Original Date: 09/14/1998
Revision Date: 01/18/2007
Best Practice : Nitric Acid Recycling Program
Nitric acid is typically used in such processes as the finishing of aluminum and stainless steel alloys; the stripping of nickel fixtures; and the plating of copper, tin, and lead. However, nitric acid is also a reportable chemical on the Superfund Amendments and Reauthorization Act, Title III under Section 313 regulations, and is targeted for reduction or elimination. In addition, the Air Force enacted a P2 initiative which called for a 50% reduction in nitric acid usage.
To comply with these directives, Raytheon Missile Systems Company (RMSC) established a Nitric Acid Recycling program. The program’s scope called for the procurement and installation of four purification units so that metallic contaminants could be removed from nitric acid solutions allowing indefinite reuse. RMSC investigated applicable technologies for recycling nitric acid, and identified diffusion dialysis and resin sorption as two possible methods.
Diffusion dialysis technology uses an anionic exchange membrane that contains positively-charged groups on a polymer matrix surface. Anions (e.g., nitrates) can pass through the membrane, but most metal cations cannot. This method allows nitric acid to diffuse through the membrane into a freshwater stream and return to the process tank, while positively-charged metal ions are blocked and remain in a low acid concentration waste solution. The second technology, resin sorption technology, relies on sorption and desorption. During the sorption step, nitric acid from the working solution passes upward through a tightly packed resin bed. Free acid is then sorbed by the resin, while the metal salt solution passes through as waste. During the desorption step, water passes downward through the resin bed to desorb the free acid from the resin, and move it back into the working solution.
RMSC chose the resin sorption technology (Figure 2-4) because this method was simpler, required fewer pumps, and cost less to maintain. The four resin sorption units cost approximately $20,000 each, and have a payback time of two years. RMSC’s Nitric Acid Recycling program has been a good environmental and business investment. The company can recover 95% of the acid used to process its components. By recycling the spent nitric acid, the company also significantly improved the process control, and reduced labor time, raw chemical purchases, hazardous waste sludge, and waste treatment costs.
Figure 2-4. Nitric Acid Recycling
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