Sciencemadness Discussion Board

Problem with my chlor-alkali cell

testimento - 12-7-2013 at 18:56

I have a project concerning the chlor-alkali cell. For those who dont know about it, it is about a normal electrolysis with two compartments separated by semi-permeable membrane in which cathode operates at pure water and anode at conc. NaCl solution.

What I've got is an anode pack made of titanium sheets, and cathode pack made of iron sheets. The "membrane" is an unglazed clay pot. Goal is to get chlorine and hydrogen gas out, lead them into a quartz chamber with UV agitator to cause them to burn into HCl gas.

The problem seems to be so far that the electrolysis doesn't seem to occur as it should be. I've made salt solution and added a little NaOH to the water comp. to make it little more conductive.

Possible problems:

-The power source gives 3.6-4VDC max, and the voltage in the solution, when the electrodes are placed on different sides of the pot, is 2.8V. ClAl cells operate at 3-3.6V for maximum efficiency.
-The solution is not heated. It's about 20C temp ambient.
-Nor it is concentrated with salt. I just made a test batch by adding about 5% max NaCl into the anodic compartment.
-And it hasn't been boiled. I've read somewhere that electrolytes should be boiled to make them saturated.

And...

-The distance between the electrodes. I've conducted a test by placing two electrodes as close as possible, technically pressing them to the walls of the clay pot, but electrolysis did not occur. When I place two electrodes at 3-5mm each other to the NaCl solution, an instant evolution of electrolysis occur, probably forming hypochlorite due to the low temp, and a literal turbulence is formed in the water due to the high power of the power source. But sources cite that distance between electrodes should not affect the electrolytic activity in the solution.

Any ideas what could fuck up?

hyfalcon - 13-7-2013 at 02:09

With pure water around one of the electrodes, I don't see how current will flow. All this has been covered before. You need to learn to search better and read. This information has already been covered many times on the board.

You need some more reading:

http://woelen.homescience.net/science/chem/exps/miniature_ch...

UTFSE

[Edited on 13-7-2013 by hyfalcon]

[Edited on 13-7-2013 by hyfalcon]

[Edited on 13-7-2013 by hyfalcon]

testimento - 13-7-2013 at 04:10

I mentioned that I put a little NaOH for the cathodic compartment to make it more conductive. Should I use other compound?

Pulverulescent - 13-7-2013 at 05:06

Quote:
Goal is to get chlorine and hydrogen gas out, lead them into a quartz chamber with UV agitator to cause them to burn into HCl gas.

If you want just HCl, why not use H2SO4/NaCl?

testimento - 13-7-2013 at 05:41

Because I need chlorine and ozone too (the app suits for o3 lysis), and it is not good idea to expect that every chemical is freely available in every country. I live in the most stringent industrialized countries on the planet, and technically all chemicals I must make myself. Basic acids may be available via company licensation process, but at this moment I cannot establish one.

I will do some testing tomorrow. I check the electronic systems, up the voltage a little bit, make concentrated solutions of chlorides and heat up the batch and see what I get.

Pulverulescent - 13-7-2013 at 08:47

Quote:
Because I need chlorine and ozone too (the app suits for o3 lysis)

Some serious study is suggested then, as you need somewhat more than the rudimentary understanding of electro-chemistry you appear to have?

Chlor-alkali animation here.

ElectroWin - 13-7-2013 at 12:56

you want to use a semi-permeable membrane, that selectively permits Na+ to flow across it but denies Cl-. it's a specialized material, i dont see how you quite get that with the unglazed pottery. also, the titanium anode should have a rare-earth oxide coating on it. platinized titanium is recommended, ruthenium MMO is do-able

Xenoid - 13-7-2013 at 12:59

Quote: Originally posted by testimento  
What I've got is an anode pack made of titanium sheets ...


Are you actually using titanium or MMO coated titanium?

You can't use titanium as an anode, it is a so-called valve metal. Ti, along with Al and various other metals become "anodised" - coated with a non-conducting layer of oxide.

For this process you will need to use MMO or graphite/carbon (try old battery electrodes or gouging rods!

testimento - 15-7-2013 at 06:50

Some pages specifically state that titanium can be used directly, and when platinum group metals or other become contact with elemental chlorine, they will form chlorides and corrode at high rate.

If it doesn't work, I can get gouging rods. But I have the titanium sheets already. TiO2 is electroconductive and it is actually used in semiconductor industry.

Semi-permeable membrane is stated here as salt bridge. The efficiency and purity of products might not be as high as with it, but the outcome is supposed to be the same. Some sources cite that clay-pot or salt-bridge can be used for the purpose.

How about PbO2, will it survive chlorine gas? I've got a full 20-pack of em, wouldn't want to do the sulfuric acid coating all over again if they pick up some chlorides, anyone know?

[Edited on 15-7-2013 by testimento]

vmelkon - 15-7-2013 at 11:24

I guess your clay is not conducting because it hasn't absorbed the salt solution yet.
You could just remove it and electrolyse the salt solution. You can even acidify it to destroy any hyprochlorite ions that form. That way, you still get some hydrogen and chlorine.

testimento - 15-7-2013 at 11:36

Thanks for the points. I made some tests how the pot soaks through, and it takes about 15-20 minutes to see the water line turning the pot darker. It has been sitting for nearly a week in the solution for now.

Few possible points I can see:

1) The solution isn't concentrated enough: it is maybe just 5% NaCl at the moment, because I was just testing the conductivity
2) The solution temperature is ambient: I am planning of bringing it up to 80C, the level some chlor-alkali cells are cited to be operating. Because the vat is plastic (100l volume), I am on my way of making electric circuit heater for the solution aside of my oven.
3) The input voltage is 4.5V DC, which should be quite enough for CA: industry operates at 3-3.6V.

I tested the conductivity of the solution, and the voltimeter says it operates at 2.6 volts(if I remember, need to verify this). This measurement was done by placing one electrode inside the pot and other outside, so electrons move through the pot wall. Is it normal that the electrolyte voltage is nearly half of the voltage compared to the electrodes?

I have a small, 3-liter castner cell prepared for use, and I might test how it would function at 600C with mixture of CaCl and NaCl by means of direct electrolysis with carbon anodes and iron cathodes. I have succesfully tested this device for producing about a kilogram of sodium metal.

If someone knows is PbO2 still suitable for chlorine processes, I'd be glad to know. :)

[Edited on 15-7-2013 by testimento]

[Edited on 15-7-2013 by testimento]

Pulverulescent - 16-7-2013 at 11:27

Quote:
I have a small, 3-liter castner cell prepared for use, and I might test how it would function at 600C with mixture of CaCl and NaCl by means of direct electrolysis with carbon anodes and iron cathodes. I have succesfully tested this device for producing about a kilogram of sodium metal.

The "Castner Process" produced Na by the hydroxide @ ~332°C and was replaced by the Downes Cell which used fused halide . . .

Pulverulescent - 16-7-2013 at 11:30

And, yes, PbO2 is well suited to 'chlorine processes'!

testimento - 16-7-2013 at 11:39

Yep, castner process is for metallic sodium which I have done, and castner-kellner cell is using liquid chloride salts(na+ca:cl) at 600-700c to create chlorine.

And thanks, I'll see about my PbO2 electrodes. :) I use the titanium plates, if they corrode away, then I'll throw them away and use lead and graphite instead. Once I've bought and cut them into proper pack, I dont have any other use for them.

Xenoid - 16-7-2013 at 12:26

Quote: Originally posted by testimento  
... I'll see about my PbO2 electrodes. :) I use the titanium plates, if they corrode away ...


These "PbO2 electrodes", are they genuine commercially made PbO2 coated titanium or are they a bunch of anode plates from an old car battery? If the latter, then because of the exposed lead surfaces they will corrode rapidly and fall to pieces.

Your titanium plate anodes will not "corrode away" unless you exceed the breakdown voltage of the NON-CONDUCTING oxide layer that they will be coated with!

testimento - 16-7-2013 at 12:51

Titanium is rated "excellent" in chlorine environment and it's corrosion rate at high temperature, pressure and concentration of wet chlorine environment is under 0.01mm/year in several tests. Platinum and other similar metals will be eaten away by elemental chlorine within days. At least the electrodes seem to conduct electricity into the solution, according to my electric meter, so long for the non-conducting layer.

http://www.parrinst.com/files/Parr_Titanium-Corrosion-Info.p...

The PbO2 electrodes are made by myself from purified lead. I milled a mould for this specific purpose and made them by spec, 3mm thick plates and coated them by electrolytic sulfuric acid bath.

I'll post some pics of the electrolysis device some day when I get chance.

Xenoid - 16-7-2013 at 13:15

testimento - you are wasting your time with these anode materials, but clearly you are a person who does not accept advice, blunder on!

testimento - 16-7-2013 at 13:21

I just checked the price for otc MMO anodes, they rate at 200EUR per 5 units of 50x50mm, which would total of 4000 euros to get the same surface area. Titanium has ridiculously low EC, only about 4% that of copper, it will need about 1500mm2 to conduct 400A of current, which must be taken into consideration with MMO's.

I'll go with the PbO2, and if the titanium shows up problems, I may coat them with PbO2 too. I've got plenty of lead, nitrates and some sulfuric acid to go with. I might even see to coat them with some commercial MMO composition if I happen to get inspired by it.

hyfalcon - 16-7-2013 at 14:56

I've got several 6" X 10" pieces of MMO from Lasered off ebay. He doesn't have anything listed at the moment, but he usually does on an intermittent basis.

I've got just over $ 20.00 US in each piece.

[Edited on 16-7-2013 by hyfalcon]

Here's one of the old auctions:

http://www.ebay.com/itm/330952002551?ssPageName=STRK:MEWNX:I...

[Edited on 16-7-2013 by hyfalcon]


Looks the same as this seller is getting 10 times as much for.

http://www.ebay.com/itm/6-X10-MMO-ANODE-MESH-EXPANDED-TITANI...

[Edited on 16-7-2013 by hyfalcon]

Fantasma4500 - 16-7-2013 at 15:07

about NaOH, this is supposedly possible to produce in bulk and low % by letting wood ashes sit in water 3 days
of what i know wood ashes contains K2CO3
i dont see where the NaOH comes from, but surely the guy isnt right as ive heard about this several other places

it can ofcourse be boiled down and at one point let cool to crystallize out (add ethanol to force it out?)
this is incase you want the NaOH

what scale are you considering with the H2 / Cl2 anyways? it goes off more powerfully than H2 and O2, tho i dont know if its in peak performance ratios (H2 / O2 is 75% 25%, 4000m/s)
i think you would like to have the cell running some time to make sure the H2 would backfire into the cell (of what i know H2 doesnt need many percentages of O2 to burn, probably around the same of acetylene, 3-5%)

but i dont really see how this setup would work, in my mind you cant have 2 solutions in 1
i mean
if the NaCl*H2O is in contact with the H2O you would get NaCl*2H2O (half concentration of original, double volume)
dont take it as NaCl can actually bind itself to water like that

as of what i know clay isnt really conductive, is it??

hyfalcon - 16-7-2013 at 16:26

Unglazed ceramic soaked with electrolyte will pass current.

testimento - 16-7-2013 at 22:37

The anode and cathode are separated to their own gas traps leading underwater and they have piping in which the gases are lead into quartz burner with UV light on it, so it should break down the Cl2 and make them go with the H2 to form HCl. It should burn like ordinary gas torch when initiated. The system will be flushed with inert gas prior to startup, of course, otherwise the H2 will backfire instantly when there'd be enough energy and bang up the cell and it could cause catastrophic failure of the quartz burner.

And about that anodization. I could call myself stupid of not remembering that I have actually anodized a ton of aluminium parts recently, but when I begun to though about it, I was thinking that anodizing will need something that oxidizes the surface, like sulfuric acid used with aluminium. If one uses plain sodium chloride solution, electolysis would form only chlorine and sodium ions, and since chlorine won't less care about Ti or TiO2, neither would sodium, it could actually work in this case. I dont see where the oxygen would come. But when used with chlorates and perchlorates, highly oxygenating stuff like chlorite, chlorate and perchlorate and every single of their ions are produced in the same container, which could induce the oxidation. In this case, no chlorine would be produced so it wouldn't neither dissolve pure platinum.

I've got to test it anyways. If it doesn't work, I'll move on to the PbO2 and see if I bother to coat the Ti plates later on.

[Edited on 17-7-2013 by testimento]

[Edited on 17-7-2013 by testimento]

ElectroWin - 17-7-2013 at 11:08

as an aside, if you really just want the NaOH,
sodium bicarbonate can be separated from mixtures of the soda ash and potash and their carbonates typically found in wood ashes, by making a saturated solution of these and bubbling CO2 into it. The NaHCO3 is less soluble and precipitates. the NaHCO3 is then washed, and boiled to release half of its CO2, and CaO added to take the remainder, leaving NaOH solution.

also, remember that electrolysis involves current density at the electrode, in amps per /m^2 or amps per sq ft if you like, and use a current limited supply rather than voltage limited

[Edited on 2013-7-18 by ElectroWin]

Oopsy_daisy - 18-7-2013 at 00:22

The voltage seems way too low to be able to push very much current through your high resistance cell. Try increasing your voltage to about 24 v and you should be able to pass at least some current through the cell.

testimento - 18-7-2013 at 09:24

24 volts? Are you shitting me? I just completed winding the transformer for 4.5VDC. I've gotta make another, if the thing needs that much voltage.. Only good thing for 24V is that the windings don't have to be oceanic cable sized... Im gonna buy a car battery charger for that if this aint gonna work.. They sell those big ones up to 100A capacity.

I made some more tests and calibrations today. The distance between electrodes is about 200mm, with saturated, 40-50C salt solution at anode and 5% NaOH at cathode. I turned the system on, giving out about 4.5VDC on load, and the electrolyte voltage was about 2.2 at the beginning, but it began to decrease slowly, reaching 0.9 volts in 5 minutes. This is a clear sign of anodic passivation. So sad, my bad. :D

The cell clearly produced a little chlorine, the smell was as recognizable as warm chlorine ever can be at spa's. Well, I've got my PbO2's prepared, I make gas traps for them and continue testing. Being little sceptic about the actual generated power of the cell, I'm expecting of producing clear amount of chlorine and hydrogen. The input power is 2000 watts with full wave rectification, which should yield about 400 amps at the power supply.

How much does the distance between electrodes effect the electrolysis effect on separated electrode cell? With chlorates the effect is enormous: my PbO2-pack with 5mm gaps between each electrode generates a whirlpool, but when put at the ends of the pool, the electrolysis is merely noticeable. How can one calculate the resistance? The clay pot doesn't seem to affect the electronic movement at all, the electrolyte voltage is same with or without it, which seems to be a good thing. The total volume of the cell is 60 liters, of which usually no more than 30-40 liters (max) is used, and of that, 7-10 liters for cathodic compartment. Should the volume affect the resistance?

[Edited on 18-7-2013 by testimento]

bfesser - 18-7-2013 at 09:46

<strong>testimento</strong>, I've recieved several U2U complaints about the coarse language in your posts. Could you please try to swear a little less?

testimento - 18-7-2013 at 09:51

Sorry. In my country it is a habit to curse a lot. I will correct my speech after this.

ElectroWin - 18-7-2013 at 09:52

really, stop thinking in terms of a voltage limited supply, and go with current limited.

testimento - 18-7-2013 at 10:03

If the device should work for longer periods of time, the efficiency must be taken into consideration. It is not sensible to build larger device which efficiency lies somewhere between 15% and 20%. It wouldn't matter if electricity was a lot cheaper, I'd be using 3-phase with aluminium busbars.

ElectroWin - 18-7-2013 at 10:18

right, so consider some kind of switching power supply (typically > 80% efficient).

some discussion of the basic techniques may be found at http://www.microsemi.com/document-portal/doc_download/129605...

[Edited on 2013-7-18 by ElectroWin]

testimento - 18-7-2013 at 11:23

If the cell doesnt begin to give out decent yield with PbO2 at 4.5VDC, I will test out 24VAC device with expected output around 20VDC to see if it would give better results.

ElectroWin - 18-7-2013 at 13:44

Quote: Originally posted by testimento  
If the cell doesnt begin to give out decent yield with PbO2 at 4.5VDC, I will test out 24VAC device with expected output around 20VDC to see if it would give better results.


youre not getting it. again,

the chemistry is about the current density, not the voltage.

example: if your anode is 10 cm^2 surface area (= 0.001 m^2), and you need 400 A/m^2 current density to produce the right chemical species, then you want the current to be 400 x 0.001 = 0.4A

and i hear of using as much as 3543.6 amp/m^2 in chlor alkali cells,...
-- http://support.olisystems.com/ApplicationBriefs/Briefs%20-%2...


[Edited on 2013-7-18 by ElectroWin]

plante1999 - 21-7-2013 at 10:46

Anyone know a place to find the chlor-alkali chem study like video? I know it is pretty uncomplete question.

There was a chemist demonstrating mercury cell on a lab scale in the video. It is an old video.

[Edited on 21-7-2013 by plante1999]

How to heat up chlorate bath in plastic tank?

testimento - 22-7-2013 at 02:23

Any ideas how to heat up chlorate bath to 60-80C when the container is plastic?

Pulverulescent - 22-7-2013 at 04:04

The usual way of heating a chlorate cell is by adjusting the working current-density to an amperage that will heat the electrolyte resistively!

bfesser - 22-7-2013 at 05:14

<strong>testimento</strong>, instead of starting a new topic for every chlorate cell question you can think of, reply to one of your <a href="http://en.wiktionary.org/wiki/Sagan" target="_blank">Sagan</a> <img src="../scipics/_wiki.png" /> existing threads.

testimento - 22-7-2013 at 13:29

Well I havent driven the test batches yet for so long the temperature would have risen, but I've gotta try and measure the temp next time. I have understood that voltage plays a major role for temp losses, and since Im driving it close to the ideal voltage (4.5VDC) with intimately placed anodes and cathodes, the temp losses may be quite minimal.

About the chlor-alkali, I tested that the both anode and cathode work, and surprisingly, some activity can be noticed at the cathode side, but anode side doesn't give any hint of chlorine. This could indicate low or no current passing through, but the voltage is same on both sides and the cathode giving hydrogen indicates that something is definitely going on there. The pot doesn't affect the current passing according to that.

Could a higher voltage be needed? Should I obtain a 24V power source and give a try with it?

ElectroWin - 22-7-2013 at 14:57

Quote: Originally posted by testimento  

Could a higher voltage be needed? Should I obtain a 24V power source and give a try with it?


why are you ignoring my replies?

Xenoid - 22-7-2013 at 15:03

testimento - you are becoming a real PITA :mad:

Please stop spamming the forum with your nonsense!

For some information on amateur anodes, please check out the Chlorate - Perchlorate site:

http://www.oxidizing.110mb.com/chlorate/anodes.html

Get in quick, as the site seems to come and go!

testimento - 23-7-2013 at 06:29

Sorry, I really missed your first post ElectroWin. I was in a hurry when I posted the chlorate temp topic and missed this and when bfesser merged the topic I didnt notice your post. I'm gonna try driving very high anode currents soon.

testimento - 9-2-2014 at 15:46

Could gallium be used in chlor-alkali cell for a substitute of mercury?

vmelkon - 12-2-2014 at 20:56

Quote: Originally posted by testimento  
Could gallium be used in chlor-alkali cell for a substitute of mercury?


I haven't tried it, but I imagine it would behave a little like Al if it comes into contact with NaOH solution.

From wikipedia
"Alkaline hydroxide solutions dissolve gallium, forming gallate salts containing the Ga(OH)4(−;) anion"

testimento - 14-2-2014 at 21:59

Well, I ordered some gallium already. So sad, gotta go with Downs cell then. :D

I have understood that in CK cell the mercury should be agitated in order to drive the amalgam into the other side, and if it's not agitated, it would just sit there and the electrolysis would come to cease.

vmelkon - 15-2-2014 at 05:58

I don't think the electrolysis would cease. Sodium mercury mixture is a metal. The problem would be that the sodium reacts with the water in the wrong place (the electrolysis cell). In industry, they have pumps to push the mercury to some other section where they flow it on graphite balls with water dumped in. The mercury than is pumped back to the cell.

I don't know if I posted this link before.
It shows how they do it in industry and also on a small scale.
http://www.youtube.com/watch?v=GtxiCZrLH8g

testimento - 15-2-2014 at 14:37

The reason I didn't go for the classic "rocking cell" is because I didn't have mercury available in reasonable price. Even a very small cell would require a minimum of a liter, or 13.5kg of mercury in order to function properly. The rocker cell idea is simply to flow the mercury from compartment to another to swirl the amalgam and allow it to surface with the differential cells. Even a tiny movement would be enough. Technical mercury is a waste product and therefore should be available quite cheap, but because the draconic laws they all end up into lockdown.

My objective is to produce chlorine gas for chlorination and other purposes, so if anyone has got ideas how to separate chlorine from the mixture, I'd be extremely happy. I dont mind the NaOH, because I can make it very cheaply from calcium hydroxide. My best bet so far is the downs cell, which technically requires to smelt fused salts (NaCl:CaCl2 42:58), or then directly calcium chloride (MP 770C) and electrolyze it with carbon anode to get chlorine out. I would pack the whole SS made cell into ceramic mantle and heat it up with nichrome to get it going. With firebrick base, aluminium oxide lining, calcium sulfate body and rockwool insulation the whole cell should actually be quite efficient and maybe even self-sustaining once it has got up to the required temperature. Pure CaCl2 has 64% of chlorine in weight.

[Edited on 15-2-2014 by testimento]