Treatment of Waste by the Molten Salt Oxidation Process at the Oak Ridge National Laboratory

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Summary Molten Salt Oxidation (MSO) process technology has been under development by Rockwell International to treat hazardous, radioactive, and mixed waste. To demonstrate technical feasibility of the process, the system has bee~ tested on a number of types of waste including simulated intermediate level waste (IL W) from the Oak Ridge National Laboratory (ORNL). The intermediate level waste streams consisted of a slurry of concentrated aqueous solutions of sodium hydroxide and wdium nitrate, with the a amount of miscellaneous combustible components such as poly-vinyl chloride (PVC), tri-butyl phosphate (TBP), kerosene. and ion exchange resins. The purposes of these tests were to destroy the organics, evaporate the water, and convert the hazardous salts (hydroxide and nitrate) into non-hazardous sodium carbonate. Results of the tests and the future possibilities for applying the MSO process to different waste streams at ORNL are discussed. Introduction The MSO process is an advanced waste treatment process which was developed to destroy hazardous wastes and convert mixed waste into a small volume of only radioactive wastes. The process was originally developed during the period of 1970-1984 when numerous waste treatment studies were made by Rockwell Internationap·2 that demonstrated excellent destruction (>99.9999%) of many hazardous wastes. Process economics were marginal at that time, but stringent regulations and a new emphasis on mixed waste disposal have led to renewed interest in the MSO process. MSO is most applicable to mixed wastes conSisting of combustible liquids and solids. The process can be used as a primary treatment in which the wastes arc oxidized directly in the molten salt vessel, or as a secondary treatment, in which waJtes high in inert material are oxidized in a rotary kiln or similar device, with the off-gcu being routed through the molten salt bath for polishing. Aqueous Waste Processing Of particular interest Co nuclear reprocessing operations are wastes high in water content but also containing organics such as tri-butyl phosphate (TBP) and kerosene. These wastes are typical of the aqueous mixed wastes from Purex reprocessing. They consist of a slurry of concentrated aqueous solution of sodium hydroxide and sodium nitrate with a small amounts of miscellaneous combustible components (PVC, TEP, kerosene. and ion exchange resins). The purposes of the molten salt oxidation system are to destroy the organics, evaporate the water from the waste, and retain the radioactive elements in the salt. The evaporation of the water content results in a large volume reduction in the waste. Because the waste itself has little heating value, a fuel must be added to it. Methanol was chosen as the fuel since it is highly miscible with water and concentrated aqueous solution. Since water and molten salts may interact violently, and the wastes contained strong oxidizing spent NaN~,, a series of tests was required to determine if such a material could be safely treated. A test program was performed consisting of: 1) oxidation of methanol-water mixtures, 2) oxidation of methanol-water­salt mixtures, and 3) oxidation of the simulated intermediate level waste slurry. A bench-scale (15.2-cm diameter) molten salt oxidation system was used for these studies. The system was located inside an enclosed walk-in hood with process controls located outside the hood.