Aperfeiçoamento da deslignificação com oxigênio e do branqueamento de polpa celulósica Kraft de eucalipto para dissolução

Abstract

The demand for the dissolving pulp production has increased in the last decade and many pulp mills are adjusting their operations to produce it. One important parameter which differentiates the dissolving pulp from paper pulp is its high content of alpha cellulose and low levels of hemicelluloses, lignin and ash. Moreover, dissolving pulp is also characterized by low polydispersity, high brightness and reactivity to chemical reagents specific. The amount and different types of hemicelluloses is the most significant chemical trait of such pulps and has an impact on processability and final product quality. Currently, techniques for the production of soluble pulp are: (1) prehydrolysis kraft, which consists of chips prehydrolysis with water or steam before of the kraft cooking, and (2) acid sulphite cooking. In both cases, hemicelluloses are solubilized and depolymerized, and it is possible to achieve high content of alpha cellulose, being the crystallinity also affected. Brazil has a huge potential for pulp paper production, being the fourth largest producer in the world; however, the production of dissolving pulp is limited. The demand for dissolving pulp is growing and the knowledge regarding its production from eucalypt is scarce. This work aimed at optimzinge the oxygen delignification and bleaching steps for a prehydrolysis kraft dissolving pulp from Eucalyptus spp. using elemental chlorine free (ECF) and conventional sequences, to attain desirable market qualities. The main findings of this work were: (1) dissolving pulp bleaching (viscose grade) was feasible by using conventional and ECF sequences, reaching the quality required for this type of pulp, except for mineral content, (2) it is recommended the intensive oxygen delignification in order to have flexibility in controlling the kappa number and especially the pulp viscosity; (3) the dissolving pulp bleaching sequence is perfectly feasible using a ECF sequence with three stages, D(EP)D, which achieved 92% ISO of brightness; (4) using the ECF sequence with the four stage D(EP)DP, may be interesting due to the final peroxidation which deliver a lower viscosity (final pH 10), and low OX level of the bleached pulp; (5) the application of the conventional sequence D(EH)DP produced very satisfactory bleaching results, and good pulp quality (excellent viscosity control by controlling the final pH of the (EH) stage; (6) the use of a chelation stage at the end of the bleaching, in sequences such as D(EP)DPQ and D(EH)DPQ reduced the ash, iron and calcium pulp content, but this step is only recommended if the concentration of these minerals are out of the specification for the dissolving pulp; (7) it is not recommend the use of elemental chlorine in the first stage of bleaching sequences such as (DC)(EP)DP or (DC)(EH)DP; and (8) in the sequences D(EH)DP e D(EH)DPQ it is not recommend operating the D 0 stage at pH < 1.7 due to the pulp quality fall under the limit.