Method in the bleaching of pulp and a device for the bleaching of pulp

Abstract

Bleaching device for pulp includes a container to which a supply of pulp and bleaching chemicals containing peroxide as well as discharge of bleached pulp are connected. The device comprises an oxygen measuring device a temperature measuring device and/or a carbon monoxide measuring device, which are arranged to measure oxygen concentration, temperature and/or carbon monoxide concentration, respectively, from the contents of the container such as from the gas volume of the same, or from a substance flow coming from the container, such as gas flow. The measuring device is connected to a data processing unit which is coupled to a controller for automatic control of the process. The process is especially a high consistency pulp bleaching of mechanical pulp, in which the consistency of pulp in the reaction area is at least 25%, advantageously at least 30%.

Description

[0001] The invention relates to a method in the bleaching of pulp which is of the type presented in the preamble of the appended claim 1. The invention also relates to a device for the bleaching of pulp which is of the type presented in the preamble of the appended claim 17.

[0002] Plant-based, typically wood-based pulp containing cellulose fibres is bleached with chemicals with the aim of increasing the brightness of the pulp, so that the pulp would fulfil the requirements set for fibrous raw material in the production of certain paper and paperboard grades and other fibrous products.

[0003] The pulp to be bleached may be produced of wood raw material in many ways but typically the production involves a mechanical treatment. Mechanical pulps, of which groundwood and refiner groundwood can be mentioned as examples, contain lignin that produces the colour which is primarily removed in bleaching. The invention is not, however, restricted solely to the bleaching of mechanical pulps, but it can also be used in the bleaching of fibrous pulps which are produced purely in chemical way.

[0004] The bleaching method to which the invention relates is peroxide bleaching. Especially in the treatment of mechanical pulps peroxide bleaching has the advantage that it bleaches the ligneous fibrous pulps at the same time preserving the lignin, if the bleaching conditions are relatively mild (35 to 55°), i.e. the yield is good (G. A. Smook, Handbook for Pulp and Paper Technologists, TAPPI 1989; pp. 167-168). The concept of peroxide bleaching refers to bleaching with an agent that produces perhydroxyl ions. Such an agent is hydrogen peroxide, but other peroxide compounds are also possible, such as sodium peroxide and sodium percarbonate.

[0005] In the bleaching of pulp a general problem is the control of the process in such a manner that the desired brightness is attained. The bleaching result is determined by means of measurements conducted on the pulp sample, whereby the accurate dosage of the bleaching chemical is attained.

[0006] For example the European patent 287626 discloses such an control method based on the measurement of the brightness of the pulp, in which two-stage bleaching is utilized in such a manner that the brightness of the pulp obtained at the first stage is utilized for the control at the second stage.

[0007] It is known method to use high consistency of pulp (approximately 30 to 40%) in peroxide bleaching. Different high consistency peroxide bleaching processes are described e.g. in U.S. Pat. Nos. 4,938,842 and 5,525,195.

[0008] The bleaching of pulp with hydrogen peroxide at a relatively low consistency (8 to 18%) and at a temperature of over 100° C. that is used for chemical pulp is known from the publication WO-96/01920. This system requires that pressure is maintained in the bleaching.

[0009] A known control and adjustment system that is used in continuous peroxide bleaching of mechanical pulp (groundwood and refiner mechanical pulp) is based on measuring the brightness of the pulp and the residual peroxide contained in the pulp after a delay of 10 minutes (G. V. Lippert Pulp & Paper Canada 94 (1993), 4, p. 40 to 44) from a sample taken before the bleaching tower. The peroxide dose in bleaching is controlled on the basis of this information. Other chemicals are proportioned to the peroxide dose by means of a fixed mathematical formula. This control system, although it is installed in all bleaching plants, is not normally used. At present, the peroxide dose is adjusted manually on the basis of the measurement of brightness after the bleaching tower and/or after a delay of 10 minutes.

[0010] The publication WO 99/53301 discloses a sensor by means of which it is possible to measure hydrogen peroxide concentrations in a sample taken from the bleaching process. The sensor contains an enzyme that changes the hydrogen peroxide into water and oxygen that can be measured. At the same time the oxygen concentration (background oxygen) of the sample is measured to eliminate the measurement error due to this oxygen concentration.

[0011] One purpose of the invention is to introduce a new way of monitoring or controlling the bleaching process of pulp, which, when combined e.g. to sampling of pulp that is entering the bleaching process or has undergone bleaching, can be utilized to better monitor the bleaching process. Another purpose of the invention is a method by means of which it is possible to obtain real-time information on the state of the process and by means of which it is possible to implement feedback control, if necessary.

[0012] Yet another purpose of the invention is also a method in which it is possible to maximise the efficiency of bleaching especially in the peroxide bleaching of mechanical pulp (groundwood, refiner mechanical pulp) by adjusting and controlling the amount of perhydroxyl ions affecting the bleaching result. The invention can also be applied to other at least partly mechanical pulps, such as chemimechanical pulps.

[0013] To attain these purposes, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the appended claim 1. It has been observed that the concentration of oxygen coming from the bleaching process, the temperature of the bleaching process and/or the carbon monoxide concentration of the bleaching process that can be measured for example from a space which is in immediate contact with the process (process area where the bleaching chemicals and pulp are mixed to each other), correlates with the variables of the bleaching process. As a result of this it is possible to obtain information on the state of the bleaching process continuously or at fixed intervals, if desired, by measuring the oxygen concentration, temperature and/or carbon monoxide concentration. The oxygen concentration is measured directly from the process, either from a space (air volume) which is in immediate communication with the process area or in the process area itself (liquid volume). The temperature and/or the oxygen concentration measurement takes place directly from the process, either from a space (gas volume) which is in immediate communication with the process area, or in the case of temperature also from the process area itself (liquid volume). The measurement can be connected to the process automation to control the bleaching process. The concentrations in the space connected to the process area can be measured by conveying gas from this space to the measurement device. It is possible to measure the oxygen concentration, temperature and/or carbon monoxide concentration of the vent gas coming from the bleaching process, for example. The temperature can also be measured directly from the gas volume by means of a temperature sensor.

[0014] The measurement result of the oxygen concentration, temperature and/or the carbon monoxide concentration is advantageously utilized to adjust the alkali/peroxide ratio of the process area.

[0015] In the following, the invention will be described in more detail with reference to the appended drawings, in which

[0016] FIGS. 1a and 1b schematically illustrate the bleaching process and the control of the same according to the invention, and

[0017] FIGS. 2 to 17 illustrate the effect of the process changes on the oxygen concentration, temperature and carbon monoxide concentration of the vent gas.

[0018] FIG. 1a shows a continuously operating bleaching tower T to the upper part of which pulp to be bleached (arrow M) and bleaching chemicals (arrow C) are conveyed. The figure is not intended as a detailed illustration of the bleaching apparatus, and for example the act of mixing pulp and bleaching chemicals together before the bleaching tower is not shown. The purpose of the figure is primarily to illustrate the control principle. Bleached pulp (arrow B) is taken out from the lower part of the bleaching tower (T) by means of arrangements, which are known in the field and which are therefore not described in more detail. The bleached pulp is conveyed to further processing.

[0019] In bleaching, the aim is to increase the brightness of the pulp. Maximum brightness is attained when peroxide forms perhydroxyl ions in peroxide bleaching according to the following reaction (hydrogen peroxide as an example):

H2O2+OH—→H2O+OOH—

[0020] The amount of perhydroxyl ions is increased by using alkali, in practice NaOH solution. The alkali/peroxide ratio can be utilized for controlling the amount of perhydroxyl ions. However, in peroxide bleaching peroxide may be decomposed according to the following reaction:

H2O2→2H2O+O2

[0021] The reaction is very exothermic, and it takes place easily when catalysts are present, for example metals present in the pulp catalyse the decomposition reaction. Peroxide decomposition reduces the bleaching efficiency and increases the consumption of bleaching chemicals.

[0022] The peroxide bleaching is most often conducted at high consistency 30 to 40%, so that peroxide reacts with the coloured lignin groups of the mechanical pulp as efficiently as possible. As a bleaching reactor a high consistency pulp bleaching tower T is used, in which a certain residence is maintained.

[0023] In the upper part of the bleaching tower T, above the process area (the pulp and bleaching chemicals mixed with each other), in a volume of a given size, there prevails a fixed oxygen concentration which is measured with an oxygen sensor X. The information received from the oxygen sensor is processed in a data processing unit U and on the basis of the information the amount of alkali and/or peroxide or the ratio of the same is adjusted in the flow of bleaching chemicals C entering the bleaching tower T.

[0024] On the basis of the oxygen concentration measured from the upper part of the bleaching tower T the alkali/peroxide ratio of the bleaching solution is adjusted either by changing the amount of alkali or peroxide in the supply of bleaching chemicals C by controlling the devices dosing these agents. The oxygen concentration in the space above the pulp to be bleached clearly indicates the efficiency of bleaching at that time. The oxygen concentration can be measured from the gas volume in the upper part of the bleaching tower or from the vent gas flow coming out of the tower.

[0025] If the amount of alkali is too high, the decomposition of peroxide into oxygen and water is stronger. For example a maximum value as a set value can be given to the measured oxygen concentration. If the oxygen concentration exceeds this maximum value, adjustment of the alkali and peroxide ratio begins. If the aim is to keep the amount of supplied peroxide in the set value determined by the final brightness, the amount of alkali is adjusted.

[0026] By maximizing the amount of perhydroxyl ions by means of information received from the measurement of oxygen concentration in the peroxide bleaching and/or by minimizing the amount of O2 molecules, it is possible to improve the bleaching efficiency of mechanical pulp and ensure a successful bleaching also in disturbance situations. The method also enables a more efficient control of the bleaching process, e.g. if changes occur in the metal concentration of the pulp suspension, because metals catalyze the decomposition of peroxide into oxygen.

[0027] Undesired decomposition reaction of peroxide is shown in the oxygen concentration measured from the upper part of the bleaching tower or in the oxygen concentration measured from the vent gas coming therefrom, as an increased amount of oxygen. The monitoring can be arranged without an automatic feedback control for example in such a manner that such unexpected increase in the oxygen concentration (e.g. above a fixed threshold value) automatically gives an alarm, indicating that a disturbance has occurred in the bleaching process, wherein it is possible to take action to amend the situation.

[0028] In the tests relating to the invention it has been observed that a change in the alkali/peroxide ratio affects the amount of oxygen in the vent gases of the bleaching tower, and this is illustrated in FIGS. 2 to 6. In the upper part of the bleaching tower, the concentrations of gases correspond to the concentrations of gases in the air if oxygen is not emitted from the process, i.e. the oxygen concentration produced by the bleaching process is the difference between the measured oxygen concentration and the normal oxygen concentration. According to the results, an increase in the alkali ratio increased the amount of oxygen and vice versa. FIG. 2 shows the effect of the increase in the alkali ratio from 0.74 to 0.87, and FIG. 3 the effect when the ratio was increased from 0.87 to 1.00. In FIG. 4 the ratio is reduced from 1.0 to 0.7. In these tests the amount of hydrogen peroxide was 3%. FIGS. 5 and 6 show the increases from 0.62 to 0.73 and from 0.73 to 0.85 with a hydrogen peroxide dose of 4%. The results also shown that a change in the peroxide dose affected the amount of oxygen, which is illustrated in FIG. 7 (reduction of the dose from 4% to 3% decreased the oxygen concentration). FIG. 8 illustrates the fact that an increase in the amount of dilution water (increase in the amount of impurities) increases the amount of oxygen in the vent gases of the bleaching tower, and FIG. 9 illustrates the opposite effect of increasing the bleaching consistency (reduction in the amount of impurities) on the oxygen concentration of the vent gas of the bleaching tower. In FIG. 8, the reduction in the oxygen concentration after the “peak” results from the fall of temperature of the bleaching reaction after the change.

[0029] The invention can also be applied to bleaching processes using peroxide and taking place at lower consistencies. At such consistencies in which there is a large amount of water in the process area, it could be possible to measure oxygen directly from the water, because the amount of dissolved oxygen behaves in a similar way in the changes of the bleaching process as the amount of oxygen in the air.

[0030] In the following, the use of two other variables in the control of the bleaching process is described with reference to FIG. 1b and FIGS. 10 to 17.

[0031] In a volume of a given size in the upper part of the bleaching tower T and above the process area (the pulp and bleaching chemicals that are mixed with each other), there prevails a fixed temperature which is measured with a temperature sensor S, and the mixture of gases contained therein has a fixed carbon monoxide concentration which is measured with a carbon monoxide sensor V (the temperature and the carbon monoxide concentration correlate with the oxygen concentration of the aforementioned space). The information received from the temperature sensor S and the carbon monoxide sensor V is processed in a data processing unit U, and on the basis of the information the amount of alkali and/or peroxide or the ratio of the same is adjusted in the flow of bleaching chemicals C entering the bleaching tower T.

[0032] In FIG. 1 broken lines illustrate the possibility of conveying a gas flow from the upper part of the bleaching tower, the composition of the flow corresponding to the composition in the volume in the upper part, to measurement of the concentration of the gas components to a measuring device A, which can be a gas analyzer. In addition to carbon monoxide, in this measuring device it is possible to analyse other gas components of the upper part of the bleaching tower as well, such as oxygen and carbon dioxide.

[0033] On the basis of the temperature and/or carbon monoxide concentration measured from the upper part of the bleaching tower T the alkali/peroxide ratio of the bleaching solution is adjusted by changing the amount of either alkali or peroxide in the supply of bleaching chemicals C by controlling the devices dosing these agents. The temperature and/or carbon monoxide concentration in the space above the pulp to be bleached clearly indicates the efficiency of bleaching at that time. The temperature and/or the carbon monoxide concentration can be measured from the gas volume in the upper part of the bleaching tower or from the vent gas flow coming out of the tower. In view of the measuring technique, it is advantageous to measure at least the carbon monoxide concentration from the flow which is conveyed out of the gas volume, i.e. from the vent gas flow itself. Thus, the measuring device that is used operates as a sort of a gas analyzer, and by means of the same it is possible to measure concentrations of other gases as well by means of methods suitable for them. The temperature can be measured from the same flow. The temperature can, however, be measured as close to the process as possible, i.e. directly from the gas volume by means of a sensor T placed therein.

[0034] If the amount of alkali is too high, the decomposition of peroxide into oxygen and water is stronger and the temperature and the carbon monoxide concentration of the exhaust gas are high. For example a maximum value as a set value can be given to the measured temperature and/or carbon monoxide concentration. If the temperature and/or the carbon monoxide concentration exceeds this maximum value, adjustment of the ratio of alkali and peroxide begins. If the aim is to keep the amount of supplied peroxide in the set value determined by the final brightness, the amount of alkali is adjusted.

[0035] By maximizing the amount of perhydroxyl ions by means of information received from the measurement of temperature and/or carbon monoxide concentration in the peroxide bleaching and/or by minimizing the amount of O2 molecules, it is possible to improve the bleaching efficiency of mechanical pulp and ensure a successful bleaching also in disturbance situations. The method also enables a more efficient control of the bleaching process e.g. if changes occur in the metal concentration of the pulp suspension, because metals catalyze the decomposition of peroxide into oxygen. Undesired decomposition reaction of peroxide is shown as an increased value of the temperature and carbon monoxide concentration measured from the upper part of the bleaching tower or the temperature and carbon monoxide concentration measured from the vent gas flow coming therefrom. The monitoring can be arranged without an automatic feedback control for example in such a manner that such an unexpected rise in the temperature and/or carbon monoxide concentration (e.g. above a fixed threshold value) automatically gives an alarm, indicating that a disturbance has occurred in the bleaching process, wherein it is possible to take action to amend the situation.

[0036] In the tests relating to the invention, it has been observed that a change in the alkali/peroxide ratio affects the temperature and carbon monoxide concentration of the vent gases of the bleaching tower, and this is illustrated in FIGS. 10 to 13. The carbon monoxide concentration was measured with the IR method by means of a gas analyzer coupled to the gas flow. According to the results, an increase in the alkali ratio raised the temperature and increased the carbon monoxide concentration and vice versa. FIG. 10 shows the effect of the increase in the alkali concentration from 0.55 to 0.9 on the temperature measured from the vent gas flow on a horizontal time scale, and FIG. 11 shows the same effect on the carbon monoxide concentration of the vent gas flow. The moment of change is denoted with a vertical line. FIGS. 12 to 13 show the effects on the same variables when the alkali ratio was reduced from 0.7 to 0.55. In the tests shown in FIGS. 10 to 13 the amount of hydrogen peroxide was 3%. FIG. 14 illustrates the act of increasing the peroxide dose from 3% to 4% with a constant alkali ratio of 0.62, wherein the temperature of the gas in the upper part of the tower rose only slightly (the first vertical line) The same FIG. 14 also illustrates the act of increasing the alkali ratio from 0.62 to 0.7 at a peroxide dose of 4% (the second vertical line) which is clearly shown as a rise in the temperature. FIG. 15 shows how the changes shown in FIG. 6 and made in the same order affect the carbon monoxide concentration. It can be observed that the carbon monoxide concentration changes in a similar manner as the temperature in FIG. 6. FIG. 16 shows the reduction of the peroxide dose from 4% to 3% and the change in the alkali ratio from 0.62 to 0.7 conducted at the same time, wherein the rise of temperature caused by the increase in the alkali ratio is more significant than the fall of temperature caused by the reduction in the peroxide dose. FIG. 17 shows the change in the carbon monoxide concentration when the peroxide dose is reduced from 4% to 3%, and the alkali ratio is at the same time changed from 0.62 to 0.7. It is observed that the decreasing effect that the reduction of the peroxide dose has on the carbon monoxide concentration is more significant than the effect of the larger alkali ratio that increases the carbon monoxide concentration.

[0037] In the invention it is possible to use both the measurement of temperature and measurement of carbon monoxide or only one of them. When the measurements of both variables in the process are used, more measurement information is obtained, by means of which it is possible to monitor the state of the bleaching process, for example FIGS. 16 and 17 show that the temperature and the carbon monoxide concentration change in different ways when two input variables of the process (alkali ratio and peroxide dose) are changed at the same time.

[0038] The measurement of temperature and/or carbon monoxide according to the invention can also be applied to bleaching processes using peroxide and taking place at lower consistencies. At such consistencies in which there is a large amount of water in the process area, the temperature could also be measured directly from the water, because a change in the temperature caused by the decomposition of peroxide behaves in a similar way in the changes of the bleaching process as a change of temperature in the air.

[0039] The oxygen concentration, the use of which in the control of process is described above, can be measured e.g. with the same measuring device A (gas analyzer) as the carbon monoxide concentration. The oxygen concentration can also be measured from the gas volume in the upper part of the bleaching tower or in lower consistencies also directly from the water in the process area. The information obtained from the measurement of oxygen concentration can be combined with the temperature and/or CO concentration measurement information and thus it is possible to obtain more information on the state of the process.

[0040] In the measurement of the above-mentioned variables it is possible to apply measuring principles known as such. There are several known temperature sensors. In the carbon monoxide sensor it is possible to utilize principles known-from carbon monoxide alarm devices. In a gas analyzer connected to the gas flow coming out of the gas volume of the container it is possible to apply principles known from gas analyzers to determine concentrations of different gases.

Claims

1. A bleaching process for pulp comprising:

contacting cellulose-containing fibrous pulp with peroxide in a process area;

measuring oxygen concentration, temperature and/or carbon monoxide concentration in the process area; and

adjusting the bleaching process based on results from measuring oxygen concentration, temperature and/or carbon monoxide concentration.

2. The process according to claim 1, wherein that the oxygen concentration, temperature and/or carbon monoxide concentration is measured from a gas volume in a container containing the process area.

3. The process according to claim 2, wherein the oxygen concentration, temperature and/or carbon monoxide concentration is measured from an upper part of a bleaching tower in a process area

4. The process according to claim 2, wherein the oxygen concentration, temperature and/or carbon monoxide concentration is measured from a vent gas flow coming out of the bleaching tower.

5. The process according to claim 1 or 2, wherein the oxygen concentration is measured and the process is adjusted on the basis of the results obtained from this measurement.

6. The process according to claim 1 or 2, wherein the temperature and/or the carbon monoxide concentration is measured and the process is adjusted on the basis of the results obtained from this measurement.

7. The process according to claim 4, wherein the temperature is measured directly from the gas volume in the container containing the process area, from an upper part of the bleaching tower by means of a sensor placed therein, and the carbon monoxide concentration is measured from a vent gas flow coming out of the container.

8. The process according to claim 7, wherein the carbon monoxide concentration is measured from a vent gas flow from a bleaching sensor.

9. The process according to the results from measuring oxygen concentration, temperatuer and/or carbon monoxide concentration are utilized to control an alkali/peroxide ratio of the process area.

10. The process according to claim 9, wherein one of the amounts of alkali and peroxide is kept constant and the other is changed.

11. The process according to claim 1, wherein the oxygen concentration, temperature and/or the carbon monoxide concentration is monitored by means of a measurement conducted at intervals or continuously and the process is controlled automatically on the basis of the results.

12. The process according to claim 1, wherein a set value is defined for the oxygen concentration, and by means of this value a feedback control of the process is performed.

13. The process according to claim 11, wherein a set value of the oxygen concentration is a fixed maximum value, and the control is performed when this maximum value is exceeded.

14. The process according to claim 1, wherein a set value is defined for the temperature and/or carbon monoxide concentration, and by means of this value a feedback control of the process is performed.

15. The process according to claim 13, wherein that the set value of the temperature and/or the carbon monoxide concentration is a fixed maximum value, and the control is performed when this maximum value is exceeded.

16. The process according to claim 1, wherein the process is a high consistency pulp bleaching, in which the consistency of pulp in the reaction area is at least 25%.

17. The process according to claim 16, wherein the process is a high consistency pulp bleaching in which the consistency of pulp on the reaction area is at least 30%.

18. A bleaching device for pulp, comprising a container that includes a supply for pulp and bleaching chemicals containing peroxide, and a discharge for bleached pulp, wherein the bleaching device contains an oxygen measuring device, a temperature measuring device and/or a carbon monoxide measuring device, which is/are effective for measuring oxygen concentration, temperature and/or carbon monoxide concentration, respectively, from the contents of the container.

19. The bleaching device according to claim 18, wherein it comprises at least an oxygen measuring device.

20. The device according to claim 19, wherein the oxygen measuring device is arranged to measure the oxygen concentration from the gas volume of the container or from a gas flow coming from the container.

21. The bleaching device according to claim 18, wherein it comprises at least a temperature measuring device and/or a carbon monoxide measuring device.

22. The bleaching device according to claim 21, wherein the carbon monoxide measuring device is a measuring device connected to a gas flow coming out of the container.

23. The bleaching device according to claim 22, wherein the measuring device is a gas analyzer.

24. The bleaching device according to claim 22, further comprises a measuring device which is arranged to measure the concentration of a gas component other than carbon monoxide, from the contents of the container.

25. The bleaching device according to claim 24, wherein the measuring device is effective for measuring oxygen.

26. The device according to claim 25, wherein the measuring device is a gas analyzer, connected to the gas flow coming out of the container.

27. The bleaching device according to claim 26, wherein the gas analyzer is effective for measuring oxygen.

28. The device according to claim 18, wherein the oxygen measuring device, the temperature measuring device and/or the carbon monoxide measuring device is/are connected to a data processing unit (U) which is connected to a controller for automatic control of the process.

29. The device according to claim 28, wherein the device comprises both a temperature measuring device and a carbon monoxide measuring device.

30. The device according to claim 29, wherein the temperature measuring device is a temperature sensor placed in a gas volume of the container.