Treatment of Chrome Plating Wastewater

A. Having experienced electrolytic reduction why would you consider anything else? If the electrolytic process does not also precipitate the chromium then you may need to tune it some. By definition, an electrolytic process will not give you re-useable water, as some conductivity (i.e. dissolved solids) are required simply to conduct the electricity. You can follow with another technology to remove them.

A. We're pondering also different options for water treatment. One of those was the use of e-cells, but we were uncertain on it. Is there a way to find out more about the e-cells?

Thanks Chris!

A. Hi Imran. I assume you are speaking about wastewater treatment rather than about how to do trivalent chrome plating.

At low pH, sodium bisulfite will convert hex chrome to trivalent chromium. This process evolves toxic sulfides in proportion to how low the pH is, so keep it as high as practical without stopping the process. Probably about 3.5-4.0. The yellow color will be gone and the water will be greenish.

The chrome will then precipitate out at normal wastewater treatment pH. If you are treating only chrome, the pH should be about 8.5, but you can go higher to remove other dissolved metals without stopping the precipitation of chrome.

Wastewater treatment, however, involves dangerous chemicals and the potential to evolve toxic gases, and operators should receive hands-on training, not proceed based on brief internet messages. Good luck!

A. Electrocoagulation using steel plates and DC current works well without any chemical additions and will remove the chromium as a precipitate at any pH over about 6.

A. You can use also the Catalytic Precipitation, ECP, without any chemicals, without sludge or precipitate, without sacrificial electrodes. No dependence on pH or temperatures, or concentration. The results are oxides and minerals filtered like solids directly from water at the time of treatment process. You will have good water in a few minutes (hours), depending on flow rate and wastewater analysis.

Regards,

A. Hi Rodrigo. In my experience, reduction of hexavalent chromium to trivalent is one of the easier processes because you can so easily track it visually. If there is any yellow/amber/orange tone whatsoever, the chromium has not been reduced to trivalent and will not precipitate. If the color has turned to light green-blue, it will.

If you have sufficient treatment time, I would not run the pH down to 2, but would try to keep it closer to 4 -- the reason being that a lot of noxious sulfur gases are released at low pH, and that drives you towards wanting to go light on the necessary bisulfite; whereas at the higher pH you can add sufficient bisulfite with less gassing.

Regards,

A. Hi Ashok. You must analyse the wastewater to see what the TDS is comprised of before you can make any plans.

For example, you cannot practically precipitate chlorides, so any attempt to reduce the TDS if it's high in chlorides might only increase it. Different metals precipitate best at different pHs, which is a possible optimization. Low pH pretreatment with chelate breakers may be needed. Please give us an analysis and some details. Thanks.

Regards,