Claims
- 1. A process for electrolyzing an aqueous catholyte solution comprising an alkali metal bisulfite to produce an alkali metal hydrosulfite in an electrolytic membrane cell having a cation exchange membrane separating a cathode compartment from an anode compartment, a porous cathode having a face adjacent the membrane, a back, a porous structure conjoining the face and the back, a first catholyte zone adjacent to the back of the cathode, a second catholyte zone separated from the first catholyte zone by a barrier means, and a cathode-membrane gap between the porous cathode and the cation exchange membrane, which process comprises feeding at least 50 percent of the volume of the aqueous alkali metal bisulfite catholyte to the first catholyte zone and through the porous cathode, the porous cathode having a ratio of total surface area to the projected surface area of at least about 30:1.
- 2. The process of claim 1 in which the alkali metal bisulfite is sodium bisulfite or potassium bisulfite; and the alkali metal hydrosulfite is correspondingly sodium hydrosulfite or potassium hydrosulfite.
- 3. The process of claim 2 in which a pH of the aqueous catholyte solution is maintained at from about 5.0 to about 6.5
- 4. The process of claim 3 in which the aqueous catholyte solution is circulated at a rate which prevents a pH change of greater than about 0.5 unit per pass through the cathode compartment.
- 5. The process of claim 4 in which the ratio of total surface area to the projected surface area of the porous cathode is at least about 50:1.
- 6. The process of claim 4 in which the porosity of the porous cathode is at least 60 percent.
- 7. The process of claim 3 in which a cell temperature in the range of from about 0.degree. to about 35.degree. C. is maintained.
- 8. The process of claim 4 in which a current density is in the range of from about 1.0 to about 4.5 kiloamps per square meter.
- 9. The process of claim 1 in which the aqueous catholyte solution and an aqueous anolyte solution are circulated at rates which maintain the differential pressure across the membrane at no greater than about 5 psi.
- 10. The process of claim 1 in which an aqueous anolyte solution selected from the group consisting of alkali metal hydroxides, alkali metal persulfates, and alkali metal halides is fed to anode compartment.
- 11. The process of claim 10 in which the aqueous catholyte solution and the aqueous anolyte solution are circulated at rates which maintain the differential pressure across the membrane at no greater than about 5 psi.
- 12. The process of claim 10 in which the aqueous catholyte solution is circulated at a rate which prevents a pH change of greater than about 0.5 unit per pass through the cathode compartment.
- 13. The process of claim 12 in which the ratio of total surface area to the projected surface area of the porous cathode is at least about 50:1.
- 14. The process of claim 12 in which the ratio of total surface area to the projected surface area of the porous cathode is from about 80:1 to about 100:1.
Parent Case Info
This application is a Continuation-in-Part of U.S. Ser. No. 892,518, filed Aug. 4, 1986, now U.S. Pat. No. 4,793,906, issued Dec. 27, 1988.
The present invention relates to the electrochemical process for the manufacture of aqueous solutions of hydrosulfites. More particularly, the present invention relates to the electrochemical production of concentrated hydrosulfite solutions at high current densities.
Many unsuccessful attempts have been made at developing a process for manufacturing alkali metal hydrosulfites such as sodium hydrosulfite or potassium hydrosulfite electrochemically that can compete with conventional reduction processes using either sodium amalgam or metallic iron. The electrochemical process for making hydrosulfite results in the reduction of bisulfite ions to hydrosulfite ions. For this process to be economical, current densities must be employed which are capable of producing concentrated hydrosulfite solutions at high current efficiencies.
Further, where the solutions, which are strong reducing agents effective as bleaching solutions, are to be used in the paper industry, the undesirable by-product formation of thiosulfate as an impurity must be minimized. At high concentrations of hydrosulfite, however, this by-product reaction becomes more difficult to control.
Additionally, electrochemical routes to hydrosulfite produce aqueous solutions which are unstable and decompose at a rapid rate. This high decomposition rate of hydrosulfite appears to increase as the pH decreases or the reaction temperature increases. One approach to control the decomposition of hydrosulfite is to decrease the residence time of the hydrosulfite solution in the process. This can be accomplished by reducing the overall system volume and/or maintaining the current density as high as possible up to a critical density above which secondary reactions will occur due to polarization of the cathode.
Some of the processes of the prior art, which claim to make hydrosulfite salts electrochemically, require the use of water-miscible organic solvents such as methanol to reduce the solubility of the hydrosulfite and prevent its decomposition inside the cell. The costly recovery of the solvent and hydrosulfite makes this route uneconomical.
The use of zinc as a stabilizing agent for hydrosulfites in electrochemical processes has also been reported, but because of environmental considerations, this is no longer practiced commercially.
More recently, U.S. Pat. No. 4,144,146 issued Mar. 13, 1979 to B. Leutner et al describes an electrochemical process for producing hydrosulfite solutions in an electrolytic membrane cell. The process employs high circulation rates for the catholyte which is passed through an inlet in the bottom of the cell and removed at the top of the cell to provide for the advantageous removal of gases produced during the reaction. Catholyte flow over the surface of the cathodes is maintained at a rate of at least 1 cm per second where the cathode has a mesh spacing of 5 mm or less. The process is described as producing concentrated solutions of alkali metal hydrosulfites at commercially viable current densities; however, the cell voltages required were in the range of 5 to 10 volts resulting in high power consumption and high power costs and hence increased product costs. There is no indication of the concentrations of thiosulfate impurity in the product solutions.
Therefore, there is a need for an electrochemical process for producing aqueous solutions of alkali metal hydrosulfites having low concentrations of alkali metal thiosulfates as impurities at high current densities and at reduced cell voltages.
It is an object of the present invention to provide an electrochemical process for producing aqueous alkali metal hydrosulfite solutions having low concentrations of alkali metal thiosulfates as impurities.
Another object of the present invention is to provide an electrochemical process for producing concentrated alkali metal hydrosulfites which operates at high current densities.
These and other objects of the invention are accomplished in a process for electrolyzing an aqueous catholyte solution comprising an alkali metal bisulfite to produce an alkali metal hydrosulfite in an electrolytic membrane cell having a cation exchange membrane separating a cathode compartment from an anode compartment, a porous cathode having a face adjacent the membrane, a back opposite, and a porous structure conjoining the face and the back, and a cathode-membrane gap between the porous cathode and the cation exchange membrane, which process comprises directing at least 50 percent of the volume of the aqueous alkali metal bisulfite catholyte through the porous cathode and transverse to the face and back of the cathode, the porous cathode having a ratio of total surface area to the projected surface area of at least about 30:1.
According to the invention, it has been found that directing the flow of the catholyte through the porous cathode to maximize contact between the catholyte and the cathode results in significant improvements in the electrochemical process for producing aqueous alkali metal hydrosulfite solutions.
US Referenced Citations (9)
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 56-87682 |
Jul 1981 |
JPX |
| 1045675 |
Oct 1966 |
GBX |
Non-Patent Literature Citations (4)
| Entry |
| Ephraim, Inorganic Chemistry, 1943, pp. 549-550. |
| Oloman, C., "The Preparation of Dithionites by the Electrolytic Reduction of Sulphur Dioxide in Water", J. Electrochem. Soc.: Electrochemical Technology, Dec. 1970, pp. 1604-1609. |
| Kovacs, L., S. Muthukumaraswami, S. Krishnamurthy, R. Thangappan and H. V. K. Upuda, "Electrochemical Reduction of Sodium Bisulphite to Sodium Hydrosulphite in a Fluidized Bed Electrode", Journal of Indian Technology, vol. 14, Apr. 1976, pp. 184-188. |
| Gizetdinova, N. A., "Electrochemical Synthesis of Sodium Dithionite", Siberian Scientific-Research Institute of Paper and Cardboard, Translated from Zhurnal Prikladnoi Khimii, vol. 52, No. 12, pp. 2741-2744, Dec. 1979, original article submitted Aug. 7, 1978. |
Continuation in Parts (1)
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Number |
Date |
Country |
| Parent |
892518 |
Aug 1986 |
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Patent Grant
4992147
