1. Dye pre reduction: This is the first step of the dyeing process, usually using sodium hydroxide (caustic soda) and sodium hydrosulfite (sodium hydrosulfite) as reducing agents to reduce insoluble vat dyes to soluble sodium salts of the chromophore under alkaline conditions. During this process, the carbonyl groups in the molecular structure of the dye are reduced to hydroxyl groups, generating cryptochromic acid, which then reacts with sodium hydroxide to form cryptochromic sodium salts.

2. Dip dyeing or roll dyeing: Pre reduced dyes are used to dye fibers through dip dyeing or roll dyeing. Dip dyeing is suitable for yarns and knitted fabrics, while roll dyeing is more suitable for woven fabrics. In this process, the sodium salt of the chromophore has a direct effect on cellulose fibers, first adsorbing on the surface of the fibers, and then diffusing into the interior of the fibers to complete the dyeing of the fibers.

3. Water washing: After dyeing is completed, water washing is required to remove dyes and chemicals that are not fixed on the fiber surface.

4. Oxidation: The chromophore of the dyed fiber needs to be oxidized by air or oxidants (such as hydrogen peroxide, sodium perborate, etc.) to transform the dye from a reduced state to an oxidized state, thereby restoring its original insolubility and restoring its original color.

5. Soap washing and post-treatment: After oxidation is completed, the oxidized floating color on the fiber surface is removed by boiling with soap, while causing dye molecules to aggregate on the fiber surface, forming microcrystals to enhance the color and fastness of the dyed fabric. Finally, wash again with water to remove residual soap and unset dyes.

During the entire dyeing process, it is necessary to strictly control process parameters such as temperature, time, pH value, etc. to ensure the uniformity and fastness of dyeing. At the same time, it is necessary to choose appropriate dyeing methods and processes based on the characteristics of dyes and the types of fibers.

Please note that this is only a basic dyeing process with vat dyes, and the actual dyeing process may vary due to factors such as the type of dye, fiber properties, and dyeing equipment. Therefore, in practical operation, adjustments and optimizations need to be made according to specific circumstances.

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