FI91896C - Method of oxygen bleaching - Google Patents

Description

91896

Method in connection with oxygen bleaching - Sått vid oxygen-blekning

The present invention relates to a process for the oxygen bleaching of a fibrous material 5, in which the pulp passes through a first predetermined lignin removal zone at a lower temperature in the leaching zone as well as at a higher pre-leaching outlet.

In an article entitled "Oxygen bleaching Kinetics at ultra-low consistency", Trappi Journal, December 1987, C.L. Hsu and Jeffrey S. Hsieh describe experiments with oxygen bleaching, 15 to investigate the effect of temperature on viscosity. The result shows that the high temperature in the initial stage of oxygen bleaching has a negative effect on the viscosity.

However, in oxygen bleaching, too low a reaction temperature of 20 to achieve a normal kappa number range cannot be used. For this purpose, temperatures of about 100 ° C or special measures are required. Thus, it has been proposed to control the temperature during oxygen bleaching of a medium-thick mass, cf. Article C.E. Courchene and 25 V.L. Magnotta: "Improvement of medium consistency oxygen bleaching through temperature control", 1984, "Oxygen delignification", ss. 11 - 5. Oxygen lignin removal was performed in a laboratory-style horizontal tubular reactor fed 30 vapors at various lengths to form two or more zones with different temperatures, with the first zone having the lowest temperature. Experiments showed that controlling the temperature at a low initial temperature in a horizontal tubular reactor improved the yield and viscosity-35 while keeping the residence time unchanged. However, the described temperature control cannot be successfully applied on an industrial scale because it is difficult to reach the exact temperature limit of 2,91896 between the two target temperature zones. This is due to e.g. because it is difficult to mix the steam with the mass evenly and efficiently evenly, and also because the horizontal tubular reactor 5 with the screw for transferring the mass has an upper space which is not filled with the mass but which forms a temperature-dependent suspension the entire length of the horizontal tubular reactor.

The object of the present invention is to avoid the above-mentioned problems completely, and to provide, in connection with oxygen bleaching, a method by which the idea of an initial low-temperature oxygen lignin removal can be utilized industrially and by which the conditions of different lignin removal zones the dewatering zones are given a constant lower and correspondingly a constant higher temperature without the disturbing vapor phase formed on top of the lignin depletion zones.

20 What is new in the present invention is that the pulp is fed through a first vertical reactor having a lignin removal zone at said low temperature temperature, and then to a second vertical reactor having a second high temperature lignin removal zone at a higher temperature. from the bleaching of pulp has been received earlier, oxygen bleaching edeltåvåsså chaise-telysså, and working or, if necessary, the SITA såådetåån såådetyllå Vapor syottåmisellå ensimmåistå reactor edeltåvåsså line mixer 30, and the second and working lignin poistovydhykkeen the outside temperature såådetåån syottåmållå såådetys t-vapor-vålisesså reactors in the line mixer.

35 The invention will now be described in more detail with reference to the drawing, in which II.

3,91896

Fig. 1 schematically shows a bleaching plant for carrying out the method according to the invention; and

Fig. 2 is a graph showing the relationship between the number of pieces and the vis-5 cosity.

The bleaching plant shown in Fig. 1 is intended for oxygen bleaching in two different stages, and then includes a first reactor 1 and a second reactor 2. The pulp is fed from the storage tank 3 to the first reactor 1 by means of a pump 5 at the outlet of the storage tank 3 via line 4 and mixer 6. , in which case the mixer is a device for feeding treatment substances into the line 4. The mixer 6 can comprise a liquefaction device e.g. for rapid and uniform mixing of various additives into the pulp. The mixer can preferably consist of a "Kamyr MC-mixer". The mass to be bleached is thus medium thick, i.e. about 6-15%.

The reactors 1, 2 are connected to each other by a line 7 containing 20 agitators 8 of the same type as described above. The mass of oxygen lignin removed from the second reactor 2 is fed to the blow tank 9 via line 10.

At the outlet of the storage tank 3, 25 cellulose preservatives, e.g. MgSO 4, are added via line 11. A basic substance such as NaOH or oxidized white liquor is fed to the first high-efficiency mixer 6 via line 12 and oxygen gas via line 13. In addition, a line 14 for high-pressure steam is connected to the mixer 6. The second mixer 15 is provided with lines 15, 16 connected for the supply of oxygen gas and high pressure steam, respectively. It is also possible to arrange a line 17 for the addition of a basic substance in the first step to the removed oxygen-lignin mass.

35

The bleaching plant further comprises suitable measuring and charging means (not shown) for measuring the temperature of both reactors 91896 4 and for controlling the feed of steam to the mixers, so that the correct different temperatures are maintained in the reactors in accordance with the present invention.

The first reactor 1 then comprises a first lignin removal zone with a lower temperature in the range of 70 to 90 ° C, preferably 75 to 85 ° C, while the second reactor contains a second lignin removal zone with a higher temperature in the range of 90 to 125 ° C. - 110 ° C. The terms "lower" and "higher" thus refer to the relative temperature ratios of the lignin removal zones. The temperature difference between the lignin removal zones should be about 20 to 40 ° C, preferably about 30 ° C.

15 In cases where the pulp to be bleached is at a sufficient temperature to be obtained in the previous oxygen bleaching treatment and which corresponds to the term "lower temperature", i.e. in the range 70-90 ° C, it is therefore not normally necessary to feed steam in addition to the mixer 6, 20 I do not state that the temperature is constant or mainly constant.

The following example illustrates the invention.

Example 25

Oxygen lignin removal in two steps was performed in the bleaching plant of Figure 1. The temperature was changed in three different series of experiments so that in the first experiment the temperature of the first stage (first lignin removal zone) was 75 ° C, in the second experiment 85 ° C, and in the third experiment 105 ° C, while the temperature in the second stage (second lignin removal zone) was 105 ° C. in three experiments. The pulp number of softwood entering oxygen bleaching was 28.7, the viscosity was 1141 dm3 / kg, and the consistency was 10%. The initial pressure (gauge pressure) in both lignin removal zones, i.e., in steps 1 and 2, was about 0.5 MPa, and in all experiments the treatment time in step 1 was 15 minutes.

II

5 91896 t and 45 minutes in step 2. Pump 5 was supplied with 5 kg MgSO 4 per tonne of dry pulp on line 11. Each experiment was repeated with the only difference in the first case being 20 kg NaOH per tonne dry pulp, in the second case 25 kg NaOH and in the third case 30 kg NaOH per tonne of dry pulp, except in the first experiment, which used 30 kg of NaOH in the second case and 35 kg of NaOH per tonne of dry pulp in the third case (instead of 25 and 30, respectively). It is generally accepted that a higher maternal dosage produces a lower kappa number and viscosity under otherwise the same conditions. In addition, a fourth experiment was performed in which oxygen lignin removal was performed in two steps at 105 ° C for 60 minutes with a base addition of 30 kg of NaOH per ton of dry pulp.

15 The results are shown in Figure 2. Thus, the values marked on the right are related to the lowest female dose (20 kg) in the three experiments, while the values on the left are related to the highest female dose (35, 30, 30 kg).

20

The results show that the high temperature at the initial stage of deoxygenation has a negative effect on the viscosity, and that the lower temperature at the initial stage produces a deoxygenated pulp with a better viscosity and a number of pieces within the normal and desired range. In addition, the graph shows that a lower temperature at the initial stage of oxygen lignin removal produces a viscosity of 15 to 30 units higher measured at the same kappa number, compared to oxygen lignin removal in only one step and at a high temperature. This effect can be exploited either to produce a pulp with better strength properties or to reduce the number of pieces of oxygen lignin removed by 1-2 units while maintaining the viscosity. The latter procedure is interesting from an environmental point of view, since it implies a reduction in chlorine consumption in the subsequent bleaching plant and thus a corresponding reduction in emissions of organochlorine compounds.

Since the lignin removal zones are spaced apart from each other, namely in each of the reactors 1, 2, no interfering effluent phase is formed on or between the lignin removal zones, and different lignin removal phases are obtained. 10 reactors 1, 2, whereby the mixers produce a uniform mixing of steam into the pulp, so that a constant, lower temperature in the first reactor can be maintained without problems, and a constant, higher temperature in the second reactor can also be maintained without problems.

15

II

Claims (3)

1. Seed for oxygen bleaching of fibrous cellulose pulp, in which the pulp passes an understood delignification zone 5 with a predetermined low temperature and a second delignification zone a predetermined high temperature which is higher in the understood delignification zone, characterized in that the pulp is fed through an understanding vertical reactor (1) containing said low-temperature delignification zone having a low temperature in the range of 70 to 90 ° C, and then through a second vertical reactor 82) containing said second delignifying zone having a high temperature in the range of 90 to 125 ° C, wherein the temperature in the understood delignification zone is about 20 - 40 ° C lower than 15 in the second delignification zone, and that the temperature in the understood delignification zone is either maintained at the temperature obtained by the bleaching pulp obtained during a previous treatment. oxygen bleaching, or if necessary controlled by controlled supply steam of a mixer (6) located in the conduit (4) for understanding the reactor (1), and the temperature of the second delignification zone is controlled by controlled supply of steam to a mixer (8) located in the conduit (7) between the two the reactors (1, 2). 25 2. A method as claimed in claim 1, characterized in that the preferred temperature ranges are 75 - 85 ° C and 95 - 100 ° C, respectively, whereby the temperature in the understand delignification zone is preferably about 30 ° C lower than in the second. 3. A composition according to claim 1 or 2, characterized in that the mass is of average consistency. t