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Selective sodium removal from aqueous waste streams with NaSicon ceramics
Separation and Purification Technology 15 (1999) 231–237
Selective sodium removal from aqueous waste streams with NaSICON ceramics S. Balagopal, T. Landro, S. Zecevic, D. Sutija, S. Elangovan *, A. Khandkar Ceramatec, Inc., 2425 South 900 West, Salt Lake City, UT 84119-1517, USA Received 24 April 1998; accepted 1 July 1998
Abstract Recent developments in the synthesis and application of the sodium ion conducting polycrystalline Nasicon ceramics allow for selective removal of sodium from aqueous wastes at ambient temperatures by electrochemical salt splitting. In the presence of an applied electric field, sodium ions are transported through the Nasicon structure. The size and electroneutrality constraints allow for selective transport of sodium ions, and exclude other monovalent, divalent and trivalent ions present in the impure reactants from migrating through the membrane. The sodium transport efficiency for generating pure NaOH from nitrate and sulfate industrial wastes is greater than 90%. These ceramic membranes provide the added benefit of very low parasitic losses due to absence of fouling by precipitants. Electro-osmotic transport of H O through the membrane which is common to polymeric membrane technology is also not observed. 2 While the initial electrochemical evaluation of the ceramic membranes showed high sodium selectivity over other metal cations, the need for improvements in sodium conductivity, long term stability, and durability in strong acid was identified. A new series of Nasicon compositions have shown considerable improvements in properties and exhibit the potential for large-scale, industrial applications. © 1999 Elsevier Science B.V. Keywords: Conducting ceramics; Nuclear waste; Salt splitting; Sodium conductor
1. Introduction The use of sodium conducing b◊-alumina ceramic [1] was extensively studied in the 1970s as an electrolyte in sodium–sulfur batteries. The production of fine-grained sodium b◊-alumina which exhibits sodium conductivities of 0.1 S cm−1 at 300°C is among the earliest investigations of electrochemical technologies at Ceramatec. As a result of the instabilities of b◊-alumina against moisture, the development effort was concentrated on the * Corresponding author. E-mail: [email protected]
sodium-ion selective ceramics, Na Zr P O 3 2 3 12 (NZP) and Na RESi O , originally known as 5 4 12 sodium super-ionic conductors [2] (or Nasicon or RE-Nasicon for the rare-earth silicates). They possess nearly as high a conductivity and have the advantage of exhibiting three-dimensional transport of sodium ions. However, the stability of Nasicon materials against molten sodium was found to be inadequate for sodium–sulfur battery application. Based on the early work by Miller and coworkers [3], Ceramatec started the investigation of Nasicon family of materials for use in aqueous applications. Sodium conducting electrolytes have technologi-
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cal importance in various industrial applications. For example, the organic membrane electrolytes are commercially used in the chlor–alkali industry to generate chlorine and caustic NaOH. However, in such applications the incoming sodium chloride feed has only ppm level of impurities. In the case of industrial byproduct streams containing sodium salts, the most commonly used organic membrane, perfluoro-sulfonic-acid-based NafionA, is not selective to a specific cation, and therefore transports protons and other cations from the impure reactants. Certain cations can precipitate during migration though the membrane if pH gradient exists. Precipitation of solids in the membrane causes fouling, and loss of conductivity [4]. Another application of critical importance is the separation of sodium from radioactive waste stored in the Department of Energy (DOE ) waste tanks. Removal of non-radioactive sodium is beneficial in lowering the weight and disposal cost of the waste. During the separation of Na+ and Cs+ by electrodialysis with a NafionA 417 membrane [5], cesium ions transported faster than sodium ions. Since the radioactive wastes are comprised of radioisotopes of cesium and strontium, the organic membranes will not provide any advantage in separation of sodium from radionuclides. Nasicon ceramic membranes demonstrate a distinct advantage over polymeric membranes in the treatment of impure byproducts from industrial processes. Since the conductivity of Nasicon [6,7] and RE-Nasicon [8] is attributable to the hopping of Na+ ions on vacancies through the lattice with a single mobile ionic species [Fig. 1(a)], they act as a sodium selective electrolyte. The oxygenshared and corner-linked polyhedra, provides the structural stability towards chemical and physical degradation. The sodium conducting channels in the structure [Fig. 1(b)] reject anions (e.g., AlO− , PO3− , SO2− etc.), owing to electroneu2 4 4 trality constraints and larger monovalent ions ( K+, Cs+, Ag+, Rb+, etc.) and divalent and trivalent metal ions due to size constraints. Based on the discussions above, Nasicon membranes offer significant benefits over the polymer membrane in the separation of sodium from aqueous solutions of sodium salts. Ceramatec has
undertaken the development of materials compositions, membrane fabrication, characterization, and electrochemical testing under conditions suitable for end-use applications under various DOE co-funded programs.
2. Applications of Nasicon 2.1. Salt splitting of sodium sulfate Nasicon ceramics are being investigated for use as electrolytes for recycling of waste sodium sulfate salt-cake produced in the pulp and paper industries, particularly in bleached paper mills [9]. The economics of this process will become increasingly beneficial as environmental regulations become more stringent. Currently, in the United States, about 800 000 tons year−1 of sodium sulfate are sewered or landfilled. Recycling of this waste can produce 550 000 tons year−1 of caustic soda for use in the bleach process and 675 000 tons year−1 of sulfuric acid for chlorine dioxide production and pH adjustment in the mill. The electrolysis of aqueous solution of sodium sulfate using a Nasicon based ceramic membrane to give sulfuric acid and sodium hydroxide is carried out according to the overall reaction given by: 2Na SO +6H O=2H SO +4NaOH 2 4 2 2 4 +2H +O . 2 2 Long-term test results show excellent stability in conductivity of RE-Nasicon membranes in caustic solutions. However, initial results show that the conductivity of RE-Nasicon membranes degrades quickly due to corrosion in acidic solutions. In order to reduce the degradation in the acidic medium, a RE-Nasicon membrane with a porous coating (10 mm) of an acid resistant sodium composition (s =10−5S cm−1), was tested in an 25°C electrochemical cell for over 1000 h in a batch mode with a pH 0.2 anolyte solution containing 1 M sodium sulfate. The catholyte was 1 M NaOH. The coated layer provided mechanical support to bulk RE-Nasicon membrane from degradation, but ultimately the membrane was chemically attacked after 1000 h of performance from ion-
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Fig. 1. (a) Projection of the (NZP) structure on the 011 crystallographic plane [6 ]; and (b) the basic NZP structure.
exchange induced corrosion. The sodium transport efficiency (<60%), and conductivity of the Dy-Nasicon fell rapidly during the course of the test. Thus, further development of materials composition and/or coating was identified as a developmental issue.
2.2. Electrosynthesis of crude tall oil from black liquor soap Chemical acidulation of soap requires the use of high-grade sulfuric acid and disposal of an increasingly unwanted by-product, sodium sulfate. Electrochemical acidulation of black liquor soap to tall oil and an acidified aqueous phase was generated with the Dy-Nasicon membrane (3 cm2 surface area) [9]. The process involves separation of sodium ions from the black liquor soap by transport through the ceramic membrane. The cell was operated at 4 V and current density of 50 mA cm−2. A final yield of 600 ml of crude tall oil from 1 l of black liquor soap was achieved indicating the viability of the electrochemical process.
2.3. Nuclear waste applications Wastes from nuclear defense materials production are dominated by sodium based salts. Approximately 1 billion liters of radioactive wastes are stored in tanks at Hanford, Savannah River, and Oak Ridge. These wastes consist mainly of sodium salts contaminated by aluminum, radioactive cesium, strontium, and residual sodium hydroxide from the neutralization of acidic waste streams. Therefore, selective removal of sodium from these wastes with ion conducting Nasicon membranes provides a dual benefit: waste disposal volumes are diminished, and valuable chemicals (NaOH ) are made in the process that can be used on-site. This greatly reduces the volume and the disposal cost of the radioactive waste. A simulated (non-radioactive) waste containing sodium hydroxide, sodium nitrate, sodium nitrite, and sodium aluminate was neutralized by sodium removal using a RE-Nasicon membrane. A single membrane electrochemical cell using Dy-Nasicon was tested using a Hanford waste simulant anolyte feed. Sodium transport efficiency of the ceramic membranes was determined and the endurance
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tests conducted [10,11]. The results are shown in Fig. 2, where the initial sodium current efficiency was near 100%, and then dropped to a steady level of 40%. At a pH of below 12, gibbsite (AlOH ) 3 precipitated at the anode, but the precipitate did not affect the current density. The drop in sodium transport efficiency, due to degradation of Dy-Nasicon membrane, emphasizes the need for the development and testing of new membrane compositions with high conductivity and stability.
Fig. 2. Long-term testing of the single Dy-Nasicon membrane.
3. Characterization of novel Nasicon compositions 3.1. Conductivity measurement A new series of Nasicon compositions were synthesized to enhance the ionic conductivity and electrochemical stability to ion-exchange induced corrosion. The ionic conductivity of the new Nasicon series was determined by linear sweep voltammetry (LSV ) using a four-electrode potentiostat (Solartron SI 1286 Electrochemical Interface) and two reference saturated-calomel electrodes (SCE). The reference electrodes were placed in custom-designed Luggin capillaries whose tips touched the membrane on each side. Two outlets of the potentiostat were connected to the reference electrodes, the other two outlets connected to the anode and cathode of the electrochemical cell. The voltage sweep rate and the voltage drop across the membrane were limited to 30 mV s−1 and 2 V, respectively. The measurements were carried out at temperatures ranging from 22 to 65°C compatible with the temperature range of the intended aqueous applications. The results are shown in Fig. 3, in which Nasicon compositions are designated as NASD, NASE and NASG. The literature data reported by Shannon et al. [12] are also shown for comparison. Fig. 3 demonstrates that the new Nasicon series have considerably higher ionic conductivity values. For example, the NASG composition has a conductivity more than one order of magnitude higher than that for Dy-Nasicon [8,12].
Fig. 3. The sodium ionic conductivity of the new Nasicon series measured by the LSV technique.
3.2. Chemical durability testing of new membranes The chemical durability of new membrane series was analyzed in the absence of electrical field in caustic solution. Dense membranes of NASD, NASE, and NASG, were immersed in 1.5 M NaOH solution and tested at 40°C. The membranes were periodically taken out of solution, washed and the weight was monitored. The results of this test, shown in Fig. 4, indicate that the membranes are very stable in caustic, and the weight loss is less than 0.1% per 1000 h of testing. This test was performed for a duration of 150 days.
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Fig. 4. The chemical durability testing of the Nasicon membranes in 1.5 M NaOH at 40°C.
3.3. Testing of new generation of Nasicon in waste simulants Tests were conducted using a single membrane cell configuration to determine membrane lifetime, sodium transport efficiency and the conductivity of the NASD, NASE and NASG compositions. The NASD membrane cell (1.4 mm thick) was operated in batch mode in NaOH solution, with fresh solution periodically replacing the replenished anolyte. The cell was continuously operated at 4.5 V and 40°C for 5000 h. Fig. 5 shows the test result, where the sodium ionic current is compared to the total current. The sodium transport efficiency achieved is very high (>90%) and remains
constant up to 3000 h, and drops below 90% between 3000 and 5000 h of testing. The transport efficiency was measured periodically when a fresh batch of anolyte was introduced. Though the membrane was operated at a relatively small current density of 25 mA cm−2, it is evident from this test that the sodium transport efficiency is very high and steady at 90%, and most importantly, that the NASD membrane was structurally stable during the entire duration of the test. This successfully demonstrates the potential of Nasicon membranes in alkaline waste treatment. A NASG membrane cell (1.4 mm thick) was operated using a simulated Savannah River Site (SRS ) anolyte, and 1.5 M NaOH catholyte. The composition of the simulant used is shown in Table 1. The purpose of this test was to validate the influence of various cations and anions present in the anolyte on the electrochemical performance of the ceramic membrane. This test was operated at 50°C, and constant current density of 200 mA cm−2, significantly higher than the previous test. The results are shown in Fig. 6, in which the sodium current density is compared with total current density. High sodium current efficiency (90%) was observed again which remained constant during the 1000 h of testing. The total applied voltage in this test was unusually high (8 V ) because of the cell design where the electrodes were positioned far away from the membrane. The steady state conductivity of the NASG membrane during the test, based on the voltage measurement across the membrane, was 2×10−2 S cm−1. The sodium mass balance of anolyte and the catholyte Table 1 Simulant composition: Savannah River Site (SRS )
Fig. 5. The electrochemical performance of the NASG membrane in SRS stimulant.
Composition
Concentration (M )
NaNO 3 NaOH Na CO 2 3 Na SO 2 4 Na HPO –7H O 2 4 2 NaNO 2 NaCl NaF Na C O 2 2 4
1.01 2.63 0.16 0.14 9×10−3 0.6 0.02 0.02 1×10−3
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of these performance characteristics clearly demonstrate the potential for recycling sodium from nuclear waste streams. 3.4. Testing in radioactive wastes
Fig. 6. The electrochemical performance of NASD100 in caustic, at 4.5 V.
in this experiment did not show any loss of sodium. During the course of this test, Al(OH ) precipi3 tated in the anolyte when the pH of the solution dropped below 12. This, however, did not influence the performance of membrane. A NASG membrane with a thickness of 0.7 mm was tested in an electrochemical cell using 5 M NaOH anolyte, at 400 mA cm−2, and 50°C. The results are shown in Fig. 7, in which the sodium current remains higher than 90% of the total current despite the higher current density. This test indicated that the membrane resistance can be significantly reduced by reducing its thickness. Overall bulk resistance of the membrane is further reduced by increasing electrolyte temperature. All
Prior tests [11] on Dy-Nascicon membrane did not show any radiation damage upon exposure to relatively large amounts of radiation. To further demonstrate the applicability of these membranes, a scaffold containing eight NASD membranes, with a total area of 25 cm2, as well as a scaffold containing 58 membranes with a total area of 820 cm2 were constructed for bench scale testing. Both scaffolds were tested at Westinghouse Savannah River Technology Center (SRTC ). Hobbs [13] recently completed the electrochemical separation of caustic from actual radioactive waste from their treatment plant with both NafionA and the smaller NASD scaffold. High electrical efficiencies were observed with the scaffolded NASD membranes with negligible H O transport 2 in comparison to the NafionA membrane. Moreover, no 137Cs was transported from the anolyte to the catholyte validating the NASD membrane to be Na ion selective. In contrast, in a NafionA cell, ≥60% of 137Cs migrated from the anolyte into the catholyte. The ceramic membrane was also very stable in low-level gamma irradiation in comparison to the NafionA membranes. Over the time frame of the tests, neither membrane exhibited radiation damage as would be expected due to short duration of tests.
4. Conclusions
Fig. 7. The electrochemical performance of the NASG membrane in caustic NaOH.
Significant advances in ceramic Nasicon membranes have been made. Benefits in applications for separation of sodium from industrial and nuclear wastes at moderate temperatures have been clearly demonstrated. The new series of Nasicon membranes developed at Ceramatec posses high ionic selectivity and conductivity (2×10−2 S cm−1 at 50°C ). Independent tests at SRTC demonstrate separation of Na from radionuclides at high conversion efficiencies. The NASD membranes were tolerant to low level gamma irradia-
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tion. Salt splitting of acidic wastes from the paper and pulp industry into high-value chemicals such as NaOH and H SO , and the acidulation of black 2 4 liquor soap are potential applications for Nasicon membranes for recycling of industrial wastes into value added products. In comparison to RE-Nasicon, which showed chemical degradation, and loss of sodium transport efficiency (measured as the ratio of cell current due to sodium transport to the total current through the system) in acidic and caustic wastes, the new Nasicon series (NASD, NASG) show excellent stability. Cells with these membranes operated successfully in excess of 5000 h at very high sodium transport efficiency (>90%) and at moderate current densities below 50 mA cm−2. The sodium transport efficiency of higher than 90% was also achieved with a NASG cell at high current densities of 400 mA cm−2. The NASD and NASG membranes are also resistant to fouling in caustic and acidic precipitants and have shown no evidence of multivalent anion migration as commonly observed with the NafionA membranes.
Acknowledgments The work performed at Ceramatec was partially funded by the US Department of Energy through cooperative agreement no. DE-FCO2-95CE41158, and under a sub-contract from Battelle, Pacific Northwest National Laboratories, under contract DE-AC06-76RLO 1830. We would like to express our gratitude to Dr Dean Kurath of Pacific Northwest Laboratories, Richland, Washington, and Dr David Hobbs of Savannah River
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Technology Center, Aiken, South Carolina for many helpful discussions.
References [1] M.S. Whittingham, R.A. Huggins, Natl. Bur. Stand. ( US ) (special publication) 364 (1972) 139. [2] J.B. Goodenough, H.Y.P. Hong, J.A. Kafalas, Fast Na+-ion transport in skeleton structures, Mater. Res. Bull. 11 (1976) 203. [3] J.R. Rasmussen, G.R. Miller, R.S. Gordon, Electrorefining of sodium using sodium beta-alumina ionic conductors, Electrochemical Society Meeting, 1982. [4] G.W. Hollenberg, D.E. Kurath, K.P. Brooks, S. Balagopal, T. Landro, Ceramic membranes for separation of sodium, J. Am. Ceramic Soc. (submitted ). [5] M.F. Buehler, W.E. Lawrence, J.D. Norton, Evaluation of aqueous Na+/Cs+ separation by electrodialysis, PNL-9022, Battelle Pacific Northwest National Laboratory, Richland, WA, 1993. [6 ] L. Hagman, P. Kierkegaard, The crystal structure of NaMeIV(PO ) , MeIV=Ge, Ti, Zr, Acta Chem. Scand. 22 2 43 2 (1968) 1822. [7] J. Alamo, R. Roy, Crystal chemistry of the NaZr (PO ) 2 43 structure family, J. Mater. Sci. 21 (1986) 33. [8] S.H. Balagopal, J.H. Gordon, A.V. Virkar, A.V. Joshi, US patent no. 5 580 430, 1996. [9] D.P. Sutija, S. Balagopal, T. Landro, J. Gordon, Ceramic cleansers: environmental applications of sodium superionic conducting ceramics , Electrochem. Soc. Interface 5 (4) (1996) 26. [10] D.E. Kurath, K.P. Brooks, G.W. Hollenberg, D.P. Sutija, T. Landro, S. Balagopal, Caustic recycle from high-salt nuclear wastes using a ceramic-membrane salt-splitting process , Sep. Purif. Technol. 11 (1997) 185. [11] D.E. Kurath, G.W. Hollenberg, J. Jue, J. Smith, A.V. Virkar, S. Balagopal, D.P. Sutija, Salt splitting using ceramic membranes, Sep. Sci. Technol. 32 (1997) 557. [12] R.D. Shannon, B.E. Taylor, T.E. Gier, H.Y. Chen, T. Berzins, Ionic conductivity in Na YSi O type silicates, 5 4 12 Inorg. Chem. 17 (4) (1978) 958. [13] D.T. Hobbs, Caustic recovery from alkaline nuclear waste by an electrochemical separation process, Sep. Purif. Technol. (submitted ).
Selective sodium removal from aqueous waste streams with NaSICON ceramics S. Balagopal, T. Landro, S. Zecevic, D. Sutija, S. Elangovan *, A. Khandkar Ceramatec, Inc., 2425 South 900 West, Salt Lake City, UT 84119-1517, USA Received 24 April 1998; accepted 1 July 1998
Abstract Recent developments in the synthesis and application of the sodium ion conducting polycrystalline Nasicon ceramics allow for selective removal of sodium from aqueous wastes at ambient temperatures by electrochemical salt splitting. In the presence of an applied electric field, sodium ions are transported through the Nasicon structure. The size and electroneutrality constraints allow for selective transport of sodium ions, and exclude other monovalent, divalent and trivalent ions present in the impure reactants from migrating through the membrane. The sodium transport efficiency for generating pure NaOH from nitrate and sulfate industrial wastes is greater than 90%. These ceramic membranes provide the added benefit of very low parasitic losses due to absence of fouling by precipitants. Electro-osmotic transport of H O through the membrane which is common to polymeric membrane technology is also not observed. 2 While the initial electrochemical evaluation of the ceramic membranes showed high sodium selectivity over other metal cations, the need for improvements in sodium conductivity, long term stability, and durability in strong acid was identified. A new series of Nasicon compositions have shown considerable improvements in properties and exhibit the potential for large-scale, industrial applications. © 1999 Elsevier Science B.V. Keywords: Conducting ceramics; Nuclear waste; Salt splitting; Sodium conductor
1. Introduction The use of sodium conducing b◊-alumina ceramic [1] was extensively studied in the 1970s as an electrolyte in sodium–sulfur batteries. The production of fine-grained sodium b◊-alumina which exhibits sodium conductivities of 0.1 S cm−1 at 300°C is among the earliest investigations of electrochemical technologies at Ceramatec. As a result of the instabilities of b◊-alumina against moisture, the development effort was concentrated on the * Corresponding author. E-mail: [email protected]
sodium-ion selective ceramics, Na Zr P O 3 2 3 12 (NZP) and Na RESi O , originally known as 5 4 12 sodium super-ionic conductors [2] (or Nasicon or RE-Nasicon for the rare-earth silicates). They possess nearly as high a conductivity and have the advantage of exhibiting three-dimensional transport of sodium ions. However, the stability of Nasicon materials against molten sodium was found to be inadequate for sodium–sulfur battery application. Based on the early work by Miller and coworkers [3], Ceramatec started the investigation of Nasicon family of materials for use in aqueous applications. Sodium conducting electrolytes have technologi-
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cal importance in various industrial applications. For example, the organic membrane electrolytes are commercially used in the chlor–alkali industry to generate chlorine and caustic NaOH. However, in such applications the incoming sodium chloride feed has only ppm level of impurities. In the case of industrial byproduct streams containing sodium salts, the most commonly used organic membrane, perfluoro-sulfonic-acid-based NafionA, is not selective to a specific cation, and therefore transports protons and other cations from the impure reactants. Certain cations can precipitate during migration though the membrane if pH gradient exists. Precipitation of solids in the membrane causes fouling, and loss of conductivity [4]. Another application of critical importance is the separation of sodium from radioactive waste stored in the Department of Energy (DOE ) waste tanks. Removal of non-radioactive sodium is beneficial in lowering the weight and disposal cost of the waste. During the separation of Na+ and Cs+ by electrodialysis with a NafionA 417 membrane [5], cesium ions transported faster than sodium ions. Since the radioactive wastes are comprised of radioisotopes of cesium and strontium, the organic membranes will not provide any advantage in separation of sodium from radionuclides. Nasicon ceramic membranes demonstrate a distinct advantage over polymeric membranes in the treatment of impure byproducts from industrial processes. Since the conductivity of Nasicon [6,7] and RE-Nasicon [8] is attributable to the hopping of Na+ ions on vacancies through the lattice with a single mobile ionic species [Fig. 1(a)], they act as a sodium selective electrolyte. The oxygenshared and corner-linked polyhedra, provides the structural stability towards chemical and physical degradation. The sodium conducting channels in the structure [Fig. 1(b)] reject anions (e.g., AlO− , PO3− , SO2− etc.), owing to electroneu2 4 4 trality constraints and larger monovalent ions ( K+, Cs+, Ag+, Rb+, etc.) and divalent and trivalent metal ions due to size constraints. Based on the discussions above, Nasicon membranes offer significant benefits over the polymer membrane in the separation of sodium from aqueous solutions of sodium salts. Ceramatec has
undertaken the development of materials compositions, membrane fabrication, characterization, and electrochemical testing under conditions suitable for end-use applications under various DOE co-funded programs.
2. Applications of Nasicon 2.1. Salt splitting of sodium sulfate Nasicon ceramics are being investigated for use as electrolytes for recycling of waste sodium sulfate salt-cake produced in the pulp and paper industries, particularly in bleached paper mills [9]. The economics of this process will become increasingly beneficial as environmental regulations become more stringent. Currently, in the United States, about 800 000 tons year−1 of sodium sulfate are sewered or landfilled. Recycling of this waste can produce 550 000 tons year−1 of caustic soda for use in the bleach process and 675 000 tons year−1 of sulfuric acid for chlorine dioxide production and pH adjustment in the mill. The electrolysis of aqueous solution of sodium sulfate using a Nasicon based ceramic membrane to give sulfuric acid and sodium hydroxide is carried out according to the overall reaction given by: 2Na SO +6H O=2H SO +4NaOH 2 4 2 2 4 +2H +O . 2 2 Long-term test results show excellent stability in conductivity of RE-Nasicon membranes in caustic solutions. However, initial results show that the conductivity of RE-Nasicon membranes degrades quickly due to corrosion in acidic solutions. In order to reduce the degradation in the acidic medium, a RE-Nasicon membrane with a porous coating (10 mm) of an acid resistant sodium composition (s =10−5S cm−1), was tested in an 25°C electrochemical cell for over 1000 h in a batch mode with a pH 0.2 anolyte solution containing 1 M sodium sulfate. The catholyte was 1 M NaOH. The coated layer provided mechanical support to bulk RE-Nasicon membrane from degradation, but ultimately the membrane was chemically attacked after 1000 h of performance from ion-
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Fig. 1. (a) Projection of the (NZP) structure on the 011 crystallographic plane [6 ]; and (b) the basic NZP structure.
exchange induced corrosion. The sodium transport efficiency (<60%), and conductivity of the Dy-Nasicon fell rapidly during the course of the test. Thus, further development of materials composition and/or coating was identified as a developmental issue.
2.2. Electrosynthesis of crude tall oil from black liquor soap Chemical acidulation of soap requires the use of high-grade sulfuric acid and disposal of an increasingly unwanted by-product, sodium sulfate. Electrochemical acidulation of black liquor soap to tall oil and an acidified aqueous phase was generated with the Dy-Nasicon membrane (3 cm2 surface area) [9]. The process involves separation of sodium ions from the black liquor soap by transport through the ceramic membrane. The cell was operated at 4 V and current density of 50 mA cm−2. A final yield of 600 ml of crude tall oil from 1 l of black liquor soap was achieved indicating the viability of the electrochemical process.
2.3. Nuclear waste applications Wastes from nuclear defense materials production are dominated by sodium based salts. Approximately 1 billion liters of radioactive wastes are stored in tanks at Hanford, Savannah River, and Oak Ridge. These wastes consist mainly of sodium salts contaminated by aluminum, radioactive cesium, strontium, and residual sodium hydroxide from the neutralization of acidic waste streams. Therefore, selective removal of sodium from these wastes with ion conducting Nasicon membranes provides a dual benefit: waste disposal volumes are diminished, and valuable chemicals (NaOH ) are made in the process that can be used on-site. This greatly reduces the volume and the disposal cost of the radioactive waste. A simulated (non-radioactive) waste containing sodium hydroxide, sodium nitrate, sodium nitrite, and sodium aluminate was neutralized by sodium removal using a RE-Nasicon membrane. A single membrane electrochemical cell using Dy-Nasicon was tested using a Hanford waste simulant anolyte feed. Sodium transport efficiency of the ceramic membranes was determined and the endurance
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tests conducted [10,11]. The results are shown in Fig. 2, where the initial sodium current efficiency was near 100%, and then dropped to a steady level of 40%. At a pH of below 12, gibbsite (AlOH ) 3 precipitated at the anode, but the precipitate did not affect the current density. The drop in sodium transport efficiency, due to degradation of Dy-Nasicon membrane, emphasizes the need for the development and testing of new membrane compositions with high conductivity and stability.
Fig. 2. Long-term testing of the single Dy-Nasicon membrane.
3. Characterization of novel Nasicon compositions 3.1. Conductivity measurement A new series of Nasicon compositions were synthesized to enhance the ionic conductivity and electrochemical stability to ion-exchange induced corrosion. The ionic conductivity of the new Nasicon series was determined by linear sweep voltammetry (LSV ) using a four-electrode potentiostat (Solartron SI 1286 Electrochemical Interface) and two reference saturated-calomel electrodes (SCE). The reference electrodes were placed in custom-designed Luggin capillaries whose tips touched the membrane on each side. Two outlets of the potentiostat were connected to the reference electrodes, the other two outlets connected to the anode and cathode of the electrochemical cell. The voltage sweep rate and the voltage drop across the membrane were limited to 30 mV s−1 and 2 V, respectively. The measurements were carried out at temperatures ranging from 22 to 65°C compatible with the temperature range of the intended aqueous applications. The results are shown in Fig. 3, in which Nasicon compositions are designated as NASD, NASE and NASG. The literature data reported by Shannon et al. [12] are also shown for comparison. Fig. 3 demonstrates that the new Nasicon series have considerably higher ionic conductivity values. For example, the NASG composition has a conductivity more than one order of magnitude higher than that for Dy-Nasicon [8,12].
Fig. 3. The sodium ionic conductivity of the new Nasicon series measured by the LSV technique.
3.2. Chemical durability testing of new membranes The chemical durability of new membrane series was analyzed in the absence of electrical field in caustic solution. Dense membranes of NASD, NASE, and NASG, were immersed in 1.5 M NaOH solution and tested at 40°C. The membranes were periodically taken out of solution, washed and the weight was monitored. The results of this test, shown in Fig. 4, indicate that the membranes are very stable in caustic, and the weight loss is less than 0.1% per 1000 h of testing. This test was performed for a duration of 150 days.
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Fig. 4. The chemical durability testing of the Nasicon membranes in 1.5 M NaOH at 40°C.
3.3. Testing of new generation of Nasicon in waste simulants Tests were conducted using a single membrane cell configuration to determine membrane lifetime, sodium transport efficiency and the conductivity of the NASD, NASE and NASG compositions. The NASD membrane cell (1.4 mm thick) was operated in batch mode in NaOH solution, with fresh solution periodically replacing the replenished anolyte. The cell was continuously operated at 4.5 V and 40°C for 5000 h. Fig. 5 shows the test result, where the sodium ionic current is compared to the total current. The sodium transport efficiency achieved is very high (>90%) and remains
constant up to 3000 h, and drops below 90% between 3000 and 5000 h of testing. The transport efficiency was measured periodically when a fresh batch of anolyte was introduced. Though the membrane was operated at a relatively small current density of 25 mA cm−2, it is evident from this test that the sodium transport efficiency is very high and steady at 90%, and most importantly, that the NASD membrane was structurally stable during the entire duration of the test. This successfully demonstrates the potential of Nasicon membranes in alkaline waste treatment. A NASG membrane cell (1.4 mm thick) was operated using a simulated Savannah River Site (SRS ) anolyte, and 1.5 M NaOH catholyte. The composition of the simulant used is shown in Table 1. The purpose of this test was to validate the influence of various cations and anions present in the anolyte on the electrochemical performance of the ceramic membrane. This test was operated at 50°C, and constant current density of 200 mA cm−2, significantly higher than the previous test. The results are shown in Fig. 6, in which the sodium current density is compared with total current density. High sodium current efficiency (90%) was observed again which remained constant during the 1000 h of testing. The total applied voltage in this test was unusually high (8 V ) because of the cell design where the electrodes were positioned far away from the membrane. The steady state conductivity of the NASG membrane during the test, based on the voltage measurement across the membrane, was 2×10−2 S cm−1. The sodium mass balance of anolyte and the catholyte Table 1 Simulant composition: Savannah River Site (SRS )
Fig. 5. The electrochemical performance of the NASG membrane in SRS stimulant.
Composition
Concentration (M )
NaNO 3 NaOH Na CO 2 3 Na SO 2 4 Na HPO –7H O 2 4 2 NaNO 2 NaCl NaF Na C O 2 2 4
1.01 2.63 0.16 0.14 9×10−3 0.6 0.02 0.02 1×10−3
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of these performance characteristics clearly demonstrate the potential for recycling sodium from nuclear waste streams. 3.4. Testing in radioactive wastes
Fig. 6. The electrochemical performance of NASD100 in caustic, at 4.5 V.
in this experiment did not show any loss of sodium. During the course of this test, Al(OH ) precipi3 tated in the anolyte when the pH of the solution dropped below 12. This, however, did not influence the performance of membrane. A NASG membrane with a thickness of 0.7 mm was tested in an electrochemical cell using 5 M NaOH anolyte, at 400 mA cm−2, and 50°C. The results are shown in Fig. 7, in which the sodium current remains higher than 90% of the total current despite the higher current density. This test indicated that the membrane resistance can be significantly reduced by reducing its thickness. Overall bulk resistance of the membrane is further reduced by increasing electrolyte temperature. All
Prior tests [11] on Dy-Nascicon membrane did not show any radiation damage upon exposure to relatively large amounts of radiation. To further demonstrate the applicability of these membranes, a scaffold containing eight NASD membranes, with a total area of 25 cm2, as well as a scaffold containing 58 membranes with a total area of 820 cm2 were constructed for bench scale testing. Both scaffolds were tested at Westinghouse Savannah River Technology Center (SRTC ). Hobbs [13] recently completed the electrochemical separation of caustic from actual radioactive waste from their treatment plant with both NafionA and the smaller NASD scaffold. High electrical efficiencies were observed with the scaffolded NASD membranes with negligible H O transport 2 in comparison to the NafionA membrane. Moreover, no 137Cs was transported from the anolyte to the catholyte validating the NASD membrane to be Na ion selective. In contrast, in a NafionA cell, ≥60% of 137Cs migrated from the anolyte into the catholyte. The ceramic membrane was also very stable in low-level gamma irradiation in comparison to the NafionA membranes. Over the time frame of the tests, neither membrane exhibited radiation damage as would be expected due to short duration of tests.
4. Conclusions
Fig. 7. The electrochemical performance of the NASG membrane in caustic NaOH.
Significant advances in ceramic Nasicon membranes have been made. Benefits in applications for separation of sodium from industrial and nuclear wastes at moderate temperatures have been clearly demonstrated. The new series of Nasicon membranes developed at Ceramatec posses high ionic selectivity and conductivity (2×10−2 S cm−1 at 50°C ). Independent tests at SRTC demonstrate separation of Na from radionuclides at high conversion efficiencies. The NASD membranes were tolerant to low level gamma irradia-
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tion. Salt splitting of acidic wastes from the paper and pulp industry into high-value chemicals such as NaOH and H SO , and the acidulation of black 2 4 liquor soap are potential applications for Nasicon membranes for recycling of industrial wastes into value added products. In comparison to RE-Nasicon, which showed chemical degradation, and loss of sodium transport efficiency (measured as the ratio of cell current due to sodium transport to the total current through the system) in acidic and caustic wastes, the new Nasicon series (NASD, NASG) show excellent stability. Cells with these membranes operated successfully in excess of 5000 h at very high sodium transport efficiency (>90%) and at moderate current densities below 50 mA cm−2. The sodium transport efficiency of higher than 90% was also achieved with a NASG cell at high current densities of 400 mA cm−2. The NASD and NASG membranes are also resistant to fouling in caustic and acidic precipitants and have shown no evidence of multivalent anion migration as commonly observed with the NafionA membranes.
Acknowledgments The work performed at Ceramatec was partially funded by the US Department of Energy through cooperative agreement no. DE-FCO2-95CE41158, and under a sub-contract from Battelle, Pacific Northwest National Laboratories, under contract DE-AC06-76RLO 1830. We would like to express our gratitude to Dr Dean Kurath of Pacific Northwest Laboratories, Richland, Washington, and Dr David Hobbs of Savannah River
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Technology Center, Aiken, South Carolina for many helpful discussions.
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