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testimento
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Problem with my chlor-alkali cell
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?
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hyfalcon
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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]
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testimento
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I mentioned that I put a little NaOH for the cathodic compartment to make it more conductive. Should I use other compound?
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Pulverulescent
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| 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.
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If you want just HCl, why not use H2SO4/NaCl?
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones"
A Einstein
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testimento
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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.
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Pulverulescent
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| 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.
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones"
A Einstein
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ElectroWin
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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
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Xenoid
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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!
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testimento
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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]
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vmelkon
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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.
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testimento
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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]
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Pulverulescent
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| 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.
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The "Castner Process" produced Na by the hydroxide @ ~332°C and was replaced by the Downes Cell which used fused halide . . .
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones"
A Einstein
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Pulverulescent
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And, yes, PbO2 is well suited to 'chlorine processes'!
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones"
A Einstein
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testimento
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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.
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Xenoid
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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!
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testimento
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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.
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Xenoid
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testimento - you are wasting your time with these anode materials, but clearly you are a person who does not accept advice, blunder on!
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testimento
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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.
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hyfalcon
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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]
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Fantasma4500
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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??
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hyfalcon
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Unglazed ceramic soaked with electrolyte will pass current.
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testimento
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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]
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ElectroWin
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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]
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Oopsy_daisy
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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.
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testimento
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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. 
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]
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