Review Article

Electrochemical dyeing : A noval dyeing Approach

Mr. Amit kumar & Sahil Sharma(Managenent Trainee)

amittc2@gmail.com

AADTT (Advanced Academy for Development of Textile Technologist)

Abstract

There is a continuing need for improving the eco-efficiency of critical textile wet processes

specially in dyeing. For example, dyestuffs such as sulphur and vat dyes, especially indigo,

play an important role in today's dyeing industry. The present use of this dye category is

based on the application of sodium to attain a water soluble form of the dye (= leuco dye) by

reduction but the disposal of dyeing baths causing various problems, because the reducing

agents necessary will finally be oxidized into species that can hardly be regenerated. These

sulphite, sulphate, thiosulphate and toxic sulphide heavily contaminate waste water from

dyeing plants. Therefore, many attempts are being made to replace the environmentally

unfavourable sodium dithionite by ecologically more attractive alternative. In this reference,

electrochemistry presents a very effective technique. The process uses an electric current

instead of chemical reducing agents, giving it a number of technical, economic and ecological

benefits. Electrochemical dyeing process results in, product savings, less chemicals with

special safety requirements, unsurpassed environmental compatibility. Moreover new process

also facilitates dye bath monitoring in real time, the high quality of the dyed fabric is ensured.

The dye liquor used in electrochemical dyeing can be reused several times without altering

the dyeing effect and contamination of dye house effluent is negligible.

Keywords

Leuco Vat dyes, reducing agents, reduction, electrochemical, environmental friendly, effluent

Introduction

The major dye classes used for colouration of cotton/cellulosics include reactive, vat, sulphur,

azoic, direct and solublised vat. The worldwide consumption of different dye classes is given

as:

DYES 1988 1992 2004

Sulphur 90000 70000 70000

Direct 74000 60000 68000

Vat 36000 21000 22000

Indigo 12000 12000 12000

Azoic 28000 18000 13000

Reactive 60000 109000 178000

TOTAL 300000 290000 354000

Table shows that reactive dye consumption increases, and next important dye class are vat

and sulphur.These dyes are insoluble in water; before application they are solublised using

suitable reducing agent and alkali. But due to their non ecofriendly and polluting nature, an

alternative reducing method using instead of conventional reducing agent called

electrochemical dyeing getting emphasis these days.

REDUCING AGENT FOR VAT DYE

Sodium dithionite(sodium hydrosulphite)is commercially reducing agent.And other reducing

agents are thiourea dioxide,sodium borohydride.But due to commercial problem,they are no

used.

SODIUM DITHIONITE

It’s Chemical formula is Na2S2O4.It reduces all vat dyes at a temperature range from

30°C to 60°C and above. The reducing property of sodium dithionite is due to

libration of hydrogen as follows:

Na2S2O4 + 4H2O 2NaHSO4 +6H-

Na2S2O4 + 2NaO 2Na2SO3 + 2H-

The amount of reducing agent and NaOH required for a specific vat dye to reduce and bring

it to soluble depends on number of C=O groups and how compact the structure is.

Sodium dithionite is unstable,gets decomposed oxidatively and thermally to several

byproducts. Some byproducts are acidic in nature, and need overdosing of alkali over

stoichiometric requirement. The stability of solution decreases with increased

temperature, increased surface exposed to air and decrease agitation of bath.

And Salts of sulphur also in form of sulphates and sulphites contaminate sewage,

lower its pH and corrosive action in pipes.

REDUCING AGENT FOR SULPHUR DYES

The sulphide reducing agents are sodium sulphide(Na2S),sodium

hydrosulphide(Na2H2S4),and sodium poly sulphide(Na2Sx where x can be from one to

six).But problems associated with sulphide based reducing agents are Contamination

of effluent with sulphur and Libration of H2S which gives foul smell of rotten eggs

and is toxic when healed.

Some ecofriendly reducing agents are also there like hydroxyacetone,ironcomplexes,glucose,

mercaptethanol but there are problem of using these related to stability,reduction time and

cost.

But now-a days we try to establish the newer method due to some disadvantages of

older methds and wealthy procedure of newer one.(like electrochemical dyeing).

Electrochemical method fulfill the entire requirement for dyeing, it provides sufficient

negative reduction potential in the range of 450m V to -1000m V to reduce the dyes.

It is a reproducible process and gives similar or improved dyeing result compared

with the technique already in use.It requires low concentration of chemicals and non-

toxic chemicals and economic technique for recycling of chemicals and washing

water. The main key point is it is adaptable to equipment and apparatus in use at

present time.

ELECTROCHEMICAL DYEING

In this,chemical reducing agents are replaced by electrons from electric current, and effluent

contaminating substances can be dispensed with altogether. In conventional method, reducing

power of chemicals can not be regained ,so recycling of bath is not possible. So due to

residual chemicals and unused dye in disposable liquor. problems will generate due to non

eco friendliness.Then electrochemical dyeing introduced as a big boon for textile dyeing.This

process was developed by BASF.

The first attempt was made in this context by reducing the amount of sodium dithionite by

application of direct voltage.This results powerful reducing agent species due to the

decomposition of hydrosulphite and production of free radical ion,SO2.

S2O4 2SO2-

In this case also recycling is not possible.Then its one step ahead is electrochemical dyeing.

THE CLASSIFICATION OF ELECTROCHEMICAL DYEING CAN BEEN SHOWN

AS:

DIRECT ELECTROCHEMICAL DYEING

In this technique,the dyes have been reduced by direct contact of electrode and dye.But,the

partial reduction of dye should be carried out by conventional method,then complete

reduction is done by electrochemical process which facilitates improved stability of reduced

dye.

In order to start the process,an initial amount of leuco dye has to be generated by a

conventional reduction i.e.by adding a small amount of reducing agent.Once the reactions

have set in,it is not needed anymore and the further process is self sustaining.This system is

sucessful in the case of sulphur dye.

MECHANISM OF REDUCTION OF DYE MOLEULE BY DIRECT METHOD

The basic reduction of indigo molecule by any method.

Direct electrochemical reduction of indigo molecule

DIRECT ELECTROCHEMICAL REDUCTION OF SULPHUR MOLECULE

PROBLEM IN DIRECT ELECTROCHEMICAL METHOD:

1) The concentration of dye required to get a specific shade is higher than the conventional

reducing process.

2) The dyestuff particles must come in contact with the electrode surface to get reduced.But

atmospheric oxygen present in the dye solution reoxidises the dyestuff molecules.

3)The surface area of the cathodic is constraint.

DIRECT ELECTROCHEMICAL REDUCTION OF INDIGO ON GRAPHITE

ELECTRODES

It has been found that graphite granules can act as electrode material for the direct

electrochemical reduction of indigo in aqueous suspension.Optimised conditions of 10gpl

was achieved for indigo. Results are favourable since very cheap and stable material.In

addition,an electrochemical fixed or fluidized bed reactor is a reasonable economic reactor

design for this task.Due to the high hydrogen over voltage on graphite under the applied

conditions,no chemisorptions or only very weak,chemisorption of hydrogen is possible.

Therefore a normal electron transfer seems to be the relevant process for the reduction of

indigo.Unfortunately the reduction rate is very low.It should be possible to accelerate the

relevant process i.e.electron transfer kinetics and probably also the absorption rate

1)by the selective generation of quinine like functionalities on the surface of the used graphite

electrode.

2)Oxidative pre treatment can increase the content of oxygen-containing groups on the

surface of carbon.e.g.soaking with hydrogen peroxide.

3)By covalent bonding of quinoid molecules onto the graphite surface.Thus,electron transfer

mediators,which can undergo fast electron transfer with the electrode and also with the

substrate(indigo),are immobilized on the carbon electrode.In the particular case of indigo

reduction,anthraquinone have been used as redox active molecules.

INDIRECT ELECTROCHEMICAL DYEING

It was first patented by THOMAS BECHTOLD in 1993.Here the dye is not directly reduced

at electrode; rather a reducing agent is added that reduces the dye in conventional manner

which in turn gets oxidized after dye reduction. The oxidized reducing agent is reduced

subsequently at the cathode surface which is then further available for the dye reduction. This

cycle is continuous for dyeing operation. In electrochemistry, the agent which undergoes

reduction and oxidation cycles is known as reversible redox system and is called a mediator.

The primary object of redox system is to generate a continuous regenerable

reduction potential in the dye liquor. Therefore, addition of conventional reducing agent is

not needed. After the dyeing cycle, the unexhausted dye gets precipitated by air oxidation and

can be removed by filteration. then the liquor containing the mediator can be recycled for

subsequent dyeing operation.This is the biggest advantage in terms of eco nature and cost of

process.

A suitable redox catalyst for indirect electrochemical dyestuff reduction

has to fulfill various requirements:

1) No reaction with solvent

2) No affinity for the fibre material

3) Rapid conversion at the electrode surface

4) No catalysis of side reactions(change in colour)

5) No problems in effluent

6) Non toxic

7) Only a small loss,if any,in activity during the useful life;therefore the maximum possible

number of reaction cycle.

8) Inexpensive

9) Adequate solubility

THIS CAN BE ONLY USED IN DISCONTINUOUS EXHAUST DYEING PROCESSES

AND IN CONTINUOUS DYEING PROCESS,IT CAN BE USED TO STABLISE THE

DYE BATH BY DIMINISHING THE AMOUNT OF LEUCO INDIGO OXIDISED

DURING DYEING DUE TO CONTACT WITH AIR.

(Reason is that the huge electrode surface of 500m2

is necessary to achieve a feasible rate of

dye reduction at an industrial scale.)

PICTORICAL REPRESENTATION OF MECHANISM OF REDUCTION

OF THE DYE BY INDIRECT ELECTROCHEMICAL METHOD

A general reaction for indirect electrochemical dyeing process using Fe(ll)-TEA complex is

established as follows:

In the first reduction step,the iron complex is cathodically reduced in a reversible

electrochemical process.The reduced complexes diffuse from the electrode towards

the dyestuff particles.At the surface of the dispersed particle oneelectron charge

transfer charge occurs and a dye radical anion gets formed which is an intermediate

step to form completely reduced dye dianion.

The completely reduced dye dianion can be formed by two routes,Since most of the

dye require two electrons for the reduction of each molecules,two dye radical anions

are assumed to give one fully reduced dye dianion molecule and to regenerate one dye

molecule in its oxidized form.The other possible mechanism for the formation of

reduced is such that one electron transfer may takes place from a reduced mediator to

a dye radical anion and forms a fully reduced dye anion molecule as a reaction

product.In an additional reaction,the equilibrium between the fully reduced dianion

molecules and the insoluble oxidized form of dye is built up.

A GENERAL REACTION FOR SUCH AN ELECTROCHEMICAL PROCESS

IS SHOWN AS:

Fe3+

L +e- Fe

2+ + L 1 cathodic process

Fe2+

L +dye Fe3+

L +dye.-

2 one-electron transfe

2dye dye

2- + dye 3 disproportionation

Dye.- + Fe

2+L dye

2- + Fe

3+ L 4 secondary reduction

Dye2-

dye + 2e- 5 oxidation

Where L denotes the associated TEA/Ligand

MEDIATOR SYSTEMS

The great advantages of this technique is the direct information about the state of reduction in

the dye bath,which is available by redox potential measurement,and that control by the

adjustment of the cell current is possible.

REDUCTION CAPACITY OF THE MEDIATOR SYSTEM

In this theory as per Nernst equation,any desired reduction/oxidation potential (upto the

liberation of hydrogen gas)can be achieved in solution by combining an appropriate

proportion of reduced and oxidized species.At any given time in an electrochemical

system,both the oxidized and reduced pair is available in the solution.By varying the voltage

provided,the concentration of these oxidized and reduced species can be adjusted and

ultimately the potential prevailing in the solution

Nernst equation:

E = Eo +RT/nF . lnCox/Cred

Where,

Eo=Standard potential of the redox pair(under the experimental conditions)in Mv,E=Potential

prevailing in the solution(mV), R=Gas constant(8.314j/Kmol), F=Faraday

constant(96500°C), T=Temperature(K), n=Electrochemical valency, Cox=Concentration of

the oxidized form of redox pair(mol/l), Cred=Concentration of the reduced form of the redox

pair(mol/l)

The reduction potential which can be realized industrially in solution is always close to the

normal potential of the redox pair used at the electrode and the mediator capacity is expressed

in terms of cathodic peak potential.

DIFFERENT MEDIATOR SYSTEMS:

Some of the mediator systems which can be used as reversible redox systems for the

electrochemical dyeing are given as:-

ORGANIC REDOX SYSTEM

Organic compounds with anthraquinonoid basic structure are used as a mediator for

indirect electrochemical dyeing.They enabled the reduction of sulphur dyes and vat

dyes with leuco potentials below the cathodic peak potential of the mediator system.

The anthraquinonoid systems have cathodic peak potentials upto a level of -850mV

DISADVANTAGES:

In these redox system,the relatively low rate of conversion at the electrodes.

Indirect redox system

Inorganic compounds used are metal complex salts.The reducing action of

metal salts of low valency levels were used earlier for vat dyes.These have

several disadvantages such as precipitation of dye bath and,heavy metal

content of the effluent due to large quantities employed.The most suitable iron

complex system is iron salt/TEA which allows potentials of up to -1050mV to

be achieved under dyeing conditios in homogeneous solution.The iron

complex can be prepared either from Fe(ll)salts or from Fe(lll)salts.The

potential which can be achieved allows a reduction of all the usual vat dyes

without use of additional use of reducing agent.

CATHODIC PEAK POTENTIAL OF DIFFERENT ANTHRA QUINONE SYSTEM

WITH 4GPL NaOH w.r.t. Ag/Agcl/3 M Kcl REFERENCE

ORGANIC COMPOUNDS CATHODE PEAK

POTENTIAL(mV)

ORGANIC COMPOUND CATHODE PEAK

POTENTIAL(mV)

1,2 dihydroxy anthra quinone -880 1,2,5,8 tetra hydroxyl anthra

quinone

-885

1,4 dihydoxy anthra quinone -760 1 amino 2 carboxy anthra

quinone

-750

1,8 dihydoxy anthra quinone -770 1 amino anthra quinone 2

sulphonic acid

-720

Anthraquinone 2 sulphonic

acid

-750 Anthra quinone 2,6 sulphonic

acid

-560

Anthra quinone 1,5 sulphonic

acid

-750 1,2 dihydoxy anthra quinone3

sulphonic acid

-760

Dyeing procedure The electrolysis was carried out under galvanostaticconditionby maintaining the constant

current and optimizing the time required for achieving maximum current efficiency.During

the electrolysis, the conversion of ferric ions was estimated by iodometric titration at different

intervals of time. After achieving maximum conversion of ferric ions (no further increase in

FeII concentration with time), the required quantity of selected dye was added into

thecatholyte. The electrolysis was continued for another 30 min for solubilization of the dye

molecules. Then, the pretreated fabric sample was introduced into the dye bath.

Both electrolysis and electrochemical dyeing were carried out at 300 ± 2 K. Experiments

were carried out at relatively large liquor ratio value, such as 1:330, 1:80, and1:60. In all the

experiments, 0.8 wt% of the fabric on the dye was used in the dyeing recipe. The dyeing was

carried out by exhaustion method for 30 min with constant stirring.After completion of

dyeing,the fabric sample was washed with cold water and exposed to air, for

oxidation/fixation of dye molecules. Then the fabric was soaped at boil, rinsed with cold

water, and air dried.

DIFFICULTIES TO ESTABLISH INDIRECT ELECTROCHEMICAL DYEING

PROCESS:-

The actual reduction of dye should be carried separately into electrochemical cell and

the reduced dye is then circulated separately into a conventional dyeing unit,e.g.jigger

To keep the dye in reduced form,it is necessary to reduced the oxidized mediator at

the cathode.This is possible only through continuous circulation of the dye liquor

from the dyeing equipment to the electrochemical cell.

The design of the cathode should have maximum enough surface area to reduce the

mediator.

A three dimensional electrode with large surface area occupying small space in the

electrochemical cell should be designed.The kind of cell will have the advantages of

carrying out the dye reduction with minimum volume of mediator.

A cell with minimum area of separator(semi permeable membrane,separatinganolyte

from catholyte)is necessary.It will ensure minimum cost of separator and will not

allow the reoxidation processes to takes place due to bleeding of the

separator,diffusion of oxygen.Thereoxidation at the separator may cause a chemical

short circuit,thus requiring further enlargement of cathode area.

ADVANTAGES:

The indirect electrochemical reduction technique using mediators permits

direct control of the dye bath potential without the undesirable connection between

temperature and reduction potential.

ELECTROCHEMICAL DYEING WITH INDANTHREN DYES

The system contains a vessel which contains a reduction system made.Stirring is

done by magnetic stirrer.At the bottom of the dyeing vessel a copper cathode is

there.The cathode provide electrons as the power supply switch ON.

Factors governing electrochemical dyeing process

Effect of dye concentration and material to liquor ratio

Even dyeing was noticed for all MLR values, but depth of the dyeing varied with increasing

MLR. When compared the better shade was obtained with MLR value of 1:60. Thus, the

liquor amount can be reduced from 240 to 60 litters. The low MLR value saves the

consumption of chemicals, water, and cost of the dyeing process.

Effect of electrode material

Both the conversion efficiency and the current efficiency values obtained for the electro-

generation of FeII-oxalate-gluconate system employing different cathode and anode

materials. Both platinum and stainless steel may be employed as anode material. Stainless

steel is the material of choice as anode due to cost consideration. Copper plate as well as

wounded copper coil could be employed as the cathode material with reasonable

efficiency.

Effect of electrolyte composition

Studies were carried out to determine the optimum concentration of ferric sulfate, oxalic

acid, and Ca-gluconate. The conversion efficiency increases when the ligand

concentrations are significantly higher than the Fe(III) concentrations. When the

ferric sulfate concentration increased beyond 25 mM by retaining the oxalic acid

concentration at 100 mM and calcium gluconate concentration at 50 mM, the conversion

efficiency decreases probably due to the insufficiency of ligands for complete complex

formation.

Effect of Complexing agents

The mediator system used in electrochemical dyeing is not stable under highly alkaline

condition. Industrial vat dyeing is preferably carried out above pH 12 and very few

Fe(III) complexes are stable in such alkaline conditions. The mediator system get

precipitate out so not works properly. In presence of excess gluconate, the Fe(III)-oxalate

complex does not lead to any hydrolysis or precipitation. The pH of ferric-oxalate-

gluconate system could be raised beyond 13.

Effect of the current intensity

We can achieve fast and more dye reduction by utilizing higher current values but heating

effect and limited freedom for reaction. When operating with a galvanostatic mode, the

current intensity is a crucial factor. Indeed, low current values will result in high selectivity of

the electrochemical reaction but long lasting electrolyses. High current values will produce

short experiments but a bad selectivity and a temperature increase due to joule effect.

Requirement satisfied by the electrochemical dyeing process

High number of dyeing cycle of the mediator without cleavage or losses in the

activity.

High current efficiency and high current density of reversible redox system.

High reaction rate between reduced mediator and oxidized dyestuff.

Sufficient conductivity of catholyte without change in the dyeing results.

Cheap electrode material for cathode and anode.

Simply constructed cell and easy maintenance.

Minimum side reactions e.g. dyestuff destruction due to formation of radicals.

ELECTRO CATALYTIC HYDROGENATION

Electro catalytic hydrogenation is a recently introduced reduction method for vat and

sulphur dyes and it proves to be a promising and attractive alternative in terms of

economic and ecological aspects.

The process consists of a sequence of reduction steps which differ principally from

those of the previously described indirect and direct electrochemical vatting processes

via the dye radical.These involve electron-transfer from the cathode to either a

mediator or the leuco radical anion of vat dye.In contrast to this

mechanism,electrochemical hydrogenation is a process in which adsorbed

hydrogen,produced in situ by electrolysis of water,reacts with adsorbed organic

substrates(i.e. vat dye)at the electrode surface.

Exclusive benefits we get from electrochemical dyeing

Liquor Recycling possible

In electrochemical dyeing experiment the dye liquor recycling loop is repeated at least 9 to 10

times. The same catholyte solution after dyeing was air oxidized and filtered to remove the

insoluble dye molecule (10–15% fresh electrolyte).The conversion as well as current

efficiencies remain reasonably same during each of this recycling experiment. The dye

intensity measured as K/S values were also found to be reasonably stable in all the 10 dyeing

recycles.

Stability of electrolyses

The pH is an important parameter influencing the performance of vat dyes reduction. The Vat

dyes are found in different form depending on the dye bath pH. In conventional reduction,

this parameter must be constantly controlled because the decomposition of dithionite

generates a decrease of the pH values which can cause some disturbances.DyebathpH is more

stable in the electrochemical process than in the conventional one. Hence, there is no need for

the addition of alkali during electrolyses.

Saving

An estimation of chemical savings can be made on basis of the charge flow required to

reduce a certain amount of dyestuff. For sulphure dye the chemical saving calculated as

0.644 kg dry SB1 (Sulphur Black 1) was reduced by a charge flow of 57.5 Ah at a voltage of

maximum 7.5 V. This corresponds to an amount of 3.33 mol reducing equivalents per 1 kg

dry SB1 and to an energy consumption of 0.67 kWh/kg dry SB1 filter cake. When sulphide

reducing agents are used instead of cathodic reduction the amount of Na2S can be calculated

from Eq.

According to Faraday’s law 3.33 moles equal an amount of 129 g Na2S, which will be

required to reduce 1 kg of dry SB1 filter cake.

Online dye bath Analysis

Controlling the bath composition is of great interest. ECD allows us to avoid a useless

prolongation of the reduction time and to prevent the phenomena of over or partial reduction

which are frequent and difficult to control when reduction is carried out by dithionite.And the

final dyed fabric is of better quality because full control over dyeing parameters is possible.

Economy

The entire textile industry get affected because savings in the chemical costs as chemical

wastes reduced by 80% and reduction of waste water recycling cost at the same time

water savings around 85% so economical.

Health

Several times bath recycling is possible with this dyeing method so it is an environmentally

sound Process. Also the toxic nature sulphates and sulphites are not there in effluent so no

adverse effect to aquatic life.

Dyeing Process

Fully controlled dyeing parameters, maximum Process reliability through control of

reducing potential as needed ranging from 0 to 1200 mV just by varying the current. Dye

Reduction Rate is Very good (10mg dye/min) and dye pick up may go up to 85-90%.

Quality of Products

Better overall fastness property is observed so comparatively more consumer

satisfaction.

LIQUOR RECYCLING IN ELECTROCHEMICAL DYEING

The possibility of restoring the reducing power of a used dye bath is an attractive one in these

days of heightened concerns over dyehouse effluents. Naturally ,if reuse of the mediator

system with different days of intended, the residual dye has to be removed from the dye

liquor. This is a more straightforward proposition with vat dyes, because of the insolubility

of their oxidised form in aqueous solutions,and their ability to form suspensions, which can

be removed by a filtration process from the oxidized dye liquor.After completing the dyeing

procedure,the remaining dye and reduced mediator can be oxidized by bubbling air through

the solution to form, the insoluble form of dye.

Recycling of dyeing liquor is then achieved by pressure filtration through a PTFE membrane

with a pore size of 0.45 or 1.2µm at a pressure of 1-2 MPa(10-20 bar).However the dye bath

has to be circulated through the electrochemical cell at a rate that must follow the threshold

conversion rate in the cell.The complete filtration should be ensured to avoid contamination

of cell by impurities released from the goods.In an electrochemical dyeing,the liquor

recycling loop was repeated nine times.The dyeing experiments showed good reproducibility

in the colour of dyed goods,confirming that electrochemical regeneration of reducing agent

can be achieved for many cylces without a measurable loss in electrochemical activity.

Two process engineering concepts for continuous electrochemical dyeing which makes

liquor recycling liable:

a)Closed circuit

b)Mediator concentrate technique

CLOSED CIRCUIT TECHNIQUE:

This technique is called as a closed circuit technique because the content of dye bath are

circulated through the electrochemical cell in this technique,With this technique,the mediator

and vat dyes like indigo can be recovered from wash water.The washing water is passed

through an ultra filtration is unit to remove the insoluble dye.The filtrate of the ultra filtration

subjected to nano filtration where the concentration of mediator is increased to a final value

of 0.6 mol/l of Fe(lll) complex.The Fe(ll)/Fe(lll) ratio in the dye bathis maintained and the

prevailing solution dye liquor potential is maintained.The advantages of this system is that it

allows almost any desired amount of reduction equivalents to be admitted into the dye bath at

constant concentration ratios.

MEDIATOR CONCENTRATE TECHNIQUE

In this mechanism, the content of dye bath are not circulated through the electrochemical cell,

nor does a dyes solution flow through it. The technique is analogous to the metering of

dithionite solutions in indigo, vat and sulphur dyeing. As far as the dye bath potential is

concerned, metering is similar to that of closed circuit technique. The quantity of mediator

required in the dye bath is correlated with the composition of dye bath liquor as in case with

conventional reducing agent.

The great advantages of this technique is that the reduced mediator is stored in tank.The

mediator from this tank can be supplied to several installations with different colouristic

settings.

By doing comparison between two,it reveals that the closed circuit technique offer great

advantages.With the closed circuit technique,

we can utilize rapid process control by measuring the potential and controlling the cell

current.

The composition of the dye bath is independent of dye consumption,so that the risk of

tailing is reduced.

Effect of recycling of electrolyte dye bath

The same catholyte solution after dyeing was air oxidized and filtered to remove the insoluble

dye molecule. This electrolyte, after addition of 10–15% fresh electrolyte, could be recycled

at least 10 times.The conversion as well as current efficiencies main reasonably same during

each of this recycling experiment. The dye intensity measured as K/S values were also found

to be reasonably stable in all the 10 dyeing recycles.

In all experiments discussed, so far, the electrochemical reduction was allowed to continue

during the dyeing experiment. The dyeing process as well as recycling experiments was also

carried out to study the dyeing efficiency without applying any electric current. Such a

procedure is, indeed, necessary for continuous process development, where the dyeing unit

should be separated from electrolyzer unit.

ADVANTAGES OF ELECTROCHEMICAL DYEING PROCESS

Product saving

Less chemicals with special safety requirements

Better fastness properties

Waste water limits easily achieved.

Maximum process reliability through control of the redox potential

Existing dyeing machine can be retrofitted.

Unsurpassed environmental compatibility.

Reduction of water and waste water cost through recycling.

Shorter and more reliable dyeing processes.

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