Optimization of Novel Bleaching Sequences for Eucalyptus Kraft Pulp

Transcription

1 Optimization of Novel Bleaching Sequences for Eucalyptus Kraft Pulp Leonardo Clavijo: Assistant Professor, Universidad de la República, Uruguay, Christian Järnefelt: R&D Engineer, Andritz Oy, Finland, Tapani Vuorinen: Full Professor, Aalto University, Finland, Abstract In recent decades the pulp and paper industry has been reinventing to achieve an environmentally more friendly production process and to reduce costs. The decrease in water consumption as well as a lower production of organochlorine compounds has been the key to this conversion. In this work two novel sequences for bleaching Eucalyptus Kraft pulp that do not use chlorine dioxide in the first bleaching stage were studied and compared with a reference sequence normally used in the pulp and paper industry. This allow effluents generated in the acidic stage be reused to increase the mill closure and hence emissions to the environment could be diminished. The sequences A(E OP )DP and A(E OP )DD were optimized. The conditions of each stage were found and a 9% ISO brightness in the final pulp was achieved. Comparing these two novel sequences, the best results were obtained when hydrogen peroxide is used in the final stage. The final brightness, brightness reversion, ClO 2 consumption, chlorate formation and AOX formation have better values for the A(E OP )DP sequence. The chemical consumption, brightness reversion, chlorate formation and AOX formation in the A(E OP )DP sequence are lower than in the reference sequence (A/D)(E OP )DP. It was possible to bleach Eucalyptus Kraft pulp to 9% ISO brightness with only 5.3 Kg/BDT of ClO 2, the brightness reversion was 2% ISO, and the AOX content in the filtrates was only 3 g/bdt. The ClO 2 conversion to ClO 3 - was 23%. The A(E OP )DP bleaching sequence has great potential to become a effective bleaching sequence at the mills because the lower chemical consumption and better value for environmental parameters. Keywords: Eucalyptus; A-stage; Chlorine dioxide; Chlorate; ECF bleaching. Introduction In recent decades the pulp and paper industry has been reinventing to achieve an environmentally more friendly production process and to reduce costs. The decrease in water consumption as well as a lower production of organochlorine compounds has been the key to this conversion. Bleaching chemical demand, bleaching yield, water consumption, effluent load and treatability and pulp organic chlorine compounds (OX), brightness stability, refinability and strength are parameters that decide bleaching technology. [1][2] The novelty in the sequences A(E OP ) D 2 and A(E OP )DP is that the first chlorine dioxide stage (D ) is eliminated. In this way the effluent from the A-stage can be recirculated improving the closure of the bleaching plant. In da Costa [3] work the novel A(E OP )D(PO) sequence is compared with several bleaching sequences. In Medina [4][5] work, the novel A(E OP )D N D is also compared with several bleaching sequences. The results in both works show that in the novel bleaching sequences the ClO 2 consumption is remarkably lower than in the reference. An important reduction of the total AOX in the effluent can be achieved with this sequences that makes this technology more environmentally friendly, and in the A(E OP )D(PO) sequence the OX content in the pulp is much lower than in the reference, as in an ECF-light sequence. According to da Costa [3] work the physical properties of the pulp obtained with this novel sequence are not very different than reference sequence properties, but the brightness reversion is considerably lower.

2 Experimental Oxygen delignified eucalyptus pulp with a kappa number of 1.8, intrinsic viscosity of 965 ml/g, ISO brightness of 53.%, and HexA content of 55. eq/bdt, was used in the laboratory bleaching trials. First, the pulp was bleached with the A(E P ) bleaching sequence. The conditions for the A-stage were selected in order to obtain three different percentages of hexenuronic acid removal (8%, 65% and 5%). In all the trials the temperature was fixed to 95ºC. The different percentages of HexA removal were achieved by changing the reaction time of the A-stage. The extraction stage was selected with the usual conditions reported in the literature. For the D-stage also three conditions were used. The different conditions are listed in Table 1. The numbers in A and E P labels denote the reaction time for the A-stage. The numbers in D labels signify the reaction time for the A-stage and the chlorine dioxide dose expressed as active chlorine. Table 1: Conditions for the first set on trials. A 45 A 6 A 12 E P 2-12 D 6-12 D D 6-16 D D 6-2 D 45-2 Consistency (%) Temperature (ºC) Time (min) Final ph ClO 2 dose (Kg actcl/bdt) H 2 2 dose (Kg/BDT) H 2SO 4 dose (Kg/BDT) NaOH dose (Kg/BDT) After selecting the best conditions for the A(E P ) sequence, the final stage D 2 or P were run with two conditions for each one. The conditions for these stages only differ in the bleaching chemical dose, as is shown in Table 2. Table 2: Conditions for the second set on trials. D 2-1 D 2-2 P 1 P 2 Consistency (%) Temperature (ºC) Time (min) ph ClO 2 charge (Kg actcl/bdt) D2=D1/3 D2=D1/ H 2 2 charge (Kg/BDT) NaOH dose (Kg/BDT) The pulp obtained with these novel sequences was compared with pulp obtained by actual procedures. As a reference the (A/D)(E P ) P bleaching sequence was selected. This sequence is one of the current sequences that are used in the modern mills for bleaching eucalyptus, and it is the sequence that is used in the mill that supplied the oxygen delignified pulp for this work. The conditions for the reference sequence are listed in Table 3. After each stage the kappa number, ISO brightness and viscosity were measured and UV-Raman spectroscopy analysis was done for each pulp. From the filtrates ph, residual chemicals and TOC were determined. After each A-stage HexA content was measured from the pulp according to the HUT procedure. AOX content was measured after, D 2 and P stages in the novel sequences and after each stage in the reference sequence. Chlorate concentration was determined after chlorinated stages in all the sequences by iodimetric titration. [6]. During the first set of experiments the acidic and extraction stages were run in a stainless steel rotating reactor and pulp was preheated in a water bath. Deionized water was used for adjusting the consistency to the desired value. The stage was done in a titanium MC

3 Intensity (a.u.) mixer reactor CMC 24 High Intensity Mixer. In this case, the pulp was preheated in a microwave oven. The final D 2 stage was also done in the titanium MC reactor and the final P stage was run in a stainless steel MC reactor, similar to the previous but coated with Teflon. The reference sequence was performed in the MC reactors, using the titanium reactor for (A/D) and stages and the stainless steel reactor for the (E P ) and P stages. Table 3: Conditions for the reference sequence. A/D E P D P Consistency (%) Temperature (ºC) 95/ Time (min) 12/ ph ClO 2 charge (Kg actcl/bdt) H 2 2 charge (Kg/BDT) H 2SO 4 dose (Kg/BDT) NaOH dose (Kg/BDT) Results and Discussion A-stage The results of the analysis of the pulps obtained after the A-stage with 3 different residence times are listed in Table 4. The UVRR spectra of the pulps obtained after each treatment are shown in Figure 1. The pick at 16 cm -1 represent the lignin content and the pick at 166 cm -1 represent the HexA content of the pulp. Table 4: Results after the A stages. O 2Pulp A_12 A_6 A_45 Final ph: Kappa Number: ISO Brightness (%) Viscosity (ml/g) Viscosity Drop (%) TOC (g/bdt): HexA (eq/bdt) HexA removal (%) Yield (%) : compared to the oxygen delignified pulp (FP) FP A_45 A_6 A_ Raman shift (cm -1 ) Figure 1: Raman spectra of Oxigen delignified pulp (FP), A_45 pulp, A_6 pulp and A_12 pulp. As it can be expected the kappa number decreased with the hydrolysis time. A

4 reduction of 5.9 units was achieved for the maximum hydrolysis time. The reduction of one kappa number unit corresponds to 8.7 eq/bdt of hexenuronic acid removal. The brightness after the A-stage did not change significantly. This behavior was also observed in a previous work. [7]. The HexA reduction after A_45, A_6 and A_12 stage was 45, 67 and 8% respectively. The viscosity drop for this stage was high especially for the longest hydrolysis time. This fact can be explained because the temperature control of the reactor has to be done manually, and although the mean temperature was around the target value (95 ºC), there were some peaks that achieved 13-15ºC. When the A-stage was done in the MC reactor the viscosity drop was lower. E P -stage The results of the analysis done to the pulp obtained after the E P -stage are listed in Table 5. During this stage almost the same brightness (64%ISO) and the same kappa number reduction (.8 units) was obtained, so the HexA content of the pulp has no influence in the brightness and the kappa number reduction ( ) that can be achieved during this stage. Table 5: Results after the E P stages. E_12 E_6 E_45 Final ph: H 2O 2 consumed (kg/bdt) Kappa Number: (Kappa number) ISO Brightness (%) Viscosity (ml/g) Viscosity Drop (%) TOC (g/bdt): Yield (%) : means kappa number after E-stage minus kappa number after A-stage. 2 : calculated respect to the oxygen delignified pulp. The viscosity drop for this stage was also high. As this stage was done in the same rotating reactor as the A-stage, the high viscosity drop can be explained due to the temperature control of the reactor. Also the Teflon coating of the reactor was damaged in some places, and this can raise the decomposition of hydrogen peroxide in free radicals that can damage the pulp. -stage After the stage the following results, shown on Table 6, were obtained. Table 6: Results after stage Final ph: ClO 2 consumed (Kg actcl/bdt) Kappa Number: ISO Brightness (%) Viscosity (ml/g) Viscosity Drop (%) TOC (g/bdt): ClO 3 - content (g/bdt) % conversion ClO 2 to ClO AOX content (g/bdt) Yield (%) : calculated respect to the oxygen delignified pulp 452

5 Intensity (a.u.) Intensity (a.u.) According to Table 6 the lowest kappa number was achieved in the _1216, _122 and _62. Also in these stages the highest brightness values were obtained. As it is showed in Figure 2 the lignin and HexA contents of these pulps were essentially the same. 12 1,6 1 8 O2 delig pulp D_62 D_1216 D_122 1,4 1,2 1 D_62 D_1216 D_122 6,8 4,6,4 2, Raman shift (cm -1 ) Raman shift (cm -1 ) Figure 2: UVRR spectra of the D_122, D_1216 and D_62 pulps. Considering that the kappa number was the same in these three conditions, the brightness was larger than 8%, and the chlorate formation varied significantly, it was interesting to determine if an intermediate stage between _1212 and _1216 would show a similar kappa number, ISO brightness and possible lower chlorate formation and higher viscosity. Therefore a new trial with 14 Kg actcl /BDT was carried out. The whole sequence was redone from the oxygen delignified pulp. The results are presented in Table 7. Table 7: Results of the _1214 stage _1214 Final ph: 4.3 ClO 2 consumed (Kg actcl/bdt): 14. Kappa Number: 1. ISO Brightness (%) 8.8 Viscosity (ml/g) 689 Viscosity Drop (%) 1 29 TOC (g/bdt): 241 ClO - 3 content (g/bdt) % conversion ClO 2 to ClO 3 26 AOX content (g/bdt) 29.5 Yield (%) : calculated respect to the oxygen delignified pulp With these conditions a similar kappa number, acceptable ISO brightness and lower chlorate content in the filtrates were achieved. Also the viscosity was higher than for _1212 and _1216 pulps. This fact can be explained because the A and E P stages was done in MC reactors with better temperature control and better mixing. If it was possible to reach the target brightness (9%) with only 14 Kg actcl /BDT, then one of the main advantages of this sequence, referred to the saving in chemicals, is achieved. Consequently, these conditions were considered as the optimal for the stage, and with this pulp the final D 2 and P stages were done. D 2 and P Final stages The results obtained after D 2 and P stages can be seen in Table 8. In both cases, the low kappa number obtained was characteristic for a fully bleached pulp, however the brightness had not achieved the target value of 9%ISO. The viscosity drop was high.

6 Table 8: Results after D 2 and P stages D 2_1 D 2_2 P_1 P_2 Final ph: ClO 2 consumed (Kg actcl/bdt) H 2O 2 consumed (Kg/BDT) Kappa Number: ISO Brightness (%) Brightness reversion (% ISO) Viscosity (ml/g) Viscosity Drop (%) TOC (g/bdt): ClO - 3 content (g/bdt) % conversion ClO 2 to ClO AOX content (g/bdt) Yield (%) : calculated respect to the oxygen delignified pulp In both cases, the low kappa number obtained was characteristic for a fully bleached pulp, however the brightness had not achieved the target value of 9%ISO. The viscosity drop was high. When the chlorine dioxide charge was increased the consumption of ClO 2 by the pulp was almost the same and so the ISO brightness got the same value. It seems to indicate that the brightness has reached a ceiling, and cannot be improved with a dosage of ClO 2 at reasonable levels. Also in this instance, when the hydrogen peroxide charge was duplicated the change in brightness was low. The quality parameters obtained for the final pulps were lower than expected. Particularly the final target brightness of 9% was not achieved, and the brightness reversion was high. Also the viscosity drop was significant in all the cases. In order to improve the brightness after the stage, the hydrogen peroxide charge in the extraction stage was increased from.5 to.6% and the temperature was augmented from 8ºC to 85ºC. Although there is evidence in the literature that the use of oxygen in the extraction stage has low influence when eucalyptus is bleached with conventional sequences, oxygen was used in this stage with a pressure of 3 KPa in order to reach the maximum brightening value after E-stage. With the purpose to improve the viscosity losses during the extraction stage and final P stage, magnesium sulfate was used with doses of.2 % in both cases, which according to the literature is the optimal value for eucalyptus bleaching [8]. The stage was also performed with 14 Kg actcl /BDT with a final ph of 4. The peroxide charge in the P stage was increased from.6 to.7% A(E OP ) P and A(E OP ) D 2 sequence The whole sequence was redone with the conditions listed in Table 9. All the trials were performed in MC reactors at 1% of consistency. Table 9: Conditions for the new trials. A-stage E OP-stage -stage P-stage D 2-stage Temperature (ºC): Time (min): Final ph: ClO 2 dose (Kg actcl/bdt): H 2O 2 dose (Kg/BDT): H 2SO 4 dose (Kg/BDT): MgSO 4 dose (Kg/BDT): NaOH dose (Kg/BDT) O 2 dose (KPa):

7 Intensity (a.u.) Intensity (a.u.) The results after each stage are listed in Table 1 and the UVRR spectra of each pulp obtained with these new conditions is showed in Figure 3. Table 1: Results of the new AE OPP and AE OPP sequences. A_12n E OP_12n _1214n P_1214n D 2_1214n Final ph: ClO 2 consumed (Kg actcl/bdt) H 2O 2 consumed (Kg/BDT) Kappa Number: ISO Brightness (%) Brightness reversion (% ISO) SCAN Viscosity (ml/g) Viscosity Drop (%) TOC (g/bdt): HexA (g/bdt) HexA removal (%) ClO - 3 (g/bdt) % conversion ClO 2 to ClO AOX (g/bdt) OX in final pulp (g/bdt) Yield (%) : calculated respect to the oxygen delignified pulp O2 delig. pulp A_12n EOP_12n D1_1214n P_1214n O2 delig. pulp A_12n EOP_12n D1_1214n Raman shift (cm-1) Raman shift (cm-1) Figure 3: UVRR spectra of the pulp after each stage in the sequences AE OPP (left) and AE OPD 2 (right) During the A-stage the removal of hexenuronic acids was smaller than in the first trial, which can be explained by a better temperature control in the MC reactor. The same kappa number and the same brightness were achieved, but the viscosity drop was lower and this fact also can be explained by the better control of temperature. However, the TOC content in the filtrate was larger than the first time indicating a better removal during this stage. This fact can be explained by the better mixing using the MC reactor where, although the temperature was lower, the effect of the better mixing improves the mass transfer from the pulp. In the E OP -stage the kappa number reached almost the same value as before, however the brightness had a larger value. The viscosity losses during this stage decreased significantly, which suggest that the use of MgSO 4 for this bleaching sequence is mandatory. The TOC value in this stage is a slight larger than before. In the -stage the same kappa number was obtained than before. The brightness was higher. The intrinsic viscosity was more than 1 ml/g higher than before, bringing the viscosity losses to an acceptable value. The TOC and AOX content had the same value than before, and the chlorate content was slightly lower. In the P-stage the same kappa number than before was obtained. The brightness was higher, achieving the target brightness of 9%ISO. The brightness reversion was remarkably

8 smaller, only 2.1%ISO instead of the 4.2%ISO obtained before. The TOC value was also significantly reduced and this fact can be explained by a smaller degradation of the pulp because of the use of magnesium sulfate. In the D 2 -stage the kappa number was almost the same value as before. The final brightness was 89%ISO quite near to the target value, but the brightness reversion was high (4.9%ISO). The viscosity drop was also improved from earlier. Comparison of bleaching sequences Kappa number In Figure 4a the kappa number after each stage is shown. Although the kappa number was higher in the first two stages for the reference sequence, after the stage the kappa number had the same value for the three sequences. In the final stage the novel sequences got a lower kappa number than the reference sequence. Brightness For the three first stages the brightness was higher for the reference sequence because the use of chlorine dioxide in the first stage. However, after the last stage the brightness is almost the same for the three sequences around 9% ISO. The brightness of the A(E OP )DP bleaching sequence is 9.4%. Results can be seen in Figure 4b. Brightness reversion Although the final brightness values were very similar for the three sequences, the brightness reversion was different for all of them. The largest brightness reversion was obtained for the A(E OP )DD sequence and the novel AE OP DP had the lowest value. This fact is in agreement with the literature where it has been reported that the P stage and the use of MgSO 4 help improve brightness reversion. Results are showed in Figure 4c. Chlorate formation The reference sequence had the largest chlorate content in the bleaching filtrates (Figure 4d). This is because of the larger ClO 2 consumption for this sequence. The chlorate formation in the A(E OP )DP bleaching sequence is very small, making this sequence more attractive from the environmental point of view. AOX formation It has to be noticed that the values of AOX formation in the bleaching filtrates for the whole sequences were very low compared to the literature. The same fact has been observed at the mill that supplied the pulp. For the two novel sequences the AOX content in the filtrates showed lower values, almost 5 % less compared to the reference sequence. The A(E OP )DP sequence generated the lowest AOX content in the filtrates, making this sequence more attractive from the environmental point of view. Result can be seen in Figure 5a. OX in final pulps The OX content in the final pulp was similar for the novel sequence A(E OP )DP and the reference sequence (A/D)(E OP )DP. The higher value for the A(E OP )DD sequence is due to the final ClO 2 stage. This fact also makes the A(E OP )DP sequence more attractive too. Result are showed in Figure 5b. Consumption of chemicals The main advantage of the A(E OP )DP sequence is the lower consumption of ClO 2. With this sequence a saving of 38% is achieved and it is possible to bleach the pulp with only 14 Kg actcl /BDT. However the generation of H 2 O 2 needs to be 22% higher and the consumption of NaOH is 8% higher for the A(E P )DP sequence than for the reference sequence. (Figure 5c)

9 Dose (g/kg of pulp) AOX formation (g/bdt pulp) OX in final pulp (g/bdt) Brightness reversion (% ISO) Chlorate formation (g/bdt pulp) Kappa number Brightness (%ISO) 6 5 (a) 1 9 (b) % 5.% 4.% A or (A/D) EOP or EP D1 D2 or P Stage (c) A or (A/D) EOP or EP D1 D2 or P Stage (d) 3.% 15 2.% 1 1.% 5.% Sequence Figure 4: a) Kappa number after each stage, b) Brightness after each stage, c) Brightness reversion in the final pulp, d) Chlorate formation in the filtrates for the three sequences made. Sequence (a) 12, 1, (b) 5 8, 4 6, , 2, 25 2 Sequence (c), Sequence ClO2 H2O2 H2SO4 NaOH Bleaching chemical Figure 5: a) AOX content in the filtrates, b) OX content in the final pulp, c) Consumption of chemicals, in the three sequences made.

10 Conclusions The A(E OP )DP bleaching sequence has a great potential to become a bleaching sequence at the mills because: The remarkably lower chemical consumption than actual sequences at the same target brightness level. The small brightness reversion of the pulp. An important reduction in the total AOX formed in the effluent, making this sequence more environmentally friendly. The low chlorate formation. The possibilities to improve the closure of the mill reusing the A-stage filtrate in the recovery cycle. The biological treatment plant can be smaller because of the lower volume of the bleaching effluents. References 1. Pikka O., Vehmaa J. (27), Advances in eucalyptus pulp bleaching technology, III International Colloquium on Eucalyptus Pulp (ICEP), Belo Horizonte, Brazil, March Colodette J., Gomez M., Rabelo M., Eiras K., Gomes A., Oliveira K. (28), Eucalyptus kraft pulp bleaching: state-of-the-art and new developments, Tappi Journal 7(2), pp. 18A-18M. 3. da Costa M., Fonseca M., Pimenta D., Colodette J. (21), Processos de branqueamento com estágio de hidrólise ácida a quente (A hot ) para polpa kraft-o 2 de Eucalyptus ssp, 34º Congresso Anual de Celulose e Papel ABTCP 21, São Pablo, Brasil, October 22 a Medina J. (27), Characterization of efficient ECF bleaching parameters of Eucalyptus pulp, HUT Department of Forest Products Technology; Master Thesis work. 5. Medina J., Lehtimaa T., Tervla P., Vehmaa J., Pikka O., Vuorinen T. (28), Chlorate reduction in Eucalyptus pulp bleaching, TAPPI 28 Engineering, Pulping & Environmental Conference, Portland USA. 6. Wartiovaara I., (1982), The influence of ph on the stage of a D/CE bleaching sequence, Paperi ja Puu, 64(9), pp 539-4, Clavijo L., Cabrera N., Rauhala T., Kiutunen S., Vuorinen T., (29); Changes in a Eucaliptus Kraft Pulp During a Mil Acid Treatment at High Temperature ; ABTCP PI 42º Pulp and Paper International Congress and Exhibition, San Pablo Brasil. 8. dos Santos L., Barbosa G., Mallet S., Filho J., Maciel P. (1997), Otimização da temperatura no estagio E OP A experiêcia industrial da Jari, O Papel, Junho 1997 pp Acknowledgements To the Academia Nacional de Ingeniería (National Academy of Engineering) who distinguished the thesis which originated this work, with the first prize in the competition for Master s Thesis.