Abstract
The fiber properties after oxygen delignification and kraft pulping were studied by looking into the chemical characteristics and morphology. The effect of the two processes on the fibers was evaluated and compared over a wider kappa number range (from 62 down to15). Wide-angle X-ray scattering, nuclear magnetic resonance and fiber saturation point were used to characterize the fiber network structure. Fiber morphology and fiber dislocations were evaluated by an optical image analysis. The total and surface fiber charges were studied by conductometric and polyelectrolyte titrations. The fiber wall supramolecular structure, such as crystallinity, size of fibril aggregates, pore size and pore volume, were similar for the two processes. The selectivity, in terms of carbohydrate yield, was equal for kraft cooking and oxygen delignification, but the selectivity in terms of viscosity loss per amount of delignification is poorer for oxygen delignification. Clearly more fiber deformations (2–6% units in curl index) in the fibers after oxygen delignification were seen. Introduction of curl depended on the physical state of the fibers, i.e. liberated or in wood matrix. In the pulping stage, the fiber continue to be supported by neighboring fibers, as the delignified chips maintain their form. However, in the subsequent oxygen stage the fibers enter in the form of pulp (liberated fibers), which makes them more susceptible to changes in fiber form.Graphic abstract
Highlights
Oxygen delignification is a well-established unit process in kraft pulping
The amount of phenolic hydroxyl groups in lignin remaining in the fibers increases as the delignification reactions proceed during kraft cooking to approximately kappa number 50–60
Kraft cooking and oxygen delignification process of softwood had the same selectivity when it comes to dissolution of cellulose and hemicellulose
Summary
Oxygen delignification is a well-established unit process in kraft pulping. The first implementation was at SAPPIs Enstra mill in South Africa in the 19700s (McDonough 1986; Sixta et al 2006). Extended delignification was one of the approaches, as less lignin coming into the bleach plant greatly reduces the formation of chlorinated organic substances. The modified kraft cooking concept was introduced, which allowed pulping to be prolonged to lower kappa number while keeping pulp viscosity at sufficiently high level. Pulp yield suffered if the kraft cooking was extended to very low kappa numbers, below 20 for softwood. It is more beneficial to prolong delignification in the oxygen stage rather than in cooking, with regard to pulp yield. By addition of magnesium sulphate, as it was suggested by McDonough (1986), the decrease in pulp viscosity could be avoided and oxygen delignification stages were introduced in most pulp mills during the 19800s
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