US3779876A

US3779876A - Process for the preparation of glyoxylic acid

Info

Publication number: US3779876A
Application number: US00281742A
Authority: US
Inventors: D Michelet
Original assignee: Rhone Poulenc SA
Current assignee: Rhone Poulenc SA
Priority date: 1971-08-20
Filing date: 1972-08-18
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18
Also published as: JPS4829720A; ATA716372A; GB1364761A; DD98281A5; BR7205614D0; BE787770A; CH541535A; DE2240731B2; FR2151150A5; AT326624B; DE2240731C3; CA974477A; NL7211021A; DE2240731A1; IT964101B

Abstract

Glyoxylic acid is prepared by a process facilitating recovery of glyoxylic acid, the process being electrolytic reduction of oxalic acid in an electrolysis cell in which A. THE CATHODE IS SOLID AND METALLIC WITH A HYDROGEN OVERVOLTAGE WHICH IS GREATER THAN THE POTENTIAL FOR THE REDUCTION OF OXALIC ACID TO GLYOXYLIC ACID, B. THE SEPARATING DIAPHRAGM IS A CATION EXCHANGE MEMBRANE, C. THE CATHOLYTE COMPRISES AN AQUEOUS SOLUTION OF OXALIC ACID WHICH IS FREE OF A STRONG INORGANIC ACID, D. THE CATHOLYTE MOVING IN A CLOSED PATH BY BEING PASSED INTO THE CATHODE COMPARTMENT, OVER THE SURFACE OF THE CATHODE, BEING REMOVED FROM THERE AND BEING RETURNED TO THE CATHODE COMPARTMENT, AND E. THE TEMPERATURE OF THE CATHOLYTE IS BETWEEN 0* AND 70*C.

Description

United States Patent [191 Michelet PROCESS FOR THE PREPARATION OF GLYOXYLIC ACID [75] lnventorz Daniel Michelet,

Sain'te-Foy-Les-Lyon, France [73] Assignee: Rhone-Poulenc S.A., Paris, France [22] Filed: Aug. 18, 1972 211 App]. No.: 281,742

[30] Foreign Application Priority Data Aug. 20, 1971 France 7130395 52 U.S. Cl. 204/76, 204/77 51 Int. Cl... c070 29/06, 007: 51/40, C07c 53 02 [58] Field of Search .1 ..204/75-77 56] 0 References Cited I UNITED STATES PATENTS 798,920 9/1905 Von Portheim 204/76 1,013,502

1/191 2 Liebknecht 204776 Primary Examiner-F. C. Edmundson Att0rney-.lohn W. Malleyet a1.

[57] ABSTRACT Glyoxylic acid is prepared by a process facilitating recovery of glyoxylic acid, the process being electrolytic reduction of oxalic acid in an electrolysis cell in which a. the cathode is solid and metallic with a hydrogen overvoltage which is greater than the potential for the reduction of oxalic acid to glyoxylic acid,

b. the separating diaphragm is a cation exchange membrane,

c. the catholyte comprises an aqueous solution of oxalic acid which is free of a strong inorganic acid,

(1. the catholyte moving in a closed path by being passed into the cathode compartmentfover the surface of the cathode, being removed from there and being returned to the cathode compartment,'and

e. the temperature of the catholyte is between 0 and 70C.

12 Claims, No Drawings PROCESS FOR THE PREPARATION OF GLYOXYLIC ACID The present invention relates to a process for the preparation of glyoxylic acid by the cathodic reduction of oxalic acid.

The preparation of glyoxylic acid by the cathodic reduction of oxalic acid in sulphuric acid solution has been known since 1903 (German Patent Specification No. 163,842). This cathodic reduction is carried out at a relatively low temperature (below about 40C), higher temperatures leading to the formation of glycollic acid [8. Avery et al., Ber. 32, 2237 (1899); German Patent Specification Nos. 163,842 and 194,038; H. Nakata, Chem. Abs.,25, 2904].

The presence of a strong inorganic acid (generally sulphuric acid) in the catholyte is very important; in 1926, Mohrschulz (Zeitsch. Fur Elektrochem., 32, 449, paragraph 5e) states that the optimum concentration of sulphuric acid is about 2 to 2.5% and that the use of concentrations, which are lower by 0.5%, leads to the formation of only a minimum amount of glyoxylic acid.

German Pat. Specification No. 204,787, which is a Patent of Addition to 194,038 mentioned above describes the use of hydrochloric acid as the electrolyte but the other characteristics of the process remain essentially the same. I industrially, the isolation of the glyoxylic acid from the reactionmedium is complicated by the presence of the strong inorganic acid. 1f the reaction mixture consists of an aqueous solution of oxalic and glyoxylic acids concentrating the solution and cooling it to to 5C precipitates the oxalic acid, leaving a solution of glyoxylic acid which can be either sold commercially as it is, or can be concentrated to obtain crystalline glyoxylic acid. 1f the reaction mixture consists of an aqueous solution of oxalic, glyoxylic and sulphuric acids, it is necessary, in addition to the'cooling, to remove the sulphuric acid from the mixture, for example through precipitation with the aid of alkaline earth hydroxides or salts (German Pat. 163,842, Example 1; Belgian Pat. 757,106, Example 2). German Pat. Specification No. 204,787 describes aiding the removal of this auxiliary electrolyte by replacing thesulphuric acid by hydrochloric acid, which can. be removed by evaporation. However, hydrochloric acid has other disadvantages, principally due to its corrosive character; it is also difficult to remove completely by simple evaporation, be cause the evaporation cannot be carried out at a high temperature because of the risks of degrading the glyoxylic acid.

Finally, processes for the preparation of glyoxylic acid by the cathodic reduction of aqueous solution of oxalic acid, which are free from sulphuric acid, have been described: E. Baur [Zeitsch. fur Elektrochem;

25, 104-5 (1919)] carried out such a process with solid electrodes, but the chemical yields obtained are not very good, confirming the need for sulphuric acid (see Mohrschulz above). Another process which can use aqueous solutions of oxalic acid which are free from sulphuric acid, is described in Belgian Pat. Specification No. 757,106; this process consists essentially of carrying out the electrolysis in an electrolyser which comprises:

a. a mercury cathode,

b. an anode compartment partially immersed in the cathode mercury,

c. an ion exchange membrane forming one of the walls of the anode compartment, and A d. a tube immersed in the mercury, enabling catholyte to be supplied in such a way that it forms a thin layer between the cathode mercury and the ion exchange membrane. This process proceeds more satisfactorily than that of E. Baur, but possesses disadvantages which are inherent in the type of apparatus: compact electrolysers .of the filter press type cannot be produced, mercury, especially in vapour form presents hazards because of its toxicity; the production of industrial electrolysers with horizontal mercury electrodes necessitates the use of large amounts of mercury and installations of large surface areas, also implying large investments; finally, it is difficult to ensure evenness of flow of the thin film of catholyte.

The present invention provides a process for the preparation of glyoxylic acid by cathodic reduction of oxalic acid, which comprises performing the cathodic reduction in an electrolysis cell having a cathode compartment containing a catholyte and solid metallic cathode with a hydrogen overvoltage, which is greater than the potential for the reduction of oxalic acid to glyoxylic acid, and an anode compartment containing an anolyte and an anode, the cathode and anode compartments being separated by a cation exchange membrane diaphragm, the catholyte moving in a closed path by being passed into the cathode compartment, over the surface of the cathode, being removed from there and returned to the cathode compartment, the catholyte being at a temperature of 0 to 70C, preferably 5 to 35C and comprising an aqueous solution of oxalic acid, which is free of strong inorganic acid, but can contain glyoxylic-acid.

The cathode, and anode and the separating diagraphm are preferably coated in parallel planes; advantageously, several of the elementary electrolysis cells can be combined in the manner of a filter press.

Examples of material forming the cathode surface are lead; solid amalgams of lead; alloys of lead with silver, antimony, tin or bismuth; and cadmium. The cathode interior and surface are usually of the same material, e.g., a lead plate, but can be different, e.g., a lead plate'with a load amalgam surface.

Anode of the electrolysis cells usually consists of a' solid electrically conducting material which is electrochemically stable in the anolyte and under the operating conditions considered. Examples of such materials are metals and metalloids such as platinum, platinised titanium, graphite, lead and its alloys, particularly with silver, antimony or tin.

Any known cation exchange membrane can be used to separate the catholyte from the anolyte, but membranes of the homogeneous type and membranes of the heterogeneous type are preferred. Thes membranes can optionally be reinforced with a screen. For carrying out electrolysis operations over a long period, it is naturally preferred to use membranes, which do not swell and which are stable to the action of the various constituents of the catholyte and the anolyte. Examples of such membranes are those described in the specifications of US. Pat. No. 2,681,320 and French Pat. Nos. 1,568,994, 1,575,782, 1,578,019, 1,583,089 and 1,584,187. The permeation selectivity of the membranes used (defined in and measured as in French Pat.

Specification No. l,584,l87) is preferably greater than The catholyte can consist essentially of water and oxalic acid with, optionally, glyoxylic acid; the catholyte can contain oxalic acid without glyoxylic acid only at the start of electrolysis; in the same way, the catholyte can contain glyoxylic acid without oxalic acid only at the end of electrolysis. The concentrations of oxalic and glyoxylic acids can be either constant when the reaction is carried out continuously, or variable when the reaction is carried out discontinuously or at the start of a continuous operation. In all cases, the concentration of oxalic acid is less than the saturation concentration at the temperature of electrolysis; generally, this concentration is greater than 2% by weight, and preferably greater than 3% when the current density is high, these values relating particularly to the constant concentration when the reaction is carried out continuously and to the final concentration when the reaction is carried out discontinuously. The concentration of glyoxylic acid is usually between 3 and 25% by weight, and preferably between and 15% by weight, these values relating particularly to the constant concentration of glyoxylic acid when the reaction is carried out continuously and to the final concentration of this acid when e i is ri d Q Qi QP "E9". lL-

The catholyte can also contain reaction by-products in small amounts, e.g., generally less than 1% by Wei An aqueous acid solution is preferably used as the anolyte, though any other anolyte capable of providing electrical conductivity between the two electrodes can be used. Aqueous solutions of sulphuric or phosphoric acids are usually employed in a concentration generally of 0.l to 5 m ols/litre, and pre ferably 0 5 to 2 mols/litre.

The current density at the cathode is preferably 3 to 50 A/dm especially lO to 3 5 A/dn 1 g g The flow of the catholytein a closed circuit is usually achieved by means of a pump; the circuit can in addition contain attached devices such as a heat exchanger or an expansion vessel. The expansion vessel enables oxalic acid to be added to the catholyte and also some catholyte to be withdrawn in order to extract the glyoxylic acid.

At least one spacer is preferablypresent iii'i'fi'iiitfie and/or cathode compartments; these spacers serve to prevent deformations of the cation exchange membrane, and contact between this membrane and the electrodes and also help to render the catholyte of uniform concentration. These spacers are generally manufactured from synthetic polymers which are chemically inert and which do not conduct electricity; they can be made in the form of interlaced, interwined, knotted or welded yarns (e.g., woven fabrics, grids or nets) or they can be in the form of plates possessing holes or grooves. In practice, these spacers are oriented along planes which are parallel to those of the electrodes and the separating diaphragm.

In the absence of a spacenThe s'b'tVifififi' speed of the catholyte (i.e. the speed in the cathode compartment, assumed to contain no spacer)is usually greater than 1 cm/second, and preferably greater than cm/second.

The catholyte is preferably degassed in the expansion 5 vessel with the aid of a stream of inert gas, e.g., nitrogen.

At the end of electrolysis, the glyoxylic acid is isolated by the known means, especially in the manner described above by concentrating and cooling the catho- 10 lyte, optionally under reduced pressure. The cooling,

which results in precipitation of the oxalic acid, is carried out at temperatures which are generally below 8, and preferably not more than 5C; the degree of concentration and the cooling temperature naturally vary according to the degree of purity desired for the glyoxwhich do not contain strong inorganic acid, e.g., sulphuric or hydrochloric acid and hence facilitate work up and recovery of the glyoxylic acid: it allows electrolysis cells to be produced which are compact and easy to dismantle; it allows the gases which are produced at the anode, especially oxygen, which are capable of creating regions of lower or even zero current density, to be removed easily. It makes it possible to use high current densities and to achieve easily the supply of electricity in series between the various elementary electrolysis cells in an assembly of several cells; it makes it possible to use cells with vertical electrodes; finally, due to the constant geometrical shape of the preferred electrolysis cells, the anolyte and the catholyte can be circulated very rapidly, enabling lower concentrations of oxalic acid to be employed and, as a result, better degrees of conversion in continuous operation to be obtained.

The following Examples illustrate the invention. The chemical yields indicated are yields of glyoxylic acid relative to the oxalic acid converted. Concentrations of solutions expressed as a percentage denote, unless otherwise stated, the number of grams of solute per 100 cm of solution; however, these concentrations in g/ 100 cm differ only slightly from concentrations in (weight/weight) because the solutions employed in the Examples generally have a density of about 1.

EXAMPLE l The reduction of oxalic acid to glyoxylic acid is carried out in an electrolysis cell possessing the following characteristics: both electrodes are rectangular plates of lead, the usable surface area of each of which is 0.8 dm the the cation exchange membrane separating anode and cathode compartments is of the heterogeneous type consisting of a cross-linked, sulphonated styrene/divinylbenzene copolymer, dispersed in a matrix of vinyl chloride polymer, and it is reinforced with a woven fabric of polyethylene glycol terephthalate. The substitution resistance of the membrane is 7 9 cm (measurement made in 0.6 M KCl solution) and its permeation selectivity is 77.5%; the distance from the electrode to the membrane is 3 mm; two pumps cause the catholyte and the anolyte to circulate in the electrolysis cell; and the anolyte and the catholyte circulating in external circuits each contain an expansion vessel equipped with supply and removal pipe- ,lines; the catholyte circuits also contain a cooling The electrolysis conditions are as follows:

current density: 17.5 A/dm voltage: 5.2 V

temperature: betweeen 25 and 28C linear speeds of the anolyte and the catholyte over their respective electrodes: about 1.5 m/second duration: 6 hours anolyte: sulphuric acid in a 10% aqueous solution initial concentration of oxalic acid in the catholyte:

initial concentration of glyoxylic acid in the catholyte: 3,48%

volume of the catholyte: 1.3 l

oxalic acid added to the catholyte: 0.4 I/hour of a 9.65% aqueous solution of oxalic acid.

Sufficient catholyte is removed to keep constant the volume of the catholyte which is circulating.

At the end of the electrolysis, the catholyte which is still circulating and the catholyte, which was removed during the electrolysis are combined, and a solution is obtained in which the concentration of oxalic acid is 4.5% and that of glyoxylic acid is 3.8%.

Current yield: 83%

Chemical yield: 93.6%

EXAMPLE 2 An electrolysis is carried out in the same apparatus as in Example 1, under the following conditions:

current density: 17.5 A/dm voltage: 5.2 V temperature: between 25 and 28C linear speeds of the anolyte and the catholyte over their respective electrodes: 1.5 m/second duration: 9 hours 30 minutes initial volume of the catholyte: 1.3 l initial concentrations in the catholyte:

of glyoxylic acid: 8.9% of oxalic acid: 5.13% oxalic acid added to the catholyte: 0.187 l/hour of a 15.7% solution of oxalic acid. Sufficient catholyte is removed to keep its volume constant. I

At the end of the experiment,-the catholyte, which is still circulating, and the catholyte, which was removed during the electrolysis, are combined; 3.2 l of a solution are thus obtained,.which contains 8.6% of glyoxylic acid and 4.3% of oxalic acid.

Electrical yield: 86% Chemical yield: 92.7% Degree of conversion: 69.5% Crystalline glyoxylic acid is prepared from the above solution by the following method. This solution is concentrated at 30C in vacuo, cooled to 0C and filtered;

the filtrate has a glyoxylic acid content of 45% (weight/weight) whilst the precipitate has an oxalic acid content of 99.5% (weight/weight). These operations are repeated until a solution containing 60% (weight/weight) of glyoxylic acid is obtained. This solution is stored for 24 hours at 5C; a precipitate forms which is filtered off: white crystals of 95.7% pure glyoxylic acid monohydrate are thus obtained.

Example 3 The reduction of oxalic acid to glyoxylic acid is carried out in an electrolysis cell similar to that of Example 1, but the usable surface area of each electrode of which is 2.5 dm

The electrolysis conditions are as follows:

current density: 14 A/dm voltage: 4.55 V

temperature: 2122C linear speed of the electrolytes over the electrodes:

about 1 m/second catholyte introduced initially: 7 l of a 3.64% strength solution of oxalic acid.

This solution is electrolysed for 7 hours 15 minutes, adding fresh catholyte at a rate of 0.542 l/hour with a 14.08% solution of oxalic acid and simultaneous removal of catholyte sufficient to keep the volume of the catholyte constant. The following catholyte then contained 3.07% oxalic acid and 3% glyoxylic acid.

Electrolysis is then carried out continuously for 24 hours, supplying fresh catholyte at a rate of 1.14 l/hour with a 8.5% solution of oxalic acid and simultaneous removal in corresponding amounts as before. At the start of this second stage of electrolysis, the volume of the catholyte was reduced to 7 1; this second stage is a stage of continuous operation in that the concentration of glyoxylic acid remains substantially constant (about 3%).

The catholyte at the end of the experiment contains 4.28% oxalic acid and 3.03% glyoxylic acid.

At the end of the experiment, the catholyte which is still circulating and the catholyte which has been removed are combined to give a total volume of 37.85 1, containing 1,200 g glyoxylic acid and 1,422 g oxalic acid. Electrical yield: 79.6% Chemical yield: 85.5%.

Example 4 In this experiment, the apparatus is the same as in Ex ample 3.

Electrolysis conditions:

current density: 14 A/dm voltage: 4.5 V

temperature: 20C

linear speed of the electrolytes over the electrodes: 1 m/second. The catholyte is continuously freed from gas in the expansion vessel by a stream of nitrogen of about l/hour.

Catholyte originally introduced: 6.3 l of a 3.82% strength solution of oxalicacid.

This'solution is electrolysed for 7 hours, supplying it at a rate of 0.495 l/hour with a 15.85% solution of oxalic' acid with removal of the catholyte sufficient to keep its volume constant.

Electrolysis is then carried out continuously for 34 hours 30 minutes (the concentrations remaining substantially constant), supplying the catholyte at the rate of 0.810 l/hour with 10.58% oxalic acid. During this second period, the volume of the catholyte is kept constant at 9 l. The catholyte at the end of the experiment contains 4.8% glyoxylic acid and 3.79% oxalic acid.

At the end of the experiment, the catholyte, which is still circulating, and the catholyte, which was removed during the electrolysis, are combined to give 38.72 1 of solution containing 1,727 g glyoxylic acid and 1,400 g oxalic acid. Electrical yield: 86.4% Chemical yield: 89.3%

EXAMPLE 5 In this experiment, the assembly is the same as in Example 3, but the cathode is a lead alloy containing 5% of silver.

Electrolysis conditions:

current density: 14 A/dm voltage: 4.6 V

temperature: 20C

linear speed ofthe electrolytes over the electrodes:

1 m/second degassing of the catholyte by means of nitrogen: 200

l/hour catholyte introduced initially: 6 l of a 3.64% solution of oxalic acid.

This solution is electrolysed for 7 hours 40 minutes, supplying fresh catholyte at a rate of 0.495 l/hour with a 16% solution of oxalic acid and simultaneous removal as in previous Examples. Electrolysis is then carried out under conditions of continuous operation for 10 hours, supplying the catholyte at a rate of 0.790 l/hr. with a 10.7% solution. During this period the volume of the catholyte is kept constant and equal to 9 1.

At the end of the experiment, the catholyte contains 4.22% glyoxylic acid and 4.4% oxalic acid.

After having combined the catholyte,which is still circulating and the liquid, which was removed during the experiment, a total volume of 18.65 1 is obtained oontaining 734 g glyoxylic acid and 801 g oxalic acid. Electrical yield: 86.5% Chemical yield: 97.8%

Example 6 The apparatus described in Example 3 is used with the following electrolysis conditions current density: 25 A/dm voltage: 5.45 V

temperature: 20",C

speed of the electrolytes over the electrodes: 1 m/second degassing of the catholyte with nitrogen at a rate of 200 to 300 l/hour catholyte introduced initially: 6.6 l ofa 3.7% solution of oxalic acid.

This solution is electrolysed for 5 hours minutes, supplying the catholyte with 0.8 l/hour of a solution containing 16.7% by weight of oxalic acid and removal to constant volume as in previous Examples. Electrolysis is then contained for 14 hours 45 minutes, supplying the catholyte with 1.460 l/hour ofa 10.65% solution of oxalic acid. During this last period, the volume of the catholyte is kept at 9 1. At the end of the experiment, the catholyte contains 4.64% glyoxylic acid and 4.18% oxalic acid.

The catholyte, which is still circulating, and the catholyte, which was removed are combined and 34.4 1 of solution are obtained containing 1,524 g glyoxylic acid and 1,382 g oxalic acid. Electrical yield: 88.3% Chemical yield: 96.3%.

EXAMPLE 7 The assembly described in Example 1 is used.

The cathode is a plate of pure lead which has been amalgamated at the surface with mercury (0.5 cm Hg for a total surface area of 2 dm). The usable cathode surface area is 0.8 dm

Electrolysis conditions:

current density: 12.5 A/dm voltage: 4.3 V

temperature: 25C

speed of the electrolytes over the electrodes: 1.5

m/second catholyte introduced initially: 1.5 l of a 4.65% solu j tion of oxalic'acid.

This solution is electrolysed for 8 hours suppying the catholyte with 17.85 g/l of oxalic acid and removal to constant volume as in previous Examples. At the end of the experiment, the volume of the catholyte is 1.7 1 containing 5.8% glyoxylic acid and 3.93% oxalic acid. Electrical yield: 89% Chemical yield: 82.4%.

EXAMPLE 8 in this embodiment, an assembly of the filter press type is produced, with 4 cells of 2.5 dm each one being similar to that described in Example 3.

These 4 cells are supplied with electrolyte in parallel, and current in series, two by two.

current density: 15 A/dm voltage: 9.24 V (for 2 cells supplied in series) temperature: 2830C circulation speed of the electrolytes over the electrodes: 1 m/second catholyte introduced initially: 8 l of a 3.46% solution of oxalic acid.

Electrolysis is carried out for 7 hours 35 minutes, supplying 0.660 l/hour of a 36.7% solution of oxalic acid, and removal to'constant volume as in previous Examples.

23 mols of oxalic acid are used as starting material and 7.5 mols of oxalic acid are recovered unused; hence 15.5 moles oxalic acid reacted. 15.2 mols glyoxylic acid are produced. Electrical yield: 75% Chemical Yield: 98.5%.

1 claim:

1. Process for the preparation of glyoxylic acid by cathodic reduction of oxalic acid, which comprises performing the cathodic reduction in an electrolysis cell having a cathode compartment containing a catholyte and a solid metallic cathode which a hydrogen overvoltage, which is greater than the potential for the reduction of oxalic acid to glyoxylic acid, and an anode compartment containing an anolyte and an anode, the cathode and anode compartments being separated by a cation exchange membrane diaphragm, the catholyte moving in a closed path by being passed into the cathode compartment, over the surface of the cathode, being removed from there and returned to the cathode compartment, the catholyte being at a temperature of 0 to C and comprising an aqueous solution of oxalic acid, which is free of strong inorganic acid.

2. A process according to claim 1, wherein in the cathode at least the surface thereof consists of acomposition selected from the group consisting of cadmium, lead, a solid amalgam of lead, and an alloy of lead with a metal selected from the group consisting of silver, antimony, tin and bismuth.

3. A process according to claim 1 wherein the concentration of oxalic acid in the catholyte is greater than 2% by weight.

4. A process according to claim 3, wherein catholyte contains between 3 and 25% by weight of glyoxylic acid.

5. A process according to claim 3, wherein the cathode current density is 3 to 50 Aldm 6. A process according to claim 1, wherein oxalic acid is added continuously to the catholyte and catholyte is withdrawn continuously in order to extract the glyoxylic acid from it.

7. A process according to claim 1, wherein the catholyte having at least a liquid phase is withdrawn, the liquid phase is cooled to a temperature of at most 5" C to deposit a precipitate and the precipitate is filtered off.

8. A process according to claim 1, wherein the anolyte circulates in a manner similar to that of the catholyte, so that the pressure on either side of the cation exchange membrane is substantially the same.

9. A process according to claim 1, wherein at least one of the anode and cathode compartments contains a spacer.

10. A process according to claim 9, wherein the cathode compartment contains a spacer and the apparent speed at which the catholyte circulates in the cathode compartment is greater than 1 cm/sec.

11. A process according to claim 1 wherein the cathode compartment is free from any spacer and the speed at which the catholyte circulates in the cathode compartment is greater than 10 cm/sec.

12. A process according to claim 1, wherein the cathode is selected from the group consisting of lead, an alloy of lead with 5% silver, and lead with an amalgamated lead surface, the cathode current density is 12.5 25 Aldm the cation exchange diaphagm is of the heterogeneous type consisting of a cross-linked sulphonated styrene divinylbenzene copolymer dispersed in a matrix of vinyl chloride polymer, the catholyte and anolyte are both circulated outside the cell, the speed at which the catholyte and anolyte pass over the cathode and anode respectively is cm/sec, the catholyte temperature is 20 30C, the catholyte is degassed with nitrogen, and oxalic acid is added continuously to the catholyte and catholyte is withdrawn continuously in order to extract the glyoxylic acid from it.

Claims

2. A process according to claim 1, wherein in the cathode at least the surface thereof consists of a composition selected from the group consisting of cadmium, lead, a solid amalgam of lead, and an alloy of lead with a metal selected from the group consisting of silver, antimony, tin and bismuth.
3. A process according to claim 1 wherein the concentration of oxalic acid in the catholyte is greater than 2% by weight.
4. A process according to claim 3, wherein catholyte contains between 3 and 25% by weight of glyoxylic acid.
5. A process according to claim 3, wherein the cathode current density is 3 to 50 A/dm2.
6. A process according to claim 1, wherein oxalic acid is added continuously to the catholyte and catholyte is withdrawn continuously in order to extract the glyoxylic acid from it.
7. A process accordiNg to claim 1, wherein the catholyte having at least a liquid phase is withdrawn, the liquid phase is cooled to a temperature of at most 5*C to deposit a precipitate and the precipitate is filtered off.
8. A process according to claim 1, wherein the anolyte circulates in a manner similar to that of the catholyte, so that the pressure on either side of the cation exchange membrane is substantially the same.
9. A process according to claim 1, wherein at least one of the anode and cathode compartments contains a spacer.
10. A process according to claim 9, wherein the cathode compartment contains a spacer and the apparent speed at which the catholyte circulates in the cathode compartment is greater than 1 cm/sec.
11. A process according to claim 1 wherein the cathode compartment is free from any spacer and the speed at which the catholyte circulates in the cathode compartment is greater than 10 cm/sec.
12. A process according to claim 1, wherein the cathode is selected from the group consisting of lead, an alloy of lead with 5% silver, and lead with an amalgamated lead surface, the cathode current density is 12.5 - 25 A/dm2, the cation exchange diaphagm is of the heterogeneous type consisting of a cross-linked sulphonated styrene divinylbenzene copolymer dispersed in a matrix of vinyl chloride polymer, the catholyte and anolyte are both circulated outside the cell, the speed at which the catholyte and anolyte pass over the cathode and anode respectively is 100 - 150 cm/sec, the catholyte temperature is 20 - 30*C, the catholyte is degassed with nitrogen, and oxalic acid is added continuously to the catholyte and catholyte is withdrawn continuously in order to extract the glyoxylic acid from it.