DE1804956C3 - Electrolysis device for the continuous regeneration of salt solutions of trivalent chromium to hexavalent chromium compounds - Google Patents

Description

The invention relates to an electrolysis device for the continuous regeneration of Salt solutions by oxidation of the trivalent chromium to hexavalent chromium compounds, consisting of an in a cathode compartment and an anode compartment divided electrolytic cell in which facilities are provided are to the solution to be treated continuously from the cathode compartment to the anode compartment transfer, and a plurality of electrode plates supported by frames in a filter press-like arrangement are held and form a series of cell units, the plates so in series are interconnected that one side of the plate acts as an anode and the other as a cathode, and where A diaphragm is arranged between adjacent electrode plates, each cell unit in one Anode and a cathode compartment separates, and which has at least one narrow opening, which at one such point is arranged and has a sufficient area that one after the other through the cathode ^ space and through the anode space a controlled flow of the solution without electrical short-circuiting the cell takes place.

As is well known, solutions of ovalent chromium compounds like chromic acid, when mixed with sulfuric acid are powerful oxidizing agents. These solutions are especially for the oxidation of fused ring-shaped polynuclear hydrocarbons in quinones suitable, which in turn contains valuable starting materials and intermediates for the production of dyes,

Form drugs and the like. Furthermore, solutions that are available in the form of oxygen are usually used of chromic acid, to the chain-cleavage oxidation of relatively long-chain The unsaturated fatty acids and oils used

ίο Oxidation of fused ring-shaped hydrocarbons and the oxidative splitting of double bonds are carried out in such a way that at increased Temperature, the oxidation solution is mixed in batches with the hydrocarbon or fatty acid

is will.

One of the advantages of chromic acid oxidation is that the spent chromium sulfate solutions can be regenerated electrolytically back to CrOr solutions. Which bring about the regeneration electrochemical reactions are transformed into an electrolytic one Line performed by the bipolar type. The present invention is now concerned with such Chromic acid regeneration process, one of its main objectives being the improvement of the electrolytic Cell lies.

In a cell typical for the oxidation of trivalent chromium salts, electrolytic decomposition occurs of the water, a direct current voltage is applied to the electrodes. The hydrogen atoms formed in the process

result in molecularea hydrogen, which can be obtained from the Lets the cell escape, while the oxygen causes the formation of lead dioxide, which in turn presumably the trivalent chromium is oxidized to the ovalent state. Under these circumstances, care must of course be taken be carried that the migration of the freshly oxidized chromic acid within the cell into the Area of the cathode where it would be reduced again is avoided. For this purpose were up Now between the anode and cathode compartments of the cells, porous ceramic diaphragms act as partitions used.

However, these diaphragms are the main source of wear in the cells. While others possible difficulties, such as B. the corrosion of the

Electrodes, which can be avoided by choosing suitable operating conditions, tend to be ceramic Diaphragms, although they are selected based on their chemical resistance, in continuous operation, to powder too

so fall apart. In addition, they are not for large-scale either Systems usable.

Furthermore, the reduced chromium salts and metallic chromium tend to operate continuously to accumulate in the cathode compartment and therefore undesirable on the various surfaces of the rooms To form precipitates.

The electrolytic regeneration of chromium salt solutions is already well known (see e.g. Gmelins Handbook of Inorganic Chemistry, 8th edition, 1962.

Volume 52, part a, page 679, last paragraph). From the book "The technical electrolysis of non-metals", by Jean Biiliter, Vienna 1954, Springer Verlag page 104, it is also already known, the electrolyte continuously first in the cathode then through the anode compartment respectively.

Electrolytic cells of the filter press type are already known (e.g. from US patents 22 19 342 and 21 72 415), but have been used until now

not used for the oxidation of chromic acid solutions.

The invention relates to an electrolysis device for the continuous regeneration of Salt solutions by oxidation of the 3-valent chromium to δ-valent chromium compounds, consisting of an in a cathode compartment and an anode compartment divided electrolytic cell in which facilities are provided are to continuously t'ie solution to be treated from the cathode compartment into the anode compartment transfer, and a plurality of electrode plates supported by frames in a filter press-like arrangement are held and form a series of cell units, the plates so in series with one another are connected that one side of the plate as the anode and the other acts as a cathode, and a diaphragm is arranged between adjacent electrode plates that separates each cell unit into an anode and a cathode compartment, and that at least has a narrow opening, which is arranged on such a slope and has a sufficient surface area, that successively through the cathode compartment u> ni through the anode compartment a controlled flow of the Solution takes place without electrical short-circuiting of the cell, which is characterized in that the Diaphragm made of polytetrafluoroethylene or polytrifluoroethylene and has a pore diameter extending from 1 to 300 microns

It has been found that the problems associated with the continuous operation of electrolytic regeneration occur from trivalent chromium salts to ovalent chromium compounds, thereby overcome can be that within the electrolysis device according to the invention, the diaphragm made of polytetrafluoroethylene or polytrifluoroethylene and a pore diameter extending from 1 to 300 microns having.

The invention is illustrated in the drawings, with pumps for simplicity. Valves and the like have been omitted. It shows

F i g. 1 shows a plan view of an electrolytic cell with filter press-like arrangement and

F i g. 2 shows a cross section of the cell according to FIG. 1.

The in the F i g. The continuous cell shown in FIGS. 1 and 2 is composed of eight hollow frames F which form four cell units A to C between the electrode plates 146 and 14a. In this embodiment the electrodes are arranged so that, with the exception of electrodes 14a and 14, one side of each electrode 14 forms the anode while the opposite side of the electrode forms the cathode. This fact becomes clear when one looks at the anode spaces A 'and the cathode spaces C in the drawing.

From Fig. 1 it can be seen that the electrolytic cell shown therein has the advantage that the disadvantage that Tension down. comes in omission. Namely, the voltage of the cell unit is usually between 4 and 8 volts direct current, which is a transformer and a large number of electrical §Q Requires connections These can be switched off by the series arrangement of FIG. The current is applied to the two ends of the series of plates and the cell voltage is determined by the number of the cells connected in series, each of the electrodes 14 is preferably made of lead. Jiickel-plated anodes and iron cathodes have also been used, but the use is not These metals are not recommended due to the corrosion that occurs.

As already stated, the electrolytic cell of FIG. 1 and 2 especially designed for continuous operation in order to effect the circulation of the electrolyte, the chromium salt solution is introduced into the cathode compartment C through the costs associated with the head piece 20 inlet lines 24 through these feeder type is the circulation of the cathode compartment contents, the flow through the diaphragm and corresponding dwell time in the anode space. In order to establish the flow through the diaphragm, the polytetrafluoroethylene membrane 16 is provided with one or more small openings 15 which allow unrestricted flow at these points. Although it was initially assumed that an opening in the diaphragm would make the system conductive by short-circuiting the unit (<e, the operation of the unit has shown that the effect of the opening is negligible) - '; the opening is in the Arranged near the bottom of the Ka'hodenraurr.s so that the feed solution must pass through this space. All compounds reduced in the cathode space fluid are oxidized again in the anode space. The area of the opening is dimensioned so that the flow from the cathode compartment into the anode compartment is not greater than the speed at which the anode compartment liquid is withdrawn. The residence time in the anolyter. is one volume cell unit per 0.5 to Ii hours. The regenerated chromium solution can be vacuum removed through manifold 18 or, alternatively, as is preferred, collected by means of overflow provided for each anode compartment. The hydrogen formed at the cathode 14 is discharged through the pipe system 12. In continuous operation, the electrical current applied at 21 and 22 flows continuously through the catholyte, the diaphragm and the anolyte and causes the anodal oxidation of the ionized chromium. To maintain the desired reaction temperature, the frames 23 can be cooled by circulating water in the cavities.

The polytetrahaloethylene from which uas diaphragm is made can be a polymer of tetrafluoroethylene or trifluorochloroethylene. These polymers are described in U.S. Patents 2,393,967 and 2,600,202. The materials useful in the invention are permeable and of sufficient molecular weight to be solids. The diaphragms are preferably about 0.25 mT (0.01 inches) thick. The cell temperature should be about 92 ⁰ C amount. The residence time in the cell should be one hour, ie it should be long enough to achieve a 50 to 80% conversion of the reduced chromium ions into the hexavalent state. A permeable layer is to be understood as meaning a porous layer whose porosity is dimensioned such that the passage of electrons is permitted while the flow of ions and hydrogen is inhibited, an average porosity of about 50% being preferred. For the purposes of this invention, the pore size should be in the range of 1 to at most 300 microns. In this way the reduction of the regenerated chromic acid at the cathode is suppressed to a minimum value, whereby the requirements with regard to the energy and the anode area are due

of the prevented ion flow take their lowest values. The size of the pores is up to one permeable membrane preferably in the range from 50 to 150 μ, with a value of about 100 μ for the average pore diameter is most preferred. It should be noted that it is also not porous Polytetrafluoroethylene fibers woven into a cloth or were linked, could be used successfully. In this case, the only ones in that The resulting structure, there are cavities and the spaces between the individual threads. The fabric used has always been in the appropriate one up to now Dimensions made so that it had a minimum electrical resistance and a sufficient one Had a barrier effect. Fibrous membranes of this type are available from the industry.

Consideration of the results when using different foiyieiraiiuoräinyieii-D'iaphfagmcfi cmäilenen illustrates some aspects of the present invention. In this way, the preferred pore size of 50 to 150 μ set. It then continued to be necessary that Void content of the polytetrafluoroethylene diaphragm to investigate what to do with diaphragms with a void content due to their porosity of 30, 40 and 50% happened. For this purpose, an electrolytic cell was used for batch operation a resting catholyte used. After passing the current for one hour, the anolyte was on his The content of hexavalent chromium ions was examined, whereby the following values were obtained. The values with regard to the required energy expressed in kW and the anode area are based on the calculated Based on 0.45 kg sodium dichromate per hour.

Table I.

At 6.5 V

Cathode area - ¹ Ze of the anode area

Diaphragm
Void content (%)
Pore size (μ)
Thickness (mm)
Required kW
Required
Anode area

50-150

0.25

1.42

0.25

40

50-150

030

1.41

0.24

50

50-150

0.25

1.73

0.21

After it was determined in this way that the optimum values for large-scale regeneration represent a pore size of 50 to 150 μ and a void content of 40%, the Behavior of such a diaphragm at different voltages still in intermittent operation compared.

Table II

tension
(V)

Required
kW

Required anode area

4.5 1.10 0.62 5.5 Ul 0.29 6.5 1.42 0.24

From Table II it can be seen that the optimal Voltage for the system consisting of a filler press type with a resting catholyte in Dependence on the electricity costs (required kW) or capital investment (required anode area) was between 5.5 and 6.5 volts.

The electrolytic cell used to determine the values listed in Tables I and II, such as the on the type shown in Fig.2. By introducing the batch in the AhödenfäUme on the way over the Cathode rooms became continuous operation, from an experiment will be described below, began. The values in this and in all of the following tables are based on the calculated values Based on various requirements per 0.45 kg of sodium dichromate.

Cell voltage;
Diaphragm:

Electrodes:

Electrode distance:

Batch:

Cell volume:

example 1

7 volts

3.18 mm polytetrafluoroethylene

(40% voids)

Lead plates

53.98 mm

4.89 g of CrAOOg solution

500 mm

Flow rate: 1 cell volume (500 ml) / h / cell

Test duration

Current density

(Amp / cm ² )

Kwh / kg (Na ₂ Cr ₂ O ₇ )

m ² anodes / kg (Na ₂ Cr ₂ CVh)

0.0716
0.0647
0.0659

0.0592
0.0631
0.0651

0.0722
0.0795
0.0662

0.0593
0.0540
0.0584

0.0521
0.0573
0.0603
0.0638

6.76
7.00
8.52

7.77
5.48
5.68

6.53
5.63
5.53

4.82
4.49
4.96

4.88
7.06
8.20

1.31 1.49 1.79

1.83 1.21 U21

1.25 0.98 1.15

1.13 1.15 1.13

1.24 1.71 1.88 137

Average current yield - 72.7%

The values present in Example 1 describe a more continuous regeneration period with an extreme Length. Even at the end of 85 hours, there was no need to interrupt operations. This Example proves not only the effectiveness of this method, but also the value of the invention used diaphragms.

The flow rates were now measured in the cells varies in such a way that less than the cell capacity of 500 ml per hour per line was removed, resulting in a longer residence time in each full cell. The values for the Flow rates are given in Example 2.

18th 04 956 Kwh / kg 8th m ² anode / kg 7th (Na ₂ Cr ₂ O;) (Na ₂ Cr ₂ OVh) Be : i s ρ i e I 2 6.76 1.31 Flybar wind wedge Experimental daters Current density 7.00 1.49 (H) (Anip / cm ² ) 8.52 1.79 500 ml / h / cell 10 0.0716 15th 0.0647 5.49 0.88 20th 0.0659 5.06 0.87 Medium current bulge - 59.4% 5.89 0.99 350 ml / h / cell 5 0.0862 4.68 0.82 10 0.0806 15th 0.0828 5.66 0.94 20th 0.0789 5.21 0.89 Average current yield - 82.6% 5.89 1.07 175 ml / h / cell 5 0.0832 5.85 1.09 10 0.0815 15th 0.0763 20th 0.0741 Average current yield - 76.7%

For this cell and under these conditions there is a somewhat longer residence time, as is the case with a Flow rate of 350 ml is reached, more desirable.

The feasibility of regenerating solutions that have not yet been fully reduced (i.e., the containing various amounts of hexavalent chromium ions) was examined in the following experiments:

5
10
15th
20th - 59.4% Example 3 Kwh / kg
(Na ₂ Cr ₂ O ₇ ) m ² anode / kg
(Na ₂ Cr ₂ 0 ₇ / h) - 69.5% Current density
(Amp / cm ² ) 6.76
7.00
8.52 131
1.49
1.79 0.0716
0.0647
0.0659 5.57
10.4 1.24
1.36 % Cr (VI) test duration
(in the supplied
th solution)
(H) 0.0622
0.0718 11.0
732
10.6
8.88 2.58
1.78
233
134 0 10
15th
20th
Average current yield - 0.0588
0.0616
0.0627
0.0634 10 5
10
Average current yield - 21.1

Average current yield - 46.0%

Furthermore, with the intention of reducing the reduction in the catholyte, the ratio of Increased anode to cathode area, which was done by isolating part of the cathode. as As a result of this measure, an increase in the current density was found. Presumably one increases Increase of the current density on the cathode the hydrogen ion content, -which the molecular hydrogen, that emerges from the system is formed more quickly. The batch contained 33.28% hexavalent chromium ions. A minor effect is indicated by a ratio of 2.6 to 1.

Example 4

relationship
Anode: cathode

Test duration
(h) current density
(Amp / cm ² )

Kwh / kg
(Na ₂ Cr ₂ O ₇ )

m ² anode / kg (Na ₂ Cr ₂ O ₇ Zh)

1: 1 5

15th
Average current yield - 173%

2.6: 1 5

15th
Average current yield - 26.0%

0.0674 0.0682 0.0690

0.0825 0.0756 0.0737 313
273
17.8

11.8
18.6
243

6.56 5.55 3.57

137
338 4.54

9
continuation 18 04 956 0.0716
0.0647
0.0659
0.0593
0.0573
0.0538
0.0539 Kwh / kg
(Na ₂ Cr ₂ O ₇ ) 10 m ² anode / kg
(Na ₂ Cr ₂ (Vh) Current density
(Amp / cm- ') 27.1
30.1 8.42
9.66 Ratio of test daiier
Anode: cathode
(H) 0.0444
0.0431 Kwh / kg
(Na ₂ Cr ₃ O ₇ Zh) m * anode / kg
(Na ₂ Cr ₂ CVh) 8: 1 5
10
Average current yield - Experimental current density
duration
(h) (Amp / cmO 6.76
7.00
8.52
6.61
6.28
5.70
6.11 1.31
1.50
1.79
1.55
1.53
1.43
1.57 Electrode gap 10
15th
20th
- 59.4%
5
10
15th
20th 57.2 mm away,
6.4 mm diaphragm
Average current yield -
28.7 mm - 15.2%

Average current yield - 70.2%

The values of Example 4 show that when the electrodes are brought closer together, the running costs can be reduced, but at the same time a larger anode area is required.

Example 5

An electrolytic cell of the filter press type with eight units was essentially similar to that already described in Considered figure created and for the regeneration of a solution of trivalent chromium sulfate The frames were made of aluminum and coated with a polychlorotrifluoroethylene film. This material is commercially available. The establishment was at an average cell temperature operated at more than 95 "C for about 95 hours, the process being based on solutions with more when 60% chromium was used in the hexavalent form. Even under these drastic conditions there was no corrosion of the frames. The frames mentioned were previously for numerous shorter ones Experiments have been used and have been in use since that time without any adverse phenomena have revealed.

From the point of view of corrosion resistance, especially at elevated temperatures, are the only ones examined construction materials that are different from sodium dichromate-sulfuric acid mixtures Have proven unassailable, lead, polytetrafluoroethylene (Teflon) and polychlorotrifluoroethylene, with the The latter polymer is preferred for coating frames, as it is lighter on it than non-porous Layer can be applied.

Example 6

It was the regeneration of a used chromium salt solution from naphthalene oxidation in one Cell with polytetrafluoroethylene diaphragms with a Thickness of 6.4 mm and a porosity of 40% studied The chromium salt solution consisted of sodium sulfate, chromium sulfate, water and a small amount of sulfuric acid and contained 7 to 8 chromium per 100 g of the solution.

During the electrolysis, the current densities found for the solution were extremely low and were in

jo in the region of 0.0095 amps ^{2 of} the electrode, the regeneration being negligible. Diluting the solution with water increased the density only slightly, ie to 0.0148 AmpVcrn ² , and increased the Cr ¹ "to Cr ^vl conversion rate to less than 5% per hour However, grams of 98% H ₂ SO ₄ per 100 g of solution produced a noticeable increase in current density to 0.052 amps / cm ² and allowed oxidation to take place at an acceptable rate so that conversion rates in excess of 20% per hour could be obtained.

Further tests carried out with the cell according to the invention demonstrated the important requirement of Addition of sulfuric acid to obtain good regeneration results. Although the real one Sulfuric acid concentration required to achieve a maximum current density with the Change in other factors may vary, but it has been shown that the optimal concentration in the

so is in the region of 15 to 20% by weight. A gradual decrease in this concentration obviously leads to a gradual decrease in the current density, while conversely higher proportions of acid occur lead to solubility problems, as these cause the precipitation of sulfates. Hence exist based on these considerations, limitations of 3 and 23% for the procedure.

B e i s ρ i eIe 7 and 8

μ The cells were essentially according to the Drawing compiled with the exception that in one case, namely in Example 7, for the diagram a 6.4 mm thick polytetrafluoroethylene membrane with a porosity of 50% was used, while for

6.5 the further example 8 a thinner membrane with the same porosity was used. In the following Table III shows the values obtained with the two diaphragms

11 12

Table IH

Dependence of the electrolytic oxidation of Chi'om solutions on the diaphragm thickness

Thickness of the
Diaphragms

Cell unit voltage

(V)

Anode current density

(Amp / cm ² ) electrode
m ² / kg

(Na ₂ Cr ₂ CVh)

energy
(Kwh / kg)

(Na ₂ Cr ₂ CVh)

Conversion (o / o)

Example 3
Example 4

32 mm
0.25 mm

7.0 6.0

0.083 0.295 0.86
0.30

5.11
5.33

46 52

The generally better results obtained with the thinner membrane can be seen if you consider the higher anode current density (A.S.D.) made possible by this and the even lower anode current density Cell voltages required electrode surface.

Examples 9-17
These tests were used in a filter press porosity cell. The tension and the

uuiuiigciuiiii, uic uci

UCl ^ CIUH [IUIIg

use wurueii varies, in eggs ιauene iv sniu tue

was similar except that 20 results of these experiments are summarized.
Polytetrafluot'ethylene membranes with different

Table IV

Dependence of the electrolytic oxidation of Cr " ¹ on the porosity of the diaphragm

example

Type of diaphragm *)

Cell unit

(V)

Stay
Time Anode
current
density Electrode,
mVkg energy
(Kwh / kg) conversion (H) (Amp / cm ² ) (Na ₂ Cr ₂ CVh) (Na ₂ Cr ₂ (Vh) (%) 0.73
0.74
0.84 0.26
0.29
0.31 0.45
0.41
0.40 4.85
5.47
6.35 36.5
40.4
46.4 1.22
1.28
1.44 0.11
0.14
0.12 0.79
0.61
0.74 436
4.89
5.65 34.5
47.6
47.2 0.53
0.50
0.57 0.23
0.27
0.30 0.37
0.33
0.30 4.31
4.94
5.39 38.7
41.0
52.4

5
6th
7th
(-70%)

9
10
(-30%)

highly porous

low porosity

50% porous

5.0 5.5 6.0

5.0 5.5 6.0

5.0 5.5 6.0

*) The porosity or the void content, which was measured for the various membranes, represents the sum äaj den closed pores and the through pores determined by density measurement methods. This measurement has a certain value insofar as for each porosity value a certain ratio of open to continuous There are pores through which the diffusion of electrons takes place. In the 50% porous material lies the preferred concentration of functional or continuous pores.

The best results are evidently achieved with a membrane with a porosity of approx. 50%, obtain. With larger porosities the results were, even with a longer residence time of the anolyte and at slightly higher current densities, generally worse, suggesting that possibly some diffusion of the hexavalent chromium ions in the catholyte back with one at the Cathode subsequent reduction takes place in the case of diaphragms with lower porosity were received much lower current densities and it was

larger electrode areas required, resulting in a

rather slow process resulted. This goes out

a comparison of the relationship between the

Conversion and residence time in the examples with low porosity and the corresponding values of

Examples 15-17 stand out.

Examples 18-20

A filter press type assembly consisting of two units, as essentially described in the drawing, has been put together. It had the following dimensions: electrode gap 5.08 cm; Anode area 234 cm ² per cell unit; Cathode area equal to anode area; Diaphragm a 0.25 mm thick polytetrafluoroethylene

film with a porosity of 50%. The cell was at different temperatures and a cell voltage of 5.0 volts. The cell was cooled to the maintain desired operating temperatures. The results obtained are shown in Table V. shown

Table V
Dependence on electrolytic oxidation
on the temperature 0.209
0.166
0.139 of chromium (III) solutions in one electrode
m- ^ kg
(Na ₂ Cr ₂ O ₇ / h) two-line Filter press unit Examples of cell temperature anode current
density
("C) (amp / cm- ') Dwell time
of the anolyte
(H) 0.40
0.45
0.59 energy
Kwh / kg
(Na ₂ Cr ₃ O ₇ Zh) conversion
(0/0) 15 703
16 50.8 0.61
0.62
0.63 435
4.20
4.23 32.8
27.7
23.1

At a certain cell voltage, the current density (Ampfern ³ ) increased with temperature, while the required plate area decreased, π whereby a much greater Cr (III) to Cr (VI) conversion was obtained at a certain dwell time.

The cell according to the invention is cells with ceramic Diaphragms which crack under their own weight in such use are superior and can still be used in circumstances where other diaphragms fail. The inventive Cell allows long-term continuous operation at previously unattainable levels Current densities and yields.

The device according to the invention has proven to be suitable to a considerable extent Process for the oxidation of various organic substances with sodium dichromate to the corresponding Oxidation products a regeneration * to introduce. Such substances can be, for example Naphthalene, anthracene, camphene, nitrotoluene, ditolysul hair dryer, unsaturated fatty acids and O-toluenesulfonamide to be named.

1 sheet of drawings

Claims (3)

Patent claims: 1. Electrolysis device for the continuous regeneration of salt solutions by oxidation of trivalent chromium to ovalent chromium compounds, consisting of an electrolytic compartment divided into a cathode compartment and an anode compartment Cell in which facilities are provided to continuously remove the solution to be treated from the To transfer cathode compartment into the anode compartment, and a large number of electrode plates, which through Frame held in a filter press-like arrangement and a number of cell units form, wherein the plates are connected in series so that one side of the plate as The anode and the other acts as a cathode, and there is a diaphragm between adjacent electrode plates is arranged that separates each cell unit into an anode and a cathode compartment and which has at least one narrow opening which is arranged at such a point and one Sufficient clearance has that one after the other through the cathode compartment and through the anode compartment controlled flow of the solution takes place without electrical short-circuiting of the cell, thereby characterized in that the diaphragm consists of polytetrafluoroethylene or polytrifluoroethylene and has a pore diameter extending from 1 to 300 microns 2. Apparatus according to claim 1, characterized in that the diaphragm? (16) made of polytetrafluoroethylene with a particle size range of 50 to 150 microns. 3. Apparatus according to claim 1, characterized in that the opening in the diaphragm (16) in Near the bottom of which is arranged.