Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-06-08
2002-04-09
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S407000, C204S220000, C204S221000
Reexamination Certificate
active
06368487
ABSTRACT:
The present invention relates to an electrolytic cell which can be used for electrochemically producing alkali metal from alkali metal amalgam, where for the purpose of the invention the term “alkali metal” denotes sodium and potassium.
The invention further relates to a process for electrochemically preparing alkali metal from alkali metal amalgam, employing said electrolytic cell.
Sodium is an important inorganic basic product which is used, for example, to prepare sodium amide, sodium alcoholates and sodium borohydride. It is produced industrially via the Downs process by electrolysis of fused sodium chloride. This process has a high energy consumption of ≧10 kWh/kg of sodium (Büchner et al., Industrielle Anorganische Chemie [Industrial inorganic chemistry], 2nd Edition, Verlag Chemie, p. 228 et seq.). Moreover, the process has the serious drawback that the electrolytic cells, when switched off, are destroyed owing to solidification of the salt melt. Furthermore, the sodium metal obtained via the Downs process has the drawback that for reasons inherent to the process it is contaminated with calcium whose residual level can merely be reduced by subsequent purification steps, but can never be entirely eliminated.
Potassium likewise is an important inorganic basic product which is used, for example, for preparing potassium alcoholates, potassium amides and potassium alloys. These days it is prepared on an industrial scale in particular by reduction of potassium chloride with sodium. In the first instance this produces NaK which is then subjected to fractional distillation. A good yield is achieved by potassium vapor being continuously removed from the reaction zone, thus shifting the equilibrium to the potassium side (Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition 1998, Electronic Release) . A drawback is that the process is operated at high temperature (870° C.). Moreover, the potassium formed contains about 1% of sodium as an impurity and therefore requires additional purification by a further rectification. The biggest drawback is that the sodium used is expensive. One of the reasons for this is that sodium is produced industrially via the Downs process by electrolysis of fused sodium chloride, requiring an energy input of at least 10 kWh/kg of sodium. This corresponds to about 5.3 kWh/kg of potassium (assuming a yield of 100%).
Sodium amalgam and potassium amalgam are intermediates which are produced in large quantities by chlor-alkali electrolysis using the amalgam process and which, as a rule, immediately after they have been prepared are converted, using water, into aqueous alkali metal hydroxide solution. The alkali metal-depleted or alkali metal-free alkali metal amalgam is normally immediately recycled into the chlor-alkali electrolysis. To maintain the sodium amalgam as a liquid, the sodium concentration of this solution must be kept to values of less than 1 wt %, preferably to values in the range of from 0.2 to 0.5 wt %. To maintain a potassium amalgam as a liquid, the potassium concentration of the solution is less than 1.5 wt %, preferably in the range of from 0.3 to 0.6 wt %. The amalgams produced on an industrial scale essentially contain metallic impurities such as, for example, copper, iron, potassium (in sodium amalgam), sodium (in potassium amalgam), lead and zinc in concentration ranges of from 1 to 30 ppm.
GB 1,155,927 describes a process according to which, employing a solid sodium ion conductor such as e.g. &bgr;-Al
2
O
3
, employing amalgam as the anode and sodium as the cathode, sodium metal can be produced electrochemically from sodium amalgam. Implementation of the process described in GB 1,155,927 does not, however, lead to the results described therein regarding sodium throughput, product purity and current density. Moreover, the system described therein exhibits unstable behavior within a few days if the temperature range claimed is adhered to.
Electrolytic cells which are used in an electrochemical process for preparing alkali metal from alkali metal amalgam and which include a solid ion conductor are often unsuitable for long-term continuous operation. One of the reasons for this is that the solid ion conductor becomes mechanically unstable after a certain operating time.
It is therefore an object of the present invention to provide an electrolytic cell which does not have these drawbacks.
A further object of the present invention is to provide a process for the electrochemical production of alkali metal from an alkali metal amalgam employing said electrolytic cell, said process permitting energetically more favorable production of sodium than the Downs process and/or energetically more favorable production of potassium than the industrial process discussed at the outset. Moreover, the process is to be integratable into the existing coordinated arrangement of a chlor-alkali electrolysis in accordance with the amalgam process, while avoiding those drawbacks which arise in the implementation of the process according to GB 1,155,927. For this to be achieved, the following essential requirements must be met:
The alkali metal reaction on the anode side must satisfy the balance requirements of product integration with the chlor-alkali electrolysis. That means that the outflow concentration of alkali metal in the amalgam of the chlor-alkali electrolysis corresponds to the inflow concentration in the alkali metal electrolysis according to the invention. Moreover, the amounts of amalgam recirculated between chlor-alkali electrolysis and alkali metal electrolysis according to the invention must be kept to an order of magnitude which is justifiable in technical and economic terms. As a rule this is achieved if 50% of the alkali metal content of the inflowing amalgam are converted in the alkali metal electrolysis. The sodium metal in the first instance must be produced in such purity that further process steps to remove mercury can be dispensed with and the drawback of calcium contamination, which obtains in the Downs process, is avoided. The potassium metal in the first instance must be produced in such purity that further process steps to remove mercury can be dispensed with and the sodium content is lower than with the reduction using sodium, where the potassium produced in the first instance contains 1% of sodium. The process is to be capable of industrial-scale implementation and must therefore permit sufficiently high current densities and space-time yields. On the grounds of production building statics, safety, environmental protection and tied-up capital, the equipment needs to be designed so as to manage with a relatively small volume of mercury. The process is to be amenable to stable continuous operation and to tolerate without problems the customary metallic impurities encountered in industrial alkali metal amalgam. The term “alkali metal amalgam” denotes a solution, of an alkali metal in mercury, which is liquid at the reaction temperature.
The present invention therefore relates to an electrolytic cell comprising an agitated, alkali metal amalgam-containing anode, 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.
The present invention also relates to a process for producing an alkali metal using said electrolytic cell.
The liquid electrolyte is expediently chosen so as to be stable with respect to alkali metal. The liquid electrolyte used is preferably not consumed in the electrolytic reaction. In a particularly preferred embodiment, the liquid electrolyte used is an electrolyte melt.
In a preferred embodiment, the present invention therefore relates to an electrolytic cell as described hereinabove, wherein the liquid electrolyte is an electrolyte melt.
Depending on which alkali metal is produced using the electrolytic cell according to the invention, different electrolyte melts are expediently employed as the liquid electrolyte. Preferential use in the electrolytic cells according to the inventio
Guth Josef
Huber Günther
Pütter Hermann
Schierle-Arndt Kerstin
Schläfer Dieter
BASF - Aktiengesellschaft
Gorgos Kathryn
Keil & Weinkauf
Nicolas Wesley A.
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