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Electrodialysis Reduces Waste in the Treatment of Pyrochemical Salt Residues

Many of the processes used to treat residues from plutonium recovery and purification operations require the addition of large quantities of reagents. The result is substantial volumes of waste solutions that require additional treatment before they can be disposed of. Examples of such processes include aqueous treatment methods for pyrochemical salt residues. Typically, the chloride-based salts and other residues are dissolved in concentrated hydrochloric acid, additional reagents are added to adjust the plutonium oxidation state, then the plutonium is separated from the remainder of the matrix via ion exchange and/or solvent extraction. While the plutonium is collected in concentrated form, the original dissolver solution, rinse solution, eluant solution, and/or back-extraction solution remain to be treated before disposal.

Figure 2:Electrodialysis of a sodium nitrate solution. The positively charged sodium ions in the feed solution migrate through the cation exchange membrane toward the negatively charged cathode. The electrochemical reaction taking place at the cathode is the reduction of water to form hydrogen gas and hydroxide ions. The net result is formation of sodium hydroxide in the catholyte strip solution. The negatively charged nitrate ions in the feed solution migrate through the anion exchange membrane toward the positively charged anode. The electrochemical reaction taking place at the anode is oxidation of water to form oxygen and free protons. The net result is formation of nitric acid in the anolyte strip solution.

As part of a research effort to find more efficient processes that generate less waste, we have been investigating the use of electrodialysis to treat process residues. Electrodialysis uses an applied electrostatic field coupled with ion exchange membranes to split salts into their respective acid and base components. This process is shown schematically in Figure 2. The electrodialysis process has a very high efficiency and can be run until essentially all of the ions have been stripped from the feed solution. Its major industrial application is to create potable water from sea water by lowering the salt concentration in the feed solution down to the parts-per-million level.

Electrorefining (ER) salts, because of their relatively uncomplicated chemical composition, were the first materials chosen to demonstrate the applicability of electrodialysis to the treatment of plutonium process residues. These salts are predominantly a 1-to-1 mixture of sodium and potassium chloride salts containing small quantities of plutonium in the form of metal and salts, and lesser amounts of americium and other heavy metals salts.

A proposed flow sheet for treating ER salts by electrodialysis is shown in Figure 3. The salt is first dissolved in a minimum amount of approximately 1 M hydrochloric acid. The resulting solution is then passed through a three-compartment electrolysis cell. As the feed solution flows through the cathode compartment of this cell, the acidic solution is slowly neutralized as hydroxide is generated at the cathode. As the pH increases, the heavy metals, including plutonium, undergo a homogeneous precipitation and can be collected on a filter configured into the recycle loop. At this point the relatively small amounts of heavy metals and actinides will have been separated from the large residue matrix and can be further treated in concentrated form for disposal or recovery of the actinides.

Figure 3. Proposed Electrodialysis/ER Salt Flow Sheet.

The feed solution, which now contains only trace quantities of actinide elements, is then sent to the central compartment of a conventional electrodialysis cell for a salt-splitting operation. Here hydrochloric acid is generated in one strip solution while a mixture of potassium and sodium hydroxides is generated in another. The hydrochloric acid stream can be recycled to the head-end process for dissolution of the next ER salt to be processed and/or to be used for other chloride processes. Before it is finally disposed of, the electrogenerated sodium/potassium hydroxide solution can be used as a final polishing agent for both chloride- and nitrate-based waste solutions for further removal of actinides.

In the electrodialysis flow sheet the only reagent added is water; dissolution of the salt is accomplished by hydrochloric acid recycled from the electrodialysis operation. Separation of plutonium from the salt solution is carried out by homogeneous precipitation using electrogenerated hydroxide ions. No additional oxidizing or reducing agents, washing, or eluent solutions are required. The net result is a tremendous decrease in the quantity of reagents used and a corresponding decrease in the quantity of waste generated. Also since a majority of the solutions generated are recycled back into the system, there will be a greater probability for removal of the actinides from the process solutions with each successive cycle.

The proposed electrodialysis flow sheet was demonstrated in a "cold" experiment using neodymium as a surrogate for plutonium, and iron as a representative for all heavy metals that might be present in an actual ER salt residue. In the first electrolysis stage greater than 99.99% of the neodymium and iron were stripped from the feed solution and collected on the filter. In the second electrolysis stage the salts were stripped from the feed solution down to the parts-per-million level, and the final compositions of the strip solutions were approximately 6 M mixed sodium/potassium hydroxide and 3 M hydrochloric acid. The concentration of the alkaline solution is sufficient to use łas is˛ in the final polishing step of the waste stream solution. The acid solution is more concentrated than necessary to recycle to the head end of the process to dissolve the next ER salt, but it could be diluted with a quantity of the deionized feed solution. Other aqueous-chloride-based processes use much more concentrated hydrochloric acid solutions, requiring a preconcentration of the strip solution before use. A separate process to accomplish this task is currently under development.

Future work planned for this project calls for performing "hot" tests on actual ER salt residues. Once the electrodialysis technology has been demonstrated on this chemically noncomplex system, more chemically complex residues will be investigated. Of particular interest are the calcium-based direct oxide reduction salts, which contain many more heavy metal impurities and in greater concentrations than in the ER salts. However, a greater challenge than removing the actinides may reside in developing a flow sheet that can handle the calcium ion, which has only limited solubility in alkaline media.

In summary, preliminary investigations have demonstrated the applicability of electrodialysis to the treatment of selected plutonium process residues. The net benefits in the use of this technology are a tremendous decrease in the quantity of reagents used and corresponding decrease in volumes of waste generated compared to the aqueous recovery processes currently in use.

Wayne H. Smith, and Douglas E. Wedman of NMT-6 are the principal researchers.


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