Known for it's lustrous silvery white color and a ability to take on a high polish, resist corrosion and withstand high temperatures, chrome has a wide range of uses.
In industrial wastewater systems, chromium is commonly found in it's highly soluble CR+6 oxidation form (known as hexavalent chrome or hex-chrome). Hex-chrome is a known carcinogen and heavily regulated by the EPA. Over the years, treatment methods have been established to remove this highly toxic mineral prior to discharging the wastewater to local muncipalities or surrounding bodies of water.
In order to remove the hexavalent chrome (CR+6) it's oxidation state must be reduced to trivalent chrome (CR+3). Once this reduction is complete the trivalent chrome can be treated in a similar method to other metal bearing wastewaters. To achieve this conversion the hex-chrome bearing wastewater's pH is lowered to between 2-3 and a reducing agent is added.
Typically sulfuric acid is used to reduce the pH to 2-3 to ensure the rate of reaction is sufficiently fast. At a pH of 2 the reaction is almost instantaneous, at a pH of 3 the reaction takes approximately 30 minutes. Sodium metabisulfite, ferrous sulfate, sodium bisulfite or sulfur dioxide gas are commonly used for reducing agents. There are advantages and disadvantages to each of these agents that should be taken into consideration during selection. For most small to mid size dischargers sodium metabisulfite is a good option. Below is the reaction that takes place between the hex-chrome and sodium metabisulfite:
2H2CrO4 + 3NaHSO3 + 3H2SO4 -> Cr2(SO4)3 + 3NaHSO4 + 5H2O
The progress of the reduction reaction can be monitored utilizing an ORP meter. The reading that corresponds to a complete reaction is not absolute. Trial and error is required to determine what reading corresponds to near complete conversion of hex-chrome. In general a reading of somewhere between 250 mV and 350 mV is a good starting point.
Effective control of a chromium reduction process is critical to ensure that the reduction proceeds quickly, safely and completely. In practice a well controlled conventional chromium reduction process is capable of producing an effluent with less than .1 mg/L CR+6. Contact Salt Creek Technologies for assistance with the design, selection, operation and troubleshooting of your chrome treatment system.