j_sum1
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Fun and games with chromium recycling
This might end up being a rant rather than having a specific question, but here goes.
The objective
I have had sitting around a quantity of chromium waste that I have always intended to recycle. Most of it comes from cleanup of various experiments
using dichromates as oxidisers. It includes waste from alcohol oxidations, rinsing glassware, tests for primary and tertiary alcohols and all manner
of other stuff. In all cases the waste material was reduced to Cr(III) with whatever was convenient: alcohols, ascorbic acid, thiosulfate,
metabisulfite. (Not all of this mess is of my creation.) I boiled it down to reduce volume last time I moved house and so about 500mL of a dark green
sticky paste the consistency of toffee has been sitting in a beaker for several years.
The ultimate goal has been to convert it back to a relatively pure chromium (VI) compound, or possibly a range of compounds.
Obvious two step process: oxidise and then precipitate/crystallise/boil down to isolate a desired product. Obviously with a lot of filterng and
washing steps involved as well.
Problem #1 β oxidations
I managed to dissolve everything to about 4L of dark green liquid and proceded to oxidise in small batches. Available oxidisers are hydrogen
peroxide, laundry bleach, Pool grade calcium hypochlorite. I experienced issues with all of these.
Peroxide was abandoned early on since it forms a peroxo complex with Cr(VI). This is unstable and I found that in short order the mixture would
revert back to the dark green of Cr(III). I was not seeing the orange and red hues I was aiming for. Besides, peroxide is not that easy for me to
get and I consider it valuable. So not the best choice.
Calcium hypochlorite had its own issues. Firstly, it is not terribly soluble and I was reticent to increase the volume any more than I had to.
Secondly, the product, calcium chromate forms a passive coating around the granules and so the reaction would halt. Thirdly, I found I was getting
incomplete oxidation in both acid and alkali. Oxidation potential is significantly higher in acid conditions. But there was the uncomfortable
problem of Cl2 production. Dichromate and chlorine are close on the electrochemical series and I often found my reaction going backwards rather than
forwards as Cl- was oxidised to Cl2.
I found more success with sodium hypochlorite bleach than calcium hypochlorite. I still had incidence of Cl2 production and the reaction going in
reverse. But the process seemed to work better. Two additional problems arose. The first was that the bleach contained a lot of NaOH to stabilise
it, with the result that a lot of additional acid was required. The second was that the only bleach I had available contained a lot of surfactant
crap. (The most recent round of covid panic caused a run on supermarket bleach, particularly the cheap brands. Therefore I had to take what I could
get.)
Eventually I obtained several litres of dark amber liquid that contained a lot of chromium 6 and I thing only a little chromium 3.
Problem #2 β isolation
I have had a good study of the solubility curves of chromates and dichromates of various cations thgat are likely to be present. I thought initially
I would be able to add ammonium chloride and precipitate out some ammonium dichromate. I was successful with one batch and yielded a few grams of
nice orange crystals that decompose in the characteristic fashion, but it was clear that I was not going to have good recovery with this option.
Another possibility was to isolate calcium chromate as an intermediate. I quickly found out that calcium chromate dissolves quite readily in a
saturated brine solution. My mixture contains a lot of Na+ and so the calcium chromate does not precipitate.
My third approach was to target the most soluble dichromate through successive steps of boiliing down and filtering and keeping the filtrate. Sodium
dichromate is several times more soluble than any of the others. I added a quantity of sodium chloride to my mix to ensure an abundance of sodium
ions. Repeated boiling and filtering steps have reduced 3 litres of liquor down to a couple of hundred mL. (I still have a 2L flask that I am yet to
start.) I have filtered off several batches of crystals. These were pale yellow, but after stirring them with brine for a while and refiltering I
have been left with near white crystals that I have been able to discard. So, progress is being made.
But the new problem:
As the volume is progressively reduced, I am finding that the chromium is reducing back to Cr(III). The liquor is not the amber / orange it once was
but is now a deep green with yellow hue when swirled in the flask. Evidently at higher concentration and temperature, the chloride ions present are
being oxidised to Cl2 during boiling.
Feels like I am back to square 1 β a dark green concentrated mixture with multiple cations and anions present and Cr in the (III) oxidation state.
And only dodgy bleach to get it to Cr(VI).
The only way forward I see is to abandon the boiling down and go for a slow evaporation. But I suspect I will still not avoid the conversion to (III)
and I suspect that the mixture will be sufficiently deliquescent to make this impractical.
I would welcome any good suggestions for a practical way forward.
Follow up project
A number of years ago I rescued from a a discard pile at a school a container that was once CrO3. The contents are about 250g of a thick black tarry
substance.
Previous tests I have done on it indicate that it has a lot of Cr in high oxidation state, but that there is also some other crap in there. Including
something containing sulfur. (I managed to extract some granules of elemental sulfar last time I played with it.) Who knows what the history of this
stuff is. I think it should be a bit easier to work with than my current mess and I would not mind recovering something useful if at all possible.
But that is depenent on me finding a solution to the present problem.
(Postscript)
Just for fun, I know that my hardware store HCl that I have been using on this project has contamination with Ti (of all things). Cleaning my frit
with peroxide has produced a rich orange peroxo complex. I gess this means that there wil be some Ti(III) or more likely Ti(IV) in my witch's brew.
I am beginning to wonder if recycling is actually worth it. There is a lot of effort to make chems that I could simply buy. But I do like the
concept of waste avoidance and revovery of useful elements wherever possible.
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Sulaiman
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electrolysis can oxidise or reduce almost anything,
a (possibly temporary) option could be to electroplate out the Cr as metal ?
CAUTION : Hobby Chemist, not Professional or even Amateur
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Jome
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I was gonna write "perhaps you should've just precipitated everything as Cr(OH)3, then heated with KNO3?" But then I remembered old trauma, filtering
large volumes of almost colloidal solids leads to a lot of suffering, clogged filters and what not.
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j_sum1
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Quote: Originally posted by Jome  | | I was gonna write "perhaps you should've just precipitated everything as Cr(OH)3, then heated with KNO3?" But then I remembered old trauma, filtering
large volumes of almost colloidal solids leads to a lot of suffering, clogged filters and what not. |
I have enough Cr2O3 already. I also have CrO3. And I also have a decent amount of K2CrO4. What I don't have is dichromates.
My thoughts are that the waste material is such a horrible mixture that it may simply not be worth doing anything with except to render inert
(Cr(III)) and discard. My goal here has been responsible processing of waste rather than a need for chems.
I am sure that as an ore, what I have would be considered valuable and Cr recovery would be a no-brainer. Probably either by electrolysis as Sulaiman
suggested or through some high temperature smelting process. But using wet processes which is what I am doing, the way forward might just be too
problematic. I am skilled enough to identify problems but really have not been successful in solving the issues in a manner that is economical in
time or other chems.
I will sit on it for a bit longer and will read any further ideas that come through this thread. But it seems likely that I will simply reduce and
dump.
Which leaves the question of what I do with the material that once was CrO3. I think that it should dissolve fairly easily and I can convert to
ammonium dichromate with reasonable yield using ammonium sulfate. If I recover something, that wil be nice. But based on current experience I won't
be attempting to extract 100% of the Cr. I think practical compromises are called for.
This is a thread in responsible practices. Which means I am interested in practical recovery of valuable matrials and safe, environmentally
responsible disposal of waste. In this particular project I am coming to the conclusion that the balance between these two objectives might lie in a
different position from where I thought it might. But I may well have missed something obvious and I am open to ideas.
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B(a)P
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What about trying to precipitate out chromium carbonate with the use of sodium carbonate.
You can then react your chromium carbonate with hypochlorite to get fairly pure chromate.
Edit - Apologies please ignore this, I realised shortly after posting this is not going to work because of where you are starting. For this approach
to work you would need to be starting from chromium chloride and as you said you don't want to be heading back the beginning again.
[Edited on 6-4-2022 by B(a)P]
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Deathunter88
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| Quote: Originally posted by j_sum1 |
I have enough Cr2O3 already. I also have CrO3. And I also have a decent amount of K2CrO4. What I don't have is dichromates.
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FYI chromate can be easily converted to dichromate by just acidifying the solution.
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unionised
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OK, it's an expensive way to do it but...
K2CrO4 + CrO3 --> K2Cr2O7
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Lion850
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Interesting read. For your CrO3 why not try to make some Lithium dichromate. Itβs a beautiful orange color. I know you have a lot of Lithium 
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Lion850
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j_sum1 below is the link to my lithium dichromate synthesis. The color has darkened a bit since this post, it is a bit more red. You should be able to
add some of your lithium metal direct to a solution of CrO3?
http://www.sciencemadness.org/talk/viewthread.php?tid=157976...
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Fantasma4500
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by my experience CrO3 is somewhat hygroscopic and will actually melt plastic bags- not sure if it pulled the water out of LDPE plastic
it should still be CrO3- or well a mix of CrO3+H2CrO4 as it forms when it gets water
seperation is best done turning it into copper chromate- in case you suppose that there are impurities
forget about vacuum filtering this stuff, just take a big bucket and cloth and let it have a go
CuCrO4 + NaOH = Cu(OH)2 + NaCrO4
Cu(OH)2 + (heat, 70*C?) = CuO (ppt)
or you could maybe distill the H2CrO4?
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