Study On The Recycled Process Of Electrochemical Reduction Of Vat Dyes

Vat dye is reduced by electrochemical reduction using clean energy instead of the traditionalhydrosulfite. It is a development direction in the future due to the elimination of sulfate, sulfitewaste liquor and sulfur dioxide pollution. Electrochemical reduction is divided into three types,including direct electrochemical reduction, indirect electrochemical reduction and electrocatalytichydrogenation reduction. In this thesis, we studied the indirect electrochemical reduction becausethe highest current efficiency is only80%and the reaction rate is limited by the low ion-radicalconcentration for the direct electrochemical reduction, as well as the cost is high, and the catalysthas shot life, low activity, conversion rate and current efficiency for the electrocatalytichydrogenation reduction.In the present study of the electrochemical reduction of dyeing,it is difficult forelectrochemical reduction of dyeing to be applied into large-scale industrial production. To solvethis problem, we used the Vat Red R as the dye and three kinds of medium materials(there aretriethanolamine, NaOH, Fe2(SO4)3) for more detailed studies using dip dyeing process, so as toachieve the purpose of reduce the production cost, reduce waste water discharge.First of all, under the condition of the same dye and NaOH concentration, we conducteddyeing experiments both in indirecy electrochemical dyeing process and in the traditional sodiumhydrosulfite dyeing process.It was proved that the color depth, chroma, levelness and lightnesswhich were tested by the colour matching instrument basically could compare with the cottonfabric which dyed in the process of the traditional sodium hydrosulfite dyeing process.Then, adequate waste dyeing liquid was collected and used in the electrochemical dyeingprocess, the surface of dyed fabric was uneven. In order to recycle waste dyeing liquid, the dyemust be oxidized by oxygen and filtrated as far as possible.By calculating the chemical consumption and dye consumption during dyeing process, we found that10%more mediummaterials (triethanolamine, NaOH, Fe2(SO4)3) and100%Vat Red R were needed to recycle wastedyeing liquid.It was approved that the K/S value of the fabric dyed in the recycled process wereas same as the fabric dyed in the initial process. And the color depth, chroma, levelness andlightness which were tested by the colour matching instrument basically could compare with thecotton fabric which dyed in the process of the traditional sodium hydrosulfite dyeing process.During the recycled dyeing process,90%of medium materials were saved.Because of thelimitation of time,we only recycled the waste liquid of the indirect electrochemical dyeing forseven times.In addition, by adopting the dyeing process, the processed waste liquid can also be used forVat yellow G, Vat blue RS. And the color depth, chroma, lightness and fastness were well whenwe operated electrochemical reduction for cotton fabric using Vat Blue RS, Vat Red G under theabove condition. When we matched the dyes,the results were also well.Finally, we also recycled the vat dyes during filtration process and tried to reuse it becausevat dyes were relatively expensive. The color depth, chroma and lightness were still the same, thewashing fastness and rubbing fastness were4-5grades. The results demonstrated that the fitratedvat dyes can also be recycled.The significance of this thesis were: firstly, the consumption of medium materials(triethanolamine, NaOH, Fe2(SO4)3)was saved by90%during the recycled processes. We savedthe cost and eliminated environmental pollution, accordingly. Secondly, the recycled can be usednot only for monochrome dyeing but also for the combination of vat dyes,so this study makes itpossible to use the recycled indirect electrochemical dyeing process in a widely range.