Study On The Separation And Reuse Of Indigo Dye From Waste Cotton Denim

As a vat dye with a history of more than 3,000 years,indigo is widely used in the denim textile industry,with an annual demand of 50,000-60,000 tons.The current indigo synthesis and recycling methods are not applicable to the current deteriorating environment,and there is an urgent need to find a green and environmentally friendly way to synthesize or recycle indigo to meet the current needs of environmental protection and promote the recycling of indigo dye resources.In this project,waste cotton denim textiles were used as the base material to systematically study the separation and reuse of indigo.Firstly,this paper analyzed the effects of sodium hydroxide on fabric weight,insurance powder on fabric weight,temperature,time,bath ratio and other factors on the dye separation effect as well as the effect of increasing the whiteness of fabric through single-factor experiments,and concluded that the effects on dye separation from strong to weak are: insurance powder on fabric weight > temperature > sodium hydroxide on fabric weight > time > bath ratio.Then the separation conditions were optimized through a three-level four-factor orthogonal experiment with dye separation rate and strength loss rate as evaluation indexes,and the optimal solution to ensure a small fabric strength loss rate and a high dye separation rate was: sodium hydroxide to fabric weight of 10 %,insurance powder to fabric weight of 10 %,separation temperature of 60 ?,and separation time of 20 min.Under these conditions,the dye separation rate of cotton denim was 87.33 % and the strength loss of the fabric was22.22 %.Secondly,the physical and chemical properties of indigo in the separation solution,fabrics before and after separation were analyzed by ultraviolet-visible spectrophotometer(UV-Vis),fourier transform infrared spectrometer(FTIR),X-ray powder diffractometer(XRD),and thermogravimetric analyzer(TGA),It can be seen that the cryptochrome indigo groups in the separation solution did not change,and the physicochemical properties of the recovered indigo dye did not change,proving that the recovered indigo can be used for secondary dyeing and utilization.The groups in the cellulose fibers in the fabric were basically undamaged and no new chemicals were produced,which basically achieved the purpose of separating the dye without damaging the fabric.Then the content of indigo in the fabric was tested by violet-visible spectrophotometer at different temperatures with the separation time,and the kinetic fitting of the data was carried out,and it was found that the separation process of indigo was in accordance with the tertiary kinetics,and the tertiary kinetic fitting by the two-stage fitting method could better explain the separation process of indigo from the fabric in the sodium hydroxide/insurance powder system.Thirdly,the staining properties such as diffusivity of indigo purified from the isolate and standard indigo dye,initial staining,homogenization and migration of the isolate and standard indigo dye were compared,and it can be seen that the staining properties of the isolate can achieve similar staining properties of standard indigo dye.Then,the content of indigo dye in the separation liquid under the best conditions was analyzed by UV-Vis spectrophotometer.According to the content of sodium hydroxide,hydrosulfite and indigo in the separation liquid,the separation liquid was simulated by using standard indigo.The color properties,mechanical properties and color fastness of yarns dyed with separation solution and simulated separation solution were compared,and the dyeing effect is close to that of indigo,which proves that the indigo in the separation liquid can be applied to the secondary dyeing.