Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-02-05
2002-09-03
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S542000
Reexamination Certificate
active
06444107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for the simultaneous production of acid and base of high purity through the electrodialytic splitting of a corresponding salt in aqueous solution using an electrodialysis cell. The invention also relates to an electrodialysis cell for carrying out the method.
2. Description of the Related Art
In a number of chemical process steps, salt solutions accumulate and, as such, are not directly used further or cannot or should not be introduced into a drainage canal as waste either. Furthermore, salt solutions with high concentrations are obtained in leaching processes of salt deposits or in the leaching of salts which are already conveyed as well as pure prepared salts. In many cases, it is in the interest of chemical engineering and economy to produce from such salt solutions more highly refined valuable substances in the form of acids and bases corresponding to the ions of the respective salt. Electrolytic or electrodialytic methods are frequently used for that purpose. The known electrodialytic methods used for that purpose operate with a three-chamber system (see report of Fraunhofergesellschaft: Institut für Grenzflächen- und Bioverfahrenstechnik [Fraunhofer Association: Institute for Interface and Bio-Material Processing], April 1999, “Elektrodialyse mit bipolaren Membranen” [Electrodialysis with Bipolar Membranes]).
In that respect, the salt solution which is to be prepared electrodialytically is conducted through a middle chamber of an electrodialysis cell being formed of three chambers. The cations travel from that cell, under the influence of the electrical field, through a cationic exchanger membrane into an adjacent chamber which contains the cathode, and form the base there with cathodically developed OH
−
-ions. Accordingly, the anions travel through an anionic exchanger membrane into the adjacent anode chamber on the other side and form the corresponding acid with the H
+
-ions developed anodically there. However, the production methods of acids and bases from salt solutions, which operate according to that method, have disadvantages. One disadvantage resides in the fact that unwanted reactions with anions take place at the anode, which lead to the contamination of the acid being formed. In that way, for example, hydrohalic acids, formed in the anode chamber with free halogens which are produced at the anode by the discharging of halide ions, are contaminated and their service value is therefore reduced. Moreover, the anode can be corrosively attacked or the ion exchanger membranes can be damaged by the halogen being released. Another disadvantage resides in the fact that the anodically formed acids are frequently not sufficiently concentrated and are therefore of little value in terms of chemical engineering and commerce.
In U.S. Pat. No. 4,212,712 a method is described for the electrodialytic production of a more highly concentrated sodium hydroxide solution from sodium chloride solutions in a three-chamber cell. However, with that method, hydrochloric acid is not directly electrodialytically formed, but instead, chlorine is separated anodically. The intermediate chamber lying between the anode chamber and the cathode chamber is separated from both adjacent chambers by cationic exchanger membranes which, in addition to being permeable to the Na
+
-ions, are permeable to water to differing degrees. The permeability to water is less towards the cathode region than the anode region into the intermediate chamber. Through the use of that configuration it is possible to generate a comparatively concentrated sodium hydroxide solution in the cell. In the authoritative literature it is also mentioned that it would be possible to use, in place of one intermediate chamber, two or more such intermediate chambers which are equipped in the direction of the cathode chamber with cationic exchanger membranes, that are permeable to water to an increasingly poorer extent. In that way, the sodium hydroxide solution in the cathode chamber is concentrated even more. However, in practice such a solution is not used because of the associated difficulties in achieving a satisfactory efficiency of flow.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and an electrodialysis cell for the simultaneous production of acid and base through the electrodialytic splitting of a corresponding salt in an aqueous solution using an electrodialysis cell, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and with which unwanted anode effects can be avoided and acids and alkalis can be produced with comparatively high concentrations and high purity. In particular, it is an object of the method according to the invention to produce, from sodium chloride solutions, hydrochloric acid of high purity and in concentrations which were heretofore not accessible with electrodialytic measures on an industrial scale.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the simultaneous production of acid and base of high purity by the electrodialytic splitting of a corresponding salt in aqueous solution with an electrodialysis cell, which comprises providing a cathode chamber having a cathode, an inlet opening and at least one outlet opening for fluids. A salt chamber is separated from the cathode chamber by a cationic exchanger membrane. The salt chamber has an inlet opening and an outlet opening for conducting a salt solution. An acid is formed in an acid chamber separated from the salt chamber by an anionic exchanger membrane. The acid chamber does not contain an anode. An anode chamber is separated from the acid chamber by a cationic exchanger membrane through which protons required for forming the acid pass from the anode chamber into the acid chamber. The anode chamber has an inlet opening and an outlet opening for a liquid proton carrier flowing through the anode chamber. The anode chamber has a hydrogen-consuming anode for converting hydrogen into protons to an extent required for forming the acid. An electrical voltage is applied between the anode and the cathode for maintaining an electrodialytic process. Cations of a salt travel under the effect of the electrical field, from the salt chamber, through the cationic exchanger membrane into the cathode chamber and form a base there with OH
−
-ions produced by catholytic splitting of water into hydrogen and OH
−
ions. Simultaneously, anions of the salt travel from the salt chamber, under the effect of the electrical field, through the anionic exchanger membrane into the acid chamber and form the acid there with protons formed analytically from hydrogen at the hydrogen-consuming anode.
With the objects of the invention in view, there is also provided an electrodialysis cell for the simultaneous production of acid and base of high purity from a corresponding salt by electrodialysis, comprising a cathode chamber having a cathode, an inlet opening and at least one outlet opening for fluids. A salt chamber is separated from the cathode chamber by a cationic exchanger membrane. The salt chamber has an inlet opening and an outlet opening for conducting a salt solution. An acid chamber in which an acid is formed is separated from the salt chamber by an anionic exchanger membrane and does not contain an anode. An anode chamber is separated from the acid chamber by a cationic exchanger membrane through which protons required for forming the acid pass from the anode chamber into the acid chamber. The anode chamber has an inlet opening and an outlet opening for a liquid proton carrier flowing through the anode chamber and a hydrogen-consuming anode for converting hydrogen into protons to an extent required for forming the acid. A device applies an electrical voltage between the anode and the cathode for maintaining an electrodialytic process. The device simultaneously causes
Bienhüls Christian
Härtel Georg
Künzel Jürgen
Bell Bruce F.
Parsons Thomas H.
SGL Technik GmbH
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