Method For Preparing Dissolving Pulp By Totally Chlorine-Free Bleaching Of Poplar Kraft Pulp

Abstract

The application relates to a method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, including: preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as raw material; ZOQP bleaching of poplar kraft pulp to obtain dissolving pulp, wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching. In this way, washing and bleaching wastewater can be recycled in entire preparation process described above, achieving zero discharge. The dissolving pulp prepared by the method has no residual toxic substances, and exhibits that all indexes exceed the requirements of qualified products in dissolving pulp industry standard (QB/T 4898-2015), and most of indexes meet those of excellent products; thus, it can replace imported dissolving pulp. The use of the method significantly solved the problems of large pollution in the production and shortage of raw materials for viscose fibers.

Claims

1. A method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, comprising: (1) preparing poplar kraft pulp by pre-hydrolysis kraft process with poplar as a raw material; and (2) performing ZOQP bleaching on the poplar kraft pulp to obtain dissolving pulp; wherein Z denotes ozone bleaching, O denotes oxy delignification, Q denotes chelation treatment, and P denotes hydrogen peroxide bleaching.

2. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein the pre-hydrolysis kraft process comprises procedures of hot water pretreatment and the kraft cooking.

3. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein the hot water pretreatment comprises steps of: mixing poplar pieces with water according to a liquid ratio of 1:4-1:7, heating up to 150-180 C., and holding for 60-100 min.

4. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein the kraft cooking comprises steps of: performing cooking for 60-100 min at the liquid ratio of 1:4-1:7, the temperature of 150-180 C., the amount of alkali (calculated as Na.sub.2O) of 20-25%, and the degree of vulcanization of 20-25%.

5. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein Z-stage comprises steps of: adjusting poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35 C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%.

6. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein O-stage comprises steps of: placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO.sub.3, and then and performing the reaction at 90-110 C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%.

7. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein Q-stage comprises steps of: placing the pulp treated in O-stage in a closed environment, adding 0.2-0.6% of chelating agent DTPA, mixing the pulp and DTPA uniformly, and then performing a chelate treatment at 60-80 C. for 20-40 min.

8. The method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp according to claim 1, wherein P-stage comprises steps of: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction for 70-100 min at the temperature of 80-100 C.; after the reaction is completed, the pulp is took out, and then is washed to neutrality and dried to obtain dissolving pulp.

9. A dissolving pulp prepared by the method according to claim 1.

10. An application of dissolving pulp according to claim 9 in the preparation of viscose fiber, acetate fiber, nitrocellulose or cellulose ether.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanation of the present application. Unless defined otherwise, all technical and scientific terms used in the application have the same meaning as commonly understood by one of ordinary skill in the art to which the application belongs.

[0027] It should be noted that the terminology used herein is only for describing specific embodiments and is not intended to limit the exemplary embodiments according to the present application. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise, and it should also be understood that when the terms comprising and/or including are used in the specification, they indicate that there are features, steps, operations, devices, components, and/or combinations thereof.

[0028] As described in the background, the current TCF bleaching of wood dissolving pulp is not good in efficiency and has high cost. Therefore, the present invention provides an environmentally-friendly method for preparing dissolving pulp by totally chlorine-free bleaching of poplar kraft pulp, in which poplar pieces are used as raw materials, comprising the following steps:

[0029] Cutting the naturally air-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm;

[0030] then, hot water pretreatment of the poplar pieces: mixing the poplar pieces with water according to the liquid ratio of 1:4-1:7, heating up to 150-180 C., and holding for 60-100 min;

[0031] subsequently, cooking the pre-treated poplar pieces by the kraft process for 60-100 min at the liquid ratio of 1:4-1:7, the temperature of 150-180 C., the amount of alkali (based on Na.sub.2O) of 20-25%, and the degree of vulcanization of 20-25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp; and

[0032] then, performing the totally chlorine-free ZOQP bleaching on the poplar kraft pulp (Z-ozone bleaching, O-oxygen delignification, Q-chelation treatment, P-hydrogen peroxide bleaching) to obtain dissolving pulp,

[0033] Z-stage includes steps of: adjusting poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35 C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%;

[0034] O-stage includes steps of: placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO.sub.3, and then performing the reaction at 90-110 C. for 30-50 min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%;

[0035] Q-stage includes steps of: placing the pulp treated in O-stage in a closed environment, adding 0.2-0.6% of chelating agent DTPA , mixing the pulp and DTPA uniformly, and performing a chelate treatment at 60-80 C. for 20-40 min; and

[0036] P-stage includes steps of: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100 C. for 70-100 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp. Various indexes of the dissolving pulp are measured.

[0037] All chemical reagents used in the present invention are chemically pure.

[0038] In the present application, the method for preparing dissolving pulp comprises steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to obtain dissolving pulp, and specifically comprises the following steps:

[0039] (1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

[0040] (2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water according to the liquid ratio of 1:4-1:7, heating up to 150-180 C., and holding for 60-100 min.

[0041] (3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 60-100 min at the liquid ratio of 1:4-1:7, the maintaining temperature of 150-180 C., the amount of alkali (based on Na.sub.2O) of 20-25% and the degree of vulcanization of 20-25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain hot water pretreated poplar kraft pulp.

[0042] (4) Ozone bleaching (Z-stage): adjusting hot water pretreated poplar kraft pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of diethyltriamine pentaacetic acid (DTPA), mixing them uniformly, and then performing the reaction at 20-35 C. for 10-30 min; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%.

[0043] (5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO.sub.3, and then performing the reaction at 90-110 C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%.

[0044] (6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.2-0.6% of chelating agent DTPA , mixing the pulp and DTPA uniformly, and then performing a chelate treatment at 60-80 C. for 20-40 min.

[0045] (7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100 C. for 70-100 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

[0046] The specific steps comprise:

[0047] (1) Cutting the naturally air-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

[0048] (2) Mixing the poplar pieces obtained in step (1) with water according to the liquid ratio of 1:4-1:7, heating up to 150-180 C., and holding for 60-100 min.

[0049] (3) Cooking the poplar pieces pretreated in step (2) for 60-100 min at the liquid ratio of 1:4-1:7, the maintaining temperature of 150-180 C., the amount of alkali (based on Na.sub.2O) of 20-25% and the degree of vulcanization of 20-25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain hot water pretreated poplar kraft pulp.

[0050] (4) Performing ozone bleaching (Z stage) on the pulp obtained in step (3): adjusting the pulp to the concentration of 25-30% and pH of 1.5-3.0, adding 1.0-3.0% of ozone and 0.3-0.7% of DTPA, mixing them uniformly, and then performing the reaction at 20-35 C. for 10-30 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 8-15%.

[0051] (5) Performing oxygen delignification (O-stage) on the pulp obtained in step (4): placing the pulp in step (4) in a closed environment, introducing oxygen to an oxygen pressure of 0.4-0.8 Mpa, adding 1.0-3.0% of NaOH and 0.3-0.7% of MgCO.sub.3, and then performing the reaction at 90-110 C. for 30-50 min while stirring at a speed of 60 r/min; after the reaction is completed, the pulp is washed to neutrality and is adjusted to the concentration of 8-15%.

[0052] (6) Performing chelation treatment (Q-stage) on the pulp obtained in step (5): placing the pulp obtained in step (5) in a closed reactor, adding 0.3-0.6% of diethyltriaminepentaacetic acid (DTPA), mixing the pulp and DTPA uniformly, and then performing a chelate treatment in a water bath at 60-80 C. for 20-40 min while stirring at a speed of 60 r/min.

[0053] (7) Bleaching the pulp obtained in step (6) (P-stage) as follows: after the reaction in Q-stage is completed, adding 1.0-3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.4-0.9% of NaOH, 1.0-2.0% of sodium silicate and 0.1-0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 80-100 C. for 70-100 min; after the reaction is completed, the pulp is taken out and washed to neutrality.

[0054] (8) Drying the pulp obtained in step (7) to obtain dissolving pulp, which is mainly used in the preparation of viscose fiber, acetate fiber, nitrocellulose or cellulose ether.

[0055] The technical solutions of the present application will be described below by specific examples. In the following examples, all percentages (%) refer to mass percentages.

EXAMPLE 1

[0056] Dissolving pulp is obtained by steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to produce dissolving pulp, and specifically by the following steps:

[0057] (1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

[0058] (2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water in a reactor according to the liquid ratio of 1:5, heating up to 165 C., and holding for 90 min.

[0059] (3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 90 min at the liquid ratio of 1:5, the temperature of 165 C., the amount of alkali (based on Na.sub.2O) of 22% and the degree of vulcanization of 24%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp.

[0060] (4) Ozone bleaching (Z-stage): adjusting the poplar kraft pulp to the concentration of 28% and placing it in a reactor, adding 2% of ozone and 0.5% of DTPA, adjusting the pH to 2.5 with acetic acid, and then performing the reaction at 25 C. for 20 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 10%.

[0061] (5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in an oxygen bleaching tank, introducing oxygen to an oxygen pressure of 0.6 Mpa, adding 2.5% of NaOH and 0.5% of MgCO.sub.3, and then performing the reaction at the temperature of 100 C. for 40 min while stirring at a speed of 60 r/min; after the bleaching is completed, the pulp is washed to neutrality and is adjusted to the concentration of 10%.

[0062] (6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.5% of chelating agent DTPA, stirring at a speed of 60 r/min to mix the pulp and DTPA uniformly, and performing a chelate treatment at 70 C. for 30 min.

[0063] (7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 2.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.8% of NaOH, 1.5% of sodium silicate and 0.2% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 90 C. for 90 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

TABLE-US-00001 TABLE 1 Indexes of excellent/ qualified products in Measured Items Unit industry standards indexes Alkali resistance % 93.0/90.0 93.1 Polypentose % 4.0/4.5 3.5 Ash content % 0.12/0.15 0.09 Iron content mg/kg 15.0/20.0 14.8 D.sub.65 brightness % 82.0 82.7 Dichloromethane extract % 0.30/0.40 0.37 Intrinsic viscosity mL/g 380~500/320~500 442 Dustiness 0.05 mm.sup.2~3.0 mm.sup.2/kg 60/80 42.4 mm.sup.2 >3.0 mm.sup.2 none none

[0064] Comparison of indexes of dissolving pulp prepared by ZOQP bleaching of prehydrolyzed poplar kraft pulp and those of products in industry standard

EXAMPLE 2

[0065] Dissolving pulp is obtained by steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to produce dissolving pulp, and specifically by the following steps:

[0066] (1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces with a length of 3-6 cm and a thickness of 2-5 mm.

[0067] (2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water in a reactor according to the liquid ratio of 1:6, heating up to 170 C., and holding for 70 min.

[0068] (3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 100 min at the liquid ratio of 1:5, the temperature of 170 C., the amount of alkali (based on Na.sub.2O) of 20% and the degree of vulcanization of 22%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp.

[0069] (4) Ozone bleaching (Z-stage): adjusting the poplar kraft pulp to the concentration of 25% and placing it in a reactor, adding 1% of ozone and 0.3% of DTPA, adjusting the pH to 2.0 with acetic acid, and then performing the reaction at the temperature of 20 C. for 30 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 12%.

[0070] (5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in an oxygen bleaching tank, introducing oxygen to an oxygen pressure of 0.4 Mpa, adding 1.5% of NaOH and 0.3% of MgCO.sub.3, and then performing the reaction at the temperature of 90 C. for 50 min while stirring at a speed of 60 r/min; after the bleaching is completed, the pulp is washed to neutrality and is adjusted to the concentration of 12%.

[0071] (6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.3% of chelating agent DTPA , stirring at a speed of 60 r/min to mix the pulp and DTPA uniformly, and then performing a chelate treatment at 60 C. for 40 min.

[0072] (7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 1.5% of hydrogen peroxide directly to the pulp for bleaching, and also adding 0.6% of NaOH, 1.0% of sodium silicate and 0.1% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 95 C. for 100 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

TABLE-US-00002 TABLE 2 Indexes of excellent/ qualified products in Measured Items Unit industry standards indexes Alkali resistance % 93.0/90.0 91.6 Polypentose % 4.0/4.5 3.9 Ash content % 0.12/0.15 0.13 Iron content mg/kg 15.0/20.0 15.9 D.sub.65 brightness % 82.0 82.3 Dichloromethane extract % 0.30/0.40 0.39 Intrinsic viscosity mL/g 380~500/320~500 481 Dustiness 0.05 mm.sup.2~3.0 60/80 60 47.7 mm.sup.2 >3.0 mm.sup.2 none none

[0073] Comparison of indexes of dissolving pulp prepared by ZOQP bleaching of prehydrolyzed poplar kraft pulp and those of products in industry standard

EXAMPLE 3

[0074] Dissolving pulp is obtained by steps of: preparing poplar kraft pulp by hot water pretreatment and the kraft process with poplar pieces as raw materials; and then performing ZOQP bleaching on the poplar kraft pulp to produce dissolving pulp, and specifically by the following steps:

[0075] (1) Material preparation: Cutting the naturally-dried and purified poplar into poplar pieces having a length of 3-6 cm and a thickness of 2-5 mm.

[0076] (2) Hot water pretreatment of poplar pieces: mixing the poplar pieces with water in a reactor according to the liquid ratio of 1:4, heating up to 175 C., and holding for 60 min.

[0077] (3) Kraft cooking of poplar: cooking the poplar pieces pretreated with hot water for 80 min at the liquid ratio of 1:5, the temperature of 160 C., the amount of alkali (based on Na.sub.2O) of 25% and the degree of vulcanization of 25%; after the cooking is completed, the resulting substance is washed and concentrated to obtain poplar kraft pulp.

[0078] (4) Ozone bleaching (Z-stage): adjusting the poplar kraft pulp to the concentration of 30% and placing it in a reactor, adding 3% of ozone and 0.7% of DTPA, adjusting the pH to 3.0 with acetic acid, and then performing the reaction at the temperature of 25 C. for 10 min while stirring at a speed of 60 r/min; excess ozone generated during the reaction enters the absorption device filled with KI solution through the air outlet for absorption; after the reaction is completed, the pulp is washed to neutrality and concentrated to the concentration of 15%.

[0079] (5) Oxygen delignification (O-stage): placing the pulp bleached in Z-stage in an oxygen bleaching tank, introducing oxygen to an oxygen pressure of 0.8 Mpa, adding 3.5% of NaOH and 0.7% of MgSO.sub.4, and then performing the reaction at the temperature of 110 C. for 30 min while stirring at a speed of 60 r/min; after the bleaching is completed, the pulp is washed to neutrality and is adjusted to the concentration of 15%.

[0080] (6) Chelation treatment (Q-stage): placing the pulp treated in O-stage in a closed reactor, adding 0.6% of chelating agent DTPA , stirring at a speed of 60 r/min to mix the pulp and DTPA uniformly, and performing a chelate treatment at 80 C. for 20 min.

[0081] (7) Hydrogen peroxide bleaching (P-stage): after the reaction in Q-stage is completed, adding 3.0% of hydrogen peroxide directly to the pulp for bleaching, and also adding 1.0% of NaOH, 2.0% of sodium silicate and 0.3% of magnesium sulfate, and then performing the bleaching reaction at the temperature of 95 C. for 80 min; after the reaction is completed, the pulp is taken out, and then is washed to neutrality and dried to obtain dissolving pulp; various indexes of the dissolving pulp are measured.

TABLE-US-00003 TABLE 3 Indexes of excellent/ qualified products in Measured Items Unit industry standards indexes Alkali resistance % 93.0/90.0 93.8 Polypentose % 4.0/4.5 3.1 Ash content % 0.12/0.15 0.09 Iron content mg/kg 15.0/20.0 12.8 D.sub.65 brightness % 82.0 83.1 Dichloromethane extract % 0.30/0.40 0.33 Intrinsic viscosity mL/g 380~500/320~500 401 Dustiness 0.05 mm.sup.2~3.0 60/80 60 39.9 mm.sup.2 >3.0 mm.sup.2 none none

[0082] Comparison of indexes of dissolving pulp prepared by ZOQP bleaching of prehydrolyzed poplar kraft pulp and those of products in industry standard

[0083] Finally, it should be noted that the above examples are only preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the foregoing embodiments, it can still modify the technical solutions described in the foregoing embodiments, or equivalently replace some of them for those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. Although the specific embodiments of the present invention are described above, they do not limit the scope of protection of the present invention. Those skilled in the art should understand that based on the technical solution of the present invention, various modifications or deformations that can be made by those skilled in the art without creative effort are still within the protection scope of the present invention.