Field of Technology The preseoi invention relates to the deiignifi cation of fibre suspensions. The present invention in particular relates to a method of enhancing deiignifi cation of a fibre suspension from alkaline pulping of a plant-based fibre raw material, according to the preamble of Claim 1 .

Background Art

Usually, removing iignin from lignocellulose-based fibre material is industrially carried out in at least two stages: first, the raw material is cooked in an alkaline cooking liquor to a predetermined kappa number, after which the fibre suspension thus obtained is fed to deiignification, where most of the remaining lignin is separated by using oxidising chemicals. The pulp contained in the reject of the deiignification is usually bleached which, for its part, further reduces the amount of lignin.

Before the fibre suspension, which is derived from cooking, is fed to deiignification, it is washed in order to reduce the percentage of the dissolved material generated by the cooking. In a similar way, the fibre suspension obtained from the deiignification is washed before the pulp is separated and subjected to bleaching. To ensure proper water economy, the aqueous filtrate of the washing of the latter pulp is typically recirculated to be used in the washing of the pulp obtained by cooking, where it is used as washing water, in which case, the need for a fresh water feed is small.

As far as mass transfer is concerned, industrial oxygen deiignification is a complicated three- phase system. The desired oxidation reactions of lignin occur in the solid wall of the celiulosic fibre. The objective is to bring the lignin into a soluble form and to extract it from the fibre wail and into the surrounding alkali solution.

With regard to lignin oxidation, which takes place in the fibre wall, it is important that enough oxygen is available. If not enough oxygen is available in the fibre wall, besides the wanted reactions, also condensation of phenolic structures occur, which is detrimental to the outcome, and which reduces the reactivity of the lignin and complicates the delignification of the fibre wall,

The oxygen which is fed in gaseous form into the process is first mixed into the fibre suspension. The first precondition for the occurrence of the oxygen delignification reactions is that the oxygen dissolves into the fibre suspension liquid. Only then can the oxygen react - either with the lignin in the fibre walls, which is the main purpose, or with the material which is the fibre suspension liquid, and which consumes oxygen, which is contrary to the main purpose.

In the conditions of oxygen delignification (typically, fibre consistency 10 %, a temperature of 100 °C and an oxygen press re of 5 bar), the amount of dissolved oxygen in the fibre suspension liquid can be "at any one time" 1.5 kg/tonne of pulp.

The hold-up of oxygen is, therefore, very small compared to the amount which is typically dosed into the oxygen delignification process, 1 1-20 kg/tonne of pulp. In principle, the liquid phase has to "recharge" its oxygen content 8-13 times, in order that the total amount of dosed oxygen can be consumed. If there is a substantial amount of oxygen-consuming material in the fibre suspension liquid, this material will consume the bulk of the dosed oxygen thus initially reducing the amount of oxygen available to the lignin in the fibre wall.

In the oxygen delignification process according to present-day technology, oxygen is used very inefficiently, and a significant part of the oxygen is consumed in totally unwanted reactions, it is possible that insufficient availability of oxygen in the fibre wall leads to condensation reactions of the lignin. Condensed lignin no longer easily reacts in the conditions of oxygen delignification, in which case the oxygen delignification slows down or even stops.

This reaction route is also described in the literature (see uitunen, S. et ai, Figure la, reaction route R6). in oxygen deligniiication according to the prior art, it is very difficult to bring the kappa number level for softwood paper pulp below 10. In this situation, an important factor is that insufficient oxygen reaches the fibre wall, In a situation according to the prior art, after the cooking of softwood, the lignin percentage of the fibres, expressed as a kappa number, is approximately 30. In the oxygen deiignification process, typically approximately 50 %, i.e. 1 5 kappa number units, are removed, the resulting kappa number level of the cellulose pulp which goes to bleaching being approximately 15, Also, new cooking methods for bleachable cellulose pulp have been developed, in which methods the kappa number after cooking can be 60-70. These are particularly demanding in the process of deiignification. in these cases, too. the desired kappa number after oxygen deligniiication should be the same as current levels, i.e. approximately 15. This means that in the oxygen deiignification process, up to 55 kappa number units must be removed. Reaction products of the lignin in the fibre wall are transferred into the filtrate 3 to 4 times more than are currently transferred. In such a situation, there is an even greater risk than in the current situation thai oxygen is consumed during the reactions of the material which is dissolved in the fibre suspension, Poor solubil ity of the oxygen, combined with the oxygen consumption of the compounds which are dissolved in the fibre suspension, constitute a restriction to the transfer of oxygen into the fibre wall, and a problem to the industrial process,

Application publication WO 20121271 1 1 Al describes a method in which lignin is removed from the washing filtrate in the oxygen deiignification, by using membrane filtering or another filtering, or by precipitating lignin by adjusting the pH value of the filtrate to a lower value and removing the precipitated material by filtering.

The described separation methods are technically effective solutions, but have serious deficiencies in other respects. The investment costs of membrane filtration are relatively high and this technology is not yet fully proven. The adjustment of the pH value and the precipitation of lignin are, in turn, rather simple operations, because after the precipitation, the filtrate is returned to the feed of the oxygen deiignificaiion. Therefore, the consumption of chemicals which is caused by the separation process that is carried out by precipitating, is high.

The described separations are not particularly effective in reducing the oxygen consumption in the oxygen deiignificaiion process. Incorporation of the separation processes into the overaii balance of operations of the pulp mill is difficult. Tims, the reference publication does not reveal, among other things, to where the separated material is ultimately brought, and how the liquid balance of the whole mill can be affected.

Application publications US 2002/0088567 and EP 0831 170 and FT patent application 961856 and US patent publication 6106667 describe solutions for recycling the washing water used in the displacement washing during the bleaching of pulp.

Summary of Invention

Problem to be solved It is an aim of the present invention to eliminate at least some of the problems associated with the prior art and to provide a method of enhancing deiignificaiion of the fibre suspension obtained from alkaline cooking of vegetable-based raw material.

Solution to Problem

The present Invention is based on the idea that the filtrate of the oxygen deiignificaiion process, used for washing the. reject of the cooking process, is treated with at least one oxidising chemical, in conditions - for example in the case of oxygen or oxygen gas, at high temperature and pressure - which reduce the percentage of the material that consumes the oxygen in the filtrate, in particular, the percentage is reduced to a level low enough to result in no significant oxygen consumption of this material takes place in the conditions of oxygen deiig fication.

The treattnetit which is perfonned with an oxidising chemical can be carried out, for example, in the same conditions as in a conventional oxygen deiigmfication of fibres, but it is also possible to operate in conditions that are more intense than conventional oxygen

deiigmfication, for example at a higher temperature or pressure, or a combination of these.

After the treatment, the filtrate can be recycled and combined with the fibre suspension obtained from cooking.

More specifically, the method according to the present invention is characterized by what is stated in the characterizing part of Claim 1.

Advantages of invention

Considerable advantages are obtained by means of the present invention. Thus, the efficiency of the oxygen deiigmfication of the fibres is improved by means of a separate oxidising treatme of the filtrate in the oxygen stage, in such a way that the oxygen consuming material, which is in the fi bre suspension of the oxygen deiigmfication, and which is outside the fibre wall, is reduced.

The oxygen to be dosed for the oxygen deiigmfication can be used more efficiently for the main purpose of reducing the kappa number, in which case the condensation reactions of the lignin are prevented, and ultimately, an oxygen deiigmfication process is achieved which results in a lower kappa number level compared to the prior art.

It is not necessary to remove the material, which presents difficulties for the oxygen deiigmfication process, from the filtrate, but instead it is chemically modified into a form which does not negatively impact upon the oxygen delignifi cation. The material no longer reacts or consumes the chemicals dosed for the oxygen deiigmfication. In particular, with the method according to the present invention, it is possible to reduce the amount of oxygen-consuming material, which is generated specifically during the oxygen delignificaiion stage arid which is almost completely recycled back to the oxygen

delignification, during the circulation of the filtrate in the post-washing.

With regard to the apparatus, oxidising of the circulating filtrate is simple to carry out.

In particular, by arranging a separate oxidising unit for the reject of the bleaching unit, in which oxidising unit considerably more intense conditions are employed than in the bleaching unit, and by returning the thus treated filtrate stream to form the washing water that displaces the brown pulp, considerable advantages are achieved, which are described in more detail below.

The following examines the preferred embodiments with reference to the accompanying drawings.

Brief Description; of the Drs mgs

Figure ί shows a process flowchart of a process according to the prior art,

Figure 2 shows a process flowchart of a process according to one embodiment of the new technology, and

Figure 3 shows a process flowchart of a process according to a second embodiment of the new technology. Embodiments

As described above, in the present technology, cooked fibres are washed in order to reduce the percentage of dissolved material obtained from cooking, after which the suspension of dissolved fibres is fed to delignificaiion, where iignin is separated from the fibres by .means of an oxidising chemical, In this case, washing of the cooked fibres is mainly or entirely carried out using the filtrate of the washing after the delignification process. In the present technology, it has been found that

- because the route of the oxygen is as follows: dissolution -> delay time in the fibre suspension liquid -> mass transfer into the fibre wall -> reactions during the oxygen deiignification process,

the oxygen consuming material in the fibre suspension liquid has time to react with part of the oxygen before there is enough time for the oxygen to enter the lignin in the fibre wall. How much extra oxygen-consuming material there is in the fibre suspension liquid, is shown in the examples below.

In connection with the present technology, it has been also found that the oxygen-consuming material in the fibre suspension liquid is partly "cooking-generated" dissolved organic material, the percentage of which depends on the efficiency of the washing process, which is carried out after the cooking and prior to the oxygen deiignification. The amount of this material may be, for example 20-30 kg/tonne of pulp, expressed as COD. The remainder of the oxygen-consuming material is material in the filtrate of the oxygen dei ignification, which material returns to the feed of the oxygen deiignification, and the amount of which may be, for example, 40-100 kg/ tonne of pulp, expressed as COD. When the mass suspension is washed after the oxygen deiignification process, typically approximately 90 % of the material in the pulp suspension liquid is displaced into the washing filtrate.

During the oxygen deiignification process, l ignin reaction products are generated, which dissolve in the liquid of the fibre suspension. Accordingly, approximately 90 % of this material is transferred into the post-washing filtrate. When the filtrate of the post-washing is returned to form the washing liquid of the washing that precedes the oxygen deiignification, correspondingly approximately 90 % of the material, which is dissolved in the wash filtrate, remains in the liquid of the fibre suspension, which liquid is fed to the oxygen deiignification. When this cycle is circulated as a continuous process, almost all the organic and oxygen- consuming lignin reaction product material, which is generated during the oxygen deiignification, returns again to the oxygen deiignification. The problematic nature of this situation is emphasised, if the aim, in the oxygen deiignification, is to remove a greater proportion of the fibre wall lignin than occurs in the prior art, It should also be pointed out that in a pulp mill it is not possible to open the post-wash filtrate, circulation and remove the circulating wash filtrate wholly or partly from the counter-current washing, because this would be completely impractical for the water use, energy efficiency and discharge of waste water from the pulp mill. Thus, according to the present invention, from the fibre suspension that is fed to the oxygen deiignification, the components that cause the chemical oxygen demand (COD) are removed as efficiently as possible.

One feature is to wash the reject of the cooking as efficiently as possible.

In one embodiment, the washing is carried out as a counter-curreni washing, in a way which is known per se.

In particular, the cooked fibres are washed applying the counter-current principle, by using a displacing washing solution, in order to reduce the percentage of cooking-derived and dissolved material. Here, "displacing washing solution" means that the washing solution replaces the liquid in the fibre.

Most suitably, the displacing washing solution used in the washing is water which is mainly comprised of a filtrate of the post- washing deiignification, or which is preferably entirely comprised of it. In the present context, the filtrate is also referred to as "circulation filtrate".

Another feature is to keep the COD value of the washing water used in the washing as low as possible. In particular, in the present technology, the COD value of the filtrate is actively reduced before it is fed to the pulp washing. In one embodiment, the COD value of the liquid phase that is fed to the delignification is at least 20 , preferably at least 30 %, in particular at least 40 % lower than the COD value of the liquid phase generated in the cooking. In one. embodiment of the present solution, enhancement of the oxygen delignification is achieved in. such a way that the wash filtrate to be circulated is oxidised with gaseous oxygen, in which case the COD percentage of the circulation filtrate can be significantly reduced. When the oxygen-treated filtrate is taken to the washer, which precedes the oxygen stage, to form the washing liquid, in principle no such organic material at all which couid consume oxygen in the conditions of the oxygen delignification, is returned to the feed of the oxygen stage. As a result, the oxygen that is dosed into the oxygen stage of the delignification works more efficiently and more selectively in reducing the residua! iignin in the fibre.

Based on the above, in one embodiment a method is generated to produce bleached fibre, according to which method the fibre suspension is fed to a first delignification stage, where the fibre suspension is delignified under oxidising conditions, in which case from the fibre suspension that is obtained from the delignification, the fibres comprised in it and, correspondingly, the liquid phase comprised in it, are separately recovered, after which the fibres are fed to the bleaching, the liquid phase is fed to the second delignification stage, where it is treated under oxidising conditions, and the liquid phase thus obtained is circulated and combined with the fibre suspension generated in the cooking, prior to feeding it into the first delignification stage.

In one embodiment, the displacing washing solution used during the washing that precedes the delignification is treated with the chemical to be oxidised. The displacing washing solution is most suitably largely (to at least 50 % by weight, in particular at least 70 % by weight, most suitably to at least 90 % or to more than 90 %) filtrate of the post-delignifi cation washing. In a more preferred embodiment, the oxidation treatment of the circulation filtrate, for example oxygen treatment, is carried out by raising the temperature. In one embodiment, the treatment of the displacing washing solution is carried out at an elevated pressure and at a temperature that is higher than during the delignification of the fibres. Most suitably, the operating pressure is 5 bar or higher (for example 5-20 bar) and the operating temperature is over 100 °C, for example approximately 12.0-150 °C,

In one embodiment, the treatment of the displacing washing solution is carried out using oxygen gas, and possibly, a peroxide chemicai, such as hydrogen peroxide.

The heating of the circulating filtrate can preferably be carried out by using an indirect low- pressure steam. The temperature of the filtrate can be raised as high as the temperature restrictions of the brown pulp washers, which precede the oxygen stage, allow, Raising the temperature lowers the volume of the direct MP (Medium Pressure.) steam that is dosed into the oxygen stage, and raises the washing temperature of the brown pulp, which has a beneficial effect on the efficiency of the washing.

The solution enhances the delignification ev en in the prior art. The solution is of particular importance for the totai oxygen consumption, if the cooking kappa number is raised above the current value and the new oxygen delignification technology is utilised. In this case, it is possible to achieve a considerable saving in the amount of oxygen which is added into the actual oxygen stage, and dosed at a pressure that is significantly higher than the current pressure.

In one embodiment, the lignin content of the fibres coming to the delignification, expressed as kappa numbers, when softwood pulps are used, is over 25, most suitably 30-70,

Typically, the !ignin content of the fibres leaving the delignification, expressed as kappa numbers, when softwood pulps are used, is 8-20, most suitably 12-1 7.

Preferably, the oxidation treatment of the filtrate removes a significant amount of the oxygen- consuming pan comprised in it. hi general, at least enough is removed to reduce the consumption of oxygen by 1 kg Qy ^' BDT, in particular at least 2 kg <¼/BDT (bone dry tonne, i.e. an absolutely dry tonne of pulp). As the examples demonstrate, it is possible to reduce the oxygen consumption by up to 6,8-3.5 kg O ₂ BDT..

The embodiments are further examined in more detail using the accompanying drawings, in which case Figure 1 shows a solution according to the prior art, and Figure 2 and 3 show two improvements according to the present technology.

In ail applications, the fibre suspension which is generated in the cooking, is led to the first washing unit, 10, 20, 30, where it is subjected to a counter-current washing, by using filtrates of subsequent washings, for example as a displacement liquid for the filtrate of the oxygen delignification. The washing liquid is therefore a filtrate of the second washing unit 12, 22, 32, which is first collected m a filtrate tank 13, 23, 33, from where it is then circulated to the washing unit 10, 20. 30.

Oxygen gas is fed into the fibre suspension that comes from the washing, before the suspension is led into the oxygen delignification unit 11, 21 , 31. The temperature of the fibre suspension is raised to the desired level, for example by feeding Medium-Pressure steam (pressure approximately 10 bar) prior to the. delignification. The steam is most suitably added as a "direct steam", in which case its condensate is mixed into the fibre suspension, Typically, the delignification is carried out in tower-like pressure reactors that operate according to the plug-flow principle, the number of which reactors may be one or several reactor units connected in series.

In the present technology, the following conditions prevail in the delignification 21 , 31: fibre consistency: approximately 1-15 %, i particular approximately 5-12 %, temperature approximately 80-105 °C, in particular approximately 85-100 ⁰ C, and pressure

approximately 3-] 0 bar, in particular approximately 3-8 bar (oxygen pressure). The dwell time is approximately 10-360 minutes, in. particular approximately 20-180 minutes. If there are several delignification reactors, the conditions, particularly the temperature and the pressure, may vary between them. A typical oxygen dose is approximately 5-25 kg (¾ ADT, most suitably approximately 10-20 kg O2/ADT (kilo oxygen per dried tonne of puip). Usually, the operating temperature and pressure in the first reactor are higher than in the following units or in the following reactors. In the examples presented below, the operating temperature in the first reactor is approximately 90-95 °C, and below 90 °C in the second reactor. The dwell times vary but are generally approximately 5-1 80, most suitably approximately 10-90 minutes/reactor.

In contrast to the prior art. in the present technology, the circulating filtrate of the washing unit 22, 32 is treated before it is returned to the washing of the pulp that, is generated during the cooking. Oxidation treatment can be carried out under the same conditions as in units 21 and 31, but preferably the operating temperatures and pressures are higher, in order to enhance the oxidation. It is possible to operate in this way also because the filtrate does not comprise significant amounts of fibres, the degradation of which should be avoided.

After the delignification, the reject is washed with a counter-current washer 12, 22, 32, after which the fibrous part is separated from the water and brought to further treatment, such as bleaching, whereas the filtrate is circulated to the washing of the pulp from the digester, as described above. Mostly, fresh washing water is fed into the counter-current washer 12, 22, 32. Instead of or in addition to the fresh water, it is also possible to use cleaner condensates from the of evaporation unit or, for example, alkaline filtrate liquids from the bleaching, The washing that is carried out after the oxygen delignification reactor (for example using washer 12) takes place in one application, for example, at the same consistency as the oxygen delignification reaction, Washing can be carried out at other consistencies, too, depending on the washer. However, a consistency of approximately 10 % is quite typical in the industry. In the washing that takes place after the oxygen delignification, part of the reaction, products of the oxygen delignification are transferred into the filtrate tank 13, in which the filtrate is collected to be used as washing liquid in the counter-current washing, in the washer that precedes the oxygen stage (for example washer 30 in Figure 1). The percentage is determined by the efficiency of the washing. A typical efficiency is approximately 90 %, in which case the same percentage of the reaction products is transferred to the filtrate.

When, during the washing that precedes the oxygen stage, the washing liquid used is this filtrate liquid, which comprises as much as 90 % reaction products of the oxygen

deli g if i cation, and which products come from the tank 13, it may be assumed that in the washer 10, a washing displacement takes place, which is similar and has a "washing yield" of 90 % (provided that the washer 10 is similar to the washer 12).

Based on the balance of the counter-current washing, the amount of the washing liquid at washer 10 is approximately the same as at washer 12, i.e. approximately 11.5 m ³ /BDT. At the washer which precedes the oxygen deiignification, approximately 90 % of the liquid that is in the filtrate tank 13 is transferred into the washable fibre suspension. Thus, in this example, at least 90% * 90 % = 81 % of the oxygen deiignification reaction products are returned to the oxygen deiignification reactor.

According to one embodiment, the washing that takes place in the washer is a "displacement wash". This means mat the clean washing water, like a piston pushes away the dirtier liquid, in which; case the fibre suspension liquid is replaced with the washing liquid. However, this action of displacement is not entirely idea l, because the washing liquid and the dirtier liquid of the fibre suspension are somewhat mixed with each other. The washing can be mathematically described by the "Norden's washing theory".

The washing efficiency according to the theory of orden is expressed as the "E value", which indicates how many perfectly mixed mass transfer stages can be performed using a washer, hi an ideal wash, i.e. a perfect displacement, the E value is infinite. In this case, the dirtier liquid of the fibre suspension is replaced to 100 % by washing liquid, and all the dirty liquid of the washable fibre suspension is transferred to the filtrate tank of the washer. If the E value™ 1 , this means that the washing liquid and the fibre suspension liquid are mixed with each other and no displacement takes place. However, such a washing transfers more than 50 % of the dirty liquid of the washable fibre suspension to the filtrate but, correspondingly, a little less than 50 % remains in the fibre suspension that is removed from the washer, which fibre suspension continues to the bleaching.

If the E value of the washer is 4.5. such a good displacement takes place that approximately 90 % of the dirty liquid of the washable fibre suspension is transferred into the filtrate, and approximately 10 % is a mixture devoid of washing liquid. Accordingly, approximately 90 % of the liquid in the fibre suspension is comprised of washing liquid and approximately 10 % is liquid of the washable fibre suspension. Such an E value of 4.5 is very typical for the washers commonly used in the industry (for example, the "DD washer").

Because the washer 10 is preceded by a number of counter-current connected washing steps, which are needed to cany out the washing between the cooking and the oxygen

delignification, and because, in these washing steps, the E value is typically over 10, almost every tiring is returned to the oxygen delignification, re. over 99 % of the reaction products which are brought to the washer 10 from the filtrate tank 13, which means approximately 90 % of all the reaction products that are generated during the oxygen delignification. Figures 2 and 3 show two embodi ments, in the first of which (the case of Figure 2) the oxidation is earned out at a lower temperature than in the second (the case of Figure 3). Thus, Figure 2 shows a solution, in which the water coming from the filtrate tank 23 is passed through the heat exchanger 25 to the oxidation unit 24. In the heat exchanger, the temperature of the water is raised to the level necessary for the oxidation, for example approximately 95- 115 °C, in, particular approximately 100-105 °C. This level is thus somewhat higher than what is observed in the del ignification 21. The pressure is approximately 4-8 bar. The reaction time is most suitably 10-180 minutes.

After the heat exchanger 25, oxygen is fed into the filtrate, as shown in the figure. The oxygen treatment of the filtrate can also be carried out at a temperature substfintialiy higher than the actual oxygen delignification stage. This is illustrated in Figure 3. It shows the heat exchanger 35, which is used to raise the temperature of the filtrate to be fed to the oxidation unit 34, to a desired level, for example approximately 130 °C. Prior to the oxidation unit, in this case, too, oxygen gas is fed into the filtrate, as shown in the figures. After the oxidation at a higher temperature, the filtrate can be directly led to the washer 30, but i is preferable to recover the heat. Therefore, the filtrate is preferably fed to the second heat exchanger 36, which is used to heat the circulating filtrate coming from the filtrate tank.

The pressure in the oxidation unit 34 is approximately 8-12 bar. The reaction time is most suitably 1 0-180 minutes,

A typical oxygen dose is approximately 0.5-20 kg {¾/ADT, most suitably approximately 1~ 15 kg 0 ₂ /ADT.

The oxidation units 24, 34 used can be similar to those pressure reactors used in the delignification 21 , 31 . it is also possible to apply other types of chemical reactors, such as mixing tank reactors. Preferably, the apparatuses selected are able to withstand those higher temperatures and pressures, which in some applications axe observed in the units 24, 34.

It should be noted that the solutions shown in the drawings represent only two applications. Within the framework of the present invention, there are other ways to operate, too.

Consequently, other strong oxidation chemicals can be used in the oxidation, if they fulfil the conditions of the case in question, in particular the conditions of the circulating filtrate ( H, temperature). One or more of the following chemicals are examples of the oxidising chemicals used: oxygen, ozone, hydrogen peroxide, peracetic acid, other peracids and peracid esters, and chlorine dioxide.

The described oxygen treatment can also be carried out on the filtrates from the washing that is between the cooking and the oxygen delignification, in which case it is possible to affect the amount of the above-mentioned cooking-derived and oxygen-consuming material in the pulp suspension that, is fed to the oxygen delignification. The following non-limiting example illustrates the new technology.

Example

Starting situation and description of the test conditions

The following examples illustrate the functionality of the applications according to Figures 2 and 3. An amount, adequate for the oxygen tests, of the filtrate from the washing stage that follows the oxygen deligmfication of a softwood pulp mill, was collected. The oxygen delignification was carried out in conditions which were typical for paper cellulose, in an oxygen deligni ication system that was comprised of two reactors:

» Temperatures: first reactor 92 °C and second reactor 85 ^C C

* Reaction time 40 ÷ 40 min

« Oxygen dosage 18.5 kg/tonne of pulp

» Alkali dosage 16.6 kg NaQH/tonne of pulp

The drop of the kappa number occurring in the oxygen delignification was 29 -> 15, i.e. typical for the desired delignification level for softwood paper pulp,

Because the efficiency of the washing after the oxygen delignification stage is high, the displacement ratio of the washing stage and the washing yield are at least 85-90 %.

Therefore, the above-mentioned filtrate of the washing stage, from which filtrate a sample was taken for oxidation tests, comprises almost all the organic material that was dissolved i the oxygen delignification stage. In addition, the filtrate also comprises organic material, which is initially sourced from the cooking process.

Total dissolved dry matter 2.5 p-%

UV iigniii (wave length 280 8.3≠ CODQ (Standard ISO 6060) 15.0 g/1 * COD _Mn (Standard SFS 3036) 7056 mg/1

* CODwn ("kappa number method" *) 2774 mg 1

*) Here, the "kappa number method" means that the consumption of potassium permanganate, from which COD is calculated, is determined under the same conditions in which the kappa number of the pulp is determined, for example according to the standard SCAN-C1 :77, i.e. at room temperature, for a period of 10 min. In the determination of the actual CQI½ _n , according to the standard SFS 3036, the corresponding conditions are 20 minutes and temperature 100 °C.

Oxygen tests were conducted for the washing filtrate under the following conditions:

» Temperatures 100 or 130 °C

* Oxygen pressure 5 or 10 bar

* Reaction time varied between 10 and 90 min

· Also, in part of the test points, hydrogen peroxide was dosed, whereby the hydrogen peroxide in the filtrate was 0.5 g/ ^' l at the beginning of the reaction.

The mixing tests were carried out in a pressurised mixing reactor (blade agitator), having a mixer speed of 150 rpm, The heating times of the reactor were 14 mm (target temperature 100 °C) and, correspondingly, 20 mm (target temperature 130 °C). During the test, the pressure of the reactor was adjusted manually by adding oxygen. During the adjustment of the pressure in the reactor, the water vapour pressure at the reaction temperature in question was observed, in order to obtain the desired oxygen, pressure.

During the oxidation test, liquid samples were taken from the reactor at various points in time (10, 30, 60 and 90 minutes). The UV-1 ignin, COD _c „ COD _Mn and CODM _B were determined from the samples by using the "kappa number method". The method, based on potassium permanganate, CODM , of determining the oxygen consumption is a more sensitive indicator to explain the impacts on the oxygen consumption in the oxygen deiignifi cation than the dichromate method, CODo. Results

Table 1 shows the results of the oxidation tests, when the reaction temperature v/as 130 °C and the oxygen pressure 10 bar, and no peroxide was added.

Table 1: Results of the oxidation tests (reaction temperature 130 °C and oxygen pressure 10 bar, no peroxide)

When the temperature is lowered to Ϊ 00 °C and the oxygen pressure is maintained at 10 bar. the result obtained is a more moderate oxygen consumption, as shown in Table 2.

Table 2: Results of the oxidation tests (reaction temperature 100 °C and oxygen pressure 10 bar, no peroxide)

Table 3 sho ws the results obtained when a pressure of 5 bar is used at t C.

Table 3; Results of the oxidation tests (reaction temperature 130 °C and oxygen pressure 5 bar, no peroxide)

By adding peroxide, it is possible to achieve a marginal enhancement of the oxidation, as shown in Table 4.

Table 4; Results of the oxidation tests (reaction temperature 130 °C a d oxygen pressure 5 bar, peroxide content 0.5 g/1)

The impact, of the oxygen treatment of the fiitfate on the oxygen delignification process can be evaluated by examining the results of the most effective treatment, i.e. tire case of a temperature of 130 °C and a pressure of 10 bar. The effects of the 90 min treatment are selected.

The filtrate of the oxygen deligmfication is used as the washing water in the washer that precedes the oxygen stage, in which case the filtrate of the oxygen stage displaces away the dirtier filtrate, typically at an efficiency of 85-90 %. In this case, the filtrate of the oxygen stage (i.e. case-specifically, filtrate which is untreated or which is oxidised according to the present invention, is almost completely returned to the oxygen stage). The consistency of the oxygen stage is typically 10 %, in which case the amount of liquid per tonne of pulp is 9 m ^J BDT (BDT - bone dry tonne, i.e. absolute dry tonne of pulp). Thus, it is possible to examine the situation in order to calculate, by using the oxygen stage filtrate that has the volume in question, the effects per each tonne of pulp.

What should also be considered are the washing steps before and after the oxygen de gnifieation (in Figures 1-3, the washing steps 10, 20, 30, i.e. the washing before the oxygen deligmfication reactor, and 12, 22 and 32, i.e. the washing after the oxygen dehgnifieation reactor).

The amount of the washing water is generally greater than the amount of the liquid comprised in the fibre suspension (thus here, 9 m ^J BDT). Here, the concept referred to is a dilution constant. It means the amount of washing liquid added in relation to the amount of liquid comprised in the fibre suspension, which makes the total amount of washing liquid greater tha the amount of the liquid comprised in the fibre suspension. Often, this dilution constant is 2-3 m ³ /BDT, in which case the total amount of washing water is 9 + 2 ... 3 m ³ BDT = 1 1 ... 12 m ³ /BDT.

The dilution constant is 2.5 nr'/BDT, i.e. 1 1.5 m'/BDT washing water is brought to the washer 12. Clean (fresh) water is used as the washing liquid in the washing. Lignin:

In the oxidation, the lignin content is reduced 8,3 ~> 5.6 g/L i.e. Alignin = 2.7 g/1 = 2.7 kg/m ^" (Table 1 ).

When calculated per tonne of pulp, die result is Alignin = 2.7 kg/m * 9 m /BDT = 24.3 kg BDT. Converted into percentage units, in other words, 2.4 % by weight.

In the oxygen delignifi cation of softwood pulp, typically a yield loss of 2-3 % by weight occurs, which almost completely is sourced from the lignin that is removed from the pulp. Thus, it can be established that by using oxidation, it is possible to remove, from the filtrate which by-passes the oxygen stage, approximately the same amount of lignin that dissolves from the oxygen delignificaiion pulp into the filtrate. It is quite clear that if such dissolved lignin is returned, to for example 90 %, along with the filtrate back to the feed of the oxygen stage, part of the oxygen would be consumed for further oxidation of this dissolved lignin, i.e. for a totally useless side reaction.

How much oxygen the oxidation of the dissolved lignin consumes can be estimated by how much the CODo and CODM _H are reduced during the oxidation. In this way, it is also possible to estimate, how much excess oxygen is consumed during the actual oxygen stage, when the filtrate is returned, according to the prior art, without the oxidation treatment according to the present invention.

Effect of the oxidation, i.e. the reduction of the COD, as shown in Table 1 ;

ACOD _M;i = 7056 -> 5408 = 1648 mg/i - 1.648 g/1 === 1.648 kg/m3. When the oxide consumption is expressed per tonne of pulp, as previously, the result A OD i;, 1.648 kg/m ³ * 9 m ³ /BDT = 14.8 kg/BDT

If it is assumed that the washing yield is 90 %, 13.5 kg/BDT of the COD is returned to the feed of the oxygen delignification reactor.

It is possible that both the CODo and the COD _H overestimate the effect on the oxygen consumption, therefore the situation should be examined with regard to a CODM„ conducted using the "kappa number method". ACODM _IS fkappa number method") = 2774 -> 1 932 = 842 mg/1 - 0.842 g/i - 0.842 kg/m ³ .

Expressed per tonne of pulp gives:

ACOD n = 0.842 kg m ³ * 9 m ³ /BDT - 7.6 kg BDT

If the washing yield of the oxygen delignification is approximately 90 %, approximately 6.8 kg COD/BDT is returned to the feed of the oxygen delignification.

Thus, the result is that by using oxygen delignification it is possible to remove oxygen- consuming material from the filtrate, to an amount which is equivalent to 6.8-13.5 kg

O2 BDT. This clearly shows that in the oxygen delignification according to the prior art, in which, without a separate oxidation treatment, oxygen-consuming material is brought back, a very significant part of the oxygen, which is dosed into the oxygen delignification, is consumed in side reactions and not for removal of residual lignin, which should be the purpose.

As described above, the new technology renders it possible to achieve a situation where the COD value of the liquid phase which is fed to the delignification is lower than the COD value of the liquid phase obtained during the cooking. Industrial Applicability

The present solution is suitable for the removal of the lignin in an initial material that is made of a vegetable-based fibre raw material. More preferably, the solution is suitable for cases in which, from a pulp which is cooked to a high kappa number, the lignin should be removed before bleaching. The new technology can be applied to the production of pulps which are suitable for production of paper and cardboard products, but. it can also be used to produce, for example, dissolving pulp and similar pulps. Reference Number List

10. 20. 30 washing unit

11. 21. 31 oxygen deiignifi cation unit

12, 22, 32 washer

13. 23 , 33 filtrate tank

14, 24, 34 oxidation unit

25. 35 heat exchanger

36 heat exchanger Citation List

Patent Literature:

WO2012/12711 1A1

US 2002/0088567

EP 0831170

FI 961856

US 6106667. Non-Patent Literature:

Kuitunen, S. et al, "Lignin oxidatior! mechanisms under oxygen delignification conditions. Part 3. Reaction pathways and modeling", Holzforschung 65 (2011), pp. 587-599.