FI118430B - Peroxide bleaching of pulp, for the production of paper and fibrous products, involves measuring the oxygen concentration, temperature and/or carbon monoxide concentration in reaction area, based on which process is monitored - Google Patents

Abstract

The method involves contacting a cellulose based fibrous pulp (M) with peroxide (C) in a container (T). The oxygen concentration, temperature and/or carbon monoxide concentration in the reaction area is/are measured, and the process is monitored based on the result. The concentrations of oxygen and carbon monoxide are measured by measuring devices (A, V) and temperature is measured by sensor (S), from the gas volume in the upper part of bleaching container or from the vent gas flowing from the bleaching tower. The consistency of pulp in the reaction area is at least 25% preferably at least 30%. An independent claim is also included for the pulp bleaching device for.

Description

118430

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 pure chemical 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-rich abrasive materials while maintaining: V: 25 lignin at relatively light bleaching conditions (35-55 ° C), i.e.: 1 · 1: good yield (GA Smook, Handbook for Pulp and Paper Technol., ···. Gists, 6th ed., TAPPI 1989; pp. 167-168). Peroxide bleaching is the process of bleaching with a substance which forms perhydroxyl ions. Such substances are hydrogen peroxide, but other peroxy compounds may also be present, such as sodium peroxide and sodium percarbonate.

m '1 A common problem with pulp bleaching is adjusting the process to achieve the desired brightness. The bleaching result is determined by measurements on a pulp sample that allows the bleaching chem .1 ··! The dosage of 35 cabbages is correct.

• · M1 * • · • «• · · ♦» • · · »· ·» 118430 2

For example, European Patent 287626 discloses such a method of controlling the brightness of a pulp, which utilizes two-stage bleaching, whereby the brightness of the pulp from the first stage is used to control the second stage.

5

It is known to use high pulp consistency (about 30-40%) in peroxide 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 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 (ground 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.

20 Peroxide bleaching dose control is 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 manually adjusted based on the measurement of post-bleaching tower lightness and / or: V: 25 after a 10 minute delay.

The purpose of the invention is to provide a novel way to monitor or control the pulp bleaching process, e.g., combined with sampling bleaching. from the oral or bleached pulp can be used to * * * '* 30 better follow the bleaching process. It is also an object of the invention to provide a method which provides real-time information on the state of the process and which, when necessary, provides feedback control.

* · • «·

It is also an object of the invention to provide a method, in particular, a mechanical • ♦. · **. The peroxide bleaching of a 35% pulp (grit, pulp) can be maximized by "*" controlling the amount of peroxide ions that affect the bleaching result. The invention can also be applied to other at least partly mechanical pulps, such as chemimechanical pulps.

To accomplish these objects, the method 5 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 temperature and / or carbon monoxide concentration of the bleaching process, which can be measured, for example, from the state immediately associated with the process (the process area where bleaching chemicals and pulp are mixed), correlate with the variables of the bleaching process. As a result, by measuring the temperature and / or the carbon monoxide content, information on the state of the bleaching process can be obtained continuously or at certain intervals. The measurement of temperature and / or carbon monoxide concentration takes place directly from the process, either from the space directly connected to the process area (gas-15 volume) or, in the case of temperature, from the process area itself (liquid volume) and can be combined with process automation to control the bleaching process. Concentrations from the space adjacent to the process area can be measured by introducing gas from this volume into the measuring device. The temperature and / or carbon monoxide content of the carbon monoxide gas from the bleaching process 20, for example, can be measured. The temperature can also be measured directly from the gas volume with a temperature sensor.

Based on the measurement of temperature and / or carbon monoxide concentration, the alkali / peroxide ratio of the process area is preferably adjusted.

• *: * · *: The invention will be described in more detail below with reference to the attached silicon. cartilage with · * «•». ···. 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 carbon monoxide) temperature and carbon monoxide concentration.

• · * »**

Figure 1 shows a continuous bleaching tower T with »». ···. 35 bleached pulp (arrow M) and bleaching chemicals (arrow [** C) are conducted. The diagram is not intended to be a detailed description of the bleaching apparatus, and, for example, the mixing of pulp and bleaching chemicals prior to the bleaching tower is not shown. The purpose of the diagram is to illustrate the principle of adjustment. From the bottom of the bleaching tower (T), bleached pulp (arrow B) is withdrawn by arrangements known in the art and therefore not described in more detail. The mass of the Val-5 band is led to further processing.

The purpose of bleaching is to increase the lightness of the pulp. Maximum brightness is obtained when in peroxide bleaching, the peroxide forms perhydroxyl ions according to the reaction below (for example, hydrogen peroxide): 10 H2O2 + OH '<-> H2O + OOH *

The amount of perhydroxyl ions is increased using alkali, virtually NaOH solution. The alkali / peroxide ratio can control the amount of perhydroxyl-15 ions. However, in peroxide bleaching, the peroxide decomposition can take place according to the following reaction: H 2 O 2 h 2 H 2 O + 2 O 20 The reaction is very exothermic and occurs readily in the presence of catalysts, e.g. Degradation of peroxide reduces bleaching efficiency and increases the consumption of bleaching chemicals.

Peroxide bleaching is usually carried out at a high consistency • · · * · *: 30-40% to allow the peroxide to react as efficiently as possible mechanically. with its colored lignin groups in the mass. A thick pulp bleaching tower T is used as the bleaching reactor, for which a certain delay is maintained.

In the upper part of the bleaching tower T, above a process area (intermixed pulp and bleaching chemicals), there is, within a certain volume, a certain temperature measured by the temperature sensor S and the gas mixture contained therein has a certain concentration of carbon monoxide carbon monoxide probe V (temperature and carbon monoxide concentration increased by · ·. **. 35 correlate with the oxygen content of the above space). The data obtained from the temperature sensor S and the carbon. ** · * limonoxide sensor V is processed by the data processing unit U and is used to control the effect of alkali and / or peroxide on the flow of bleaching chemicals C to the bleaching tower T. or their ratio.

The dashed lines illustrate the possibility of introducing from the top of the bleaching tower 5 a flow of gas having a composition corresponding to the volume of the top to measure the concentration of the gas components in a measuring device A, which may be a gas analyzer. In addition to carbon monoxide, this gauge can also analyze other gas components such as oxygen and carbon dioxide in the top of the bleaching tower.

10

Based on the temperature and / or carbon monoxide concentration measured at the top of the bleaching tower T, the alkali / peroxide ratio of the bleaching solution is controlled by varying the amount of either alkali or peroxide in the bleaching chemicals C supply by controlling the metering devices. The temperature and / or carbon monoxide concentration in the space above the pulp to be bleached clearly indicates the current efficiency of bleaching. The temperature and / or carbon monoxide concentration can be measured from the gas volume at the top of the bleaching tower or from the flow of carbon monoxide from the tower. It is advantageous for the measuring technique to measure at least the carbon monoxide concentration from the flow which is discharged from the gas volume, e.g., from the flow of carbon monoxide gas. In this case, the measuring device used acts as a kind of gas analyzer, and can be used to measure the concentrations of other gases as well; methods. The temperature can be measured from the same flow. However, the temperature can be measured as close to the process as possible, i.e., .ll directly from the gas volume with the sensor S placed therein.

• l

If too much alkali is present, the peroxides are more degraded to oxygen and water, and the bleaching tower has a high temperature of carbon monoxide and carbon monoxide. The measured temperature and / or carbon monoxide -!: '·· 30 dip content can be set to a value eg eg a maximum value.

If the temperature and / or carbon monoxide concentration rises above this maximum, the alkali / peroxide ratio is adjusted. If you fed peroxy-! it is desired to keep the amount of din at a constant value, e.g.

«« «35 *: · *! By maximizing the peroxide bleaching data from temperature and / or carbon monoxide measurement data, the peroxide 118430 6 and / or minimizing the amount of O 2 -mole molecules generated in the peroxide decomposition reaction can improve mechanical pulp bleaching efficiency and also ensure bleaching success in disturbances. The method also allows more effective control of bleaching, e.g.

5 if changes in the metal content of the pulp suspension occur because the metals catalyze the decomposition of the peroxide to oxygen. The undesired peroxide decomposition reaction is reflected in an elevated value measured at the top of the bleaching tower and the temperature and carbon monoxide concentration measured from the flow of carbon monoxide from the bleaching tower. The Val-10 control can be arranged without automatic feedback control, for example, so that such an unexpected rise in temperature and / or carbon monoxide concentration (e.g., above a certain threshold) will automatically trigger an alarm to indicate that there is a malfunction in the bleaching process. to fix it.

15

Changes in the alkali / peroxide ratio in the experiments relating to the invention have an effect on the temperature and carbon monoxide concentration of the bleaching tower carbon monoxide, and this is illustrated in Figures 2-5. The carbon monoxide concentration was measured with a gas analyzer coupled to the gas stream by IR. According to the results, raising the alkaline ratio increased the temperature and carbon monoxide concentration and vice versa. Figure 2 shows the effect of increasing the alkaline ratio from 0.55 to 0.9 on the temperature measured on the carbon monoxide stream on a horizontal time scale, and Figure 3 has the same effect on the carbon monoxide concentration in the carbon monoxide stream. The moment of change • V: 25 is indicated by a vertical line. Figures 4 and 5 show the effects on the same quantities when the alkaline ratio was lowered from 0.7 to 0.55. Referring to Figures 2-5, ···. in these experiments the amount of hydrogen peroxide was 3%. Figure 6 shows. !!!: raising the peroxide dose from 3% to 4% at a constant alkaline ratio of 0.62, which created only a slight increase in the temperature of the gas at the top of the tower (en- **** ' vertical line). Figure 6 also shows the alkaline ratio. increase from 0.62 to 0.7 at a dose of 4% peroxide (second vertical line), * | **: which is clearly evident as the temperature rises. Figure 7 shows the pre- ..! · .. · the effect of certain changes made in the same order on the carbon monoxide content. It can be seen that the carbon monoxide content ·, ···. 35 changes as well as the temperature in Figure 6. Figure 8 shows a reduction in the peroxide dose from 4% to 3% and a change in the ί * 'alkaline ratio from 0.62 to 0.7, causing an increase in the alkaline ratio. 118430 7 m the increase in temperature is greater than the decrease in temperature caused by the reduction of the peroxide dose. Figure 9 shows the change in carbon monoxide concentration as the peroxide dose is reduced from 4% to 3% while the alkaline ratio is changed from 0.62 to 0.7. It is observed that the effect of lowering the peroxide dose on the carbon monoxide concentration is greater than the effect of increasing the carbon monoxide concentration on a higher alkaline ratio.

Both the temperature measurement and carbon monoxide measurement, or just one of them, can be used in the invention. By using both process measurements of the process, more measurement data is available to monitor the state of the bleaching process, for example, Figures 8 and 9 show that temperature and carbon monoxide content change differently when two process inputs (alkaline ratio and peroxide dose) are changed 15 simultaneously.

The invention can also be applied to lower bleaching processes using peroxide. In these thickeners, where the process area is high in water, the temperature 20 could also be measured directly from the water, since the change in temperature caused by peroxide decomposition behaves similarly to changes in the air bleaching process.

In addition, it is possible in the invention to measure the oxygen content "· *: 25 of the gas volume at the top of the bleaching tower, from the bleaching tower. ·· *. From the gas stream or, at lower consistency, also directly from water in the process area such as at the time of filing this application, · · · * in the secret Finnish patent application '*' * 20001058. The oxygen content can be measured, for example, with the same measuring apparatus A (gas analyzer) as the carbon monoxide concentration.

I., "Oxygen content data can be combined with temperature and / or CO content measurement data from the same process * to provide more information on the process status.

The measurement principles for the above variables may be applied using known measurement principles. Numerous temperature sensors are known. The carbon monoxide sensor may use the known principles of carbon monoxide detectors. In a gas analyzer connected to a gas flow out of the gas volume of the container, known principles from gas analyzers can be applied to determine the concentrations of various gases.

5 • · 1 • • • • • • • 1 ♦ ♦ 1 * · • • • • 1 1 »» • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1 1 »» · · · · · · · · · · · · · · · · · · · · · · · · · · ··· ·

Claims (15)

118430 A process for bleaching pulp, wherein the cellulosic fibrous fiber 5 is bleached by contacting it with the peroxide in the process area and controlling the bleaching process by measuring the results obtained from the process, characterized in that the gas in the container containing the process area measuring the temperature and / or carbon monoxide content of the bleaching tower carbon monoxide flow and monitoring the results obtained from this measurement. Method according to Claim 1, characterized in that the temperature is measured directly from the gas volume by means of a sensor 15 (S) placed therein and the carbon monoxide content is measured from an outflow such as a flow of carbon monoxide gas. . Method according to claim 1 or 2, characterized in that: ;;; that the measurement results are used to control a process such as process area; Y 20 alkali / peroxide ratio. • · • · • * • · · Method according to Claim 3, characterized in that one of the al-* · "·: amounts of potassium and peroxide is kept constant and the other is altered in the adjustment of: ***: ··· 25 A method according to claim 3 or 4, characterized by: ···! that the temperature and / or carbon monoxide content is monitored periodically or continuously, and the process is automatically adjusted based on the results. : 30 A method according to any one of the preceding claims, characterized in that a setpoint is set for the temperature and / or the carbon monoxide content, and thereby provides a process feedback control. 35 118430 Method according to Claim 6, characterized in that the setpoint for the temperature and / or the carbon monoxide content is a fixed maximum value, and the adjustment is performed when this maximum value is exceeded. A process according to any one of the preceding claims, characterized in that the process is a thick mass bleaching process wherein the pulp has a consistency in the reaction area of at least 25%, preferably at least 30%. A pulp bleaching apparatus having a reservoir (T) having a supply of pulp (M) and bleaching chemicals (C) containing 10 peroxide and removing bleached pulp (B), characterized in that the apparatus comprises a temperature measuring device and / or a carbon monoxide measuring device which: is arranged to measure temperature and / or carbon monoxide concentration, respectively, from the gas volume of the tank (T) or the gas flow from the tank. 15 Apparatus according to Claim 9, characterized in that the carbon monoxide measuring device is a gas stream flowing out of the tank (T). such as a measuring device (A) associated with a flow of carbon monoxide, such as ·. ··; a gas analyzer. : .v 20 Apparatus according to claim 9 or 10, characterized in that it also has a measuring device arranged to measure the concentration of the gas component other than carbon monoxide from the gas volume of the tank (T) or from the gas flow from the tank, in particular 25 oxygen concentration. »· • · f Apparatus according to claim 11, characterized in that a measuring device connected to a gas flow such as a gas gas flow: V (A), such as a gas analyzer, is arranged to measure at least one other gas component in addition to carbon monoxide: oxygen. * · *! ···. Apparatus according to one of the preceding claims 9 to 12, characterized in that the temperature measuring device and / or the carbon monoxide measuring device is connected to a data processing unit (U) connected to 35 controllers for automatic control of the process. 118430 Apparatus according to one of the preceding claims 9 to 13, characterized in that the apparatus comprises both a temperature measuring device and a carbon monoxide measuring device. Apparatus according to one of the preceding claims 9 to 14, characterized in that the temperature measuring device is a temperature sensor (S) located in the gas volume of the tank (T). t 9 · 1 2 3 ·· • 1 * · · • • • • · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·· • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • f «I • · * ♦ · & t 1« · • 1 ·· · · M 2 • · 3 118430