plating, anodizing, & finishing Q&As since 1989
I work at an Industrial Wastewater facility that treats metal plating waste. I have recently had a problem with cyanide. We treat it with a two stage process; adding calcium hypochlorite at pH 10.5, then after mixing (many times overnight), we lower the pH to 8.5 and allow more mix time. Then we transfer the basin to another system for polymer addition, sedimentation, filtration, etc.
One problem we have had lately is chrome in our cyanide basin. After the usual treatment, we have to lower the pH to 3.0 and add sodium metabisufite. This water passes through a neutralization tank that raises the pH back to about 10.5. Lately, our in house CN results, which are done on a HACH DR-2010 without distillation have differed greatly with the results from our contract lab. I have noticed that we are generally a decimal point lower than they are. (Our 0.002 is their 0.02, etc).
Is it possible that after I oxidize the CN maybe I am not completely allowing the resulting cyanates to be released as CO2 and Nitrogen? Am I reducing cyanates to cyanide when I add sodium metabisulfite to treat the chrome?
The HACH method, while not an approved method without distillation, has served us very well for years. Why are we having trouble now?
Thanks in advance for any responses.Derek Burton
- Atlanta, GA USA
HAVE YOU TRIED RETAINING A SAMPLE FOR ANALYSIS PRIOR TO DROPPING THE PH FOR CHROME REDUCTION? THIS WOULD ELIMINATE THE GUESS WORK OF THE EFFECTS OF THE BISULFITE. YOU DIDN'T MENTION WHAT VALUES YOU NORMALLY OBTAIN FOR CN. OUR INdependent LAB USING EPA METHOD 335.2 GENERALLY SHOWS TOTAL CYANIDE OF <.O2 MG/L. IT IS NOT UNCOMMON FOR OUR HACH UNIT TO BE A DECIMAL POINT LOWER. THE METHOD WILL DETECT A SMALLER NUMBER.
SEVERAL YEARS AGO WE WERE REQUIRED BY NY STATE AND THE USEPA TO MOVE OUR SAMPLE POINT FOR CYANIDE TO THE POINT OF TREATMENT NOT TO OUR DISCHARGE BECAUSE OF THE FEAR THAT THE LONGER THE TIME IN SOLUTION THE LOWER THE CYANATE NUMBER WOULD BE.
WITH YOUR ADDED PROCESS STEPS, ARE YOU ALLOWING THE NUMBER TO LOWER BELOW WHAT YOU HAVE HISTORICALLY RUN?
HOPE I HELPED!WILLIAM F. MORGAN
ESPEY MFG AND ELECTRONICS - SARATOGA SPRINGS, NY USA
The fact that you are a factor of x10 when analysing for cyanide in your samples may be due to the fact that your sample contains a complex chrome cyanide compound that your test kit is unable to measure.
Most test kits only measure free and weakly bound cyanide complexes. Those cyanide complexes that are strongly bound like iron and I believe chromium will require a pre-treatment such as distillation or UV photolysis to break the complex cyanide bond and free the cyanide for measurement in your test kit. Try leaving a sample on the window sill and expose to UV radiation for 2 days and then measure cyanide with your test kit - you should see an increase in cyanide concentration if my theory is correct.
Best of luck.Dave Morris
We have had similar problems with the colour comparison tests. If we take a sample with cyanide and zero chrome, do our Hypochlorite treatment and test for chrome afterwards we find chrome. If we take a chrome effluent and reduce the chrome6 we find cyanide afterwards. I do believe it is the treament method (the reduction chemical for the chrome and oxidation for the cyanide) which interferes with the colourimetric tests. We have not been able to establish if only an overdose of treatment chemical causes this.
All the bestTrudy Kastner
Chemserve Systems - Durban, South Africa
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