Chemistry: electrical and wave energy – Apparatus – Electrophoretic or electro-osmotic apparatus
Reexamination Certificate
1997-05-05
2001-04-17
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrophoretic or electro-osmotic apparatus
C204S632000, C204S633000, C204S537000, C204S538000, C204S295000, C205S344000, C205S510000, C205S511000, C205S512000
Reexamination Certificate
active
06217733
ABSTRACT:
The invention relates to a bipolar membrane which can be used for the electrodialysis of aqueous electrolytes.
Bipolar membranes are constituent components of electrodialysis cells. The latter are well known in the art, where they are used, in particular, for the manufacture of acids and bases from their salts. In these electrodialysis methods, the bipolar membranes are immersed in aqueous electrolytes, and they are the seat of a dissociation of water under the action of an electric field. An effort is generally made to reduce the voltage needed for the dissociation of water in the bipolar membrane.
In methods generally used to manufacture bipolar membranes, a cationic ion exchange membrane and an anionic ion exchange membrane which have previously undergone a conditioning are juxtaposed. To this end, in British Patent Application 2122543, a description is given of bipolar membranes obtained by juxtaposing a cationic ion exchange membrane and an anionic ion exchange membrane which have undergone a conditioning treatment such that they possess, along their contiguous common surface, an internal zone containing metal cations and anions selected, in particular, from tetraborate, metaborate, silicate, metasilicate, tungsten, chlorate, phosphate, sulfate, chromate, hydroxyl, carbonate, molybdate, chloroplatinate, chloropaladate, orthovanadate and tellurate anions. For the conditioning treatment, treatment of the ion exchange membranes with aqueous solutions of indium sulfate, sodium sulfate, cerium sulfate, copper sulfate, manganese sulfate, chromium chloride, ruthenium chloride and rhodium chloride may be mentioned specifically. In International Application WO 89/01059, a method is described for manufacturing a bipolar membrane, according to which a cationic ion exchange membrane and an anionic ion exchange membrane are treated separately with an alkaline solution of a salt of a metal other than sodium or potassium, the two ion exchange membranes are thereafter juxtaposed and the assembly thereby obtained is then treated with an alkaline aqueous solution. In this known method, the alkaline solution used is sodium hydroxide solution and the metal salt is generally a metal chloride. Ceric sulfate may be mentioned.
The bipolar membranes obtained by means of the known methods described above are generally characterized by a good mechanical cohesion, a moderate electrical resistance and a moderate voltage for the dissociation of water.
The objective of the invention is to provide bipolar membranes having improved performances relative to those of the membranes obtained by means of the known methods described above, especially bipolar membranes affording a low voltage of dissociation of water.
Consequently, the invention relates to a bipolar membrane which can be used for the electrodialysis of aqueous electrolytes, comprising two ion exchange membranes, anionic and cationic respectively, which are juxtaposed along a common surface, the bipolar membrane being characterized in that it comprises, along the abovementioned common surface, a gel based on hydrated metal sulfate and/or sulfite, excluding indium, cerium, manganese and copper sulfate gels.
The bipolar membrane according to the invention comprises an anionic ion exchange membrane and a cationic ion exchange membrane. Anionic ion exchange membrane is understood to denote a thin, nonporous sheet selectively permeable to anions and impermeable to cations. A cationic ion exchange membrane is a thin, nonporous sheet selectively permeable to cations and impermeable to anions. Hereinafter, the anionic ion exchange membrane will be designated anionic membrane and the cationic ion exchange membrane will be called cationic membrane. In the bipolar membrane according to the invention, the ion exchange membranes must be made of a material which is inert with respect to acidic or basic aqueous solutions. Cationic membranes which can be used in the bipolar membrane according to the invention are, for example, sheets made of fluorinated polymer containing functional groups derived from sulfonic acids, from carboxylic acids or from phosphonic acids, or mixtures of such functional groups, these groups playing the part of fixed cationic sites of the membrane. Cationic membranes of this type which can be used in the bipolar membrane according to the invention are those known under the name RAIPORE (PALL RAI) and under the brand name MORGANE (SOLVAY), especially the membranes RAIPORE R-4010, MORGANE CDS and MORGANE CRA. Anionic membranes which can be used in the bipolar membrane according to the invention consist of sheets made of a polymeric material which is inert with respect to acidic or basic aqueous solutions and which comprises quaternary ammonium groups playing the part of fixed anionic sites. Anionic membranes which can be used in the bipolar membrane according to the invention are the membranes RAIPORE R-1030, RAIPORE R-4030 and MORGANE ADP.
The thickness of the ion exchange membranes will affect the mechanical and electrochemical properties of the bipolar membrane according to the invention. The optimum thickness of the ion exchange membranes will be the outcome of a compromise between sufficient mechanical strength (property favored by large thicknesses) and a low transverse electrical resistance (property favored by small thicknesses). In practice, the thickness of the ion exchange membranes is generally greater than 10 &mgr;m, and preferably at least equal to 20 &mgr;m. It is generally less than 250 &mgr;m and rarely exceeds 200 &mgr;m, the most suitable thicknesses generally being from 30 to 150 &mgr;m.
In the bipolar membrane according to the invention, the anionic membrane and the cationic membrane are juxtaposed so that they are in contact with one another substantially over the entire area of a common surface. According to the invention, the bipolar membrane comprises, along this common surface, a gel based on hydrated metal sulfate and/or sulfite. The expression “metal sulfate and/or sulfite” denotes any metal salt derived from sulfuric acid or sulfurous acid. The expression “gel based on hydrated metal sulfate and/or sulfite” means that the gel is formed from metal sulfate and/or sulfite, and that it can comprise anions other than sulfate anions and sulfite anions, for example, in general, hydroxyl anions. In the bipolar membrane according to the invention, the gel constitutes a hydrophilic and electroconductive medium along the common junction surface of the two ion exchange membranes. While not wishing to be bound by a theoretical explanation, the inventors think that this gel promotes the dissociation of water molecules when the bipolar membrane is used in the electrodialysis methods defined above. The gel must consequently be in contact with the two ion exchange membranes along at least a portion of the abovementioned common junction surface, this portion generally representing at least 25 (preferably at least 50) % of said common junction surface. It may be located entirely in only one of the two ion exchange membranes to the exclusion of the other, or alternatively overlap the abovementioned common surface and belong simultaneously to both ion exchange membranes. Preferably, the gel is located at least partially (for example in the proportion of at least 50%) in the anionic ion exchange membrane. In practice, the gel extends over only a portion of the thickness of the or of each ion exchange membrane, said portion representing, for example, a few percent of the total thickness of said ion exchange membrane. The properties of the bipolar membrane according to the invention will depend on the quality of the gel, and especially on its water absorption capacity and its electrical conductivity. Indium sulfate, cerium sulfate, manganese sulfate and copper sulfate gels are excluded from the invention. This expression is understood to mean that the gel based on hydrated metal sulfate and/or sulfite may possibly contain indium sulfate gel, cerium sulfate gel, manganese sulfate gel or copper sulfate gel, but in the trace state in w
El Moussaoui Rachid
Hurwitz Heinz
Bell Bruce F.
Knobbe Martens Olson & Bear LLP
Solvay ( Societe Anonyme)
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