EP0426832B1 - Process for reducing dyes - Google Patents

Abstract

In a process for reducing dyes in aqueous solution, a pair of electrodes is immersed in the solution. The cathode potential is maintained below the value at which hydrogen is evolved. A reducing agent is then used, the redox potential of which, increased by the charge transfer overvoltage required for the reduction of the oxidized form of the reducing agent to the reduced form at the cathode, is less than the cathode potential.

Description

The invention relates to a process for the reduction of dyes in aqueous solution with pH> 9, using a reducing agent with a redox potential of over 400 mV, which is present in a reduced and oxidized form, a pair of electrodes being introduced into the solution, the Cathode potential is kept below the value at which hydrogen evolution occurs.

In textile finishing, vat dyes for dyeing cellulose fibers have a considerable market share (approx. 12.5%, world consumption approx. 25,000 t / year). This dye class is one of the high-quality dyes, particularly due to its high fastness properties. When used in dyeing, the primarily non-fiber-insoluble dye particles are reduced to their alkali-soluble leuco form by reduction. The reduced dye has a high affinity for the substrate and is now quickly absorbed by the dye. When the pull-up phase has ended, the leuco form is oxidized to fix the dye, forming the water-insoluble pigment. The basic chemical structure of the dyes is often anthraquinone or indigoide. In terms of quality, sulfur dyes are inferior to vat dyes, but their price is very cheap, so that they have a relatively large market share in cellulose dyeing (25%, 50,000 t / year). The sulfur dyes are used analogously to the vat dyes, with the reduction of the sulfur dyes being possible even at lower redox potentials.

Many textile dyes of other classes of dyes have azo groups in their coloring molecule parts. These azo groups can be reductively irreversible cleave what can be used to destroy dyes (peeling off and correcting incorrect stains).

Reducing agents are also used to destroy excess bleaching agents, reductive bleaching (wool) and reductive wastewater treatment (decolorization).

The main reducing agent for vat dyeing and for the reductive cleavage of azo dyes is Na₂S₂O₄ sodium dithionite ("Hydro"), which has a reduction potential of approximately -1000 mV in an alkaline environment. Sulfinic acid derivatives (Rongalit types BASF) are used for reductions at higher temperatures (steaming processes, HT processes) (reduction potential at 50 ^o C approx. -1000 mV). Sulfinic acid derivatives can be activated through the use of heavy metal compounds such as Ni-cyano complexes, Co complexes etc. The use of anthraquinone compounds as accelerators for the reducing agents used has been proposed, but is practically not carried out.
Other reducing agents are thiourea dioxide (-1100 mV), hydroxyacetone (-810 mV) and sodium borohydride (-1100 mV). With regard to the required reduction potential (approx. -600 mV), indigo lies between the vat dyes and sulfur dyes. In addition to "hydro", hydroxyacetone / sodium hydroxide solution can also be used here as a reducing agent. Historically, iron vitriol (FeSO₄) lime vats, zinc lime vats and fermentation vats were used.
Because of the lower reduction potential required, other reducing agents can also be used for sulfur dyeing. The main reducing agents are Na₂S and NaHS (reduction potential approx. -500 mV). Mixtures of glucose and sodium hydroxide were also used.

In various Indian works (cf. "Dyeing with less chemicals" EH Daruwalla in TEXTILE ASIA, September 1975, pages 165-169), a process of the type characterized at the beginning has been proposed, in which the consumption of sodium dithionite is reduced by applying a DC voltage . This reduction is due to the fact that the reducing agent is converted at the cathode into a form which has an increased reducing power. Due to the reaction with the dye, this substance breaks down into the same products as the sodium dithionite itself. These products cannot be regenerated with the voltage applied to the cathode. This voltage is in any case at a level which is only usable with the mercury electrode used, but which would already lead to harmful hydrogen evolution in practical electrode materials.

From DE-A-13 95 67 and essentially also from the similar FR-A-319 390 a method is known in which the reducing agent sodium dithionite is produced by the action of the electric current from sodium bisulfite, which is now able to Reduce indigo. In the known method, the reduction is started in the acidic pH range and the pH value only migrates to the weakly alkaline range in the course of the process, as a result of which a vat is formed. According to the knowledge of the electrolysis of sulfite solutions, the cathodic reduction of the sulfite comes to a standstill, so that there is no longer a reversible redox system. The electrochemical formation of sodium dithionite from sodium hydrogen sulfite / sulfite is therefore only successful in acidic solutions, in alkaline solutions are not a reversible redox system in the sense of a mediator.

The reducing agents currently used lead to various disadvantages in their application: Na₂S₂O₄ is a relatively expensive chemical that has to be imported by many countries. In the dyeing processes, a large excess of Na₂S₂O₄, based on the amount theoretically required for the reduction, must be used. In the dye bath, the oxygen present in the liquor must first be removed, only then can the dye reduction begin. During the dyeing process, atmospheric oxygen from the environment continuously consumes Na₂S₂O₄. The quantities used are approx. 1.25 to 2.5 kg of reducing agent per kg of dye.

The high amounts used lead to an accumulation of oxidation products of the reducing agent in the dyeing liquor. This means that the dyeing liquor can only be recycled in very few cases. The amount of reducing agent must be in the dyebath sufficient for complete reduction to complete the dyeing process. The dye bath is therefore drained off with a relatively large amount of reducing agent. The oxidation therefore takes place in a new treatment bath, since otherwise the entire excess of reducing agent still present in the dye bath must also be oxidized.

The reducing agent bath leads to considerable oxygen consumption in the wastewater, which leads to wastewater problems. When using sulfides as reducing agents, the procurement costs are relatively low, but the wastewater problem is becoming increasingly important here, since not only oxygen depletion, but also considerable toxicity and odor problems occur.

The invention has for its object to avoid the disadvantages of the previous reducing agents. This is achieved in that a reducing agent is used whose redox potential (half-stage potential), increased by the charge transfer overvoltage for the return of the oxidized form of the reducing agent to the reduced one below the cathode potential.

According to the invention, the dye is therefore not reduced directly at the electrode, which has already been proposed, but has not proven successful. Rather, a reducing agent is used which reduces the dye in a conventional manner, is oxidized in the process and reaches the cathode in this oxidized form, where it is returned to its original state. Redox systems of this type are called mediators in electrochemistry. The use of such mediators for the reduction of dyes was not obvious for several reasons. So far, mediators have hardly been watery per se

Solution used, in the alkaline range only very exceptionally, and not at all above a pH value of 9. On the other hand, the substances used hitherto for the reduction of dyes cannot be used for the process according to the invention, since their oxidation products could only be converted into the ground state at cathode voltages at which an unreasonable development of hydrogen at the cathode would long ago take place.

The cathode thus reduces the reversible redox system which, in turn, is able to reduce the dye after the reduction potential of the dye has been reached. By setting the optimal redox potential in solution, color shifts, such as those caused by over-reduction, can be avoided. The upstream reversible redox system has the task of generating a continuously regenerable reduction potential in the dye liquor, as a result of which no further reducing agent has to be added to the dye liquor. The proportion of reducing agent consumed by air oxidation is continuously renewed at the cathode. There are no secondary products from the addition of reducing agents in the dyeing liquor. Enrichment by the usually necessary addition of reducing agent does not occur either. After removing the unfixed dye (centrifugation, filtration, ..), the dye bath can be reused, only the liquor volume lost with the goods having to be replaced. Chemical consumption in the usual sense does not occur. Even the dye reoxidation can be carried out in the dye bath, which according to the literature should lead to an improvement in the rub fastness of the dye (doubtful). This procedure is not economically justifiable with the reducing agents currently used, since at the end of the dyeing process large amounts of reducing agent remain in the dye liquor and draining the dye liquor is more cost-effective. A closed recycling of the entire dyeing liquor without time-consuming reprocessing is out of the question, also because of the ongoing enrichment with secondary reducing agent products.

The use of indirect electrochemical reduction therefore not only reduces the costs of reducing chemicals, but also enables the dyeing liquors to be closed for the first time after the residual dye has been removed. With the exception of the rinse water, dyeing free of waste water is possible. It is precisely the dye liquors that are currently heavily contaminated with chemicals that can be completely recycled.

Various upstream redox systems can be used for indirect electrochemical dye reduction:
As organic compounds with which the redox system can be implemented, in particular those with an anthrachinoid basic structure have been investigated. Experiments with anthraquinone mono- and disulfonic acids, hydroxyanthraquinones and mixed substituted products enabled the reduction of sulfur dyes and vat dyes with the corresponding potential. The quantities of anthrachinoid compound used are between 0.5. 10⁻³ mol / l and 3. 10⁻³ mol / l, with concentrations of about 1.5. 10⁻³ mol / l are cheap. To assess the required quantities of redox catalyst, the oxygen input from the air must also be taken into account. The amount of catalyst required can be reduced by a closed apparatus.

Inorganic compounds for the invention Can be used, one has to look above all under the metal complex salts. For example, the system Fe (II / III) triethanolamine sodium hydroxide solution is suitable as a reduction mediator. The achievable potentials of up to -980 mV enable the reduction of all common vat dyes, indigoid dyes, sulfur dyes, azo dyes without the use of other reducing substances.

The person skilled in the art, who is familiar with the teaching of the invention, can certainly be expected to find further reducing agents which can be used as mediators under the specified process conditions. It is important that the activity of these substances decreases at most slightly during the period of use, so that a large number of reduction cycles is guaranteed. Rapid conversion should take place on the electrode surface. The catalysis of side reactions by the reducing agent should be excluded. Of course, low toxicity is also required for technical applications.

The reduction effect of the various redox systems is always characterized in this description by their half-step potential. As such, a certain relationship between the reduced and the oxidized form of the substance used arises at each potential. For technically usable systems, however, a certain resilience must be given, the reduction potential achieved must not collapse immediately. In practice, this means that one will work in the area in which reduced and oxidized species are present in approximately the same amount. In order to determine this potential, it is not necessary to wait for an equilibrium to form, but rather it is also possible to dynamically develop the

Determine peak potentials of the Cv curves between which the half-wave potential lies.

The invention is then explained in more detail using a device for carrying out the method and by means of some application examples. The device for carrying out the method is shown schematically in the single drawing. However, the dyeing and decolorization processes for textiles described in the application examples do not fall as such under the subject matter of the claims.

The device shown comprises a container 11, on the bottom of which there is a working cathode 1 made of copper. A magnetic stirrer 8 is located above the working cathode 1 to accelerate the removal of the reduction products. A reference electrode 4 (Ag / AgCl) is provided for measuring the cathode potential by means of the voltmeter 5. The potential in solution is measured using a separate measuring electrode 3 made of copper or platinum, which is connected to the reference electrode. As a result, the potential increase in the solution can be tracked as a result of the reduction system that is building up.

It is essential that the working anode 2 is shielded by a diaphragm 7 in order to avoid reoxidation at the anode in a known manner. A container 10 filled with textiles to be dyed is introduced into the electrolysis chamber on the cathode side with respect to the diaphragm 7, through which the solution is sucked by means of the liquor circulation pump 9, whereupon it returns to the container 11.

By using cathode material with high hydrogen overvoltage, depending on the alkali content, a power potential of up to -1200 mV can be realized at the cathode by means of the power supply unit 6, without the development of hydrogen.

In the experiments described below, the temperatures were between 40 and 50 ^o C, but in itself the entire temperature range from 20 to 90 ^o C could be used.

Application example 1 Reduction of a vat dye - indanthrene blue GC

Process conditions:
Pull-out procedure liquor ratio 1:20
Product weight: 6.6 g Bw (100%) fleet volume 130 ml
Color depth: 3% (197 mg dye)
Dye bath: 4 g / l NaOH, 2 g / l triethanolamine, 0.5 g / l Fe₂ (SO₄) ₃ The working cathode consists of Cu (area 36 cm²), the working anode consists of Pt (area 10 cm²). The working potential of the Cu cathode is -1150 mV against an AgCl reference electrode. The goods are wetted with the lye at 40 ° C. After adding the redox system and switching on the working current (approx. 35 mA), the potential in the solution rises to -940 mV within 20 minutes and is held there for 1 hour. The reduced dye on the goods is oxidized by rinsing. The dyeing is completed by boiling soap according to the dye manufacturer's instructions.

The color depth achieved during coloring corresponds to the guide values of the dye manufacturers.

Example of use 2 Reduction of a sulfur dye - hydrosol light green 3B

Process conditions:
Pull-out procedure liquor ratio 1:20
Product weight: 6.68 g Bw (100%) fleet volume 135 ml
Color depth: 5% (334 mg dye)
Dyebath: 8 g / l Na₂CO₃, 4 g / l triethanolamine, 0.5 g / l Fe₂ (SO₄) ₃ The working cathode is made of Cu (area 36 cm², the working anode is made of Pt (area 10 cm²). The working potential of the Cu cathode is -1150 mV against an AgCl reference electrode. The goods are wetted with the alkali at room temperature. After the addition of the redox system and switching on the working current (approx. 30 mA) the potential in the solution rises to over -800 mV within 20 min and is held there for 40 min. During this time the dyeing temperature was increased to approx. 60 ° C, the working current rises to 60 mA, the potential in the solution reaches -870 mV. The reduced dye on the goods is oxidized by rinsing. The dyeing is completed by boiling soap according to the instructions of the dye manufacturer The color depth achieved corresponds to the guide values of the dye manufacturers.

Example of use 3 Reduction of an azo dye - Remazolbrillantrot BB

Process conditions:
Peeling test liquor ratio 1:20
Product weight: 5.76 g Bw (100%) fleet volume 115 ml
Color depth: initial color 10 g color / kg goods (KKV-colored)
Dyebath: 8.8 g / l NaOH, 4 g / l triethanolamine, 0.5 g / l Fe₂ (SO₄) ₃
The working cathode consists of CU (area 36 cm²), the working anode consists of Pt (area 10 cm²). The working potential of the Cu cathode is -1150 mV against an AgCl reference electrode. At RT, the goods are wetted with the lye. After adding the redox system and switching on the working current (approx. 20 mA), the potential in the solution rises to -450 mV within 20 min. As the temperature rises to 55 ° C, the potential rises to -800 to -900 mV and is held there for 1 hour. The azo dye on the goods is almost completely destroyed, which is normally achieved by treatment with NaOH / Na₂S₂O₄.

Example of use 4 Reduction of an indigo dye BASF Brillantindigo 4B-D

Process conditions:
Pull-out procedure liquor ratio 1:20
Goods weight: 7.0 g Bw (100%) fleet volume 140 ml
Color depth: 4% (280 mg dye)
Dyebath: 1.4 g / l NaOH, 30 g / l Na₂SO₄, 4 g / l triethanolamine. 0.5 g / l FeSO₄. 7H₂O
The working cathode consists of Cu (area 36 cm²), the working anode consists of Pt (area 10 cm²). The working potential of the Cu cathode is -1150 mV against an AgCl reference electrode. The goods are wetted with the barrel at RT. After adding the redox system and switching on the working current (approx. 10-20 mA), the potential in the solution rises to over -870 mV within 60 min, especially after the addition of Na₂SO₄. During this time the dyeing temperature is increased to approx. 45 ° C. The reduced dye on the goods is oxidized by rinsing. The dyeing is completed by boiling soap according to the dye manufacturer's instructions.

The color depth achieved during coloring corresponds to the guide values of the dye manufacturers.

Application example 5 Reduction of a sulfur dye - Hydron Blue 3R

Process conditions:
The reduction of the dye was recorded colorimetrically and evaluated.
Dye bath: 4 g / l NaOH, 0.5 g / l anthraquinone-1,5-disulfonic acid, 10 mg / l hydron blue 3R
The working cathode is made of Cu (area 88 cm²), the working anode is made of Pt (area 6 cm²). The working potential of the Cu cathode is -850 mV against an AgCl reference electrode. After adding the redox system and switching on the working current (approx. 10-20 mA) the reduction of the dye is monitored colorimetrically. Already at room temperature, the upstream anthraquinone system is reduced to approx. 34% within 20 minutes (reaching the half-step potential), the sulfur dye now added is immediately reduced quantitatively. After the working current has been switched off, the reoxidation of the sulfur dye can be observed.

Claims (6)

1. Process for reducing dyes in aqueous solution with pH >9 using a reducing agent having a redox potential of above 400 mV and which is present dissolved in reduced and oxidized form, an electrode pair being introduced into the solution, whose cathode potential is kept below the value at which hydrogen evolution occurs, characterized in that use is made of a reducing agent, whose redox potential (half-wave potential), increased by the charge transfer overvoltage, is below the cathode potential for the return of the oxidized form of the reducing agent to the reduced form taking place on the cathode.
2. Process according to claim 1, characterized in that use is made of a cathode of Cu, Zn, Pb or high-grade steel.
3. Process according to claim 1 or 2, characterized in that use is made of a reducing agent with an anthraquinonoid basic structure.
4. Process according to claim 3, characterized in that use is made of 0.5·10⁻³ mole/l to 3·10⁻³ mole/l, preferably approximately 1.5·10⁻³ mole/l, of the anthraquinonoid compound.
5. Process according to claim 1 or 2, characterized in that a metal complex salt is used as the reducing agent.
6. Process according to claim 5, characterized in that a mixture of 0.5·10⁻³ mole/l to 5·10⁻³ mole/l of iron (II) or iron (III) salt with triethanol amine is used.

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