WO1994011569A1

WO1994011569A1 - A method in delignifying and/or bleaching of cellulose pulp

Info

Publication number: WO1994011569A1
Application number: PCT/SE1993/000953
Authority: WO
Inventors: Otto Sten Axel Gustaf Lindeberg
Original assignee: Mo Och Domsjö Aktiebolag
Priority date: 1992-11-11
Filing date: 1993-11-10
Publication date: 1994-05-26
Also published as: SE9203366D0; AU5439594A; SE9203366L

Abstract

When using oxidative bleaching agents, such as peroxide, to delignify and/or bleach ligning containing cellulosic pulp, difficulty explained swings are sometimes obtained in the bleaching results. This problem is solved by the present invention, which relates to a method in which the cellulose pulp is pretreated with a complexing agent in the presence of liquid and sodium ions and then treated with an oxidative bleaching agent. The method is characterized by adding a process foreign magnesium in any form to the cellulose pulp prior to and/or during the pretreatment process, in a quantity which is determined at least partially by the amount of sodium ions present.

Description

A method in delignifying and/or bleaching of cellulose pulp

Technical field

The invention relates to a method in delignifying and/or bleaching of lignocellulosic pulp with oxidative bleaching agent. The invention is primarily intended for application in delignification and/or bleaching of chemical cellulose pulps with oxidative bleaching agent. Sulfite pulp, sulfate pulp and polysulfide pulp are examples of such pulps. The term sulfate pulp also includes cellulose pulp which is digested at high sulfidity, and cellulose pulp digested in accordance with a counterflow method in which white liquor is added also during an advanced stage of the cook, and cellulose pulp in which the actual sulfate cook is preceded by chemical treat¬ ment of the lignocellulosic material, for instance wood chips. Other examples of chemical cellulose pulps are alkaline cellulose pulps which are cooked in the presence of catalysts, for instance in the presence of a quinon compound, such as antraquinone. Other chemical cellulose pulps are those referenced SS-AQ (mini-sulfide-sulfite-antra-quinone) , Si-Sa-Si (sulfite-sulfate-sulfite) and PS-Si (polysulfide- sulfite), all of which are referred to in the journal "Paperi ja Puu", 5/1989, pages 509-513.

Secondarily, the invention can be applied in conjunction with delignification and/or bleaching of semichemical cellulose pulps with oxidative bleaching agent, and in conjunction with bleaching high yield pulps, such as CTMP (chemithermo echanical pulp) and mechanical pulp with oxidative bleaching agent.

Background art Recent environmental requirements placed especially on industries which manufacture bleached chemical cellulose pulp has resulted in a change in the choice of bleaching agent. Partly conventional chlorine bleaching has been replaced progressively with alkaline oxygen delignification of cellulose pulp and partly the use of chlorine containing bleaching agents (particularly chlorine and hypochlorite) in the subsequent cellulose pulp treatment stages/bleaching stages has been reduced and replaced with oxidative bleaching agents, such as peroxide and oxygen. Ozone bleaching is now on the threshold of a break through as a commercial bleaching method. The important advantage gained with the use of such bleaching agents from an environmental aspect is that the waste liquors deriving from these bleaching stages can be incorporated in the chemical recovery cycle, which has long been used in sulfate pulp mills for instance, and which is now used also in many sulfite pulp mills for instance.

When using these bleaching agents to delignify pulp and also to greatly increase pulp brightness, bleaching waste liquor for instance can be advantageously passed in counterflow from the bleaching department via the alkaline oxygen bleaching stage and the cooking stage of the lignocellulosic material (the wood) to evaporation and subsequent combustion of a concentrated waste liquor, that contains dissolved substances and residues from the cooking stage, delignification stages and bleaching stages. This results in a marked decrease in waste liquor emissions to the recipient. Although this is beneficial from an environmental aspect, the increased extent to which the system is closed in the manufacture of bleached chemical cellulose pulp also results in problems, among other things because the quantities in which different metal ions are present tend to increase in diverse liquids with which the cellulose pulp comes into contact. As a result, the concentration of diverse metal ions in the cellulose pulp will also increase. It has been found that when bleaching cellulose pulp with oxidative bleaching agents, such as peroxide, transition metals for instance, such as manganese and iron, prevent a good bleaching result from being obtained with the aid of peroxide.

It has long been understood that, for instance, transition metals should be rendered safe or removed from the cellulose pulp and/or the pulp suspension before adding peroxide. Complexing agents, such as EDTA, DTPA, NTA, etc. , are suitable chemicals for this purpose. Swedish Patent Applications 8902058-0 (467 006) and 9001449-9 (466 061) both describe for instance a method in which the cellulose pulp is treated with a complexing agent at temperatures above room temperature and at a pH within the interval 3.1 to 9.0 prior to carrying out an alkaline peroxide bleaching stage. According to the described method, it is advantageous to wash the complex bound metals from the cellulose pulp prior to the bleaching stage.

Many metals are present in the system in addition to the two mentioned above, for instance sodium and magnesium. The liquid encompassing the cellulose pulp will often have a high sodium ion concentration, among other things because sodium is often the base used at the cooking of the lignocellulosic material (wood chips) to cellulose pulp and/or because sodium hydroxide is often added to the system in at least one location in the bleaching department (extraction stages and alkaline bleaching stages). The reason why magnesium is present in the system is explained by the fact that the lignocellulosic material (wood chips) taken from nature contains magnesium, and, for instance, because magnesium sulfate is often added to the system as a protector when bleaching cellulose pulp, such as in oxygen bleaching and peroxide bleaching. ₄

Disclosure of the invention

Technical problem

When delignifying/bleaching cellulose pulp with oxidative bleaching agent, such as peroxide, in i.a. pulp mills in which the degree to which the system is closed has been increased (resulting among other things in a reduction in the discharge of contaminated liquid from the bleaching department to the recipient), the bleaching results obtained will often vary with time. These varying bleaching results may either be due to the fact that a given charge of bleaching agent has imparted varying brightness to a cellulose pulp or that a given desired brightness of the cellulose pulp has required a marked variation in the amount of bleaching agent charged.

The solution The present invention provides a solution to the aforesaid problem and relates to a method in delignifying and/or bleaching of lignocellulosic pulp, wherein the cellulose pulp is treated with complexing agent in the presence of liquid and sodium ions and in which process foreign magnesium in any form is added to the cellulose pulp prior to and/or during the pretreatment, and wherein the cellulose pulp is then treated with oxidative bleaching agent. The method is characterized by that said magnesium being added in a quantity which is determined at least partially by the amount of sodium ions present so that after the pretreatment stage the pulp will contain this substance in an amount corresponding to at least 50 mg Mg/kg absolute dry pulp, including any magnesium that was already present in the pulp. By process foreign magnesium is meant magnesium that is not already present in the system. As before mentioned, system associated magnesium has its origin, for instance, in the starting material (the wood chips) and bleaching waste liquors that circulate in the system. Part of the process foreign magnesium added in the pretreatment stage will, in time, become system associated magnesium. The process foreign magnesium is advantageously delivered directly to the pulp or to the pulp suspension, immediately prior to its introduction into the pretreatment vessel. A second method is to add the process foreign magnesium directly in the pretreatment vessel. A third method is to add process foreign magnesium to a liquid which is used to thin or dilute the pulp prior to and or during the pretreatment stage. Still another method is to add process foreign magnesium to a liquid that is used to wash the pulp prior to or subsequent to said pretreatment stage, i.e. in conjunction with said stage.

A suitable magnesium for addition to the cellulose pulp prior to and/or during the pretreatment stage is magnesium sulfate, for instance MgS0 . 7H₂0. Naturally, the magnesium added can be chosen from a large number of other magnesium compounds, the choice of magnesium being primarily determined by the solubility and price of the chemical concerned. The magnesium, preferably in the form of a salt, is conveniently dissolved in a given quantity of water or in a given quantity of process liquid taken from the system, and the solution is then added to the cellulose pulp or to the pulp suspension or to a process liquid which is then itself added to the cellu- lose pulp or the pulp suspension. Although not preferred, it is fully possible to add the magnesium salt in a solid form at said locations. Metallic magnesium can also be added, even if that is not suitable from a practical and economical aspect. As earlier mentioned, such bleaching agents as oxygen, ozone and peroxide are used more and more in the bleaching of chemical cellulose pulp for instance. One or two or these bleaching agents will normally be included in different bleaching sequences, and bleaching sequences are found which contain all three of these bleaching agents.

In the case of sulfate pulp, bleaching sequences such as O-P-D-D, 0-Z-P-D, O-Z-P-D-E-D, 0-P-Z-D, O-P-Z-D-E-D and O-P- Z-P have been proposed and are also beginning to be used in practice. Furthermore, similar bleaching sequences are found in which peroxide is used repeatedly, i.e. the bleaching sequence includes several peroxide bleaching agents.

Subsequent to digestion, sulfite pulp will normally contain less lignin than sulfate pulp in the same location, and therefore the sulfite pulp bleaching sequence is often shorter. It is even possible to fully bleach sulfite pulp, i.e. to a brightness of 90% ISO, while using the short bleaching sequence 0-P. This short bleaching sequence is sometimes supplemented with a D-stage.

Although the aforegiven symbols are used as a matter of routine in bleaching processes, an explanation of what is meant by these symbols will be given below. 0 oxygen

P = peroxide Z = ozone D = chlorine dioxide

E = sodium hydroxide (alkaline extraction stage).

It has long been proposed in the literature, and also practiced in recent times, that a treatment stage with complexing agents (Q) is introduced upstream of peroxide bleaching stages and even upstream of ozone bleaching stages. It is preferred to introduce the cellulose pulp treatment stage according to the present invention prior to the peroxide stage or stages that are included in the aforesaid bleaching sequences. The inventive treatment stage is introduced to the system at said location even in those cases where one or more peroxide stages are included in bleaching sequences not mentioned here. A certain effect is also obtained when the inventive cellulose pulp treatment stage is introduced prior to ozone bleaching stages. When the inventive cellulose pulp treatment stage is introduced prior to an ozone bleaching stage which, in turn, is followed by a peroxide bleaching stage, a clearly improved bleaching result is obtainable first in the peroxide bleaching stage, which is the oxidative bleaching stage to which the inventive pretreatment is best suited. When the inventive pulp treatment stage is introduced prior to an ozone bleaching stage which is followed by the series of bleaching stages peroxide, ozone and peroxide an improved bleaching result can be similarly obtained primarily in the two peroxide bleaching stages.

The present invention is mainly based on the understanding that the amount of sodium present in the pulp fibres and in the liquid surrounding said fibres immediatly prior to and when adding a complexing agent and when said complexing agent is in contact with the pulp has a signi¬ ficant influence on the bleaching result. Thus, when applying the invention, it is necessary to estimate the amount of sodium that is bound in the cellulose pulp and that is contained in the liquid present in and/or surrounding the pulp fibres when the pulp is to be pretreated in accordance with the invention. The amount of sodium at this location is determined by a large number of factors. One important factor is whether or not sodium was used as a base when digesting e.g. wood chips to produce cellulose pulp, and if so how effective the pulp was washed after the cooking (digestion) stage. Another important factor is if the bleaching sequence includes alkaline bleaching stages and extraction stages (where alkali in the form of sodium hydroxide is normally added), and if so how recycling of the waste liquors through the bleaching department was effected. A third important factor is the location of the peroxide bleaching stage, i.e. at the beginning of the bleaching sequence, in the middle of said sequence or at the end thereof.

The amount of sodium present in said liquid will vary from mill to mill because of these factors, among other things. A given mill which employs a given bleaching sequence and a given degree to which the system is closed will normally lie on a given level with regard to the amount of sodium contained in said liquid. However, it is not unusual for the amount of sodium present in said liquid to swing around a given average sodium content, as a result of diverse swings in the process used to produce bleached cellulose pulp. The amount of sodium present will thus vary from case to case, mainly within a range of 0.1 to 15 grams per liter of liquid. The amount of sodium bound in the pulp is in a certain way proportional to the amount of sodium in said liquid. The amount of sodium that accompanies the pulp suspension into the complexing stage of an imaginary sulfate pulp bleaching sequence in the form of O-Q-P-Z-P has been calculated. These show that strict counterflow circulation results in a sodium content of 6 grams per liter of liquid, whereas a partial circulation results in a sodium content of 2 grams per liter of liquid.

It has surprisingly been found that the negative effect of the sodium (including the sodium bound to the pulp) on the peroxide bleaching stage that follows the pretreatment stage can be eliminated by a suitable addition of magnesium to the pretreatment (complexing) stage. Magnesium is added to the cellulose pulp prior to and/or during the pretreatment stage in an amount corresponding to 0.1-10, preferably 0.2-5 mmol/liter liquid, partially in dependence on the amount of sodium contained in the pulp in the liquid respectively.

The amount of magnesium contained by the pulp after the pretreatment stage and when the pulp comes into contact with the peroxide will mainly depend on the amount of magnesium present in the aforementioned liquid. As earlier mentioned, the cellulose pulp may contain magnesium (and normally in varying quantities) already when it is transported to and introduced into the pretreatment stage, and with regard to those quantities of magnesium in the cellulose pulp recited below it is the sum of any magnesium that is already present in the cellulose pulp and the magnesium that is taken up by the pulp as a result of the magnesium addition to the pre¬ treatment stage. It has been found that different types of cellulose pulps often require a given smallest magnesium content after the pretreatment stage and in the peroxide bleaching stage, in order to achieve a sufficiently good bleaching effect of the peroxide. In some cases, a positive effect is achieved when magnesium is added to the pretreat- ment stage in accordance with the invention even when the sodium content of the pulp suspension is relatively low. The effect achieved by the magnesium will also depend on the manganese content of the pulp at a given magnesium content. The magnesium content of some cellulose pulps will preferably be at least 50 mg Mg/kg absolute dry pulp, while in the case of other cellulose pulps the magnesium content will be at least 90 mg Mg/kg absolute dry pulp. It has also been surprisingly found that at least certain pulps obtain a marked improvement in viscosity at a magnesium content of 50 mg per kg absolute dry pulp, whereas a clearly improved brightness is first obtained with a magnesium content of 90 mg per kg absolute dry pulp.

Suitable parameters for the cellulose pulp pretreatment stage coincide essentially with those pretreatment stage parameters disclosed in the two earlier mentioned Swedish patent applications. Although it is quite possible to use a broad pH-range in the pretreatment stage, for instance a pH from 1-9, an absolute preferred range is 4-7. The amount of magnesium as seen totally in the cellulose pulp and in relation to certain other metals in said pulp will depend partly on the pH applied in the pretreatment stage. The temperature may vary within the range of 20-150°C, although a temperature range of 50-95°C is preferred. Time is related to temperature to some extent and may vary within the range of 1-1000 minutes, although a preferred time range is from 30-300 minutes. A general rule is that the longer the time the better the pretreatment effect. The pulp consistency may vary within the range of 1-40%, preferably within the range of 3-18%.

The complexing agent used for the transition metals, such as manganese and iron, during the pretreatment stage may be of any known kind. Preferred complexing agents are nitro¬ genous polycarboxylic acids, such as EDTA (ethylenediamine- tetraacetic acid), DTPA (diethylenetriaminepentaacetic acid) and NTA (nitrilotriacetic acid) or phospohonic acids and polyphosphates. The complexing agent is charged for instance in an amount corresponding to 0.1 to 10 kg per tonne of absolute dry pulp. The amount of complexing agent charged will depend to some extent on the consistency of the pulp during the pretreatment process. A mixture consisting of several complexing agents may, of course, be used. Although the cellulose pulp may be passed directly from the pretreatment stage to a subsequent peroxide bleaching stage, it is preferred to introduce a washing stage between these two stages. The pulp may be washed in any suitable know manner. For instance, the liquid surrounding the pulp fibres may be displaced more or less completely with the aid of clean water or with the aid of chemical lean waste liquor. The pulp consistency may alternatively be increased radically with the aid of a press, so as to remove the major part of the complex bound metals from the pulp. A combination of these two pulp washing procedures may be used. Any known peroxide may be used in the following bleaching stage, for instance. Suitable peroxides are sodium peroxide, hydrogen peroxide and peracetic acid. Both acid and alkaline conditions can prevail during the bleaching process. It is quite possible and in some instances even preferred, for instance when bleaching sulfite pulp, to add one or more protectors to the peroxide bleaching process. Furthermore, it is advantageous to add a protector to the peroxide bleaching process when the peroxide stage follows immediately after acid treatment stages, for instance when the pulp is bleached with ozone, which is often effected at a pH within the range of 2-5, i.e. when the bleaching scheme includes the sequence Z-Q-P. This also applies when the bleaching scheme includes the sequences Z-P and Q-Z-P. Magnesium sulfate is an example of an appropriate protector. The addition of magnesium sulfate to the peroxide bleaching process can occasionally impair the bleaching result. Other protectors are also useful. However, it is necessary to decide on the protector (investigate) from case to case, sine the choice of protector is influenced among other things by the type of pulp concerned, the choice of bleaching sequence and the position of the peroxide bleaching stage or stages in the bleaching sequence. The inventive method has been found effective in the bleaching of a chemical pulp with oxidative bleaching agent, preferably peroxide bleaching agent, and optimal results are obtained with such a chemical pulp, which is produced from hardwood.

Advantages

The invention enables optimal use of oxidative bleaching agents, for instance different peroxide bleaching agents. By this is meant that after the bleaching stage the cellulose pulp will exhibit a low kappa number (low residual lignin content) and consequently a high brightness with low peroxide consumption. A high pulp viscosity is also retained with optimal use of the peroxide.

Best embodiments Certain parts of the inventive method are described in more detail in the following with reference to batchwise tests carried out in laboratory apparatus with the intention of simulating cellulose pulp bleaching on a technical scale.

Example 1 A mill produced birch sulfate pulp was taken from the pulp system subsequent to the pulp having been bleached with oxygen and washed. The pulp was then washed carefully in the laboratory with deionised water. All of the tests reported in this patent application refer to mill manufactured pulps that have been washed carefully in the laboratory with deionised water.

That starting pulp had a kappa number of 13.5 and an intrinsic viscosity of 1181 dm³/kg and a brightness in % ISO of 48.2. All of the kappa numbers, viscosities and brightness recited in this patent application have been measured in accordance with respective SCAN-C 1:77, SCAN-CM 15:88 and SCAN-C11:75.

With the intention of simulating the presence of the sodium that is normally found in a pulp suspension in the pulp mill, there was added to the pulp having a consistency of 10% sodium sulphate in an amount corresponding to 8.0 grams of sodium per liter of liquid. 2 kg complexing agent in the form of EDTA were added for each tonne of absolute dry pulp, and the pH was adjusted to 5.0. The treatment tempera¬ ture was 90°C and the time 60 minutes.

Three test series were carried out. In one test series, the pulp was pretreated completely in accordance with the aforesaid, meaning that no magnesium was added to the pulp. This test series was thus not carried out in accordance with the invention, but constitutes an example of the standpoint of techniques.

In a second test series, magnesium sulfate was added to the pulp in addition to the aforesaid chemicals, in an amount corresponding to 0.05%, calculated as Mg^2* of absolute dry pulp. This becomes 2.3 when calculated as mmol Mg²⁺/liter of liquid. The addition of magnesium sulfate was tripled in a third test series.

Thus, the pulp samples were pretreated in accordance with the invention in the second and third test series. After pretreating the different pulp samples in the aforedescribed manner, tha samples were washed carefully with deionised water.

The pulps were then bleached with hydrogen peroxide at a pulp consistency of 10%. The temperature was 90°C and the time 240 minutes. The hydrogen peroxide was added in an amount corresponding to 25 kg per tonne of absolute dry pulp. The alkali addition varied between 13 and 16 kg NaOH, calculated per tonne of absolute dry pulp. The amount of peroxide that remained after the bleaching stage ( = residual peroxide) was determined by iodometric titration in accordance with a standard method described in "Textbook of Quantitative Inorganic Analysis", The MacMillan Company, third edition 1952, Kolthoff and Sandell, page 600. The analysis liquid was pressed from the pulp samples. The pulp samples were then washed carefully with deionised water, whereafter the kappa number, viscosity and brightness of the samples were determined. Samples were taken from the pretreated and washed pulps with the intention of determining the calcium, magnesium, manganese, iron an sodium contents of the pulp, all in mg/kg absolute dry pulp, by means of atom absorption. The same analyses were made on the starting pulp, i.e. pulp which had not been pretreated and to which no sodium sulfate had been added.

The results obtained are set forth in Table 1 below. Table 1

As will be seen from tests 1-4, which constitute an example of the standpoint of techniques, the brightness of the starting pulp can be increased from its initial 48% ISO to the interval 72-73% ISO, by adding 25 kg of hydrogen peroxide for each tonne of absolut dry pulp.

In the case of tests 5-8, which represent an application of the present invention, a brightness slightly in excess of 79% ISO was obtained with the same hydrogen peroxide addi¬ tion, without all peroxide being consumed. The kappa number has also been reduced slightly in comparison with tests 1-4, and above all the viscosity of the pulp has become consider- ably higher, on average by about 60 units.

In the case of tests 9-12, in which the magnesium addition was further increased during the pretreatment pro¬ cess and which also represent application of the invention, the brightness has been further increased by about one unit. When studying the amounts in which different metals were present in the starting pulp it will be seen that sodium in particular is the dominant metal, followed by calcium. Magnesium and manganese lie on a completely different and lower level, while iron is present only in trace quantities. When pretreating the pulp in accordance with the standpoint of techniques, tests 1-4, the highest percentage reduction was obtained in the case of manganese, followed by calcium and magnesium. The addition of sodium sulfate to the pulp suspension has quite naturally resulted in a significant increase in the sodium content of the pulp. The skilled person has earlier been wary of transition metals in particular, such as manganese, and has directed the pretreatment of cellulose pulp so as to remove (or render harmless) manganese for instance from the cellulose pulp to the greatest possible extent. In the case of the tests concerned, the manganese content of the pulp has been lowered from 90 mg/kg pulp to just beneath 8 mg/kg. Despite this extensive removal of manganese from the pulp, the result of the peroxide bleaching process was still unsatisfactory. This shows that the manganese content of the pulp is far from being the only parameter that influences the result of the peroxide bleaching. It is important to note that the magnesium content of the pulp in tests 1-4 was reduced from 133 mg/kg in the starting pulp to 30 mg/kg. Compensation of this reduction by adding magnesium sulfate during the pulp pretreatment process so as to raise the magnesium content of the pulp to 103 mg/kg resulted surprisingly in a marked improvement in pulp brightness after the peroxide bleaching stage. It should also be noticed that the sodium content of the pulp is still very high, more specifically 2600 mg/kg. A threefold increase in the addition of magnesium to the pulp during the pretreatment stage resulted in slightly more than a twofold increase in the magnesium content of the pulp, while the sodium content of the pulp decreased to 2260 mg/kg. However, this elevated magnesium input during the pretreatment stage and the elevated magnesium content of the pulp resulted in an increase in pulp brightness of only about 1 unit after the peroxide bleaching. The results achieved indicate that there is a threshold with regard to the magnesium content of the pulp, and _.that this threshold must be exceeded if an acceptable result is to be obtained in the peroxide bleaching.

Example 2

The following tests were carried out with an oxygen bleached hardwood sulfate pulp. The hardwood comprised a mixture of 50% birch and 50% eucalyptus.

The test procedure and the analyses were identical to those recited in Example 1, with the exception that the pretreatment temperature was 70°C, the time 180 minutes, the pulp consistency 12% and the pH-value 7. Further, 0.025% magnesium in the form of magnesium sulfate calculated on absolute dry pulp was added in the pretreatment stage, besides the addition of 0.05% Mg. With regard to the peroxide stage, the sodium hydroxide charge was 16 kg NaOH throughout, calculated per tonne of absolute dry pulp.

Three tests series were carried out. The first test series was carried out in accordance with the standpoint of techniques, i.e. without adding magnesium in the pretreatment stage, and the two following series were carried out in accordance with the invention with the aforesaid magnesium additions.

The results achieved are set forth in the following Table 2. Table 2

Charge of Mg Ca Mg Mn Fe Na Kappa- Vise. Bright. Resid.p. Test % mmol/1 mg kg mg/kg mg/kg mg/kg mg/kg No. dm³/kg % ISO %

With regard to what happens to the pulp during pretreatment in accordance with the standpoint of techniques, it will be seen from these tests and also from the tests reported in Example 1, that, among other things, the presence of large quantities of sodium ions leads to an increase in the sodium content of the pulp and a reduction in the magnesium content of the pulp. This reduction in the magnesium content of the pulp is compensated for by adding magnesium during the pretreatment stage in accordance with the invention. Furthermore, the pulp now contains more magnesium than the starting pulp.

With regard to the result of the peroxide bleaching it will be seen that a magnesium addition as low as 0.025% (tests 15,16) results in a kappa number reduction of about 1 unit, an increase in viscosity of about 40 units and an increase in brightness by a full 9 units, while a considerable part of the peroxide charged to the system remains unconsumed in comparison with the pretreatment and peroxide bleaching of pulp in accordance with the standpoint of techniques (tests 13, 14). A twofold increase in the magnesium addition (tests 17, 18) results in a further marginal reduction of the kappa number, a clear further increase in viscosity and a further, slight increase in brightness, while still more peroxide remains unconsumed after the bleaching process.

These tests show that a splendid result is obtained at pH = 7 during the pretreatment stage, which is fully in line with the aforesaid, preferred pH-range of 4-7 during the pretreatment stage, and that the inventive pretreatment stage also provides a splendid result in the case of those hardwoods which do not exclusively originate from birch.

Example 3

The starting pulp used in the following tests was identical to the starting material recited in Example 2.

In other respects all condiditions were the same as those recited in Example 2, with the exception that the pretreatment stage was effected at pH = 5 and the most important difference that the time was cut to 10 minutes. In Example 1 the pretreatment time was 60 minutes and in Example 2 the pretreatment time was 180 minutes.

The result achieved are set forth in Table 3 below. Table 3

Charge of Mg Ca Mg Mn Fe Na Kappa- Vise. Bright. Resid.p. Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg No. dm³/kg % ISO %

It will be seen from the metal analyses that when the pretreatment time is as short as 10 minutes, the manganese content of the pulp will be somewhat higher, 11-13 mg/kg, than is normal, 7-8 mg/kg. Furthermore, the magnesium charged to the system is unable to bind in the pulp to the same extent as with a pretreatment time of 180 minutes. An addition of 0.025% Mg (tests 21, 22) results in a pulp magnesium content of 60 mg/kg in comparison with a magnesium content of 70 mg/kg of the starting pulp and the magnesium content of 92 mg/kg obtained in the earlier tests (15,16). The difference becomes even more noticeable when adding

0.050% Mg, which resulted in the contents of 90 mg/kg (tests 23, 24) and 263 mg/kg (tests 17, 18).

Despite the short pretreatment time, the addition of 0.025% Mg during the pretreatment stage (tests 21, 22) resulted in a considerable bleaching improvement in comparison with known techniques (tests 19, 20). A twofold increase in the magnesium addition results in further bleaching improvement, particularly with respect to brightness. The fact that the inventive pretreatment step was not carried out in an optimal fashion is also reflected by the low residual peroxide contents (tests 21, 22, 23, 24).

It is evident form these tests that a pretreatment time of 180 minutes is far better than a pretreatment time of 10 minutes, which supports what has been said in the aforegoing, viz. that the positive effect obtained when pretreating pulp in accordance with the invention is enhanced with increasing treatment time up to a point at which the effect flattens out.

Example 4

A further number of tests were carried out on starting pulp of the kind recited in Example 2.

The difference between the following tests and the tests reported in Example 2 is that the temperature during the pretreatment stage was lowered from 70°C to 25°C and that the pretreatment stage in these tests was carried out at pH 5. Table 4

Charge of Mg Ca Mg Mn Fe Na Kappa- Vise. Bright. Resid.p. Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg No. dmVkg % ISO %

Starting pulp -

25 0 0

26

27 0.025 1.4

28 29 0.050 2.8

30

As will be seen from Table 4, the inventive method provides a clear, positive effect even when the temperature in the pretreatment stage is as low as 25°C, as will be seen by comparing the bleaching result from tests 27, 28, 29, 30 with the bleaching results of tests 25, 26.

These tests also show that a higher addition of magnesium during the inventive pretreatment stage need not necessarily lead to a better effect. With regard to the properties of the peroxide bleached pulp, an addition of 0.025% Mg (tests 27, 28) has, in this case, led to a similar result as that obtained with an addition of 0.050% Mg (tests 29, 30), or possibly a marginally better result. Furthermore, the lower magnesium addition has led to an unusually high residual quantity of peroxide; of the 25 kg of hydrogen peroxide initially charged for each tonne of pulp, 8 kg peroxide per tonne of pulp remained unconsumed.

Example 5

The following tests were also carried out on the staring pulp recited in Example 2.

These tests can be seen as a continuation of the tests reported in Example 4, with the difference that pulp consis- tency was lowered from 12% to 3% and that in these tests the pH during the pretreatment stage was 7. The results obtained are set forth in Table 5 below.

Table 5

Charge of Mg Ca Mg Mn Fe Na Kappa- Vise. Bright. Resid.p. Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg No. dm³ kg % ISO %

In all tests, including those above, the complexing agent EDTA has been charged to the pretreatment stage in an amount of 2 kg for each tonne of absolute dry pulp. A reduction in pulp concentration during the pretreatment stage from 12% to 3% implies a reduction of almost 80% in the EDTA- content of the pulp suspension. It will be seen from the metal analyses of the pulp after the pretreatment stage that, for instance, the manganese content was twice as high as normal, probably as a result of the low content of complexing agent in the pulp suspension.

It is also evident from the results that the inventive method led to a markedly improved bleaching result (tests 33, 34, 35, 36) in comparison with the method according to the standpoint of techniques (tests 31, 32), despite a pulp consistency as low as 3% during the pretreatment stage, leading for intstance to a higher than normal manganese content of the pulp. The result obtained with an input of 0.025% magnesium was at least equally as good as the result obtained with an input of 0.050% magnesium also in these tests. Example 6

The starting pulp used in the following tests was an oxygen bleached birch sulfate pulp.

The tests and the pulp analyses were identical to those reported in Example 1, with the exception that only one charge of sodium hydroxide was used, in the peroxide bleaching, namely 16 kg NaOH calculated per tonne of absolut dry pulp. In other regards, 0.05% magnesium was charged to the system calculated per tonne of absolute dry pulp, either in the pretreatment stage or in the peroxide bleaching stage or in both of these stages.

The results achieved are set forth in Table 6 below. Table 6

Charge of Mg Ca Mg Mn Fe Na Charge of Kappa- Vise. Bright. Resid.

Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg Mg % No. dm³ kg % ISO %

Start.pulp - 37 0 0

38

43 0.05 2.3 44 0.05 2.3

As will be seen from the Table, the first four tests were carried out in accordance with the standpoint of techniques. The most common procedure when practising the standpoint of techniques is to add magnesium to the peroxide bleaching stage after pretreating the pulp in the sole presence of a complexing agent (tests 39, 40). This addition leads to a marginal reduction in kappa number and a pronounced increase in viscosity, while increasing brightness by 5 units. A very low residual peroxide concentration is obtained. All this in comparison with tests 37, 38, in which no magnesium was added to the pulp. The last four tests were carried out in accordance with the invention. The bleaching results obtained were clearly best in tests 41, 42, in which no magnesium was added to the peroxide bleaching. In comparison with the best method according to the standpoint of techniques (tests 39, 40), the kappa number was further reduced by almost one unit while the brightness increased by a further 4.5 units and the residual peroxide content was comparatively very high.

The addition of magnesium to the peroxide bleaching stage (tests 43,44) when practising the inventive method results in impaired bleaching results. For instance, the brightness is impaired by two units and the residual peroxide content is less than half the original.

Nevertheless, these tests show a much better bleaching result than the bleaching results obtained in the best tests (39, 40) carried out in accordance with the earlier technique.

The aforesaid is valid for a birch sulfate pulp and should therefore presumably be valid for hardwood sulfate pulps in general.

Example 7

The starting pulp used in the following tests was an oxygen bleached softwood sulfate pulp. The tests were identical in other respects with the tests reported in Example 6. The results achieved in these tests are set forth in Table 7 below. Table 7

Charge of Mg Ca Mg Mn Fe Na Charge of Kappa- Vise. Bright. Resid.p.

Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg Mg % No. dm³/kg % ISO %

Start.pulp -

The pattern is repeated from Example 6 with regard to the bleaching results obtained. The best results were obtained in those tests (49, 50) that were carried out in accordance with the present invention, where no magnesium was added to the peroxide bleaching stage. The next best results were achieved in those tests (51, 52) that were carried out in accordance with the present invention and in which magnesium was added both in the pretreatment stage and in the peroxide bleaching stage, followed by the best tests (47, 48) carried out in accordance with the standpoint of techniques, i.e. in which magnesium was added to the peroxide bleaching stage.

The difference between the bleaching results obtained with softwood sulfate pulp are much smaller in this example than the differences between the results obtained when bleaching hardwood sulfate pulp.

Example 8

The starting pulp used in the tests reported in this example was a sulfite pulp that had been digested with sodium as the base, followed by an oxygen reinforced alkali (extraction) stage.

These tests are also identical with the tests reported in Example 6. The only difference between the tests lies in the starting pulp, i.e. the pulp on which the tests were carried out.

The results achieved are set forth in Table 8 below.

Table 8

Charge of Mg Ca Mg Mn Fe Na Charge of Kappa- Vise. Bright. Resid.p. Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg Mg % No. dm³/kg % ISO %

it will be seen from the metal analyses that the metal content of the sulfite pulp differs markedly from the metal content of the sulfate pulp (both softwood and hardwood) with very low values of all the metals that were determined.

The first four tests were also carried out in accordance with the standpoint of techniques and the four following tests in accordance with the invention.

When comparing the tests 53, 54 according to the standpoint of techniques with the tests 57, 58 carried out in accordance with the invention it is found that the addition of magnesium to the pretreatment stage results in a viscosity increase by about 90 units and that the residual peroxide content is twice as high, while brightness is about one unit lower.

In order to achieve an optimal result when practising the inventive method, it is necessary to add magnesium to the peroxide bleaching stage (tests 59, 60) in addition to the pretreatment stage. It will be seen from the Table that good bleaching results were also obtained in the reference tests (55, 56), in which magnesium was added to the pulp only in the peroxide bleaching. However, the inventive method (tests 59, 60) resulted in a slightly higher viscosity and a slightly lower peroxide consumption than the best tests (55, 56) carried out in accordance with the standpoint of techniques, whereas brightness is slightly lower.

Thus, when practising the present invention on sulfite pulp, it is of greatest importance to also add magnesium to the peroxide bleaching stage.

Example 9

The starting pulp used in this example was a birch sulfate pulp produced by extended cooking and thus having a commensurately low kappa number.

The tests reported in this example differ from all other tests reported hitherto insomuch that no sodium sulfate was added to the pulp samples.

In these tests, the pulp present in the pretreatment stage was supplied with a complexing agent in the form of EDTA, in an amount corresponding to 2 kg per tonne of absolute dry pulp. The pulp had a consistency of 10% and a pH of 6. The temperature was 90°C and the time 60 minutes, with the exception of two tests in which the time was extended to 960 minutes. Reference is made to the following text and to Table 9 with regard to the addition of magnesium in the pretreatment stage.

Subsequent to pretreating the various pulp samples in the aforedescribed manner, the samples were washed carefully with deionised water.

The pulps were then bleached with hydrogen peroxide at a pulp consistency of 10%. The temperature was 90°C and the time 240 minutes. The hydrogen peroxide was added in an amount corresponding to 25 kg per tonne of absolute dry pulp. The alkali charge corresponded to 16 kg NaOH per tonne of absolute dry pulp.

Magnesium in the form of magnesium sulfate was added to the pulp suspension, either in the pretreatment stage or in the bleaching stage or in both stages, as evident from the Table.

The results are set forth in Table 9 below. Table 9

Charge of Mg Ca Mg Mn Fe Na Charge of Kappa- Vise. Bright. Resid. Test % mmol/1 mg/kg mg/kg mg/kg mg/kg mg/kg Mg % No. dm³/kg % ISO %

When analysing the pulp to determine its metal content, a residue was found that had not been found in any of the earlier described pulps. Later research into the manner of 0 manufacture of the pulp in the pulp mill showed that talcum and sludge having a high calcium carbonate content is added as a matter of routine when digesting the wood. These substances are normally added in relatively small quantities, although when manufacturing this particular starting pulp it 5 is thought that either talcum or sludge, or both of these substances, were added in quantities many times the ususal quantity by mistake. It will be seen from the Table that the amount of calcium present in this starting pulp (birch sulfate pulp) is five times greater than the contents of the 0 earlier used starting pulps (birch sulfate pulp).

Furthermore, in order to reach the pH value 6 during the pretreatment stage, it was necessary to add to the pulp suspension much more acid than was required in the earlier tests, which was a further sign that this starting pulp was 5 unusual.

Talcum is comprised of a large number of chemical compounds. Quantitatively, silicone dioxide (Si0₂) dominates, followed by magnesium oxide (MgO). The sodium content of the starting pulp was low and extremely low in the treated pulps. The remaining, still not yet mentioned metal contents are all relatively high, with the exception of the magnesium content in tests 69, 70, in which the pulp was pretreated for 960 minutes.

It is assumed that the complexing agent charge of 2 kg per tonne of absolute dry pulp is too low for this "impure" pulp.

Of those tests reported in the above Table, tests 61, 62, 63, 64 and 69, 70 were carried out according to the standpoint of techniques, whereas tests 65, 66, 67, 68 were carried out in accordance with the invention.

As will be seen from the tests overall, the pulp was difficult to bleach and also the starting pulp had a low brightness. In those tests (61, 62) in which no magnesium was added to any of the stages, the pulp brightness was about 68% ISO. The addition of magnesium in the peroxide bleaching stage (tests 63, 64) impaired the brightness by two units and the kappa number was marginally higher. Extremely long pretreatment times in the pretreatment stage (tests 69, 70) resulted in still lower pulp brightness after the peroxide bleaching, even though the kappa number was lowered slightly.

The addition of magnesium to the pretreatment stage in accordance with the invention (tests 65, 66) resulted in a brightness which was a full 7 units higher than the brightness achieved in the best tests (61, 62) carried out in accordance with the standpoint of techniques, and the kappa number was lowered by almost a full unit. Furthermore, a certain amount of residual peroxide was obtained with these tests, which was not the case in any of the other tests.

The addition of magnesium to both the pretreatment stage and the peroxide bleaching stage (tests 67, 68) greatly lowered the brightness of the pulp in comparison with the best tests (65, 66) carried out in accordance with the invention. A somewhat elevated viscosity was obtained, however.

This test series shows that the inventive method provides a clearly improved bleaching effect even in certain cases which are not connected with a high sodium content of the pulp and/or a high sodium content of the liquid which is enclosed in and surrounds the pulp fibres. As evident from the aforegoing, the inventive method is also effective in those instances in which something unusual occurs during the manufacture of the cellulose pulp, for instance when digesting wood to pulp under extraordinary conditions, for instance totally wrong additions of additive chemicals. Additive chemicals, such as talcum and sludge, for instance, are often added to the system to maintain the resin dispersed in the liquid phase during the digestion process and the following treatment stages.

The inventive method has also been in a few runs tested on mechanical pulps. No clearly positive bleaching effect could be shown however, although it was found that mechanical pulp that had been pretreated in accordance with the invention did not require the addition of waterglass during the following peroxide bleaching stage, which is more or less a necessity when peroxide bleaching mechanical pulps in accordance with conventional methods.

Claims

29 CLAIMS

1. A method in delignifying and/or bleaching of lignin containing cellulosic pulp, in which the pulp is pretreated with a complexing agent in the presence of liquid and sodium ions and in which process foreign magnesium in any form is added to the cellulose pulp prior to and/or during the pretreatment, and then is treated with oxidative bleaching agent, c h a r a c t e r i z e d b y that the magnesium is added in a quantity which is determined at least partially by the amount of sodium ions present so that after the pretreat- ment stage the pulp will contain this substance in an amount corresponding to at least 50 mg Mg/kg absolute dry pulp, including any magnesium that was already present in the pulp.

2. A method according to Claim 1, c h a r a c t e r i z e d b y adding magnesium in a quantity of 0.1-10, preferably

0.2-5 mmol/liter liquid.

3. A method according to Claims 1-2, c h a r a c t e r i z e d b y adding magnesium so that after the pretreatment stage the pulp will contain this substance in an amount corresponding to at least 90 mg Mg/kg absolute dry pulp, including any magnesium that was already present in the pulp.

4. A method according to Claims 1-3, c h a r a c t e r i z e d b y adding magnesium in the form of magnesium sulfate.

5. A method according to Claims 1-4, c h a r a c t e r i z e d b y that the cellulose pulp is a chemically produced hardwood pulp.

6. A method according to Claims 1-5, c h a r a c t e r i z e d b y carrying out the pretreat¬ ment at a pH of 1-9, preferably 4-7.

7. A method according to Claims 1-6, c h a r a c t e r i z e d b y washing the pulp after the pretreatment.

8. A method according to Claims 1-7, c h a r a c t e r i z e d b y that the complexing agent is EDTA.

9. A method according to Claims 1-8, c h a r a c t e r i z e d b y that the oxidative bleaching agent is a peroxide.