FI117562B - Pulp bleaching process and pulp bleaching plant - Google Patents

Description

. ) 1 117562

Method for pulp bleaching and pulp bleaching equipment

The invention relates to a process for bleaching pulp of the type set forth in the preamble of claim 1. The invention also relates to a pulp bleaching apparatus of the type set forth in the preamble of claim 9.

Chemical pulps containing vegetable cellulosic fibers, most commonly of wood origin, are bleached with chemicals in order to increase the whiteness of the pulp 10 in order to meet the requirements of the fiber raw material for the manufacture of certain paper and board grades or other fiber products.

Bleached pulp can be produced from wood raw material in many ways, 15 but most often involves mechanical treatment. Mechanical pulps, such as grinder and pulp, contain lignin, the color of which is mainly removed by bleaching. However, the invention is not limited to bleaching mechanical pulps, but can also be used for bleaching purely chemically obtained abrasive pulps.

The bleaching process to which the invention relates is peroxide bleaching.

Peroxide bleaching, in particular for mechanical pulp processing, has the advantage of bleaching lignin-containing fiber pulps while maintaining .v 25 lignin at relatively light bleaching conditions (35-55 ° C), i.e.: good yield (GA Smook, Handbook for Pulp and Paper Technology). gists, 6th edition, TAPPI 1989; pp. 167-168). Peroxide bleaching is the process of bleaching with a substance which produces perhydroxyl ions. Such- ···. these substances are hydrogen peroxide, but other peroxy compounds can also. 30 such as sodium peroxide and sodium percarbonate.

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A common problem in pulp bleaching is adjusting the process to achieve the desired brightness. The bleaching result is determined by measurements on a sample of pulp to determine the correct dosage of the bleaching chemical.

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For example, European Patent 287626 discloses such an adjustment method based on the measurement of the brightness of the pulp, in which two-stage bleaching is utilized, whereby the brightness of the pulp from the first stage is used to control the second stage.

It is known to use high pulp consistency (about 30-40%) in peroxide-5 bleaching. Various peroxide bleaching processes at high consistency have been described e.g. U.S. Patent Nos. 4,938,842 and 5,525,195.

Bleaching of pulp at a relatively low consistency (8-18%) and at a temperature above 100 ° C with hydrogen peroxide is known for chemical pulp 10 as used in WO-96/01920. This system requires maintaining pressure in the bleaching process.

One known control and control system for continuous peroxide bleaching of mechanical pulp (groundwood and pulp) is based on the measurement of pulp brightness and residual peroxide in pulp after a 10-minute delay (GV Lippert Pulp & Paper Canada 94 (1993) 4: 40-44). ) from a sample taken before the bleaching tower. The peroxide dose of bleaching is controlled based on this information. Other chemicals are related to the peroxide dosage by a specific mathematical formula. This control system, although installed in all bleaching plants, is not generally used. Peroxide dosing is currently controlled manually based on the measurement of the brightness after the bleaching tower and / or after the 10 minute delay.

It is an object of the invention to provide a new way of monitoring or adjusting the pulp bleaching process, which, for example, combined with bleaching: ***: oral or bleaching pulp, can be used to better monitor the bleaching process. Purpose of the Invention. there is also a method that provides real-time information about the process state of the process, and where necessary, provides feedback control.

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It is also an object of the invention to provide a method whereby the efficiency of bleaching can be maximized by adjusting and controlling the effect of bleaching performance, particularly in mechanical peroxide bleaching (grinding, pulp). 35 amount of perhydroxyl ions. The invention can also be applied! ···! other at least partly mechanical pulps, such as chemimechanical ♦ · '·' pulps.

In order to achieve these objects, the method according to the invention is essentially characterized in what is set forth in the characterizing part of the appended claim 1. It has been found that the concentration of oxygen from the bleaching process, which can be measured, for example, from the space immediately associated with process 5 (the process area where bleaching chemicals and pulp are mixed together), correlates with the variables in the bleaching process. As a result, by measuring the oxygen content, information on the state of the bleaching process can be obtained continuously or at intervals desired. Oxygen concentration measurement is performed directly from 10 processes, either from the space (air volume) directly connected to the process area or from the process area itself (liquid volume), and can thus be combined with process automation to control the bleaching process.

Based on the oxygen concentration measurement, the alkali / peroxide ratio of the process area is preferably adjusted.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 schematically illustrates the bleaching process and its adjustment according to the invention, and Figures 2-9 illustrate the effect of process changes on oxygen content.

*, v 25 Figure 1 shows a continuous bleaching tower T, with a pulp to be bleached (arrow M) and bleaching chemicals (arrow C) at the top. The diagram is not intended to be a detailed description of the bleaching system, and, for example, a mixture of pulp and bleaching chemicals. before the bleaching tower is not shown. The purpose of the diagram is to further illustrate the principle of adjustment. From the lower *** part of the bleaching tower (T), the bleached pulp (arrow B) is taken out by arrangements known in the art and therefore not further described. The bleached pulp is subjected to further treatment.

»» • · 1; 35 The purpose of bleaching is to increase the whiteness of the pulp. Maximum Brightness • 1 1 · .1 ··. is obtained when in peroxide bleaching the peroxide forms perhydroxyl ions according to the reaction below (for example, hydrogen peroxide): 1 · 1 · 1 · 1 · 117562 4 H2O2 + OH- e * H2O + OOH

The amount of perhydroxyl ions is increased using alkali, virtually NaOH solution. The alkali / peroxide ratio can control the amount of perhydroxyl-5 ions. However, in peroxide bleaching, the peroxide decomposition can take place according to the reaction below: H 2 O 2 h 2 H 2 O + 02 10 The reaction is very exothermic and occurs readily in the presence of catalysts, e.g. Decomposition of peroxide reduces bleaching efficiency and increases the consumption of bleaching chemicals.

Peroxide bleaching is usually carried out at a high consistency of 30-40% in order to react as effectively as possible with the colored lignin groups of the mechanical pulp. The bleaching reactor used is a thick pulp bleaching tower T, for which a certain delay is maintained.

20 At the top of the bleaching tower T, the process area (intermixed

pulp and bleaching chemicals) above a certain volume has a specific oxygen content, measured by an oxygen sensor S. The data from the oxygen sensor is processed in the data processing unit U and used to adjust the bleaching chemicals C to the bleaching tower T

• · 25 the amount or ratio of alkali and / or peroxide in the flow.

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Based on the oxygen concentration at the top of the bleaching tower T, the alkali / peroxide ratio of the bleaching solution is controlled either by changing the alkali; or the amount of peroxide in the feed of bleaching chemicals C by controlling these. * ··. 30 substance dispensing devices. The oxygen content in the space above the bleached pulp clearly indicates the current efficiency of bleaching. The oxygen concentration can be measured from the gas volume at the top of the bleaching tower or from the stink gas flow from the tower.

• · * * · i 35 If there is too much alkali, the peroxides are more degraded to oxygen and water. The set oxygen concentration can be set, for example, • to a maximum value. If the oxygen concentration exceeds this maximum * * · maximum, the alkali / peroxide ratio will be adjusted. If the · · * · 5 117562 amount of peroxide supplied is to be kept at a constant value such as the final brightness, the amount of alkali is adjusted.

By maximizing the peroxide bleaching data from the oxygen content measurement 5 by perhydroxyl ions and / or by minimizing O 2

the number of wedges can be improved by the bleaching efficiency of the mechanical pulp

six and ensure successful bleaching even in the event of disturbances. The method also allows more effective control of bleaching, for example, if changes in the metal content of the pulp suspension occur because the metals catalyze the decomposition of the peroxide to oxygen. The undesirable peroxide decomposition reaction is manifested in an increased oxygen content in the oxygen concentration measured at the top of the bleaching tower or in the oxygen concentration measured from the effluent gas stream therefrom. Monitoring can be arranged without automatic feedback control, e.g., such that an unexpected increase in oxygen concentration (e.g., above a certain threshold value) will automatically trigger an alarm to indicate that there is a malfunction in the bleaching process, and action may be taken to remedy it.

Experiments relating to the invention have found that the change in the alkali / peroxide ratio affects the amount of oxygen in the bleaching gases of the bleaching tower and is illustrated in Figures 2-6. The concentrations of gases at the top of the bleaching tower correspond to the concentrations of the gases in the air if oxygen is not secreted from the process, i.e. the oxygen concentration caused by the bleaching process is the difference between the measured oxygen content and normal oxygen concentration. According to the results, raising the alkaline ratio increased the amount of fhape and vice versa. Figure 2 shows the effect of raising the alkaline ratio ·: ··. * From 0.74 to 0.87, and Figure 3 has the effect of increasing the ratio · · · ·. from 0.87 to 1.00. In Figure 4, the ratio is reduced from 1.0. "·. 30 to 0.7. In these experiments, the amount of hydrogen peroxide was 3%. Figures 5 and 6 show the increments of 0.62-0.73 and 0.73-0, 85 peroxide dose 4%.

... The results also show that alteration of the peroxide dose had an effect on the * · »** amount of oxygen illustrated in Figure 7 (dose reduction from 4% to 3% reduced oxygen concentration). Figure 8 shows •: *; 35 illustrates that increasing the amount of dilution water (increasing the purity of impurity * * · *. ** ·.) Increases the amount of oxygen in the bleaching tower [*. *, And Figure 9 illustrates increasing the bleaching consistency • · · * · * ' decrease) the opposite effect on the oxygen content of the bleaching tower carbon monoxide. Figure 8: The decrease in oxygen concentration after the 'peak' is due to the decrease in the bleaching reaction temperature after the change.

The invention can also be applied to lower bleaching processes using peroxide. In these densities, where the process area is high in water, oxygen could also be measured directly from the water, since the amount of dissolved oxygen behaves similarly to changes in the bleaching process as the amount of oxygen in the air.

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Claims (10)

1. A process of bleaching pulp, in which cellulose-containing fiber pulp is bleached by contacting it with peroxide in a process area and the bleaching process is controlled by means of measurement results from the process, characterized in that an oxygen content of a space of the process area or space associated with the process area is measured, which space comprises oxygen from the bleaching process, and the process is followed by results from this measurement. 10 Process according to claim 1, characterized in that the oxygen content is measured from gas volume in a container containing the process area, such as the upper part of a bleaching tower, or from a flow coming from a container, such as the exhaust gas flow of a bleaching tower. 15 Process according to claim 1 or 2, characterized in that the process is controlled by the measurement results, such as an alkali / peroxide relation of the process area. 20 4. Process according to claim 3, characterized in that one of the amount of alkali and peroxide is poured constantly and the other is changed in the control of. relationship. • · · * · · • • • »* 1 1:; i ,: Process according to claim 3 or 4, characterized in that the oxygen content is controlled with a measurement that is carried out at intervals or continuously, and the process is automatically controlled on the basis of * · ·. results. • · · • · • * Process according to any of the preceding claims, characterized in that a set value is determined for the oxygen content, and with • · ♦ · .1 · ♦. With this in mind, an in-line government of the process is implemented. 7. Method according to claim 6, characterized in that the set value of the oxygen content is a certain maximum value, and the control is carried out when this: maximum value is exceeded. • · · · · 1 · * · «117562 Process according to any of the preceding claims, characterized in that the process is a thick pulp coating, wherein the consistency of the pulp at the reaction range is at least 25%, preferably at least 30%. 5 9. Pulp bleaching plant comprising a container (T), to which is connected pulp feed (M) and peroxide containing bleaching chemicals (C) and removal of the bleached pulp (B), characterized in that the plant comprises an oxygen sensor (S). ), which is arranged to measure the oxygen content from the contents of the container (T), as from its gas volume, or from material flow coming from the container, such as gas flow. Plant according to claim 9, characterized in that the oxygen sensor 15 (S) is connected to a data processing unit (U), which is connected to a regulator to control the process automatically. • · • · · '' • ♦ * · • 1 ♦ • · · ····; ' • · · • · · · · 1 · •••• A · • · ··· '• · • V •• I • · · • · • · ··· • · • · · · · · · ^ · · · T »! • · · · · · · · · · · 1 · · · 1 · · · · · · · · · · · · · ···