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Sodium Hydroxide Production from Electrolytic Cell with Ceramic Membrane

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Sodium Hydroxide Production from Electrolytic Cell with Ceramic Membrane Europaisches Patentamt European Patent Office OffOffice europeen des brevets © Publication number : 0 559 400 A2 12) EUROPEAN PATENT APPLICATION © Application number : 93301493.8 © int. ci.5: C25B 9/00, C25B 1/16, C25B 1/22 (g) Date of filing : 26.02.93 @ Priority : 28.02.92 US 843509 @ Inventor : Joshi, Ashok 3377 South 2410 East Salt Lake City, UT 84105 (US) @ Date of publication of application : Inventor : Liu, Meilin 08.09.93 Bulletin 93/36 1346 East 4170 South Salt Lake City, UT 84124 (US) Inventor : Bjorseth, Alf @ Designated Contracting States : Staverbakken 14 AT BE CH DE DK ES FR GB GR IE IT LI NL PT N-1300 Sandvika (NO) SE Inventor : Renberg, Lars c/o Storal Swedish Match, PO Box 16100 S-103 22 Stockholm (SE) © Applicant : CERAMATEC INC. 2425 South 900 West _ Salt Lake City, Utah 84119 (US) © Representative : Rees, David Christopher et al Kilburn & Strode 30 John Street London WC1N 2DD (GB) © Sodium hydroxide production from electrolytic cell with ceramic membrane. (57) An electrolytic cell (10) for producing caustic soda from sodium salts through a thin membrane (15) of ceramic or ceramic composite electrolyte. The membrane 15 separates anode and cathode chambers (12,11) and conveniently comprises NASICON. O O o> 10 10 LU Jouve, 18, rue Saint-Denis, 75001 PARIS EP 0 559 400 A2 The present invention relates to the production of caustic, and concurrently, strong acids via electrolysis of a sodium salt in an electrolytic cell. Electrolytic cells employing composite polymeric membranes are currently used to produce caustic solu- tions. Examples of such electrolytic cells are disclosed in an article entitled "Use Electrodialytic Membranes 5 for Waste Recovery", Nicholas Basta, Chemical Engineering, March 3, 1986 and an article entitled "The Be- haviour of Ion Exchange Membranes in Electrolysis and Electrodialysis of Sodium Sulphate", J. Jorissen and K.H. Simmrock, Journal of Applied Electrochemistry 1991. Such cells have polymeric membranes which are not efficient at higher concentrations of caustic and acid as well as at higher conversion ratios of salt to acid although they are resistant to strongly acidic and basic solutions. The operational life of such polymeric mem- 10 branes is very sensitive to impurity contaminations. For example, such cells are limited generally to caustic production at a maximum concentration of about 20% and to salt to acid conversion of about 50%. Higher caus- tic concentrations or salt to acid conversions severely limit the efficiency of the process. For instance, at a fifty percent conversion of salt to acid, the current or energy efficiency is only about sixty percent. According to one aspect of the invention, there is provided an electrolytic cell for the production sodium 15 hydroxide from a sodium salt derived from a strong acid comprising: an anode chamber; a cathode chamber; an acid resistant, electronically conductive anode in the anode chamber; an alkali resistant, electronically con- ductive cathode in the cathode chamber; and a membrane separating the anode chamber from the cathode chamber; characterised in that the membrane is solid, impervious, sodium ion conducting, electronically insu- lating, and resistant to acid and alkali. 20 The invention also extends to an electrolytic process for producing concentrated sodium hydroxide solution from an aqueous sodium salt solution having anions derived from a strong acid, which comprises: introducing electrons into an aqueous catholyte solution containing sodium hydroxide; removing electrons from an aqueous anolyte solution containing sodium cations and anions producing oxygen gas in the anolyte; producing hydro- gen gas in the catholyte; and producing an acid from the anions in the anolyte; from an acid; characterised by 25 conducting sodium ions through a solid integral, monolithic anolyte/catholyte separator containing mobile so- dium ions in its lattice to produce a concentrated solution of sodium hydroxide in the catholyte solution. The electrochemical cell of the present invention is particularly useful for producing, through electrolysis, a concentrated caustic (NaOH) solution from a sodium salt solution, preferably concentrated, in which the salt has an anion derived from a strong acid, especially sulphuric acid. The cell has an anolyte chamber, a catholyte 30 chamber, a pair of electrodes and a cation-conducting ceramic membrane, such as NASICON, separating the anolyte chamber from the catholyte chamber. Contrary to polymeric membranes, electrochemical cells having a ceramic membrane display high current efficiency such that at a one to one, proton to sodium ion, conversion rate (fifty percent), the current efficiency is about 94% or greater. At about 80% salt to acid conversion, the current efficiency is about 86%; polymers 35 drop to about 20% at this level. The cell of this invention operates at a relatively low temperature, generally less than 100°C, to produce a concentrated caustic solution in the catholyte chamber and a relatively concentrated acid, for example, sul- phuric acid, in the anolyte chamber. The cell can operate either continuously or batchwise. Hydrogen gas is produced in the catholyte chamber while oxygen is produced in the anolyte chamber through the electrolysis 40 of the water present in each chamber. The cell may also be used to produce caustic and chlorine from NaCI solutions. The cell is unique in its ability to employ a unitary ceramic or a composite ceramic membrane which is durable in a concentrated caustic solution and a strong acid. The ability of the cell to produce caustic in con- centrations up to about 50% NaOH is very advantageous. Conventional polymeric membrane electrolytic cells 45 are limited to caustic concentrations of about 20% NaOH, and so requires considerable evaporation to achieve concentrated caustic or dry, solid NaOH. Evaporation is an energy-inefficient operation. In addition, unlike polymeric membranes which can be damaged by trace amounts of alkaline earth metal cations (Ca++, Mg++), ceramic membranes are relatively less affected by impurity contaminants. Accordingly, for the systems based on polymeric membranes, pretreatment of a feed sodium salt solution to remove con- 50 taminants is an expensive process. The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is a schematic representation of a caustic electrolytic cell having a unitary (monolithic) ceramic membrane; 55 Figure 2 is a schematic representation of a caustic-producing electrolytic cell employing tubular ceramic electrolytes; Figure 3 is an elevation of a thin-film ceramic electrolyte sandwiched between two porous support mem- bers; 2 EP 0 559 400 A2 Figure 4 is a graph showing results from a laboratory cell for NaOH production of the ceramic membrane; Figure 5 is a schematic representation of an electrodialysis cell employing sodium ion-conducting mem- branes alternating with proton-conducting membranes; Figure 6(a) is a graph illustrating initial impedance spectra for a typical NASICON tube; 5 Figure 6(b) is a graph illustration initial ionic conductivity of the NASICON tube as a function of temperature; Figure 7(a) is a graph illustrating the impedance evolution of NASICON tubular cell; Figure 7(b) is a plot of the tube resistivity determined from the spectra of Figure 7(a); Figure 8 is a graph illustrating typical l-V characteristics of a tubular cell at two testing temperatures; Figure 9 is a plot of cell performance as a function of increasing cell temperature; 10 Figure 10 is a plot of cell performance as a function of NaOH concentration in the catholyte; Figure 11 is a plot illustrating the effectiveness of certain electrode materials; Figure 12 is a graph illustrating the characteristics of several electrolytic cells having NASICON tubes of differing conductivity; Figure 13 is a schematic section through a composite ceramic-polymer electrolyte; 15 Figure 14 is a schematic section through a polymer-wrapped ceramic tube electrolyte; Figure 15 is a graph illustrating cell performance using sodium chlorate at an operating temperature of 50°C; Figure 16 is a graph illustrating cell performance using sodium chlorate operated at room temperature; Figure 17 is a graph illustrating the effect of salt to acid conversion on current efficiency of polymeric mem- 20 branes; and Figure 18 is a graph illustrating the effect of NaOH and acid concentrations on the current efficiency of polymeric membranes. Figure 1 shows a cell having a corrosion resistant container 10, a catholyte chamber 11, an anolyte cham- ber 12, an anode 13, preferably porous WC, a cathode 14, preferably porous silver, and a ceramic sodium ion- conducting electrolyte 15. In the cell of Figure 1, the ceramic membrane is a very thin unitary ceramic such 25 as NASICON (Trade Mark), which is structurally supported by the cathode and the anode, each of which is porous. NASICON is a cation-conducting ceramic material having the following composition:- Na2.94Zr1.49P0.8O10.85 While a thin ceramic ion-conductive membrane is very effective and efficient, it is not necessary for either 30 the cathode or anode to contact the electrolyte. Both the catholyte and anolyte are ion-conductive so that from an electrical standpoint the electrodes may be remote from the sodium ion-conductive membrane. In such an event, a thin-film dense ceramic membrane may be deposited on a porous substrate, which does not have to be an electrode. A sodium salt, such as an aqueous solution of sodium sulphate, is charged into the anolyte chamber. A 35 very dilute solution of NaOH is charged into the catholyte chamber. It is desirable to start with an electrolyte solution of some sort in the catholyte chamber since pure water is not a very good electrolyte. An example of an overall electrolytic reaction, using sodium sulphate, is as follows: Equation (1): Na2So4 + 3H20 2NaOH + H2S04 + V2O2 + H2 The half-cell reactions may be summarised as follows: 40 Equation (2): (anode) Na2S04 + H20 -» 2Na+ + H2S04 + 2e + 1/202t Equation (3): (cathode) 2Na+ + 2H20 + 2e -» 2NaoH + H2t The above-identified reactions are electrolytic reactions, i.e.
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