Use of secondary condensates from black liquor evaporation as process water in D o bleaching

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1 Use of secondary condensates from black liquor evaporation as process water in D o bleaching By M. Sankari, M. Sillanpää, K. Ala-Kaila and O. Dahl Abstract: In this research, different volatile organic compounds and condensate fractions were added to the softwood kraft pulp suspension prior to chlorine dioxide bleaching. Results showed that most of COD in condensates originated from different volatile organic compounds, which did not affect bleaching results or had only minor effects on pulp properties. Most probably sulphur-based components, which also contribute to COD in the condensates, are the most harmful components for bleaching. M. SANKARI Oulu, M. SILLANPÄÄ Oulu, mervi.sillanpaa@oulu.fi K. ALA-KAILA Oy Metsä-Botnia Ab Rauma, O. DAHL Oulu, LACK LIQUOR evaporation produces a B large volume of condensate. Stripped condensates are largely used in pulp mills worldwide in the brown stock washing area, in the O-stage washing area and in the recausticizing cycle. Additionally, part of the condensates has a potential to be used in bleach plant [1], particularly in the EO and EOP bleaching stages. The recycling of condensates is an important step towards the closure of pulp mill water circuits, or at least a good way to reduce the fresh water consumption. In modern pulp mills, alkaline bleach plant filtrates are recycled to brown stock and post-oxygen washing [2-4]. This leads easily to an excess of condensates and to the question whether this excess could be used even in the D-stages. When condensates are used in brown stock and post-oxygen washing, part of the wash water continues to the next bleaching stage and the effect of this carry-over is not clear. There exists only limited amount of published data concerning the effect of condensates on the bleaching of kraft pulp. Annola et al. [5] performed laboratory tests, which showed that peroxide bleaching and ozone bleaching were not greatly influenced by the use of black liquor condensate as a wash water prior to TCF bleaching. They concluded that there would be a small increase in reagent consumption and possibly a small loss in viscosity in the peroxide stage due to the contaminants present in the condensate. Both hydrogen peroxide and ozone bleaching effectively oxidized the organic compounds present in the evaporator condensate. Niemelä et al. [6] used also different black liquor evaporation condensates on a laboratory scale as wash water during hydrogen peroxide, chlorine dioxide, ozone, and peroxyacetic acid bleaching. It was shown that condensates did not affect the typical pulp properties (kappa number, viscosity, and brightness), and did not increase off-flavour of the pulps. The results make it also reasonable to assume that the typical secondary condensates would not increase the consumption of bleaching chemicals. Typical contaminants in black liquor condensates are alcohols (mainly methanol), ketones, organic acids (e.g. resin acids, fatty acids), terpenes, phenols and dissolved gases like methane and ethane [7]. The condensates are also contaminated with odorous compounds such as hydrogen sulphide and other total reduced sulphur (TRS) compounds. Additionally, there may be black liquor carry-over and fibres [8]. Therefore, to avoid problems before using certain fractions of evaporator condensates in different parts of the pulp mill, the condensates should be stripped to remove efficiently odorous TRS-gases and alcohol contaminants. Emilsson et al. [9] reported that about 90% of COD-causing compounds in black liquor based condensates can be removed by stripping, and that by replacing normal fresh water with stripped condensates the peroxide consumption in TCF bleaching can be reduced and strength of the pulp improved due to lower amount of harmful metals in condensates. Although there exists few good studies about condensate reuse, the information concerning the behaviour of secondary condensates in ECF bleaching is still limited. EXPERIMENTAL Altogether, two sets of bleaching experiments were carried out to study the effect of different compounds on pulp properties in chlorine dioxide bleaching. In the first set of experiments, different volatile organic compounds were added to the pulp suspension before the bleaching. The other set was done by mixing the pulp suspension with different condensate fractions arising from black liquor evaporation. Material and Chemicals: The bleaching experiments were performed with softwood kraft pulp taken from a local pulp mill after the second oxygen delignification stage. The consistency of the pulp was about 6%, and after 24 hours it was dewatered to a consistency of about 37%. In order to homogenize the thickened pulp, pulp cakes were torn into small pieces by hand and mixed thoroughly. The exact consistency value of the pulp was measured according to the SCAN-C 3:78 standard. The pulp was divided and placed in plastic bags that were hermetically sealed and stored in a cold room (below + 4 C) until they were used in the experiments. The main properties of the initial pulp samples were as follows: kappa number 14.2, ISO brightness 40.3% and viscosity 850 dm 3 /kg :5 (2004) T 113 Pulp & Paper Canada

2 TABLE I. Factors used in the experiments and their contributions to COD load. Factor Abbreviation kgcod/kg Formic acid Fo 0.3 Acetic acid Aa 1.1 Propionic acid Pr 1.7 Methanol Me 1.6 Ethanol Et 2.1 Acetone Ac 2.3 TABLE III. Conditions in the D0 laboratory bleaching. Consistency, [%] 10 Temperature, [ C] 60 Reaction time, [min] 45 ph 2 to 3 Pressure, [bar] 1 Dosage of ClO 2, [kg/o.d.t] a 26 Amount of pulp in VOC tests, [g o.d.] 50 Amount of pulp in condensate tests, [g o.d.] 29 a as kilogram equivalent chlorine per tonne of oven dry pulp. D 0 TABLE II. Properties of the secondary condensates used in the experiments. SCA SCB SCC ph Conductivity a,[ms/m] Colour b, [mg/l Pt] COD, [mg/l] 1,400 2,200 51,100 Na, [mg/l] c K, [µg/1] c ,950 Ca, [µg/l] d a SFS-EN 27888, Temperature 25 C b SFS-EN ISO 7887, Part 4; filtration throw 0.45 µm membrane (In SCC white precipitation was observed even after filtration) c SFS 3017, Measured with Perkin Elmer 4100 FAAS (filtration 0.45 µm) d SFS 3018, Measured with Perkin Elmer 4100 FAAS (filtration 0.45 µm) TABLE IV. Analysis of the responses of the experiments. Response Standard Details ISO brightness SCAN-CM 11:95 Without EDTA; using a Minolta 525i spectrophotometer Kappa number SCAN-C 1:77 Smaller reagent amounts Viscosity SCAN-CM 15:99 With manual time-keeping TABLE V. Worksheet for the volatile compound experiments. Exp. No Formic Acid Acetic acid Propionic acid Methanol Ethanol Acetone Kappa ISO brightness Viscosity kgcod/tp kgcod/tp kgcod/tp kgcod/tp kgcod/tp kgcod/tp number [%] [ml/g] The chemicals in the experiments were as pure (analytical grade or better) and as new as possible in order to avoid contamination. Distilled water was used in the analysis. Every chemical was diluted with distilled water and standard solutions were prepared by weighing. Factors in Volatile Compounds Experiments: The aim of the experiments was to determine the effects of different volatile organic compounds (VOC) on the bleaching performance in D0 bleaching. These compounds (factors) are also the main components in the secondary condensates [7], Table I. Each compound was added so, that its COD load on the experiments was 10 or 30 kgcod/bdtp. These two levels of the factors were based on our previous experiments [10]. The actual amount of compound was calculated using the information presented in Table I. The experiments were designed with the MODDE 3.0 computer program (Umetri AB). The statistical program is used to decrease the amount of performed tests without losing reliability of the results. Because the number of factors studied was large, linear screening experiments were carried out using two-level fractional factorial designs. The confidence level was 95%, and the validity of the fitted empirical model was tested with the analysis of variance (ANOVA). A whole worksheet for the volatile compound experiments is shown in Table V. Factors in Condensate Experiments: The aim of the second part of the experiments was to study the effect of different mill condensates in D 0 bleaching. Three different secondary condensate fractions (SCA, SCB, SCC) were collected from different positions of mills scale evaporators and used in the experiments. Fraction SCA was a stripped fraction, fraction SCB was a collective fraction and fraction SCC Pulp & Paper Canada T :5 (2004) 37

FIG. 1. Scaled and centred coefficients for ISO brightness. Bleaching experiments with different volatile organic compounds FIG. 2. Scaled and centred coefficients for kappa number.

ISO brightness of the bleached pulp as a function of the added COD. Condensate experiments with 95% confidence FIG. 5. Kappa number of the bleached pulp as a function of the added COD.

3 FIG. 1. Scaled and centred coefficients for ISO brightness. Bleaching experiments with different volatile organic compounds FIG. 2. Scaled and centred coefficients for kappa number. Bleaching experiments with different volatile organic compounds FIG. 3. Scaled and centred coefficients for viscosity. Bleaching experiments with different volatile organic compounds FIG. 4. ISO brightness of the bleached pulp as a function of the added COD. Condensate experiments with 95% confidence FIG. 5. Kappa number of the bleached pulp as a function of the added COD. Condensate experiments with 95% confidence was condensate before any stripping. Main properties of these three condensates are shown in Table II. Condensate experiments were done without experimental design. The added amounts of the condensates were based on the COD load, which they contribute to when replacing part of the process water in bleaching. The selected levels were 2 and 5 kgcod/bdtp for SCA, 5 and 10 kgcod/bdtp for SCB, and 10 and 30 kgcod/bdtp for SCC. Bleaching Procedures: The D 0 bleaching stage was carried out with a set of experiments in the laboratory. The bleaching experiments with volatile organic compounds were divided into four or five days each and were performed in random order according to the experimental design. In the bleaching experiments with condensates, selected experiment was done twice per day. The bleaching conditions are listed in Table III. The bleaching experiments were begun by diluting the pulp with distilled water. Volatile organic compounds or condensates were added and the pulp was mixed properly for five minutes. The ph FIG. 6. Viscosity of the bleached pulp as a function of the added COD. Condensate experiments with 95% confidence of the suspension was adjusted to about 2.5 with 1 mole/l or 4 mole/l sodium hydroxide or with 0.5 mole/l sulphuric acid. Before adding chlorine dioxide, distilled water was added so that the final consistency of the pulp suspension would be 10%. After addition of chlorine dioxide, the pulp was mixed again for one minute before being closed hermetically and placed in a water bath. After the reaction time, a filtrate sample was pressed from the pulp suspension to measure the final ph. The pulp was diluted to a 4% consistency with de-ionized water, mixed for :5 (2004) T 115 Pulp & Paper Canada

4 sec and filtered to a consistency of about 20%. After that, it was washed twice with 10 times its own weight of de-ionized water. In the volatile compounds experiments, highly volatile compounds (methanol, ethanol and acetone) were added to the pulp suspension and mixed properly for five minutes before the addition of chlorine dioxide. In addition, three blank bleaching tests with de-ionized water were included in these experiments. Only de-ionized water was added to the pulp and it was bleached like the other samples. Determination of Pulp Properties: Typical pulp properties, ISO brightness, kappa number and viscosity, were determined according to standard procedures as responses describing the actual bleaching result. The standards and exceptions in the standard methods are mentioned in the Table IV. The kappa number was measured and the laboratory sheets for ISO brightness were prepared on the same day when the pulp was bleached. ISO brightness was measured the next day. Viscosity was measured later after every sample had been bleached. Determination of Filtrate and Condensate Properties: The COD of the solutions was measured by HACH s closedtube method (dichromate oxidation) with two parallel determinations. Organic acids (formic acid, acetic acid and propionic acid) from condensates were analyzed with Merck-Hitachi HPLC with UV- VIS detector (L-4250). The column was Transgenomics ICSep COREGEL 87H3. Sulphur compounds (HS, MM, DMS and DMDS) were analyzed with Perkin Elmer 8500 gas chromatography with head space injection nitrogen as a carrier gas. The column was 10% carbowax 20M 80/100 Supelcoport. Hydrogen sulphide (HS) and methyl mercaptane (MM) were analyzed only qualitatively. RESULTS Volatile Compound Experiments: There were no outliers, i.e., experiments whose results differ much from the other experiments on the experimental data of ISO brightness. After optimization of the Modde-models, the residuals were normally distributed. The model was fitted with a multiple linear regression (MLR). The reproducibility of the experiments was not very good, so it was difficult to fit good model to the data. Only acetic acid had some positive effect to the brightness as can be seen in Fig. 1. Effects of other factors were zero when taking the error bars into account. Coefficients in Fig. 1 are calculated as the change in average response when a factor goes from its low level to its high level. Those coefficients are scaled and centred (divided by the standard deviation of the response, for example ISO brightness) to make the coefficients comparable between the responses. There were two outliers (experiments Table VI. Chemical composition of the original condensates and filtrates samples after bleaching. Sample a Formic acid b HS MM DMS Acetone Methanol Ethanol DMDS mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l Blank 53 (±6) n.d n.d n.d <8 <10 A2 59 (±11) <0.4 n.d n.d 16 n.d 8 10 A5 48 (±12) <0.4 n.d n.d 16 n.d n.d 20 B5 68 (±2) <0.4 n.d n.d 16 n.d n.d <10 B10 50 (±4) <0.4 n.d n.d 16 n.d n.d 10 C10 49 (±6) <0.4 n.d n.d 16 n.d n.d n.d C30 65 (±26) <0.4 n.d n.d n.d <10 SCA n.d <0.4 n.d <8.5 n.d n.d SCB 174 (±1) <0.4 n.d n.d n.d <80 n.d 110 SCC 576 (±211) ref. ref. 6,490 n.d 11,760 c n.d 4,240 a A2, A5 = 2 and 5 kgcod/bdtp SCA; B5, B10 = 5 and 10 kgcod/bdtp SCB; C10, C30 = 10 and 30 kgcod/bdtp SCC b Confidence interval from two analyses is in brackets c Measured with HPLC n.d. = no detection ref. = references 5 and 13) in the experimental data of kappa number. After taking off those outliers, the Modde-model became acceptable. It was difficult to fit a good model to the data, because the difference between the various experiments was so small. Only acetic acid and propionic acid had minor negative effect on the kappa number, Fig. 2. The effects of the volatile organic compounds on the viscosity were zero, in practice, at a 95% confidence level, Fig. 3. Many of these compounds contribute to a considerable amount of COD, Table I, for example, methanol, but they did not have any effect on the kappa number, viscosity or ISO brightness. Condensate Experiments: Every secondary condensate fraction had an effect on ISO brightness and kappa number. The brightness of the pulp was decreased, Fig. 4, and the kappa number was increased, Fig. 5, as a function of COD in the used condensates. The higher kappa number and lower brightness are probably explained by the oxidisable material present in condensate. However, no effect was observed on the viscosity of the pulp, Fig. 6. Chemical Composition of Original Condensates and Filtrates From Bleaching Tests: Results of the chemical composition of the samples are depicted in Table VI. Filtrates after each bleaching with condensates (A, B, C and blank) were also analyzed as original condensates. Acetic acid or propionic acid was not found in any samples (after bleaching or from original condensates). Some traces of terpene compounds (pinenes and karenes) were found in every sample; in SCC the amount of those compounds was obvious. Because samples C10 and C30 did not contain sulphur compounds (dimethyl sulphide, MDS and dimethyl disulphide, DMDS), which should originate from SCC, we conclude that they were released or oxidized during sample preparation or actual bleaching. This observation is consistent with some earlier findings [5,11,12]. DISCUSSION In volatile compound experiments, only acetic acid had some effect on both the pulp brightness and kappa number. Propionic acid had a minor negative effect on the kappa number. When the statistical deviation in the experiments was taken into account, the detected effects were almost negligible. Condensates (mainly SCB and SCC fractions) lowered the pulp brightness (Fig. 4) and raised the pulp kappa number (Fig. 5). Condensates did not appreciably contain those compounds, which were used in the volatile compound experiments, so it could be stated that various sulphur and terpene compounds in condensates were most likely the components that affected the bleaching results. This finding is in good agreement with the results reported by Niemelä et al. [12]. When the applied foul condensate in their bleaching experiments contained dimethyl sulphide and dimethyl disulphide, they observed a reduction in pulp brightness. When cleaner condensates without these sulphur compounds were applied, the effect was less pronounced. However, we must remember that condensates might contain also other compounds, which were not analyzed in these experiments, but which could affect the bleaching result. For example, fatty and resin acids are known to affect the brightness in the D 0 stage [10]. Secondary condensates can be used as process water in bleaching, but in that case the condensates should be clean enough. Traditionally the quality of condensates has been evaluated measuring either conductivity or COD of the filtrates. Those measurements can be adequate, but better results can be obtained by measuring the Pulp & Paper Canada T :5 (2004) 39

5 colour of the condensates as Dahl et al. have earlier proposed [13]. The colour measurement takes into account lignin and coloured sulphur compounds, but, for example, disregards colourless methanol. Lignin and methanol both contribute to COD but are known to have opposite effect on bleaching result [10,14]. CONCLUSIONS No effects of the volatile organic compounds on the pulp viscosity were detected. Many of these compounds have a high COD, but they had no clear effect on the pulp properties (kappa number, viscosity or ISO brightness). Only acetic acid and propionic acid had minor effects on the pulp. The attained results indicated that when the COD load of condensate was higher, the bleaching results after D 0 stage were poorer. The brightness of the pulp decreased and the kappa number increased as the COD of the condensate was higher. However, again no effect on the pulp viscosity was observed. COD characterization results indicated that most of COD in condensates originated from methanol and formic acid. However, those components did not affect bleaching results. So, the most harmful Résumé: Dans la présente étude, diverses fractions de condensats et de composés organiques volatils ont été ajoutées à une suspension de pâte kraft de résineux avant le blanchiment au bioxyde de chlore. Les résultats ont indiqué que la plus grande partie de la DCO dans les condensats provenait des divers composés organiques volatils, ce qui n avait aucun effet sur les résultats du blanchiment et seulement un effet mineur sur les propriétés de la pâte. Fort probablement, les composantes soufrées, qui contribuent aussi à la DCO des condensats, sont les éléments les plus nuisibles lors du blanchiment. Reference: SANKARI, M., SILLANPÄÄ, M., ALA-KAILA, K., DAHL, O. Use of secondary condensates from black liquor evaporation as process water in D o bleaching. Pulp & Paper Canada 105(5): T (May, 2004). Paper presented at the 2002 International Pulp Bleaching Conference in Portland, OR, on May 19, Not to be reproduced without permission of PAPTAC. Manuscript received June 09, Revised manuscript approved for publication by the Review Panel on September 10, Keywords: CONDENSATES, BLACK LIQUORS, EVAPORATION, CHLORINE DIOXIDE BLEACHING, WATER SUPPLY, KRAFT PULPS, SOFTWOODS, CHEMICAL OXYGEN DEMAND, VOLATILES CONTENT, SULFUR COMPOUNDS. components for bleaching were probably sulphur-based components, such as DMDS and DMS, which also contribute to COD in the condensates. The condensates from black liquor evaporation can be used as process water in D 0 bleaching. However, the amount of DMS and DMDS compounds should be minimized to avoid problems in ECF bleaching. The use of secondary condensates in bleaching is a true option to decrease the demand of fresh water thus contributing to the move towards the closure of water cycles in kraft pulp mills. ACKNOWLEDGEMENTS This study was initiated under a contract between the, National Technology Agency of (TEKES), Metso Automation, Metsä-Botnia, Andritz Ahlstrom and UPM-Kymmene. LITERATURE 1. PEKKANEN, M., KIISKILÄ, E. Options to close the water cycle of pulp and paper mills by using evaporation and condensate reuse. Proc., 1996 Minimum Effluent Mills Symposium, Atlanta, (1996). 2. SÄVELIN, P., BERGQVIST, L. Closure of the EO stage in the ECF bleach plant. Proc., 1997 TAPPI Minimum Effluent Mills Symposium, San Francisco, 67 (1997). 3. JORONEN, A., LÖPPÖNEN, R., PIKKA, O., VILP- PONEN, A. Fractional washing in TCF and ECF bleaching-mill experiences. Proc., 1998 International Pulp Bleaching Conference, Helsinki, 219 (1998). 4. LIIAS, P., MERIKALLIO, T. Metsä-Rauma Process-A forerunner in TCF quality. Proc., 1998 International Pulp Bleaching Conference, Helsinki, 269 (1998). 5. ANNOLA, T-L., HYNNINEN, P., HENRICSON, K. Effect of condensate use on bleaching. Paper and Timber 77(3): (1995). 6. NIEMELÄ, K., SAUNAMÄKI, R., RASIMUS, R. Use of black liquor evaporation condensates at different bleaching stages. Proc., th International symposium on wood and pulping chemistry, Yokohama, Japan, (1999). 7. BLACKWELL, B.R., MACKAY, W., MURRAY, F., OLDHAM, W. Review of kraft foul condensates: Sources, quantities, chemical composition and environmental effects. Tappi J. 62(10): (1979). 8. SEBBAS, E. Reuse of kraft mill secondary condensates. Proc., 1987 TAPPI Engineering Conference, New Orleans, (1987). 9. EMILSSON, K., HÅKANSSON, M., DANIELSON, G. Extended stripping and usage of evaporator condensates at Värö mill, Sweden. Proc., 1997 Minimum Effluent Mills Symposium, San Francisco, (1997). 10. VIIRIMAA, M., DAHL, O., NIINIMÄKI, J., PERÄMÄKI, P. Effects of certain chemical components on the bleachability of pulp in the D0 stage. Proc., th International symposium on wood and pulping chemistry, Nice, France, Vol. III: poster presentations, (2001). 11. HYNNINEN, P. On the reactions of methyl mercaptan and its oxidation products in air- or steam stripping of sulfate mill condensates. Paperi ja Puu 53: 159 (1971). 12. NIEMELÄ, K., SAUNAMÄKI, R., RASIMUS, R. Studies on the use of black liquor evaporation condensates at different bleaching stages. Proc., 10th International conference CELLUCON 98, Turku,, (1998). 13. DAHL, O., TIRRI, T., NIINIMÄKI, J. Evaporation of acidic effluent from ECF bleaching: pilot tests. Proc., 1998 TAPPI Environmental Conference, Vancouver, (1998). 14. VIIRIMAA, M., DAHL, O., NIINIMÄKI, J., ALA- KAILA, K., PERÄMÄKI, P. Identification of the wash loss compounds affecting the ECF bleaching of softwood kraft pulp. Appita J. 55(6): (2002) :5 (2004) T 117 Pulp & Paper Canada

T 282. WORKING GROUP CHAIRMAN Junyong Zhu SUBJECT

T 282. WORKING GROUP CHAIRMAN Junyong Zhu SUBJECT NOTICE: This is a DRAFT of a TAPPI Standard in ballot. Although available for public viewing, it is still under TAPPI s copyright and may not be reproduced or distributed without permission of TAPPI. This