Improved selectivity in kraft cooking through changesin the cooking liquor concentration profiles

Abstract: In the production of bleached kraft pulp, extendeddelignification in the cooking stage is a method of reducingthe load on the receiving waters. A low lignin content (lowkappa number) also means a reduced need for bleachingchemicals. The extent to which the pulp can be delignified inkraft cooking is limited by the poor selectivity towards theend of the cook. In order to identity the most importantfactors affecting the selectivity, expressed as viscosity andyield versus kappa number, cooks were canied out in acontinuous liquor flow digester. Varying the liquor compositionthrough the digester was found to be a technique well suited toa study of the effects of different time profiles for theconcentrations of active cooking chemicals, dissolved ligninand hemicellulose.Addition of dissolved lignin during different parts of thecook showed that the lignin has no or a slighfly positiveeffect during the initial phase and the beginning of the bulkdelignification phase, but that it has a negative effect duringthe final part of the cook. Furthermore, it was demonstrated,in accordance with results obtained by other methods in theliterature, that the selectivity is improved by a low andrelatively uniform concentration of effective alcali and by ahigh concentration of hydrogen sulfide ions, especially duringthe initial delignification and during the first part of thebulk delignification phase.The negative effect of the dissolved wood substance on theselectivity was also confirmed by continuous liquor flowcooking simulating the concentration proffiles of activecooking chemicals in conventional cooking, but without theaddition of lignin or hemicellulose to the cooking liquor.Cooking with a rnimum of dissolved wood substance in thecooking liquor gives a viscosity 200-250 din3/kg higher thanthat achieved in conventional cooking. The increase inviscosity is mainly due to the decreased concentration ofdissolved lignin.It was shown that the presence of dissolved lignin in analkaline solution leads to an increase in the alkalinity whenthe temperature is raised. Taking this effect into account, theinfluence of the dissolved lignin on the delignification ratewas investigated. At conditions during the initial phase thatlead to sorption of sulfide by the wood, the negative effect ofthe dissolved lignin is counteracted. Under such conditions,the dissolved lignin caused an increase in the delignificationrate during the bulk phase but a decrease during the finalphase.Two of the principles for improved selectivity,viz.levelled out concentration of effective alkali anda low concentration of dissolved lignin during the final partof the cook, were first realised in mill-scale batch cooking aspart of the present investigation. Changes in the cookingliquor concentration profiles were achieved by split whiteliquor charges and liquor displacement. The modified processwas run successfully to a kappa number 25 % below the previoustarget. With the results of the present work as a significantcontribution, the industrial processes have been furtherdeveloped by others, but there is a potential for furtherimprovement by simultaneously fulfilling all the principles forimproved selectivity.Keywords:Black liquor, Dissolved lignin, Cellulose,Effective alkali, Extended delignification, Hydrogen sulfideion, Hydrolysis, Kappa number, Kinetics, Kraft cooking,Selectivity, Softwood, Sulfidity, Yield, Viscosity, Xylan.