EP0578946B1 - Electrochemical process for reducing oxatic acid to glyoxylic acid - Google Patents

Abstract

The present invention describes a process for preparing glyoxylic acid by electrochemical reduction of oxalic acid in aqueous solution in divided or undivided electrolytic cells, characterized in that the cathode comprises from 50 to 99.999% by weight of lead and the aqueous electrolysis solution in the undivided cells or in the cathode space of the divided cells additionally contains at least one salt of metals having a hydrogen overvoltage (overpotential) of at least 0.25 V, based on a current density of 2500 A/m<2>, and at least one mineral acid or organic acid. The process of the invention has the advantage that a highly pure, expensive lead cathode is not necessary and industrially available lead-containing materials can be used, for example alloys which, besides lead, contain at least one of the metals V, Sb, Ca, Sn, Ag, Ni, As, Cd and Cu. Periodic rinsing with nitric acid is not necessary.

Description

The present invention relates to a process for the production of glyoxylic acid by electrochemical reduction of oxalic acid.

Glyoxylic acid is an important intermediate for the production of technically relevant compounds and can be produced either by a controlled oxidation of glyoxal or by an electrochemical reduction of oxalic acid.

The electrochemical reduction of oxalic acid to glyoxylic acid has long been known and is generally carried out in aqueous, acidic medium, at low temperature, on electrodes with high hydrogen overvoltage, with or without the addition of mineral acids and in the presence of an ion exchange membrane (DE-AS 458 438) .

Satisfactory results have not been achieved with the electrolysis processes of oxalic acid which have been customary up to now on an industrial scale or in experiments with a longer electrolysis time, since the current yield decreased significantly in the course of the electrolysis (DE-AS 347 605) and the evolution of hydrogen increased.

In order to counter these disadvantages, the reduction of oxalic acid on lead cathodes was carried out in the presence of additives, for example tertiary amines or quaternary ammonium salts (DE-OS 22 40 759, 23 59 863). The concentration of the additive is between 10⁻⁵% and 1%. This additive is then contained in the product glyoxylic acid and must be separated by a separation process. No details are given in the documents mentioned about the selectivity of the process.

In Goodridge et al., J. Appl. Electrochem., 10, 1 (1980), pp. 55-60, various electrode materials are examined with regard to their current efficiency in the electrochemical reduction of oxalic acid. It has been shown that a high purity lead cathode (99.999%) is best suited for the purpose mentioned.

International patent application WO-91/19832 also describes an electrochemical process for the production of glyoxylic acid from oxalic acid, in which, however, high-purity lead cathodes with a purity of more than 99.97% are used in the presence of small amounts of lead salts dissolved in the electrolysis solution. In this process, the lead cathodes are periodically rinsed with nitric acid, which reduces the life of the cathodes. Another disadvantage of this process is that the oxalic acid concentration must be kept constantly in the range of the saturation concentration during the electrolysis. The selectivity is only 95%.

US Pat. No. 4,692,226 mentions that lead or one of its alloys, preferably with Bi, is used as the cathode material for the electrochemical reduction of oxalic acid to glyoxylic acid. No further details are given. A 99.99% lead cathode is used in the examples.

The object of the present invention is to provide a process for the electrochemical reduction of oxalic acid to glyoxylic acid which avoids the disadvantages mentioned above, in particular has a high selectivity, reaches the lowest possible oxalic acid concentration at the end of the electrolysis and a cathode with a high one Long-term stability used. Selectivity is understood to mean the ratio of the amount of glyoxylic acid produced to the total amount of products formed during the electrolysis, namely glyoxylic acid plus by-products, for example glycolic acid, acetic acid and formic acid.

The object is achieved by a method as is the subject of independent claim 1; preferred embodiments are the subject of dependent claims 2-13.

Of particular interest are cathodes which consist of 66 to 99.96% by weight, preferably 80 to 99.9% by weight, of lead and 34 to 0.04% by weight, preferably 20 to 0 , 1 wt .-%, consist of other metals.

Surprisingly, a large number of lead-containing materials can be used as cathodes. In particular, in contrast to WO-91/19832, no high-purity lead has to be used. This has the advantage that conventional inexpensive lead alloys can be used as cathodes. Alloy components are V, Sb, Cu, Sn, Ag, Ni, As, Cd, Ca, in particular Sb, Sn, Cu and Ag. Of interest are, for example, alloys which consist of 99.6% by weight of lead and 0.2% by weight of tin and silver. Of particular interest are conventional lead alloys such as pipe lead (material no. 2.3201, 98.7 to 99.1% Pb; material no. 2.3202, 99.7 to 99.8% Pb), shot lead (material no. 2.3203, 94.5 to 96.8% Pb; material no.2.3205, 93 to 95% Pb; material no.2.3208, 91.5 to 92.5% Pb), hard lead (material no.2.3212, 87 to 88 % Pb), white metal with 70 to 80% Pb, lettering metal, for example PbSn5Sb28 with 67% Pb, metallurgical lead (99.9 to 99.94% Pb) or fine copper lead (99.9% Pb).

The method according to the invention is carried out in undivided or preferably in divided cells. The usual diaphragms made of polymers or other organic or inorganic materials, such as glass or ceramics, which are stable in the aqueous electrolysis solution, are used to divide the cells into anode and cathode compartments. Ion exchange membranes, in particular cationic cation exchange membranes made of polymers, preferably polymers with carboxyl and / or sulfonic acid groups, are preferably used. The use of stable anion exchange membranes is also possible.

The electrolysis can be carried out in all customary electrolysis cells, such as, for example, in beaker or plate and frame cells or cells with fixed bed or fluidized bed electrodes. Both the monopolar and the bipolar circuit of the electrodes can be used.

It is possible to carry out the electrolysis both continuously and batchwise.

All materials on which the corresponding anode reactions take place can be used as anode material. For example, lead, lead dioxide on lead or other carriers, platinum, metal oxides on titanium, for example titanium dioxide doped with noble metal oxides such as platinum oxide, are suitable for the development of oxygen from dilute sulfuric acid. Carbon or titanium dioxide on titanium doped with noble metal oxides are used, for example, for the development of chlorine from aqueous alkali metal chloride solutions.

Aqueous mineral acids or solutions of their salts, such as, for example, dilute sulfuric or phosphoric acid, dilute or concentrated hydrochloric acid, sodium sulfate or sodium chloride solutions, can be used as anolyte liquids.

The aqueous electrolysis solution in the undivided cell or in the cathode compartment in the divided cell contains the oxalic acid to be electrolyzed in a concentration expediently between about 0.1 mol of oxalic acid per liter of solution and the saturation concentration of oxalic acid in the aqueous electrolysis solution at the electrolysis temperature used.

Salts of metals with a hydrogen overvoltage of at least 0.25 V (based on a current density of 2500 A / m) are added to the aqueous electrolysis solution in the undivided cell or in the cathode space of the divided cell. Such salts are mainly the salts of Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, Tl, Ti, Zr, Bi, V, Ta, Cr, Ce, Co or Ni, preferably the salts of Pb , Sn, Bi, Zn, Cd and Cr. The preferred anions of these salts are chloride, sulfate, nitrate or acetate.

The salts can be added directly or can also be generated in the solution, for example by adding oxides, carbonates, in some cases also the metals themselves.

The salt concentration of the aqueous electrolysis solution in the undivided cell or in the cathode compartment of the divided cell is advantageously from about 10 auf to 10 wt .-%, preferably to about 10⁻⁵ to 0.1 wt .-%, each based on the total amount aqueous electrolysis solution.

Surprisingly, it was found that metal salts can also be used which form poorly soluble metal oxalates after addition to the aqueous electrolysis solution, for example the oxalates of Cu, Ag, Au, Zn, Cd, Sn, Pb, Ti, Zr, V, Ta, Ce and Co. In this way, the added metal ions from the product solution can be removed very easily by filtration after the electrolysis to the saturation concentration.

Phosphoric acid, hydrochloric acid, sulfuric acid, nitric acid, trifluoroacetic acid, formic acid or acetic acid are added to the aqueous electrolysis solution in the undivided cell or in the cathode compartment in the divided cell. It is preferred to add mineral acids, particularly preferably nitric acid.

The concentration of the abovementioned acids is between 10⁻⁶ and 10% by weight, preferably between 10⁻⁶ and 0.1% by weight. If acids are added to the catholyte or to the electrolyte of an undivided cell in the concentrations given above, the current yield surprisingly remains above 70% even after several batchwise tests, while the current yield in the absence of the acid is clearly below 70%. At the beginning of the electrolysis, the addition of acid can initially be dispensed with if there are simultaneously salts of the above-mentioned metals in the aqueous electrolysis solution. This is the case, for example, with the first batch-batch approach or with a continuous procedure up to the passage of about 90% of the amount of charge theoretically to be transferred, based on the proportion of oxalic acid that is in the electrolysis cycle at the beginning of the electrolysis. However, if no acid is added in subsequent experiments or in the subsequent period of electrolysis, the current yield drops from experiment to experiment.

Flushing the cathode with 10% nitric acid to regenerate the cathode, as is proposed in the aforementioned international patent application WO-91/19832, leads to a strong removal of the lead cathode and thus to a shortening of the cathode service life.

There is no need for rinsing with nitric acid in the process according to the invention, which represents a considerable advantage of the process according to the invention. Surprisingly, the addition of the acids described above in the concentrations given above does not lead to significant corrosion of the lead cathode.

The current density of the method according to the invention is expediently between 10 and 5000 A / m, preferably 100 to 4000 A / m.

The cell voltage of the method according to the invention is dependent on the current density and is expediently between 1 V and 20 V, preferably between 1 V and 10 V, based on an electrode spacing of 3 mm.

The electrolysis temperature can range from - 20 ° C to + 40 ° C. Surprisingly, it was found that at electrolysis temperatures below + 18 ° C, even at oxalic acid concentrations less than 1.5% by weight, the formation of glycolic acid as a by-product can be less than 1.5 mol% compared to the glyoxylic acid formed. The proportion of glycolic acid increases at higher temperatures. The electrolysis temperature is therefore preferably between + 10 ° C and + 30 ° C, in particular between + 10 ° C and + 18 ° C.

The catholyte flow rate of the process according to the invention is between 1 and 10,000, preferably 50 and 2000, in particular 100 and 1000, liters per hour.

The product solution is worked up using customary methods. In the case of discontinuous operation, the electrochemical reduction is stopped when a certain turnover has been reached. The resulting glyoxylic acid is separated from any oxalic acid still present in accordance with the prior art mentioned above. For example, the oxalic acid can be selectively fixed to ion exchange resins and the aqueous oxalic acid-free solution can be concentrated in order to obtain a commercial 50% by weight glyoxylic acid. In the case of a continuous procedure, the glyoxylic acid is continuously extracted from the reaction mixture by customary methods and the corresponding equivalent proportion of fresh oxalic acid is added simultaneously.

The reaction by-products, in particular glycolic acid, acetic acid and formic acid, are not or not completely separated from the glyoxylic acid by these methods. It is therefore important to have a high selectivity in the To achieve procedures to avoid complex cleaning processes. The process according to the invention is characterized in that the proportion of the sum of by-products can be kept very low. It is between 0 and 5 mol%, preferably below 3 mol%, in particular below 2 mol%, relative to the glyoxylic acid.

The selectivity of the process according to the invention is all the more remarkable in that, even at a low final oxalic acid concentration, that is to say in the range of 0.2 mol of oxalic acid per liter of electrolysis solution, the proportion of by-products is preferably below 3 mol%, based on glyoxylic acid.

The particular advantage of the cathode used according to the invention is that a high-purity, expensive lead cathode can be dispensed with and conventional, technically available lead-containing materials can be used. Periodic rinsing with nitric acid can also be dispensed with, so that the lead wear is kept very low and a long service life of the cathode can be achieved in the technical process.

In the following examples, which explain the present invention in more detail, a divided circulation cell is used, which is constructed as follows:

Kathode:

Blei (99,6 %) mit Anteilen Zinn (0,2 %) und Silber (0,2 %)

Anode:

dimensionsstabile Anode für Sauerstoff-Entwicklung auf Basis Iridiumoxid auf Titan

Kationaustauschermembran:

2-Schichtmembran aus Copolymerisaten aus Perfluorsulfonylethoxyvinylether + Tetrafluorethylen. Auf der Kathodenseite befindet sich eine Schicht mit dem Äquivalentgewicht 1300, auf der Anodenseite eine solche mit dem Äquivalentgewicht 1100, beispielsweise ®Nafion 324 der Firma DuPont;

Abstandhalter:

Polyethylennetze

Circulation cell with 0.02 m electrode area, electrode spacing 3 mm.

Cathode:

Lead (99.6%) with tin (0.2%) and silver (0.2%)

Anode:

Dimensionally stable anode for oxygen development based on iridium oxide on titanium

Cation exchange membrane:

2-layer membrane made from copolymers of perfluorosulfonylethoxy vinyl ether + tetrafluoroethylene. On the cathode side there is a layer with the equivalent weight 1300, on the anode side one with the equivalent weight 1100, for example ®Nafion 324 from DuPont;

Spacers:

Polyethylene nets

The quantitative analysis of the components was carried out by means of HPLC, the chemical yield is defined as the amount of glyoxylic acid produced, based on the amount of oxalic acid consumed. The current yield relates to the amount of glyoxylic acid produced. The selectivity has already been defined above.

Example 1 (comparative example)

without the addition of salts and acid

Electrolysis conditions:

Current density: 2500 A / m Cell voltage: 5 - 8 V Catholyte temperature: 16 ° C Catholyte flow: 400 l / h Anolyte: 2 normal sulfuric acid

Starting catholyte:

2418 g (19.2 mol) oxalic acid dihydrate in 24 l aqueous solution final catholyte after 945 Ah: Total volume: 25.2 l 0.30 mol / l oxalic acid 0.44 mol / l glyoxylic acid Chemical yield 95% Current efficiency 62%

The example shows the unsatisfactory current yield, although a fresh lead cathode was used.

Example 2 (comparative example)

with the addition of lead salts, without the addition of acids

Electrolysis conditions as example 1 Starting catholyte (a)

• (a) 2418 g (19.2 mol) oxalic acid dihydrate in 24 l aqueous solution and addition of 1.76 g lead (II) acetate trihydrate (40 ppm Pb⁺)
  After passing through 950 Ah, a sample was taken to determine the current efficiency, the catholyte was drained, 1300 ml of water were added to the anolyte and a fresh catholyte solution (b) was introduced.
• (b) 2418 g (19.2 mol) of oxalic acid dihydrate in 24 l of aqueous solution and addition of 0.022 g of lead (II) acetate trihydrate (0.5 ppm Pb⁺).
• (c) and (d): Fresh catholyte solution was added two more times as in (b), (c) and (d).

• (a): 81 %
• (b): 70 %
• (c): 67 %
• (d): 60 %

The course of the current yield was as follows:

• (a): 81%
• (b): 70%
• (c): 67%
• (d): 60%

After 4 discontinuous tests and 3800 Ah transferred charge, corresponding to 76 hours of electrolysis time, the current yield had dropped from 81% during test (a) to 60% during test (d). The current efficiency of experiment (d) was in the range of the current efficiency found on a fresh lead cathode without the addition of salts or acids (see Example 1).

Example 3 (comparative example)

Rinsing with 10% nitric acid. Follow-up experiment on example 2

The electrochemical cell was rinsed with 5 l of 10% HNO₃ for 20 minutes at about 20 ° C. The content of lead (II) ions after the rinsing process was 0.88 g / l, which corresponds to a lead wear of 4.4 g.

The example confirms the strong corrosion of the lead cathode when rinsing with nitric acid.

Example 4:

with the addition of lead salts and nitric acid electrolysis conditions as example 1

Starting catholyte (a)

• (a) 2418 g (19.2 mol) oxalic acid dihydrate in 24 l aqueous solution and addition of 1.76 g lead (II) acetate trihydrate (40 ppm Pb⁺)
  After passing through 945 Ah, a sample was taken to determine the current efficiency, the catholyte was drained into a collecting container, 1300 ml of water were added to the anolyte and a fresh catholyte solution (b) was poured in:
• (b) 2418 (19.2 mol) oxalic acid dihydrate in 24 1 aqueous solution and addition of 0.022 g of lead (II) acetate trihydrate (0.5 ppm Pb⁺) and 0.86 ml of 65% HNO₃ (33 ppm)
  The process steps described above under a) were repeated three times and fresh catholyte solution (c), (d) and (e) were used as in (b).

• (a) 78 %
• (b) 80 %
• (c) 71 %
• (d) 72 %
• (e) 71 %.

The course of the current yield was as follows:

• (a) 78%
• (b) 80%
• (c) 71%
• (d) 72%
• (e) 71%.

The weight of the cathode increased slightly during the electrolysis from 1958.3 g before experiment a) to 1958.9 g after experiment e).

Final catholyte in the collection container Total volume: 127 l 0.22 mol / l Oxalic acid (28 moles) 0.52 mol / l Glyoxylic acid (66 mol) 0.0031 mol / l Glycolic acid (0.39 mol) 0.0004 mol / l Formic acid (0.05 mol) 0.0002 mol / l Acetic acid (0.03 mol) Chemical yield 97% Power consumption 4725 Ah Current efficiency 75% selectivity 99.3%

After an initial current yield of 78% during test (a), this rose to 80% during test (b), in order to then stabilize at values just above 70% in the subsequent tests.

Example 5:

with the addition of lead salts and nitric acid

Electrolysis conditions as example 1: Starting catholyte (a)

(a) 2418 g (19.2 mol) oxalic acid dihydrate in 24 l aqueous solution and addition of 0.022 g lead (II) acetate dihydrate (0.5 ppm Pb⁺) and 0.86 ml 65% HNO₃ (33 ppm ).

After passing through 945 Ah, a sample was taken to determine the current efficiency, the catholyte was drained into a collecting container, 1800 ml of water was added to the anolyte and a fresh catholyte solution (b), corresponding to the catholyte solution (a), was poured in and the process steps described three times ( b), (c) and (d) repeated.

• (a) 86 %
• (b) 73 %
• (c) 70 %
• (d) 75 %

The course of the current yield was as follows:

• (a) 86%
• (b) 73%
• (c) 70%
• (d) 75%

Final catholyte in the collection container: Total volume 101 l 0.20 mol / l Oxalic acid (20.2 mol) 0.53 mol / l Glyoxylic acid (53.5 mol) 0.0010 mol / l Glycolic acid (0.10 mol) 0.0004 mol / l Formic acid (0.04 mol) Chemical yield 95% Power consumption 3780 Ah Current efficiency 76% selectivity 99.7%

Example 6:

with the addition of nitric acid, without the addition of lead salts

Electrolysis conditions as example 1 Starting catholyte:

2418 g (19.2 mol) oxalic acid dihydrate in 24 l aqueous solution and addition of 0.86 ml 65% aqueous HNO₃

Final catholyte after 945 Ah: Total volume 25.2 l 0.15 mol / l Oxalic acid (3.8 mol) 0.60 mol / l Glyoxylic acid (15.1 mol) 0.0010 mol / l Glycolic acid (0.026 mol) 0.0004 mol / l Formic acid (0.010 mol) Chemical yield 98% Current efficiency 85% selectivity 99.7%

This example shows that when 65% nitric acid is added, it is not necessary to add lead salts, since a sufficient amount of Pb from the electrode material dissolves. A Pb⁺ concentration of 0.5 ppm was measured in the aqueous electrolysis solution.

Example 7:

Catalytic effect of added metal salts

Before each experiment, the cathode was rinsed with 2 liters of 10% nitric acid for about 10 minutes at about 25 ° C.

Electrolysis conditions as example 1

During the experiment, the amount of hydrogen developed cathodically was measured.

Starting catholyte:

• a) ohne Zusatz eines Metallsalzes
• b) mit 1,46 g Blei(ll)acetat-Dihydrat
• c) mit 1,67 g Zinkchlorid
• d) mit 1,85 g Wismut(lll)nitrat-Pentahydrat
• e) mit 2,01 g Kupfer(ll)sulfat-Pentahydrat und
• f) mit 2,85 g Eisen(ll)-chlorid-Tetrahydrat

403 g (3.2 mol) oxalic acid dihydrate in 4000 ml water

• a) without the addition of a metal salt
• b) with 1.46 g of lead (II) acetate dihydrate
• c) with 1.67 g zinc chloride
• d) with 1.85 g bismuth (III) nitrate pentahydrate
• e) with 2.01 g of copper (II) sulfate pentahydrate and
• f) with 2.85 g of iron (II) chloride tetrahydrate

• a) 23,6 l
• b) 13,1 l
• c) 11,8 l
• d) 18,7 l
• e) 5,4 l
• f) 17,6 l

After passing through 171 Ah, the amount of hydrogen developed cathodically was as follows:

• a) 23.6 l
• b) 13.1 l
• c) 11.8 l
• d) 18.7 l
• e) 5.4 l
• f) 17.6 l

The example shows the catalytic effect of the added metal salts regardless of the acid concentration. The metal salts bring about a significant reduction in the evolution of hydrogen compared to experiment a).

Example 8:

The electrolysis was carried out analogously to Example 4, but a lead-antimony alloy, material no. 2.3202 with a lead content between 99.7 and 99.8%.

The electrolysis was stopped after experiment (d).

• (a) 82 %
• (b) 71 %
• (c) 72 %
• (d) 72 %

The course of the current yield was as follows:

• (a) 82%
• (b) 71%
• (c) 72%
• (d) 72%

Chemische Ausbeute 96 % Stromverbrauch 3780 Ah Stromausbeute 74 % Selektivität 99,1 % Final catholyte in the collection container

Chemical yield 96% Power consumption 3780 Ah Current efficiency 74% selectivity 99.1%

Example 9:

The electrolysis was carried out analogously to Example 4, but a lead-antimony alloy, material no. 2.3205 with a lead content between 93 and 95%.

After experiment (c), the electrolysis was ended.

• (a) 76 %
• (b) 73 %
• (c) 74 %

The course of the current yield was as follows:

• (a) 76%
• (b) 73%
• (c) 74%

Final catholyte in the collection container Total volume 76 l 0.21 mol / l Oxalic acid (16 mol) 0.52 mol / l Glyoxylic acid (39 moles) 0.0046 mol / l Glycolic acid (0.35 mol) 0.0006 mol / l Formic acid (0.04 mol) 0.0011 mol / l Acetic acid (0.08 mol) Chemical yield 95% Power consumption 2835 Ah Current efficiency 74% selectivity 98.9%.

Claims (13)

1. A process for preparing glyoxylic acid by electrochemical reduction of oxalic acid in aqueous solution in divided or undivided electrolytic cells, wherein the cathode comprises from 50 to 99.999% by weight of lead, the missing proportion up to 100% by weight comprising at least one of the metals V, Sb, Ca, Sn, Ag, Ni, As, Cd and Cu, and the aqueous electrolysis solution in the undivided cells or in the cathode compartment of the divided cells in addition contains at least one salt of metals having a hydrogen overpotential of at least 0.25 V, based on a current density of 2500 A/m, and nitric acid, phosphoric acid, sulfuric acid, hydrochloric acid, trifluoroacetic acid, formic acid or acetic acid.
2. The process as claimed in claim 1, wherein the cathode comprises from 66 to 99.96% by weight, preferably from 80 to 99.9% by weight, of lead and from 34 to 0.04% by weight, preferably from 20 to 0.1% by weight, of other metals.
3. The process as claimed in claim 1 or 2, wherein the cathode, in addition to lead, comprises at least one of the metals Sb, Sn, Cu and Ag.
4. The process as claimed in at least one of claims 1 to 3, wherein the cathode comprises 99.6% by weight of lead, 0.2% by weight of Sn and 0.2% by weight of Ag.
5. The process as claimed in at least one of claims 1 to 3, wherein the cathode comprises from 93 to 95% by weight of lead and from 7 to 5% by weight of antimony.
6. The process as claimed in at least one of claims 1 to 5, wherein the aqueous electrolysis solution contains from 10⁻⁶ to 10% by weight, preferably from 10⁻⁶ to 0.1% by weight, of the acid.
7. The process as claimed in claim 6, wherein the acid is nitric acid.
8. The process as claimed in at least one of claims 1 to 7, wherein the concentration of the salts of metals having a hydrogen overpotential of at least 0.25 V, based on a current density of 2500 A/m, in the aqueous electrolysis solution in the undivided cell or in the cathode compartment of the divided cell is from 10⁻⁶ to 10% by weight, preferably from 10⁻⁵ to 0.1% by weight, based on the total amount of the aqueous electrolysis solution.
9. The process as claimed in at least one of claims 1 to 8, which comprises using, as the salts of metals having a hydrogen overpotential of at least 0.25 V, based on a current density of 2500 A/m, the salts of Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, Tl, Ti, Zr, Bi, V, Ta, Cr, Ce, Co, Ni, preferably of Pb, Sn, Bi, Zn, Cd, Cr, or a combination thereof, especially Pb salts.
10. The process as claimed in at least one of claims 1 to 9, wherein the current density is between 10 and 5000 A/m, preferably between 100 and 4000 A/m.
11. The process as claimed in at least one of claims 1 to 10, wherein the electrolysis temperature is between -20°C and +40°C, preferably +10°C and +30°C, especially +10°C and +18°C.
12. The process as claimed in at least one of claims 1 to 11, wherein the oxalic acid concentration in the electrolysis solution is between 0.1 mol per liter of electrolysis solution and the saturation concentration of oxalic acid in the electrolysis solution at the electrolysis temperature used.
13. The process as claimed in at least one of claims 1 to 12, wherein, for the first batch in the case of a discontinuous mode of operation or, in the case of a continuous mode of operation, until approximately 90% of the electric charge to be transferred theoretically, based on the proportion of oxalic acid present in the electrolysis circulation at the start of the electrolysis, have passed through, the addition of the mineral acid or the organic acid is dispensed with.