PROFITABLE AND ENVIRONMENTALLY SOUND PRODUCTION OF BLEACHED PULP

Author

Monica Bokström

Company and address

Metso Paper Sundsvall AB, Sundvall 85194, Sweden

email

monica.bokstrom@metso.com

Keywords

ECF, TCF, ozone, bleaching, oxygen delignification

1. ABSTRACT

The closure demands on bleach plants have lead to significant development of bleaching techniques to reduce the effluent volumes as well as the bleaching chemical consumption, without harming the pulp quality.

Today there are several options for bleaching and the proper bleaching sequence to use for retrofitted or new bleaching lines can differ significantly depending on the demand on the pulp quality, chemical costs and effluent restrictions.

A fundamental property of highly closed and efficient modern bleach plants is that these use presses as washers throughout the bleach plant. The presses give efficient barriers between the stages, which is important for low chemical consumption and control of the bleaching to uniform quality. The water consumption as well as the effluent volumes are low with presses as washers, without problems with scaling.

The most common first step is to improve the oxygen delignification process to make it possible to better preserve the pulp yield and strength. This can be done by replacement of the residual delignification in cooking by extended oxygen delignification. This strategy can be very profitable, if applied in a proper way, giving both higher pulp yield and improved bleachability of the pulp. With the OxyTracTM process, these positive effects can be used efficiently. Even further improvement in the bleachability with higher delignification degree to lower kappa numbers before bleaching is one of the special features of this process for oxygen delignification.

Another very interesting process is the ZeTracTM process for high consistency ozone bleaching that can be applied to both ECF and TCF bleaching. With this new system the capital cost has been considerably reduced for HC ozone bleaching. The process is unique in giving low bleaching chemical costs at high degree of closure. The high brightness ceiling and low brightness reversion with the ZeTrac sequences are also important for highest pulp quality.

Some examples of mill applications of these processes are presented in this paper.

2. INTRODUCTION

The development work of Metso Paper fiberline has been focused on developing a fiberline producing high quality pulp, meeting environmental targets and being flexible to production of both ECF and TCF in the same equipment. It should also handle a large production in one single line and use the same type of washers in all positions and the same screens in fine and bleached pulp screening. Important examples of this development are shown in a number of mill installations, of which some are given below.

The installations of the OxyTrac process for extended oxygen delignification and the press based bleach plant have shown to be most efficient to fulfill the requirements for bleaching and closure. The light ECF sequences based on high consistency ozone, ZeTrac can give low effluent load and maintained pulp strength. These sequences are also very interesting economically, as the costs of installation and chemicals of the sequence are significantly lower than of the more conventional alternatives.

3. PRESSES AS WASHERS IN THE BLEACH PLANT

With the introduction of presses as washers in the bleach plant, the wash liquor volume has been significantly reduced. Special advantages of using presses as washers in the bleach plant are that these washers give efficient barriers. Since the outlet pulp consistency is high, it separates low and high pH, as well as low and high temperatures. This is a major advantage from a process control point of view.

When presses are introduced in the bleach plant, the consumption of alkali is reduced. In an ECF mill, bleaching with the sequence D(EO)DD, with filters as washers a typical consumption of alkali is about 13-15 kg NaOH/adt. If the washing after the D0 stage is performed by a press instead, the alkali consumption is reduced to 8-10 kg/adt. This finding has been further studied in laboratory scale. The results of this study are shown in (figures 3.1-3.4), where press washing after the D0 stage has been compared with filter washing. The washing was followed by an EO stage where fresh alkali was added and EO filtrate was used as dilution liquor. The study shows that the requirements of fresh alkali (NaOH) to reach a certain final pH (pH 11) after the EO stage are lower with press washing compared with filter washing. As can be seen, this is valid both for hardwood and softwood.

Figures 3.1 and 3.2

Figures 3.3 and 3.4

The explanation of the reduced consumption of alkali with presses is that the amount of dilution liquor added after washing to a high consistency pulp (30% pulp consistency) is about 5 m3/adt while it is only 1 m3/adt for a pulp at an outlet consistency of 14%. Since the dilution liquor contains residual alkali, the requirement of fresh alkali will be reduced when the washer after the D0 stage is a press instead of a filter. If D1 filtrate is used as wash liquor, alkali is also needed to neutralize the acid D1 filtrate in the pulp after the washer. Since the acid liquor volume in the pulp after a press is much smaller than in the pulp after a filter, the requirement of alkali will be lower.

4. THE BLEACH PLANT AT SCA ÖSTRAND AND OxyTrac DELIGNIFICATION

An interesting case is the bleach plant at the SCA Östrand mill in Sweden (see figure 4.1). In 1995, the mill replaced the old bleaching lines with a new bleach plant for TCF pulp with the design capacity of 1100 adt/d for softwood and 1250 adt/d for hardwood pulp in the same line. The bleaching sequence is Q(OP)(Zq)(OP) with TwinRoll™ presses after all stages. The filtrates from the two (OP) stages and the (Zq) stage are recirculated to the post oxygen washing. The filtrate taken out is from the Q stage and there is also a purge from the acid loop ahead of the ozone stage. The bleach plant effluent data are less than 7 m3/adt and less than 12 kg COD/adt.

Figure 4.1

The mill produces high quality TCF pulp also at full brightness. One important step to make this possible was the rebuilt of the single stage oxygen delignification system to the two -stage oxygen delignification process for extended oxygen delignification (see figure 4.2).

Figure 4.2

The process conditions in the process are developed to give improved selectivity to the conventional oxygen delignification. This has made it possible to extend the delignification without sacrificing the fiber quality, up to 55-70% for softwood pulps and up to 50-55% for hardwood pulps.

The standard conditions for the process are given in (figure 4.2). In the first stage, with a residence time of 30 min., the temperature is maintained at a relatively low level, 80-85 oC, and the pressure at a relatively high level, 8-10 bars. A high chemical concentration in this initial phase of delignification combined with these conditions favors delignification over cellulose degradation, which means improved selectivity. The delignification in the second stage needs a higher temperature and a longer time, about 90-100 oC and 60 min. As the temperature level in the second stage is higher than in the first stage, the alkali concentration and the pressure (i.e., the oxygen concentration) should not be too high for minimized cellulose degradation. All the chemicals, alkali and oxygen, should thus be charged ahead of the first stage, to maintain a high chemical concentration in this first stage, and there is no split of the chemicals between the reactors. The pulp is heated between the reactors to the required temperature level for the second stage.

Together with the process development, we have also developed a control strategy to even out variations from the digester. The important factors used for the control are the production rate, the inlet kappa number, the chemical charges and the temperature in the second stage. The efficiency of the control system is, of course, very important to minimize the chemical consumption in the bleach plant.

The installation of the process at SCA Östrand has given a high degree of delignification. For softwood pulp, the digester kappa number has been raised to 28-30 from about 24, and the kappa number of pulp for bleaching has been reduced to 10 from about 14. This is a significant change compared with the delignification in one stage. For the hardwood pulp, the digester kappa number is about the same but the outlet kappa number is about one unit lower with this process. There is a significant improvement on the selectivity for both qualities of pulp, which has resulted in improved bleached pulp strength, reduced chemical consumption, lower effluent volume and COD discharge. In addition to these rather easily measurable effects in mill scale, laboratory tests have shown that a conversion like this from a single stage system to OxyTrac gives an improved yield (see figure 4.3).

Ffigure 4.3

In time, with more installations of this process in full scale, there was also an additional effect discovered, which has later on been confirmed by laboratory tests: The process gives an improved bleachability of the pulp.

The improved bleachability with the process can be seen both for softwood and for hardwood pulps. The improvement of the bleachability develops more with higher delignification, i.e. low kappa number before bleaching.

Figure 4.4

Figure 4.4 shows the bleachability of a eucalyptus pulp after conventional single stage delignification and after the new process. The delignification degree on this kind of pulp is only extended by about 0.5 kappa unit. Anyhow, the figure shows that after extended delignification, the requirement of chlorine dioxide for bleaching is substantially reduced. At 90% ISO, the required kappa factor is reduced from 4.0 to 3.2. Similar results are also seen for softwood pulps. Figure 4.5 also shows that the bleachability improves with higher delignification degree, which is very interesting as a lower inlet kappa number, with a conventional system, usually increases the required kappa factor to reach the target brightness.

Figure 4.5

5. THE NEW ZeTrac PROCESS

Metso Paper has recently introduced a new improved high consistency ozone process called ZeTrac. The new system has considerably reduced the capital outlay as the plug screw feeder, the refiner fluffer and one washing stage have been eliminated. These changes also reduce the operating (energy) and maintenance costs of the system. As one washing stage has been eliminated directly after the ozone stage, the washing is made after the extraction stage, i.e., after (Ze) or (Z(EO)). The filtrate is then fully recyclable to the brown stock or post oxygen washing system. With a forceful ozone stage in the beginning of the bleaching sequence, the bleaching costs and the effluent volumes and load can thus be significantly reduced.

The new high consistency ozone system is shown in figure 5.1. The press shredder screw has been modified to give a very well-fluffed pulp. The pulp drops directly into the paddle reactor where the ozone is introduced. Gas flows are controlled by two fans, of which one is placed on the shredder hood and the other in the residual gas handling system. At the reactor outlet the pulp is diluted with  alkaline liquor giving a very rapid change in the pH level, thereby avoiding calcium oxalate scaling.

Figure 5.1

In figure 5.2, a simplified flow sheet of the sequence (Z(EO))DD is given. As can be seen, the bleach plant is compact with this sequence as some washers are removed.

Figure 5.2

The high consistency ozone stage is preceded by an acid loop where the pulp is acidified and where metal ions are removed from the pulp. As there is no washing between the ozone stage and the (EO) stage, the filtrate is alkaline. This filtrate can thus be recycled to the post oxygen washing system without risk of scaling. Applying this sequence on a pulp after the oxygen delignification and with a charge of 5-6 kg ozone per ton will give a kappa number level of 2-3 after extraction. The chlorine dioxide charge required for final bleaching is thus low, giving low load to the effluent. With presses as washers, the total effluent volume will be less than 7 m3/t including 2 m3/t taken out from the acid loop ahead of the ozone stage. In (figure 5.2) there is a comparison between water consumption and effluent load for a conventional ECF and the light ECF ozone sequence applied on a hardwood pulp and with the same kappa number after the extraction stage. The effluent volume is reduced by 30% and the COD load by 40%. However, in the normal case, the kappa number after extraction will be lower with the ozone sequence giving even lower COD discharge.

The sequence (Z(EO))DD is designed for pulps that are relatively difficult to bleach. For pulps that are more easily bleached, it can be sufficient with only a short extraction stage, e, and only one D stage for final bleaching (see figure 5.3). A bleach plant like this one has the benefits of very low installation and operating costs, but also a reduced bleaching chemical cost and reduced effluent load. These features of the process are unique as other alternatives for significantly reduced environmental impact from bleaching will increase the cost of bleaching.

Figure 5.3

An example of a eucalyptus pulp of kappa number 10 after the oxygen delignification is given in figure 5.4. Applying an alternative sequence, Q(OP)DD for reduced effluent discharge compared with the standard ECF sequence D(EOP)DD, the Q(OP)DD sequence will increase the costs of bleaching chemicals. With the ozone sequence both reduced effluent and lower chemical costs are achieved.

Figure 5.4

The costs of bleaching ECF can thus be significantly reduced by this (Z(EO))DD sequence compared with a standard ECF alternative, especially if the pulp is difficult to bleach to high brightness. The ozone sequence also raises the brightness ceiling (see figure 5.5). As ozone is very efficient in reducing hexenuronic acids in the pulp, the brightness reversion is also lower (see figure 5.6).

Figure 5.5

Figure 5.6

The first installation of the ZeTrac process was in the Burgo Ardennes mill in Belgium, which started up the ozone stage in the end of year 2000 (see figure 5.7). The next start-up will be the Oji Paper Nichinan mill in Japan, and in the end of year 2002, the VCP Jacarei mill in Brazil will apply this high consistency ozone technique in their new line, (see figure 5.8). As there are so many benefits of this new technology we expect many more mills to apply this technique in the near future.

Figure 5.7

Figure 5.8

Figure 5.8 Bleaching at VCP Jacarei mill

6. CONCLUSION

The fiberline of today must produce high quality pulp and at the same time meet the environment targets. These requirements have been met by the introduction of the extended oxygen delignification, by the high consistency ozone bleaching and by the introduction of presses within the bleach plant. TwinRoll presses are efficient washers that give barriers to temperature, pH and metal ions, which are favorable for the processes and scaling management. Press washing after a D0 stage has shown to significantly reduce the need for fresh alkali in the EO stage.

The OxyTrac process for oxygen delignification gives increased delignification power with improved selectivity and pulp bleachability. ECF bleaching including a high consistency ozone stage, ZeTrac, followed by an extraction stage without an intermediate washing stage, the water consumption and the COD discharge can be reduced considerably compared with a conventional ECF sequence. Since the process gives an alkaline filtrate without any chlorides that can be recirculated to the post oxygen washer. This process also gives quality advantages as high brightness, brightness stability and significantly reduced costs of bleaching.

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