Cooking of chemical pulp in sulphur-free liquor rontainmq a dellain fication-acoeJeratmg coumpound

TECHNICAL FIELD:

The present invention relates to a process for producing chemical pulp from comminuted cellulose- containing material without using sulphur-containing cooking liquids. The process according to the invention includes at least one impregnation stage followed by at least one cooking stage, with a deligniflcation- acceleratmg compound, such as an anthraqumone, being supplied to at least one of the said stages. In the process according to the invention, a first cooking liquid which has been extracted from a position downstream of the impregnation stage is supplied to the impregnation stage, with the said first cooking liquid primarily containing OH ^" ions as the active substance. The returning of extracted cooking liquid makes it possible to achieve a higher concentration of OH ^" ions in the cooking stage which succeeds the said impregnation stage than is possible in the case of conventional soda-anthraqumone cooking. According to a preferred embodiment, a metal-complexing agent is supplied to the impregnation stage or to another stage located upstream of the cooking stage.

The process according to the invention is suitable for producing chemical pulp from essentially all the generally available raw materials for cellulose production. Examples of suitable raw materials are softwood chips derived from fir and pine and hardwood chips derived from birch, aspen and eucalyptus. The process according to the invention is also suitable for producing pulp from non-Scandinavian raw materials such as southern pine or loblolly pine and can also be used for producing pulp from straw, reed-grass, esparto grass and other alternative raw materials.

The process according to the invention is suitable both for producing pulp which i s intended for

ECF or TCF bleaching and for producing unbleached high- yield pulp.

STATE OF THE ART: Recently, techniques for producing pulp have to an ever greater extent been directed by the environ¬ mental demands imposed by authorities, environmental organizations and customers. This trend has led to pulp manufacturing processes in which there is a reduced use of chlorine chemicals during bleaching, an increased degree of closure and a decreased consumption of energy.

A familiar problem associated with producing pulp by the sulphate method or the sulphite method is the problem of troublesome smells in the vicinity of the pulp mill and, in certain cases, even at a distance of several tens of kilometres. These smell problems are principally caused by various sulphur compounds which are produced when pulp is cooked using sulphur- containing cooking liquids. Examples of such malodorous compounds are hydrogen sulphide (H S) and methyl- mercaptan (CH,SH) . Various solutions, such as pur:fyιng exhaust gases and improving the sealing of the pulp manufacturing process, have been proposed for remedying the smell problems.

A disadvantage of using sulphur-containing cooking liquids for cooking pulp is that a relatively complicated chemical recovery system is required in order to be able to recirculate the chemicals. The presence of sulphur also impedes efforts, which have been in progress for a long time now, to close the pulp manufacturing process.

Another disadvantage of sulphur-containing pulp cooking is that sulphur discharges from the pulp manufacturing process contribute to the ever increasing acidification of the environment.

It is very clear, therefore, that it would be advantageous to completely eliminate the use of sulphur in pulp production.

For this reason, various processes which do not make use of sulphur-containing cooking liquids have already been proposed.

For example, patent specification SE-C2-502 667 previously disclosed a method for producing chemical pulp from comminuted cellulose-containing material, which method comprised cooking the fibre material with a cooking liquid. The object of the invention which is described is stated as being that of producing a chemical pulp which, after the cooking, already has a substantially reduced content of transition metals and at the same time has considerably improved properties, especially, but not exclusively, with regard to tear resistance, viscosity, yield, kappa number and brightness.

According to the abovementioned patent specification SE-C2-502 667, these advantages are achieved by means of a pretreatment, prior to the cooking, with a compound which has the ability to complex with metals which are naturally present in the fibre material. The pretreatment with complexing agents is stated to give, within the same kappa number interval, a lower content of metals such as manganese, a tear resistance which is at least 10 higher, a viscosity which is at least 5" higher and a yield which is at least lϊ higher as compared with the corresponding parameters for a pulp produced without the said pretreatment. The method which is described for producing chemical pulp is stated to be suitable for any method for producing chemical pulp, including anthraquinone/hydroxide pulp (NaOH/KOH) as well. No cooking liquids containing sulphur are used in a pulp manufacturing process of this nature.

It is thus previously known to pretreat chips with complexing agents and then carry out isothermal cooking using a cooking liquid which principally contains OH ^" as the active substance in the cooking, and using anthraquinone as a delignification-accelerating additive.

However, it is not evident from the said patent specification, SE-C2-502 667, how this abovementioned cooking without sulphur chemicals should be carried out in practice. It is a previously known fact that conventional soda-anthraqumone cooking requires a high cooking temperature in order to achieve satisfactory ligmn dissolution. The high temperature gives rise to a high consumption of energy and to a serious risk of carbohydrate breakdown.

ACCOUNT OF THE INVENTION:

An object of the present invention is therefore to provide a process for producing chemical pulp from comminuted cellulose-containing material, which process yields a perfectly satisfactory pulp quality at a competitive cost without any sulphur-containing cooking liquid having to be used.

This object of the invention is achieved by the process according to the invention including at least one impregnation stage followed by at least one cooking stage with at least one concurrent cooking zone and at least one countercurrent cooking zone and, as the conclusion to the cooking stage, at least one extended displacement zone, with a delignification-acceleratmg compound being supplied to at least one of the said stages and a first cooking liquid, which has been drawn off from a position downstream of the impregnation stage, being supplied to the impregnation stage, with the cooking liquid essentially containing OH ^" ions as the active substance, and by the concentration of OH ^" ions in the said concurrent cooking zone being higher than m the said impregnation stage, preferably at least 20" higher and most preferably at least 50 higher.

The concentration of OH ^" ions at the beginning of the concurrent cooking zone of the cooking stage must exceed 0.5 mol/1, and preferably 0.7 mol/1. Most

preferably, the concentration of OH ions must exceed 1.0 mol/1.

Under certain circumstances, at can be advantageous to make a further addition of OH ^" ions to the previously mentioned first cooking liquid before it is supplied to the said impregnation stage.

As previously mentioned, a delignification- acceleratmg compound, for example anthraqumone, is supplied when the process according to the invention is being implemented.

It would be evident to the skilled person that several types of unsubstituted or substituted qumones or hydroqumones can also be used as the delignification-accelerating compound, such as anthraqumone, benzoqumone or naphthoqumone, or salts of these or other similar compounds, since such compounds have been found to give a similar effect.

1, 4-Dιhydro-9, 10-dιhydroxyanthracene has been found to be especially suitable for use as the delignification-acceleratmg compound when the invention is being implemented.

When the process according to the invention is being implemented, the best results have been obtained when the major part of the delignification-acceleratmg compound is added to the previously mentioned concurrent cooking zone.

In this connection, the total delignification- acceleratmg compound added should exceed 1 kg per tonne of bone dry wood and preferably exceed 1.5 kg per tonne of bone dry wood. The optimum amount to be added is between 1.8 and 2.5 kg per tonne of bone dry wood.

The quantity of delignification-acceleratmg compound which is added to the impregnation stage mutt be less than 50 ⁽ ό by weight of the total amount added and preferably less than 30. by weight. The best results are obtained if less than 10, by weight of the total amount of the delignification-acceleratmg compound added is added to the impregnation stage.

It has also been found that the total quantity of delignification-acceleratmg compound which is added must exceed 0.5o by weight of the total quantity of effective alkali (EA) , calculated as NaOH, which is added to the process. Preferably, the total quantity of delignification-accelerating compound should exceed 1" by weight, and most preferably be from 1.1 to 1.5" by weight, of the total quantity of effective alkali (EA) which is added. The fact that the abovementioned first cooking liquid is returned to the impregnation stage makes it possible to achieve a higher concentration of OH ^" ions m the cooking stage, especially at the beginning of the said concurrent cooking zone, as compared with conventional soda-anthraqumone cooking. Furthermore, returning the cooking liquid means that the heat energy losses are minimized.

Taken together, this provides a more efficient delignification process, thereby making it possible to achieve a second mam object of the invention, namely that of providing a process for producing chemical pulp, which process requires a lower temperature during cooking than do previously known processes for producing pulp without sulphur-containing cooking liquids, for example soda-anthraqumone cooking accordance with the state of the art.

The lower temperature level which can be used in the process according to the invention results in lower energy consumption and in there being less risk of carbohydrate breakdown.

It has been found that, while the temperature in the impregnation stage, when implementing the invention, may be within the interval 80-140°C, it should preferably be between 100 and 130°C, and should most preferably be between 120 and 127°C.

While the temperature the subsequent cooking stage may, when cooking softwood, be within the interval 150-170°C, it should preferably be between 158 and 163°C.

When cooking hardwood, the temperature m the said cooking stage may be within the interval 140-165°C and should preferably be between 155 and 158°C.

It has been found to be especially advantageous if the cooking is carried out isothermally in at least two adjacent zones in the said cooking stage.

According to a preferred embodiment of the invention, a metal-complexmg agent is supplied to the impregnation stage or to another stage upstream of the cooking stage. In this context, a metal-complexmg agent is understood as being a compound which has the ability to bind metals which are harmful to the pulp production process m the form of metal complexes which can, if required, be removed from the process. Thus, metal complex-containing extraction liquid which has been extracted from the impregnation stage can be removed from the process or conveyed to evaporation. In this connection, an excess of cooking liquid which has been extracted from the cooking stage can be conveyed to evaporation at the same time.

Examples of compounds which are suitable for use as metal-complexing agents are DTPA, diethyl- tπammepentaacetic acid, and EDTA, ethylenediamine- tetraacetic acid, but other similar compounds can also be used. DTPA has been found to be most advantageous for use in the process according to the invention.

According to a preferred embodiment of the invention, a second cooking liquid, having a lower concentration of OH ^" ions than is present in the said first cooking liquid, is supplied to the previously mentioned extended displacement zone.

When the process according to the invention is being implemented, the kappa number after the cooking stage is between 15 and 30, and preferably between 18 and 25, when softwood pulp which is intended for bleaching is being cooked.

The kappa number is between 9 and 25, and preferably between 12 and 18, when hardwood pulp which is intended for bleaching is being cooked.

The kappa number is between 40 a d 100, and preferably between 50 and 85, when high-yield pulp which is not to be bleached is being cooked.

It has been found that pulp which has been produced by the process according to the invention and which has then been oxygen-delignifled m a manner familiar to the skilled person can be bleached to a brightness exceeding 80" ISO both by means of ECF bleaching and TCF bleaching (ECF: elemental chlorine- free, TCF: totally chlorine-free) .

When higher brightness levels are required, for example higher than 85'. ISO, m certain cases higher than 88V ISO, it has been found that, while pulp which has been produced by the process according to the invention can indeed be bleached to such high brightness levels, the consumption of bleaching chemicals is then greater than is the case when bleaching conventional oxygen-delignifled sulphate pulp. When pulp which has been produced by the process according to the invention is being ECF- bleached, it has been found advantageous for the bleaching sequence to comprise the bleaching stages

(DQ) (PO) . When carrying out TCF-bleachmg, a bleaching sequence is advantageously used which comprises the bleaching stages (Q) (OP) (ZQ) (PO) .

For the rest, the process according to the invention can be implemented using apparatus and equipment which are suitable for preparing pulp and which are familiar to the skilled person.

The process according to the invention makes it possible to achieve a more efficient cooking process as compared with sulphur-free cooking in accordance with the state of the art, together with the previously mentioned advantages arising from the ability to use lower process temperatures.

BRIEF DESCRIPTION OF THE FIGURES:

The attached Figures 1, 2, 3 and 4 show typical results from beating experiments which were carried out using unbleached pulps which were obtained by processes according to the invention (Nos. ITC 1666 and ITC 1668) . For comparison, corresponding results are shown for pulps which were produced by conventional ITC sulphate cooking (No. ITC 1544) and by soda- anthraqumone cooking (ITC 1618) in accordance with the state of the art.

The attached Figures 5, 6, 7 and 8 show results from beating experiments using bleached pulps which were produced by processes according to the invention and which, after oxygen-delignification, were bleached by ECF bleaching using the bleaching sequence (DQ) (PO) and by TCF bleaching using the bleaching sequence Q (OP) (ZQ) (PO) , respectively. In this context, Fig. 1 shows the tensile index as a function of the number of revolutions when beating m a PFI beater,

Fig. 2 shows the tear index as a function of the number of revolutions when beating m a PFI beater, Fig. 3 shows the tear index as a function of the tensile index,

Fig. 4 shows the tensile index as a function of the density, Fig. 5 shows the tensile index as a function of the number of revolutions when beating in a PFI beater,

Fig. 6 shows the tear index as a function of the number of revolutions when beating in a PFI beater, Fig. 7 shows the tear index as a function of the tensile index, and

Fig. 8 shows the tensile index as a function of the density.

EXAMPLES :

In that which follows, the invention is illustrated using process data and results from a number of laboratory experiments. The experiments are reported in Tables 1 to 7 below. The tables record cooking experiments using processes according to the invention and experiments using conventional isothermal (ITC) sulphate cooking together with experiments using isothermal (ITC) soda-anthraquinone cooking in accordance with the state of the art. In addition, results are recorded from a number of oxygen- delignification and bleaching experiments.

In the experiments, softwood chips were cooked on a laboratory scale in a standardized manner as described below, thereby simulating a continuous cooking process.

Presteaming:

Scandinavian softwood chips corresponding to 1.8 kg of bone dry chips were added to a circulation digester and subjected to steaming at 110°C for 5 minutes under a pressure of 1.0 bar.

Impregnation: Impregnation liquid was allowed to circulate through the digester at 125°C for 45 minutes at the same time as a nitrogen pressure of 10 bar was applied.

In the two experiments using the process according to the invention, cooking liquid which had been extracted, and returned, from the subsequent cooking phase was used for the impregnation. This gave a liquid:wood ratio of 5.5:1. In addition, a etal- complexing agent, DTPA, was added during the impregnation at the rate of 2 kg per tonne of bone dry wood.

In the experiment using isothermal (ITC) soda- anthraquinone cooking in accordance with the state of the art, an OH ^" ion-containing impregnation liquid was used, with the liquid:wood ratio being 3.6:1. No metal-

complexing agent was added to the impregnation in this experiment. However, a delignification-acceleratmg compound, 1, -dιhydro-9, 10-dιhydroxyanthracene, was added at the rate of 1 kg per tonne of bone dry wood. Only conventional white liquor was added the experiment using conventional isothermal (ITC) sulphate cooking, with the lιquιd:wood ratio being 3.6:1. As a consequence, neither a delignification-acceleratmg compound nor a metal-complexmg agent was added in this experiment.

Concurrent cooking:

After the impregnation had been carried out, the nitrogen pressure was released from the digester and the temperature was raised to the cooking temperature at the same time as further cooking liquid was added. The dwell time during this phase was 60 minutes.

In the two experiments using cooking m accordance with the invention, all the free liquid was extracted after the impregnation and replaced with cooking liquid containing OH ^" ions, such that a lιquιd:wood ratio of 4:1 was obtained. In addition, the previously mentioned delignification-acceleratmg compound was added during the concurrent cooking at the rate of 2 kg per tonne of bone dry wood.

In the experiment using isothermal (ITC) soda- anthraquinone cooking in accordance with the state of the art, further cooking liquid was added, such that the liquid:wood ratio became 4:1. In addition, the previously mentioned delignification-acceleratmg compound was added at the rate of 0.5 kg per tonne of bone dry wood.

In the experiment using conventional isothermal (ITC) sulphate cooking, only conventional white liquor was added, such that a lιquιd:wood ratio of 4:1 was obtained.

Countercurrent cooking:

After the concurrent cooking had been completed, a countercurrent cooking was initiated with 7.2 1 of cooking liquid containing 25 g/1 effective alkali (EA) , calculated as NaOH, being pumped in gradually and being allowed to displace the corresponding quantity of used cooking liquid from the concurrent cooking. The temperature was the same as for the concurrent cooking except m experiment No. ITC 1668, where the tempera-ture, after having been at a lower level during the concurrent cooking, was raised during the countercurrent cooking. The concentration of the cooking liquid in the countercurrent phase was adjusted such that approximately 15-25 g/1 effective alkali (EA) remained when the countercurrent cooking had been completed.

In the experiment using isothermal (ITC) soda- anthraqumone cooking in accordance with the state of the art, the delignification-acceleratmg compound was additionally added during the countercurrent cooking at the rate of 0.5 kg per tonne of bone dry wood.

Extended displacement washing - Hi-Heat:

In the extended displacement phase, the exchange of chemicals between the exhausted cooking liquid and the supplied cooking liquid continued at the same temperature as during the concurrent cooking and the countercurrent cooking. A cooking liquid containing 10 g/1 effective alkali (EA) , calculated as NaOH, was added in this context. In this way, 10.3 1 of cooking liquid were displaced over a period of 180 minutes.

Finally, the fully cooked chips were transferred to a propeller-equipped disintegrator for defibenng for a period of 15 minutes. After the resulting, unscreened pulp had been washed and thickened, its yield was determined. After that, the pulp was screened in two stages, with the coarser rejects being separated off in a perforated screen basket having a hole diameter of 1. mm under a water

pressure of 150 kPa. The remaining fine rejects were separated off on a vibrating diaphragm screen having a 0.15 mm gap-width.

Process conditions during the cooking and the results which were obtained using processes according to the present invention are given in the enclosed Table 1, see experiments No. ITC 1666 and ITC 1668. For comparison, the said Table 1 records the corresponding process parameters and results for a cooking experiment which was carried out using conventional isothermal

(ITC) sulphate cooking, see experiment No. ITC 1544.

The said Table 1 also records the corresponding process parameters and results for a cooking experiment which was carried out using isothermal (ITC) soda- anthraqumone cooking in accordance with tne state of the art, see experiment No. ITC 1618.

Enclosed Table 2 records process parameters from two oxygen-delignification experiments, m this case for pulp which was cooked by conventional ITC sulphate cooking and for pulp which was cooked by a process according to the invention. The other pulps were oxygen-delignified in a similar manner, after which the pulps were bleached.

Table 3 records process parameters and results from ECF bleaching experiments using the bleaching sequence (DQ) (PO) . In this context, the abbreviation ECF stands for elemental chlorine-free and may, like the abbreviations used in the bleaching sequence, be regarded as being familiar to skilled persons in the field. In the experiments recorded m Table 3, pulp which was cooked by conventional ITC sulphate cooking, or pulp which was cooked by isothermal soda- anthraquinone cooking in accordance with the state of the art, was bleached after oxygen-delignification. Table 4 likewise records corresponding process parameters and results from an ECF bleaching experiment using the bleaching sequence (DQ) (PO) , but in this case for an oxygen-delignified pulp which was cooked by a process according to the invention.

Table 5 likewise records results from bleaching experiments with an oxygen-delignified pulp which was cooked m accordance with the invention; however, m this case, a TCF bleaching sequence, i.e. Q(OP) (ZQ) (PO) , was used. In this context, the abbreviation TCF stands for totally chlorine-free, which, like the bleaching sequence, is probably familiar to skilled persons in the field.

Table 6 records the pulp properties of the unbleached pulps which were obtained by means of the cooking experiments recorded in Table 1.

Finally, Table 7 records the pulp properties of pulp which was cooked by a process according to the invention and which, after oxygen-delignification, was bleached by means of ECF bleaching, using the bleaching sequence (DQ) (PO) , or by means of TCt bleaching using the bleaching sequence Q(OP) (ZQ) (PO) .

CONCLUSIONS: Based on that which is recorded in the enclosed figures and tables, the following conclusions can be drawn.

Unbleached softwood pulp which has been produced by the process according to the invention exhibits, at the same kappa number, yields and tear resistances which are of the same level as those for a sulphate pulp produced by conventional ITC sulphate cooking.

Pulp which has been produced m accordance with the invention exhibits a tear resistance which is clearly higher than that of pulp which has been produced by isothermal (ITC) soda-anthraqu one cooking m accordance with the state of the art.

It is also evident that processes in accordance with the invention make it possible to achieve the same kappa number as that obtained by conventional ITC sulphate cooking at an H factor of the same order of size.

It is evident from the experiment using isothermal soda-anthraqumone cooking in accordance with the state of the art that, m this case, a substantially higher H factor is required order to achieve the same kappa number, as compared with cooking m accordance with the invention.

This means, therefore, that the process in accordance with the invention, unlike isothermal soda- anthraqumone cooking in accordance with the state of the art, makes it possible to lower the cooking temperature to the same level as for conventional ITC sulphate cooking, thereby ensuring substantial energy savings and reduced risk of carbohydrate breakdown.

Based on the beating experiments which were carried out in a PFI beater, it appears that unbleached pulp which has been cooked in accordance with the invention requires more beating revolutions than do the two reference pulps which were cooked accordance with the state of the art. The bleaching experiment using the (DQ) (PO) bleaching sequence (ECF bleaching) shows that a perfectly satisfactory brightness can be achieved for pulp which has been produced in accordance with the invention. The bleaching experiment using the

Q(0P) (ZQ) (PO) bleaching sequence (TCF bleaching) likewise shows that, in this case too, a perfectly satisfactory brightness can be achieved for pulp which has been produced in accordance with the invention. In summary, the results obtained show that the process m accordance with the invention makes it possible to produce both unbleached and bleached pulps having perfectly satisfactory pulp properties without any use of sulphur-containing cooking liquids and at a substantially lower cooking temperature than in the case of isothermal soda-anthraqumone cooking m accordance with the state of the art.

In the foregoing text, the present invention has been illustrated with the aid of examples

comprising process data and results from a number of experiments. However, the invention is in no way restricted to that which has been stated in this context and can naturally be varied within the scope of the attached patent claims.

Table 1

(AQ : delignification-acceleratmg compound)

Table 2 OXYGEN-DELIGNIFICATION

Laboratory-scale oxygen-delignlfication of pulp which has been cooked using conventional ITC sulphate cooking or in accordance with the invention.

PULP FROM COOK NO. , ITC 1544, ITC 1666,

TYPE OF COOKING SULPHATE (ITC) SULPHUR-FREE

IN ACCORDANCE COOKING IN

WITH THE STATE OF ACCORDANCE WITH

THE ART THE INVENTION

Oxygen-delignification No. S-1429 S-1459

PRIOR TO OXYGEN-

DELIGNIFICATION

Kappa number 23.8 22.8

Viscosity, drnVkq 1326 1018

OXYGEN-DELIGNIFICATION

Dry substance content, t 10 10

Temperature, °C 95 95

Time, minutes 60 60

Initial pressure, bar 5 5

NaOH added, kg/BDMT 20 20

MgSO„ added, kg/BDMT 3 3

AFTER OXYGEN-DELIGNIFICATION

Final pH 11.4 11.0

Kappa number 10.3 11.1

Viscosity, dm /kg 103j 884

Table 3

Table 4

Table 5

Table 6

PULP PROPERTIES - UNBLEACHED PULP AFTER COOKING

PULP TYPE IN ACCORDANCE IN ACCORDANCE

WITH THE STATE WITH THE

OF THE ART INVENTION

Pulp No. ITC ITC ITC ITC

1544 1688 1666 1668

Kappa number 23.8 25.1 22.8 23.8

Viscosity, dm ³ /kg 1326 940 1018 1022

Weighted mean fibre length, mm 2.37 2.36 2.49 2.52

Fibre strength - zero span, Nm/g 149 137 145 148

Interpolated pulp properties at a tensile index of 80 kNm/kg:

PFI beater, revolutions 900 1100 1400 1600

Schopper-Riegler number, °SR 15.5 16.5 16.5 17.0

Density, kg/m 630 640 620 620

A r resistance, sec/100 ml 3.0 4.0 3.0 4.0

Burst index, MN/kg 6.0 5.3 5.6 5.7

Tear index, Nm /kg 19.5 17.4 20.2 19.2

Table 7

PULP PROPERTIES - OXYGEN-DELIGNIFIED SOFTWOOD PULP

PRODUCED BY THE PROCESS ACCORDING TO THE INVENTION,

AFTER ECF BLEACHING AND AFTER TCF BLEACHING

BLEACHING SEQUENCE (DQ) (PO) Q(OP) (ZQ) (PO)

Bleaching experiment No. B-3158 B-3159

Kappa number 0.9

Viscosity, dmVkg 721 661

Brightness, ISO 89.1 88.9

Weighted mean fibre length, mm 2.16 2.22

Fibre strength - zero span, Nm/g 112 105

Interpolated pulp properties at a tensile index of 80 kNm/kg: PFI beater, revolutions 1300 1400 Schopper-Riegler number, ^C SR 17.5 19.5 Density, kg/m 660 660 Air resistance, sec/100 ml 4.0 4.6 Burst index, MN/kg 6.2 6.4 Tear index, Nm ^* 7kg 17.4 16.5