The industry is desperately in the need of newer and very efficient dyeing/finishing and functional treatments of textiles. There is growing awareness and readiness to adapt new perspective on industrial upgradation of Cleaner Production Programme, such new technologies help enterprises achieve green production and cost reduction at the same time. Green Production has become necessary for enterprises under the upgrade and transformation policy. Therefore there is an urgent need to promote new technologies in textile dyeing and finishing, injecting new thoughts to the industry.
Electrochemical Process Technology
Electrochemistry refers to the use of electrical energy in initiating chemical reactions, replacing traditional aid agents in direct chemical reactions. Taking sulphur dyes as example, in traditional technology, sulphides (such as sodium sulphide, Na2S) are used as reducing agents. Although reduction process is fast and direct, large amount of chemical energy is wasted and wastewater with high chemical oxygen demand (COD) value is produced, making long-term operation inefficient. If direct electrochemical reduction is adopted, no reducing agents are needed and the COD value of wastewater can be largely reduced, hence lowering the cost of wastewater treatment.
Direct electrochemical reduction is undoubtedly more efficient than the traditional technology, and the underlying chemical principle is also simple. However, as the stability and oxidising/reducing power of different chemical substances are not the same, dyes may not be directly and effectively reduced by electrodes. Hence the scope of utilising direct electrochemical reduction is quite narrow. The principle of indirect electrochemical reduction is the same, but in operation another strong oxidising/reducing agent acts as medium, which makes the technology more applicable to different kinds of dyes. Taking indigo as example, traditional technology takes sodium dithionite (Na2S2O4) as a reducing agent, and the product should be re-oxidised in the air afterwards to fix the colour. Just like traditional reduction of sulphides, large amount of chemical energy is wasted and wastewater with high COD value is produced.
Enterprises attempt to reduce the amount of sodium dithionite used in order to lower production cost, but such attempt produces other difficulties as well. For example, injecting nitrogen can reduce the oxidation of sodium dithionite but is too expensive. Adding aldehydes or directly powering with electricity can improve the reducing power of sodium dithionite, but the problem of wastewater remains.
If indirect electrochemical reduction is adopted, the medium can replace sodium dithionite as the reducing agent. The medium can provide both oxidizing and reducing substances and can regenerate so that both waste and pollution can be reduced. Past experiments show that reduction by electrolysis can save about 90 per cent of production cost when compared with reduction by sodium dithionite.