Process and a dissolving pulp manufactured by the process

Abstract

The invention relates to processes for manufacturing pulp and more specifically to improved processes for manufacturing dissolving pulp. The processes have primarily been developed to be used in connection with large scale kraft processes, i.e. they have been designed to be incorporated into a plant. A liquor derivable from the process and comprising xylan, lignin, alkali and water is also disclosed as well as a dissolving pulp produced by the process.

Claims

1. A process for manufacturing dissolving pulp comprising the steps of: a) selecting a wood based raw material, said wood based raw material having a xylan content of from 12 weight % or more; b) adding a cooking liquor comprising white and/or black liquor to the wood based raw material; c) digesting the wood based raw material composition obtained from step b) in a kraft cooking process; d) oxygen delignifying the pulp obtained from step c); e) adding industrial white liquor comprising Na.sub.2CO.sub.3, NaHS and NaOH to the pulp obtained from step d) wherein said pulp has a xylan content of 8 weight % or more and wherein the temperature is lowered and kept from 50 C. to 65 C. from 5 minutes to 15 minutes and wherein the alkali concentration in the liquid phase of the obtained pulp suspension is in the range of 70 g/l to 100 g/l; and f) removing a major part of the alkali and dissolved xylan as a liquor flow from the pulp obtained from step e) by directly dewatering the pulp without dilution, wherein the removed liquor flow has the same xylan and alkali content as the liquid phase in step e); and g) after step f) subjecting the dewatered pulp to washing and pressing in a washing press device 1-5 times.

2. The process according to claim 1, wherein said white liquor added in step e) has a suspended solids content of 20 mg/l or less.

3. The process according to claim 1, wherein said white liquor has a combined Na.sup.+ and K.sup.+ concentration of from 3 to 5 mol/l, OH concentration of from 2.25 to 3.5 mol/l, HS.sup. concentration of from 0.2 to 1.0 mol/l, and a CO.sub.3.sup.2 concentration of from 0.10 to 0.60 mol/l.

4. The process according to claim 1, wherein the pulp obtained from step g) is filtered in a wash filter.

5. The process according to claim 1, wherein the temperature in step e) is 60 C. or lower.

6. The process according to claim 1, wherein the alkali concentration in step e) is in the range of from 85 to 100 g/l.

7. The process according to claim 1, wherein said process comprises a washing step between step d) and step e), said washing step comprising washing the pulp obtained from step d) in a washing device.

8. The process according to claim 1, wherein xylan and alkali removed in step f) are recirculated as a liquor flow and used as an alkali source in step e) and/or wherein xylan and alkali are removed as a liquor flow and used as an alkali/xylan source in another process for manufacturing pulp.

9. The process according to claim 1, wherein xylan and alkali removed in step f) are recirculated as a liquor flow and used as an alkali source in step d).

10. The process according to claim 9, wherein said liquor flow is oxidized before being used in step d).

11. The process according to claim 8, wherein the alkali concentration of said liquor flow is in the range of from 60 to 90 g/l without any supplementary addition of alkali.

12. The process according to claim 11, wherein said liquor flow is used as an alkali source in another process for manufacturing pulp.

13. The process according to claim 1, wherein said process comprises a step before step b), which step comprises activation of the wood based raw material by using steam until a P-factor of from 25 to 200 has been reached.

14. The process according to claim 1, wherein said process comprises a step after step g), which step is a combined depolymerization and bleaching step.

15. The process according to claim 14, wherein said combined depolymerization and bleaching step is performed by adding ozone, or by adding hypochlorite or by adding chlorine dioxide and sulfuric acid.

16. The process according to claim 14, wherein said combined depolymerization and bleaching step is performed at a temperature of from 80 to 99 C. and at an effective acid charge of from 5 to 20 kg H.sub.2SO.sub.4/air dried tons.

17. The process according to claim 14, wherein the obtained pulp after the combined bleaching and depolymerization step comprises 6 weight % or less xylan.

18. The process according to claim 1, wherein 90% or more of the alkali and dissolved xylan is removed from the pulp obtained from step e) after steps f) and g) are both performed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The processes disclosed herein will be described in more detail with reference to the appended drawings wherein:

(2) FIG. 1 shows a process diagram of a process as defined herein,

(3) FIG. 2 shows a process diagram of a process for manufacturing dissolving pulp,

(4) FIG. 3 shows a process diagram for a process for manufacturing pulp, comprising a linked process for manufacturing pulp, and

(5) FIG. 4 shows the result after running samples of dissolving pulp obtained from the process as defined herein.

DETAILED DESCRIPTION

(6) FIG. 1 schematically shows the process for manufacturing dissolving pulp as defined herein. The wood based raw material may be activated by performing a steam treatment on the wood based raw material and after the steam treatment white liquor may be added to the vessel and a traditional kraft cooking process may be performed. The kraft cooking process is followed by an oxygen delignifying step and a cold caustic extraction step (CCE-step). In the CCE-step, the oxygen delignified pulp is treated with alkali. The alkali source is industrial white liquor as set out herein. Suitable but not limiting parameters for the CCE-step are a temperature of from 30 to 50 C., a NaOH concentration of from 70 to 95 g/l and a time interval of from 15 to 30 minutes. The CCE-step will reduce the xylan content in the pulp to less than 6 weight % such as to from 6 weight % to 2 weight %. Accordingly, the process as defined herein comprises the steps of kraft cooking, oxygen delignification and cold caustic extraction followed by a washing step including an initial dewatering step performed directly after the CCE-step. A steam activation step may optionally be performed before the kraft cooking step. The process may comprise further steps such as depolymerisation and bleaching to desired viscosity and brightness level. The liquor removed from the pulp by the dewatering step coupled to the CCE-step has a high alkali and hemicellulose (xylan) concentration. As disclosed herein, the alkaline hemicellulose stream from the dewatering step may be recirculated and/or removed and used in other processes and applications.

(7) FIG. 2 is a schematical representation of a kraft process as defined herein, including an optional steam activation step. In the figures, each rectangle represents a process step, and any accompanying washing step.

(8) In the depolymerisation step (DA), the pulp may be treated with sulfuric acid at a temperature of from 80 to 99 C. The effective amount of sulfuric acid may be from 5 to 20 kg/ADT and this step may be performed for 60 to 180 min. Before this treatment, the pulp may be treated with chlorine dioxide (D) which means that there will be chlorine dioxide present in the pulp. The obtained pulp has excellent properties, such as low viscosity, high brightness and a narrow molecular weight distribution.

(9) The DA-step may be performed by using a chlorine dioxide charge in kg/ADT of 1.8 times the kappa number and a temperature of around 90 C. and an end pH of about 2.0. The DA-step may be performed during about 140 minutes.

(10) The alkaline extraction step fortified with oxygen and hydrogen peroxide (EOP) may be performed according to the following, but not limiting, parameters: pH is about 10.4, O.sub.2 is 4 kg/ADT, temperature is about 80 C.

(11) The chlorine dioxide/complexing agent step (D/Q) may be performed according to the following, but not limiting, parameters: MgSO.sub.4 0.6 kg/ADT, EDTA 1 kg/ADT, temperature 80 C. and a pH of 4.5.

(12) The pressurized hydrogen peroxide step (PO) may be performed according to the following, but not limiting, parameters: pulp consistency 10 weight %, end-pH 10.5-11.0, temperature 105 C., O.sub.2 is 3 kg/ADT, residual H.sub.2O.sub.2 3.0 kg/ADT and MgSO.sub.4 1.0 kg/ADT.

(13) The drying of the pulp may be performed to a dry content of 90-93% and the pulp may be cut into sheets and stacked in bales.

(14) FIG. 3 discloses the basic concept of a kraft process system according to the present disclosure, and including a parallel kraft pulping line in which the alkali and xylan containing liquor removed by the washing step after the CCE-step is used in the kraft cooking process. After the cooking step in which the alkali is consumed and the hemicellulose is precipitated onto the wood fiber the pulp may be bleached in a conventional way to a desired target brightness.

(15) FIG. 4 shows that the dissolved pulp as manufactured according to the present disclosure has a higher degree of fibrillation than the reference sample pulp manufactured using a conventional method.

ABBREVIATIONS

(16) mol/l mol/liter

(17) H.sub.2SO.sub.4 sulfuric acid

(18) ADT air dried tons

(19) EDTA ethylenediaminetetraacetic acid

(20) O.sub.2 oxygen

(21) Na.sub.2CO.sub.3 sodium carbonate

(22) NaHS sodium hydrosulfide

(23) NaOH sodium hydroxide

(24) Na+ sodium ion

(25) HS.sup. hydrosulfide ion

(26) K+ potassium ion

(27) OH.sup. hydroxide ion

(28) CO.sub.3.sup.2 carbonate ion

(29) H.sub.2O.sub.2 hydrogen peroxide

(30) MgSO.sub.4 magnesium sulfate

(31) ml/g milliliter/gram

(32) kinks/mm kink is defined as an abrupt change in the fiber curvature

(33) D Chlorine dioxide

(34) A Acid

(35) Q Complexing agent

(36) PO Pressurized hydrogen peroxide

(37) EOP Alkaline extraction fortified with oxygen and hydrogen peroxide

(38) The present disclosure is further illustrated by the following non-limiting examples.

Example 1

(39) Silver birch wood containing 25% xylan was cooked to pulp according to a Rapid Displacement Heating (RDH)-process to a kappa number of 17 in an industrial digester system. Cooking temperature was 160 C., the H-factor 350 and the residual alkali 10 g/l. After cooking, the pulp was screened before oxygen-delignification in a two-step industrial process. The temperature in the first reactor was 85 C. and 102 C. in the second reactor. The total alkali charge was 23 kg/ADT, total oxygen charge 15 kg/ADT and the magnesium sulfate charge was 3 kg/ADT. After the oxygen delignification, a pulp sample was taken out at the wash press and additionally washed in order to proceed with the pulp in the lab. The pulp had, after the oxygen delignification, a kappa number of 9.3, a brightness of 59.8% ISO and a viscosity of 1008 ml/g.

(40) The pulp was then treated with industrial white liquor with high ionic strength at a consistency of 10%, effective alkali concentration of 95 g/l, at a temperature of 40 C. for minutes. The liquors and pulp were pre-heated to the process temperature before mixing and treated in plastic bags. After the treatment, the free liquor was pressed out and the pulp was subsequently washed with diluted filtrate at alkali concentrations of 13 g/l, 3 g/l and with water in a sequence in order to simulate an industrial washing sequence. The resulting pulp had a xylan content of 5.5 weight % and a R.sub.18-value of 97.8%. The filtrate, which was pressed out directly after the CCE-treatment, had an effective alkali concentration of 83 g/l and a dissolved xylan concentration of 28.8 g/l.

(41) The pulp, after the white liquor treatment, had a great potential as a dissolving pulp, however the viscosity and brightness needed to be adjusted. This was performed in a combined chlorine dioxide and acidic step. In an industrial process, it is important that the pH in the step does not drop too much below 2.0 as this increases the risk of severe corrosion on the equipment. Instead other parameters than the acidic charge were adjusted in order to meet the demands of a dissolving pulp. The DA-step was conducted at 95 C. at an active chlorine charge of 6.1 kg/ADT and a sulfuric acid charge of 10 kg/ADT. The residence time was 165 min and the treatment resulted in a pH of 1.9. After the DA-step, an extraction step was performed at 80 C., alkali charge of 5.5 kg/ADT and a hydrogen peroxide charge of 2 kg/ADT for 120 minutes. This resulted in a pulp with a brightness of 85.7% ISO and 390 ml/g in intrinsic viscosity.

(42) The final step was a Q PO treatment with alkali charge of 20 kg/ADT and a hydrogen peroxide charge of 10 kg/ADT. The temperature was 110 C. and the residence time 150 minutes. The pulp was thereafter analyzed and a good dissolving pulp was obtained with a R.sub.18-value of 97.6%, xylan content of 4.4 weight % and a viscosity of 383 ml/g.

(43) The final pulp was also analyzed for other relevant parameters and the results are shown in Table 1. The metal ion content is an important property for a dissolving pulp and this content is very low and a reason for this is the acidic treatment at a pH of approximately 2 in the combined DA-step, which protonises the pulp acids and therefore lowers the metal ion content.

(44) TABLE-US-00001 TABLE 1 Characterization of the pulp after the different treatments O.sub.2 CCE DA EOP Q PO Viscosity, ml/g 1008 997 389 383 Brightness, % ISO 59.8 67.3 85.7 92.3 Kappa number 9.3 3.4 R.sub.18, % 97.8 97.6 R.sub.10, % 94.7 xylan, % 23.6 5.5 4.4 Ash content, % 0.12 Acetone extractives, 0.13 % Fe, ppm 1.5 Mn, ppm <0.1 Mg, ppm 26 Si, ppm 20 Ca, ppm 22 Ni, ppm 0.1 Cu, ppm 0.2

Example 2 Mill Process

(45) In this example a kraft mill using 4 batch digesters at 325 m.sup.3 each was used. The raw material comprised of 93% Silver birch and 7% of other hardwoods, mainly aspen. The wood chips were steamed to a P-factor of 100 and the activation was terminated with the addition of white liquor to the bottom of the digester, immediately followed by a white and black liquor mixture until a cooking liquor:wood based raw material ratio of 3.7:1 was reached. The cooking step was performed with liquor circulation at 160 C. until a H-factor of 400 was reached. Typical properties of the pulp after the digestion was; viscosity: 1100 ml/g, brightness: 45% ISO, kappa number: 13.

(46) The oxygen delignification was performed in a two-step reactor, using a total oxygen charge of 23 kg/ADT, without any additional charge of alkali. A charge of 1 kg MgSO.sub.4/ADT was used to minimize the degradation reactions. The temperature in the two steps was 86 C. for 30 min and 105 C. for additional 60 min. After this treatment, the properties of the pulp were: brightness: 56% ISO, kappa number: 9.

(47) Since the wood was activated using steam, the alkali charge in the CCE-step could be lowered. White liquor was charged so that a concentration of effective alkali was 85 g/l at a temperature of 45 C. for 20 minutes. The resulting pulp slurry was dewatered in a press before dilution and treatment in two wash presses and one wash filter in a sequence. After washing the properties of the pulp were: viscosity: 770 ml/g, brightness: 61.9% ISO.

(48) The filtrate after the press was analyzed and the xylan content was 24.3 g/l, lignin content was 4.6 g/l resulting in a xylan/lignin ratio of 5.3:1. The weight average molecular weight of xylan was determined to 30.0 kg/mol, corresponding to a degree of polymerization of 227.

(49) Since the pulp still contained some hydrosulfide ions after washing, chlorine dioxide was charged first and then just after, sulfuric acid was charged. The DA-step was performed in a small up-flow tower coupled with a larger down-flow tower. The temperature was 91 C., chlorine dioxide charge was 21 kg/ADT and sulfuric acid charge was 24 kg/ADT. About 9 kg of the sulfuric acid charge was used for neutralization and the rest was used as active charge. After washing, the pulp was treated in an extractions step at 80 C., 4 kg O.sub.2/ADT, 2 kg H.sub.2O.sub.2/ADT and an alkali charge to reach a final pH of 10.4. The properties of the pulp after these treatments were; viscosity: 420 ml/g, brightness: 86% ISO.

(50) To reach the target brightness, the pulp was treated in a Q PO sequence. The chelating step was performed with 0.5 kg/ADT of EDTA with 0.6 kg/ADT of magnesium sulfate at a temperature of 80 C. After washing, the PO-step was conducted with 10 kg of H.sub.2O.sub.2/ADT, kg NaOH/ADT, 1 kg MgSO.sub.4/ADT and 3 kg O.sub.2/ADT. The temperature in the bottom of the reactor was 95 C. After this final treatment, the pulp was dried in a drying machine to a dry content above 90% as set out above, cut into sheets and stacked in bales.

(51) TABLE-US-00002 TABLE 2 Characterization of the pulp after the different treatments in the mill. Cook DA EOP Q PO Viscosity, ml/g 1100 418 422 Brightness, % ISO 45.1 86.3 91.5 Kappa number 13.2 0.9 0.9 R.sub.18, % 92.1 96.3 96.8 R.sub.10, % 88.7 92.7 93.4 xylan, % 15.8 5.0 4.1 Ash content, % 0.85 0.12 0.06 Acetone extractives, 0.86 0.16 0.26 % Fe, ppm 2.1 1.0 1.0 Mn, ppm 27 <0.5 <0.5 Mg, ppm 68 56 77 Si, ppm 17 11 6 Ca, ppm 950 66 38 Cu, ppm <0.5 <0.5 <0.5

Example 3 Comparison with Commercial Dissolving Pulps

(52) Different commercial pulps (paper pulps and dissolving pulps) were collected and analysed using L&W FiberTester and CP-MAS .sup.13C NMR spectroscopy [Wollboldt et al. 2010 (Wood Sci. Technol. 44:533-546)]. The uniqueness of the dissolving pulp produced using the method as defined herein is illustrated in the measured data as shown in Tables 3 and 4 below.

(53) TABLE-US-00003 TABLE 3 Data from analyses of fibre dimensions with L&W Fiber Tester and WRV-measurements. Kinks Shape factor WRV Pulp (kinks/mm) (%) (g/g) Comm. Birch paper KP.sup.1 0.530 90.5 1.22 Comm. Eucalypt paper KP.sup.2 0.614 91.1 1.10 Birch DP Example 2.sup.3 (P = 0) 1.542 78.3 0.98 Birch DP Example 2.sup.3 (P = 100) 1.510 79.5 0.90 Comm. PHK eucalypt.sup.4 1.076 87.2 0.95 Comm. sulphite beech.sup.5 1.266 83.9 0.80 .sup.1Birch paper kraft pulp .sup.2Eucalypt paper kraft pulp .sup.3Birch dissolving pulp prepared according to Example 2 .sup.4Commercial prehydrolysis kraft eucalypt dissolving pulp .sup.5Commercial beech sulphite dissolving pulp

(54) TABLE-US-00004 TABLE 4 Data from analyses with CP-MAS .sup.13C NMR spectroscopy. Lateral fibril Fibril aggregate Cellu- Crystal- width dimension lose linity Pulp (nm) (nm) II (%) index (%) Comm. Birch paper KP.sup.1 4.4 15.3 4.2 54.5 Birch DP Example 2.sup.2 (P = 0) 4.9 22.7 17.8 59.0 Birch DP Example 2.sup.2 (P = 100) 5.2 18.1 8.8 60.7 Comm. PHK eucalypt.sup.3 4.7 14.3 0.2 61.1 Comm. Sulphite beech.sup.4 4.7 14.3 6.7 57.3 .sup.1Birch paper kraft pulp .sup.2Birch dissolving pulp prepared according to Example 2 .sup.3Commercial prehydrolysis kraft eucalypt dissolving pulp .sup.4Commercial beech sulphite dissolving pulp

(55) As is evident from Table 3, a main difference between the dissolving pulp produced according to the method as defined herein and the commercial dissolving pulps, is the high kink value and the low shape factor of the pulp produced according to the invention. Furthermore, from Table 4 it is evident that pulps which have been produced according to the present method have elevated contents of cellulose II as a result of the high alkali charge in the CCE-step and that the lateral fibril aggregate dimensions are significantly larger than for the commercial pulps analysed.

(56) In order to obtain comparative values e.g. when measuring LFAD in pulp by using NMR, it is important that the analyzed pulps are dried to the same extent. All tested commercial pulps were therefore dried in a drying machine to a dry content above 90%.

Example 4

(57) FE-SEM

(58) After coating with a thin layer of Au/Pd, the pulp samples were examined by high-resolution scanning electron microscopy at a 350 magnification with a Hitachi S4000 SEM (FE-SEM) applying an acceleration voltage of 6 kV. For preservation of the surface structure of moist pulps, the method of rapid freezing in liquid N.sub.2 and normal freeze-drying described by Okamoto and Meshitsuka, 2010 (Cellulose 17:1171-1182) was applied.

(59) The result of the SEM analysis is shown in FIG. 4. FIG. 4 shows that the dissolving pulp fibers made according to the process of the invention are curly and have a high kink as measured by image analysis as disclosed herein. The curly pulp fibers may be formed into bulky pulp sheets that absorb liquid easily and are easy to disintegrate in a dry state.