FI91896B - Method for oxygen bleaching - Google Patents

Description

91896

Method for oxygen bleaching - Sätt vid oxygen-blekning

The present invention relates to a process for oxygen bleaching of fibrous material 5, wherein the pulp passes through a first lignin removal zone at a predetermined lower temperature and a second lignin removal zone at a predetermined temperature higher than the first lignin removal zone.

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, it is not possible to use too low a reaction temperature in oxygen bleaching to achieve a normal piece number range. Temperatures of about 100 ° C or special measures are required for this purpose. Thus, temperature control has been proposed in connection with oxygen bleaching of a medium-thick pulp, cf. Article C.E. Courchene and 25 V.L. Magnotta: "Improvement of medium consistency oxygen bleaching through temperature control", 1984, "Oxygen delignification", ss. 11 to 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. The experiments showed that controlling the temperature at a low initial temperature in a horizontal tubular reactor improved the yield and viscosities while maintaining the residence time. However, the described temperature control cannot be successfully applied on an industrial scale because it is difficult to achieve a precise temperature limit of 2,91896 between the two target temperature zones. This is due to e.g. that it is difficult to mix steam quickly and efficiently with the pulp evenly, and also that a horizontal tubular reactor with a screw for transferring the pulp has an upper space which is not filled with pulp but which forms a vapor phase which interferes with temperature control along the entire horizontal the length of the tubular reactor.

It is an object of the present invention to obviate the above-mentioned problems completely, and to provide a method for utilizing the idea of the initial low temperature of oxygen lignin removal in industrial bleaching, and to regulate and control the temperatures of the various lignin removal zones so that the lignin removal zones are constant lower and constant higher temperature, respectively, without the interfering vapor phase formed on the lignin removal zones.

A novel aspect of the invention is that the pulp is fed through a first vertical reactor having said lower temperature lignin removal zone, and then to a second vertical reactor having said second higher temperature lignin removal zone, and that the temperature in the first lignin removal zone is maintained at that temperature. the pulp has been obtained in a previous pre-oxygen bleaching treatment, or that it is controlled, if necessary, by a controlled supply of steam to the mixer in the line prior to the first reactor, and that the temperature of the second lignin removal zone is controlled by a controlled supply of steam to the line between reactors.

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

3,918 96

Figure 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 a pump 5 at the outlet of the storage tank 3 via line 4 and mixer 6. a device for supplying treatment agents to the line 4. The mixer 6 may comprise a liquefaction device e.g. for rapid and uniform mixing of various additives into the pulp. The mixer may 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 a stirrer 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 via line 11, 25 cellulose preservatives, e.g. MgSO 4, are added. A basic substance such as NaOH or oxidized white liquor is fed to the first high-power 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. A line 17 can also be provided for adding the basic substance in the first step to the removed oxygen lignin mass.

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The bleaching plant further comprises suitable measuring and control means (not shown) for measuring the temperature of both reactors 4 91896 and for adjusting the steam supply to the mixers so that the correct different temperatures are maintained in the reactors according to 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, preferably 10 95 - 110 ° C. Thus, the terms "lower" and "higher" refer to the relative temperature relationships 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 Thus, in cases where the pulp to be bleached is at a sufficient temperature obtained during the previous oxygen bleaching treatment and which corresponds to the term "lower temperature", i.e. in the range 70-90 ° C, it is not normally necessary to supply additional steam to the mixer 6, provided 20 en that the temperature is constant or substantially 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 ligninization zone) was 75 ° C, in the second experiment 85 ° C, and in the third experiment 105 ° C, while in the second stage (second ligninization zone) the temperature was 105 ° C in all experiments. 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.

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91896 5 tia 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 true that a higher base 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 base dose (20 kg) in the three experiments mentioned, while the values on the left are related to the highest base dose (35, 30, 30 kg).

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The results show that the high temperature in the initial stage of oxygen lignin removal has a negative effect on the viscosity, and that the lower temperature in the initial stage produces a deoxygenated pulp with better viscosity and a number of pieces within the normal and desired range. The diagram further shows that the lower temperature in 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 stage and at a high temperature. This effect can be utilized 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 unchanged. The latter procedure is interesting from an environmental point of view, as 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 their reactors 1, 2, no interfering vapor phase is formed on or between the lignin removal zones, and efficient regulation and control of the temperatures of the different lignin removal zones 8 is provided. 1, 2, wherein the mixers produce a uniform mixing of steam into the mass so that a constant, lower temperature in the first reactor can be maintained without problems, and likewise a constant, higher temperature can be maintained in the second reactor without problems.

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Claims (3)

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