Precipitate the heavy metal from EDTA complex ??

[Jeffrop]

We want to precipitate the nickel ion from complex EDTA-nickel by hydroxide (OH from lime) but did not work ! The EDTA-Ni is so stable, Ni ion can not form a complex with OH to be precipitate ! !.

We try to break down the EDTA-Ni complex before precipitate by oxidation (HNO3/H2O2, Hypochlorite, O3/UV, thermal hydrolysis ..., ) but Ni can not be free from EDTA !

Any body who know how to precipitate the heavy metal from EDTA complex (EDTA-m) please let we know !!.

Thanks

 

[bimr]

Sulfide precipitation is a proven method for this application. Metal sulfides are extremely insoluble and will precipitate even in the presence of EDTA.

The problem with sulfide precipitation is the potential release of hydrogen sulfide if the process went acidic. The Permutit Company (now US Filter) patented a process many years ago called sulfex. This process more or less minimizes the problem of hydrogen sulfide generation by tieing up the sulfide ion with iron.

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-1-4012/chap4.pdf

 

[ChrisDerrick]

I have strugled with this exact problem in the past. How are you generating the waste? Are there high concentrations of other metals such as iron, zinc, chromium, etc. The sulfide method probably won't be efficient unless you remove all the iron, and any other metals first. H2S can be avoided by adding bleach or peroxide after the filtration process. You may try using an anti-chelating agent such as Sodium Dimethyldithiocarbamate. Most wastewater treatment chemical suppliers produce a line of anti-chelating agents.

 

[Jeffrop]

Hi ChrisDerrick,

Thanks for your feedback, the EDTA-Ni complexe waste generating by the plating line. The concentration of other metal (Fe, Cr, Zn ..) is low (less than 10 ppm).
We already try with suffite but did not work, Ni is still presente after precipitation.

We will try with Sodium Dimethyldithiocarbamate.

Regards

Jeff

 

[bimr]

Jeffrop,

Did you say that you tried sulfite? Sulfite will not do anything. You have to use sulfide.

From his post, ChrisDerrick does not seem to be familiar with sulfide precipitation and in particular the sulfex process. For example, in the sulfex patent, iron is used to tie up the sulfide iron, so his post about removing iron does not make any sense. There is no excess h2s when using the sulfex process. No wonder he says that he is struggling.

It is possible that it may be economical to precipitate the metals in the OH form first, then precipitate the metals in the sulfide form second. This would depend on a number of things, concentration of metals, flow rate, size of operation, etc. and the decision can not be made without more information.

 

[kenvlach]

bimr is correct. I have used sodium sulfide in a polishing step, but actually prefer DTC (dimethyldithiocarbamate). The reaction is rapid, visible and can be monitored by ORP.

Two caveats:
1) You should not have any significant quantity of oxidant in the effluent, as both sulfide and DTC are reducing agents (and fairly expensive).
2) An important detail for 2-stage precipitation, i.e., hydroxide precipitation followed by DTC or sulfide: You must separate the solids formed in the 1st stage before treating the liquid in the 2nd stage. Otherwise, as the DTC-metal complex forms, metal hydroxide precipitate will dissolve to combine with the freed EDTA. Typically, the 1st stage is followed by clarification and filter pressing, although the equipment of course depends upon volume.

I still don’t understand while hypochlorite oxidation doesn’t work for you. I use an excess of calcium hypochlorite (65%, granular) at pH 10-11. This works for Ni in the presence of both ammonium hydroxide and EDTA complexants.

 

[Jeffrop]

Dear Kenvlack, Bimr and ChrisDerrick,

Thanks you for your interesting our subject and your suggestions !

Yes, we try with sulfide (Na2S) but did not work (sulfide only, not combine hydroxide/sulfide), after precipitation and sludge separation, we still have Ni in water ! We also try using sodium hypochlorite 12% but we got same result.

No oxidant in our effluent. We will try the Kenvlack's suggestion "2-stage precipitation (hydroxide/DTC)" and repeat the experience (hydroxide/sulfide) from Bimr and will let guy know the result.

Regards

Jeffrop

 

[bimr]

It is hazardous to use sulfide at low pH levels because of the possiblity of H2S. Sulfide precipitation or hydroxide pH are not chemically possible at low pH. Check the pH curve on the attached link:

http://www.remco.com/wasteh2o.htm

It is recommended that either you hire someone familiar with this chemistry or spend some time to learn it yourself.

 

[Jeffrop]

Hi Bimr

Tks for your recommandation !

Jeffrop

 

[ChrisDerrick]

Bimr:

I have to take exception with your response to my suggestions. I am quite familiar with sulfide precipitation and the sulfex process. However, my experience with EDTA chelated wastes is limited to boiler treatment wastes having iron concentrations as high as 10,000 mg/l and nickle concentrations closer to 10 mg/l. The sulfix process is certainly not very efficient until all or most of the excess iron is removed.

As you suggested lime/NaOH are very effective for removing the bulk of the metals leaving the highly chelated Ni, Zn, Cr, etc. The sulfix process can then be attempted in a polishing step to meet final discharge standards.

Try to attack the problem next time and not my character!

 

[bimr]

I have not attacked anyone's character. I only repeated what Chris Derrick said in his post, ie "I have strugled with this exact problem in the past". Perhaps, people should reread their posts before sending them.

In the sulfex process, you do not have to remove "all the iron". You do not have to remove any iron. Of course, the sulfex process is more efficient if high concentrations of metals are precipitated first as a hydroxide. But that depends somewhat on the overall size (flow rate) of the system. For a small low flow system, it is probably more efficient to do a one step sulfex process.

Hydrogen sulfide gas generation is avoided in the sulfex process by an elevated pH as well as the presence of iron. Use of iron is integral to the sulfex process. The sulfex process will not work without iron. Sulfex is iron sulfide.

The high alkalinity of the sulfex wastes solids eliminates the possiblity of hydrogen sulfide in the filtrate.

 

[Jeffrop]

Dear Kenvlack, Bimr and ChrisDerrick

The EDTA waste treatability test was done with two stages precipitations, here is the result,

1- Reference test:

One organic waste water sample (no EDTA) was treated by hydroxide precipitation at pH 9.6, using cationic polymer .
* The heavy metals before treatment is (mgr/l) : Al: <0.3, Cr: <0.3, Cu: <0.3, Fe: 8.7, Mo: <0.3, Ni: 5.5, Pb <0.3 and Zn <0.3.
* After the precipitation and separate the sludge the metals level is (mgr/l) Al: <0.3, Cr <0.3, Cu: <0.3, Fe: 0.7, Mo: <0.3, Ni: 0.7, Pb: <0.3 and Zn: <0.3
It clear for me the test was well done, all the metals were removed by precipitation.

2- EDTA waste treatability test:

We repeal the same hydroxide precipitation as about with same organic waste water but additional 5% the EDTA waste (V/v)
* Sample #1: before precipitation The heavy is (mgr/l) Al:< 0.3, Cr: <0.3, Cu: <0.3, Fe: 18, Mo: <0.3, Ni: 4.4, Pb <0.3 and Zn <0.3.
* Sample #2: after the precipitation and separate the sludge, the metals level is (mgr/l) Al: <0.3, Cr <0.3, Cu: <0.03, Fe: 4.3, Mo: <0.3, Ni: 4.2, Pb: <0.3 and Zn: <0.3
It clear that EDTA was complexed with Fe and Ni, and Fe and Ni cant not precipitated.

3- Second stage with sulfide (Na2S)

The sample #2 was flitted and treated by sulfide precipitation at pH 9.4 with 3.0 gr Na2S in 150 ml sample
* After the precipitation and separate the sludge the metals level is (mgr/l) Al: <0.3, Cr <0.3, Cu: <0.3, Fe: 0.4, Mo: <0.3, Ni: 4.2, Pb: <0.3 and Zn: <0.3
It's clear that Fe was precipitated but not Ni.

3- Second stage with sulfide (DTC)

The sample #2 was flitted and treated by DTC precipitation at pH 9.0 with 1.0 gr DTC in 150 ml sample
* After the precipitation and separate the sludge the metals level is (mgr/l) Al: 1.0, Cr <0.3, Cu: <0.3, Fe: 0.8, Mo: <0.3, Ni: 4.2, Pb: <0.3 and Zn: <0.3
One again Fe was precipitated but not Ni.

Why Ni not precipitated ? the pH not corrected ? Na2S or DTC feed not good enough ?

Could your guy pls review and advice

Regards

 

[bimr]

Several questions and comments:

1. What chemical are you using to raise the pH. If lime is used instead of NaOH, you can also expect lower metal concentrations.
2. Were all of the analyses filtered before testing? To achieve low metal concentrations, it is imperative to filter.
3. It is sometimes difficult to select the best polymer. Not many polymers are effective with these metal precipitation processes. Is the polymer optimized?
4. For EDTA at high pH, calcium, magnesium, and iron are all preferred over the hazardous divalent metals such as copper, nickel, and zinc. Adding any one of these (for example calcium chloride) would increase the precipitation of the divalent metals. Have you tried feeding calcium chloride?
5. Have you optimized the chemical dosages?

 

[Jeffrop]

Dear bimr

Here is the answer your question:

1- Lime was using to raise the pH.
2- Yes all samples were filtered before go to an other stage.
3- Polymer cationic was using, this polymer done good result with waste sample no EDTA.
4- No, calcium chloride was not using in our test.
5- Not yet, we will.

Thanks

 

[bimr]

Sound like you are making some progress. I would try adding calcium chloride. The EDTA prefers calcium over nickel. I would also optimize the pH. As the pH rises, more calcium and more nickel will precipitate.

 

[ChrisDerrick]

Bimr:
Seems we got into a tussle over semantics and not real treatment technology. I should have said most of the iron and not All. This may be fodder for an inependent letter but.... I have a boiler treatment waste chelated with acetic acid, iron at 5-10,000 mg/l. Chrome +3 at 15-20 mg/l(Cr+6<0.10 mg/l). Discharge limit is 4.5 mg/l total chrome. Batch treatment with lime, NaOH, and polymer,(pH-12.5-13.0) filter press, followed by carbon polish produces results from <0.1 to 8.5 mg/l. of toal chrome and <1.0 mg/l iron. Now, the batch treatment is consistant but the lab data varries a lot. Any ideas on why? Is it analysis bias, or varability in the waste stream? Carbamates are minimally effective and Cr2S3 is relatively soluable. I can not understand why one batch is very low in chrome and the next is very high. Any Ideas? Suggestions? Any ideas would help. Thanks!

 

[bimr]

If the wastes are from the same boiler, then the results may be questionable. If the wastes are from different boilers, then one would not expect the batch chemicals or the metal content to be the same since the boiler metallurgies may be different.

 

[ChrisDerrick]

bimr:

The waste is all from the same boiler and was stored in one tank prior to treatment. This was a newly built boiler and thus were cleand to remove &quot;mill scale&quot;. No one is really sure how such high levels of chrome made it into the waste water in the first place.

I used a colormetric method for analzing Cr+3 and Cr+6 in the field. However, i found it to be less than reliable due to the difficulties in deciding when the proper colors had been reached (the permanginate would turn brown instead of pink even at very high dilution) I dont know exactly how the laboratory tests for chrome but could their test suffer the same type of difficulties?

The treated waste went into a second tank and was analyzed prior to discharge. It met the discharge requirements but was higher that I would have expected from the data gathered in the benchwork and batches with results similar to the benchwork. That would indicate some of the batches really did have higher chrome levels as the lab data found. Unless, the final result was incorrectly high.

So you can see I am grasping at straws to understand the mechanismn of the chrome removal from the citric acid chelation. I thought it was simple a matter of removing the chrome as the hydroxide. Can you think of anyting that would cause such a variety of chrome concentrations after treatment?

Thanks

 

[FrancisM]

A proven solution can be provided by GE Betz, a chemical company who specializes in water treatment chemicals.

Try them.