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Reducing Chrome 3 Levels

We have tried the lime slurry method for reducing chrome 3 (trivalent chrome) but our levels only dropped from 4.3 to 4.1 ppm.

Do you have another suggestion for the reduction.

Wayne Lewis
CHC Composites - Gander, Nf., Canada

Chrome precipitation occurs best in a pH range of minimal solubility. The best way to precipitate chrome is to adjust the pH to approximately 8.5 pH and use a flocculant to coagulate the mixture. Allow some time to settle. This method may produce a cloudy supernate if detergents and/or surfactants are present.

The process is simplified using sodium hydroxide instead of lime. Lime is slow to dissolve and will overshoot the desired pH range. Usually, a coagulant increases the efficiency and will produce a clear - chrome free supernate.

Good luck

Paul Cravens
Southern Water Treatment Co. - Friendswood, Texas

I use a product called "dipoz aid" manufactured by Chemetall Oakite.

I first lower pH to <2 with Nitric and add dispoz aid #1 then raise pH to ~8 and add dipoz aid #2 and a coagulant called "Kleer Aid Polymer". The Cr3 settles and leaves me an effluent containing less than 1ppm of Cr. I'm sure that sodium bisulphate or another manufacturer's product will work just as well as my Dispoz Aid, but that's what I use.

Benjamin Curto - Ponderay, Idaho, USA

First I would lower the pH to 2.0, then I would add Sodium Metabisulfite. Let this mix for about 15 minutes. Then I would add Magnesium Oxide to raise the pH to 8.5. Add a Flocculent and the Chrome will fall out of solution.

Randy Mattson
- Seattle, Washington, U.S.A.

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