO2

There's not a lot of literature that I could find regarding this. The main problem seems to be the reliable formation of a
layer of beta-MnO2, vis-a-vis gamma-MnO2, which is inactive.

Supposedly the thermal decomposition of manganese nitrate leads to the beta form, whereas other precursors don't.

Also, in addition to erosion, I think I read where, depending on conditions, the beta form may slowly revert to the gamma
form over time. I'm not sure about this or about how much of a problem it really is in practice.

It would seem to be cheap and not very toxic (although manganese compounds can be neurotoxic if inhaled).

****************************************

Silicon

I found a patent that mentions its use as an anode for perchlorates, and a news group discussion where someone claims it worked, but that's all I could find. I would speculate that the difficulty of working with it may be one reason it apparently never became an important anode material.

****************************************

Graphite

This is probably the cheapest and easiest anode material to make *chlorates* with. According to what I've read in some
patents, getting a yield of 100 kg of chlorate will, depending on conditions, erode between 0.4 and 1.0 kg of material from an *untreated* graphite anode. If the graphite is impregnated with various resins to make it "waterproof", the loss can supposedly be reduced to 0.1 to 0.2 kg per 100 kg of chlorate.

Supposedly graphite will make perchlorate, although I've found hardly any documentation to that effect. I assume it will do so at low efficiency under relatively extreme conditions, e.g., low chloride and high chlorate concentration at higher voltage, compared to other materials.

In fact, if it is desired to make chlorate that is free from perchlorate contamination (especially in a "continuous"
process), graphite may be the material of choice, since all of the other materials seem to be much more likely to make
perchlorate under a given set of conditions.

I should say I have some ideas on how to treat graphite to make it better, and I'll probably post these ideas in the future.

*****************************************

"Mixed Metal Oxides"

This stuff, the active, outer layer of the so-called "Dimensionally Stable Anode" or "DSA", is looking pretty good to me.

After reading what I believe are the original two patents, and a few full text journal articles that I found on the internet, this seems the way to go for perchlorate production at home. (In fact the patent does recommend it for perchlorate production, but the recommendation is buried deeply within the body of the patent).

As I understand it, what has become known as a "DSA" has several distinguishing features of interest. The first is the
substrate of titanium or niobium for example, upon which an oxide layer is formed, I guess by anodizing. Regardless of
whether the next layer (the outer layer), is platinum, a single platinum group metal oxide, or mixed metal oxides, the point
is that intermediate oxide layer conducts current from the substrate to the outer layer, but doesn't conduct current to the brine. IOW, if the active outer layer gets dissolved in a spot, supposedly the intermediate oxide layer is a nonconductor, as far as the brine is concerned, and accordingly acts as a barrier to prevent the substrate from being corroded. So, rather than sudden catastrophic failure, the anode fails by the slow loss of active surface area as the outer layer erodes or dissolves or whatever it does.

Another feature is the use of metal oxides, rather than pure platinum, for example, as the active layer. Supposedly, when used in thin layers, the platinum group metal oxides are approximately as conductive as the pure metals themselves, while at the same time they are two orders of magnitude more resistant to corrosion by the brine and the electrolysis products. Moreover, ruthenium and palladium for example are much cheaper than platinum, so they are cheaper to make.

As I understand it, there is generally a platinum group metal oxide and a non platinum group metal oxide such as TiO2,
present in the outer layer. Although the anode would work without the TiO2 in the outer layer, it is better with it because it supposedly makes the RuO2 or PdO2 or whatever more chemically stable.

What I don't understand is why these "Dimensionally Stable Anodes" don't seem to be used in industrial perchlorate
production; or at least, why there's not much evidence that they are. This has me somewhat baffled.

In any case, it seems there are lots of companies out there using DSAs in pool chlorinators, an application for which they
seem to work very well. And the journal articles I found demonstrate that they work very well for chlorate production.

Since there seems to be many places selling generic DSA equipped pool chlorinator replacement cells, complete with anode, cathode and electrode terminals, I am probably just going to buy one of these to experiment with for perchlorate production.
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[*] posted on 30-12-2005 at 19:49


Perhaps you should include titanium, which in many cases is as good an inert electrode as you can ask for.
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[*] posted on 31-12-2005 at 06:43


Titanium is generally used as a cathode, or as an anode substrate; is this the use you had in mind?
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[*] posted on 31-12-2005 at 10:42


Yes, I guess the key word would have been "electrode", my apologies for being vague. As a cathode it finds use in electrolysis and especially electroytic refining (i.e. gold refining) or as a substitue for nickel. I'm sure there are situations where it is not ideal, but I'm dragging this off topic :( as I now see this is directed primarily towards the production of chlorates and perchlorates. However, I think this is a good idea jp, perhaps we should have discussion (or make a list of suitability or something like that) on cathodes and anodes.
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[*] posted on 31-12-2005 at 21:29


FYI, titanium makes a nice diode: as a cathode, it passes current normally as any other metal. As an anode, no current flows.

You can probably develop colors (as Woelen with his niobium) by anodizing to various voltages in just about any acid solution.

Tim




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[*] posted on 31-12-2005 at 23:41


Hello Fleaker,

Actually, my subject line should have been more specific.

What I'm interested in doing is tying up some loose ends (primarily involving anodes) regarding the home manufacture of chlorates and perchlorates, or at least, putting much of the relevant information in one spot, if possible.

I've found two or three dedicated web pages that relate to anodes, a few news group discussions here and there, and a few discussions in forums like this one, but none of what I've read so far has given me a warm and fuzzy feeling as to the best method of making perchlorates on a semi-serious scale at home.

I am leaning towards buying a replacement pool chlorinator cell and testing it for use in ClO4 production. Unfortunately, the cheapest one I found so far is almost $200 USD:

http://www.saltcells.com/ecomatic-sr75-generic-cell-p-481.ht...

I may also try to make one based on information in the attached patent.

As described in the patent, the inventor apparently even successfully treats a graphite substrate, creating a rugged RuO2-TiO2 coating that appears to hold up well in a subsequent test.

If this stuff can be slapped onto graphite (even if it doesn't last as long as a Ti or Nb substrate, who cares? if it's relatively cheap and easy to do) this may be the way to do it at home.

Unfortunately neither of the two patents I have go into detail regarding the tradeoffs among the various platinum group metal (PGM) oxides.

Ruthenium oxide seems to be an industry standard, but why, I wonder? Is it because it's somewhat cheaper?

From what I've read elsewhere, PdO may be the oxide of choice, at least in the sense that it generally has a low chlorine overpotential and a high oxygen overpotential, compared to the other PGM oxides; however this seems to be somewhat process dependent, mediated by both physical and chemical factors.

Pd compounds may also be less toxic than Ru compounds and somewhat easier to work with (although I think Pd oxidizes at a higher temperature compared to Ru).

Also, I don't know how the various PGM oxides compare in terms of general resistance to erosion under the conditions of interest.

If I understand correctly, the erosion occurs when small amounts of the oxides get reduced to metals, and that's supposedly why the mixture of oxides is needed - to stabilize the PGM oxide against reduction to the metallic state.

The (most likely small) differences in efficiency and ruggedness between Pd and Ru may be unimportant for a small scale home operation, with price, availability, toxicity and ease of use being more important.

Attachment: 3632498.pdf (830kB)
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[*] posted on 31-12-2005 at 23:54
Another Patent


Attached is another related patent.

I guess it's too big...it didn't upload.

[Edited on 1-1-2006 by jpsmith123]
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[*] posted on 1-1-2006 at 02:12


....... regarding the home manufacture of chlorates and perchlorates, or at least, putting much of the relevant information in one spot, if possible............

Carbon cutting electrodes (gouging rods) used by welders work well. They are a bit soft and the copper coating needs removal but the price is right.

Using the 3/8 rods as both electrodes in a 8L chlorate cell produced a couple kilos
from NaCl in a 1 1/2 days leaving about half of the anode unconsumed. The small amount of carbon at the cell bottom did not interfere with the chlorates purification.
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[*] posted on 2-1-2006 at 00:04


A few kilograms in 1.5 days? Wow...sounds like you were operating at a rather high current.
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[*] posted on 2-1-2006 at 11:14


Quote:
Originally posted by 12AX7
FYI, titanium makes a nice diode: as a cathode, it passes current normally as any other metal. As an anode, no current flows.

You can probably develop colors (as Woelen with his niobium) by anodizing to various voltages in just about any acid solution.

Tim

It is a little bit off-topic, but I want to mention that I tried electrolysis of NaCl with an anode, made of titanium (I just purchased 2.4 meters of 1.5 mm thick 99.9% Ti-wire for EUR 5,- :)). No current flows at all, when the voltage is below approximately 12V. The color of the wire shift from grey to a little brownish grey, but the color is not nice and bright as with the niobium experiments, it is more as if the wire is a little bit tarnished.

When the voltage is increased to 13V, the titanium anode dissolves/erodes, giving a white flocculent precipitate. When some dilute HCl is added, then this white precipitate does not dissolve. When some 3% H2O2 is added, then the white precipitate dissolves quickly, leaving a clear deep-orange liquid. This orange color is due to formation of a red peroxo-complex.

Summarizing, titanium is not suitable for production of chlorate, but that you all probably knew already :D.




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[*] posted on 8-1-2006 at 16:44


I'm wondering, is anyone else here interested in the idea of using a commercial (pool chlorinator) MMO anode (or complete cell) for ClO3/ClO4 production? I'm trying to get some prices, etc., from various manufacturers and/or distributors which I'll post here if there is any interest. If not, I won't waste bandwidth by posting it here.

Regards,
Joe
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[*] posted on 8-1-2006 at 23:41


Yes, I'm interested in such anodes (not complete cells) if they are not too expensive. However, shipping may make things expensive (I'm in the Netherlands). But if you can obtain price info and find a supplier, who ships internationally, that would be very nice. I'm still looking for a good anode. Graphite is nice, but far from perfect.



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[*] posted on 9-1-2006 at 05:34


I'm waiting for some info from a potential supplier right now. Unfortunately, most distributors seem to have little or no technical information at their disposal regarding chlorinator cells and electrodes.

I suspect that most manufacturers in fact use MMO coatings, but the use of imprecise language in a few instances, apparently by marketing people who don't know any better, forces me to have to ask each potential source.

As a last resort, I suppose the color of the anode may be a good indicator, since PGM oxides are essentially black, while the noble metals themselves wouldn't be.

In any case, I was originally considering buying a complete cell, but I decided against it as I believe that would be wasting money on a housing and possibly other hardware that I wouldn't use.

That's because pool chlorinator cells are designed for a relatively low salt concentration, so their electrodes are generally quite close together, to keep the voltage drop across the cell within reason when operating at the rated current.

Since I anticipate using saturated NaCl brine, and since since I'm not sure yet exactly what type of power supply I'll be using and exactly how I'll operate the cell, I think it would be best to have a setup with maximum flexibility.

Then there is also the issue of mesh vs. plate type anodes, and single plate vs multiple parallel plates. At this point, I'm leaning toward the single, relatively long plate, if for no other reason than that it should be easier to estimate current density, for purposes of fine tuning the process, e.g., when wanting to make high purity chlorate. The single plate type should also make it easier to adjust electrode spacing.
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[*] posted on 9-1-2006 at 06:53


I have a chlorinator cell on my pool. I could measure the voltage/current if it would be any help. It's supposed to produce 40g per hour of "active chlorine". This is attached to a 85000L pool and runs 8 hours per day to keep it bug free. The salt concentration is just tastable.

Anyway, if I can help in anyway then just ask.

Mike.
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[*] posted on 9-1-2006 at 19:00


Quote:
Originally posted by Pommie
I have a chlorinator cell on my pool. I could measure the voltage/current if it would be any help. It's supposed to produce 40g per hour of "active chlorine". This is attached to a 85000L pool and runs 8 hours per day to keep it bug free. The salt concentration is just tastable.

Anyway, if I can help in anyway then just ask.

Mike.


Kindly remove the chlorinator and send it to me for testing ASAP ;)

Seriously, thanks for the offer. You're in Australia? That (and also India) seems to be the land of pool chlorinators...you must almost be tripping over them...

I'm considering buying a "Sal-Chlor" generic replacement anode from one of the places in Australia. The only problem is that I have no official confirmation as of yet that it has a MMO coating.

I'm probably going to get one though, since the anode does appear to be black or nearly black from the picture I've seen.
Moreover, the price is right.

You don't perchance have a "Sal-Chlor" chlorinator, do you?
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[*] posted on 9-1-2006 at 21:40


I have previously priced 1.5mm MMO (titanium substrate) electrode wire which ran ~$8 AUS (~$6 US) per metre. Though minimun order of 25m :(

The wire was meant for cathodic protection of steel structures. Possibly a better (cheaper) source then pool chlorinators. They are available in ribbon/plate/mesh etc. I think there is also Ti/Cu alloy substrate.

[Edited on 10-1-2006 by Axt]
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[*] posted on 10-1-2006 at 00:50


Hmmm, I can see possibilities there. Thanks for that info, Axt. Does this supplier have a web site?
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[*] posted on 10-1-2006 at 02:04


Yes, the Australian manufacturer/supplier is nmtelectrodes.com, though I expect that there is suppliers world wide search for "ICCP electrodes" ( for Impressed Current Cathodic Protection) or words to that effect.
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[*] posted on 10-1-2006 at 03:47


I am not sure if this has been mentioned before; forgive me if this was already considered a long time ago.


Why not just using PbO2-coated lead plates from old car batteries? They are exactly what we want. Why all the hassle of coating, if they are freely available in old batteries?




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[*] posted on 10-1-2006 at 03:55


Battery electrodes have an exposed lead mesh substrate, they fall to bits fast.
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[*] posted on 10-1-2006 at 04:21


And what if they are very loosely held in place with a very very restricted amount of polyester-like hardener? I am sure this could be made to work, if this is the only problem!?!



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[*] posted on 10-1-2006 at 04:44


They are a rectangular lead mesh with pressed PbO2, the lead is not completely covered in PbO2, nor is the PbO2 impervious to liquids. The lead will quickly corrode away. I dont think there is any way they could be made practical.

Below is a plate from a car battery, see the exposed lead around the edge and along the mesh.

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[*] posted on 10-1-2006 at 05:22


Unfortunately it seems many people have already tried using lead-acid battery anodes, all unsuccessfully, AFAIK.

Here's a thread which describes what is probably a typical example:

http://www.sciencemadness.org/talk/viewthread.php?
tid=1425

MMO based anodes seem to be one approach that hasn't been fully explored, or at least documented, IMHO.

Since it seems that 15 or 20 amp rated MMO coated Ti anodes can be had for $75 to $100 USD, I think it's worth trying.

If they turn out to be as good for home ClO4 production as it seems they should, then it might be interesting and maybe even economical to try to coat Ti substrates at home.

Especially if the info like the following from the patent I uploaded is accurate.

***************************************

Example XVI

Two titanium rods were degreased and pickled and subsequently placed in a galvanic bath having the following composition:

100 cc. ethanol
100 cc. water
1 g. ruthenium chloride
10 g titanium chloride

and subsequently connected to a source of alternating current of 13 volts and a current density of 15 amp/m^2, temperature 20-30 degrees C., for a period of about 20 minutes.

After about 20 minutes both rods were coated with a mixture of titanium oxide and ruthenium oxide, the adhesion of which was still further improved by heating at 400 degrees C for 5 minutes.

The anode thus made is excellently suitable for use in various electrolyses at low current densities.
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[*] posted on 10-1-2006 at 05:25


Ok- well at least in Germany, car batteries are (were?) definitely not made of a mesh-material. They were solid, and I know this because I took them apart.

Regardless- I was wondering, what evidence is there that the lead will quickly corrode away?? After all, it'd be covered with PbCl2, which is quite insoluble, or simply with more PbO2. Alternatively, one could i.e. add a small amount of Na2SO4 to the NaCl solution (which is what the electrolyte is, NaCl, NaOH, NaOCl, and NaClO3), which would cover any free Pb.

Hmm. Did you try this very thing, using battery electrodes, and did you find corrosion that made the system unworkable?


Nonetheless, I can see the following solutions - dip the mesh into NC-acetone lacquer, let it soak. Take it out to dry. Then dip it into acetone again, just to remove the OUTER layer of NC, and let it dry again. A significant amount of PbO2 should be exposed now, no? Or something along those lines. Anyway. This probably belongs to the dedicated PbO2 thread, might merge it there later.




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[*] posted on 10-1-2006 at 06:35


Somewhere I heard of an older patent from 1977 that takes titanium and coating this with SnO2 with some MoOx and finally ruthenium oxide. All steps incorpate a volatile salts and application of heat of 600 to 800 C to decompose tho the oxide. So possibly pickling titanium in wamr oxalic acid, brushing on concentrated stannous chloride solution and applying a flame to decompose to SnO2. Follow this with Ammonium Molybdate solution and ruthenium trichloride. The ruthenium salts will be difficult because the carbon monoxide from the flame would reduce it to the metal. An electric oven is needed here.



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