Caustic Recycle from Hanford Tank Waste Using NaSICON Ceramic Membrane Salt Splitting Process [report]

Matthew S. Fountain, Dean E. Kurath, Gary J. Sevigny, Adam P. Poloski, J. Pendleton, S. Balagopal, M. Quist, D. Clay
2009 unpublished
ph: (865) 576-8401 fax: (865) 576-5728 email: Available to the public from the National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Rd., Springfield, VA 22161 ph: (800) 553-6847 fax: (703) 605-6900 email: online ordering: This document was printed on recycled paper. Summary A family of inorganic ceramic materials, called sodium (Na) Super Ion Conductors (NaSICON), has been studied
more » ... Pacific Northwest National Laboratory (PNNL) to investigate their ability to separate sodium from radioactively contaminated sodium salt solutions for treating U.S. Department of Energy (DOE) tank wastes. Ceramatec Inc. developed and fabricated a membrane containing a proprietary NAS-GY material formulation that was electrochemically tested in a bench-scale apparatus with both a simulant and a radioactive tank-waste solution to determine the membrane performance when removing sodium from DOE tank wastes. Implementing this sodium separation process can result in significant cost savings 1 by reducing the disposal volume of low-activity wastes and by producing a NaOH feedstock product for recycle into waste treatment processes such as sludge leaching, regenerating ion exchange resins, inhibiting corrosion in carbon-steel tanks, or retrieving tank wastes. This report presents the results of the actual waste tests associated with an NAS-GY formulation of the ceramic membrane and the electrochemical cell design incorporating the membranes. Conclusions from these experiments are as follows:  Sodium transport efficiencies ranged from 88 to 102% while average efficiencies were 93% (nonradioactive test) and 99% (both radioactive tests). The membrane appears to undergo an initial "break-in" period, which is assumed to be a transition from an unsaturated to a saturated Na channel network.  All three experiments produced Na transport rates in good agreement with theoretical Na transport rates based on applied current. Average Na separation rates of 9.6 kg/day/m 2 (simulant-1 M NaOH), 10.3 kg/day/m 2 (rad-1 M NaOH), and 10.3 kg/day/m 2 (rad-18.6 M NaOH) were observed.  The NAS-GY membrane is highly selective to sodium. No transport of any cations or anions was detected except for Na and 137 Cs. Decontamination factors on the order of 2000 were observed with respect to 137 Cs.  ICP-OES measurements clearly indicated Na ion transport, but also identified a decreased concentration of Al, Ca, and Si in the samples taken after the large voltage increases occurred.  The electrochemical cell system successfully produced a 19 M NaOH solution with no observable membrane performance loss. In addition, concentrating the NaOH product to 19 M NaOH satisfies the feedstock caustic requirement of DOE's WTP and would eliminate the need for a process evaporator when using other separation technologies.  A maximum recoverable sodium quantity could be predicted by combining both an aluminate solubility model and a gibbsite crystal growth rate. Testing shows cell voltages were relatively
doi:10.2172/951865 fatcat:7efl4e32k5gxdn7tmzrperdd6q