Hazardous hexavalent chromate is currently used in Department of Defense (DoD) plating shops for critical corrosion control and surface finishing requirements. Although many organizations are seeking alternatives to chromate, it is still the method of choice for plating application shops. In plating shop wastewater treatment plants, hexavalent chromium (VI) is reduced to trivalent chromium (III) with ferrous sulfate in order to meet discharge regulations. However, this process produces large volumes of iron/chromium hydroxide sludge for disposal. In this project, the operation of a 7 gallons per minute, automated, pilot-scale Anion Liquid Ion Exchange (A-LIX) system was validated for separation of chromate from plating wastes at Watervliet Army Arsenal and Warner-Robins Air Force Base.

Technology Description

The A-LIX technology continually extracts the chromium (VI) chromate anion from plating shop wastewater using a chromate-specific, water immiscible amine in a closed-loop process. This method produces a clean waste stream that meets chromium (VI) discharge requirements and requires no ferrous sulfate reducing agent. There is no waste sludge generation. The chromate anion is continuously stripped from the extractant phase of this process and recovered as a concentrate for recycling.

Demonstration Results

Steady state operation for periods up to ten days was conducted. Residual chromate in the waste stream was as low as 0.1 - 0.3 parts per million (ppm) while chromium levels as high as 20,000 ppm were obtained in the concentrate. The estimated capital cost for a 20 gallons per minute system was $450-490 thousand. Annual operating costs were $300-500 thousand, and payback varied between 2-4 years depending on the cost of the existing sludge disposal method.

Implementation Issues

The A-LIX technology has the potential to eliminate up to 2,000 tons per year of chromium (III) sludge and reduce the amount of new chromate purchased for electroplating by 175,000 pounds per year, resulting in a cost savings of nearly $2 million per year. This process is widely applicable, small in size, and easy to use. It will allow DoD plating facilities to meet future stringent chromium discharge limits. In addition, this technology can be used to remove other metal cations by simply altering the extracting compound. The versatility of A-LIX will allow it to be applicable to many different areas of industry. (Project Completed - 2003)