EP1059366B1 - Electrolytic cell for producing an alkali metal - Google Patents

Abstract

Electrolysis cell comprises a moving alkali metal amalgam-containing anode, an alkali metal ion-conducting solid electrolyte, and a cathode. The solid electrolyte and the cathode are separated from each other by a liquid electrolyte.

Description

The present invention relates to an electrolytic cell used for electrochemical Production of alkali metal from alkali metal amalgam can be used. According to the invention, the term “alkali metal” denotes sodium and potassium.

The invention further relates to a method for electrochemical production of alkali metal from alkali metal amalgam using this electrolytic cell.

Sodium is an important inorganic basic product that is used, for example, for Production of sodium amide, sodium alcoholates and sodium borohydride used becomes. It is technically after the Downs process by electrolysis of melted table salt. This process has a high energy consumption of ≥ 10 kWh / kg sodium (Büchner et al., Industrial inorganic Chemistry, 2nd edition, Verlag Chemie, p. 228 f). Furthermore, the process the serious disadvantage that the electrolysis cells when turned off by the Solidification of the molten salt can be destroyed. It also has the Downs process sodium metal gained the disadvantage that it is process-related with calcium is contaminated, its residual content only by subsequent cleaning steps diminished, but can never be completely excluded.

Potassium is also an important basic inorganic product, for example for the production of potassium alcoholates, potassium amides and potassium alloys is used. Today it is technically mainly through reduction obtained from potassium chloride with sodium. This creates NaK, the then fractionally distilled. A good yield is achieved in that Potassium vapor is continuously withdrawn from the reaction zone, whereby the Balance is shifted to the potassium side (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition 1998, Electronic Release). The disadvantage is that Process at high temperatures (870 ° C) works. It also contains the resulting Potassium approx. 1% sodium as an impurity and must therefore still pass through another rectification can be cleaned up. The main disadvantage is that sodium used is expensive. This is also due to the fact that sodium is technically after the Downs process obtained by electrolysis of molten table salt with an energy expenditure of at least 10 kWh / kg sodium. This corresponds to about 5.3 kWh / kg of potassium (with 100% yield).

Sodium amalgam and potassium amalgam are intermediates used in chlor-alkali electrolysis accrued in large quantities after the amalgam process and usually immediately after preparation with water to alkali metal lye be implemented. The low alkali metal or alkali metal free alkali metal amalgam is usually immediately returned to chlor-alkali electrolysis. To keep the sodium amalgam in liquid form, the sodium concentration of this solution to values of less than 1% by weight, preferably Values in the range of 0.2 to 0.5 wt .-% are kept. Potassium amalgam Keeping it in liquid form is the potassium concentration of the solution less than 1.5% by weight, preferably in the range of 0.3 to 0.6% by weight. The amalgams obtained on an industrial scale essentially contain metallic impurities such as copper, iron, potassium (in sodium amalgam), Sodium (in potassium amalgam), lead and zinc in the concentration range from 1 to 30 ppm.

GB 1,155,927 describes a process by which sodium metal can be obtained from sodium amalgam electrochemically using a solid sodium ion conductor, such as, for example, β-Al ₂ O ₃ , with amalgam as the anode and sodium as the cathode. The execution of the method described in GB 1,155,927 does not lead to the results described there with regard to sodium conversion, product purity and current density. Furthermore, the system described there behaves unstably over the course of a few days if the claimed temperature range is maintained.

Electrolysis cells used in an electrochemical process for the production of Alkali metal from alkali metal amalgam are used and a solid ion conductor are often not suitable for long periods in permanent operation to be held. One reason for this is the mechanical instability of the solid ion conductor that occurs after a certain period of operation.

It was therefore an object of the present invention to provide an electrolysis cell which does not have these disadvantages.

Another task was to create a process for electrochemical manufacture of alkali metal from an alkali metal amalgam using this To provide electrolytic cell, which is a more energy-efficient production of Sodium as the Downs process or an energy-efficient production of potassium as the technical process discussed at the beginning. Should continue the process in the existing combination of a chlor-alkali electrolysis after the amalgam process can be integrated, avoiding those disadvantages should be used in the execution of the method according to GB 1,155,927 occur. The following essential requirements must be met:

The alkali metal conversion on the anode side must meet the balance sheet requirements of the product group with chlor-alkali electrolysis are sufficient. That is, the drain concentration of alkali metal in the amalgam corresponds to chlor-alkali electrolysis the feed concentration in the alkali metal electrolysis according to the invention. Further must between the chlor-alkali electrolysis and alkali metal electrolysis according to the invention circulated quantities of amalgam in a technical and economically justifiable size. Usually this is achieved if the alkali metal content of the incoming amalgams is implemented to 50%. The sodium metal must be primary occur in such a purity that further process steps for mercury separation can be omitted and the disadvantage of the Downs process Calcium contamination is avoided. The potassium metal must primarily in one such purity arise that further process steps for mercury separation can be omitted and the sodium content is lower than in the reduction with Sodium, where the primary potassium produced contains 1% sodium. The procedure is supposed to be feasible on an industrial scale and must therefore be sufficiently high Enable current densities and space-time yields. Due to the statics of the Production building, security, environmental protection and capital lockup an apparatus concept is required, which has a relatively small mercury content gets along. The process should be stable in continuous operation and the usual metallic ones occurring in technical alkali metal amalgam Tolerate contamination undamaged. The term "alkali metal amalgam" refers to a solution of an alkali metal in mercury at the reaction temperature is liquid.

Another method for the old separation of alkali metal from the corresponding Amalgam is described in FR-A-15 55 128.

The present invention relates to an electrolytic cell comprising a moving Liquid containing alkali metal amalgam Anode, the liquid anode being stirred in and / or using a pump one under atmospheric pressure or slightly overpressure Circulation is moved an alkali metal ion conductive Solid electrolyte and a cathode, which is characterized in that the solid electrolyte and the cathode is separated from one another by a liquid electrolyte are.

The present invention also relates to a method for producing an alkali metal using this electrolytic cell.

The liquid electrolyte is expediently chosen so that it is compared to alkali metal is stable. A liquid electrolyte is preferably used, which is in the Electrolysis reaction not used up. In a particularly preferred embodiment an electrolyte melt is used as the liquid electrolyte.

In a preferred embodiment, the present invention therefore relates to a Electrolysis cell as described above, which is characterized in that the Liquid electrolyte is a molten electrolyte.

Depending on which alkali metal is produced using the electrolytic cell according to the invention, different electrolyte melts are expediently used as the liquid electrolyte. In the electrolytic cells according to the invention, NaOH melts, NaNH ₂ melts or mixtures thereof are preferably used in the production of sodium, KOH melts, KNH ₂ melts or mixtures thereof in the production of potassium.

Accordingly, the present invention relates to an electrolysis cell as described above, which is characterized in that the electrolyte melt is a NaOH melt, a NaNH ₂ melt or a mixture thereof or a KOH melt, a KNH ₂ melt or a mixture thereof ,

In a particularly preferred embodiment, these melts or Mixtures of these are used in anhydrous form. In another particularly preferred Embodiment are mixtures and again as electrolyte melts particularly preferably anhydrous mixtures used. Among them are eutectic ones Mixtures preferred.

It is of course conceivable to add one or more suitable additives to the liquid electrolyte. Examples include additives that lower the melting point. In principle, all additives which lower the melting point and which do not interfere with the use of the electrolytic cell and the method according to the invention are suitable. Preferred are melting point-lowering additives which are selected in the production of sodium from the group consisting of NaI, NaBr, Na ₂ CO ₃ and a mixture of two or more thereof, and are selected in the production of potassium from the group consisting of KI, KBr , K ₂ CO ₃ and a mixture thereof.

The anode and cathode compartments of the electrolytic cell according to the invention are separated from one another by a solid electrolyte which conducts helium-tight alkali metal ions. Ceramic materials such as NASICON®, the composition of which is specified in EP-A 0 553 400, are suitable for this purpose in the production of sodium. Glasses that conduct sodium ions as well as zeolites and feldspar are also suitable. A variety of materials can also be used in the production of potassium. Both the use of ceramics and the use of glasses are possible. For example, the following materials can be considered: KBiO ₃ (TN Nguyen et al., Chem. Mater. 1993, 5 , 1273-1276), gallium oxide-titanium dioxide-potassium oxide systems (S. Yoshikado et al., Solid State Ionics 1992, 53 -56 , 754-762), alumina-titanium dioxide-potassium oxide systems and KASICON® glasses (M. Lejeune et al., J. Non-Cryst. Solids 1982, 51 , 273-276).

Sodium β "aluminum oxide, sodium β aluminum oxide and sodium β / β" aluminum oxide are preferred or potassium-β "aluminum oxide, potassium-β-aluminum oxide and potassium β / β "alumina.

The present invention therefore also relates to an electrolysis cell, as described above, which is characterized in that the solid electrolyte is selected from the group consisting of sodium β-aluminum oxide, sodium β "aluminum oxide and sodium β / β "alumina or selected from the group from potassium-beta-alumina, potassium-beta "-alumina and potassium-beta / beta" -alumina.

Potassium β "aluminum oxide, potassium β aluminum oxide or potassium β / β" aluminum oxide can start from sodium beta "alumina, sodium beta" alumina or sodium β / β "aluminum oxide by cation exchange getting produced.

The solid electrolyte expediently has the shape of a thin-walled and nevertheless pressure-resistant, one-sided closed pipe (EP-B 0 424 673), on the open end an electrically insulating ring using a helium-tight, likewise electrically insulating glass solder connection is applied (GB 2 207 545, EP-B 0 482 785). The wall thickness of the electrolyte conducting alkali metal ions lies generally in the range from 0.3 to 5 mm, preferably in the range from 1 to 3 mm, particularly preferably in the range from 1 to 2 mm.

The cross-sectional shape of the tube closed on one side is the preferred one Circular embodiment. However, cross-sectional shapes are also conceivable enlarged surface, for example from a composite of several circular Areas can be derived.

The design of the alkali metal ion-conducting solid electrolyte with regard to its Leak tightness has decisive factors in the method according to the invention Influence, because mercury can only over leaks in the solid electrolyte or Sealing system in the liquid electrolytes and thus also in the alkali metal produced arrive because in the method according to the invention the anode potentials be adjusted so that the formation of mercury ions is excluded becomes.

As a rule, solid electrolytes are used that have leak rates of less than 10 ^-9 (mbar • l) / s in a helium leak test, i.e. are helium-tight within the detection limit.

Furthermore, the releasable sealing connections are preferably carried out so that the Liquid electrolyte and amalgam are each sealed off from the ambient atmosphere become. Detachable seals between liquid electrolyte and amalgam avoided if possible, since the removable seals are usually liquid-tight, but are not gas-tight.

In a preferred embodiment come as releasable sealing connections Flat seals for use, preferably made of graphite, for example made of unreinforced GRAPHIFLEX®. In a preferred embodiment, the Seals with an inert gas such as Flushed with argon or nitrogen to a To prevent oxygen from diffusing through. With helium-tight electrolytes and the listed sealing arrangement are alkali metals with a Mercury residual content in the range of 0.05 to 0.3 ppm obtained.

The geometry of the solid electrolyte is essentially arbitrary and can be special Process conditions are adjusted. In a preferred embodiment the solid electrolyte, as already mentioned above, has the shape of a closed on one side Pipe on. In a further preferred embodiment, this is Tube in contact with the anode on its outside. This anode compartment is more preferably, delimited on its outside by an outer tube that consists of is made of a material that is very dense and resistant to hot Is amalgam. Stainless steel and graphite are particularly suitable as materials. In a further preferred embodiment, stainless steel is selected as the material.

The outer tube can essentially have any cross section. An outer tube is preferably used which is concentric with the solid electrolyte tube is.

The annular gap between the outer tube and the ceramic tube is within the scope of the invention Flow in the longitudinal direction of the liquid anode.

As already mentioned, the annular gap has a preferred gap width of 1 up to 10 mm. More preferably, the annular gap has a width in the range of 2 to 5 mm, particularly preferably a width in the range from 2.5 to 3 mm.

Accordingly, the present invention relates to an electrolytic cell as described above, which is characterized in that the solid electrolyte is closed on one side Tube is formed, which is installed in a concentric stainless steel tube is that there is an annular gap with a gap width in the range of 1 to 10 mm.

The process according to the invention is carried out in an electrolysis cell with a moving liquid alkali metal amalgam anode operated. This is a moving liquid anode, during operation with regard to its alkali metal content is depleted, so that it is enriched with alkali metal-rich amalgam, that in a normal amalgam cell of a chlor-alkali production or by electrolysis of sodium or potassium salts with a mercury or Amalgam cathode, e.g. NaOH or KOH, can be replaced can be.

This can be done in a technically simple manner, since the liquid alkali metal amalgam is easy to promote. As a rule, the concentrated amalgam flow a normal amalgam cell in a heat exchanger the operating temperature of the method according to the invention is heated and the hot fed moving liquid anode. This is best done in one Counterflow heat exchanger so that the hot drained depleted Amalgam heated the inlet.

The replacement of depleted amalgam can be both discontinuous as well done continuously. The continuous procedure is operational easier to do. The disadvantage that usually the incoming concentrate diluted with circulated, already depleted alkali metal amalgam can be compensated for by the fact that the process has several stages is performed.

The liquid anode is expediently stirred and / or with a Pump in an atmospheric pressure or slightly overpressure Circulation moves. The caused by the sales-related exchange of amalgam Movement and / or thermal convection are compared to that in the required movement negligible and sufficient not reaching the preferred current densities.

If the liquid anode is operated without moving, as described in GB 1,155,927, only current densities of 40 to 70 A / m ² can be achieved. With an increase in the cell voltage, the current density can only be increased insignificantly because the resistance of the cell increases with increasing current density. Surprisingly, at moderate cell voltages, ie cell voltages in the range from 0.9 to 1.6 volts for sodium amalgam and from 0.95 to 2.1 volts for potassium amalgam, current densities of 250 to 3000 A / m ^{2 are} achieved when the anode is moved ,

Accordingly, the present invention also relates to a method as described above, which is characterized in that it is carried out at a current density of more than 250 A / m ² .

The anode is moved, for example, by stirring. for example Bubbling gas or using a mechanical stirrer or with a pump. A movement in the form of a forced flow is preferred, such as with an amalgam circuit driven by a pump can be achieved.

The flow rate is generally in the range of 0.03 to 1.0 m / s, preferably in the range from 0.05 to 0.6 m / s and particularly preferably in the range from 0.1 to 0.3 m / s. A higher flow rate allows in the Usually higher current densities. Another design advantage of the anode in The shape of an annular gap lies in the relatively small area of the anode Anode volume. This makes it possible to meet the demand for moderate apparatus weight and acceptable mercury circulating capacity.

Essentially all of them can be used as cathode material in the cell according to the invention suitable materials. Examples include steel, Pure nickel with, for example, the DIN material number 2.4066 or electrode graphite. In a preferred embodiment of the cell according to the invention the cathode is made of steel.

Accordingly, the present invention also relates to an electrolytic cell as above described, which is characterized in that the cathode is a steel cathode.

Suitable steels include stainless steel, austenitic steel or carbon steel. The preferred austenitic steels include Steels with the DIN material numbers 1.4541 or 1.4571 to name as preferred unalloyed steels the steels with the DIN material numbers 1.0305 or 1.0346. In a very particularly preferred embodiment of the invention Unalloyed steels are used in electrolysis cells.

In a further preferred embodiment, the cathode is designed as a rod, which is built into the solid electrolyte designed as a tube. Preferably the rod is installed in such a way that between the solid electrolyte and the Rod creates a gap with a gap width in the range of 1 to 6 mm.

Accordingly, the present invention also relates to a cell as described above which is characterized in that the steel cathode is designed as a rod, which is installed in the solid electrolyte in the form of a tube such that between a gap with a gap width in the inner wall of the solid electrolyte and the rod Range from 1 to 6 mm arises.

The cathode can of course also all in the cell according to the invention have other suitable geometries. For example, as a tube, as Wire mesh or be designed as expanded metal.

In the process according to the invention, the alkali metal is formed on the solid cathode. This increases on the rod which is designed according to the preferred embodiment Cathode in the liquid electrolyte and can be used as a pure metallic Phase are deducted.

When operating the method according to the invention, the action of Water vapor on the ceramics that conduct alkali metal ions is also essential be excluded. As a rule, this leads to the amalgam that traces water heated, the water vapor removed and only then the water-free amalgam-mercury mixture fed to the liquid anode. Removal of water vapor is conveniently by stripping with inert gas or by applying Negative pressure supported.

The present invention also relates to a method as described above, which is characterized in that it is at a temperature in the range of 260 to 400 ° C is carried out.

The current density is generally 0.5 to 10 kA / m ² , preferably 1.0 to 3 kA / m ² (sodium) or 0.3 to 3 kA / m ² , preferably 0.5 to 1, 5 kA / m ² (potassium). The current density is specifically set at the external power source, usually a line rectifier.

In a special embodiment, the electrolysis cell according to the invention integrated into the power supply of an amalgam-supplying chlorine cell, so that a additional line rectifier can be omitted.

When the alkali metal ions conducting solid electrolytes are used for the first time, a too high a ceramic resistance is observed, which in the course of further operation remains high. The resistance of the solid electrolyte can be compared be too high by a factor of 30. This is believed to be due to the lack of reactivity of the surface. The cause is in the action of water in the form of the water content of the ambient air to search. This damage can occur particularly when the ceramics or done during assembly. Therefore, the ceramic tubes are convenient after sintering under vacuum in diffusion-proof aluminum / plastic Packed composite films. The original packaging is used for storage '' Ceramic tubes in tightly closing, argon-filled metal containers locked in.

In a further preferred embodiment of the method according to the invention the solid electrolyte is conditioned in order to lower its resistance to reach.

Accordingly, the present invention also relates to a method as described above, which is characterized in that the solid electrolyte before the implementation the process is conditioned.

Here it is conceivable, among other things, for conditioning the solid electrolyte before and / or after installation in the electrolytic cell with one or more to treat chemical compounds, for example one or more to apply ion-conducting layers. In principle, all are suitable for this chemical compounds conceivable.

If sodium is produced in the process according to the invention, it is possible, for example, to treat the solid electrolyte with NaOH, NaNH ₂ , NaOR or a mixture of two or more thereof. If potassium is produced in the process according to the invention, it is possible, for example, to treat the solid electrolyte with KOH, KNH ₂ , KOR or a mixture of two or more thereof. Here, R denotes a straight-chain or branched-chain alkyl radical having 1 to 5 carbon atoms. R can likewise denote a suitable, optionally suitably substituted aryl or aralkyl radical.

Accordingly, the present invention also relates to a method as described above, which is characterized in that the solid electrolyte with NaOH, NaNH ₂ , NaOR or a mixture of two or more thereof or with KOH, KNH ₂ , KOR or a mixture of two or more of which is conditioned, where R denotes a straight-chain or branched-chain alkyl radical having 1 to 5 carbon atoms.

It is conceivable, among other things, to use the solid electrolyte with a melt alcoholic solution and / or with aqueous solution of the above compounds to treat.

Accordingly, the present invention also relates to a process as described above, which is characterized in that NaOH, NaNH ₂ , NaOR or the mixture of two or more thereof or KOH, KNH ₂ , KOR or the mixture of two or more thereof as a melt or / and as an aqueous solution or / and as an alcoholic solution.

If the solid electrolyte is designed as a tube in a preferred embodiment, it is possible to treat one side or both sides of the tube by treating them to condition chemical compounds. Of course it is too possible to condition the solid electrolyte several times in two or more steps, wherein the chemical compound or the mixture of two or more of which may be the same or different in the individual conditioning steps can.

Another possibility of conditioning the solid electrolyte and lowering the ceramic resistance is to first operate the cell with reversed polarity, ie to operate the anode first as the cathode and the cathode as the anode. In this case, the cathode, like the anode, can consist of sodium amalgam and mercury. The current density in the reversed state is linear over a period of 1 to 44 h, preferably 2 to 6 h, from 50 A / m ² to 3000 A / m ² (sodium) or from 30 A / m ² to 1000 A / m ² (potassium) increased.

The lowest ceramic resistances are obtained when starting for 1 to 24 Hours at an operating temperature of 300 to 350 ° C (sodium) or 250 to 350 ° C (potassium) first liquid alkali metal is used as anode, which then replaced by amalgam. This embodiment of conditioning is particularly preferred.

Of course, it is also possible to use the above types of conditioning perform in combination with each other, all combinations are conceivable.

In a preferred procedure, the current direction is in time intervals from 1 to 24 hours for 1 to 10 minutes reversed by Anode and cathode are short-circuited via an external resistor. The Resistance is dimensioned so that the current strength during polarity reversal is about 1.5 times corresponds to the current in operation. The yield of alkali metal recovered is complete in the process according to the invention, based on the alkali metal converted on the anode side. The current yield of the alkali metal obtained is in normal polarity mode of operation within the measurement accuracy 100%. The averaged current yield is reduced due to the intermittent polarity reversal to values in the range of 95 to 98%.

The amalgam supplied to the anode is in a preferred embodiment depleted from 0.4% by weight to 0.1% by weight of alkali metal. The not implemented Alkali metal is not lost when coupled with chlor-alkali electrolysis, because it is returned to the chlor-alkali cell and via the amalgam cycle comes back from there.

(i) Durchführung einer Chloralkali-Elektrolyse unter Erhalt von elementarem Chlor und Alkalimetallamalgam;

(ii) Durchführung eines Verfahrens, wie oben beschrieben, unter Erhalt von Alkalimetall.

The present invention thus also relates to an integrated process for the production of chlorine and alkali metal from alkali metal chloride, which comprises the following steps (i) and (ii):

(i) performing chlor-alkali electrolysis to obtain elemental chlorine and alkali metal amalgam;

(ii) Performing a process as described above to obtain alkali metal.

Fig.1:

Schematische Darstellung einer erfindungsgemäßen Eletrolysezelle, umfassend einen einseitig geschlossenen rohrförmigen Festelektrolyten, der in ein konzentrisches Edelstahlrohr eingebaut ist, wobei in den rohrförmigen Festelektrolyten selbst wiederum eine stabförmige Edelstahlelektrode eingebaut ist.

Fig.2:

Schematische Darstellung einer für den Dauerbetrieb konzipierten Apparatur, in der die erfindungsgemäße Elektrolysezelle eingebaut ist.

The present invention is explained in more detail in the following example. In this example reference is made to the two Figures 1 and 2 attached to this application

Fig.1:

Schematic representation of an electrolysis cell according to the invention, comprising a tubular solid electrolyte which is closed on one side and which is installed in a concentric stainless steel tube, a rod-shaped stainless steel electrode itself being installed in the tubular solid electrolyte itself.

Figure 2:

Schematic representation of an apparatus designed for continuous operation, in which the electrolytic cell according to the invention is installed.

Example: Apparatus (Figure 1)

The core of the cell according to FIG. 1 consisted of a cell closed on one side Tube (1) made of β "aluminum oxide (32 mm outer diameter, 210 mm length, Wall thickness 1.7 mm). At the open end was a ring made of α-aluminum oxide (2) attached helium-tight using a glass solder connection. By means of this ring (2) was the tube which was conductive with respect to sodium ions and was made of β "aluminum oxide with the opening into a concentric stainless steel tube (3) (with an inner diameter of 37 mm and a length of approx. 215 mm). The inside diameter of the steel tube was matched to the outside diameter of the ceramic tube, so that an annular gap with a gap width of 2.5 mm was created. The over the anode space defined by the annular gap and the tube length fulfilled on the one hand the Demand for an apparatus concept that uses a relatively small mercury, silver content came out. Secondly, the ring cross-section allowed one in terms of the current density very effective flow through the anode space in the axial Direction. The ring was made of α-aluminum oxide (2) with one each Flat gasket below (4) and above (5) over the housing (6) and the cover flange (7) with four clamping screws (8).

An anode power supply (9) was attached to the stainless steel container. For the Amalgam was supplied at the bottom with a pipe socket (10) for the drain at the side a pipe socket (11) welded on top. Protruded from the cover flange Rod made of stainless steel (18) as a cathode in the opening of the tube made of β "aluminum oxide.

A separating space was let into the cover flange, which could be influenced gravity the molten sodium from the heavier electrolyte melt separated.

A tube (13) was passed through the cover flange and was used for free removal of liquid sodium. The cell could be wrapped with electrical heating tapes (14) and be isolated or together with several cells in a heated Chamber to be installed.

The liquid sodium formed was over with pressure generated by reaction the heated drain pipe (13) in an inert, partially filled with paraffin oil The vessel was discharged and solidified in paraffin oil in the form of small balls.

• Dauerversorgung (19) mit trockenem vorgeheiztem Na-reichem Amalgam.
• Beheizung (20), ausgestaltet für Beheizung im Bereich von 310 °C bis 360 °C.
• Gleichstromversorgung (21).
• Definierte Strömungsgeschwindigkeit in der Anode durch einen internen, mit einer Pumpe (23) getriebenen Amalgamkreislauf (22), stufenlos einstellbar im Bereich von 0,02 bis 0,8 m/s.
• Ableitung von flüssigem Natrium (24).
• Dauerentsorgung von Na-armem Amalgam (25).
• Abgasbehandlung (26).
• Sicherheitsüberwachung besonders hinsichtlich Hg-Emission (27).

The electrolytic cell was integrated in an apparatus designed for continuous operation with the following functions (FIG. 2).

• Permanent supply (19) with dry preheated Na-rich amalgam.
• Heating (20), designed for heating in the range from 310 ° C to 360 ° C.
• DC power supply (21).
• Defined flow rate in the anode through an internal amalgam circuit (22) driven by a pump (23), continuously adjustable in the range from 0.02 to 0.8 m / s.
• Derivation of liquid sodium (24).
• Permanent disposal of low-sodium amalgam (25).
• Exhaust treatment (26).
• Security monitoring especially with regard to mercury emissions (27).

Attempt:

The commercial pipe made of sodium β "aluminum oxide was installed promptly within an hour in the laboratory atmosphere after using a vacuum package was removed. After that, both chambers of the cell flooded with argon and the cell closed. The installation in the apparatus took place 2 to 5 days later.

The apparatus was heated to 330 ° C. with a temperature increase from 20 ° C./h. The cathode space inside the ceramic tube, which was closed on one side, was then filled via a feed line with an externally melted melt of 60% by weight NaNH ₂ and 40% by weight NaOH. The anode space outside the ceramic tube was filled with liquid sodium. Over a period of 35 minutes, the current was increased once from 5 A to 40 A in increments of 5 A and then held at 40 A for 4 hours.

After 4 hours the voltage / current ratio had dropped to 0.18 V / 40 A. The anode compartment was then emptied and the amalgam circuit at 39 kg of amalgam filled. The content of the amalgam cycle was switched off Pump warmed to 330 ° C. The circuit was then started up. The sodium still in the anode compartment was rinsed out and distributed in the amalgam itself.

This first filling was discarded and the circuit was filled with fresh amalgam heated to 330 ° C. and containing 0.4% by weight of sodium. An average flow rate of 0.3 m / s was set, which corresponded to a circulation volume flow of 0.29 m ³ / h.

A cell voltage of 0.82 V was set in the de-energized state. The output voltage a DC power supply was limited to 2 volts and the Circuit closed with the cell. It was done in a time frame of 3 hours the current increased linearly from 0 A to 40 A. After that were in the time interval of 30 min each drained 7.8 kg of amalgam from the circulatory contents and replaced by fresh amalgam. It was observed that the cell voltage fluctuated between values in the range of 1.5 to 1.7 volts.

From a current of 40 A at an anode area of 200 cm ^2, a current density calculated from 2000 A / m ^2. This was twice as high as that required for industrial use of the process.

There was a constant discharge of sodium. Sodium discharge and depletion of the amalgam corresponded to Faraday's law. The attempt stayed stable for at least 4000 operating hours.

Claims (14)

1. An electrolytic cell comprising an agitated, liquid anode containing an alkali metal amalgam, the liquid anode being agitated by stirring and/or by means of a pump in a circuit under atmospheric pressure or slight positive pressure, an alkali metal ion-conducting solid electrolyte and a cathode, wherein the solid electrolyte and the cathode are separated from one another by a liquid electrolyte.
2. The electrolytic cell as claimed in claim 1, wherein the liquid electrolyte is an electrolyte melt.
3. The electrolytic cell as claimed in claim 2, wherein the electrolyte melt is an NaOH melt, an NaNH₂ melt or a mixture of these, or is a KOH melt, a KNH₂ melt or a mixture of these.
4. The electrolytic cell as claimed in any one of claims 1 to 3, wherein the solid electrolyte is selected from the group consisting of sodium β-aluminum oxide, sodium β"-aluminum oxide and sodium β/β"-aluminum oxide or from the group consisting of potassium β-aluminum oxide, potassium β"-aluminum oxide and potassium β/β"-aluminum oxide.
5. The electrolytic cell as claimed in any one of claims 1 to 4, wherein the solid electrolyte is fashioned as a unilaterally closed tube which is mounted inside a concentric alloy steel tube in such a way as to produce an annular gap having a gap width in the range of from 1 to 10 mm.
6. The electrolytic cell as claimed in any one of claims 1 to 5, wherein the cathode is a steel cathode.
7. The electrolytic cell as claimed in claim 6, wherein the steel cathode is fashioned as a rod which is mounted in the solid electrolyte as claimed in claim 4 in such a way as to produce, between the inner wall of the solid electrolyte and the rod, a gap having a gap width in the range of from 1 to 6 mm.
8. A process for producing an alkali metal using an electrolytic cell as claimed in any one of claims 1 to 7.
9. The process as claimed in claim 8, which is carried out at a current density greater than 250 A/m².
10. The process as claimed in claim 8 or 9, which is carried out at a temperature in the range from 260 to 400°C.
11. The process as claimed in any one of claims 8 to 10, wherein the solid electrolyte is conditioned.
12. The process as claimed in claim 11, wherein the solid electrolyte is conditioned with NaOH, NaNH₂, NaOR or a mixture of two or more of these, or with KOH, KNH₂, KOR or a mixture of two or more of these, where R denotes a straight-chain or branched-chain alkyl radical having from 1 to 5 carbon atoms.
13. The process as claimed in claim 12, wherein NaOH, NaNH₂, NaOR or the mixture of two or more of these, or KOH, KNH₂, KOR or the mixture of two or more of these are employed as a melt or/and as an aqueous solution or/and as an alcoholic solution.
14. An integrated process for producing chlorine and alkali metal, starting from alkali metal chloride, which comprises the following steps (i) and (ii):

    (i) performing a chlor-alkali electrolysis to obtain elemental chlorine and alkali metal amalgam;

    (ii) performing a process as claimed in any one of claims 8 to 13 to obtain alkali metal.

Images