Sciencemadness Discussion Board - Reduction of KCr(SO4)2

Reduction of KCr(SO4)2

crystal grower - 2-4-2018 at 06:03

I'm trying to get elemental Chromium for my element collection.
My starting material is chrome alum.
So far I tried electrolysis with lead anode, but the results were quite poor. The deposit was greyish in color and I'm not sure how much if any elemental Cr was produced.
The problem is probably my inability to control the voltage and current precisely.
I'd appreciate any suggestion regarding chrom alum electrolysis. Is a diaphragn/membrane necessary for this electrolyte cell? Is some other Cr compound superior for electrolysis?
I'd also like to try precipitating Cr with Al from chrome alum solution, but wanted to ask here first how much doable is it if you have any experience.

[Edited on 2-4-2018 by crystal grower]

MrHomeScientist - 2-4-2018 at 06:18

I'm not sure about electrolysis, but I got my chromium sample from a chromium(III) oxide thermite with fine mesh aluminum powder. This is complete speculation, but you might be able to do something like this: Dissolve your chrome alum in water. Add barium (or calcium) chloride, filter the solids off to get a solution of potassium and chromium chlorides. Add potassium hydroxide to precipitate chromium hydroxide (not too much, or the Cr will redissolve), filter the solids. Heat the hydroxide to convert to the oxide. Mix with aluminum powder and thermite away!
Obviously I haven't worked that out in much detail, but it might give you a starting point if you want to follow that route.

I suspect chromium is too reactive to precipitate from aqueous solution but others may know better.

crystal grower - 2-4-2018 at 07:10

"I suspect chromium is too reactive to precipitate from aqueous solution but others may know better." That's most likely true.
Thermite seems like a good option. I'll give it a try. Thanks for answer.
By the way, is there anything you'd like to notify me about Cr thermite before attempting it? Like common issue etc.

[Edited on 2-4-2018 by crystal grower]

MrHomeScientist - 2-4-2018 at 08:25

I had a lot of difficulty igniting it. My Cr₂O₃ was bought from United Nuclear, and was a very clumpy powder. I had to use a very fine aluminum powder to get it to work, 425 mesh IIRC. It was also hard to mix it up uniformly enough to keep the reaction going. You might not have those problems if your oxide is a free-flowing powder, though. It burns pretty sedately, with very bright light but not a lot of sparks.

Good luck! Let us know how it goes. I'm particularly interested if my method from alum to oxide works at all

crystal grower - 2-4-2018 at 09:59

Thanks, I'll let you know how it went.
I dont think there will be any problem regarding the precipitation (even though most of the time problems arise when you dont expect it

).
I'll just have to make some CaCl2, limestone + HCl should be fine.

MrHomeScientist - 2-4-2018 at 11:05

Damp Rid is a convenient source for anhydrous CaCl₂. It's a product meant for removing moisture from basements, closets, etc. I live in Florida where humidity is always 400% so this stuff is everywhere! I use it in my desiccator cabinet.

DraconicAcid - 2-4-2018 at 11:51

Strong reducing agents in aqueous solution will only take Cr(III) to Cr(II), IIRC.

AJKOER - 3-4-2018 at 06:38

Perhaps not CaCl2, but Calcium acetate (from vinegar acting on Ca(OH)2 or CaCO3, from say CaCl2 + washing soda, Na2CO3).

Try roasting the K and Cr acetate to obtain oxides (much easier said that done in my experience).

Less heating, dissolve the acetates in dry methanol and apply UV light to generate radicals (like .CH3 and .OH) to breakdown the acetate, and allow volatile gases to escape (fume hood or outdoors). Or, dissolve in water with added N2O and apply UV light to generate radicals (like .OH) to breakdown the acetate. Then, thermally decompose the evaporated solution. If the reduced product is Potassium formate, it decomposes above 167 C. For some helpful technical details on the photolysis of acetates, see preview at https://link.springer.com/article/10.1007%2Fs003390050595, also product formation of metal oxide at http://iopscience.iop.org/article/10.1088/0957-4484/26/26/26... and https://www.researchgate.net/publication/231407820_Photodeco... .

Note, Potassium acetate in CH3OH: 24.24 g/100 g (15 °C) and 53.54 g/100 g (73.4 °C).

[Edited on 3-4-2018 by AJKOER]

crystal grower - 3-4-2018 at 08:09

Thanks for the link AJKOER. I'll take a look at that.
Meanwhile, the method Mrhomescientist suggested seems to be working fine. Currently I'm drying the cr hydroxide and I also tried to "small-scale" heat it to get the green Cr2O3. That doesn't seem to be a problem either.
What I'm most concerned about is the thermite itself. I hope I'll be able to fire it off and recover at least some Cr metal.

crystal grower - 3-4-2018 at 08:11

By the way, what do you think about adding a bit of KNO3 to boost the reaction a notch?

MrHomeScientist - 3-4-2018 at 12:19

I only have limited experience with thermite heat boosters, but KNO₃ is supposed to work. Probably couldn't hurt. I'm not sure what the ratios would be though. I miss blogfast, he's the thermite expert

The one I have used is calcium sulfate, which, coincidentally, you produced as a byproduct during your chrome alum conversion! It's reaction with extra aluminum in the mix produces a lot of heat, and produces aluminum sulfide and possibly some sulfur. The sulfide reacts with moisture to form H₂S, so be careful with the slag. I used plaster of Paris in the past, but mine was heavily contaminated with calcium carbonate. Washing this with sulfuric acid until the bubbling stops fixes that problem.

Glad to hear you're making progress!

Foeskes - 4-4-2018 at 05:27

I got something like 50% yield from potassium dichromate boosted chromium oxide thermite. Be careful though don't use too much potassium dichromate.

crystal grower - 4-4-2018 at 05:59

What's the role of K2Cr2O7 in this reaction?
Sorry if it's a dumb question.

Foeskes - 4-4-2018 at 17:30

It reacts with Aluminium producing a lot of heat, since Aluminium's reaction with Cr2O3 is usually not energetic enough to be continuous or sometimes fails to melt chromium(something like 1907°C)

crystal grower - 4-4-2018 at 20:36

Ok, thanks for clarification.

MrHomeScientist - 5-4-2018 at 06:09

Dichromate is a great idea for a chromium thermite heat booster! Do you know the reaction equation with aluminum? I couldn't find it, but admittedly I didn't have time to look for very long.

In general, heat boosters are oxidizers that react with excess aluminum to provide extra heat. Depending on the products in this case, dichromate might end up making more chromium and increase your yield!

crystal grower - 5-4-2018 at 09:21

This is the reaction:
K2Cr2O7 + 4Al → 2Cr + 2KAlO2 + Al2O3
It should be initiated at 800-900°C which is a significant difference compared with Cr2O3, which I believe, needs about 1300°C.
That could like really helpful

. Thanks for this idea.

crystal grower - 6-4-2018 at 11:51

I've just performed the reaction.
Suprisingly I was able to initiate the reaction quite easily using a sparkler and a little of S + Al.
The problem is the heat wasn't enough to melt the chromium properly, so it will be quite a job to separate some of it. I'll probably try it again, using more Cr2O3 and also more K2Cr2O7 when I get some on hand.
Still I'm quite happy as this was my first go at thermite type reaction and I was able to fire it off with a goddamn sparkler

. Thanks for all the info, I'll let you know when I try it again.

MrHomeScientist - 9-4-2018 at 12:33

Awesome! Was your sulfur mixed throughout as a heat booster, or did you just use it in a pile on top as an igniter? How large was your total thermite? A larger charge might help keep the heat up. I'm also interested to see how the dichromate performs.

Glad the sparkler worked too; I've heard that being used but haven't tried it myself. That makes for a nice fuse to give you time to get away, too.

Also, pics!

Foeskes - 9-4-2018 at 22:00

I found my old thread on the procedure: https://www.sciencemadness.org/whisper/viewthread.php?tid=77...
The amount of booster you need probably depend on the size of the particle.
The larger the Aluminium the more you need. Again be careful with Cr(VI), you can probably use other oxidizers like KClO3 or KClO4
I can't really do thermites for a while, my Mg ribbon went missing.

[Edited on 10-4-2018 by Foeskes]

crystal grower - 10-4-2018 at 05:07

Quote: Originally posted by MrHomeScientist

Awesome! Was your sulfur mixed throughout as a heat booster, or did you just use it in a pile on top as an igniter? How large was your total thermite? A larger charge might help keep the heat up. I'm also interested to see how the dichromate performs.

Glad the sparkler worked too; I've heard that being used but haven't tried it myself. That makes for a nice fuse to give you time to get away, too.

Also, pics!

I used the sulfur just as an igniter, like you said. Maybe it would be a good idea to mix it with the thermite.
The whole mixture was only about 80g, so the total heat probably wasn't anything great.
The sparkler is indeed a safe way to ignite it.
I don't think pics of the results would be very interesting, it's just a big mess really. But I may try.

Also one more question, I've read that pottery grade Cr2O3 is often calcinated and won't Dissolve in most acids. Is it true for thermite too, or will it ignite just fine?

MrHomeScientist - 10-4-2018 at 05:35

Not sure about the pottery stuff; I've never had access to a good pottery supplier. My intuition says it won't matter because, after all, you're heating the powder to high temperature yourself anyway.

In Foeskes' thread, his total charge was only 43g and he got a nice single lump of metal, using dichromate as a heat booster. Since that's half the size of yours, I think that's further evidence that you need more heat!

Foeskes - 10-4-2018 at 06:19

What I learnt is that for my coarse Al powder I get better chance of complete reaction if I spread the Al/S charge all over the thermite. One annoying thing about this mix is the H2S produced.
I finally found out why my newer thermites don't work, they added CaCO3 to the oxide, so not only my measurements are off but it also takes up some of the heat that is required to ignite. I wasted a entire container of Al.

[Edited on 11-4-2018 by Foeskes]

crystal grower - 10-4-2018 at 08:22

Quote: Originally posted by Foeskes

What I learnt is that for my coarse Al powder I get better chance of complete reaction if I spread the Al/S charge all over the thermite. One annoying thing about this mix is the H2S produced.
I finally found out why my newer thermites don't work, they added CaCO3 to the oxide, so not only my measurements are off but it also takes up some of the heat that is required to ignite. I wasted a entire container of Al.

[Edited on 10-4-2018 by Foeskes]

Hmm, yeah H2S is a really disgusting garbage.
Well, looks like I have to give it a second go, and try to improve my mixture.