Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
2002-03-08
2003-02-04
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06515102
ABSTRACT:
The present invention relates to a process for recovering fluoropolymers from fluorine-containing ion exchange membranes used for e.g. electrolysis of sodium chloride.
Fluorine-containing ion exchange membranes are widely used as diaphragms for electrolytic cells for the production of sodium hydroxide by electrolysis of sodium chloride. As such a fluorine-containing ion exchange membrane, a laminated ion exchange membrane having from 2 to 4 laminated films of a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups, or one having such a laminated ion exchange membrane reinforced by a woven fabric made of polytetrafluoroethylene (hereinafter referred to as PTFE).
Further, for the purpose of preventing deposition, on the membrane surface, of a gas generated during electrolysis and reducing the voltage for electrolysis, one having inorganic particles of e.g. silicon carbide or zirconium oxide coated on the surface of a laminated ion exchange membrane, is used. On the other hand, on the surface of the ion exchange membrane, precipitates composed mainly of iron oxide or a hydrate thereof (hereinafter referred to as surface precipitates) are likely to deposit during the electrolysis.
Heretofore, when the performance of ion exchange membranes used for the electrolysis decreased, it was common that they were dismounted from the electrolytic cell and used or disposed for e.g. land filling as wastes. However, in recent years, it is desired to recover and reuse fluoropolymers constituting the ion exchange membranes as materials for ion exchange membranes or membranes for fuel cells, from the viewpoint of avoiding the influence over the environment.
In order to recover and reuse a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups from ion exchange membranes, it is necessary not only to separate the two but also to remove the above-mentioned woven fabric and inorganic particles from the ion exchange membranes.
The following methods have therefore been known as a method for recovering fluoropolymers from fluorine-containing ion exchange membranes. A method wherein laminated fluorine-containing ion exchange membranes comprising two or more fluoropolymer layers having e.g. carboxylic acid groups and/or sulfonic acid groups, are converted to acid-forms or alkali metal salt-forms and then immersed in a water-soluble organic solvent to elute the respective fluoropolymers, and the respective fluoropolymers are recovered from the eluted solution (JP-B-3-14860), or a method wherein laminated ion exchange membranes comprising a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups, are immersed in a fluoroalcohol to elute the fluoropolymer having sulfonic acid groups (JP-A-2000-86809).
However, in these methods, it is difficult to remove inorganic particles, whereby there will be a problem that the obtainable fluoropolymers are of low purity.
It is an object of the present invention to provide a process for efficiently recovering a fluoropolymer having carboxylic acid groups and a fluoropolymer having sulfonic acid groups, respectively, in high purity, by removing inorganic particles from an ion exchange membrane comprising the fluoropolymer having carboxylic, acid groups, the fluoropolymer having sulfonic acid groups and the inorganic particles deposited on the surface.
The present invention provides a process for recovering fluoropolymers, which comprises contacting an ion exchange membrane comprising a fluoropolymer having carboxylic acid groups (hereinafter referred to as a C-polymer) and a fluoropolymer having sulfonic acid groups (hereinafter referred to as a S-polymer) and having inorganic particles deposited on the surface, with a solvent which is a good solvent for the C-polymer and a good solvent for the S-polymer, to separate a solution having the C-polymer and the S-polymer dissolved therein, and the inorganic particles, then esterifying the C-polymer in the solution to form precipitate of an ester of the C-polymer, and fractionating and recovering the precipitate and a solution having the S-polymer dissolved therein by solid-liquid separation.
Here, the inorganic particles include not only particles of e.g. silicon carbide or zirconium oxide deposited on the surface of the ion exchange membrane to prevent attachment of gas, but also surface precipitates, etc. deposited during the electrolysis. Further, in this specification, the C-polymer includes not only one in the form of an acid-form but also one wherein a part or whole of the fluoropolymer having carboxylic acid groups is in the form of a salt-form. Likewise, the S-polymer includes not only one in the form of an acid-form but also one in which a part or whole of the fluoropolymer having sulfonic acid groups is in the form of a salt-form.
According to the present invention, inorganic particles on the surface of an ion exchange membrane are preliminarily removed, and then the C-polymer and the S-polymer are recovered, whereby the C-polymer and the S-polymer can be obtained in high purity. Further, the C-polymer in the solution having the C-polymer and the S-polymer dissolved therein, is esterified to form precipitate of an ester of the C-polymer, whereby the C-polymer and the S-polymer can easily be separated and recovered by solid-liquid separation.
Further, in a second aspect, the present invention provides a process for recovering fluoropolymers, which comprises treating an ion exchange membrane comprising a C-polymer and a S-polymer and having inorganic particles deposited on the surface, with a solvent to let it swell and to remove the inorganic particles, then contacting it with a solvent which is a good solvent for the C-polymer and a good solvent for the S-polymer to obtain a solution having the C-polymer and the S-polymer dissolved therein, esterifying the C-polymer in the solution to form precipitate of an ester of the C-polymer, and fractionating and recovering the precipitate and a solution having the S-polymer dissolved therein by solid-liquid separation.
According to this process, the ion exchange membrane is swelled by a solvent, whereby inorganic particles can easily be separated from the ion exchange membrane, and the C-polymer and the S-polymer can be obtained in high purity.
Further, in this specification, the solvent which is a good solvent for the C-polymer and a good solvent for the S-polymer, will be hereinafter referred to simply as a good solvent. Further, the solvent to let the ion exchange membrane swell thereby to remove inorganic particles, will be hereinafter referred to as a swelling solvent.
The following compounds may be mentioned as preferred good solvents in the present invention. An alkyl alcohol such as methanol, ethanol, n-propanol or i-propanol, a solution which is a mixed solution comprising the above alkyl alcohol and water, wherein the content of water is at most 30 mass%, an amide such as N,N-dimethylformamide, N,N-dimethylacetoamide or N-methylpyrrolidone, a ketone such as acetone or 2-butanone, an ether such as diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, 1,2-dimethoxyethane, tetrahydrofuran or 1,4-dioxane, a nitrile compound such as acetonitrile, a sulfur-containing compound such as dimethylsulfoxide or sulfolane, and a fluorocompound such as 1,1-dichloro-2,2,3,3,3-pentafluoropropane, 1,3-dichloro-1,1,2,2,3-pentafluoropropane, 1,1-dichloro-2,2,2-trifluoroethane, 1,1,1,2,3,3-hexafluoropropane, 1,1,1,2,3,4,4,5,5,5-decafluoropentane, perfluoro(n-butylmethyl)ether, 1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,3,3-pentafluoro-1-propanol, 2,2,2-trifluoroethanol or 2,2,3,3-tetrafluoropropanol.
Especially, when the good solvent is an alkyl alcohol such as methanol, ethanol, n-propanol or i-propanol, or a mixed solution comprising such an alkyl alcohol and water, such a good solvent can be used as it is for the esterification reaction of the C-polymer, such being preferred. Particularly preferred is methanol or an aqueous methanol solution wherein the content of wat
Fukatsu Yutaka
Kumai Seisaku
Mihara Harumi
Tokura Ryota
Asahi Glass Company Limited
Boykin Terressa M.
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