Sciencemadness Discussion Board

Chloroalkali process

chucknorris - 13-12-2012 at 08:17

What have I read about the chloroalkali process, one needs two electrodes and some sort of membrane in a pot where brine and water are flowed and then one will get H2 and Cl2 and sodium hydroxide.

Im intending to make device that can produce hydrochloric acid and sodium hydroxide, so what materials could be applied for the electrodes? Would platinum do? They must not corrode during process. And where the hell could one look for that ion exchange membrane?

MrHomeScientist - 13-12-2012 at 10:58

I've actually done this using a simple clay flower pot as the membrane and graphite electrodes, and it definitely works once you give the clay a few minutes to soak in the solution. I sealed the hole in the bottom with caulk and placed it inside a larger plastic bucket, and filled both with saturated salt solution. Putting the anode in the flower pot produces NaOH in there and HCl in the bucket (with some chlorine gas escaping). I used about 6V, didn't measure the current. It was more a proof of concept experiment.
I've since learned that you don't need salt water in the anode compartment, just fresh water. Pure distilled water would probably require adding a small amount of dissolved ions so it conducts at the beginning. The graphite electrodes start to corrode fairly quickly and leave black specks in the solution (which could be easily filtered out). Platinum would certainly be the best, but I doubt you need to go that extreme. I don't know enough about electrochemistry to comment beyond that though.

[Edited on 12-13-2012 by MrHomeScientist]

elementcollector1 - 13-12-2012 at 11:14

I believe the NaOH is produced at the cathode, actually. And yes, this experiment works.
You can use pencil anodes (literally a pencil that's been sharpened on both ends) and a metal cathode (preferably something non-amphoteric, such as nickel or to some extent copper).
MrHomeScientist, did you happen to notice the huge voltage and current drop that's supposed to happen with these flowerpot setups? I'm thinking of producing my own NaOH and KOH this way, and want to know how energy and cost-efficient this is (an ironic proposition for the home chemist).

MrHomeScientist - 13-12-2012 at 11:32

You're right it is the cathode, my mistake. Damn terminology!

Now that you mentioned it I actually used a steel cathode (- electrode) and a graphite anode (+ electrode) in my setup. I ran it for 24 hours and the caulk had started to degrade too, so for a more permanent setup a better method of filling the flower pot hole would need to be found.

Since this was just a proof of concept run, I didn't measure voltage or current. I could set it back up and take a look this weekend if I get some time. I was trying it out because my neighbor wanted to make her own lye for soap making, but since she has small children and dogs I didn't recommend she try this because of the chlorine offgasing. I wondered if I could replace the anode with steel to make iron chloride and hopefully reduce fumes. You'd need to replace the anode often, of course. She didn't care about the acid half of the reaction so the contamination wouldn't be an issue.

chucknorris - 13-12-2012 at 11:40

Could platinum coated materials be used as anode&cathode materials? Im looking for process that could generate *enough* HCl + NaOH in single run, for ex. using 100 liter combined anode-cathode space. Also Im planning of using at least 3kW of power imput to achieve sufficient production scale, because it cannot be ran for weeks non-stop.

I discovered that the claypot setup can also be made with mercury at the bottom, but mercury is quite expensive. But what makes the claypot setup work? How does it works as membrane? The Na atoms must dissolve through it into the cathode section with pure water.

elementcollector1 - 13-12-2012 at 11:45

Of course, if you swap the water and saltwater's spots in the setup, you can get a mix of pure HCl and HOCl, which can be pretty powerful.

I once used an old steel bar (it had previously been used for metalworking, so it was coated in oil as well as impregnated with it) as cathode, and to my surprise, it rusted orange ferric hydroxide. Not sure what happened there...

I was hoping to one day build a larger-scale setup (for which flowerpots would be ideal), but I have a feeling I'd have to either suck it up and get a carbon welding electrode or just start taping pencil leads together to form a massive anode.

chucknorris - 13-12-2012 at 12:30

Nope, my intention is to flow the Cl2 and H2 gases in common pipe and chamber to react them with UV light source to form HCl and then bubble them in cold water to make 37% HCl solution. Apart from this, I want to collect the NaOH, not because I cannot get it elsewhere(4eur per kg for me otc), but its against my principle to waste useful products that are produced in equimolar amounts compared to the principal products. Secondarily, bubbling chlorine gas to NaOH yields hypochlorite, which is as well needed in big quantities in my stock.

elementcollector1 - 13-12-2012 at 12:50

You know, that reaction is explosive, dangerously so. Glass would not be an ideal container for this.It'd be awesome if we could invent transparent steel to withstand this, but maybe put a UV LED (if such a thing exists) in there to catalyze the reaction. That would take some engineering, though...

chucknorris - 13-12-2012 at 13:03

Yep its quite exothermic, but I wouldn't say *explosive*. Steady flow of both gases into the reactor should generate quite a nice flame, and wether the reactor is cooled, it shouldn't cause that big trouble.

And if it did, the chemist was prepared for catastrophic failure and then he just simply changes the reactor vessel to material that can withstand the tremedous reaction forces. :D

elementcollector1 - 13-12-2012 at 18:29

Quote: Originally posted by chucknorris  
And if it did, the chemist was prepared for catastrophic failure and then he just simply changes the reactor vessel to material that can withstand the tremedous reaction forces. :D

... You're funny!
What color is the flame?

chucknorris - 14-12-2012 at 00:35

I suppose it looks like this:

http://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Hyd...

Waffles SS - 14-12-2012 at 02:46

Animations of Chloroalkali process
(for watching animations click on pictures)









[Edited on 14-12-2012 by Waffles SS]

watson.fawkes - 14-12-2012 at 08:07

Quote: Originally posted by Waffles SS  
Animations of Chloroalkali process
These are very good animations, excellent examples of visualization of the process flow. Even if you aren't interested in this process as such, it's worth watching the animations to see how they represent the various material flows.

vmelkon - 15-12-2012 at 14:46

Here a full video of a company that does chloralkali.
They have all 3 types of cells.
http://www.youtube.com/watch?v=GtxiCZrLH8g

chucknorris - 16-12-2012 at 13:43

Oh great thanks :)

nannah - 30-12-2014 at 05:22

How do you hook up the Hg electrode? I mean, how do you usually avoid to electrify the solution, you couldnt have used clips, right? :/
I just cant figure out how its done. :)

Ps. I am thinking about stoppering my flower pot instead of coulking it. So i wonder, what material stopper are good non reactive?

Im sorry if my questions seems stupid, but i need to ask. :)

macckone - 30-12-2014 at 09:25

Couple of notes. The clay pot method is a diaphram process not a membrane
process. Correspondingly, the liquid level on the sodium hydroxide side should
be lower than the chlorine side to prevent bleach and chlorate formation.

The water is intended to have electricity flowing through it.

Plug materials for clay pots can be any non-conductive material that withstands
sodium hydroxide and chlorine. A good choice is PVC as it is readily available.
Polypropylene also works.
Silicone caulks are horrible for this process as they are attacked by sodium hydroxide and chlorine.
Some HEPA vacuum bags are made of polypropylene.
They make good diaphram materials when layered.

Types of anode materials: Carbon (graphite or glassy carbon), Titanium, Platinum.
Cathode materials: steel or titanium (or any number of other things not attacked by sodium hydroxide or subject to hydrogen embrittlement)

nannah - 30-12-2014 at 09:58

I haven't done any electrochemical reactions yet, but i have made colloidal silver in the past, and when doing that it is important that your alligator clips doesnt touch the water. Just the silver.

I have figured that the same thing applies when you are doing different electrochemical reactions too, or am i wrong there?

I.e if i would want to make some colloidal mercury, (i know, i know, not a good idea) how would you "clip" the Hg cathode?

I dont have much knowledge of the chloralkali process, but it sounds interestering. I am going to try and make some Br from NaBr as soon as when i have all "ingredients". :)
Anyway, i have thought much about the best way to seal the bottom of a flower pot, and i have come to the conclusion that a stopper thats made out of a non reactive material is probably best.
I will check and see if i can get some PP or PVC stoppers around here.

Thanks m8. :)






macckone - 30-12-2014 at 13:16

You want whatever is going into the liquid to be a compatible electrode.
If your alligator clip is steel it is fine with sodium hydroxide but not with chlorine.

In the mercury case the connecting wire is usually steel and connected through
the bottom of the mercury chamber.
Alternatively the mercury may not be actually charged directly but
the electrode placed in the hydroxide chamber and the liquid carrying current
to the mercury separator which acts as an ion permeable membrane.
However the mercury needs to be circulated to prevent solidification as
solid mercury amalgam.

vmelkon - 31-12-2014 at 12:49

Industrially, the connection to the mercury is done through the bottom of the tank.

What I did in a small test a few years ago was to stick a graphite pencil into the mercury. Cut the top of the pencil so that you can put your aligator clip there. The graphite doesn't react with Hg but I noticed that a micro-droplet was stuck to it when I took it out. No big deal.
Anyway, it works.
These small spots would form on the mercury which would fizz around. This must have been the K-Hg amalgam (I was electrolysing a KCl).

If I wanted, I would have melted polyethylene onto a iron rod and stuck that into the Hg. I suppose the advantage there is that iron is a better conductor compared to graphite.

I think it was taking about 5 min for the amalgam spots to form. Once I stopped the electricity, the fizzing would be clearly visible.

nannah - 31-12-2014 at 13:29

If i understand it correctly, could you possibly amalgamate graphite with Hg, and use the rod as an electrode in a electrolysis? :)

From what i have read, it seem like if you were to do a electro reduction, Hg is the way to go. But which metal are the next best/next best for oxidation and reduction?
I havent gotten to the potentials yet, i know i should figure that out by myself, but i just want to hear what you have to say.

Metacelsus - 31-12-2014 at 15:47

Carbon won't amalgamate with mercury.

macckone - 31-12-2014 at 18:55

Steel is the other conductor that doesn't amalgamate.


[Edited on 1-1-2015 by macckone]

S.C. Wack - 31-12-2014 at 20:35

Just a little bit of
certain industrial cells.
a small mercury cell, as simple as it gets.
rubber, secret of chlorine cell construction.
"Above 5 per cent of H2 in Cl2 may explode, so this condition must be avoided."

Attachment: jce_116_1953.pdf (5.3MB)
This file has been downloaded 666 times

blogfast25 - 1-1-2015 at 08:43

Very interesting document. 5 % hydrogen max? That's really low. 'Helped' by moisture, no doubt.

forgottenpassword - 1-1-2015 at 10:33

Interesting indeed. I didn't know that sodium amalgam could be made by electrolyzing aqueous NaCl! I may never buy sodium again...

Magpie - 1-1-2015 at 10:40

I was thinking same. I wonder if the mercury could be removed from the amalgam by distillation?

forgottenpassword - 1-1-2015 at 10:48

I presume so, but using the amalgam "as is" seems equally viable.

blogfast25 - 2-1-2015 at 04:52

Quote: Originally posted by Magpie  
I was thinking same. I wonder if the mercury could be removed from the amalgam by distillation?


Yes, of course. But here the object is simply to recycle both the mercury AND the Na (as NaOH), not to prepare Na metal.