WO2005059241A1

WO2005059241A1 - Method for the acidification of pulp prior to ozone bleaching

Info

Publication number: WO2005059241A1
Application number: PCT/SE2004/001775
Authority: WO
Inventors: Lars-Åke LINDSTRÖM; Pär-Erik LARSSON
Original assignee: Metso Paper, Inc.
Priority date: 2003-12-18
Filing date: 2004-12-01
Publication date: 2005-06-30
Also published as: SE0303436L; SE0303436D0; SE526241C2

Abstract

A method for ozone bleaching pulp at high pulp concentration and a temperature in the interval 20-60°C in an ozone stage (9), which is preceded by an acidification reactor (4), in which the pulp is acidified at a pulp concentration of 15-25%, and in the later part of the same reactor is cooled via dilution liquid added in a dilution zone (5), so that the temperature of the pulp out from the reactor is lower than the temperature during the acidification and adapted to the temperature desired in the ozone stage. The acidification is carried out at a temperature of above 70°C, but at the highest 95°C at an acidification time of 30 minutes and at the highest 80°C at an acidification time of 120 minutes, where the highest temperature at staying-times between 30 and 120 minutes is obtained by linear interpolation.

Description

Method for the acidification of pulp prior to ozone bleaching

This invention relates to an improved method for ozone bleaching lignocellulosic material at high pulp concentration. The method is especially suitable for bleaching lignocellulosic pulps manufactured from hardwood or one-year plants by alkalic digesting processes.

Different ozone bleaching methods are previously known where the conditions for the ozone treatment can vary within pretty wide limits, according to the below summary:

pH: 1,5 - 5

Pulp concentration: 1 - 45%

Treatment time: 0,5 - about 20 minutes

Temperature: 20 - 70°C

Pressure: from slight vacuum to pressurized systems

It was found that ozone bleaching at high pulp concentration (HC) can yield high selectivity, uniform final pH and uniform outgoing kappa number and brightness. The HC-technique further offers the possibility of high batching and consumption of ozone and, thus, high delignification.

Ozone bleaching is carried out in the acid pH-range. To enable the pulp to show the acid pH suitable for ozone bleaching, the pulp from the alkalic digesting process must be acidified before the pulp is moved to the ozone bleaching stage. In a standard process the acidification is carried out as an integrated part of the ozone bleaching method. The pulp concentration in such a stage typically is about 10% and the acidification is carried out during a time below about 10 minutes. At the same time the temperature in the acid partial stage is adjusted so that the pulp assumes the temperature desired in to the ozone reactor.

Acid processing stages of different kind are per se known previously. Their object has been in different ways to improve subsequent bleaching processes, for example by reducing the metal ion content in the pulp or reducing the hexenurone acid content in the pulp.

For functioning well, the ozone stage requires that the temperature is not too high. If the temperature is too high, the ozone can disintegrate to oxygen gas, which implies higher specific ozone consumption for a given bleaching work, and undesirable side reactions, for example cellulose disintegration, can occur to an increasing extent. At previously known ozone bleaching methods the bleaching is carried out at a temperature of 20 - 70°C.

It was found that the effectiveness of the ozone stage can be improved substantially by intensifying the acidification, i.e. by higher temperature and longer staying-time. But then the difficulty arises to achieve a sufficiently low temperature in the ozone stage, which is advantageous for minimizing side reactions causing deteriorated ozone utilization.

Through SE 518 080 a bleaching method is known where the pulp is processed at a high temperature and then is bleached in at least one bleaching stage. Even if it is stated that the hot A-stage (acid stage) can be followed by a Z-stage (ozone stage), nothing is said about how the temperature problem arising in this case is solved.

Several difficulties, thus, arise when it is tried in an integrated ozone bleaching method at high pulp concentration to combine a more effective acidification stage requiring higher temperature and a longer staying-time than the usual acidification stage with a subsequent ozone stage. The relatively high temperature of the acidification stage does not agree with the requirement of the ozone stage of a low temperature. The low temperature in the ozone stage is necessary to prevent the pulp from being subjected to unnecessary high disintegration with resulting reduced viscosity and lower pulp strength. The low temperature is also necessary for achieving an ozone utilization as good as possible. The reason is, as already mentioned, that ozone tends to disintegrate to oxygen gas to an increased extent at higher temperature.

A new method, which meets these apparently opposing requirements, is to carry out the acidification stage in a manner which implies that the acidification is intensified without serious disintegration of the pulp by delivering a pulp with sufficiently low temperature to the ozone stage. This can be achieved by dividing the acidification into two phases. The first phase is carried out at a pulp concentration which is significantly higher than usual. In the second phase the pulp is diluted and cooled. By carrying out the first phase at high pulp concentration, the heat content in the pulp is relatively low which practically renders it possible to cool the pulp with dilution liquid to a suitable temperature, before it is dewatered and moved to the ozone stage. The pulp, thus, need not be cooled to a temperature suitable for the ozone stage in a separate step, but this can be carried out by dilution to a later part of the acidification stage. If the pulp had been acidified at a lower concentration, the heat content in the pulp would have been so great, that it would have been practically impossible to lower the temperature sufficiently much by dilution directly to the reactor.

The temperature in the acidification stage is adapted for intensifying the acidification treatment without deteriorating the pulp properties. It is not wanted to have too high a temperature, partly because this can imply increased disintegration of the cellulose, partly because too high a temperature would make it impossible to lower the temperature in a simple way sufficiently much to meet the temperature requirements in the ozone reactor. By increasing the temperature during the acidification treatment compared with the standard method, where the temperature is 20 - 70°C, but yet not so high that it will be practically impossible to lower the temperature of the pulp before the subsequent ozone bleaching stage, an improved integrated ozone bleaching method is achieved.

The invention, thus, solves the problems in that it in an effective way combines an intensified acidification step requiring higher temperatures with a subsequent ozone stage without requiring any cost-increasing intermediate stages for giving the pulp the temperature required for the ozone stage. The acidification time can be 30 - 120 minutes. By having a temperature in the acidification stage, which is higher than 70°C, but at the highest 95°C, at an acidification time of 30 minutes, and at highest 80°C at a staying-time of 120 minutes, and where the maximum temperature at staying-times therebetween is obtained by linear interpolation, an intensified acidification stage is achieved which is adapted to the subsequent ozone bleaching stage with respect both to temperature and cellulose disintegration.

The method is described in the following in detail. Pulp is washed and dewatered to a pulp concentration of about 30% (25 - 35%). Thereafter a tempered liquid is added, containing acid, which is mixed with the pulp so that the pulp concentration is lowered to about 20% (15- 25%), a temperature of at least 70°C and a pH of 2 - 3,5 is obtained. In order to avoid too great disintegration of the pulp, the temperature shall not exceed 95°C at 30 minutes acidification time and 80°C at 120 minutes acidification time. The maximum temperature in the acidification stage at times between 30 and 120 minutes is obtained by linear interpolation.

After acidification the pulp is diluted with temperature-controlled washing liquid in the dilution zone of the reactor to a pulp concentration suitable for transportation to subsequent stages (3 - 12%). The temperature after the dilution zone, T₀uτ, shall be lower than the temperature in the reactor T_R, and adapted to the temperature requirement in the ozone reactor, 20 - 60°C. The pulp is dewatered to > 35% in a press and bleached with ozone in known manner. After the reaction with ozone the pulp is washed for the next processing stage. The pulp is possibly alkalized before its washing.

The described ozone bleaching method can be preceded and/or followed by one or several delignifying or bleaching stages.

The presses used for dewatering and washing can be of different types, such as for example constriction washing press, dewatering press, double-wire press or screw press.

By this method a substantially more effective ozone utilization than in existing methods is achieved.

Example

Table 1 shows the results of ozone bleaching where the intensity in the acidification stage has been varied.

Table 1. Ozone bleaching of hardwood sulphate pulp; kappa number 11.5, viscosity 983 dm³/kg, brightness 53.4% ISO Pre-treatment Standard HC Improved bleaching method Pulp concentration % 10 20 20 20 20 Staying-time min 20 80 50 120 120 Temperature °C 20 85 90 80 80 H S0₄ kg/odt 9 9 9 12 9 Final pH 2.8 2.8 ^■ 2.8 2.4 2.7

| Ozone stage | Pulp concentration % 42 42 42 42 42 Staying-time sec 95 94 94 93 92 Temperature °C 40 40 40 40 40 0₃ kg/odt 4.98 4.99 4.98 4.98 4.99 0₃ rest kg/odt 0.08 0:07 0.08 0.17 0.17 0₃ consumption kg/odt 4.90 4.92 4.90 4.81 4.82

I Alkalization | Pulp concentration % 12 12 12 12 12 Staying-time min 10 10 10 10 10 Temperature °C 55 55 55 55 55 NaOH kg/odt 12 12 12 12 12 Final pH 10.6 11.4 11.2 11.5 11.6

I Analyses | Brightness %ISO 67.6 69.5 68.7 71.8 71.4 Kappa number 3.7 2.6 3.0 2.0 2.1 Viscosity ml/g 857 775 793 740 742 Delta kappa (kappa number reduction) 7.8 8.9 8.5 9.5 9.4 Delta kappa kappa/kg 0 ptp 1.59 1.81 1.73 1.98 1.95 /0₃ consumption

The results from the Table are shown in Fig. 4, where the ozone consumption pro reduced kappa number unit is plotted against temperature in the ozone stage. The Figure shows that the ozone utilization improves at increased temperature in the ozone stage and that the intensified acidification results in improved ozone utilization.

The invention is described in the following in greater detail with reference to Figures 1-4.

Figure 1 shows a commercial method for ozone bleaching carried out at high pulp concentration (standard HC). The acidification is carried out as an integrated part of the ozone bleaching method at a pulp concentration of about 10% during a time below about 20 minutes. At the same time the temperature is adjusted in the acid stage so that the desired temperature in the ozone reactor is obtained. With reference to the number designations in the Figure the acidification is carried out in a reactor (13). The pulp is diluted before the treatment in the reactor to a concentration of about 10% with filtrate recycled from a press (15) located after the acidification reactor (13). The filtrate is temperature-controlled (14) and acid is added (12) before it is mixed into the pulp. After acidification the pulp is dewatered in a press (15) and then moved to the ozone stage (16). The pulp can then, before a_possible extraction stage (17), be diluted with filtrate, the temperature of which is adjusted via a temperature control (18). The pulp is dewatered and then washed in a press (19).

Figure 2 illustrates the effect of.the temperature on viscosity losses in the ozone stage at the bleaching of a hardwood sulphate pulp with kappa number 10 according to the sequence a (Ze), where the a-stage (acid stage with short staying-time) is carried out at 5% pulp concentration, pH = 2,5 and time 20 minutes. The ozone addition was 5.7 kg O₃/odt, and the time about 5 minutes. After the ozone bleaching an e-stage (alkaline extraction stage) was carried out at 12% pulp concentration, time 10 minutes and pH = 11.5. It can be read from the Figure that a lower temperature at the ozone bleaching yields a lower viscosity loss.

Figure 3 shows schematically the improved ozone bleaching method according to the invention. The pulp is washed and dewatered in a press (1) to a pulp concentration of 25-35%. Thereafter liquid and acid are added so that the pulp concentration is lowered to 15-25%» and a temperature of above 70°C and a pH of 2 - 3.5 are obtained. The addition can be made through a tempered liquid containing acid or through separate additions of dilution liquid (2), acid and steam (3). The pulp is acidified in a reactor (4). The highest temperature at the acidification depends on the acidification time. At an acidification time of 30 minutes the temperature shall not exceed 95°C, and at an acidification time of 120 minutes the temperature shall not exceed 80°C. After the acidification phase dilution liquid is added to a dilution zone (5) in the later part of the reactor. The dilution liquid shall have such a temperature that the temperature of the pulp out from the reactor is lower than during the acidification and adapted to the temperature requirement in the ozone reactor. The temperature of the dilution liquid is controlled via a temperature control (6). The pulp concentration from the reactor shall be 3-12%, preferably 4-10%. The pulp is then dewatered in a press (7), so that a pulp concentration of 35-50% is obtained. The pulp is then bleached with ozone at a temperature of 20-60°C in a reactor (8). After the reaction the pulp is washed in one stage (9) before the next processing stage. The pulp can possibly be alkalized in one stage (10) before it is washed. The pulp can then be diluted with filtrate from the press (9), which can be temperature-controlled (11). Figure 4 shows how the ozone utilization is deteriorated at increased temperature in the ozone stage and how the intensified acidification causes a more effective ozone utilization. The points connected by a dashed line show tests where the acidification has taken place at a pulp concentration of 5%, pH 2.5, temperature 20°C and time 20 minutes (i.e. standard method). The encircled points above the line represent points with intensified acidification according to Table 1.

The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the patent claims.

Claims

1. A method for ozone bleaching of lignocellulosic material, where a) the pulp is dewatered to a pulp concentration of 25-35% in a press (1) whereafter liquid and acid are mixed with the pulp b) the pulp is acidified in a reactor (4) where the pH is in the interval 2 - 3.5 c) the pulp is dewatered in a press (7) to a pulp concentration of 35-50%o, and then d) is bleached with ozone in one stage (8) where the temperature is in the interval 20-60°C, characterized in, that the acidification is carried out at a pulp concentration of 15- 25%> and at a temperature of above 70°C, but at the highest 95°C at an acidification time of 30 minutes and at the highest 80°C at an acidification time of 120 minutes, where the highest temperature at staying-times between 30 and 120 minutes is obtained by linear interpolation, and that dilution liquid is added to a dilution zone (5) in the later part of the acidification reactor (4), so that outgoing pulp has a concentration of 3-12% and a temperature, which is lower than the temperature during the acidification and adapted to the temperature desired in the ozone bleaching stage (8).

2. A method as defined in claim 1 , characterized in, that the pulp after the ozone bleaching stage is washed in a press (9).

3. A method as defined in claim 2, characterized in, that the pulp is alkalized in one stage (10), before it is washed in the press (9).

4. A method as defined in claim 1 , characterized in, that the ozone bleaching method can be preceded by one or several delignification or bleaching stages.

5. A method as defined in claim 1 or 4, characterized in, that the ozone bleaching method is followed by one or several bleaching stages.