The third generation of polyoxometalates: The basis for a commercially feasible closed-mill delignfication technology

A new technology based on the use of polyoxometalates (POMs) as delignification agents is under development. These reagents are chlorine free and can be used under conditions wherein they oxidize lignin and chromophores in wood pulp fibers while leaving the cellulose undamaged. Their promise is enhanced by the fact that they can be reactivated with oxygen, in a separate stage, under conditions that result in oxidation of the organic byproducts of the delignification process. Thus, they can be continuously recycled in closed systems that can provide the basis for a new class of closed mill technologies in which the consumable oxidant is oxygen, and the primary byproducts are carbon dioxide and water. Such systems are inherently the product of well-integrated subsystems, each of which accomplishes a specific task. The two key tasks that have been the points of focus for the program are delignification of kraft pulps by POM solutions under anaerobic conditions, and regeneration of the oxidative capacity of spent POM liquors simultaneously with oxidation of the organic byproducts to water and carbon dioxide. The first group of POMs investigated provided the basis for establishing the requirements for each of the tasks. They led to demonstration of selective delignification and effective re-oxidation and mineralization. The current generation of POMs have been optimized for effectiveness in both tasks. Furthermore they are readily synthesized and they are inherently self-buffering. Their performance in the context of traditional bleaching is indicated by the capacity to reduce the kappa levels of softwood kraft pulps from 30 to below 5 while retaining viscosity above 20. More recently it has been shown that they can effectively delignify kraft pulps at much higher kappa levels in the 100 to 110 range, and soda AQ pulps at kappa levels about 120. Thus POM delignification technology is applicable both for achieving TCF closed mill systems in the traditional bleaching context and, perhaps more significantly, for cost effective incremental expansions of the capacity of recovery-boiler-limited pulp mills.