EP0008470B1 - Process for the electrolysis of aqueous alkali metal halide solutions - Google Patents

Abstract

An improvement in a process of electrolyzing an aqueous solution of an alkali metal halide in a membrane cell in which a pH above 1.0 is maintained in the anode chamber, the alkali metal halide solution is removed from the anode chamber and its concentration and pH are increased is described. The improvement resides in thereafter decreasing the pH of at least a portion of the resultant solution to below 1.0 and thereafter adjusting the pH to 1.0 to 6.0 and returning the so adjusted solution to the anode chamber.

Description

The invention relates to a method for the electrical analysis of aqueous alkali halide solutions in membrane cells at pH values above 1.0 in the anode compartment, the alkali halide solution being passed through the anode compartment and zones for concentration with alkali halide and for pH adjustment.

Until recently, essentially two methods, namely the amalgam and the diaphragm method, have been available for the electrolysis of alkali halide solutions, in particular sodium chloride solutions. The amalgam process has the advantage of permitting the production of highly concentrated alkali lye of high purity, but due to the use of mercury requires high costs for environmental protection. The diaphragm process does not require such expenditure, but only enables the production of alkali lye of considerably lower concentration, which also has considerable alkali halide contents.

After it was possible to create so-called ion-exchange membranes, which are chemically resistant, hydraulically impermeable and essentially only permeable to cations, membrane processes are becoming increasingly important. It can be assumed that the membrane process will be the process of the future.

In the membrane process, the anode and cathode spaces of the electrolysis cell are separated by an ion exchange membrane, through which essentially only the alkali ions can pass. These are electrically neutralized at the cathode and form alkali water and hydrogen in the cathode compartment with water. Halogen ions cannot pass through the membrane and are therefore only released in the anode compartment in the form of halogen gas.

When using the currently known ion exchange membranes, however, it cannot be avoided that some of the OH ions formed in the cathode space migrate through the membrane into the anode space. For example, in the electrolysis of sodium chloride _^2/3 of efficiency losses of 5 to 20% obtained in dependence on the quality and duration of the membrane are about due to the entry of the OH ions in the anode compartment. Here, the OH ions form with halogen gas - depending on the pH of the anolyte chlorine-oxygen acids or their salts, especially hypochlorite and chlorate, which can only be destroyed by adding acid.

The formation of the chlorine oxygen acids or their salts is ultimately responsible for the fact that the solubility of the alkali halide decreases. This makes the separation potential - in extreme cases around 50 mV - less noble.

In order to eliminate the disadvantage described above, it is known to carry out the electrolysis in the membrane cell with acidic anolyte, with so much hydrochloric acid being added to the brine that the penetrating OH ions are just being neutralized [D. Bergner “Electrolytic chlorine generation using the membrane process”, Chemiker-Zeitung 101 (1977), pages 433 to 447].

It is also known to measure the acid addition in such a way that the pH of the anolyte is in the range from about 1 to 5, preferably 3.0 to 4.0 (DE-A-2 409 193) or at about 2 ( DE-A-2 631 523). In the process according to US Pat. No. 3,948,737, the pH of the anolyte should not be above 4.5, preferably between 2.5 and 4.0, values of pH = 1 and below also being permitted. In the process known from US Pat. No. 4,040,919 for the electrolysis of alkali halide solutions in a membrane cell, the anolyte is adjusted to a pH in the range from 0 to 3 and preferably from 1 to 2.5 by means of hydrochloric acid. Similarly, in the process of GB-A-1 174264 in the case of alkali metal chloride electrolysis in a diaphragm cell, the alkali metal halide solution enriched with alkali metal halide is adjusted to a pH of 0.2 to 4.5 and preferably 1.5 to 4. The temperature of the anolyte is kept in the range of 85 to 100 ° C.

Although it is advisable to set lower pH values with a view to the destruction of the chlorine oxygen acids or their salts, it is disadvantageous that when very high hydrogen ion concentrations are set, hydrogen ions migrate through the membrane into the cathode compartment and reduce the lye current yield there by reaction with the alkali metal hydroxide solution. The halogen current yield is thus improved, but at the same time the lye current yield is deteriorated (D. Bergner 1. c.).

The object of the invention is to provide a process which is simple to carry out, avoids the disadvantages of the known processes and leads to advantageous results with regard to both halogen yield and alkali yield.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that only a partial stream of the enriched alkali metal halide solution is adjusted to a pH below 1.0.

It is particularly advantageous to treat only a partial flow of at most 20%, preferably from 8 to 15%, and to choose the addition of acid in such a way that, after combination with the main flow, the desired range from 1.0 to 6.0 pH is achieved.

For example, a partial stream of 8% has to be separated from the alkali halide solution, which is usually obtained with a pH of about 11 from the zones of saturation with alkali halide and the precipitation and filtration of the impurities, and adjusted to pH = 0.4, if a final pH of 1.5 is sought after reunification. On the other hand, under otherwise identical conditions, a partial flow of 15% should only be set to a pH of 0.67. A part Stream of 10% should be brought to a pH of 0.6 if, after combining with the main stream, a final pH of 1.7 is desired.

In order to destroy the halogen-oxygen acids or their salts as quantitatively as possible and to reduce the formation of CIO _z , it is advantageous to adjust the partial flow to a pH below 0.8 and to adjust it depending on the desired final pH Choose value accordingly small.

The pH is preferably set below 1 at a temperature above 70 ° C., in particular in the range from 80 to 90 ° C., since this promotes decomposition.

• Durch die Ansäuerung, insbesondere eines Teilstromes, unter einen pH-Wert von 1, vorzugsweise unter 0,8, werden die Halogensauerstoffsäuren bzw. deren Salze praktisch quantitativ zerstört.

The method according to the invention does two things:

• Acidification, in particular of a partial stream, below a pH of 1, preferably below 0.8, destroys the halogen oxygen acids or their salts practically quantitatively.

As a result of the adjustment of the anolyte to a pH in the range from 1.0 to 6.0, there is only a slight formation of halogen oxygen acids or their salts, which do not have a very disadvantageous effect on the current efficiency. In this respect, particularly favorable conditions are achieved if, in a further advantageous embodiment of the invention, the pH of the electrolyte to be fed to the anode chamber is set to a value in the range from 1 to 2.5.

In the preferred embodiment of the invention, a partial stream is in turn branched off from this stream for the purpose of virtually complete destruction of the halogen oxygen acids or their salts, so that ultimately a steady state is established in which as much halogen oxygen acids are destroyed by the partial stream treatment as are formed in the anode compartment. For example, when a partial flow of 10% is separated off, the pH is adjusted to 0.6 and the pH of the anolyte after recombining is 1.1, a content of chlorine-oxygen acid or its salts of 20 g / l (calculated as sodium chlorate ) maintained.

A further advantageous embodiment of the invention consists in not degassing the electrolyte emerging from the anode space of the membrane cell before strengthening with alkali metal halide, but instead to adjust it to a pH of about 7 to 10 by adding alkali metal hydroxide solution. As a result, the dissolved halogen gas, which is present in small amounts, is converted into halogen oxygen acids or their salts, which are largely eliminated anyway by the acidification that occurs after the saturation and removal of the impurities.

The membrane cell itself has the known constructive elements. Polyfluorohydrocarbons with cation-exchanging groups, such as, for example, sulfonic acid (SO, H), carboxylic acid (COOH) and phosphonic acid (PO ₃ H ₂ ) groups, are suitable as membrane material. Individual fluorine atoms can also be replaced by other halogen atoms, in particular chlorine atoms. Regarding suitable membrane materials cf. also D. Bergner 1. c., page 441, right column ff.

The anodes to be used in carrying out the method according to the invention can consist of graphite. However, titanium, niobium or tantalum electrodes coated with noble metal or noble metal oxide or so-called dimensionally stable anodes, in which the electrocatalytic effect of mixed oxides of noble metals and film-forming metals, in particular titanium, originate, are particularly advantageous.

Steel and nickel, nickel in the form of the so-called porous double skeleton cathodes are particularly suitable as the cathode material.

The method according to the invention in its preferred embodiment with partial flow separation gives the possibility of changing the pH of the anolyte during operation of the membrane cell by appropriately dimensioning its quantity and its pH. In particular, signs of aging in the membrane can be compensated for by lowering the pH of the anolyte. Different membrane cells can also be supplied with anolyte of different pH values by differently dimensioning the partial and main streams.

The invention will be explained, for example, and in greater detail on the basis of the flow diagram and the exemplary embodiment.

1 shows the anode spaces of two membrane cells for sodium chloride electrolysis. Chlorine gas is discharged via line 20. The electrolyte depleted in sodium chloride reaches the treatment room 4 via lines 2 and 3, is mixed there with sodium hydroxide solution supplied via line 5 and adjusted to a pH of 7 to 10. In this process, dissolved chlorine gas is converted into hypochlorite, from which, depending on pH, temperature and time, some or all of the sodium chlorate is formed.

The solution then passes into the saturator 6 and is brought to a concentration of about 310 g / l with sodium chloride introduced over 7. In the downstream device 8, the impurities, in particular the calcium and magnesium ions, are precipitated by adding sodium hydroxide solution above 9 to a pH of approximately 11. After the treatment in the filter device 10 and discharge of the precipitated impurities via line 11 reaches the Lö _- solution in line 12 and is introduced into a partial stream 13 and a principal stream 14 split. While the main stream 14 flows in the direction of the anode compartments 1, the partial stream 13 in the device 15 is brought to a pH below 1.0, preferably below 0.8, by adding concentrated hydrochloric acid via line 16. Chlorine oxygen acids or their salts are largely destroyed with the formation of chlorine. The chlorine gas is combined with the chlorine gas originating from the anode spaces 1 of the membrane cells using a line 21

The solution then flows off via line 17 and - mixed with the solution of the main stream 14 - is fed via line 18 or 19 to the anode compartments 1. By installing appropriate control valves, variable mixing ratios and thus different pH values can be set in the solutions flowing through lines 18 and 19, respectively.

Embodiment

Two membrane cells with steel cathodes and dimensionally stable anodes based on titanium were used for the electrolysis. The membranes consisted of ethylenediamine-modified Nafion® (a product from DuPont). The applied cell voltage was 3.8 volts.

The anode compartments 1 of the membrane cells were charged with a brine which contained 310 g / l NaCl and had a pH of 1.7 and a temperature of 85 ° C. The residence time of the anolyte in the anode compartments 1 was measured in such a way that the decrease in NaCl was 25 g / l. During this time, approx. 2 g / l chlorine oxygen acids (calculated as NaClO ₃ ) were formed.

The electrolyte solution emerging from the anode compartments 1 was adjusted to pH 8 in the treatment compartment 4 with sodium hydroxide solution, then strengthened again in the saturator 6 to a NaCl concentration of 310 g / l and brought to pH 11 in the device 8 with additional sodium hydroxide solution, the contamination was felled. After filtration in the filter device 10, the electrolyte was adjusted to pH 1.7 in the initial phase of the process and returned to the anode compartments. After the concentration of chlorine-oxygen acid had increased to 22 g / l (calculated as NaClO ₃ ), a 10% partial stream of the pure sols emerging from the filter device 10 was passed via line 13 into the device 15 and there to pH 0 by adding hydrochloric acid , 6 set. As a result of this measure, the content of chlorine-oxygen acid in the partial stream was reduced to 2 g / l. The chlorine gas formed was passed via line 21 to line 20.

After combining the partial stream largely freed from chlorine-oxygen acid with the main stream of pure brine conducted via line 14, on the one hand a mixed pH of 1.7 and on the other hand an average concentration of chlorine-oxygen acids of 20 g / l (calculated as NaClO ₃ ) was obtained. This concentration was maintained throughout the process.

In contrast, if the pure brine emerging from the filter device were only raised to a pH value of 1.7, the concentration of chlorine-oxygen acid would reach 140 g / l after a comparatively short operating time. This would reduce the NaCI solubility to 270 g / l, which would increase the separation potential by 50 m / V. The resulting occurrence of side reactions would result in a considerable deterioration in the current yield.

Claims (5)

1. A process of electrolyzing aqueous solutions of alkali halides in membrane cells in which pH values above 1.0 are maintained in the anode chamber and the alkali halide solution is moved through the anode chamber, said solution being strengthened to increase its alkali halide concentration, its pH value being adjusted to a pH value below 1.0 and subsequently adjusted to a pH value in the range of 1.0 to 6.0 characterized in that a partial stream of the solution which has been strengthened is adjusted to a pH value below 1.0.

2. A process according to claim 1, characterized in that a partial stream amounting to up to 20%, preferably 8 to 15%, is adjusted to such a pH value below 1.0 that the desired pH value in the range of 1.0 to 6.0 is obtained when the partial stream has been recombined with the main stream.

3. A process according to claim 2, characterized in that the partial stream is adjusted to a pH value below 0.8.

4. A process according to any of claims 1 to 3, characterized in that the electrolyte to be fed to the anode chamber is adjusted to a pH value in the range of 1.0 to 2.5.

5. A process according to any of claims 1 to 4, characterized in that the electrolyte leaving the anode chamber is adjusted to a pH value of 7 to 10.

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