Chrome Dyeing of Wool: Reducing the Amount of Chromium in the Residual Bath

The concentration of the residual Cr(VI) and Cr(III) in the dyebath can be decreased to acceptable levels by incorporating alpha-hydroxymonocarboxylic acids in afterchrome dyeing using the normal dye to dichromate ratios, the most suitable being lactic acid. The mechanism is based on the reduction of Cr(VI) to Cr(III) which appears to take place predominantly on the wool fibre. The optimum pH range for the treatment in 3.4 to 3.6. Compounds which are substantive to the wool fibre such as sulphates, or form stable complexes with Cr(III) such as ethylenediaminetetracetic acids, which are also effective in reducing the concentration of the Cr(VI) in the bath, form stable complexes with Cr(III), adversely affecting the concentration of the residual Cr(III) in the bath