Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 572 of 707

solved metal content. These two tests, titration and metal analysis, are the basic requirements for the proper function of the acid process. The main reason to dispose of an acid tank and start with a fresh chemistry is due to dissolved metal concentrations being high enough to interfere with the acid-metal reaction. Therefore, a means of extending the bath life involves either removing the dissolved metal or converting the dissolved metal to a form that no longer interferes with the acid-metal reaction. There are three commercially available methods that deal with the dissolved metal problem.1 1. Additives to precipitate and/or sequester the dissolved metal 2. Diffusion dialysis 3. Acid sorption 1. Additives Metal precipitation/sequestering is an in-tank means of removing a portion of dissolved metal by precipitation and a portion by sequestering (possibly chela- tion). PRO-pHx™ ( is one example of such a chemical method. PRO-pHx has a proprietary formulation, but it is believed that part of the chem- ical reaction produced by PRO-pHx™ involves metal being sequestered because dissolved metal concentrations can go much higher than what would be expect- ed without any apparent loss of acid-metal activity. The high concentrations of dissolved metal are prevented from interfering with the normal acid-metal activ- ity which would indicate some form of sequestering action. In normal operating use, PRO-pHx is added to the acid tank to maintain a 1% concentration of the additive. A portion of the dissolved metal forms a precip- itate that can be filtered. The remainder of the dissolved metal stays in the acid tank but in a form that is not active. 2. Diffusion dialysis The diffusion dialysis process makes use of a membrane that allows the acid's neg- ative ions (SO4-2, NO3-2, Cl-1, etc.) to pass through while preventing the posi- tive metal ions from passing through. A typical system is 90% efficient, meaning that 90% of the acid is recovered and 90% of the metal is removed in each mem- brane pass. The results are a waste stream that is high in dissolved metal and a acid stream that can be returned to the acid tank. 3. Acid sorption The process of acid sorption works on the same principle as ion exchange in a water deionization system. The acid anions (negative charge) are captured from the acid solution stream by an ion exchange resin while allowing the positive met- al ions to pass through. Then the resin column is back-flushed with fresh water to free the acid anions. This back-flushed solution is, therefore, rich in acid and poor in dissolved metal. The acid-rich solution can then be returned to the acid tank. This method is between 80% and 90% efficient. The acid sorption process is commonly used in large aluminum anodizing sys- tems to maintain the amount of dissolved aluminum in the correct range. The economics will determine which method of acid recovery makes sense for each metal finisher. Again, that is why it is critical to know the cost of acid pur- chases and disposal to determine the payback for acid recovery systems. 571

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