Recirculação do filtrado alcalino do branqueamento para o estágio de deslignificação com oxigênio de polpa kraft e sua consequência no processo

Abstract

It was evaluated the recycle of the extraction filtrate to POW pulp. The results are presented in four chapters: (1) alkaline filtrate recycle to POW softwood pulp using OD(EPO)DED bleaching sequence; (2) alkaline filtrate recycle to POW hardwood pulp using A/D(EP)DP bleaching sequence; (3) effect of the (EP) filtrate recycle to O stage using OA/D(EP)DP bleaching sequence at the pulp and filtrate extractives amount; (4) effect of the alkaline filtrate recycle from hardwood ECF bleaching sequences at the chemical recovery cycle. In chapter 1, it was evaluated the effect of the alkaline filtrate at the pulp bleaching efficiency, chemical and physical-mechanics characteristics. The chemistry characteristics of the recycled filtrate as well as the effluents that go to the biological treatment were studied also. The pulp bleaching efficiency was affected with the alkaline recycle; one unit of EOP filtrate carryover consumes 0.08 kg/tod of chlorine dioxide. The sulfuric acid and NaOH consumption were also increased to pH adjustments. The viscosities of the bleached pulp were not affected. It was not noticed any difference at the physical-mechanical properties of the refined bleached pulp when EOP filtrate was recycle or not, by the statistics method used. The sodium, sulphate and chloride levels were increased in the dioxide and extraction filtrates with 15kg/tod of alkaline filtrate, but this increase was more in the acid filtrate. The effluent generated from the recycled sequence presented les COD, BOD, TOC, solids and color; however the BOD/COD was worst. In chapter 2, it was evaluated the bleaching efficiency when the (EP) alkaline filtrate is recycled to POW hardwood pulp using A/D(EP)DP bleaching sequence. The chemical and physical-mechanical properties of the bleached pulp were also studied. The chlorine dioxide consumption increased 13.7% when 5 kg of EP COD/tod was added to the POW pulp. The mainly effects at the bleached pulp when the alkaline filtrate was recycled were: increase of OX, brightness reversion and increase in calcium concentration The pulp viscosity, hemicellulose content and hexenuronic acids content were not affected. The physical-mechanical properties of the refined pulp were not affected, except for specific volume. In chapter 3, the effect of the (EP) filtrate recycle to O stage using OA/D(EP)DP bleaching sequence at the pulp and filtrate extractives amount was studied. The DCM and ethyl acetate extractives were analyzed at the pulp and A/D and EP bleaching filtrates. Some specific compounds increased at the pulp when 15 kg/tod of the alkaline filtrate was recycled, like the high molecular weight fatty acid and sterols. The alkaline filtrate recycle to POW pulp presented little increase at the A/D filtrate extractive amount. The compounds identified at the A/D filtrate were different from the one from (EP) filtrate, as -sitosterol, - sitostanol and stigmast-3.5-dien-7one. In general, most of the identified compounds at the (EP) filtrate during the control sequence were decreased during the 15 kg/tod carryover sequence. In chapter 4, the effect of (EP) filtrate recycle from two bleaching sequences (D(EP)DP e DHT(EP)DP) to POW pulp at the chemical recovery were studied. The (EP) kappa number was reduced by 15% at DHT stage; the viscosity reduced 22%, however the brightness decreased 3.3 % when compared with conventional Do. The Do stage generated 10 % less chloride ion at the alkaline filtrate comparing with Do stage. However, the substitution of water as POW showers by alkaline filtrate increase significantly the chloride level at the recovery cycle. The potassium concentration didn’t change. The alkaline recycle strategy depends essentially of some technology to remove the chloride and potassium from the system.