Q. What is the best way to reduce hexavalent chrome to trivalent chrome from an aluminum conversion coating rinse water? K.O.
A. The “best way” depends upon many factors such as safety concerns, volume of wastewater, discharge limits and final discharge. That being said, our experience has been that, for the vast majority of chrome reduction applications, the use of sodium metabisulfite (MBS) is best.
Sodium metabisulfite (Na2S2O5), when added to water, forms sodium bisulfite (NaHSO3), which reacts with the hexavalent chromium (Cr+6) to form trivalent chromium (Cr+3) according to the following reaction:
3NaHSO3 + 2H2CrO4 >>> Cr2(SO4)3 + 3NaHSO4 + 5H2O.
Approximately 2.8 lbs of MBS and 1.5 lbs of sulfuric acid are needed to reduce 1 lb of hexavalent chrome. Because the speed of the reaction is pH-dependent (speed decreases as pH increases), we typically design for a retention time of 15–20 min and a pH of 2–2.5.
To feed the MBS solution into the rapidly mixed chrome reduction tank, an oxidation-reduction potential (ORP) sensor and controller is used. A set point of -300 millivolts is adequate for most applications. As you become more familiar with the system, you may want to increase (become more negative) the set point in 25-milli-volt increments and check the validity of the setting. There are several suppliers of colorimetric testing kits for hexavalent chrome (sample turns purple if hexavalent chrome is present) that can be used to determine the best ORP set point; -400 millivolts is the maximum set point that most have found to be acceptable, but each wastestream needs to be evaluated.
Alternative technologies for chrome reduction include sulfur dioxide gas, sodium hydrosulfite, ferrous sulfate, scrap iron/steel/steel wool and electrochemical.