POLYPROPYLENE OR POLYETHYLENE BASED SEPARATOR FOR USE IN ELECTROCHEMICAL CELLS FOR PRODUCING ALKALI METAL FERRATES

Abstract

The primary subject of the invention is a separator for separating the anode and cathode compartments in electrochemical cells, comprising (a) a support made of polyethylene and/or polypropylene fibres, and b) a Fe(III)-containing precipitate deposited on the support according to point a). In the separator, the support may be a woven or non-woven support, preferably a non-woven textile having a surface density of approx. 5-100 g/m.sup.2, preferably approx. 15-70 g/m2, more preferably approx. 25-40 g/m.sup.2.

Claims

1. A separator for separating the anode and cathode compartments in electrochemical cells, comprising (a) a support made of polyethylene and/or polypropylene fibres, and b) a Fe(III)-containing precipitate deposited on the support according to point a).

2. The separator according to claim 1, wherein the arithmetic mean of the pore sizes in the support is approx. 1-100 micrometers, preferably approx. 10-50 micrometers.

3. The separator according to claim 2, wherein the fibre thickness in the support is 5 to 50 micrometers, preferably 10 to 25 micrometers.

4. The separator according to claim 1, wherein the support is a woven or non-woven support, preferably a non-woven textile.

5. The separator according to claim 4, wherein the thickness of the non-woven support is preferably 0.1 to 1 mm, preferably 0.15 to 0.5 mm, more preferably 0.2 to 0.3 mm.

6. The separator according to claim 4, wherein the non-woven support has a surface density of approx. 5-100 g/m.sup.2, preferably approx. 15-70 g/m.sup.2, more preferably approx. 25-40 g/m.sup.2.

7. The separator according to claim 1, wherein the Fe(III)-containing precipitate comprises Fe(OH).sub.3, Fe.sub.2O.sub.3 and FeO(OH).

8. The separator according to claim 1, wherein the Fe(III)-containing precipitate is in an air-dry state.

9. A process for producing a separator for separating the anode and cathode spaces in electrochemical cells, the process being selected from the following process variants: a) a support made of polyethylene and/or polypropylene fibres is soaked in an aqueous solution of one or more water-soluble Fe(II) salt(s) and/or Fe (III) salt(s), then immersed in an aqueous alkali solution, where in case of Fe(II) salt the use of oxygen atmosphere, preferably air atmosphere is mandatory, then after the deposition of the Fe(III)-containing precipitate, the support is rinsed with distilled water and, if desired, dried; b) a support made of polyethylene and/or polypropylene fibres is soaked in an aqueous alkaline solution, then immersed in an aqueous solution of one or more water-soluble Fe(II) salt(s) and/or Fe(III) salt(s), where in case of Fe(II) salt the use of oxygen atmosphere, preferably air atmosphere is mandatory, then after the deposition of the Fe(III)-containing precipitate, the support is rinsed with distilled water and, if desired, dried; c) a support made of polyethylene and/or polypropylene fibres is soaked in a solution of one or more ferrate salt(s), preferably Na.sub.2FeO.sub.4 and/or K.sub.2FeO.sub.4, then immersed in distilled water and, after the deposition of the Fe(III)-containing precipitate, the fabrics are rinsed with distilled water and, if desired, dried; or d) a support made of polyethylene and/or polypropylene fibres is soaked in distilled water, then immersed in a solution of one or more ferrate salt(s), preferably Na.sub.2FeO.sub.4 and/or K.sub.2FeO.sub.4, and after the deposition of the Fe(III)-containing precipitate, the fabrics are rinsed with distilled water, and, if desired, dried; e) during the electrolysis of an alkali metal ferrate salt, preferably a Na.sub.2FeO.sub.4 and/or K.sub.2FeO.sub.4 salt, under known conditions, a support made of polyethylene and/or polypropylene fibres is used to separate the anode and cathode compartments, and then after the deposition of the Fe(III)-containing precipitate on the support, the support is rinsed with distilled water and, if desired, dried.

10. The process according to claim 9, wherein the support is made of polyethylene and/or polypropylene fibres, and a Fe(III)-containing precipitate is deposited on the support, wherein the arithmetic mean of pore sizes in the support is approx. 1-100 micrometers, preferably approx. 10-50 micrometers.

11. A method for the production of alkali metal ferrates, said method comprising producing the alkali metal ferrates in an electrochemical cell wherein an anode compartment and a cathode compartment of the electrochemical cell are separated by the separator of claim 1.

12. The method according to claim 11, wherein the alkali metal is sodium or potassium, preferably sodium.

13. A method for the production of alkali metal ferrates, said method comprising producing the alkali metal ferrates in an electrochemical cell wherein anode and cathode compartments of the electrochemical cell are separated by the separator which has been prepared by the process according to claim 9.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0065] FIG. 1: Electron micrograph of a polypropylene (PP) fleece fabric which can be used as a carrier in the separator according to the invention.

EXAMPLES

Example 1

[0066] Preparation of Sodium Ferrate Solution using 2 Layers of Composite Separator (“Pre-Treated” Membrane, PP Fleece Fabric: 30 g/m.sup.2)

[0067] a) Preparation of the separator

[0068] a1) In the separator, the support is made of a non-woven PP textile (“fleece” fabric) having a surface density of 30 g/m.sup.2. The electron micrograph of the PP fleece fabric used is shown in FIG. 1. The pore sizes are in the range of 1-100 micrometers, the fibre thickness is approx. 15 to 21 micrometers and the thickness of the support is approx. 0.2 and 0.3 mm.

[0069] Pouches were formed from the fleece fabric that could be fitted to the cathodes (size: approximately 17.5 cm×28.5 cm).

[0070] a2) Pretreatment of the support:

[0071] 1. The fleece fabric was thoroughly moistened with distilled water.

[0072] 2. The fleece fabric was soaked for 3 hours in an aqueous solution containing Na.sub.2FeO.sub.4 and sodium hydroxide at a concentration of 6 g/dm.sup.3 for ferrate ions and 14 mol/dm.sup.3 for sodium hydroxide.

[0073] 3. The fleece fabric containing sodium ferrate and the deposited iron(III) precipitate was soaked in distilled water for 5 minutes and then gently rinsed.

[0074] The separator prepared as above was used directly in the electrolysis cell. Two separator pouches were used on both cathode plates, keeping a distance of 2 to 3 mm between the cathode plate and the layer closer to it, as well as between the two layers, with polypropylene (PP) spacers.

[0075] c) Properties of the electrolysis cell and electrolysis conditions

[0076] The volume of the electrolysis cell was V.sub.c=2100 cm.sup.3. The outer jacket of the electrolysis cell was tempered by a water bath at 15° C., while the surface of the anode was tempered in built-in tempering tubes with water at approx. 32° C. The material of the electron-conducting phase of the anode is white cast iron. Anode geometric surface used: 1010 cm.sup.2. The arrangement of the anode and cathode plates is a symmetrical “sandwich” where two cathode plates enclose the anode. Electrolyte solution in the cell: 16 mol/dm.sup.3 NaOH solution. Electrolysis current strength: I=22 A. Cell resistance (determined by impedance measurement): 0.173Ω. The electric potential difference of the cell (cell voltage) during electrolysis is 3.5 to 4.1 V.

[0077] Duration of electrolysis: 4 h.

[0078] Efficient electrical energy consumption: E≈335 Wh.

[0079] Anode weight loss: Δm=6.65 g.

[0080] (a) Charge efficiency calculated from weight loss:

[00001]$\eta =\frac{6.65g}{55.85g.Math.{\mathrm{mol}}^{-1}}.Math.\frac{6\times 96485C.Math.{\mathrm{mol}}^{-1}}{22A\times 4h\times 3600s.Math.h^{-1}}=0.218$

[0081] b) Concentration of ferrate ions in the solution immediately after electrolysis (determined by spectrophotometry): c.sub.FeO.sub.4.sub.2−=6.64 g/dm.sup.3.

[0082] Charge efficiency calculated on the basis of spectrophotometry:

[00002]$\eta =\frac{6.64g.Math.{\mathrm{dm}}^{-3}\times 2.1{\mathrm{dm}}^3}{1119.84g.Math.{\mathrm{mol}}^{-1}}.Math.\frac{6\times 96485C.Math.{\mathrm{mol}}^{-1}}{22A\times 4h\times 3600s.Math.h^{-1}}=0.212$

[0083] Based on the above, the estimated purity of the product is:

[00003]$y=\frac{m_{\mathrm{Fe}\left(\mathrm{ferrate}\right)}}{{\Delta m}_{\mathrm{Fe}}}=\frac{6.5g}{6.65g}\approx 98\%$

[0084] c) Concentration of ferrate ions 4 hours after electrolysis in the solution stored at 10° C. (determined by spectrophotometry)=6.27 g/dm.sup.3.

[0085] The corresponding “effective” charge efficiency is: η=0.201

Example 2

[0086] Preparation of Sodium Ferrate Solution using 2 Layers of PP Fleece Fabric (30 g/m.sup.2) Separator (Membrane Prepared “In Situ”)

[0087] The separator used and the cell are as given in Example 1. Electrolyte solution in the cell: 16 mol/dm.sup.3 NaOH solution. Electrolysis current strength: I=22 A. Cell resistance (determined by impedance measurement): 0.171Ω. The electric potential difference of the cell (cell voltage) during electrolysis is 3.4 to 4.0 V. Duration of electrolysis: 4 h.

[0088] Efficient electrical energy consumption: E≈330 Wh.

[0089] Weight loss of the anode: Δm=6.72 g.

[0090] (a) Charge efficiency calculated from weight loss:

[00004]$\eta =\frac{6.72g}{55.85g.Math.{\mathrm{mol}}^{-1}}.Math.\frac{6\times 96485C.Math.{\mathrm{mol}}^{-1}}{22A\times 4h\times 3600s.Math.h^{-1}}=0.22$

[0091] b) Concentration of ferrate ions after electrolysis in the solution (determined by spectrophotometry): c.sub.FeO.sub.4.sub.2−=6.44 g/dm.sup.3.

[0092] Charge efficiency calculated on the basis of spectrophotometry:

[00005]$\eta =\frac{6.44g.Math.{\mathrm{dm}}^{-3}\times 2.1{\mathrm{dm}}^3}{1119.84g.Math.{\mathrm{mol}}^{-1}}.Math.\frac{6\times 96485C.Math.{\mathrm{mol}}^{-1}}{22A\times 4h\times 3600s.Math.h^{-1}}=0.206$

[0093] The corresponding “effective” charge efficiency is:

[00006]$y=\frac{m_{\mathrm{Fe}\left(\mathrm{ferrát}\right)}}{{\Delta m}_{\mathrm{Fe}}}=\frac{6.3g}{6.72g}\approx 94\%$

[0094] c) Concentration of ferrate ions 4 hours after electrolysis in the solution stored at 10° C. (determined by spectrophotometry) c.sub.FeO.sub.4.sub.2−=5.97 g/dm.sup.3.

[0095] The corresponding “effective” charge efficiency is: η=0.191.

[0096] It can be seen that all the essential end-state parameters of the process are worse than the corresponding values in Example 1.

Example 3 (Reference Example)

[0097] Preparation of Sodium Ferrate Solution Without the use of a Separator

[0098] The cell used is as given in Example 1. Electrolyte solution in the cell: 16 mol/dm.sup.3 NaOH solution. Electrolytic current: I=22 A. Cell resistance (determined by impedance measurement): 0.153Ω. The electric potential difference of the cell (cell voltage) during electrolysis is 3.4 to 4.1 V. Duration of electrolysis: 4 h.

[0099] Effective electrical energy consumption: E≠300 Wh.

[0100] Anode weight loss: Δm=6.62 g.

[0101] (a) Charge efficiency calculated from weight loss:

[00007]$\eta =\frac{6.62g}{55.85g.Math.{\mathrm{mol}}^{-1}}.Math.\frac{6\times 96485C.Math.{\mathrm{mol}}^{-1}}{22A\times 4h\times 3600s.Math.h^{-1}}=0.217$

[0102] b) Concentration of ferrate ions immediately after electrolysis in the solution (determined by spectrophotometry): c.sub.FeO.sub.4.sub.2−=4.32 g/dm.sup.3.

[0103] Charge efficiency calculated on the basis of spectrophotometry:

[00008]$\eta =\frac{4.32g.Math.{\mathrm{dm}}^{-3}\times 2.1{\mathrm{dm}}^3}{1119.84g.Math.{\mathrm{mol}}^{-1}}.Math.\frac{6\times 96485C.Math.{\mathrm{mol}}^{-1}}{22A\times 4h\times 3600s.Math.h^{-1}}=0.138$

[0104] Based on the above, the estimated purity of the product is:

[00009]$y=\frac{m_{\mathrm{Fe}\left(\mathrm{ferrát}\right)}}{{\Delta m}_{\mathrm{Fe}}}=\frac{4.23g}{6.62g}\approx 64\%$

[0105] c) Concentration of ferrate ions 4 hours after electrolysis in the solution stored at 10° C. (determined by spectrophotometry) c.sub.FeO.sub.4.sub.2−=3.07 g/dm.sup.3.

[0106] The corresponding “effective” charge efficiency is: η=0.098.

[0107] It can be seen that all the essential end-state parameters of the process are significantly worse than the corresponding values in Example 1 and 2.