Potassium Dichromate recovery

laroygreen · Oct 20, 2021

Hi all,

I found a commercial waste disposal facility that would take my used chemistry. Since most of the chemicals are going to end up in water (washing, transfer, etc.) I was wondering if I could easily filter (or boil?) the water so that all the chemicals are left behind in the filter (which gets disposed of properly) and the water would be safe to enter a septic tank? Taking all that water to a facility would be limiting and likely incur unnecessary costs.

Don_ih · Oct 20, 2021

After an initial rinse, almost no chemistry ends up in washing water - so that subsequent water would be safe in a septic system. You could let your developer, bleach, fix, toner, and initial rinse water evaporate. That would leave behind dry chemicals. Of course, you need space for that.
You probably don't want to boil water with potassium dichromate in it.
No filter will get chemicals out of solution.

fgorga · Oct 20, 2021

Two thoughts...

I agree with Don. After the first wash, there is so little chemistry left, that I would not worry about subsequent washes going in the septic system. I also agree, that there is no simple, cheap method for filtration.

As for evaporating water, this might work depending on your source of heat and your patience.

You will be surprised at how much energy it takes to evaporate water. Energy costs money. Thus, if you are going to use a gas or electric stove, you may find that between the hassle of processing and the cost of the energy, this is not practical.

One free source of energy is the sun. If you can arrange to put a tray (you will need a large surface area to make this practical) in a sunny spot where children and animals can't get to it, and you can leave it there for extended periods of time this might work. I would keep reusing the tray over and over until a significant amount of solid accumulates.

I have actually tried this second method for cyanotype waste, except I did not use a sunny spot. Rather, I put the tray in the loft of my garage/barn instead of in a sunny spot. (Figuring that a protected space was more important than maximizing evaporation rate.) The rate of evaporation during the height of a New England summer was maybe acceptable. The rest of the year it was very slow.

I accumulated my initial washes in five gallon buckets and refilled the evaporation tray from those buckets as the water slowly evaporated. Thus, I had to remember to keep an eye on things. This works but it is slow and requires oversight. It also helps control the mouse population, as I would regularly find mice drowned in my five gallon buckets! The rate of evaporation from my five gallon storage buckets was negligible.

Hope this is useful,

laroygreen · Oct 20, 2021

Thank you for the input. I live near the equator and I have a solar water heater system (which I rarely use) so I was thinking I could use both those as a heat source. I was just concerned that heating up water with darkroom chemicals might cause dangerous fumes or something - however given that potassium dichromate has a boiling point of 500 deg. c, would a simple distillation setup be safe?

nmp · Oct 20, 2021

Couldn't you do this:

https://www.photrio.com/forum/threads/potassium-dichromate-disposal-uk.44053/#post-629886

:Niranjan.

Don_ih · Oct 20, 2021

laroygreen said:
however given that potassium dichromate has a boiling point of 500 deg. c,

That's the boiling point of the compound. You don't want to boil water with potassium dichromate in it - you might get that solution as a vapour. I wouldn't want to inhale it.

Raghu Kuvempunagar · Oct 20, 2021

Adding a little bit of Ascorbic Acid to the first rinse water converts the dichromate to less toxic chromium compound. Just search for more information on this process. If you are going to evaporate the rinse water, it might be better to do with the less toxic compound.

fgorga · Oct 20, 2021

laroygreen said:
Thank you for the input. I live near the equator and I have a solar water heater system (which I rarely use) so I was thinking I could use both those as a heat source. I was just concerned that heating up water with darkroom chemicals might cause dangerous fumes or something - however given that potassium dichromate has a boiling point of 500 deg. c, would a simple distillation setup be safe?

You haven't specified what process you are using, so I can't say for sure about all of the components, but dichromate is not volatile. There is a minor hazard of airborne droplets is you really got the solution to a rolling boil, but that is unlikely.

As for you question about a distillation set up... this is probably over kill. You just need to evaporate off the water. To a chemist, anyway, distillation implies recondensing and collecting the vapor, usually because you want the resulting liquid. In this case, there is really no need that I can see for condensing and collecting the water.

fgorga · Oct 20, 2021

There have been a number of posts about converting hexavalent chromium to trivalent chromium.

While trivalent chromium is less toxic than hexavalent chromium, it is still toxic and must be disposed of properly, it should not be poured down the drain. Thus this suggestion does not really help the original poster.

Raghu Kuvempunagar · Oct 20, 2021

fgorga said:
There have been a number of posts about converting hexavalent chromium to trivalent chromium.

While trivalent chromium is less toxic than hexavalent chromium, it is still toxic and must be disposed of properly, it should not be poured down the drain. Thus this suggestion does not really help the original poster.

Who said it should be poured to the drain? ! As far as I understood, OP wants to evaporate dichromate rinse water and then dispose the precipitate. If he's going to do that he might want to precipitate the less toxic compound to reduce the risk posed by any accidental exposure during the process of evaporation till the subsequent safe disposal of the compound.

laroygreen · Oct 20, 2021

Thank you all. I think I have some options to try. I live on an island and some things that seem easy elsewhere is near impossible here. Just today I tried to import a 1 liter kit for R4A chemistry and I was quoted a hazmat price of $1,244.13 USD! So now I have to pay them to either dispose of it or return it to the seller . I didn't choose alt processes, alt processes chose me .

koraks · Oct 21, 2021

Don Heisz said:
You don't want to boil water with potassium dichromate in it

Yeah, I wouldn't recommend it. While in theory water vapor will be pure without any chromate in it, I can imagine that the process of boiling, especially if it is done vehemently, will result in airborne droplets which will be the original solution.

A better approach would be to precipitate out the chromium compound. Since chromates are poorly soluble salts with many positive ions, the addition of a soluble salt that forms an insoluble complex with chromates would precipitate out the chromate salt which you could then filter out and offer to a waste disposal service. I'm not a chemist and I cannot offer a good candidate for a complexing salt, but I expect something rather mundane, safe and cheap is likely to work well. Perhaps something as simple as calcium hydroxide (in itself somewhat dangerous due to its potential to create burns, but safe if handled responsibly) would do the trick, calcium being a group II alkaline metal which should form an insoluble salt with chromate.

AgX · Oct 21, 2021

laroygreen said:
I live on an island and some things that seem easy elsewhere is near impossible here. I didn't choose alt processes, alt processes chose me .

What about not introducing Dichromate from the start?

For effluent treatment see this proposal:
https://www.photrio.com/forum/threads/dichromate-waste-treatment.135482/

Maris · Oct 21, 2021

Natural surface waters already contain chromium. Sea water has a chromium content of about 5 parts per billion which works out to half a ton per cubic kilometre of water. From a chemical point of view, not a legal point of view, adding a few grams of chromium to the ocean surrounding an island could not make the slightest difference to anything living or dead.

From a legal point of view there may strict limits on the disposal of chromium. A few grams is nothing but a few tons is an environmental disaster. Basically chromium does not go away, it goes somewhere.

A nice question is what do waste disposal companies do with chromium that the small scale user cannot do? In my past capacity as a scientist tracing toxins in the environment I have encountered companies that incinerate everything that looks problematical. Sure, the chromium (and other bad stuff) "disappears" but it actually goes up the chimney as ultrafine dust that disperses and settles in the environment so thinly that the concentration at any one point is negligible. So they hope.
Other companies dispose of toxins at sea relying on the enormous dilution factor to obviate harm. So they hope.

fgorga · Oct 21, 2021

AgX said:
What about not introducing Dichromate from the start?

This, of course, is always the best approach!

fgorga · Oct 21, 2021

@Maris

Fully agree with what you say in post #15.

However, the speciation of chromium is especially important... Cr(VI) is much more of an issue than Cr(III) from a biological/environmental point of view.

laroygreen · Oct 22, 2021

Thank you all! A few things to try to see what works best for my workflow. Just want to be safe for myself and the environment. The MSDS for Potassium Dichromate is scary reading so I'll be using gloves and respirator at all times until final wash and be careful not to contaminate my workspace.

AgX · Oct 22, 2021

fgorga said:
However, the speciation of chromium is especially important... Cr(VI) is much more of an issue than Cr(III) from a biological/environmental point of view.

That is the post about I linked to above.

Potassium Dichromate recovery

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