Method for delignifying and bleaching pulp

Abstract

Process for the delignification and bleaching of pulp, comprising a first bleaching stage with hydrogen peroxide in the presence of a molybdate or tungstate in an acidic aqueous mixture; subsequent to the first bleaching stage, a second bleaching stage with hydrogen peroxide in an alkaline aqueous mixture; and, subsequent to the second bleaching stage, a third bleaching stage with hydrogen peroxide in the presence of a molybdate or tungstate in an acidic aqueous mixture.

Claims

1. A process for the delignification and bleaching of pulp, comprising: a) a first bleaching stage in which pulp is reacted with hydrogen peroxide in the presence of a molybdate or tungstate in an acidic aqueous mixture; b) subsequent to the first bleaching stage, a second bleaching stage in which pulp is reacted with hydrogen peroxide in an alkaline aqueous mixture; and c) subsequent to the second bleaching stage, a third bleaching stage in which pulp is reacted with hydrogen peroxide in the presence of a molybdate or tungstate in an acidic aqueous mixture; wherein said process does not comprise a bleaching stage in which an oxidizing agent other than hydrogen peroxide and oxygen is used.

2. The process of claim 1, wherein in the first and third bleaching stages, the pulp is reacted with 0.1 to 5 wt % of hydrogen peroxide in the presence of 10 to 2000 ppm of molybdenum in the form of molybdate or 200 to 10 000 ppm of tungsten in the form of tungstate, based in each case on the mass of dry pulp, at a temperature of 50 to 150 C. and at a pH in the range from 1 to 7, and in the second bleaching stage the pulp is reacted with 0.1 to 5 wt % of hydrogen peroxide, based on the mass of dry pulp, at a temperature of 50 to 100 C. and at a pH in the range between 7 and 12.

3. The process of claim 1, wherein the pulp is washed after the first bleaching stage and after the second bleaching stage.

4. The process of claim 1, wherein said process does not comprise a further bleaching stage in which an oxidizing agent other than hydrogen peroxide and oxygen is used.

5. The process of claim 1, wherein the second bleaching stage is carried out with addition of oxygen at a pressure of 0.1 to 1.5 MPa.

6. The process of claim 1, wherein a complexing agent is added in the first bleaching stage, in the third bleaching stage or in the first and third bleaching stages.

7. The process of claim 1, comprising an additional stage of alkaline delignification of the pulp with oxygen before the first bleaching stage.

8. The process of claim 1, comprising an additional bleaching stage in which pulp is reacted with hydrogen peroxide in an alkaline aqueous mixture after the third bleaching stage.

9. The process of claim 1, comprising an additional stage of acidic hydrolysis with addition of a complexing agent before the first bleaching stage.

10. The process of claim 1, comprising the additional steps of: a) separating the pulp from the aqueous mixture after the first bleaching stage, the third bleaching stage or the first and third bleaching stages to give a molybdate- or tungstate-containing aqueous solution; b) contacting the molybdate- or tungstate-containing aqueous solution obtained in step a) with a water-insoluble, cationized inorganic carrier material at a pH in the range between 2 and 7, to give a molybdate- or tungstate-laden carrier material and a molybdate- or tungstate-depleted aqueous solution; c) separating the molybdate- or tungstate-laden carrier material from the molybdate- or tungstate-depleted aqueous solution; d) contacting the molybdate- or tungstate-laden carrier material with an aqueous solution at a pH in the range between 7 and 14, to give a molybdate- or tungstate-depleted carrier material and a molybdate- or tungstate-laden aqueous solution; e) separating the molybdate- or tungstate-depleted carrier material from the molybdate- or tungstate-laden aqueous solution; and f) returning the molybdate- or tungstate-laden aqueous solution obtained in step d) to the first bleaching stage, the third bleaching stage or the first and third bleaching stages.

11. The process of claim 10, wherein the molybdate- or tungstate-depleted carrier material separated in step e) is used again in step b).

12. The process of claim 10, wherein in the first and third bleaching stages the pulp is reacted with 0.1 to 5 wt % of hydrogen peroxide in the presence of 10 to 2000 ppm of molybdenum in the form of molybdate or 200 to 10 000 ppm of tungsten in the form of tungstate, based in each case on the mass of dry pulp, at a temperature of 50 to 150 C. and at a pH in the range from 1 to 7, and in the second bleaching stage the pulp is reacted with 0.1 to 5 wt % of hydrogen peroxide, based on the mass of dry pulp, at a temperature of 50 to 100 C. and at a pH in the range between 7 and 12.

13. The process of claim 12, wherein a complexing agent is added in the first bleaching stage, in the third bleaching stage or in the first and third bleaching stages.

14. The process of claim 10, wherein said process does not comprise further bleaching stages between the first and second bleaching stages and between the second and third bleaching stages.

15. The process of claim 10, wherein the second bleaching stage is carried out with addition of oxygen at a pressure of 0.1 to 1.5 MPa.

16. The process of claim 10, wherein a complexing agent is added in the first bleaching stage, in the third bleaching stage or in the first and third bleaching stages.

17. The process of claim 10, comprising an additional stage of alkaline delignification of the pulp with oxygen before the first bleaching stage.

18. The process of claim 10, comprising an additional stage of acidic hydrolysis with addition of a complexing agent before the first bleaching stage.

Description

EXAMPLES

(1) All of the experiments were carried out with kraft pulps which had been delignified with oxygen under alkaline conditions. Examples 1 to 4 used an oxygen-delignified eucalyptus kraft pulp with a brightness of 64.7% ISO, Examples 5 and 6 an oxygen-delignified spruce kraft pulp with a brightness of 48.1% ISO.

(2) The bleaching stages were each carried out under the experimental conditions indicated, with the pulp densities specified in Tables 1 to 6, the pulp being mixed with the corresponding amount of water and with the amounts of bleaching chemicals indicated in the tables, and maintained at the stated temperature in a plastics pouch within a thermostated waterbath. In a deviation from this procedure, in Examples 5 and 6, the alkaline, oxygen- and peroxide-assisted extraction Eop, the peroxide-assisted oxygen stage Op and the oxygen-assisted peroxide stage Po were carried out in a high-shear mixer at the oxygen pressure stated in each case. The amounts of bleaching chemicals stated are based on the mass of the dry pulp used in the bleaching sequence. In the case of EDTA, the amounts are based on the amount of the commercial 40 wt % aqueous solution used. For the catalysed bleaching with hydrogen peroxide in the presence of molybdate, the catalyst used was sodium molybdate in the form of an aqueous solution. The pH values at the start of a bleaching stage were determined at room temperature; the pH values at the end of the bleaching stage were determined at the temperature of the bleaching stage, in each case using a glass combination electrode.

(3) Washing took place between each of the bleaching stages, by adding demineralised water to a pulp density of 2 wt %, intensive stirring of the resulting suspension and removal of the pulp from this suspension by means of vacuum filtration and centrifugation.

(4) TABLE-US-00001 TABLE 1 Bleaching of oxygen-delignified eucalyptus kraft pulp in Example 1 with the bleaching sequence DEpDP Quantities used and bleaching Bleaching stage parameters D Ep D P ClO.sub.2 in wt % of 2.6 0.5 active chlorine H.sub.2SO.sub.4 in wt % 0.4 0.1 H.sub.2O.sub.2 in wt % 0.4 0.2 NaOH in wt % 0.4 0.4 Temperature in 90 85 80 80 C. Time in minutes 120 75 120 120 Pulp density in % 10 10 10 10 pH at start 10.7 11.2 pH at end 2.8 9.0 4.3 10.2

(5) TABLE-US-00002 TABLE 2 Bleaching of oxygen-delignified eucalyptus kraft pulp in Example 2 with the bleaching sequence AqPPmoPPmoP Quantities employed and bleaching Bleaching stage parameters Aq P Pmo P Pmo P H.sub.2O.sub.2 in wt % 2.0 0.5 2.0 0.1 2.0 H.sub.2SO.sub.4 in 0.25 0.28 0.28 wt % NaOH in wt % 1.4 1.4 1.4 Mo in wt % 0.025 0.01 EDTA in wt % 0.2 0.1 0.1 Temperature 90 85 90 85 90 85 in C. Time in 300 90 120 90 60 90 minutes Pulp 10 10 10 10 10 10 density in % pH at start 4.2 11.6 3.5 11.8 3.5 11.5 pH at end 4.0 10.7 4.0 10.5 4.1 10.7

(6) TABLE-US-00003 TABLE 3 Bleaching of oxygen-delignified eucalyptus kraft pulp in Example 3 with the bleaching sequence DEpDP Quantities employed and bleaching Bleaching stage parameters D Ep D P ClO.sub.2 in wt % of 1.86 0.2 active chlorine H.sub.2SO.sub.4 in wt % 0.5 0.15 H.sub.2O.sub.2 in wt % 0.2 0.2 NaOH in wt % 0.4 0.4 Temperature in 90 85 80 80 C. Time in minutes 120 75 120 120 Pulp density in % 10 10 10 10 pH at start 10.8 11.4 pH at end 2.8 9.1 4.5 10.2

(7) TABLE-US-00004 TABLE 4 Bleaching of oxygen-delignified eucalyptus kraft pulp in Example 4 with the bleaching sequence AqPPmoPPmoP Quantities employed and bleaching Bleaching stage parameters Aq P Pmo P Pmo P H.sub.2O.sub.2 in wt % 0.6 0.5 0.6 0.5 0.6 H.sub.2SO.sub.4 in 0.25 0.35 0.35 wt % NaOH in wt % 1.3 1.3 1.3 Mo in wt % 0.01 0.01 EDTA in wt % 0.2 0.1 0.1 Temperature 90 85 90 85 90 85 in C. Time in 300 240 120 240 120 240 minutes Pulp 10 10 10 10 10 10 density in % pH at start 4.6 11.9 4.1 11.9 3.5 11.9 pH at end 4.7 10.7 4.3 10.7 3.9 10.7

(8) TABLE-US-00005 TABLE 5 Bleaching of oxygen-delignified spruce kraft pulp in Example 5 of the bleaching sequence DEopD Quantities employed and Bleaching stage bleaching parameters D Eop D ClO.sub.2 in wt % of 2.95 1.0 active chlorine H.sub.2SO.sub.4 in wt % 0.15 0.03 H.sub.2O.sub.2 in wt % 0.5 NaOH in wt % 1.0 O.sub.2 in MPa 0.3 MgSO.sub.4 in wt % 0.1 Temperature in C. 90 80 75 Time in minutes 60 90 120 Pulp density in % 10 11 10 pH at start pH at end 2.8 10.8 3.9

(9) TABLE-US-00006 TABLE 6 Bleaching of oxygen-delignified spruce kraft pulp in Example 6 with the bleaching sequence PmoOpPmoPo Quantities employed and Bleaching stage bleaching parameters Pmo Op Pmo Po H.sub.2O.sub.2 in wt % 0.5 0.7 0.5 2.9 H.sub.2SO.sub.4 in wt % 0.25 0.25 NaOH in wt % 1.0 1.6 O.sub.2 in MPa 0.5 0.5 Mo in wt % 0.02 0.02 EDTA in wt % 0.1 0.1 MgSO.sub.4 in wt % 0.1 0.15 Temperature in 90 100 90 107 C. Time in minutes 120 75 120 160 Pulp density in % 10 11 10 12 pH at start 4.7 4.3 pH at end 5.2 10.4 5.1 10.8

(10) For the bleached pulp, the brightness of the pulp was determined in accordance with the PAPTAC Standard E.1, and the viscosity of the pulp in accordance with TAPPI Standard T 236 om 99. In addition, the loss of brightness by heat ageing and the post-colour number (PC number) were determined using the TAPPI T 260 (wet) and TAPPI UM 200 (dry) methods. The results are summarized in Table 7.

(11) In the pairs of experiments 1 and 2, 3 and 4, and 5 and 6, the conditions of the bleaching sequences were selected such that the pulp was bleached in each case to a comparable brightness. For eucalyptus kraft pulp, the bleaching sequence of the invention, by comparison with the industrial standard bleaching sequence with chlorine dioxide, achieves a lower level of fibre damage, evident from a higher viscosity. Furthermore, the pulps bleached with the process of the invention exhibited a better stability of brightness, i.e. a lower yellowing tendency, than the pulps bleached with chlorine dioxide.

(12) TABLE-US-00007 TABLE 7 Properties of the pulps bleached in Examples 1 to 6 Example 1* 2 3* 4 5* 6 Bleaching DE.sub.pDP A.sub.qPP.sub.moPP.sub.moP DE.sub.pDP A.sub.qPP.sub.moPP.sub.moP DE.sub.opD P.sub.moO.sub.pP.sub.moP.sub.o sequence Brightness 91.8 91.4 90.1 89.5 87.3 87.2 in % ISO Viscosity 14.4 16.0 15.0 17.2 15.2 12.7 in mPa*s Heat ageing, wet: Change in 2.5 1.4 2.7 0.9 3.4 2.3 brightness in % ISO PC number 0.275 0.147 0.364 0.117 0.621 0.403 Heat ageing, dry: Change in 1.9 2.0 1.9 1.4 2.8 2.3 brightness in % ISO PC number 0.201 0.224 0.245 0.188 0.498 0.403 number *not according to the invention