2.5. Indirect electrochemical dyeing:

Thomas Bechtold patented indirect electrochemical dye reduction method in 1993. Here, the dye is not directly reduced at the electrode. Rather, a reducing agent is added that reduces the dye in the conventional manner which in turn gets oxidized after dye reduction. The oxidized reducing agent is subsequently reduced at the cathode surface, which is then further available for dye reduction. This cycle is continuously repeated during the dyeing operation. In electrochemistry, the agent, which under goes reduction and oxidation cycles, is known as reversible redox system and is called a mediator.

Thus, in the system, the dye reduction does not take place due to direct contact of dyestuff with the cathode, like in direct electrochemical reduction, but it takes place through the mediator which gets repeatedly reduced due to the contact with the cathode. Therefore this system is known as indirect electrochemical dyeing.

The object of the reversible redox system primarily in the first place is to generate a continuous regenerable reduction potential in the dye liquor. Therefore addition of conventional reducing agent is not essential and therefore there is no accumulation of decomposition products of the reducing agents takes place in the indirect electrochemical dyeing. The electrochemical dyeing appears simple because after dyeing cycle, the unexhausted dye gets precipitated by air oxidation and can be removed by filtration. After the dye removal, the color containing the mediator, ligand and alkali can be recycled for subsequent dyeing operation. This appears to be most important feature in the terms of the cost and the environment friendliness of the process.

2.5.1. Difficulties to establish indirect electro chemical dyeing process:

• The actual reduction of the dye should be carried separately into electrochemical cell and the reduced dye is then circulated separately into a conventional dyeing unit.
• To keep the dye in reduced form it is necessary to reduce the oxidized mediator at the cathode.
• The design of the cell should be such that the cathode should have the maximum surface area available for the reduction of mediator.
• A three dimensional electrode with large surface area occupying small place in electrochemical cell should be designed.

2.6. 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 concern over dye house effluents. Naturally if reuse of the mediator system with different dye is intended, the residual dye has to be removed from the dye liquor. This is more straightforward proposition with vat dyes. Because of the insolubility of their oxidized form in aqueous solutions, and their ability to form suspensions, which can be removed by a filtration process form the oxidized dye liquor.

In electrochemical dyeing experiment by Thomas Bechtold the dye liquor recycling loop was repeated nine times. The dyeing experiments showed good reproducibility in the color of the dyed goods, confirming that electrochemical regeneration of the reducing agent can be achieved for many cycles without a measurable loss in the electrochemical activity.

Two process-engineering concepts for continuous electrochemical dyeing, viz. the closed circuit and the mediators concentrate technique have made liquor recycling viable

2.6.1. Close circuit technique:

This technique is called as a close circuit technique because the content of the dye bath are circulated through the electrochemical call in this technique. With this technique, the mediator and the vat dyes like indigo can be recovered from wash water. The washing water is passed trough and ultra filtration unit to remove the insoluble dye. The filtrate of the ultra filtration is then subjected to nano filtration where the concentration of the mediator is increased to a final value of 0.6 mole/liter of Fe (III) complex. The Fe (III) salt concentrate is also metered into the electrochemical cell so that the Fe (II) / Fe (III) ratio in the dye bath is maintained and the prevailing solution potential is maintained. The advantage of this system is that it allows almost any desired amount of reduction equivalent to be admitted into the dye bath at constant concentration ratios.