Polyoxometalate bleaching: a highly selective, closed-mill technology

Polyoxometalates, a class of oxidatively robust inorganic oxidants and oxidation catalysts, are currently under investigation at the Forest Products Laboratory (FPL) and at Emory University as an alternative to chlorine-based chemicals in the bleaching of softwood kraft and other pulps. Although polyoxometalate salts are used in a number of industrial processes, the feasibility of using these salts and oxygen in the commercial bleaching of chemical pulps was only recently demonstrated. A clear advantage of polyoxometalates over oxygen alone, hydrogen peroxide or ozone, is their inherently high selectivity for the residual lignin in softwood kraft pulps. Softwood kraft pulps delignified to low kappa numbers using polyoxometalates possess papermaking properties comparable to those obtained using chlorine (and/or dioxide) and alkali. The goal of ongoing research is to develop a highly selective, energy-efficient, oxygen-based polyoxometalate delignification and bleaching technology, compatible with mill closure. To this end, several unit operations are under development with the aim of constructing and optimizing an integrated, laboratory-scale bleaching facility. These include: Bleaching, pulp washing, wash water recovery, removal of salt impurities, wet oxidation of dissolved organic compounds, and aerobic oxidation of spent polyoxometalate salts to their bleaching active forms.