Description of oxygen-alkali delignification of kraft pulp using analysis of dissolved material

Abstract

The possibilities of estimating the performance chemistry of an industrial oxygen delignification process were studied by analyzing in detail dissolved pulp material and by using component material balances. One purpose was to estimate the yield and selectivity of this process by characterizing the various reaction products with respect to their origin in pulp. A further aim was to estimate the amount of oxygen that is actually consumed or needed in different reactions during the process and the effect of oxygen charge on selectivity. The research was focused on industrial oxygen delignification process in order to find a way to describe the delignification process which would take account the variables absent in laboratory experiments. The kraft pulps used were Scandinavian softwood (mixture of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies)) and hardwood (silver birch (Betula pendula)). The methods were also utilized in laboratory-scale oxygen delignification of bamboo (Bambusa procera) kraft pulps. The use of component material balances to estimate the pulp yield and selectivity of an industrial oxygen delignification process as well as of the laboratory-scale bamboo pulp process proved feasible. The material balances corresponded to a pulp yield of 97-98 % for the industrial wood pulps and yields of 95-98 % for the laboratory bamboo pulps. It was estimated that roughly 70-80 % of the softwood and ~60 % of the hardwood pulp material dissolved derived from lignin. The selectivity of bamboo pulps was comparable to that of the birch pulps. Estimations for the origin of oxygen on the basis of the dissolved reaction products revealed that 39-80 % of the charged oxygen was consumed during the oxygen delignification stage. The results suggested a molecular oxygen consumption of 0.6-1.0 kg/BDt per unit of kappa number reduction for softwood and 1.0-2.0 kg/BDt per 11kappa unit for hardwood kraft pulps. Oxidation of components (Na2S, Na2SzO3, and Na2SO3) in the oxidized white liquor (OWL) consumed a considerable portion of the molecular oxygen, especially in cases where the sulfide concentration in OWL was high. It was observed that birch kraft pulps were more susceptible to degradation, which was seen in greater amounts of dissolved hydroxy monocarboxylic acids and volatile acids compared to that of softwood pulps. In spite of the somewhat different chemical composition of bamboo pulps, compared to wood kraft pulps, the oxygen delignification of bamboo kraft pulp resulted, for example, in the formation of the same carbohydrate-derived degradation products (e.g., aliphatic carboxylic acids) as obtained during the oxygen delignification of birch kraft pulp.