The sulphur dyes (of which sulphur black is one type) are mainly used for dyeing cotton, rayon and cotton-synthetic blends. The darker shades of the sulphur dyes are preferred by textile manufacturers for the deep colours they produce in the final fabric.
The solid dye is first converted into a water soluble form so that it can be absorbed by the fibre. This is achieved using an alkaline reducing agent in solution, traditionally sodium sulphide. Once absorbed, the dye is reconverted (by oxidation) to the insoluble form to fix it on the fabric and ensure that it will not wash out. This step is often carried out using acidified dichromate (usually sodium dichromate or potassium dichromate).
Amongst synthetic dyes, it is a low cost dye and exhibits excellent wash and good light fastness. The jet black shade of sulphur black colour cannot be produced in an economical manner by use of any other class of cotton dye. Further, it has good covering properties for dyeing immature cotton. The appeal of this material colours results from its durability, flexibility of application and low cost.
Generally, woven goods are dyed on jiggers and knitted goods are dyed on winches, or jet dyeing machines, by batching. The adverse aspects of sulphur black dyeing result from the use of sodium sulphide and acidified dichromates, which are toxic and hazardous to health, both to factory staff and to end users. Consequently, many European countries have discontinued the use of sulphur colours. However, they continue to purchase a large volume of fabric dyed in this chemical shade from developing countries. Therefore, this demonstration project has been developed to retain the advantages of sulphur black dyeing, whilst reducing the hazards normally associated with it. This is achieved by substituting the hazardous reduction and oxidation chemicals with less harmful ones.
Being fairly cheap, sodium sulphide is a widely used, traditional reducing agent for sulphur black dyeing. However during processing, sodium sulphide gives off foul smelling sulphide gas which is toxic by inhalation and can be fatal even at very low concentrations.
Its use also contributes to sulphide in the effluent which can have toxic effects (beyond permissible levels) on the aquatic life in the receiving water bodies. Sulphide-bearing effluents are also corrosive and can accelerate corrosion of the equipment used for effluent treatment.
One method is spreaded to reduce or eliminate the occurrence of sulphide and chromium in the effluents: Implement chemical substitution for sodium sulphide and dichromate by less harmful substances. The substitution of sodium sulphide and dichromate by least harmful substitutes (such as glucose, peroxide, sodium perborate etc.) can thus make the sulphur black dyeing process eco-friendly. Such a modification maintains the price and quality advantage of the sulphur black dye and at the same time eliminates the adverse environmental impacts of the process.
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