paper presented at 2001 TAPPI PULPING CONFERENCE ...

paper presented at 2001 TAPPI PULPING CONFERENCEabbrevated versionHYDROGEN PEROXIDE in an ECF BLEACH PLANT: CENIBRA’s INDUSTRIAL EXPERIENCEHENRIQUE, Paulo Molinar *; COSTA, Marcelo Moreira *; CORREIA, Flávio Marcelo *; FONSECA, Maria JoséOliveira *; SANTOS, Jorge Roberto Cardoso *; LANDIM, Alexandre Brandão *; FILHO, César Leporini ***Celulose Nipo-Brasileira S.A.- CENIBRA, ** DEGUSSAABSTRACT:With the purpose of expanding production of ECF bleached eucalyptus Kraft pulp, CENIBRA evalueted inlaboratory and mill tests alternatives with the application of hydrogen peroxide in its ECF bleaching sequenceD 0 EoD 1 D 2 . The addition of hydrogen peroxide in this production line aims at a chlorine dioxide demandoptimization and an increase of the production, maintaining 90 ± 0.5% ISO brightness. The following sequenceswere investigated: D 0 EopD 1 D 2 , D 0 EoD 1 P, D 0 EoD 1 (QP) and D 0 EopD 1 P. Industrial results confirmed the laboratorytests, showing replacement rates variing from 1.1 to 2.4 kg ClO 2 /kg H 2 O 2 . More attractive replacement rates wereobtained for the addition of 2.5 kg H 2 O 2 /adt in the last bleaching stage (D 0 EoD 1 P). However, a larger substitution ofthe total chlorine dioxide demand was found with the sequence D 0 EopD 1 P (5 kg H 2 O 2 /adt divided between bothapplication points), which decreased the demand by 7.7 kg of ClO 2 /adt for kappa number 8.6 pulp. In addition, pulpand effluent characteristics are described for each alternative, as well as corrosion rates of titanium under alkalineconditions (pH < 11), with low concentrations of hydrogen peroxide (< 0.1%). Results show that the addition ofhydrogen peroxide in existent bleaching plants allows increased production without a modification of the chlorinedioxide generation at acceptable levels for operation costs and pulp properties.Key Words: ECF peroxide bleaching; sequence flexibilization; production increase; replacement ofClO 2 /H 2 O 2; titanium corrosion; Eucalyptus spp.1. INTRODUCTIONDuring the last two decades, pulp-bleaching plants underwent a large number of modifications. Bleachingprocesses using chlorine chemicals still dominate and will maintain this dominance Modern bleaching technologymakes use of oxygen and oxygen derivatives, such as hydrogen peroxide, peracids and ozone. In theirimplementation special attention must be given to the bleach plant material compatibility in order not to destroy theequipement.2. OBJECTIVEThis paper demonstrates CENIBRA’s laboratory and plant experience with the application of hydrogenperoxide in the D 0 EoD 1 D 2 type ECF bleaching sequence. The aim was to increase the production flexibility and tosubstitute a part of the chlorine dioxide with hydrogen peroxide for market brightness (90 ± 0,5% ISOThe existingcapacity for the generation of chlorine dioxide should be suffcient despite the production increase. ). The existingbleach plant should be used and not be affected by corrosion.3 - EXPERIMENTALFor comparison purposes, in the laboratory and at plant level the implementation of hydrogen peroxide in theECF - D 0 EoD 1 D 2 , bleaching sequence was investigated, aiming at a brightness of 90 ± 0,5% ISO. Thus, thebleaching sequences evaluated were: D 0 EopD 1 D 2 , D 0 EoD 1 P, D 0 EoD 1 (QP) and D 0 EopD 1 P. Table 1 shows the mostimportant conditions. In sequence D 0 EoD 1 (QP) a chelating agent at a constant dosage of 0,6 kg/adt was added,together with hydrogen peroxide and sodium hydroxide. The washing of the pulp between the bleaching stages wascarried out with 9 m 3 /adt in the laboratory and in the plant at 8 to 10 m 3 /adt in the diffusion washers as shownschematically in Figure 6.

Table 1 – The main conditions of the laboratory and plant tests.2Main ConditionsBleaching stagesD 0 Eo/Eop D 1 D 2 PConsistency %* 10 10 8 - 10 8 - 10 8 – 10Time, min 40 70 165 165 165Temperature, ºC 50 80 75 75 75pH ~3,0 11,5 var. var. 11,0H 2 SO 4 amount, kg/adt 8 - 10 - - - -O 2 amount, kg/adt - 3 - - -NaOH amount, kg/adt -

3CELULOSE E PAPEL, ABTCP. INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, ISO.SCANDINAVIANPULP, PAPER AND BOARD, SCAN Testing Committee. Stockholm, SCAN. (s.d.). STANDARDMETHODS, Examinationof Water and Wastewater (APHA/ AWWA/ WEF). 20 ed., 1998. PAPIERTECHNISCHE STIFTUNG FURFORSCHUNG UND AUSBILDUNG IN PAPIERZEUGUNG UND PAPIERVERARBEITUNG.32 PULPTESTING. “Determination of the totalHalogenated Organics OX Content”. TAPPI Standard Methods. Atlanta, TAPPI. (s.d.) and TAPPI Usefulmethods. Atlanta, TAPPI.Table 2 – The main parameters analyzed including the analytical procedures.Analyzed Parameters Methodology Analyzed Parameters MethodologyKappa Number TAPPI – 236 Blow-out index, kPa.m 2 /g TAPPI - 403Viscosity, mPa*s TAPPI – 230 Tearing index, mN.m 2 /g TAPPI - 414Brightness, % ISO ISO – 2469 Specific volume, cm 3 /g TAPPI - 411After color number (NCP) CENIBRA Air resistance, s/100 ml TAPPI - 460Permanganate Number (NP) ABTCP C4 Light dispersion coefficient, TAPPI - 425cm 2 /gPentosanas, % TAPPI-223 Opacity, % TAPPI - 425Chloride, mg/lSTANDARD Soluble extractibles in DCM, % ISO - 624METHODSRefining PFI, Total load TAPPI - 248 Total halogenated compounds PTS - RH: 012/90(OX), mg/lSheet confection TAPPI - 205 Titanium corrosion rate ASTM G1 -81/G31-72Shopper-Riegler, ºSR SCAN C19 Color, kg Pt/ adt STANDARDMETHODSDrainability Freeness, ml SCAN C21 Chemical oxygen demand (COD),kg O 2 / adtSTANDARDMETHODSTraction index, N.m/g TAPPI - 494 Adsorbable organo halogenatedcompounds (AOX), kg Cl - / adtSCAN-W94. RESULTS4.1. Laboratory tests4.1.1. Bleaching testsA sample of industrially oxygen delignified pulp with a Kappa number of 8,5 was used for the reference. 20,4 kgof chlorine dioxide as Cl 2 active/adt in stage D 0 (FK = 0,24) and 12 kg NaOH/adt in Eo were used. Figure 7 presents thelaboratory tests with several variations for the amount of chlorine dioxide in stage D 1 , however, with fixed addition of thischemical in stage D 2 (4 kg Cl 2 active/adt) and hydrogen peroxide in stage P (2,5 kg H 2 O 2 /adt) for the sequencesD 0 EoD 1 D 2 and D 0 EoD 1 P, respectively. In stage D 2, the chlorine dioxide loads are the usual ones in the process, whilst theload of hydrogen peroxide was optimized in the laboratory based the consumption under process conditions. In thelaboratory, it was observed that the change of stage D 2 for a final P results in a substitution factor of 1,82 kg ClO 2 /kgH 2 O 2 .One of the reservations in respect to a substitution of part of the chlorine dioxide with hydrogen peroxide inan ECF sequence is the potential increase of the lignin residual in the pulp. This residual can be measured by thepermanganate number (NP) and correlates with the brightness reversion of the pulp. Usually under normal processconditions, NP values lower than 1 result in better brightness stability, or, in other words, a smaller brightness lossduring an accelerated reversion test.

4FS ~ 1,82 kg ClO 2 /kg H 2 O 2929088Brightness,%ISO868482807876Pulp D 0 Eo#Kappa = 2,9Viscosity = 21,8 cPBrightness = 76,0 % ISOD 0 EoD 1 P (2,5 kg H 2 O 2 /tsa)D 0 EoD 1 D 2 (4 kg Cl 2 active/tsa)D 0 EoD x (2 - 12 kg Cl 2 active/tsa)740 2 4 6 8 10 12 14ClO 2 chargein D 1 , kg ClO 2 c/ Cl 2 activeFigure 7 – Substitution factor attained by the application of hydrogen peroxide in the last bleaching stage(D 0 EopD 1 D 2 vs. D 0 EoD 1 P) for a brightness of 90 % ISO.The values measured in Figure 8 indicate that for similar NP levels, usually under 1, pulp generated in the D 0 EoD 1 Psequence show brightness reversal values (1h at 105 ± 3 ºC), slightly lower or similar to pulp bleached withD 0 EoD 1 D 2 . This can be explained, considering that an alkaline stage in the presence of hydrogen peroxide isefficient in the removal of carbonyl groups present in carbohydrates [3] which is one of the main causes of brightnessreversion in bleached Kraft pulps.2,3Reverted points, % ISOD 0 EoD 1 D 21,9D 0 EoD 1 P1,51,10,786 87 88 89 90 91 92Final brightness, % ISOFigure 8 – Brightness reversion of pulp bleached in D 0 EoD 1 D 2 and D 0 EoD 1 P sequences.Besides the worries about the stability of the brightness, the presence of equipment constructed with titaniumcould potentially threat the use of hydrogen peroxide. The literature is not clear concerning corrosion. Therefore, aseries of corrosion tests were conducted with the titanium material, simulating industrial conditions in stage P.4.1.2. Titanium corrosion tests in alkaline media in the presence of H 2 O 2The behavior of titanium alloys, used in the bleaching stages, was one of the obstacles for a mill test.Therefore corrosion tests were made. The titanium corrosion rates in alkaline media containing small hydrogenperoxide concentrations was checked. The addition of hydrogen peroxide was simulated for the oxidative extraction(Eo) and the last stage of the D 0 EoD 1 D 2 sequence, where the rotor of the mixer and the filter mesh of the stage (D 2 )diffuser are made of grade 2 titanium.

5Titanium is well known for its excellent corrosion resistance in chlorine containing media, especially underhighly oxidative conditions. Such resistance makes it very attractive for application in bleaching processes. Its use isrecognized in the pulp and paper industry since the sixties, specifically in chlorine dioxide generation plants,diffusers, pulp washers, sodium hypochloride storage and systems containing wet chlorine [25] .Table 3 shows the test samples, the conditions of their exposure and the main properties or changes observedon their surfaces. For the corrosion rate calculation, the density of the alloy was considered being constant and equalto 4,51 g/cm³ [26] . Linear measurements were made using a caliper rule. For every weighing set, the analyticalbalance was previously calibrated. The values of the differences resulted from the third and the second decimalplace. Thus, the corrosion test precision can be reported as being as good as 0,001 mm/a.0,023Table 3 – Corrosion rates of titanium samples with and without welding, two pH values, and two different hydrogenperoxide concentrations.Identification Corrosion media Corrosion rate, mm/a # Corrosion productsCP*A pH10 ± 0,10,0060,03 ± 0,01 % H 2 O 2B pH10 ± 0,10,009No visual changes0,1 ± 0,02 % H 2 O 2C pH11 ± 0,10,03 ± 0,01 % H 2 O 20,005D pH11 ± 0,10,0160,1 ± 0,02 % H 2 O 2E pH11 ± 0,10,012White scale and change in0,1 ± 0,02 % H 2 O 2glossF pH11 ± 0,10,1 ± 0,02 % H 2 O 2* Identification of the titanium samples grade 2: no welding (A, B,C,D), normally welded (E) and weldingshowing thermally affected areas (F). The area of the test samples was 17,2 – 0,14 cm 2 .# Main corrosion test conditions: temperature 75ºC, retention time 120h. The reaction media was theindustrial filtrate from stage D 1 , containing 19,3 mg *l -1 of Ca 2+ and 2,8 mg *l -1 of Mg 2+ .The resistance of materials in any corrosive medium is related to the corrosion rates independently of thecorrosive medium to which it was exposed [27] . Reference values indicate that low corrosion rates were observed forthe pH conditions evaluated, the hydrogen peroxide concentration and the test samples. A higher corrosion rate wasfound in the welded test specimen which had thermally affected areas. This was reported also in other studies [25] . Inthis study the titanium corrosion rate at pH 10 and 5% of hydrogen peroxide showed values of 0,43 and 0,46 mm/a.Tests were carried out by immersion in autoclaves at 70 ºC during 48h, which are test conditions similar to thoselisted in Table 3. Comparing these results with those found on the test samples B, it is evident that there is a linearrelationship based on the hydrogen peroxide concentration within this mentioned range.Looking at Table 3, it will be noted that for H 2 O 2 ≤ 0,03% (~ 3 kg/adt) concentrations, corrosion rates arelow, resulting in a noteworthy corrosion resistance property [27] . Increasing the H 2 O 2 concentration in the medium, atendency towards an increase in the corrosion rate at both evaluated pH levels can be observed. Therefore, duringthe plant test an important precaution was the control of the hydrogen peroxide addition to less than 3 kg/adt. Otherpapers describe higher corrosion rates (2 and 2,5 mm/a) for pH 11 and a peroxide concentration of 0,03%. In thepresent work, the test samples were previously passivated in a H 2 O 2 /HF mix carried out at 80°C during 7h. It wasnoted that the corrosion rates are strongly influenced by pH [28] .The available literature shows controversal data regarding the titanium corrosion in alkaline media and thepresence of hydrogen peroxide. During an industrial test of 6,5 hours; hydrogen peroxide concentration up to 0,26%and a pH up to 11,2 at 80°C, a corrosion rate of 0,0025mm/a was reported. The same author also confirmed thatcorrosion tests carried out in laboratory by linear polarization resistance (LPR) show much higher corrosion values.Afterwards, trying to simulate industrial conditions, de-ionized water was exchanged for filtrate from Eo (Ca 2+ = 12mg*l -1 and Mg 2+ = 5 mg*l -1 ) and the laboratory tests were repeated. In this test the corrosion rate (0,01 mm/a) wasclose to those obtained in the plant test [29] . Probably there are titanium corrosion inhibitors present in the alkaline

6media in the presence of hydrogen peroxide even under high loads of this chemical as mentioned before. Calciumand magnesium ions [29,30,31] , as well as cellusose fibers [30] are being mentioned as titanium corrosion inhibitors.It was noted that at a pH of 10 to 11, for a hydrogen peroxide concentration varying about 0,03%, there wasno formation of scale nor changes in the characteristic gloss of the test samples (silvery). However, the samplessubmitted to a concentration of about 0,10% hydrogen peroxide at pH 10 to 11, showed changes in the metallicgloss towards a golden appearance, together with the formation of a white scale, close to the supporting hole of thesamples and at the edges. The test samples with normal welding and thermally affected also showed a change in thecharacteristic gloss and the formation of scale. It is believed that the change in gloss corresponds to a macroscopicresponse associated to an initial corrosion stage, involving the full surface of the metal. The formation of gas at thesame places where the formation of thicker and less adherent scale occurred, may be related to the decomposition ofthe hydrogen peroxide and the conversion of this first layer into the final corrosion products. Despite of thelimitations for these corrosion test, the resulting low corrosion rates at pH < 11 and < 10 kg H 2 O 2 /adt, demonstratethat the corrosion in titanium equipment during an industrial test of 6 days would be without significance.4.2. Plant test using H 2 O 2 in the D o EoD 1 D 2 sequence4.2.1. Characteristics of the process and cost evaluationThe main objective of the plant test was to achieve a brightness close to 90 %ISO, using hydrogen peroxideand replace a part of the chlorine dioxide in the D o EoD 1 D 2 sequence. For the correct evaluation of the resultsobtained during the plant test, three basic considerations were taken into account: (1) clear separation of the differentsequences D o EopD 1 D 2 , D o EoD 1 P, D o EoD 1 P(QP) and D o EopD 1 P, disregarding the intermediate phases between thesequences; (2) comparison of the sequences at the same range of final brightness, as well as (3) comparison of theconsumption of chemicals and the cost of bleaching in relation to the Kappa number.Based on the production timetables for each evaluated sequence, the consumption of chemicals wascomputed as well as the operational cost with regard to the reference sequence (D 0 EoD 1 D 2 ), as can be seen in Table4 and Figure 9, respectively. In Table 4, it is important to note the factor that relates the total consumption ofchlorine dioxide with the average Kappa number after the oxygen stage. Thus, using this factor (kg Cl 2 active /#Kappa unit) it is easier to compare the bleaching sequences, as fluctuations of the Kappa number are compensated.This factor was also used to estimate the consumption of chlorine dioxide for each tested sequence, with regard tothe accumulated average Kappa number (~8,6) during 1999. As this factor links the chlorine dioxide consumption toeach tested sequence by the Kappa unit, the same must be analyzed together with the substitution factor (FS = kgClO 2 /kg H 2 O 2 ).Table 4 – Main process results obtained in the evaluated sequences.Sequences# Kappa(pre-O 2 pulp)kg Cl 2active totalFC, kg Cl 2 active/# Kappa unit*Finalbrightness,% ISO**FS,kg ClO 2 / kg H 2 O 2 ***D 0 EoD 1 D 2 8,8 55 6,25 89,6 Control (blanc)D 0 EopD 1 D 2 9,8 51 5,20 89,6 1,14D 0 EoD 1 P 9,5 46 4,84 89,6 1,84D 0 EoD 1 (QP) 9,3 41 4,41 89,6 2,41D 0 EopD 1 P 10,0 39 3,90 90,5 1,54* Consumption factor of ClO 2 as Cl 2 active by Kappa number unit of the pre-O 2 pulp;** Average of the timetables of the different bleaching sequences;*** FS = (kg Cl 2 active consumed during the blanc test / n.º kappa) - (kg Cl 2 active consumed in the sequenceunder consideration / n.º kappa) x n.º accumulated average Kappa in 1999 (~ 8,6) : kg of H 2 O 2 /adt added:2,63 kg Cl 2 /kg ClO 2 .For a similar load of hydrogen peroxide, a higher substitution factor indicates the sequence with the highestsubstitution of chlorine dioxide. Thus, from this point of view, the most effective application of hydrogen peroxideis in the last bleaching stage, the exchange of stage D 2 for a final P, as can be seen in Table 4. This fact is easilyjustified as the gain in brightness in a hydrogen peroxide stage is directly related to the initial lignin content [9,10,11,12] .Thus, as the Kappa number after D 0 is greater than after D 1 , conditions are more favorable in the last stage for thehydrogen peroxide to act as a bleaching agent. Probably, allied to this fact, there is a smaller carry-over and a more

7adequate profile of the transition metals (Mn, Fe, Cu, Co e etc.) after the stage D 1 in relation to D 0 . Related to thetransition metals content it is very important to keep the pH in D 0 adjusted to values under 3,5, as in an acid washingstage [32] . In the sequences with a final P the substitution factor was 1,84 and 2,41 for the D 0 EoD 1 P and D 0 EoD 1 (QP)sequences, respectively. The difference in the substitution factor can be explained by the addition of the chelatingagent. Under industrial test conditions it had as the main effect the reduction of the hydrogen peroxide consumption.Very likely the action of the chelating agent inhibits the decomposition of hydrogen peroxide by transition metals[2,3,15] . In spite of the fact that the substitution factor found in the laboratory (Figure 7) was identical to the factorobtained in the industrial process (Table 4) for the D 0 EoD 1 P sequence if compared with the control stage(D 0 EoD 1 D 2 ), there was a difference in the global consumption of chlorine dioxide. This fact can be explained by thesmaller consistencies resulting from the production increase on this fiber line, which uses atmospheric diffusers aswashing elements. During the industrial test the usual consistency values were about 8%, whereas in the laboratoryan exact 10% consistency were used. Lower consistency decreases simultaneously the concentration of chemicalsand the retention time, consequently increasing the demand for chemicals.The application of hydrogen peroxide in the first extraction stage (D 0 EopD 1 D 2 ) showed a smaller substitutionfactor (FS = 1,14), while the D 0 EopD 1 P sequence had a factor in between (FS = 1,54), although a superiorbrightness to 90% ISO was easily achieved. Theoretically, the substitution factor is 0,79 kg ClO 2 /kg H 2 O 2 calculatedfrom the molecular weight of each oxidant and from the quantity of electrons involved in its oxidation [33] . Usuallylaboratory or industrial experiences report higher values in relation to the theoretical chlorine dioxide substitutionvalues for hydrogen peroxide [28,32,34,35] . An explanation for this difference is a miltiple action of hydrogen peroxidein alkaline media. There can be an oxidation of lignin and simultaneously an extraction of already oxidizedfragments, plus an increase of the brightness, which reduces the demand for chlorine dioxide in later stages [35] .100Cost, % of the reference9896949296,095,496,690,790D0EopD1D2 D0EoD1P D0EoD1(QP) D0EopD1PFigure 9 – Bleaching chemical cost of different sequences in relation to the D 0 EoD 1 D 2 reference.Figure 9 takes into consideration the maximum ClO 2 and NaOH production capacity at Cenibra and a Kappanumber of pre-O 2 pulp of 8,6 units (accumulated average in 1999). Considering a production of 1 million pulp(adt/year) it is 8,49 kg/adt ClO 2 and 16,4 kg/adt NaOH. In the cost calculation the average alkaline loss of 6,1 kgNaOH/adt was taken into account. In spite of the higher substitution factor found for just a final P (D 0 EoD 1 P orD 0 EoD 1 (QP), Figure 9 shows that the sequence with the lowest overall cost is D 0 EopD 1 P. All the other sequencesalso had an operational cost below the reference. However, this situation only holds true for the production increaseof 250.000 adt/year, without investments in the chlorine dioxide plant, for the specific case of CENIBRA. It islogical that the main reason that explains the lower costs of the sequences with hydrogen peroxide is the high cost ofchlorine dioxide produced from purchased chlorate. Taking into account the additional consumption of sodiumhydroxide (4 kg NaOH/adt), of sulphur dioxide (1,5 kg SO 2 /adt) and the capacity for chlorine dioxide without thepurchase of chlorate, the substitution factor economically feasible must be over 3,49 kg ClO 2 /kg H 2 O 2 . Therefore, itis important to note that the economical feasibility of hydrogen peroxide application in ECF sequences differs foreach situation under analysis, specifically in each plant.In addition to the economical feasibility, also the quality of the bleached pulp needs to be tested.

Also shown in Table 5 are the physical-mechanical and optical properties of the pulps. The values for thephysical-mechanical resistance were maintained within the normal range. This confirms that the use of hydrogenperoxide in the sequences did not have any significant negative effect, neither regarding the viscosity delta duringbleaching, nor regarding the properties of the bleached pulp. The main data for the properties are shown in Figures10 to 13. Considering the traction index of 70 N.m/g as a fixed parameter, it can be noted in Figure 10 that there wasa variation around 600 rotations (~8 wh in the PFI mill) in order to reach the desired resistance. Usual processvalues for the finished CENIBRA product are around 25 to 30 wh. For the same traction index the sequencesD 0 EoD 1 (QP) and D 0 EopD 1 P showed a slight increase in energy consumption, while the sequences D 0 EopD 1 D 2 andD 0 EopD 1 P showed a lower tearing resistance for a traction index of 70 N.m/g (Figure 11). Regarding the apparentspecific volume and opacity shown respectively in Figure 12 and 13, the sequence D 0 EoD 1 P had higher values for atraction index of 70 N.m/g. For a narrow viscosity range, these small variations are usually found in the process.They are mainly the result of morphological differences of the fiber, considering that the wood mix also changesduring the mill test.9130110Tractio90ninde70x,N.m/50g30D0EoD1D1D0EopD1D2D0EoD1PD0EoD1QPD0EopD1PTearing index, mN.m2/g13121110987654D0EoD1D2D0EopD1D2D0EoD1PD0EoD1QPD0EopD1P100 1000 2000 3000 4000 5000 6000 7000Number of rotations (mill PFI /total load)Figure 10 – Evolution of the traction index of bleachedpulps versus the number of rotations of thePFI mill.310 20 30 40 50 60 70 80 90 100 110Traction index, N.m/gFigure 11 – Evolution of the tear index of bleached pulpsversus the traction index 70 N.m/g for therefining curve in the PFI mill.Blow-out index, kPa.m2/g9D0EoD1D28D0EopD1D27D0EoD1PD0EoD1QP6D0EopD1P54321010 20 30 40 50 60 70 80 90 100 110Opacity, %86D0EoD1D284D0EopD1D282D0EoD1PD0EoD1QP80D0EopD1P7876747210 20 30 40 50 60 70 80 90 100 110Traction index, N.m/gFigure 12 – Evolution of the specific volume of bleachedpulps versus the traction index 70 N.m/g forthe refining curve in the PFI mill.Traction index, N.m/gFigure 13 – Evolution of the opacity of refined pulps versusthe traction index of 70 N.m/g for the refiningcurve of the PFI mill.Further to the evaluation of operational costs, physical-mechanical resistance and optical aspects, anotherimportant evaluation are the environmental aspect.4.2.3– Environmental aspectsTable 6 shows the main characteristics of the total amount of acid and alkaline effluents generated during theindustrial test. In can be seen that for a brightness close to 90% ISO (Table 4), the D 0 EoD 1 P and D 0 EopD 1 Psequences showed an improvement regarding the effluent quality. The main effluent parameters of the bleachingprocesses evaluated during the industrial test, such as COD, color and AOX also were reported by unit of Kappa

10Number of the pre-O 2 pulp (load /# Kappa). In this case, the objective was also to equalize the Kappa numbereffect of the pre-O 2 in relation to the parameters analyzed.The COD load generated in the bleaching is directly related to the selectivity of the sequence. In correlationwith the lower viscosity loss during the bleaching, the D 0 EoD 1 P sequence also showed lower COD loads per Kappanumber unit. Higher values were observed for the D 0 EopD 1 D 2 and D 0 EoD 1 (QP). This confirms the tendency foundfor the loss of viscosity (Table 5). In this Table, the D 0 EoD 1 P was the only one that showed a higher selectivity thanthe control sequence. Consequently, as the result of the higher selectivity, there is a lower loss in viscosity duringbleaching (Table 5) and a lower COD per Kappa number unit in the effluent (Table 6). The COD load per Kappanumber unit can provide a good indication also of the yield in bleaching. Some papers report this relationshipbetween bleaching yield and COD load, showing that approximately 9,9 kg COD/adt is equivalent to a loss of 1% inyield [36,37] . Using this relationship and establishing the Kappa number for the pre-O 2 to a mean value obtainedduring 1999 (8,6), the yield of the sequences can be estimated. Therefore, it was estimated that the sequenceD 0 EoD 1 P has a yield about 1% higher than the control sequence. Following the same reasoning, the other sequencesshowed yields similar or inferior in relation to the control sequence.Table 6 – Main effluent parameters for the different sequences.Sequences # Kappa Value per #Kappa Pre-O 2 unitevaluatedOxygen Pulp COD, kg O 2 /adt/# KappaColor, kg Pt/adt/# KappaAOX, g Cl - /adt/# KappaD 0 EoD 1 D 28,8 2,69 20,1 109,7(control)D 0 EopD 1 D 2 9,8 3,50 18,8 83,0D 0 EoD 1 P 9,5 1,58 7,6 77,2D 0 EoD 1 (QP)* 9,3 3,87 23,3 70,3D 0 EopD 1 P 10,0 2,50 13,0 65,2* chelating agent amount of 0,6 kg/adtRegarding the color of the effluent, the known effect of hydrogen peroxide, bleaching of the effluent wasobserved [38,39] . Among the sequences using hydrogen peroxide the only one to show higher values with regard to thecontrol sequence was the one in which simultaneously a chelating agent and hydrogen peroxide was added in thefinal P, the D 0 EopD 1 (QP) sequence. Probably this was due to the addition of the chelating agent. Alike the CODbehavior, the color of the effluent was lower for the D o EoD 1 P and intermediate values were found for theDoEopD1P sequence.AOX is also important. AOX values are related mainly to: (1) the oxygen pulp Kappa number, (2) the Kappafactor used in D 0 , (3) COD of the pulp after the O stage and (4) the total amount of chlorine dioxide [2] . Thus, thehigher the substitution of chlorine dioxide with hydrogen peroxide, the lower the AOX values will be. As can beseen in Table 6, the reduction of AOX was proportional to the chlorine dioxide input. This confirms that one of thebenefits of hydrogen peroxide in the ECF sequences is the lower generation of AOX.5. CONCLUSIONSThe laboratory tests showed an exchange of stage D 2 for a final P with an application of 2,5 kg H 2 O 2 /adt anda substitution factor of about 1,8 kg ClO 2 /kg H 2 O 2 , maintaining brightness stability of the bleached pulp.Titanium corrosion tests by immersion of test samples in an alkaline media (pH < 11), containing smallconcentrations of H 2 O 2 (< 0,1%) and using as the dilution matrix the industrial filtrate, showed corrosion rates lowerthan 0,023 mm/a.

11The results in plant scale confirmed the laboratory results, showing that a substitution factor of about 1,8kg ClO 2 /kg H 2 O 2 can be expected when changing a D 2 stage for a final P stage. The values for the substitution factorare within the range from 1,1 to 2,4 kg ClO 2 /kg H 2 O 2 .The application of hydrogen peroxide in Eop (D 0 EopD 1 D 2 ) resulted in a lower substitution factor, the bestvalues were found for sequences with a final P stage. The highest substitution factor was found for the D 0 EoD 1 (QP)sequence using a chelating agent as additional additive.The D 0 EopD 1 P sequence resulted in an intermediate substitution factor (1,5 kg ClO 2 /kg H 2 O 2 ). In thissequence there was the highest substitution of chlorine dioxide (7,7 kg ClO 2 ), due to the application of 5 kgH 2 O 2 /adt, divided equally at the two point of application. This sequence had the lowest operational cost regardingthe bleaching chemicals.Adding 2,5 kg H 2 O 2 /adt in the D 0 EoD 1 P sequence, gave the highest viscosity and the best selectivity. In linewith this fact, a lower COD load was found per Kappa number unit.The use of hydrogen peroxide in ECF sequences allows the partial substitution of chlorine dioxide. Thismakes it feasible to increase production without expansion of the chlorine dioxide plant while maintaining themarket pulp quality level.6. BIBLIOGRAPHY1. COSTA, M. M., COLODETTE, J. L. Branqueamento TCF de polpa kraft: uma revisão.In:SEMANA DECELULOSE E PAPEL, 5., 1998, Curitiba.2. DENCE, C.W., REEVE, D.W. Reaction principles in pulp bleaching In: PULPING BLEACHING:PRINCIPLE AND PRATICE, 1996, Atlanta. TAPPI. p. 113-124.3. GRATZ, J.S. Reactions of polysaccharides and lignins in bleaching with oxygen and related species. In: 1990OXYGEN DELIGNIFICATION SYMPOSIUM, Toronto TAPPI, 1990. p. 1-22.4. ANDERSON, R. Peroxide delignification and bleaching. In: INTERNATIONAL NON-CHLORINEBLEACHING CONFERENCE, 1992, Amelia Island.5. GELLERSTTEDT, G., LINDORS, E. L. On the structure and reactivity of residual lignin in Kraft pulp fibers.In: 1991 INTERNATIONAL PULP BLEACHING CONFERENCE PROCEEDINGS – Stockholm, p. 73.6. LACHENAL, D., FERNANDES, J.C., FROMENT, P. Behavior of residual lignin in Kraft pulp duringbleaching. In: 1994 INTERNATIONAL PULP BLEACHING CONFERENCE, Montreal, p.41.7. HOBBS, G.C., ABBOTTt, J. The role of radical species in peroxide bleaching processes. In: APPITAANNUAL CONFERENCE PROCEEDINGS, 46., 1992, Victoria.8. GIERER, J. The chemistry of delignification: a general concept. Holzforschung, 1982, (1), p. 43 – 51.9. VAN LIEROP, B., LIERBEGOTT, N., FAUBERT, M.G. Using oxygen and peroxide to bleach Kraft pulps, In:CPPA ANNUAL MEETING PREPINTS, 79.,1993, Montreal, p. 81.10. TROUGHTON, N.A., DESPREZ, F., DEVENYNS, J. P. 100 TCF Bleaching of softwood kraft pulps to highbrightness, In: 1994 TAPPI PULPING CONFERENCE Proceedings p. 807.11. GERMGARD, U., NORDEN, S. OZP-Bleaching of Kraft pulps to full brightness, In: 1994 INTERNATIONALPULP BLEACHING CONFERENCE Preprints, Montreal, p.53.12. GIERER,J., JANSBO, K.,YIENG, E., YOON, B. H. On the participation of hydroxyl radicals In Oxygen andhydrogen peroxide bleaching processes. In: 1991 INTERNATIONAL SYMPOSIUM ON WOOD ANDPULPING CHEMISTRY NOTES, 6., vol 1, p. 93.13. TATSUMI, K., MURAYAMA, K., TERASHIMA, N. Reaction of lignin with hydroxyl radicals, In: 1987TAPPI INTERNATIONAL OXYGEN DELIGNIFICATION CONFERENCE, p. 99.14. LAPIERRE, L., BOUCHARD, J., BERRY, R.M., LIEROP, B. Chelation prior to hydrogen peroxide bleachingof Kraft pulps: on overview. In: PREPRITINS SPRING CONFERENCE – CANADIAN PULP & PAPERASSOCIATION, 1995, Canada.15. COLODETTE, J.L. Factors affecting hydrogen peroxide stability in the brightening of mechanical andchemimechanical pulps, 1986, New York: College of Environment Science and Forest Syracuse. New York,USA. p. 273.16. COLODETTE, J.L. et al. Novos processos para branqueamento de polpa Kraft de eucalipto. In: CONGRESSOANUAL DA ASSOCIAÇÃO BRASILEIRA TÉCNICA EM CELULOSE E PAPEL, 26., 1993, São Paulo.Preprints São Paulo: ABTCP, 1993. p. 71 - 90.

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