Is the problem too much Cr⁺⁶ or too much Cr⁺³, or both?

grrun

Ozone is a powerful oxidant and (while I'm not positive) it is likely to oxidize Cr⁺³ to Cr⁺⁶ which will create another problem for your client. If the problem is too much Cr⁺³, then a better solids separation is required. Cr(OH)₃ is quite insoluble. Why don't you do a Google Search to see if you can find any thing.

grrun    

Hi Terry,

Cr[6+] is much toxic more than Cr[3+], so in the wastewater there is much Cr[6+], it will be deoxydated Cr[6+] to Cr[3+], before to precipitate them at high pH. Some chemicals can use for this purpose like as : FeSO[/4], NaHSO[/3],...in low pH ( 2-4 )

i've got to agree , you must chemically convert from hex to trivalent chome one we used was sodium mettabisoulfite.








Dear Terry Murphy,

there seems to be a luck of understanding of the chemistry of Cr(6+, 3+). By no means flocculation/sedimentation is able to reduce Cr6+ to Cr3+. You will need a reducing agent or oxidize other wastewater components by Cr6+. You will find it in textbooks or at the internet how to reduce Cr6+ in wastewater. If it is converted 100 % practically you will have to precipitate Cr3+, preferentially as Cr(OH)3. Therefore you will have to adjust the pH accordingly.

Ozone is a strong oxidizing agent. Therefore, I cannot see any positive role it might play in the present case!

Kind regards
T. Sobisch [/quote]

Hi Terry
I agree that first the highly toxic hexavalent chrome has to be reduced to chrome 3+. As we are specialised in tannery effluent treatment we have experience in the precipitation of chrome. Chrome can be well precipitated at pH 8-9 with NaOH, MgO or Lime and is then dewatered in a filterpress or centrifuge. The Chrome 3+ ends up in the sludge and the filtrate has less than 5 ppm of total chrome. If this method is suitable for you please do not hasitate to contact us.
Best regards,
Dr. Wolfram Scholz

The hexavalent cromium can be reduced for the adition of a very strong reductor agent, the most used is the Sodium meta bisullfit, The treatment for you precipioted this metal is increasing the pH to 9.0 to 9.2, whith Calcium hydrixide(Cal), the metal is precipited when hydroxide and the level of cromium 3 is reduced.

Has anyone heard of using such reducing compounds in liquid form as a rinse for parts that may have Cr(VI) on the surface or if the part is porous, converting the Cr(VI) within?  Vendor names that could help me with this?

Thanks
Eric

Dear BCSMike,

it would be more helpful if you would not only state you have a technology reducing it to non detectable levels.
Please provide information on the technology used and by which mechanism it achieves the results.

Kind regards
T. Sobisch

Dear all,
I am a newbie with BIG problem that needs your advices.
I just started the commissioning of a Cr waste water treatment plant. Following is my procedure I have done.
- Dilute Cr waste water with portable water (actual result: pH=0.99, ORP=785mV)
- Use NaHSO3 and H2SO4 to keep condition of pH=2.5 and ORP=250mV (actual result: pH=2.17 and ORP=241mV)
- Use NaOH to increase pH=8 (actual result: pH=7.98 and ORP= -124mV)
At that time the color of waste water was changed from redish brown to very dark green color.
I have not got the analysis result of inlet and outlet so I don't know how much Cr6+ was changed to Cr3+. But the problem is I could not find the sedimentation when I dosed FeCl3 finally (no floc appears).

Please help me to find the reason   ???
Thanks in advance.

Best regards,
Kei

Eproj wrote on Oct 12^th, 2006, 3:06pm:


I had the experience of providing consulting services to a company that was applying conversion coatings to aluminium, their coating was non adherent, and cracked all over.

The problem turned out to be them using recycled water in the post chromating rinse. It had excess sulfites in it, among other things.

I'd be very careful implementing a solution like you describe in production.

Hello,

Depending water characterization, you can also look to filter the Cr VI directly with Metallx. Look at : www.solmetex.com

Good luck

Alain

This post is older but I thought I would post the procedure I used for a wastestream with a high [Cr+6] for others seeking information.  The original concentration of Total Chromium was 3.7 mg/L and our final analytical yielded a Total Chromium of 0.1 mg/L.  First, the pH of the wastestream was lowered to 2.5 using Sulfuric Acid.  Next, magnesium bisulfite was added until the color of the water changed from yellow to blue (sodium bisulfite can also be used but the color change goes from yellow to clear).  At this point the majority of Cr+6 is changed to Cr+3.  Once this reaction took place the pH was adjusted to a 9 with Sodium hydroxide.  Next, a coagulant followed by an anionic polymer was applied before sending the wastestream to our DAF.  By raising the pH and applying coagulant/anionic polymer the chromium is preciptated out in its trivalent form and skimmed off with the rest of the sludge from the DAF to be sent to a lined landfill.  Hope this helps.

dear all

hexavalent chromium can be reduced to trivalent chromium by a reducing agent like ferrous sulfate  or sodium metabisulfate
the reaction take palce at acidic ph

decrease the ph with sulphuric acid 2-3

add reducing agent ferrous sulfate  1 : 12

allow it for 10 min

raise the ph to 8.5 with lime to precipitate the trivalent chromium as chromium hydroxide