Low Chrome Dyeing: Eliminating Cr(VI) in the Exhausted Bath by Using Cr(III) as the Mordant

Abstract

Previous work examined the performance of sulphamic acid and rare earth in the afterchrome dyeing of wool using the dichromate process. This paper describes a dyeing method that uses a complex of Cr(III) to afterchrome mordant dyes and so completely eliminates Cr(VI) from the effluent. 5-Sulphosalicylic acid, sodium salt (SSA) was selected as an efficient ligand for complexing with Cr(III) to form a special chelate compound (SSA-CrIII). This compound is named the "complexing carrier," since it is of great value in carrying Cr(III) into wool and transferring the Cr(III) ion to the chrome dyestuffs already taken up by the wool to make the mordanting possible. The optimum molecular ratio of SSA to Cr(III) in the chelate compound and the methodology for testing the pickup ratio of SSA onto wool have been established through much laboratory work. The mordanting results indicate that satisfactory hue, good color fastness, and improved handle are imparted to wool fibers when compared with the dyeing results obtained through the conventional mordanting process. After allowing for desorption of SAA during the alkali solubility test, the alkali solubility of the dyed wool after the new mordanting method is similar to that obtained after conventional afterchrome dyeing with dichromate.

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