Sciencemadness Discussion Board - Recovery of silver, mercury and chromium from acid mixture

Recovery of silver, mercury and chromium from acid mixture

Eithern - 10-11-2023 at 14:48

Hello, I'm new here

I work in a small environmental analysis laboratory and I like to do some experiments in the free time.
For the determination of COD we use a kit that contains H2SO4, silver and mercury sulfate (as catalyst and for removing chloride respectively) and potassium dichromate as oxidizing agent.
Until some months ago we simply disposed of the reacted material, but now I want to recover these 3 metals, because silver is precious, mercury is hard to obtain, and chromium... well it's there.
The problem is the solution is quite complex and I don't know for sure which species are present. I start by reducing the unreacted chromium with citric acid, than I slowly add NaCl to precipitate silver chloride that is removed by filtration. But after this? Adding NaOH a brown precipitate is formed, and with NaBH4 I see a not recognized grey powder.
I managed to produce some metallic mercury by collecting the undissolved crystals on the bottom of the reaction cells, washing them with a bit of water to remove the acid (there's a transition from white to yellow, and i found this information"decomposed in H2O to yellow insoluble basic sulfate and H2SO4") and then using NaBH4. But this is not feasible on large scale.
I even tried putting a piece of iron and copper but no mercury was formed, although the copper was covered by a grey substance.

Any idea for a selective separation? I've thioacetamide but maybe the chromium forms insoluble sulfides too? Plus I prefer to obtain mercury in the metallic form directly. Thanks in advance.

Bedlasky - 10-11-2023 at 15:35

I would go like this:

1. Precipitate Ag as AgCl

2. Precipitate Hg as HgS

3. Reduce Cr(VI) to Cr(III) and precipitate it with Na2CO3 as Cr(OH)3 (don't use NaOH or NH3, Cr(OH)3 is soluble in excess of the reagent)

AgCl and HgS both precipitate from acidic solutions. Cr3+ doesn't react with sulfide in acidic solution, it reacts with sulfide only in alkaline solution to form Cr(OH)3 (Cr2S3 hydrolyze in the water, the same is also true for Al2S3).

All of these compounds can be easily converted to useful soluble salts.

[Edited on 10-11-2023 by Bedlasky]

unionised - 10-11-2023 at 16:38

If you have enough sulphide to ppt the Hg then you have already reduced Cr(VI) to Cr(III)

clearly_not_atara - 10-11-2023 at 19:32

Cr6+ precipitates with barium

Sir_Gawain - 10-11-2023 at 20:38

Won’t the mercury precipitate with the silver as mercury (I) chloride?

Rainwater - 11-11-2023 at 03:07

I would use a test tube
add copper foil, slowly and in small amounts until no more disolved

Should form an almagium on the bottom of the tube of Hg and Ag
Decant the liquid, Distill the Hg almagium (375c).

Then use nitric acid to bring everything back into solution
Percipitate the silver as AgCl with HCl

Dont know what to do with the chromium.

Bedlasky - 11-11-2023 at 04:14

Quote: Originally posted by clearly_not_atara

Cr6+ precipitates with barium

There is also sulfate present. So you get mix BaSO4 and BaCrO4.

Quote: Originally posted by Rainwater

Decant the liquid, Distill the Hg almagium (375c)

No offense, but why would you recover mercury by distillation when there are plenty of safer ways?

Quote: Originally posted by Sir_Gawain

Won’t the mercury precipitate with the silver as mercury (I) chloride?

In these kits is used mercury(II) sulfate, so you can't precipitate it with chloride.

[Edited on 11-11-2023 by Bedlasky]

Rainwater - 11-11-2023 at 05:34

Quote:

No offense, but why would you recover mercury by distillation when there are plenty of safer ways?

High yields, No additional chemicals, Only method I know.
I use a steal pipe, with taped cap, insert some copper tubing as a condencer and boil away.
Mercury is a cheap and easy way to extract silver, gold and a lot of other stuff
A bunch of hillbillies dumping and polluting gave it a bad name.

[Edited on 11-11-2023 by Rainwater]

Eithern - 13-11-2023 at 13:33

Hi guys, thanks for your replies!

Quote: Originally posted by Bedlasky

I would go like this:

1. Precipitate Ag as AgCl

2. Precipitate Hg as HgS

3. Reduce Cr(VI) to Cr(III) and precipitate it with Na2CO3 as Cr(OH)3 (don't use NaOH or NH3, Cr(OH)3 is soluble in excess of the reagent)

AgCl and HgS both precipitate from acidic solutions. Cr3+ doesn't react with sulfide in acidic solution, it reacts with sulfide only in alkaline solution to form Cr(OH)3 (Cr2S3 hydrolyze in the water, the same is also true for Al2S3).

All of these compounds can be easily converted to useful soluble salts.

In regard of point 3, since I should add a lot of sodium carbonate to neutralize the acid, is it possible that sodium sulfate also form and co-precipitate?
Also, I don't know an easy method to convert HgS to metallic mercury other then roasting it.

Quote: Originally posted by Rainwater

I would use a test tube
add copper foil, slowly and in small amounts until no more disolved

Should form an almagium on the bottom of the tube of Hg and Ag
Decant the liquid, Distill the Hg almagium (375c).

I tried with iron and copper but without succes, or at least I didn't see any amalgam (the "iron" that maybe contained other metals formed a strange semi-solid matrix, the copper instead was simply covered by a grey film).

I can try your suggestion in some test tubes

But one thing... since I accumulated at least 4-5 litres of solution, I thought of reducing the volume by putting some of it in a crystallization dish on light heat. At some point there were little white crystals both on bottom and floating on the surface, and a faint white patina formed on the glass borders of the dish (like when you use concentrated HCl).
Well, could it have been HgCl2? I've read contradictory things and don't know if the mercury sulfate only complex the chloride ions or form mercury chloride instead, I hope not because it would be a problem.
The crystals on the bottom, washed and reduced with NaBH4, formed a grey powdery precipitate, but I thought all chromium salts are colored. Boh?

Bedlasky - 13-11-2023 at 13:59

Quote: Originally posted by Eithern

Hi guys, thanks for your replies!

Quote: Originally posted by Bedlasky

Sodium sulfate is soluble, so it doesn't coprecipitate (if you have enough water).

HgS can be converted to HgCl2 by treating with concentrated HCl. This is pretty useful salt for mercury chemistry. But if you want to specifically elemental mercury, you can reduce it with some strong reducing agent like SnCl2 (you must add it in excess, otherwise you get mix of Hg + Hg2Cl2).

Rainwater - 13-11-2023 at 15:14

Quote:

Depending on the consentration, it will look different.
Thats a positive result for everything below copper in the reactivity series.

RU_KLO - 14-11-2023 at 05:05

first I would try to do an analytical scheme, to confirm which metals are in your solution.

First, using ammonia (if acidic solution) take the solution to ph 7-9 (neutral - slighty basic)

Then proceed with group reagents. you could find a lot of information on the web, wiki or here is the most complete I ve found so far (just change H2S for thioacetamide)

Attachment: 4.1_MIS_and_NJS_Manual_for_Inorganic_semi-micro_qualitative_analysis.pdf (395kB)
This file has been downloaded 557 times

RU_KLO - 14-11-2023 at 05:25

Quote: Originally posted by Eithern

Also, I don't know an easy method to convert HgS to metallic mercury other then roasting it.

from reading, you could use aqua reggia to dissolve HgS.

"In dissolving HgS, aqua regia acts as an oxidising agent and also complexing agent. The NO3- ions in presence of hydronium ion oxidize the
S2- ions to free sulphur and the Cl- ions with Hg2+ ions from soluble complex-ion, HgCl4 2-

3HgS + 2NO3- +8H3O+ + 12Cl- → 3HgCl4(2-) + 2 NO +3S +12H2O

(filter the sulfur - very difficult)

then

Mercuric chloride is converted into mercury by:

A. placing copper metal in aqueous solution of HgCl2.
B. treating aqueous solution of HgCl2 with excess of stannous chloride.
C. treating aqueous solution of HgCl2 with PbCl4 solution.

(note: I dont know how to convert HgCl4(2-) ion to HgCl2...)

Of course fumehood or outside and proper protection equipment.

Rainwater - 14-11-2023 at 06:28

Quote:

But one thing... since I accumulated at least 4-5 litres of solution

After you find a small scale experments that works, you can scale up to bucket size and process everything at once.
For simple disposal i would skip copper and go stright to iron.

The test strips I use can be found here.
https://www.amazon.com/dp/B08NVRSTHV/
If no trace of heavy metals can be detected, then you know the procedure works and can scale up.

I have found that these test strips are inaccurate at ph below 5
Once your solution test negitave for copper, mercury, and chromium and are netural ph
Their safe to pour down the drain

My standard waste treatment is using copper, then iron to remove the elements according to the reactivity series.

To recover metals/salts you can try the following

A solution with a ph below 7 is desirable
A few drops of hcl may need to be added
Dont be considered if AgCl percipitates, only be concerned with ph

1) Use copper metal as described above, and let the reaction run for an extended amount of time (hours to days).
Bubbling air into the mix helps but is not required.

2) Decant the liquid and save for step 9
If you test your solution here, you should see only copper and chromium.
Now your left with very little liquid some metal fragments and sludge.

3) If you know there is mercury, distillation of the metel pieces and sludges will remove it.
Distill until dry. (375c iir)
If AgCl percipitated, it will now be AgO (no white powder, will be black)
Distillate will contane water, hg, hcl

4) The copper pieces and sludge are then treated with consentrated nitric acid
25-65% will work.
Do not use RFNA. Dilute it first.
Add in small amounts until everything is disolved. Try not to add too much.

5) Filter. Carbon, gold, and some pgm will be trapped in the filter.
If they are present. You can skip this step

6) Then, to your filtrate, add a small amount of consentrated sulfuric acid (3-4 drops, or until percipitate stops)
Filter, this will remove any lead. Probley doesn't apply to what you're doing.

7) Then add small amounts of NaCl saturated solution into your filtrate until no more white percipitate forms.
Try not to overshoot.

8) Filter off the AgCl percipitate. Rinse with water until perfectly white, do not expose to uv light, or it will decompose and turn gray

9) Take the liquids from step 2 and step 8 and raise the ph
to about 6 to 7 with NaOH or bicarbonate (foams a lot)
The solution will heat up and can boil, so add it slowly.

10) Let it sit on iron/steel for a day. Do not use stainless steel.
This will percipitate/cement out the copper you added earlier.
I dont know what state this will percipitate the chromium, but it come out with the copper.

11) decant the liquid and test for heavy metals.
If heavy metals are still present, adjust the ph down and let it sit on the iron longer.

Edit: corrected ph in step 9

[Edited on 15-11-2023 by Rainwater]