EP0008470A1 - Process for the electrolysis of aqueous alkali metal halide solutions - Google Patents

Abstract

In a process for the electrolysis of aqueous alkali halide solutions in membrane cells, work is carried out in the anode compartment at pH values above 1.0, and the alkali halide solutions are passed through zones for concentration with alkali halide and for pH adjustment after leaving the anode compartment. In order to avoid the formation of high levels of chlorine oxygen acids, which result in a considerable deterioration in the current yield, at least a partial stream of the concentrated solution is adjusted to a pH below 1.0 at elevated temperature and then to a pH in the range raised from 1.0 to 6.0, preferably 1.0 to 2.5. The partial flow is at most 20% and is adjusted to a pH below 1.0 such that the desired pH of 1.0 to 6.0 is achieved after combination with the main flow. The pH value is adjusted above 70 ° C.

Description

The invention relates to a process for the electrolysis 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 adjusting the pH.

Until recently, there were essentially two processes available for the electrolysis of alkali halide solutions, in particular sodium chloride solutions, namely the amalgam and the diaphragm process. The amalgam process has the advantage of producing highly concentrated alkali metal lye of high Allowing purity, however, requires high environmental protection costs due to the use of mercury. The diaphragm method requires this not uses, 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 anode 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 sodium chloride electrolysis, approximately 2/3 of the efficiency losses of 5 to 20% obtained depending on the quality and operating time of the membrane are due to the entry of the OH ions into the anode compartment. Here, the OH-ions with halo gangas ⁱ - depending on the pH of the anolyte - chlorine-oxygen acids or their salts, in particular hypochlorite and chlorides. advice that can only be destroyed by adding acid.

The formation of 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 by the membrane method ", 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-OS 24 09 193) or at about 2 (DE -OS 26 31 523). 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 z _{Ul also being} allowed.

Although the setting of low pH values is recommended with regard 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 there reduce the lye current yield by reaction with the alkali metal hydroxide solution. The halogen current yield is thus improved, but the alkali current yield deteriorates at the same time (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 at least a partial stream of the concentrated solution is brought to a pH at an elevated temperature 1-, 0 and then increases to a pH in the range of 1.0 to 6.0.

In principle, it is possible to adjust the entire alkali halide solution to a pH below 1 and then to bring it to the higher pH by adding alkali.

However, it is particularly advantageous to treat only a partial stream 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 stream, the desired range from 1.0 to 6.0 lying pH is achieved.

For example, a partial flow 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 desired 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 partial flow of 10% should be brought to a pH of 0.6 if a final pH of 1.7 is desired after the combination with the main flow.

In order to destroy the halogen-oxygen acids or their salts as quantitatively as possible and to reduce the risk of C10 _{2 formation} , it is advantageous to adjust the substream to a pH below 0.8 and to adjust it depending on the desired end Select pH 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.

The method according to the invention does two things:

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

As a result of the adjustment of the anolyte to a pH in the range from 1.0 to 6.0, only a slight formation of halogen oxygen acids or their salts takes place, which do not have a 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 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 reunification is 1.7, a content of chlorine-oxygen acid or its salts of 20 g / 1 (calculated as sodium chlorate) maintain.

A further advantageous embodiment of the invention consists in not degassing the electrolyte emerging from the anode space of the membrane cell prior to strengthening with alkali halide, but rather 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 construct tive elements. Polyfluorocarbons are the membrane material. with cation-exchanging groups such as sulfonic acid (SO ₃ H), carboxylic acid (COOH) and phosphonic acid (PO ₃ H ₂ ) groups. Individual fluorine atoms can also be replaced by other halogen atoms, in particular chlorine atoms. Regarding suitable membrane materials cf. also D. Bergner lc, 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, 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 value of the anolyte during operation of the membrane cell by appropriately dimensioning its quantity and its pH value. 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 is explained for example and in more detail with the aid 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 reaches the saturator 6 and is brought to a concentration of approx. 310 g / l with sodium chloride introduced via 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 treatment in the filter device 10 and discharge of the precipitated impurities via line 11 the solution reaches line 12 and is divided into a partial flow 13 and a main flow 14. 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 ^R (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 / 1 NaCl and had a pH of 1.7 and a temperature of 85 ° C. The residence time of the anelyt 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 / 1 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 / 1 and brought to pH 11 in the device 8 with further sodium hydroxide solution, the Contamination was precipitated. 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 into the device 15 via line 13 and there to pH 0 by adding hydrochloric acid , 6 set. As a result of this measure, the chlorine oxygen acid content 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, a mixed pH value of 1.7 on the one hand and an average concentration of chlorine oxygen acids of 20 g / 1 (calculated as NaClO ₃ ) on the other hand were obtained. This concentration was maintained throughout the process.

If, on the other hand, the pure brine emerging from the filter device were only raised to a pH of 1.7, the concentration of chlorine-oxygen acid would reach 140 g / l after a comparatively short operating time. This would reduce the NaCl solubility to 270 g / 1, which would result in an increase in the separation potential by 50 m / V. The resulting occurrence of side reactions would result in a considerable deterioration in the current yield.

Claims (6)

1. A process for the electrolysis 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, characterized in that at least a partial stream of the concentrated solution is adjusted to a pH below 1.0 at elevated temperature and then raised to a pH in the range from 1.0 to 6.0. 2. The method according to claim 1, characterized in that a partial flow of a maximum of 20%, preferably from 8 to 15%, is set to such a pH below 1.0 that, after combination with the main stream, the desired, in the range of 1.0 to 6.0 lying pH is achieved. 3. The method according to claim 2, characterized in that the partial stream is adjusted to a pH below 0.8. 4. The method according to claim 1, 2 or 3, characterized in that one makes the adjustment to a pH below 1.0 at a temperature above 70 ° C, preferably in the range of 80 to 90 ° C. 5. The method according to one or more of claims 1 to 4, characterized in that the pH of the electrolyte to be supplied to the anode chamber is adjusted to a value in the range from 1.0 to 2.5. 6. The method according to one or more of claims 1 to 5, characterized in that the electrolyte leaving the anode compartment is adjusted to a pH of 7 to 10.

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