An efficient photochemical bleaching of kraft pulps using total halogen-free reducing reagents

Total chlorine-free bleaching of kraft pulps was conducted by a reductive photochemical process at room temperature. Reducing reagents were screened, and bleaching conditions were optimized using excimer lasers. The laser used in the best bleaching condition was then substituted by conventional light sources to make the process more easily accessible. The most favorable condition for the bleaching of kraft pulps was irradiation with a low-pressure mercury lamp in the presence of aqueous sodium borohydride solution, which gave bleaching efficiency and the properties of pulps similar to those obtained by a conventional two-stage, elemental chlorine-free process. The photochemical background of the bleaching is discussed.