When bleaching, oxidised white liquor can also be used as an alkali source instead of externally purchased caustic soda. However, for this purpose, the sulphide contained in the white liquor must be removed; otherwise it would interfere with the bleaching process. In white liquor oxidation, the sulphide is converted to thiosulphate (partially oxidised white liquor) or to sulphate (fully oxidised white liquor). Partially oxidised white liquor is suitable for oxygen delignification, while fully oxidised white liquor is also useful for peroxide bleaching stages.
Advantages of this process:
Reduction in chemical costs (white liquor compared to caustic soda)
Closes the chemical cycle