MOLTEN SALT OXIDATION FOR THE EFFICIENT DESTRUCTION OF RADIOACTIVE, HAZARDOUS CHEMICAL, MEDICAL WASTE AND MUNITIONS

摘要

The technique of Molten Salt Oxidation, MSO, involves the simultaneous immersion of waste materials and air into a bath of molten carbonate and the subsequent conversion and release of the carbon and hydrogen present in the waste as CO2 and steam, respectively, together with unreacted nitrogen from the air: all other elements present remain and are concentrated in the melt. MSO was first employed in the 1960s and later the US Government spent over $40 million seeking to understand the process and make it faster and more efficient in the oxidation of, in particular, paper and cotton. Since these objectives were not achieved further funding was not forthcoming. Recently we have developed MSO for the oxidation of UO2 into uranate, and outlined a possible procedure by which spent nuclear fuel rods can be treated in a molten carbonate bath to separate out uranates and plutonates and subsequently precipitate out the major fission products. Developing an efficient conversion procedure for UO2 to uranates, by increasing the concentration of peroxide and superoxide ions generated in the melt, led to re-examining the MSO process for the destruction of waste materials. The earlier studies had not shown full understanding of the generation and role of these species in effective MSO. To sustain the high concentration of superoxide ions we realized that this could be achieved with the addition of nitrate ion as catalyst. Since oxygen dissolves chemically, and not physically, in molten carbonate, it enters solution as the peroxide ion, O 2- . This ion reacts with nitrate to form superoxide and nitrite, which is re-oxidized to nitrate by peroxide, thereby continuously providing superoxide to oxidize organic waste. Since all other inorganic gases that could be formed, oxides of nitrogen, sulfur, chlorine, etc., are acidic, they are retained in the alkaline carbonate melt. Using a carbonate melt containing 5% KNO3 we now found that paper, cotton and plastics could be fully oxidized, with no remaining char, as had been found previously. Clothing and paper-wipes, etc., having low and medium level radioactive contamination can now be oxidized. The radioactive elements will remain in the molten carbonate bath. The earlier work in this area had shown that all possible radioactive splashes and carryover can be retained using a double filtration system. To establish the efficiency of MSO, now with added catalyst, we used the system to attempt to oxidise the most difficult waste product accumulating today, scrap tires. EU regulations currently prohibit the burying of scrap tires and will shortly ban the burying of chipped tires. Burning tires in cement kilns will also be banned because of the high Nox emissions resulting. Using 5% KNO3 in the carbonate melt all the organic compounds used in modern tires manufacture were oxidized, leaving only carbon black, which could potentially be recycled, but after de-vulcanization, the carbon black was also fully oxidized.