Sciencemadness Discussion Board - Sulfuric acid from gypsum using diaphragm cell

Sulfuric acid from gypsum using diaphragm cell

mysteriusbhoice - 9-9-2018 at 02:35

https://www.youtube.com/watch?v=8pfKbRt2Zus
in tis video I have sucessfully converted CaSO4 into H2SO4 using electrochemical means.

j_sum1 - 9-9-2018 at 04:15

Interesting. I have a few questions. I am not sure I fully understand the process you have going here.

Firstly, I am surprised that paper is up to the task. I would have expected mechanical failure.

You have a pink solution at the end which you attribute to permanganate (which is quite a feat by itself.) Where did the manganese come from?

What are the reactions happening at cathode and anode? I am not clear on what the reduction and oxidation reactions are.

At the end you have a mix of Na2SO4 and H2SO4 in water. What is your separation method at this point?

How long did you let it run for and what concentration did you get?

mysteriusbhoice - 9-9-2018 at 08:04

the manganese could have came from the natural desert gypsum or more likely impure lead or steel nails as electrodes!!
ok this is the reaction
in the anode the SO4 ions are attracted to it and combine with water to form H2SO4 and O2 is liberated
and in the cathode Na ions are attracted to it and it also combines with water to form NAOH and H2 gas

Na2SO4 --> Na+ + SO4-2
2Na+ + 2H2O --> 2NaOH + H2
2SO4-2 + 2H2O --> 2H2SO4 + O2

2NaOH + CaSO4 --> Na2SO4 + Ca(OH)2

side reactions:
NaOH + H2SO4 --> NaHSO4 + H2O

then the NaOH that accumulates in the cathode end reacts with the CaSO4 to convert it into Ca(OH)2 and Na2SO4
which starts the reaction again.

towards the end you have a mix of Ca(OH)2 which will literally harden up and a small amount of Na2SO4 and side product NaHSO4 which is the partially neutralized salt.
Their solubility in solution drops low temperatures so freezing it will cause them to crystalize out!

Then you could just distill it off at that point, as for the concentration, my home setup is so crude and I dont really have proper measuring equipment for that since my scale broke so the best I could do is pretty much test the concentration crudely by seeing how many drops it takes to convert 1ml of my chromate solution into dichromate but the problem is that the solution of chromate I have has a bit of hydroxide mixed in it and unknown conc so until i actually dry and measure out a given quantity I dont really have a way to gauge it.

Heat is also important in the reaction between NaOH and CaSO4 hence why i used 12 volts instead of something reasonable like 3.3v or 5v

overall it would be best to have multiple cells one where the gypsum is located and the feed solution of NaOH from the cell is fed through it and instead of using paper I use a proper AEM membrane on one side and CEM membrane on the other side so the NaOH once passed through the barrier cannot go back and ruin the acid compartment, which should solve the chloride salt contamination but that wouldnt really be amateur anymore.

[Edited on 9-9-2018 by mysteriusbhoice]

mysteriusbhoice - 9-9-2018 at 08:16

One other problem I encountered was chloride contamination which hurt when concentrating the acid because the sulfuric acid reacted with the chloride in solution while boiling and produced HCl and that produced horrible fumes of both Cl2 and HCl while boiling/distilling!

My challenge was to use natural gypsum but if you want pure sulfuric acid with no contamination you can buy plaster of paris and sodium sulfate from a food additive store or even sodium bisulfate can be used in place of it.

Swinfi2 - 11-9-2018 at 02:44

Testing this as gypsum is pretty easy to aquire and H2SO4 isn't (for me). Damn i hope my electrodes are the right way round but it's fizzing so fingers crossed.

[edit] *facepalm* yup electrodes are the right polarity now, and the cell liquours show green and red on pH paper (in the wrong places but that will be fixed by more electricity.)

Will get back with more info when i find out.

Pot was charged with NaHSO4 + CaSO4.1H2O
Outer jar charged with NaHSO4 as electrolyte.
Nothing measured.

[Edited on 11-9-2018 by Swinfi2]

[Edited on 11-9-2018 by Swinfi2]

mysteriusbhoice - 11-9-2018 at 11:33

Quote: Originally posted by Swinfi2

Testing this as gypsum is pretty easy to aquire and H2SO4 isn't (for me). Damn i hope my electrodes are the right way round but it's fizzing so fingers crossed.

[edit] *facepalm* yup electrodes are the right polarity now, and the cell liquours show green and red on pH paper (in the wrong places but that will be fixed by more electricity.)

Will get back with more info when i find out.

Pot was charged with NaHSO4 + CaSO4.1H2O
Outer jar charged with NaHSO4 as electrolyte.
Nothing measured.

[Edited on 11-9-2018 by Swinfi2]

[Edited on 11-9-2018 by Swinfi2]

Nice work!!
Im now trying another commonly purchased sulfate called potassium aluminum sulfate
but the basic potassium aluminate is highly soluble and paper is not an option so this time im creating an Ion exchange membrane out of PVA glue soaked paper and Borax solution.

I tried to find flower pots but no luck because they were all glazed.
Btw if you use graphite as anodes then it will erode on high conc but atleast its lead free!
meanwhile using GSLD or straight up lead is fine too because i can recycle the PbO2 from my runs and convert it into lead acetate to make PbO2 plated graphite electrodes

btw what voltage is your cell at?

[Edited on 11-9-2018 by mysteriusbhoice]

Swinfi2 - 11-9-2018 at 12:04

Just at 5V using my phone charger and a dead USB cable.

Yeah i've yet to disassemble a lead acid bat, and the electrodes seem to be holding up well enough. I'll stop when they start getting damaged. I got lucky with my pot find, all the other unglazed pots i could find had large holes in them. unfortunately this is my first time using the pot so it's leaching out a lot but proof of concept and testing at lower V is all I was looking for. The gypsum keeps going "crusty" which I assume to mean the metathesis working, I'm keeping it stirred to avoid plaster casting the bottom of the cell. I think the initial plaster hydration should well have completed by now.

To be fair I was expecting crusty Ca(OH)2 on my electrodes but the extra Na+ really does the trick. I think I'll be running this one a good few times to regenerate my acid reserves, thanks for the method

mysteriusbhoice - 11-9-2018 at 23:55

when you get the formation of Ca(OH)2 it will be a starchy thick soup consistency like cream if you used plaster.
if this gets exposed to the CO2 in the air it forms CaCO3 which is why its a binder in concrete.

Ca(OH)2 forms when your middle cell becomes very alkaline and the reaction is slow at room temp taking 3 days to fully react but at higher temps it can take mere minutes assuming you have enough NaOH formed at high enough conc.

to seperate the NaHSO4 from the acid during the end of the run all I do is put it in the freezer and let the sulfates and bisulfates drop out of solution

Jackson - 12-9-2018 at 09:30

Can a non glazed clay pot be used instead of the paper? This seems like it can be pretty useful. Do you know how concentrated it is?

macckone - 12-9-2018 at 14:05

This is an interesting exercise. It is probably wasteful of energy but for many people who can get gypsum (everyone on the planet) but not sulfuric acid (a lot of people), it is great.

The best method I found for separating salt from acid is concentrating to 70-85% and vacuum filtering (apparently a pain in the ass) after cooling. The only way to remove the remaining salt is distillation which requires very high temps and preferably quartz or ceramic retorts. Not that you can't use borosilicate, but you may melt the borosilicate.

mysteriusbhoice - 12-9-2018 at 19:30

Quote: Originally posted by Jackson

Can a non glazed clay pot be used instead of the paper? This seems like it can be pretty useful. Do you know how concentrated it is?

a non glazed pot actually will work wonderfully but sadly I could not obtain it because every flower pot I came across was glazed but I do plan on making an Ion exchange membrane using PVA and borax (silly putty) coated on paper.

about the conc I did boil it down to 25% of its volume and then it suddenly produced sulfur like smell and that is when i stopped it.

Quote: Originally posted by macckone

This is an interesting exercise. It is probably wasteful of energy but for many people who can get gypsum (everyone on the planet) but not sulfuric acid (a lot of people), it is great.

The best method I found for separating salt from acid is concentrating to 70-85% and vacuum filtering (apparently a pain in the ass) after cooling. The only way to remove the remaining salt is distillation which requires very high temps and preferably quartz or ceramic retorts. Not that you can't use borosilicate, but you may melt the borosilicate.

It is pretty wastefull of energy because the high overvolt heats up the center cell but that makes the all in one step process a thing.

If i wanted to run this efficiently I would first convert my calcium sulfate into sodium sulfate by reacting it with NaOH on the stove or something.

use that as the electrolyte and use an IEM or a flower pot as the barrier.

run the cell at 5v instead of 12v.

then the NaOH produced in the cathode area gets stored so it can react with more CaOH in future batches.

[Edited on 13-9-2018 by mysteriusbhoice]

Jackson - 13-9-2018 at 10:12

I have a flowerpot that I’m going to try this on. I think I will have to plug up the bottom. Should I use cotton Or plug it with something like silicone caulk?

Swinfi2 - 13-9-2018 at 16:08

Probably the caulk, if it survives it will do a better job. Cotton will probably contaminate less but also be way less effective as a barrier to mixing. Good luck and just know some pots are full of organic residues.

At this point my acid is near black/brown/red from graphene and organic breakdown. I think this should stop once everything soluble has been pushed out by the current.

Next step is boil it down and freeze it out. I'll edit with an estimate of my production rate later.

Jackson - 14-9-2018 at 11:11

Would dumping some H2O2 onto the flower pot destroy the organic contamination?

[Edited on 9/14/2018 by Jackson]

macckone - 14-9-2018 at 12:02

Boiling it down to concentrated sulfuric acid and then adding peroxide definitely would destroy most organic contaminants. peroxide by itself doesn't destroy as many organic compounds as you would think.

Swinfi2 - 17-9-2018 at 18:44

Tbh I don't think it's worth the peroxide. If you need pure acid I'd probably distill it after making enough (but that's pretty dangerous). A lot of synths don't need super pure acid.

I had some other impure acid from a failed ether synthesis that was full of char so i filtered both together and attempted to freeze out the sulphate salts. (No crystals) and boiled it down to about 70% (from density of 1.62gcm) its black but clean enough to use for stuff like HCl generator and moisture traps.

Started the cell up again today with saturated bisulphate in both chambers and a good heap of plaster. But my electrodes are stripping like crazy now :/ anyone have any idea what I could use instead of graphite or plate them with? (The cathode is fine its the anode that strips)

[Edited on 18-9-2018 by Swinfi2]

mysteriusbhoice - 17-9-2018 at 20:11

Quote: Originally posted by Swinfi2

Tbh I don't think it's worth the peroxide. If you need pure acid I'd probably distill it after making enough (but that's pretty dangerous). A lot of synths don't need super pure acid.

I had some other impure acid from a failed ether synthesis that was full of char so i filtered both together and attempted to freeze out the sulphate salts. (No crystals) and boiled it down to about 70% (from density of 1.62gcm) its black but clean enough to use for stuff like HCl generator and moisture traps.

Started the cell up again today with saturated bisulphate in both chambers and a good heap of plaster. But my electrodes are stripping like crazy now :/ anyone have any idea what I could use instead of graphite or plate them with? (The cathode is fine its the anode that strips)

[Edited on 18-9-2018 by Swinfi2]

Use a lead anode just melt some fishing weights
That is what I used in the video because as long as the cell is charged it will coat in lead dioxide and won't corrode but you will have to deal with lead waste.

Fulmen - 7-10-2018 at 01:26

Quote: Originally posted by mysteriusbhoice

I do plan on making an Ion exchange membrane using PVA and borax (silly putty) coated on paper

Do you have any references or additional info on this?

If it's possible to run this process on Na2SO4 one could get almost full yield from NaHSO4. First decompose it to Na2SO4 and SO3, then process the remainder to sulfuric acid.

mysteriusbhoice - 10-10-2018 at 10:49

Quote: Originally posted by Fulmen

Quote: Originally posted by mysteriusbhoice

I do plan on making an Ion exchange membrane using PVA and borax (silly putty) coated on paper

well there is a post on how to make IEM using polyvinyl alcohol and glutaraldehyde cross linking which creates a hydrophilic polymer
and i also found this below linking a few papers on that matter
https://www.science.gov/topicpages/c/crosslinked+pva+membran...

however since all i got is poly vinyl acetate, it can cross link using something else called sodium borate which also creates a hydrophylic polymer, although i haven't tested that in a running cell I have made membranes out of it and tested if they degrade when i let them dry out etc and surprisingly they dont.

I also tested them against sulfuric acid/hydrchloric acid and they didnt break down but for some reason when soap/detergent is present they immediately break down
the polymer seems to also be resistant in basic conditions from mixing it with saturated NaOH solution.

My process for that is that I first apply the glue onto tissue paper or cotton fabric then I soak it in a sodium borate bath to cross link and its done!

Fulmen - 10-10-2018 at 11:34

Very interesting read. I had a similar idea for a plastic mesh supported membrane by spraying the PVA with borax solution. I think I will have to do some experiments as soon as I find the boric acid.

macckone - 10-10-2018 at 14:39

It is sold in stores as 20 mule team borax or other laundry booster.
Looking at the labels on various washing powders.
Some places in europe apparently ban boron compounds because of their toxicity to insects and small humans.
Sometimes it is also available as a pure substance for killing insects.

Fulmen - 11-10-2018 at 08:06

I found it (or at least some heat treating paste containing boric acid , I should have pure boric acid *somewhere* as well). I ended up using a disposable viscose rag as substrate, appr. 2mm thick. Saturated with PVA-glue and dipped in boric acid it gelled up instantly, so it looks promising.

Seems like NASA has experimented with similar membranes (not borate cross-linking though): https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197900...

Fulmen - 11-10-2018 at 11:22

Worked like a charm. I made a couple of different membranes, using different amount of PVA (by diluting with water). I'm now drying them to get the dry mass and dimensions. After all, a membrane not capable of drying out isn't really that practical.
And while it might increase electrical resistance, a dense fibrous support should act like a diaphragm for any water diffusing through? That would only improve performance, right?

Also noticed this: https://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra1...

Quote:

PVA with varying concentrations (1, 2, 4 and 6%) of glutaraldehyde has resulted in a respective 2.8, 5.6, 32 and 34% of degree of cross-linking in PVA1, PVA2, PVA3 and PVA4 membranes. Due to the reduction of available free volume in membranes, progressive improvements in membrane rigidity with impeded membrane porosity were observed with increasing cross-link densities. In succession, proton conductivities of 7.53 × 10−3, 8.4 × 10−4, 1.2 × 10−4 and 4.5 × 10−5 S cm−1 were observed from the respective PVA1, PVA2, PVA3, and PVA4 membranes. The increased cross-link density enhanced the mechanical strength but reduced other membrane properties such as water uptake and proton conductivities of the membranes.

The action of borates seems to be too fast for mixing, dipping in a borate solution seems like simplest approach. Trying out different concentrations might be worth some investigation.

Next step will be to construct a cell prototype, shouldn't be too hard using SAN "acrylic". I'll see what I can cook up during the weekend.

macckone - 11-10-2018 at 11:50

Excellent work. Usually the pva and borate are diluted and mixed so the membrane forms as it dries. It is supposedly a 'workable' gel for a couple of minutes before final drying.

What dilution are you using when mixing?
A lot of the papers don't give the amount of water used to dissolve the stuff but I have seen some that are starting with 4%.

Fulmen - 11-10-2018 at 21:26

I didn't measure, it was glue I had previously diluted for paper tube rolling. Perhaps 10%, just enough to make it liquid. I'll backtrack the percentages from the dry weight if it works.

Fulmen - 12-10-2018 at 08:40

OK, I now have 4 samples ranging from 60 to 100mg/cm^2 of PVA/borate. It's probably far thicker than it needs to be, so I'll prepare a couple of tests using wet-strong paper towel.

Jackson - 12-10-2018 at 08:41

Is it PVA glue?

Is the membrane better or worse than a clay flowerpot?

[Edited on 10/12/2018 by Jackson]

madcedar - 12-10-2018 at 15:12

Yes it's PVA glue that he is talking about, the white stuff everyone uses at home. It can be set with a 4% solution of borax in water.

mysteriusbhoice - 16-10-2018 at 00:02

From production of sulfuric acid to creating homemade IEM one thing does lead to another after all

mysteriusbhoice - 20-11-2018 at 09:59

Guys update to this project:
if you add a 3rd small compartment where the cathode goes with holes at the bottom where the alkaline liquid will collect then you can discard it every 12 hours after which you keep adding more Na2SO4 into it to continiously drop the PH on the anode side until you have a fairly concentrated solution of H2SO4.
and to get Na2SO4 you could start with CaSO4 and react it with NaOH on the stove and the liquid obtained can be used as the new electrolyte but keeping the CaSO4 slurry barrier around the paper will help improve collection.

also I am currently producing HCl using this method by adding only some sulfate ions in the initial run and instead of filling Na2SO4 into the cathode compartment after flusing out the NaOH produced I fill it with NaCl which reacts with the H2SO4 on the anode side to produce HCl.

Where I live even HCl is restricted so I have to come up with these crazy solutions to produce them.

MrHomeScientist - 21-11-2018 at 12:17

I'm quite surprised this works. How is sulfuric acid made, when calcium sulfate is so insoluble? It seems unlikely that CaSO₄ would be attacked by aqueous NaOH. Are you sure you aren't just electrolyzing the sodium sulfate you started with?
Is it required that you start with hydroxide solution in the inner container and acid in the outer container?
If this does indeed make sulfuric acid straight from gypsum, that's very cool and tremendously useful since gypsum is so common.

Swinfi2 - 21-11-2018 at 12:52

When i did this with CaSO4 i did notice the solution started to cloud where it came into contact with the air but it wasn't so easy to tell if it had gone to completion as one whiteish powder looks a lot like another. I switched over to converting MgSO4 into Na2SO4 with NaOH in a seperate step.

mysteriusbhoice - 21-11-2018 at 21:57

Quote: Originally posted by MrHomeScientist

I'm quite surprised this works. How is sulfuric acid made, when calcium sulfate is so insoluble? It seems unlikely that CaSO₄ would be attacked by aqueous NaOH. Are you sure you aren't just electrolyzing the sodium sulfate you started with?
Is it required that you start with hydroxide solution in the inner container and acid in the outer container?
If this does indeed make sulfuric acid straight from gypsum, that's very cool and tremendously useful since gypsum is so common.

At high temperature it reacts with the NaOH accumulating in the cathode compartment and I made my Na2SO4 by reacting NaOH in a cooking pot with gypsum so the NaOH in the cell attacks more CaSO4 eventually producing Ca(OH)2
This video below shows that you can make Na2SO4 from a hot solution of NaOH
https://m.youtube.com/watch?v=_R9atKBUS8M

mysteriusbhoice - 21-11-2018 at 22:02

Quote: Originally posted by Swinfi2

When i did this with CaSO4 i did notice the solution started to cloud where it came into contact with the air but it wasn't so easy to tell if it had gone to completion as one whiteish powder looks a lot like another. I switched over to converting MgSO4 into Na2SO4 with NaOH in a seperate step.

I have a new cell design that is for totally liquid reactants using Na2SO4 and no CaSO4 needed.
This cell has a cathode compartment filled with liquid and uses a porous barrier made from sand and the cathode is drained every 12 hours after the NaOH accumulates then the cathode liquor is frozen to precipitate Na2SO4 and the precipitate is put into the anode compartment to be recycled.

mysteriusbhoice - 21-11-2018 at 22:08

As for making IEMs out of PVA glue it works but I cannot use paper to hold the polymer.
I will try those really usable fiber bags next