a combination of the following sodium dithionite thiourea dioxide hydroxyacetone or mixtures or equivalents thereof the anthraquinonoid vat dye is any of or a combination of the following or their equivalents vat brown 3 c i 69015 vat black 25 c i 69525 indanthren direct black 5589 vat violet 1 c i 60010 vat red 13 c i 70320 vat red 10 c i 67000 vat yellow 2 c i 67300 vat orange 15 c i 69025 vat blue 6 c i 69825 or vat brown 1 c i 70800 cotton yarn dyed with darker shades of vat red 10 c i 67000 anthraquinonoid vat dye was used in pilot plant operation using treatment unit of the present invention in conjunction with rope dyeing process the dye composition in the dye tank was made in the following order a solution 5 g l of dispersing agent setamol ws commercially available was made next complexing agent trilon tb was added to a final concentration of 2 g l next vat red 10 c i 67000 was added to the solution to make a final concentration of 150 g l of dye finally the wetting agent was added to the solution to a final concentration of 3 g l this composition was introduced to the reaction unit at a rate of 0 10 l.