NMT Develops Acid Recycling Process

NMT is implementing projects that will ultimately help meet the goal of eliminating TA-55 hazardous and radioactive waste discharge, which now contains higher levels of nitrates, chlorides, and radionuclides than the Environmental Protection Agency mandates. By recycling nitric and hydrochloric acid before discharge from the site, NMT hopes to meet or exceed EPA environmental standards, reduce the radioactivity of effluents discharged from the site, and minimize waste by reusing acids.

The acid recycling is a two-step process. First, the waste stream is evaporated to separate volatiles (components of the waste stream that readily evaporate mostly water and acid in this instance) from dissolved radioactive salts and other inorganic salts. These nonvolatile salts are then mixed with cement for disposal. Second, the evaporated acid-water mixture is separated into a low-acid (mostly water) stream that may be safely discharged and a concentrated acid stream that is suitable for reuse.

Nitric acid is removed from the waste stream by fractional distillation, a method that separates the components in a mixture of volatiles by distilling each component at its respective boiling point. NMT demonstrated the feasibility of removing nitric acid by evaporating a simulated waste stream in the ATLAS line at TA-55. Results of this demonstration were encouraging: 82% of the nitric acid was recovered during the first pass through the ATLAS line. In the second and third passes of clean distillate through the evaporator, 43% and 21%, respectively, of the original nitric acid feed was concentrated to 10.7 M HNO₃ and 12 M HNO. The distillation column to be built will enable continuous production of >=12 M HNO₃.

NMT designed a 10-inch-diameter, 13-foot-tall stainless steel column with a reboiler and condenser for distillation of nitric acid. Once built, the column will operate in room 434 (the old evaporator room) inside the Plutonium Facility and will concentrate 500-liter batches of waste solution with a nitric acid concentration of 2 to 6 M to a concentration of 12 M HNO₃. Automated controls will enable the system to sense process changes (such as a change in acid concentration in the incoming waste stream) and to change operating conditions so that final acid concen-trations remain constant

The recycling of hydrochloric acid requires a slightly different approach. Because a simple distillation of water and hydrochloric acid limits its maximum acid composition to about 6 M HCl, concentrating hydrochloric acid beyond this level from the waste stream is somewhat more difficult than concentrating nitric acid. NMT will employ a vapor-phase membrane separator to solve this problem. The separator will employ tubes of Nafion, a DuPont product that permits water in the stream to pass through while trapping hydrochloric acid.

As national environmental regulations become increasingly stringent, improved waste treatment methods such as NMT's acid recycling at TA-55 become critically important to the welfare of both the Laboratory and the nation.

Project contributors include Tom Mills, Elaine Ortiz, Noah Pope, Wayne Punjak, Steve Schreiber, Louis Schulte, Brad Smith, Coleman Smith, Wayne Smith, and Steve Yarbro.

Figure 1. Acid recycle and recovery system.

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