Electrolytic cell containing nickel alloy anode and silver cathode

The present invention relates to an electrolytic cell comprising a nickel alloy anode and a silver cathode, which is preferably used for electrochemical preparation of 3,6-dichloro-picolinic acid.

Substitution of chlorine with hydrogen in the active chlorine compounds by electrochemical reduction is a known and very important process.

For example, it is known to prepare 2,3,5,6-tetrachloropyridine by electrochemical reduction from pentachloropyridine and 2,3,5,6-tetrachloropyridine for the preparation of various herbicides, insecticides. A similarly known process is the preparation of 3,6-dichloro-picolinic acid from tetrachloro-picolinic acid or 3,5,6-trichloro-picolinic acid.

The economics of electrochemical processes are highly dependent on the electrochemical cells used, whose energy utilization, reasonable cost, long life and selectivity in the desired reaction have a significant effect on the process. The cells suitable for the reduction of the organochlorine compounds (to replace the chlorine hydrogen) include, as a minimum, a cathode at which electrochemical dechlorination takes place, an anode at which water is converted to oxygen, and an electrolyte which initially contains the organic chlorine compound to be reduced.

However, the cells currently used for electrochemical reduction of organochlorine compounds are known to have insufficient anode. The quality of graphite anodes is highly dependent on the type of graphite used, is prone to breakage, is easily lost in activity and is not sufficiently selective. They also have the disadvantage that they contain traces of heavy metals which are washed into the electrolyte and deactivate the cathode. The electrochemical cells containing the graphite anode thus have a very short operating time. The disadvantages of stainless steel anodes are that they corrode very easily, and this corrosion not only damages the anode but also damages the electrolyte with heavy metal ions and thus deactivates the cathode. thus, the life of such cells is also very short.

Therefore, the development of an anode suitable for the reduction of electrochemical organic chlorine compounds is a very important task. For example, the appropriate anode must meet the following requirements:

1) it must not be fragile, it must be dimensionally constant,

2) they must be resistant to corrosion

(a) in an aqueous alkaline medium containing chloride ion,

(b) concentrated hydrochloric acid,

c) between cathodic and anodic potential changes,

3) do not contaminate the electrolyte and the cathode with heavy metal ions,

4) be capable of producing oxygen in an aqueous solution containing chlorine ions,

5) together with a suitable cathode, be capable of selectively replacing chlorine in organic chlorine compounds with hydrogen.

The present invention relates to an electrochemical cell comprising one or more anodes and one or more silver cathodes, suitable for the reductive production of 3,6-dichloro-picolinic acid from tetrachloro-picolinic acid or 3,5,6-trichloro-picolic acid having an anode surface of 40 -70% by weight of nickel, 5-30% by weight of bromine, 3-25% by weight of molybdenum and optionally 5-15% by weight of an alloy containing one or more of the following metals: iron, tungsten, manganese, cobalt.

The anode of nickel alloy used in the electrochemical cell of the present invention reduces the corrosion and the contamination and fragility problems that have led to the short life of the cells.

The anode used in the cell according to the invention does not contaminate the electrolyte or cathode with heavy metal ions in non-fragile, dimensionally stable, corrosion-resistant chloride ion containing aqueous hydrochloric acid and at varying anode and cathode potentials. with a suitable cathode suitable for the selective conversion of chlorine to hydrogen in organic chlorine compounds. Examples of such nickel alloys are Hastalloy C-276 (Cabot Corp.), Inconel 718 and Nimonic 115 (INCO Companies), Ildimet 200,500 and 700 (Special Metals Corporation), Rene '41 (Teledyne Corp.) and Waspaloy (United Technologies). Corp.). Preferably, the anodes having an alloy containing 50-651% nickel, 12-201% chromium and 4-201% molybdenum are present on the surface. Particularly preferred is the Hastalloy C-276 alloy, which contains 551% nickel, 161% chromium, 16% K2 molybdenum, 5% iron, 4% tungsten, 2.5% cobalt and 11% manganese.

Any cathode compatible with the medium used and capable of electrochemically converting chlorine in organic chlorine compounds to hydrogen is used in the electrochemical cell of the invention. Preferred examples are those described in 4.242.183. The silver cathode described in U.S. Patent No. 4,260,408 and particularly preferred is the expanded silver cathode disclosed in U.S. Patent 4,460,441. For both cathodes, the surface has a microcrystalline layer formed from colloidal hydrated silver oxide in an aqueous basic medium.

In use, the cells of the invention contain an aqueous alkaline electrolyte. The solution is made basic by the addition of compatible compounds capable of forming hydroxyl ions in the solution, such as alkali metal, alkaline earth metal or tetraalkylammonium hydroxide. Because the chloride ion is formed as a by-product in the reductive dechlorination process, the chloride ion is usually present, but chloride compounds such as sodium, potassium or tetraalkylammonium chloride are also frequently added. Other compatible water-soluble salts may also be used. Compatible water soluble organic solvents may also be used as co-solvents in addition to water. The ionic organic chlorine compounds used in the electrochemical process and their reaction products are also components of the electrolyte. The non-ionic organochlorine compound-2EN 201014 Β two, when used in the reductive dechlorination process, is dispersed in the electrolyte by dispersion or suspension. The term "compatible" according to the invention means that said substance in the cell is not reduced, oxidized and does not adversely affect the cell material.

The shape and dimensions of the electrochemical cell of the present invention, cathode and anode, are consistent with those used in the art for similar purposes. Cells containing other cathodes and anodes are generally preferred, since they are designed to operate continuously.

The cells of the present invention are constructed, for example, as described in Encyclopedia of Chemical Technology, 3rd Edition (1979), Vol. 8, pp. 696-705. side.

The electrolytic cell of the present invention, operated by a nickel-alloy anode, is particularly advantageous for the preparation of 3,6-dichloro-picolinic acid by electrolytic reductive dechlorination from tetrachloro-picolinic acid or 3,5,6-trichloro-picolinic acid.

The essence of this improved process is that the life of the cells is significantly increased, their productivity on the fly is increased and the production cost of the desired product is reduced. The implementation of this improved process is provided by an anode of nickel alloy, which has a higher corrosion resistance than known and used anodes and has a longer lifetime and does not contaminate either the electrolyte or the cathode, thus increasing the lifetime of the latter.

The following examples further illustrate the invention. 35

First Example

An electrolytic cell equipped with a Teflon-coated magnetic stirrer, a cylindrical silver sieve cathode, an unperforated cylindrical anode of Hastalloy C-276, a standard caliper electrode (SCE), and a thermometer and a Luggin capillary tube was filled to the desired volume. With 18% aqueous hydrochloric acid (the Luggin capillary is then removed) and the acid in the cell for approx. 15 After stirring for 1 minute, the acid is removed, the cell is rinsed with reverse osmosis (RO) and filled with 108 g of 7% sodium hydroxide (mercury grade, caustic, solution in RO water). The cathode was cathodically treated at 0.7 V vs SCE for 7 minutes, then at -1.3 V vs. SCE (6.0 ampermax), with a background current of 0.5 amperes. Subsequently, tetrachloro-picolinic acid (11.76 g, 0.0451 mol) was added to the cell in small portions for 1.5 hours by flushing with the cell liquid in 3 g portions and the resulting slurry was added to the solution.

During electrolysis the cathode potential

It is maintained at 1.3 V while the cell current varies between 0.5 and 4.7 amps. After the addition of 9 g of tetrachloro-picolinic acid, the cathode is reactivated as described above and the remaining 2.7 g is then added. Total reaction time

2.3 hours.

Dilute 50 g of the final 190.3 g of cellular liquid with 100 ml of water, adjust the pH to 0.94 with hydrochloric acid, extract the resulting extract 7 times with 50 ml of methylene chloride and combine the extracts with sodium hydroxide. dried over sulfate, filtered and concentrated under reduced pressure at 50-60 ° C to give 2.26 g of 3,6-dichloro-picolic acid as a white solid (8.6 g total).

Further electrolysis was carried out using the expanded silver cathode as described above, the results of which are summarized in the following table.

First Spreadsheet

<td rowspan="2">reactions hour</td><td rowspan="2">Current utilization,%</td><td colspan="2">3,6-dichloro-picolinic</td>

<td>yield %</td><td>Purity %</td>

<td> 2,30</td><td>ΊΊ, ί</td><td> 99,0</td><td> 98,6</td>

<td> 2,60</td><td> 74,2</td><td> 97,6</td><td> 98,2</td>

<td> 2,10</td><td> 74,3</td><td> 95,5</td><td> 98,2</td>

<td> 2,05</td><td> 75,4</td><td> 94,7</td><td> 98,6</td>

<td> 2,00</td><td> 71,0</td><td> 93,3</td><td> 98,9</td>

<td> 2,00</td><td> 73,8</td><td> 98,3</td><td> 99,8</td>

<td> 2,00</td><td> 74,2</td><td> 95,3</td><td> 97,1</td>

<td> 1,80</td><td> 74,7</td><td> 94,6</td><td> 97,3</td>

Second Example

In a continuous electrolysis cell formed from several expanded silver plate cathodes and a Hastalloy C-276 plate electrode arranged alternately, tetrachloro-picolinic acid is reduced to 3.6 dichloro-picolinic acid in a continuous mode. Electrolysis at 50 ° C, 0.1 amp / cm² below the current density while applying a Luggin voltage of less than 1.3 V to the cathode. The cathode is often reactivated according to known methods. The electrolyte contains 21% sodium hydroxide, less than 3.6% sodium chloride and 1.2% tetrachloro-picolinic acid. During the electrolysis, the concentration of sodium hydroxide and tetrachloro-picolinic acid is kept constant by adding a sufficient amount of a solution containing 25% by weight of sodium hydroxide and 12% by weight of tetrachloro-picolic acid. The cellular fluid is acidified with hydrochloric acid to form 3,6-dichloropico-3HE 201014 Β

6 for precipitation of linic acid. In this way, 3,6-dichloro-picolinic acid of relatively uniform purity is obtained in high yield.

The cell was operated for 11 months, and the electrodes were visually inspected every 3-4 months. During the 5 operations, the anode worked perfectly, with slight corrosion.