Technical field

The invention relates to a method of delignifying/- bleaching lignin-containing celllulose pulp. The cellulose pulp is produced from any known lignocellulosic material with the use of any known alkaline or acid pulping process. Also included are digestion processes that include an alkaline or neutral stage and an acid stage. A wide variety of ligno¬ cellulosic materials are available in varying quantities worldwide. A very common lignocellulosic material is wood, which is normally chopped into chips prior to the digestion or cooking process. Examples of different wood types are softwoods and hardwoods. Examples of known alkaline digestion processes are the sulfate process, the polysulfide process and processes of the soda type (sodium hydroxide) in which catalysts are used, for instance some quinone compound. The term sulfate process includes, e.g., the use of high sulfidity, the use of counterflow cooking in which white liquor is also supplied during an advanced stage of the cook, and use of a chemical treatment of the lignocellulosic material prior to the actual sulfate cook.

The sulfite method is an example of a predominant acid digestion process. In turn, the sulfite method can be divided into a single stage process, in which the pH is relatively low, and a two stage process, in which the first

stage is essentially neutral and is followed by a second strongly acid stage.

Background art

For environmental reasons, the use of such bleaching agents as oxygen gas (0), some peroxide compound (P), such as hydrogen peroxide and ozone (Z), has been proposed in recent times with regard to bleaching sulfate pulp, for instance. This has resulted in that one also in commercial scale, i.e. in full scale has introduced the use of this type of bleaching agents, including the aforesaid bleaching agents and also in the aforesaid stage sequence. Other stage sequences, such as 0-Z-P for instance, have also been proposed and used commercially. By avoidance of chlorine-containing bleaching agents, which in the end give rise to corrosive chloride, it has been possible to close the bleach plants more and more, by which is meant that the (wash) liquors can be handled in the bleach plant to an increasing extent. In traditional open system bleach plants, the washing liquors (waste liquors) deriving from respective bleaching stages, including the waste liquor deriving from extraction (E) stages, are discharged either directly to the recipient or possibly to some external cleansing measure.

When using the above exemplified oxidative bleaching agents, and then particularly when using some peroxide compound, problems are caused by the content of metals in the pulp and/or the presence of metals in general. The most problematic metals are the transistion metals, of which manganese is particularly problematic, because of the large quantities in which it is present. Manganese, for instance, is a natural constituent of the starting material, i.e. of the lignocellulosic material, e.g. in the form of wood. Furthermore, the process water used will normally contain manganese and manganese can also enter the system by the way of apparatuses used in the pulp manufacturing chain. With the intention of overcoming this problem, a complex forming stage (Q) has been introduced into the pulp

processing chain, preferably immediately prior to the peroxide bleaching stage. A complexing agent of the type EDTA, DTPA and NTA, among others, that is introduced at a suitable pH-value will capture any free manganese ions that are present and, above all, will convert the manganese from a bound form in the pulp to a water soluble complex bound form. Manganese complexes of the type Mn(EDTA) ^2" or Mn(DTPA) ^3" then form.

The aforedescribed bleaching sequences that include four treatment stages will normally only result in partial bleaching of the cellulose pulp. Normally, at least one further bleaching stage is required in order to achieve full bleaching of the cellulose pulp, i.e. bleaching to a brightness of up to 90% ISO and thereabove. The bleaching agent in this position may be chlorine dioxide (D) or peroxide (P). In this latter case, the bleaching stage may be preceded by a complex forming stage. The use of ozone (Z) in two or more bleaching stages has been proposed, in similarity with peroxide (P). Chlorine dioxide (D) may also be used in, for instance, double stages at the end of the bleaching sequence.

When using closed liquor systems in the case of long bleaching sequences, and particularly in cases when the pH-values of the different bleaching stages vary widely, it has been found difficult to avoid the precipitation of, e.g. diverse calcium containing compounds, which in turn cause clogging problems in the process. Particular problems are encountered when using very low pH-values in the acid bleaching and treatment stages. For instance, the wire cloths on the washing filters are liable to become clogged very quickly, causing production problems. The clogging problems can become so serious as to result in pipes becoming plugged to a greater or lesser extent.

The addition of diverse auxiliary chemicals, sometimes referred to as catalysts, has earlier been proposed with the intention of improving precisely the peroxide bleaching stage or stages. For instance, it is proposed in

the Canadian Patent Specification 1 190 360 that a salt of one of the substances (the metals) aluminium, zinc, molybdenum and tin are added when peroxide bleaching cellulose pulp at a pH-value of at least 10. The addition shall exceed 0.01 percent by weight calculated on the cellulose pulp.

Said patent specification recommends a pH-value above 10 during the peroxide bleaching stage. The Canadian Patent Specification 1 129 161 instead recommends maintaining the pH-value within the range of 1 to 7 in a similar peroxide bleaching of cellulose pulp. Also recommended is the presence of a metal ion chosen from the group tungsten, molybdenum, chromium, osmium and selenium in an amount corresponding to 0.5 to 20 mole percent calculated on moles of peroxide. The peroxide addition shall be 0.1-5 percent by weight, calculated on bone dry pulp.

It is not clear if any of these two methods have been commercialized, i.e. used in the everydal manufacture of peroxide bleached cellulose pulp. The reason for that, if so, is also unknown.

DISCLOSURE OF THE INVENTION Technical problem

Earlier proposed and aforedescribed treatment sequences intended to obtain a cellulose pulp of high final brightness are normally relatively long and therefore result in high costs, e.g. both fixed and flexible costs. Further¬ more, long bleaching sequences can result in problems in constructing closed liquid systems, such closed systems being advocated by many people in this field, not least by the environmental authorities. This applies in particular when pH-values vary radically, particularly downwards, in the different treatment and bleaching stages.

The solution

The present invention provides a solution to the aforesaid problem and relates to a method of delignifying/

bleaching a non-delignified/unbleached or predelignified/- prebleached lignin-containing cellulose pulp, characterized in that the cellulose pulp in one and the same stage is treated with a) complexing agent (Q); b) molybdenum-containing substance (Mo); c) a chlorine-free oxidative bleaching agent, preferably a peroxide compound (P); and in that this treatment is followed by at least one bleaching stage in which a chlorine-free oxidative bleaching agent, preferably a peroxide compound (P), is used.

That treatment of the cellulose pulp with a) + b) + c) is carried out under the conditions: pH = 2-7, preferably 4-6, pulp consistency = 3-30%, temperature = 40-130°C, time = 20-720 minutes.

The invention can be applied in both open and closed bleach plants. By open bleach plants is meant that waste liquors deriving from respective treatment or bleaching stages are discharged directly to the recipient, or are passed to and collected in some pool for external cleansing of the formed optionally total volume of waste liquor. By closed bleach pl-ants is meant that all treatment and bleaching waste liquors are handled and preferably passed in counterflow so that the waste liquor will be finally mixed with the digestion liquor, wherein the common waste liquor is evaporated and burned in a soda recovery unit. It is very much preferred to apply the invention in a closed liquid system.

With regard to the pH-value in the initial treat- ment stage, the pH of an open system for instance may be as low as 2, although it is preferred that the pH will lie within the range of 4-6. This preference is made in view of the fact that large quantities of calcium present in the pulp will precipitate at very low pH-values, for instance pH = 2, and collect in the suspension liquid and therewith lead to the aforedescribed precipitation problems and subsequent clogging problems. Through obtaining a pH-value of 4 or

thereabove the dissolution of calcium from the cellulose pulp will be slight and as a result of that it works also well to feed the resultant treatment waste liquor backwards in the system in a completely closed way in accordance with what has earlier been described. Furthermore, when seen totally, it is so that the treatment of the cellulose pulp in this stage has its optimum somewhere at or in the proximity of pH 5.

It will be understood from the aforesaid tempera¬ ture range that cellulose pulp can be treated in this stage at both atmospheric pressure and at elevated pressure. A pressure range of 0.1-0.4 MPa is suitable when opting for pressurized treatment. Regarding the treatment time it is so that the result of the treatment improves with the time. Consequently, long treatment times are applied, wherein 20 minutes is a lowest limit at which an at least respectable result can be achieved. There is no upper limit from the aspect of the result achieved, although from a practical and a system volume aspect an upper limit of 720 minutes, i.e. 12 hours, is stated. As is known, a long treatment time requires a retention vessel or tank of very large volume. Pulp mills, for instance sulfate pulp mills, often include one or two retention vessels or buffer vessels. These vessels have a retention time in the order of six to nine hours for instance, and one such already existing vessel can be used conveniently for the aforedescribed initial treatment of the cellulose pulp.

Any complexing agent effective and known to the art can be used and added to the cellulose pulp. Two examples of appropriate complexing agents are ethylene diamine tetra- acetic acid (EDTA) and diethylene triamine pentaacetic acid (DTPA). The complexing agent is added in an amount of 400-10 000 grams, calculated as 100% chemical for each tonne of bone dry pulp.

A second component that shall be added to the cellulose pulp is a substance which contains the basic element molybdenum. Any substance which contains at least molybdenum and oxygen can be added, examples of such

substances being ammonium heptamolybdate having the chemical formula (NH ₄ ) ₆ Mo ₇ 0 ₂₄ . 4H ₂ 0, and ammonium dimolybdate having the chemical formula (NH ₄ ) ₂ Mo ₂ 0 ₇ . The cation ammonium can, of course, be replaced with other cations, such as sodium for instance, which cation is preferred. Examples of other suitable molybdenum compounds are isopolymolybdates, permolybdates and permolybdic acid.

The substance shall be added in an amount of 10- 1000 grams, calculated as molybdenum for each tonne of bone dry pulp. In the preferred case, in which a closed liquid system is used, a given amount of the added substance is circulated, therewith enabling the amount charged to be kept low.

A third component that shall be added to the cellulose pulp is the chlorine-free oxidative bleaching agent. Examples of such bleaching agents are oxygen gas, ozone and some peroxide compound. A preferred bleaching agent is a peroxide compound, such as hydrogen peroxide, sodium peroxide, peroxy acetic acid, peroxosulfate, peroxodisulfate, perborate or organic peroxides. The bleaching agent shall be added in an amount of 3-50 kg, preferably 5-20 kg, calculated as 100% chemical for each tonne of bone dry pulp. The peroxide charge in this stage is normally lower than in a conventional peroxide stage, for instance. After having initially treated the cellulose pulp in accordance with the aforegoing, the pulp is preferably washed in one or more stages on any known type of washing apparatus or apparatuses, including typical presses. It is not absolutely necessary to wash the cellulose pulp at this position, although it is very much preferred as will be apparent from one of the following working examples.

The pulp is then bleached with a chlorine-free oxidative bleaching agent in a known manner. Examples of bleaching agents that can be used are oxygen, ozone and peroxide also now and peroxide being the preferred bleaching agent. Suitable peroxides are the same as those given above, with particular preference to hydrogen peroxide. The hydrogen

peroxide bleaching process can be carried out both with or without applying pressure. Suitable parameters when bleaching at atmospheric pressure are: time = 60-720 minutes, preferably = 180-300 minutes; temperature = 60-100°C, preferably = 75-98°C; peroxide charge = 1-50 kg per tonne bone dry pulp (ptp), preferably 10-40 kg ptp; alkali (NaOH) charge = 1-30 kg ptp, preferably 5-25 kg ptp; pulp consistency = 3-40%, preferably 8-18%. The alkali charge is adapted so that the pH-value of the pulp suspension at the end of the bleaching stage, i.e. the end-pH, lies in the range of 10-11.5. When pressurizing, there is applied a pressure of 0.2-1 MPa, preferably 0.4-0.6 MPa. Air and/or oxygen gas can be used to pressurize the system. Suitable parameters in this case are: time = 60-180 minutes; temperature 105-120°C, peroxide charge = 5-25 kg ptp; alkali (NaOH) charge = 2-20 kg ptp. The same conditions as those given above apply with regard to pulp consistency and adaptation of the alkali charge.

The aforesaid bleaching stage is suitably preceded by a complex forming treatment of the cellulose pulp. This treatment can be carried out in accordance with any known technique. Suitable parameters are: pulp consistency = 1-40%, preferably 3-18%; temperature = 20-150°C, preferably 50-98°C; time = 1-1000 minutes, preferably 30-300 minutes; complexing agent charge = 0.1-10 kg ptp; pH = 4-9.5, preferably 5-7.

Magnesium, for instance in the form of magnesium sulfate, may otpionally be added in an amount of 0.1-10 mmole per litre of liquid, preferably 0.2-5 mmole per litre of liquid. Although not absolutely necessary, it is preferred to wash the cellu- lose pulp after this complex forming treatment.

As earlier mentioned, the initial treatment stage described above may be carried out solely on non-delignified/ unbleached cellulose pulp or on cellulose pulp that has been predelignified/prebleached. There are a number of conceiv- able delignifying/bleaching processes that can be carried out at this position, although an absolute preference is to one or more alkaline oxygen-gas delignifying processes. One alka-

line oxygen-delignifying stage is by far the most suitable. The oxygen-gas delignification of cellulose pulp can be carried out in accordance with any known technique, including both medium consistency delignification and high consistency delignification. In the case of medium consistency oxygen-gas delignification, suitable parameters are: alkali (NaOH) charge = 1-50 kg ptp; temperature = 60-120°C; time = 20-180 minutes; pressure = 0.2-1.0 MPa. The cellulose pulp is normally washed after the oxygen-gas delignification process. Although one of the aims or targets of the present invention is to keep down the number of bleaching and treat¬ ment stages, it is fully possible to bleach the cellulose pulp with a chlorine-free oxidative bleaching agent in one or possibly two further stages after the aforedescribed termina- ting bleaching stage, so as to impart to the cellulose pulp a final brightness of 90% ISO and thereabove. Although such additional bleaching detracts from the advantage afforded by a short bleaching sequence, it substitutes this advantage with the advantage of achieving an extremely high final brightness and/or very mild conditions during said extra bleaching stage or stages.

A final brightness of such high magnitude can also be achieved by employing optimal parameters in said treatment stages, including a heavy charge of chlorine-free oxidative bleaching agent in the terminating stage, in accordance with the preferred embodiment of the invention. In this case, it is preferred to treat the cellulose pulp with a complexing agent in a separate stage prior to the terminal bleaching stage. The application of such a complex forming stage is harmless from the aspect of a closed liquid system because of the relatively high pH of said stage, for instance a pH of 5.

Advantages

It has surprisingly been found that an acceptable final brightness of cellulose pulp can be achieved with the method according to the invention by using a treatment or bleaching sequence as short as 0 - a) + b) + c)-P. That means

that the total cost of producing fully bleached cellulose pulp keeps down. Furthermore, in one preferred embodiment of the invention, it is possible to close completely the liquid system in the cellulose pulp manufacturing process. This has been confirmed with mill tests.

Best embodiment

In the following, some parts of the inventive method are described in more detail with reference to laboratory apparatus used in batch wise tests and intended to simulate bleaching of cellulose pulp on a technical scale. An account is also given of the application of the invention in the mill itself, so called mill tests.

Example 1

Pulp samples were taken from a birch sulfate line comprising batch wise digesters in a pulp mill, after a wash that followed a conventional alkaline oxygen-gas bleaching stage.

The pulp was transported to the laboratory, where it was washed one further time with de-ionized water in a careful way. The pulp exhibited the following characteris¬ tics; kappa number (χ) determined in accordance with SCAN-C 1:77 = 8.9; viscosity determined in accordance with SCAN-CM 15:88 = 879 cm ³ /g; and brightness = 59.5% ISO determined in accordance with SCAN-C 11:75. All pulp characteristics given in this patent document are determined in accordance with these analysis methods.

The pulp was treated in the laboratory as zero samples or zero series in the following manner:

One series comprising two tests (1+2) was carried out in accordance with the sequence (0)-Q ₁ -P-Q ₂ -E+P-P and one series comprising two tests (3+4) was carried out in accordance with the sequence (0)-Q ₁ -P+Mo-Q ₂ -E+P-P. 0 within parenthesis indicates that the oxygen-gas bleaching process was carried out in the pulp mill. 50 g pulp were admixed with de-ionized water in a

plastic can having a volume of 1 litre, such as to obtain a final pulp consistency of 10%. EDTA and sulfuric acid were added to the pulp suspension and the can was then stored for three hours in a water bath having a temperature of 70°C. The amount of EDTA and sulfuric acid (H ₂ S0 ₄ ) added to the pulp suspension and the measured pH-valures are set forth in Table

1 below. In this case, and in all the following Examples, the complexing agent EDTA was a commercial product that contained 40% active substance. The given amounts in which this chemical was added and also the amounts in which other chemicals were added, however, relate to a 100% substance.

This stage Q _x is the same for all tests 1-4.

In tests 1 and 2, the pulps were bleached with peroxide (P) in plastic bags, which were placed in a water bath. The pulp consistency was 12%, the time three hours, and the temperature 95°C. The only difference between tests 1 and

2 was that 2%-hydrogen peroxide was added in test 1, calculated on bone dry pulp, while 4%-hydrogen peroxide was added in test 2, calculated on bone dry pulp. The sulfuric acid was added in an amount of 0.1%, calculated on bone dry pulp. The pH-values and amounts of residual peroxide measured at the end of the stage are set forth in Table 1 below. The amount of residual peroxide present was measured by idometric 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. This applies to all amounts of residual peroxide stated in this patent document.

Tests 3 and 4 were carried out in an identical manner, with the only exception that ammonium heptamolybdate (P+Mo) having the earlier mentioned formula was added in addition to peroxide and sulfuric acid. The ammonium heptamolybdate was added in an amount of 300 ppm calculated solely as molybdenum. Reference is made to Table 1 below in other respects.

The remaining stages Q ₂ -E+P-P were carried out in tests 1-4 in an identical manner.

Plastic cans having a volume of 2 litres were used in the second complex forming treatment stage Q ₂ , as the pulp consistency was as low as 3%. The time was one hour and the temperature 60°C. The amount of EDTA and sulfuric acid charged and the measured pH-values are set forth in Table 1 below.

Plastic bags were again used in the two following treatment stages.

The E+P stage was carried out at a pulp consistency of 10%. The time was two hours and the temperature 70°C. The amount of hydrogen peroxide and sodium hydroxide charged and the end-pH, residual peroxide quantities and pulp characteristics are made apparent in Table 1 below.

The terminating peroxide bleaching stage was carried out at a pulp consistency of 12%. The time was three hours and the temperature 90°C. The amount of hydrogen peroxide and sodium hydroxide charged and the end-pH, residual peroxide quantities and pulp characteristics are made apparent in Table 1 below. The pulp samples were washed carefully with ion- exchanged water between each treatment stage. Table 1 ests Qi P resp. P + Mo

EDTA H ₂ SO ₄ Start End H ₂ O ₂ Mo End Rest.p EDTA H ₂ SO ₄ Start End

% % pH pH % ppm pH % % % pH pH

1 0.3 0.35 5.1 5.35 2 0 4.35 1.03 0.2 0.05 4.90 4.95

2 4 0 3.95 3.32 4.85 4.95

3 2 300 3.90 0.63 4.80 4.90

4 4 300 3.80 2.89 4.80 4.90

Tests E + P

H ₂ O ₂ NaOH End Rest.p. χ Vise. Bright. H ₂ O ₂ NaOH End Rest.p. χ Vise. Bright.

% % PH % cm ³ /g ISO % % PH % cm7g %ISO

1 0.5 1 11.05 0.28 — _ — 1.5 10.90 1.00 3.1 514 86.3

2 0.29 - - 10.95 0.91 2.6 458 86.3

3 11.20 0.36 2.3 783 72.4 11.15 1.14 1.4 720 87.6

4 2.1 784 74.6 1.13 1.3 702 88.2

A comparison between these two test series shows that the presence of ammonium heptamolybdate in the first hydrogen peroxide bleaching stage gives an improved bleaching result. Brightness is improved by 1.3 to 1.9%, and, above all, there is a large difference in the viscosity of the fully bleached pulp, despite the kappa number of the two pulps treated with hydrogen peroxide plus ammonium hepta¬ molybdate in the described position being more than one unit lower. However, none of these tests was carried out in accordance with the inventive method.

Part of the pulp taken from the mill was stored in a refrigerated space in the laboratory. The pulp was removed from the refrigerated space sometime after having carried out the aforedescribed tests, and was washed carefully with de- ionized water in the laboratory. The following pulp charac¬ teristics were measured on this occasion: kappa number (κ) = 9.1; viscosity = 866 cm ³ /g; and brightness = 59.5% ISO. These values are very close to the values given earlier. The pulp was also subjected to a metal analysis on this occasion. The amount of manganese (Mn), iron (Fe) and silicon (Si) present were determined by atom absorption. The values measured were: Mn = 20.8 mg/kg bone dry pulp; Fe = 6.6 mg/kg; Si = 5.2 mg/kg.

Four tests were carried out with a short treatment or bleaching sequence.

Test 5 was carried out according to (0) - b)+c)-P and the tests 6, 7 and 8 were carried out according to (0) - a)+b)+c)-P. These three tests were thus carried out in accordance with the inventive method. Appropriate parts of the tests were actually carried out in the manner earlier described. The first treatment stage was carried out at a pulp consistency of 12%. The time was three hours and the temperature 95°C. The only difference between the three tests 6, 7 and 8 carried out in accordance with the invention resided in the addition of complex forming agent in increasing quantities and also in

slightly differing amounts of sulfuric acid additions to optimize the pH-value during the treatment or bleaching stage. Other treatment parameters and measurement values determined after this treatment stage are apparent from Table 2 below. After this treatment, the pulps were washed care¬ fully with ion-exchanged water.

The peroxide bleaching stage was carried out at the earlier mentioned pulp consistency and during the same length of time, whereas the temperature was lowered slightly to 90°C. The conditions were identical for all four tests, and the amounts in which hydrogen peroxide and sodium hydroxide were added and the measured values are apparent from Table 2 below. After this stage, the pulps were washed carefully with ion-exchanged water.

Table 2

Tests b) + c) resp. a) + b) + c)

H ₂ O ₂ H ₂ SO ₄ EDTA Mo End Rest.p. Mn X Vise.

% % % ppm pH % mg/kg cm ³ /kg

5 1 0.42 0.0 150 4.00 0.06 6.05 2.8 750

6 1 0.50 0.3 150 4.15 0.25 3.56 3.1 813

7 1 0.59 0.6 150 4.30 0.12 1.14 3.3 822

8 1 0.77 1.0 150 4.45 0.00 0.81 3.2 820

Tests

H ₂ O ₂ NaOH End Rest.p. X Vise. Brightness

% % pH % cm ³ /g % ISO

5 3 2 11.00 0.00 1.5 500 82.0

6 3 2 11.15 0.00 1.9 588 83.7

7 3 2 11.20 0.36 1.9 715 88.1

8 3 2 11.30 0.42 2.0 741 87.9

As will be evident from the Table, the worst bleaching result was obtained in test 5, in which no

complexing agent was added. Brightness and viscosity were both very low. Much better results were obtained in the tests carried out in accordance with the invention. This is particularly true of tests 7 and 8 in which 0.6% and 1% EDTA were supplied respectively to the pulp. In these cases, brightness rose to 88% and the viscosity to well above 700 cm ³ /g. It can be concluded from these tests that the complexing agent must be added in a sufficiently large quantity, at least with respect to the described process parameters. It will also be evident from the Table that a charge of 0.3% EDTA gave a much poorer result than when the addition is increased to 0.6% EDTA.

A comparison between tests 7 and 8 carried out in accordance with the invention and tests 3 and 4 in Table 1 shows that the brightness of the pulps is the same, i.e. 88% ISO, whereas the viscosity of the pulps treated in accordance with the invention is slightly higher than the viscosity of the zero pulps, despite the tests carried out in accordance with the method according to the invention being restricted to three treatment stages whereas the zero pulps were treated in six stages. A brightness of 88% ISO is furthermore an acceptable final pulp brightness, which shows that the invention enables a fully bleached pulp to be produced with the use of only three bleaching or treatment stages.

Example 2

Further laboratory tests were carried out with the same pulp as above.

A number of tests simulating the method according to the invention were carried out and a study was made on the influence of increasing amounts of ammonium heptamolybdate in the first treatment stage on the bleaching result. Two tests were carried out without the addition of ammonium heptamolyb¬ date by way of reference (zero-sample).

The pulp consistency was 10% in the first treatment stage. The time was three hours and the temperature 75°C. The amounts in which hydrogen peroxide, sulfuric acid, complexing

agent and ammonium heptamolybdate were added and the measured values relating to end pH, residual peroxide, kappa number and viscosity are apparent from Table 3 below. After the stage, the pulps were washed carefully with ion-exchanged water. In two cases, the pulps were not washed in this position.

The subsequent peroxide bleaching stage took place at a pulp consistency of 10%. The time was three hours and the temperature 75°C. The amount of hydrogen peroxide and sodium hydroxide added and the measured values of end pH, residual peroxide, kappa number, viscosity and brightness will be apparent from Table 3 below. The pulps were washed carefully with ion-exchanged water after the hydrogen peroxide bleaching stage.

Table 3

Tests a) + c) resp. a) + b) + c)

H ₂ O ₂ H ₂ SO ₄ EDTA Mo End Rest.p. X Vise.

% % % ppm PH %

9 1 0.45 0.3 0 4.65 0.83 7.9 870

10 100 4.75 0.76 7.1 861

11 200 4.70 0.69 6.3 893

12 300 4.55 0.58 5.8 863

13 400 4.55 0.55 5.4 866

14 500 4.30 0.45 4.6 882

15 500 4.30 0.45 4.6 882

16 0 4.65 0.83 7.9 870

Tests

H ₂ O ₂ NaOH End Rest.p. X Vise. Brightness

% % PH % crnVg % ISO

9 3 3 11.10 1.10 5.4 674 81.4

10 tr 11.25 2.08 5.0 848 82.7

11 11.30 1.83 4.1 826 83.5

12 11.25 1.76 4.1 829 84.0

13 11.20 1.65 3.5 827 84.1

14 11.25 1.20 3.0 798 84.3

15 11.25 0.03 2.9 597 79.2

16 11.35 0.03 5.7 568 76.7

The tests 10-15 are carried out in accordance with the invention. The reason why the final brightness of the pulps remained at a value of 84% ISO in this case is because the treatment parameters, primarily the temperature, in both the initial treatment stage and in the hydrogen peroxide bleaching stage are milder than in the case of tests 7 and 8, for instance. The kappa numbers in tests 10-15 are relatively high, in the interval of 3 to 5, and the viscosity of the pulps are relatively high for the same reason. When studying the kappa number and viscosity of the pulps after the first treatment stage, it is seen that delig¬ nification of the pulp increases with increasing addition of ammonium heptamolybdate. The zero sample gave a kappa number of 7.9 and a viscosity of 870 cm ³ /g. With an ammonium hepta- molybdate addition of 500 ppm, calculated solely as molyb¬ denum, the kappa number fell to 4.6 while, surprisingly, the viscosity remained as high as 882 cm ³ /g.

The best cellulose pulp final bleaching results were obtained in the tests 10-14. As with the pulps after the first treatment stage, an increased charge of ammonium heptamolybdate results in decreasing kappa numbers of the finally bleached pulp. In parallel therewith, the brightness of the finally bleached pulp increases whereas the viscosity continues to lie at 800 cm ³ /g or thereabove. A charge of ammonium heptamolybdate of only 100 ppm, calculated as solely molybdenum, would appear to be the lowest limit, since brightness is then only raised to 82.7% ISO from the 81.4% ISO of the zero sample (test 9). On the other hand, this small amount of ammonium molybdate considerably improved the pulp viscosity.

Test 15 carried out in accordance with the inven¬ tion differs from the other tests 10-14 carried out in accordance with the invention insomuch that the pulp was not washed after the initial treatment stage. With regard to tests 10-14, the pulps are washed carefully in this position with ion-exchanged water. Although the omission of this washing stage had no effect on pulp delignification, it

lowered the brightness from 84.3% ISO (test 14) to 79.2% ISO, and the viscosity from 798 cm ³ /g (test 14) to 597 cm ³ /g. The quality of the pulp was thus seriously impaired, which was probably because an excessively large amount of manganese ions were carried over to the final hydrogen peroxide bleaching stage. The omission of a pulp washing stage in said position does not have the same serious consequences under other conditions, which explains that we in this patent document have stated, that washing of the pulp in this position is only highly preferably, and not absolutely imperative.

Washing of the pulp in this position was also omitted in the second zero sample, test 16. This resulted in a drop in final brightness from 81.4% ISO (test 9) to 76.7% ISO, and a drop in viscosity from 674 cm ³ /g (test 9) to 568 cm ³ /g.

Example 3

A further series of tests were carried out in the laboratory on the same pulp. The treatment sequence a)+ b) + c) - Q- P was used throughout, meaning that all tests were carried out in accordance with the invention. The amounts of hydrogen peroxide charged in the first treatment stage were varied, and the amounts of hydrogen peroxide charged in the hydrogen peroxide bleaching stage (P) were also varied at one and the same low hydrogen peroxide charge in the first treatment stage.

The pulp consistency was 12% in the first treatment stage. The time was three hours and the temperature 95°C. The amounts in which hydrogen peroxide, sulfuric acid, complexing agent and ammonium heptamolybdate, calculated solely as molybdenum were charged, and the measured values of end pH and residual peroxide will be apparent from Table 4 below. All pulps were washed carefully with ion-exchanged water after the treatment stage.

The second stage was a complex forming stage (Q) at a pulp consistency of 3%. The time was one hour and the

temperature 60°C. The amounts of EDTA and sulfuric acid charged and the measured values of pH, kappa number and viscosity will be apparent from Table 4 below. The pulps were washed carefully with ion-exchanged water after the mentioned treatment.

The third and final stage was a hydrogen peroxide bleaching stage at a pulp consistency of 12%. The time was three hours and the temperature 90°C. The amounts of hydrogen peroxide and sodium hydroxide charged and the measured values of end pH, residual peroxide amount, kappa number, viscosity and brightness are apparent from Table 4 below. The pulps were washed carefully with ion-exchanged water after the final treatment.

Table 4

Tests a) + b) + c) Q

H ₂ O ₂ H ₂ SO ₄ EDTA Mo End Rest.p. EDTA H ₂ SO ₄ PH X Vise.

% % % ppm PH % % % cm ³ /kg

17 1.0 0.5 0.3 150 4.00 0.15 0.2 0.05 4.9 2.7 806

18 1.5 0.5 0.3 150 4.00 0.58 0.2 0.05 4.9 2.7 795 19 2.0 0.5 0.3 150 4.05 1.03 0.2 0.05 4.9 2.7 793

20 1.0 0.5 0.3 150 4.20 0.24 0.2 0.05 5.0 3.2 835

21

22

23 π

Tests P

H ₂ O ₂ NaOH End Rest.p. X Vise. Brightness

% % PH % cm ³ /g % ISO

17 2 1.5 10.85 1.02 1.5 751 86.5

18 2 1.5 10.80 1.11 1.5 747 86.3

19 2 1.5 10.80 1.13 1.6 746 86.4

20 2 1.5 11.10 0.96 1.8 748 86.6

21 3 2.0 11.35 1.49 1.8 748 88.2

22 4 2.5 11.50 1.86 1.6 724 89.0

23 5 3.0 11.60 2.24 1.5 711 89.6 All of these tests carried out in accordance with the invention gave good results. The incremental increase in

the hydrogen peroxide charge in the final hydrogen peroxide bleaching stage also resulted in an incremental increase in the final brightness of the pulps. A hydrogen peroxide charge of 5% in the final stage, calculated on bone dry pulp, enabled the final brightness of the pulp to be brought to 90% ISO, or more precisely to 89.6% ISO.

With regard to birch sulfate pulp, the results show that a fully bleached cellulose pulp having a final bright¬ ness of 90% ISO can be produced with the relatively short treatment or bleaching sequence 0 -a) +b) +c) - Q - P, i.e. four stages, in accordance with the invention.

Example 4

Pulp samples were taken from a pine sulfate line with a continuous digester, after a wash which followed a conventional alkaline oxygen-gas bleaching stage.

The pulp was transported to the laboratory, where it was washed one more time carefully with de-ionized water. The pulp had the following characteristics: kappa number (κ) = 16.6; viscosity = 981 cm ³ /g; and brightness = 34.2% ISO. With regard to the test procedure adopted, approp¬ riate parts of two tests were carried out in the laboratory in the manner earlier described.

A zero sample 24 was carried out in accordance with the sequence (0) - a) + c) -P, and the other test 25 was carried out in accordance with the sequence (0) - a) + b) + c) - P, in accordance with the invention. The reference (0) signifies that oxygen-gas bleaching of the pulp was carried out in the mill.

The first treatment stage in the two tests 24 and 25 performed in the laboratory had the following in common. The pulp consistency was 12%, the time was three hours and the temperature 90°C. Hydrogen peroxide was added in an amount of 4%, calculated on bone dry pulp, and the complexing agent EDTA was added in an amount of 0.3%, calculated on bone dry pulp. 300 ppm of ammonium heptamolybdate was also charged in test 25 carried out in accordance with the invention, said

ammonium heptamolybdate being calculated as molybdenum on bone dry pulp. The two pulps were then washed carefully with ion-exchanged water and their kappa numbers and viscosities were measured. The aforesaid charges and measured numerical values are apparent from Table 5 below.

In the final peroxide stage, which was the same in both tests, the pulp consistency was 12%, the time = three hours, and the temperature = 80°C. Hydrogen peroxide and sodium hydroxide were added to the pulps in respective amounts of 3.5% and 2.5%, calculated on bone dry pulp. The pulp was then washed carefully with ion-exchanged water. The kappa number, viscosity and brightness of the pulp were measured. The aforesaid charges and the measured numerical values will be apparent from Table 5 below.

Table 5

Tests a) + c) resp. a) + b) + c) P

H ₂ O ₂ H ₂ SO ₄ EDTA Mo End χ Vise. H ₂ O ₂ NaOH End χ Vise. Brightn. % % % ppm pH cm ³ /g % % pH cm ³ /g % ISO

24 4 0.5 0.3 - 4.5 14.6 911 3.5 2.5 10.510.6 820 75.9 25 4 0.5 0.3 300 4.4 4.7 837 3.5 2.5 10.5 2.0 750 86.0

The use of the complexing agent EDTA, ammonium heptamolybdate and hydrogen peroxide in one and the same stage in accordance with the invention, enables an acceptable final brightness to be achieved in solely three treatment or bleaching stages, even in the case of sulfate pulp produced from pinewood, i.e. coniferous/long fibre pulp. The test pulp produced in accordance with the invention has a brightness of 86% ISO, whereas the brightness of the reference pulp is only 75.9% ISO. A surprisingly large difference also exists with regard to the lignin content of the finally bleached pulps, expressed as kappa number. More precisely, a kappa number of 2.0 as opposed to a kappa number of 10.6. Because the viscosity of the starting pulp was relatively low, the

viscosity of the finally bleached pulp produced in accordance with the invention became also relatively low.

Example 5

Tests in which the inventive method was applied were carried out in a sulfate pulp mill in which birch wood chips were digested in a number of batch digesters, to produce cellulose pulp. After the digestion, the pulp was treated normally in accordance with what is stated below. The digested pulp was screened in a closed pulp screening room and the accepted pulp was then washed on a belt washer, whereafter the pulp was delignified with oxygen- gas. Normally, 40 kg of sodium hydroxide, calculated per tonne of bone dry pulp, was added to the pulp. The tempera¬ ture was 110°C and the time one hour. The pulp was deligni- fied in one single stage in a bleaching tower, alternatively referred to as an oxygen-gas bleaching reactor, at a pulp consistency of 10% and at an oxygen-gas pressure of 0.3-0.6 MPa. The pulp was then washed in two series connected washing presses. After being washed, the pulp was passed to a storage tower and there stored for about six hours. The pulp was transferred from the storage tower to a washing filter, and thereafter to a tower in which the pulp was treated with complexing agent EDTA. EDTA was charged in an amount corres¬ ponding to 2.5 kg per tonne of bone dry pulp. 7.5 kg of sulfuric acid were also charged for each tonne of bone dry pulp, so as to maintain a pH of about 5. The pulp had a consistency of about 10%. The temperature was 73-80°C, and the time about three hours. It is important to note that all of the numerical values given shall be taken as being approximate, since they refer to mill conditions where some variations in process parameters for instance are unavoid¬ able. The process parameters can be controlled and checked much more easily in the laboratory since the treatment stages are there normally carried out intermittently, i.e. batchwise, and not continuously.

The pulp was washed in two series connected washing

filters after the complex forming treatment. The pulp was thereafter passed to a first peroxide bleaching stage. The pulp consistency was about 10%, the temperature about 85°C and the time three hours. 35 kg of hydrogen peroxide and 14 kg of sodium hydroxide were charged for each tonne of bone dry pulp. The end pH vas 10.7. The pulp was then washed on a washing filter.

The pulp was then treated with ozone, which was added to the pulp in a mixer in an amount corresponding to 3.5 kg for each tonne of bone dry pulp. 9 kg of sulfuric acid were added for each tonne of bone dry pulp, in order to obtain the correct pH-value. Subsequent to mixing in the ozone, the pulp was collected in a tank and there held for fifteen minutes. Remaining process parameters were: pulp consistency = 10%; temperature = 55°C; and pH = 2.5. The pulp was washed on a washing filter after this stage.

The terminating bleaching stage was a peroxide stage. Hydrogen peroxide and sodium hydroxide were charged in respective amounts of 15 kg and 12-15 kg for each tonne of bone dry pulp. Remaining process parameters were: pulp consistency = 12%; temperature = 75°C; time = three hours; and end pH = 10.5. The process was terminated by washing the pulp on a washing filter.

The treatment- or bleaching sequence 0-Q-P ₁ -Z-P ₂ has been described in the aforegoing. Fresh water was delivered to the pulp on the last washing filter, and from there the washing liquid was passed through the whole of the bleachery strictly in counterflow, and mixed finally with the digestion waste liqour, this combined waste liquor subsequently being evaporated and combusted in the soda recovery unit. In the period immediately prior to treatment of the cellulose pulp in accordance with the invention, as described below, it was possible to produce pulp in a system which was fully closed from a liquid aspect for a relatively long time. In previous tests carried out on a fully closed system, problems have arisen with clogging of the wire cloths on the washing filters, for instance after being used for only a day or so.

As a result, it has been necessary to pass the washing liquid in counterflow only one stage calculated from the end, i.e. from the washing filter that follows the terminating hydrogen peroxide bleaching stage to the washing filter that follows the ozone bleaching stage, and thereafter to an outlet. On these occasions, fresh water was delivered to the pulp on the washing filter after the first hydrogen peroxide bleaching stage. The washing liquid occurrent in this position could also at times be passed to the outlet, even though the washing liquid was normally passed backwards in the process, in strict counterflow.

The kappa number, viscosity and often the bright¬ ness of the pulp were determined in a number of positions, and the measured values are apparent from Table 6 below. The aforedescribed treatment of the forwardly moving pulp was changed at a given time point, by also adding ammonium dimolybdate in an amount of 300 ppm, calculated as molybdenum, and hydrogen peroxide in an amount of 10 kg per tonne of bone dry pulp in connection with the complex forming stage, in addition to adding EDTA and sulfuric acid in given amounts. Regarding the addition of these chemicals it was carried out in the following manner. Powdered ammonium dimolybdate was dissolved in water having a temperature of 50°C, to obtain a 5 percent solution. This molybdate solution and a hydrogen peroxide solution (50 percent solution) were charged in the screw of the washing filter that precedes the complex forming stage. Liquid was delivered to the washed pulp conveyed in a continuous mat in said screw to form a liquid suspension. The complexing agent EDTA, in the form of a commercial product, i.e. a solution containing 40% active substance, and a sulfuric acid solution were delivered to the high pulp consistency pump located downstream of the mentioned washing filter and functioning to press the pulp suspension into the complex forming treatment tower. A change in the subsequent hydrogen peroxide bleaching stage was also made after half of the test period had passed, this change consisting in lowering the hydrogen peroxide charge to 25 kg

per tonne of bone dry pulp. Treatment of the pulp was terminated with washing the pulp on the following washing filter.

The kappa number, viscosity and often the bright- ness of the pulp were measured in several positions, the measured values being apparent from Table 6 below. Samples of the pulp were taken for analysis from the washing filter upstream of the complex forming treatment, from the second washing filter downstream of the complex forming treatment, and from the washing filter downstream of the hydrogen peroxide bleaching stage, among others. These pulp samples were washed carefully with ion-exchanged water in the laboratory, prior to being analyzed.

As will be evident, the aforedescribed treatment or bleaching sequence 0 - a) + b) + c) - P was carried out in accordance with the inventive method. Fresh water was delivered to the pulp on the washing filter downstream of the hydrogen peroxide bleaching stage, and the washing waste liquor was passed backwards in the pulp manufacturing chain strictly in counterflow, and finally mixed with the digestion waste liquor. This waste liquor mixture was evaporated and combusted finally as concentrated waste liquor in the soda recovery unit.

The aforedescribed test period had a duration of seventeen hours.

Table 6

Unbleached Oxygen-gas pulp bleached pulp Q resp. a) + b) + c) F ₁

X Vise. % Vise. χ Vise. Brightn. χ Vise. Brightn. cm ³ /g cm ³ / g cm ³ / g % ISO cm ³ /g % ISO

Conv. bleach. technique 15.0 1000 8.0 750 10.1 729 51.5 5.7 668 78.3

Bleach, techn. accord, to the 14.0 950 10.5 729 2.7 636 66.2 1.6 601 85.8 inv.

X Vise. Brightn. X Vise. Brightn, c Vg % ISO cm ³ / g % ISO

Conv. bleach. technique 2.5 - 85.0 2.5 600 87.0

The measured values given above have been chosen from among a large number of the values that were measured during respective time periods in the manufacture of finally bleached cellulose pulp and are representative of the results achieved. The measured values obtained with pulp that has been bleached in accordance with the invention have been chosen from the second half of the test period, i.e. when the hydrogen peroxide charge in P _l was lowered to 25 kg per tonne of bone dry pulp.

It will be evident from the aforegoing that a pulp brightness of up to 85.8% ISO can be achieved when practicing the inventive method using only three treatment or bleaching stages, i.e. 0 - a) + b) + c) - P, this bright¬ ness being only marginally lower than the brightness = 87.0% ISO of the pulp that was produced in accordance with a known technique and comprising fully five treatment or bleaching stages.

The liquid closed system applied for seventeen hours while practicing the inventive method gave no indica¬ tion of crystallization of troublesome calcium compounds.

However, seventeen hours is much too short time to provide evidence that crystallization will never occur when practicing the inventive method for ever. However, it can be assumed on the basis of this indication in combination with the fact that the pH-value was at lowest about 5, meaning that calcium is not released from the cellulose pulp at all, or only to a very slight degree that the washing filters, for instance wire cloths, will not become clogged at persever practicing of the method.