WO1995003119A1 - Electrodialyzer for desalinating salt solutions and process for desalinating salt solutions by electric dialysis - Google Patents

Abstract

An electrodialyzer for desalinating salt solutions has a plurality of cation and anion exchange membranes (15, 16) arranged between the anode (13) and the cathode (14). These membranes delimit an anode chamber (17), diluate chambers (18), concentration chambers (19) and a cathode chamber (22). The input chamber is an anode pre-chamber (18) separated from the anode chamber (17) by a cation exchange membrane (15). According to a process for desalinating salt solution by electric dialysis, the initial solution, and thus the brine in the concentration chambers (19), is acidified by hydrogen ions. Hardening salt residues on the membrane surfaces may thus be reduced or completely eliminated.

Description

 Electrodial fan for desalting

Saline solutions and process for desalting saline solutions by electrodialysis

The invention relates to an electrodial vent for the desalination of salt solutions, in which a number of cathion and anion exchange membranes are arranged between the anode and cathode, which delimit an anode chamber, diluate and concentration chambers and a cathode chamber, and to a method for Desalting salt solutions by electrodialysis.

The reduction of the salt content of aqueous solutions by means of electrodialysis has been known for a long time. The corresponding apparatus consists of the anode chamber, diluate chamber, concentration chamber and cathode chamber. The individual chambers are separated from one another by membranes. The electrodialysis chambers are pumped through in parallel with the salt water. In the process, residues occur on the membrane surfaces and the cathode due to the concentration polarization. To remove the residues, use an electrode pole wash followed by washing the chambers. However, this means regular interruption of the actual desalination process and is undesirable. Furthermore, with this method no adjustment of the properties of the end product.

The object of the invention is to reduce or prevent the formation of residues on the membrane surfaces.

According to the invention, this object is achieved in that an anode prechamber is provided as the input chamber, which is separated from the anode chamber by a cathode exchange membrane.

In this construction, hydrogen ions pass from the anode chamber into the anode pre-chamber, with the result that the brine is also acidified in the concentration chambers. This reduces or completely prevents the formation of residues on the membrane surfaces.

It is also favorable that a cathode pre-chamber is provided as the starting chamber, which is separated from the cathode chamber by an anion exchange membrane. As a result, hydroxyl ions are added to the desalinated liquid so that the pH rises again.

It is also advantageous that overflow channels connect the anode prechamber with the first diluate chamber, adjacent diluate chambers with one another and the last diluate chamber with the cathode prechamber. The salt solution therefore flows through the diluate chambers one after the other, which leads to a long residence time and therefore to good desalination.

It is advantageous here that the inlet and outlet overflow channels of the individual diluate chambers are arranged close to opposite ends of each diluate chamber. The salt chamber therefore flows through the dialyzer assuming the salt content in a meandering shape. A line is also recommended, which leads from a current divider at the outlet via the anode chamber to the end product line and there admixes a branched-off part with the outlet flow. The branched-off part of the output current absorbs hydrogen ions in the anode chamber, so that the pH of the end product can be precisely adjusted.

In this context, a metering valve arrangement for setting the pH in the output stream is advantageous.

In a further embodiment of the invention it is ensured that a labyrinth pore layer is provided in the anode chamber, which layer covers part of the anode surface. This ensures that hydrogen ions can be released on the one hand to the incoming salt solution and on the other hand to the branched-off part of the output stream. The ratio of the surfaces of the labyrinth pore layer and the anode is preferably 0.5 to 0.9.

A settling container for sparingly soluble salts, which is connected to the outlet of the concentration chambers and to the outlet of the cathode chamber, is also advantageous. Here, the alkali from the cathode chamber is mixed with the acidified brine, which leads to the precipitation of the hardness-forming and generally poorly soluble salts.

In a further embodiment, a screw conveyor can be used to discharge the sparingly soluble salts precipitated in the settling tank. What remains is a residue solution free from hardness-forming salts.

It is also favorable that the residue solution softened in the settling tank can be admixed. This suggestion is recommended if the input solution has a low salt content, for example less than 1 g / l. The salt content of the input solution is hereby raised to a value necessary for proper operation without causing hardness

Salts that could contaminate the membrane surfaces can be used. This also has the consequence that intensive acid formation takes place.

In terms of the method, the object is achieved in that the input solution and thereby the brine in the concentration chambers are acidified by hydrogen ions.

It is advantageous here if a pH value <3 is maintained in the concentration chambers. Under these conditions, the hardness-forming salts are completely dissociated and cannot form deposits on the membrane. Continuous operation is possible.

It is advantageous that hydroxyl ions are added to the desalinated water in excess and the pH of the end product is regulated by admixing a branched part provided with hydrogen ions.

The acid content of the demineralized water is therefore greatly reduced and the desired pH value is regulated by a liquid containing hydrogen ions.

It is preferred that hydroxyl ions are added to the brine for the purpose of precipitating sparingly soluble salts. Manganese, calcium and magnesium can then be obtained electrolytically from these salts.

It is also recommended that the brine residue residue freed from precipitated salts be added to the input solution. solution is added. In this way, an optimum salt content can be set for operation.

In order to obtain sufficient acid formation, it is recommended that the salt concentration of the input solution be raised to about 2.5% or more.

The invention is explained below with reference to the drawing. Show it:

Fig. 1 is a schematic representation of a desalination plant according to the invention and

Fig. 2 is a schematic representation of an electrodializer according to the invention.

The electrodial generator to be described, which is the subject of the invention, is shown schematically in FIG. 2. An anode 13 and a cathode 14 are each through. a pressure plate 12 supported. They each delimit an anode chamber 17 and a cathode chamber 22. Between an anode pre-chamber 18 and a cathode pre-chamber 21 there are exchange membranes 15 and 16 which separate diluate chambers 20 from concentration chambers 19. As an example, it is stated that the exchange membrane 15 is permeable to H and Na and the exchange membrane 16 to OH and Cl. The input solution, which enters the anode prechamber 18 through a nozzle 24, passes through overflow channels 30 and all diluate chambers 20 into the cathode prechamber 21, from which the

Liquid emerges through a nozzle 27 and reaches a distributor 32. Here, a partial stream is branched off and returned via a line 33 through a nozzle 25 into the anode chamber 17. The amount of the partial flow can be adjusted by means of the metering valve 34. In the diluate chambers 20, the solution is desalinated by the diffusion of the hydrogen ions through the cathode exchange membrane 15 and the anions through the anion exchange membrane 16 in the direction of the concentration chamber 19, which runs from the last diluate chamber 20 demineralized water through the overflow channel 30 into the cathode prechamber 21, where it absorbs hydroxyl ions, which have been formed by the influence of the electric field as a result of the electrolysis reaction on the cathode 14, via the membrane 16. The number of hydroxyl ions is greater than that of hydrogen ions, because some of them migrated through the ion exchange membranes into the concentration chambers during the desalination process. Therefore, the liquid stream emerging from the cathode chamber reacts alkaline.

In order to be able to subsequently influence the pH of the output current, it is divided in the current divider 32. As already described, one part reaches the anode chamber 17 via the line 33 and the metering valve 34, where it is acidified by the hydrogen ions accumulated there. A labyrinth pore layer 31 is attached in the anode chamber and covers part of the anode surface. Thereafter, the liquid stream exits the anode chamber 17 at the nozzle 25 and is mixed with the outlet stream 36 via a line 35. As a result of this and with the position of a metering valve 37, the pH of the end product can be set precisely, one of the essential advantages of this method.

The brine collects in the concentration chambers 19 of the electrodialyzer due to the osmotic process. This is acidified by the hydrogen ions, which, as already mentioned, from the anode pre-chamber 18 and the diluate chambers 20 through the membrane migrate brane 15. This leads to a reduction in the formation of residues on the membrane surfaces.

A complete exclusion of the formation of residues of sparingly soluble hardness salts is achieved if a pH of <3 is maintained in the concentration chambers 19 by adjusting the current density and the other process parameters. Under these conditions, these salts are completely dissociated.

The brine which emerges from the concentration chambers 19 is mixed with the alkali from the cathode chamber 22. The poorly soluble salts are precipitated in a separator. The residue solution thus softened is then admixed with the input product. This increases the salt content and intensive acid formation takes place in the anode chamber of the electrodialyzer. It is thereby achieved that a pH value of <3 prevails in the concentration chambers, which is necessary in order to avoid residue formation on the membrane tops.

If the salt content of the input solution is higher, for example> 1 g / l, it is introduced directly into the anode prechamber (buffer) 18 through the nozzle 24. Hydrogen ions from the anode chamber, which collect there as a result of the oxidation reaction, diffuse into this chamber through the cathode exchange membrane 15, which separates the anode space 17 from the anode prechamber (buffer) 18. The solution in the anode pre-chamber (buffer) 18 is acidified by these diffusing hydrogen ions. This solution now passes through the overflow channel 30 into the first diluate chamber 20, where the desalination of the solution at the expense of the directed diffusion of the cathions (together with the hydrogen ions) through the cathode exchange membrane 15 and the anions through the anion exchange membrane 16 into the Concentration chamber 19 takes place. In this way, the separation of the initial solution into drinking water and brine is effected after passing through the further diluate chambers. The deionized water runs from the last diluate chamber 20 through the overflow channel 30 into the cathode pre-chamber 21, into which hydroxyl ions diffuse through the anion exchange membrane 16, which separates the cathode chamber 22 from the cathode pre-chamber 21. The hydroxyl ions have formed on the cathode 14 as a result of the electrolysis reaction and are concentrated in the cathode chamber 22.

The demineralized water, which now contains excess hydroxyl ions, flows from the cathode prechamber (buffer) 20 via the connection piece 27 into the branch 32. Here, a partial flow of the water is branched off and via the pipe 33 via the connection piece 25 of the anode chamber 17 fed. There, the desalinated water flows through the channels of the labyrinth pore layer 31 past the anode 13. It absorbs the hydrogen ions concentrated there and there is an alkali reaction. The water flow then flows back through the connecting piece 27 and the pipeline 35 into the end product line 36 and mixes there with the ^" other partial flow which has continued at the branch 32. The flow rates of the two partial flows can be adjusted by means of the metering valves 34 and 37 through which the pH of the end product can be regulated.

The labyrinth pore layer, which is inserted in the anode chamber 17, prevents the desalted partial stream from flowing through so that it takes all hydrogen ions with it. In this way it is achieved that part of these hydrogen ions is used to acidify the desalted partial stream, while the other part of the hydrogen ions is influenced by the influence of the electrical potential from the anode chamber 17 by the cathode. Exchange membrane 15 diffuses into the anode prechamber 18 and there acidifies the freshly arriving input solution, which then flows through the overflow channel 30 into the diluate chambers 20.

When desalting input solutions with a low salt content, for example <1 g / 1, we turn to Fig. 1. Containers 3, 5 and 8 are arranged in a communicating manner, the connecting line between the containers 3 and 5 being able to be closed by a valve 9 and the connecting line between the containers 5 and 8 being able to be closed by a valve 10. The containers 3, 5 and 8 are filled with the valves 9 and 10 open to a level which is predetermined by a float valve 4. Now the valves 9 and 10 are closed and NaCl is added to the settling tank 5 until a salt content measuring probe 6 indicates a concentration of about 5%. Then the valve 9 is opened and a feed pump 7 is switched on. This now takes up equal parts of liquid from the containers 3 and 5, so that the salt concentration of the liquid arriving at the pump 7 is about 2.5%. The pump 7 conveys this liquid via the feed line to the nozzle 24 into the electrodialyzer 1. There, the desalination then takes place and thus the separation of fresh water and brine. Depending on the position of a three-way valve 11, the brine applied to the nozzle 29 is either drained off directly, namely if the salt concentration in the container 5 is sufficiently high, or is fed to the latter if the salt concentration measured by the sensor 6 is too low. The three-way valve 11 is set by means of the value of the probe 6 via the salt content in the container 5. The brine flowing into the container 5 is mixed with the alkali applied to the connector 28 from the cathode chamber 22 (from FIG. 2), as a result of which the sparingly soluble salts are precipitated in the container 5, from which they are conveyed with a derschnecke 12 be removed for further processing. The desalinated water flows via the line 35 into the storage container 8, from which it can be removed via the line 36.

An important side effect is that the demineralized water is also sterilized because of the chlorine ions that form.

The highly concentrated brine remaining in the settling tank can be applied to a conventional saline. The salt can be generated by the evaporation from the incident sunlight. It is essentially in the form of sodium halide because the hardening salts have already been separated.

Accordingly, the method serves for the desalination of aqueous salt solutions, which can also contain hardness salts, by electrodialysis. The input solution is directed into the anode pre-chamber, from where it is then passed through overflow channels through the dilution chambers. These are separated from the concentration chambers by ion exchange membranes. Under the influence of the electric field, anions and cathions diffuse into the concentration chambers and form the brine there, which is continuously released. Hydrogen ions that diffuse from the anode chamber into the anode antechamber, migrate with the solution into the dilution chambers and migrate through the ion exchange membranes into the concentration chambers maintain a pH value of preferably <3, thereby residue formation of hardness salts on the surfaces of the ion exchange membranes is avoided.

Furthermore, a partial stream of the desalinated water emerging from the cathode prechamber can be branched off and sent through the anode chamber, where it contains water. absorbs ion ions. This part is then admixed with the rest of the end product. The pH of the end product can be adjusted depending on the amount of branched water that flows through the anode chamber.

In the desalination of solutions with a low salt content, the salt content of the starting solution is increased by admixing the brine from the concentration chambers. The hardness salts in the brine were precipitated from the cathode chamber by adding alkali. The process also works here without an ion exchanger at the entrance and can therefore be operated continuously since regeneration of such an ion exchanger for softening is not necessary.

The electrodialyzer has an anode and a cathode plate, a number of anion and cathion exchange membranes between them, which form the anode chamber, the dilution chambers, the overflow channels, the concentration chambers and the cathode chamber. In addition, the anode chamber is separated from the anode prechamber (buffer) by another membrane. The cathode chamber is also separated from the cathode prechamber (buffer) by an additional membrane.

Furthermore, two nozzles are attached to the anode chamber, through which a partial stream of the demineralized water is passed through it in order to take up hydrogen ions. A labyrinth pore layer is arranged in the anode chamber, which serves to distribute the proportion of the hydrogen ions in such a way that they are carried along by the desalinated water fed in as well as diffuse through the cathode exchange membrane into the anode antechamber by means of the electric field and from there through the dilution chambers and the ion exchange membranes into the concentration to migrate chambers in order to maintain a pH value of <3 as possible, so that residues on the ion exchange membranes are avoided and the process can proceed continuously. The partial stream flowing through the anode chamber is admixed with the end product in order to be able to adjust its pH. The concentration chambers are also provided with a nozzle so that the brine can be removed. At the cathode chamber there is a further nozzle which allows the alkali to be removed from it, which can be added to the brine in order to precipitate hardness salts. At the anode pre-chamber there is a nozzle for introducing the starting solution. On the cathode chamber there is a nozzle for the removal of the demineralized water, which is connected to the anode chamber via a branch with a pipeline and a metering valve. The outlet port of the anode chamber is connected via a pipeline to the drain line for the end product, to which hydrogen ions are added in order to adjust the pH.

Item 1. Electrodialyzer

Pos. 2. Feeding block of the electrodialyzer

Pos. 3. Container for the starting solution

Pos. 4. Float valve

Item 5. Settling tanks for the brine

Pos. 6. Sensor for salt concentration

Item 7. Pump

Pos. 8. Dilute container

Pos. 9. Dosing valve

Item 10. Shut-off valve

Item 11. Multi-way valve (elmag)

Item 12. Conveyor screw

Item 24. Inlet anode pre-chamber (buffer)

Item 28. Drain from cathode chamber (alkaline)

Item 29. Drainage of the brine (from chamber item 19)

Item 35. Anode outlet end product connecting line Item 36

Item 36. End product outlet

LEGEND to Fig 2.

Item 12. Pressure plates

Pos. 13. Anode

Item 14 cathode

Pos. 15. Cathion exchange membrane (at the anode)

Pos. 16. Anions exchange membrane (at the cathode)

Item 17. Anode chamber

Item 18. Anode prechamber (buffer)

Pos. 19. Ion enrichment chambers

Pos. 20. Dilution chambers

Item 21. Cathode antechamber (buffer)

Item 22. Cathode chamber

Item 23. Housing

Item 24. Inlet anode pre-chamber (buffer)

Item 25. Inlet anode chamber

Item 26. Anode chamber drain

Item 27. Drain cathode antechamber (buffer)

Item 28. Drain from cathode chamber (alkaline)

Item 29. Drainage of the brine (from chamber item 19)

Item 30. Overflow channels

Pos. 31. Labyrinth pore layer (on the anode plate)

Item 32. T-piece, for dividing the drain

Item 33. Partial flow to the anode chamber

Item 34.Dosing valve

Item 35. Anode outlet end product connecting line Item 36

Item 36. End product outlet

Item 37

Claims

claims Electrodialyzer for desalting salt solutions, in which a number of cathode and anion exchange membranes are arranged between the anode and cathode, which delimit an anode chamber, diluate and concentration chambers and a cathode chamber, characterized in that an anode antechamber (18) is provided as the input chamber which is separated from the anode chamber (17) by a cathode exchange membrane (15). 2. Electrodialyzer according to claim 1, characterized gekenn¬ characterized in that a cathode pre-chamber (21) is provided as the output chamber, which is separated by an anion exchange membrane (16) from the cathode chamber (22). 3. Electrodialyzer according to claim 2, characterized gekenn¬ characterized that overflow channels (30), the Anodenvor¬ chamber (18) with the first diluate chamber (20), neighboring diluate chambers (20) with each other and the last diluate chamber (20) with the cathode antechamber Connect (21). 4. Electrodialyzer according to claim 3, characterized in that the inlet and outlet overflow channels. le (30) of the individual diluate chambers (20) are arranged near opposite ends of each diluate chamber. 5. Electrodialyzer according to one of claims 1 to 4, characterized by a line (33, 35) which leads from a current divider (32) at the output via the anode chamber (17) to the end product line (36) and there is a branched-off output current Part added. 6. Electrodialyzer according to claim 5, characterized by a metering valve arrangement (34, 37) for adjusting the pH in the output stream (36). 7. Electrodialyzer according to one of claims 1 to 6, characterized in that in the anode chamber (17) a labyrinth pore layer (31) is provided which covers part of the anode surface. 8. Electrodialyzer according to claim 7, characterized gekenn¬ characterized in that the ratio of the surfaces of the labyrinth pore layer (31) and anode is 0.5 to 0.9. 9. Electrodialyzer according to one of claims 1 to 8, characterized by a settling container (5) for sparingly soluble salts, which is connected to the outlet (29) of the concentration chambers (19) and to the outlet (28) of the cathode chamber (22). 10. Electrodialyzer according to claim 9, characterized by a screw conveyor (12) for discharging the sparingly soluble salts precipitated in the settling tank (5). -16- 11. Electrodialyzer according to one of claims 1 to 10, characterized in that the initial solution softened residue solution can be mixed in the settling tank. 12. A method for desalting salt solutions by electrodialysis, in particular using the electrodialyzer according to one of claims 1 to 11, characterized in that the input solution and thereby the brine in the concentration chambers is acidified by hydrogen ions. 13. The method according to claim 12, characterized in that a pH value <3 is maintained in the concentration chambers. 14. The method according to claim 12 or 13, characterized gekenn¬ characterized in that the desalinated water in excess hydroxyl ions are supplied and the pH of the end product is regulated by admixing a branched, provided with hydrogen ions. 15. The method according to any one of claims 12 to 14, characterized in that hydroxyl ions are added to the brine for the purpose of precipitation of poorly soluble salts. 16. The method according to any one of claims 12 to 15, characterized in that brine residue solution freed from precipitated salts is added to the input solution. 17. The method according to claim 16, characterized in that the salt concentration of the input solution is raised to about 2.5%.