Extending Sulfuric Acid Anodizing Bath Life

              Extending Sulfuric Acid Anodizing Bath Life
WMRC
Fact Sheet
TN07-089
May 2007
Background
It has often been theorized that by keeping an acid bath clean, the acid can be re-used continuously without dumping the bath. However, the bath would need to be spiked with fresh acid to maintain its effectiveness. Numerous facilities have tried to extend the life of their acid baths using filtration but have been unsuccessful. U.S. companies treated over 15 million pounds of spent acid off-site in 2004. They treated another 1.9 billion pounds on-site.1 If it were possible to economically use acid over and over, companies would not need to treat thousand of tons of spent acid and would save millions of dollars in transportation and disposal costs, as well as help protect the environment.
A study by Ellis, Deluhery and Rajagopalan demonstrated that spent acid etch rates can be improved by using sodium silicate filtration, ultrafiltration or carbon filtration.2 During the same research project, they also demonstrated that the etching properties of acids are very sensitive to organic/colloidal contamination and, to a lesser degree, dissolved metal concentration. If organic and colloidal contaminants could be minimized in acid baths, then it might be possible to realize continuous use of acid baths without dumping the contents.
A commercial acid bath extender, combined with filtration and regular spiking of the acid bath, is advertised to remove colloidal contaminants and dissolved metals. 3 When used at a 1% concentration (volume of extender to volume of spiked acid addition), the manufacturer states that the acid bath can be used indefinitely without partially decanting or fully dumping the bath.
Given this background, the Waste Management and Research Center (WMRC) worked with All-Brite Anodizing, a small job shop anodizer located in Northlake, IL to achieve two goals. The first was to test the effectiveness of the acid bath extender. The second was to determine if sulfuric acid used in the anodizing process could be cleansed sufficiently to continue using it without decanting portions to the wastewater pre-treatment system.
Testing at All-Brite Anodizing
All-Brite provides decorative and protective anodizing services and has a typical sulfuric acid anodizing process. Prior to testing the acid bath extender, the company would discharge part of its acid tanks to its wastewater pre-treatment system and add make-up acid to the tanks about very eight weeks. The tanks were dumped when the aluminum contamination level reached 2.0 oz/gal (15 gm/L). Based on the volume dumped, fresh acid was added to maintain acid strength.
From September 2005 to October 2006, All-Brite tracked acid strength and aluminum concentrations in its two anodizing tanks. The two anodizing tanks were the same size, on the same automatic anodizing line and were filled with fresh acid at the same time. The automatic
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WMRC is a Division of the Illinois Department of Natural Resources