ORP Control for Metal Removal with sulfides

[harbor]

I'm looking for verification of an explaination that was given to me regarding the use of an ORP Controller to feed a sulfide (NaHS, DTC, etc.) to remove metals.

I'm aware that ORP measures the Oxidation/Redcuction Potential, so using an ORP Controller to feed bleach or metabisulfite to a treatment system makes sense.

Here is the reasoning I was given for using ORP to feed sulfide based metal removal chemicals.

..." your not so much measuring Oxidation/Reduction as you are measuring the water's ability to conduct electricity due to the amount of dissolved metal in the water. If the pH remains constant, as the dissolved metal concentration increases, the ORP reading increases (in mV). As the metal is precipitated and removed, the conductivity (in mV) is reduced. For example, if you have a mix of dissolved metals in the water the ORP reading may be +200 mV; as you add the sulfide the metals are precipitated and the corresponding ORP reading is decreased until all the metals have been precipitated and removed - and now the ORP is at a lower value - maybe +10 mV..."

This description seems to make sense - until I think that the meter is designed to measure Oxidation or Reduction reactions. Any enlightenment would be appreciated.

 

[ACHRISTIDE]

Although the explanation you have received is indeed creative, I do believe that it is patently incorrect. In my experience, Nalco sales reps have used this explanation when they are not quite sure why their Nalmet technology employs an Ion Selective Electrode for sulfide.

Specifically, if the explanation that you were given was correct, why wouldn't you just use a conductivity meter to determine the specific conductance of the water? Usually a cheap and reliable controller.

While the use of the type of ORP controller that you are discussing is worth considering, I have found it to be most useful and potentially reliable in extremely clean streams (e.g. printed circuit board manufacturing) Most conventional waste streams have multiple contaminants that can interfere with the probe performance. (Oil & grease, TOC, etc.)

Direct, proportionate feed is typically the best (most cost effective/reliable) way to feed these products. (Water meter w/ 4-20 mA output to activate a proportional feed chemical metering pump.)

Good luck!

 

[harbor]

ACHRISTIDE,

Thanks for your response. The proportional pump set-up makes sense except I worry that the dissolved metal concentration in the untreated waste water changes at various times (e.g. 9AM - Cu=10 ppm, 11AM - Cu=0.5 ppm, 3PM - Cu=35 ppm). The OPR meter is hyped as a cure to metal fluctations - when the metal concentration is high the chemical pump keeps pumping until the metals are gone; if the metals are low the chemical pump shuts off sooner. It seems to make sense and offers something the proportional pump maybe can't address.

My issue is with the explaination of how the ORP meter works - and how it relates to Oxidation / Reduction Potential. Any thoughts from folks who have had success with ORP control for these applications?