Caustic recovery from alkaline nuclear waste by an electrochemical separation process

Large quantities of highly-alkaline, radioactive liquid wastes are stored i n underground tanks at the Department of Energy's Savannah River, Hanford a nd Oak Ridge sites. These wastes will be pre-treated to separate radionucli des and the remaining decontaminated liquid wastes will be incorporated int o solid wasteforms for permanent disposal. Significant savings in disposal costs could result by recovering the sodium hydroxide for re-use. Laboratory tests demonstrated an electrochemical separation process for the recovery of sodium hydroxide from simulated and radioactive Savannah River Site (SRS) wastes. The separation process uses an electrochemical cell sep arated into two compartments with a cation-selective membrane. The waste so lution enters the anolyte side of the cell. Sodium ions migrate across the membrane into the catholyte. Hydroxide forms at the cathode by the reductio n of water. The catholyte product is thus sodium hydroxide or caustic. Hydr oxide oxidizes at the anode to form oxygen and water. The anolyte product i s thus a sodium hydroxide-depleted waste stream. If nitrite is present in t he waste solution, the nitrite also oxidizes to nitrate. Two different membranes were evaluated: an organic-based membrane, Nafion(R ) Type 324 or 350, and an inorganic-based membrane, Ceramatec(R) NASD. Init ial tests with the Nafion(R) membrane with simulated waste solutions indica ted that the migration of sodium across the membrane was not significantly affected by changes in the current density, temperature and the concentrati ons of nitrate/nitrite, hydroxide and aluminate. Both membranes successfull y separated caustic from radioactive SRS waste. (C) 1999 Westinghouse Savan nah River Company. Published by Elsevier Science Ltd. All rights reserved.