Study Of Graphite Felt On Indirect Electrochemical Reduction And Dyeing Process Of Vat Dyes

Reducing agents required in the dyeing process for vat and sulfur dyes cannot be recycled, and lead to many environmental problems. Traditional dyeing methods use sodium hydrosulfite（Na2S2O4） as reducing agent, but some of the byproducts produced in the hydrosulfite reduction process is sulfur compounds, which will pollute the atmosphere in the form of hydrogen sulfide; Also, this method will produce a lot of high concentrations of sulfite and sulfate which will pollute the water in the industry, and cannot meet the requirements of green chemical processes. Therefore, electrochemical dyeing method was proposed in our study for its moderate reaction conditions, controlled redox potential reductant renewable, depleting chemicals, and less wastewater discharge.Therefore, the application of direct electrochemical reduction of indigo as a novel route has been investigated by spectrophotometric and voltammetric experiments in laboratory cells. The results of experiments indicated the reaction mechanism revealed possibility of this novel route to produce water-soluble indigo, which offers tremendous environmental benefits.A graphite felt was used as the cathode material for the indirect electrochemical reduction in alkaline aqueous solution wmploying iron-triethanolamine（Fe-TEA） complex as a redox mediator. Surfact structure and specific surface area of the carbon felt electrode and its electrochemical performance toward the reduction were investigated by SEM, BET, and cyclic voltammetry, repectively. Moreover, an optimization of different operating conditions for the reduction of vat dyes was perfomed with an H-cell. In particular, the dependence of current efficiency（CE） on electrolysis time was chosen to analyze the indirect electrochemical reduction mechanism of indigo. The results showed that the graphite felt, with a high specific surface area of approximately 7289 cm2/g, is an excellent electrode material for the indirect electrochemical reduction of vat dyes and that a moderate current density, temperature, and concentration of vat dyes have the most beneficial effects on the reduction process.Firstly, indirect electrochemistry reduction device was designed. The main work consisted of designing electrolytic bath and electromagnet and connecting the whole circuit.Secondly, the indirect electrochemistry reduction process of vat dyes in appearance of additional alternating induced electric field was studied. The effects of influence factors on K/S values and CV curves were discussed by orthogonal and single factor experiments. The best reduction conditions with addit ional electric field are NaOH 15 g/L, TEA 35 g/L, Fe₂（SO₄）₃ 5 g/L, dye 2%owf, reduction voltage 3V, cathode area 100cm2, reduction time 20 min. Under mentioned conditions, highest reduction rate and color depth can be obtained. Different dyes with different concentration were also studied under the same media.Reducing agents required in the dyeing process for vat and sulphur dyes cannot be recycled and lead to problematic waste products. The application of electrochemical reduction of several vat dyes and even mixtures of them on a fixed bed cathode consisting of graphite granules has been investigated by spectrophotometric experiments in a laboratory scale pilot plant. Experiments yield information about the kinetics and show the possibility and versatility of this environmentally friendly process for production of water soluble leuco dyes. In addition to singular dyes, even dyestuff mixtures were investigated. Regarding the effect of different molecular structures on the kinetics and a preliminary understanding of the reaction mechanism, relationships between computed molecular parameters and the reduction rate are presented.