A method for the chlorine-free bleaching of pulp

Abstract

The application discloses a paper pulp chlorine-free bleaching method, which comprises the following steps: 1) ozone bleaching to obtain ozone-bleached paper pulp; 2) hydrogen peroxide bleaching to obtain hydrogen peroxide-bleached paper pulp; 3) chelation treatment to obtain chelation-treated paper pulp; and 4) hydrogen peroxide bleaching to obtain bleached pulp. The paper pulp chlorine-free bleaching method has the advantages of low temperature, normal pressure, low energy consumption, low cost, high efficiency, suitability for various paper pulp systems, green environmental protection and the like, and the bleached paper pulp has high whiteness, high viscosity, low Kappa number and no AOX; and the method is suitable for large-scale industrial application.

Description

Technical Field

[0001] This invention relates to the field of pulp and paper technology, and specifically to a method for chlorine-free bleaching of pulp. Background Technology

[0002] Conventional Light-ECF bleaching method (OZ-D1-E) p -D2 process: oxygen delignification → extraction → ozone bleaching → extraction → chlorine dioxide bleaching) and traditional ECF bleaching method (O-D1-E) p - The D2 process (oxygen delignification → chlorine dioxide bleaching → extraction → chlorine dioxide bleaching) is a common ECF (elemental chlorine-free) bleaching method. However, both of these ECF bleaching methods have significant drawbacks and cannot fully meet practical application requirements. Specifically: 1) High temperature and high pressure requirements: The O stage needs to be carried out under conditions of temperature ≥90℃ and pressure ≥0.5MPa, which not only results in high energy consumption but also high equipment investment; 2) Limited efficiency: O2 in the O stage mainly reacts with non-phenolic lignin, resulting in poor treatment of phenolic lignin, and residual phenolic lignin will affect pulp brightness; 3) Complex process and high water consumption: It involves multi-stage extraction and inter-stage switching, the system occupies a large area, and the water consumption is as high as 25m³. 3 / t~30m 3 / t;4) High ClO2 dosage and heavy environmental burden: The ClO2 dosage in section D is ≥4%, and the AOX dosage reaches 0.2kg / t~0.8kg / t, which puts great pressure on wastewater treatment.

[0003] Therefore, developing a low-temperature, normal-pressure, low-energy-consumption, low-cost, high-efficiency, and environmentally friendly chlorine-free pulp bleaching method is of great significance. Summary of the Invention

[0004] The purpose of this invention is to provide a chlorine-free bleaching method for pulp.

[0005] The technical solution adopted in this invention is:

[0006] A method for chlorine-free bleaching of pulp includes the following steps:

[0007] 1) Ozone bleaching: The pulp is concentrated and then transferred to an ozone reactor to bleach it with ozone. The pulp is then washed to obtain ozone-bleached pulp.

[0008] 2) Hydrogen peroxide bleaching: The ozone-bleached pulp is concentrated, and then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate are added. The pulp is then transferred to a hydrogen peroxide bleaching tower for hydrogen peroxide bleaching. The pulp is then washed to obtain hydrogen peroxide-bleached pulp.

[0009] 3) Chelation treatment: The hydrogen peroxide bleached pulp is concentrated, then a chelating agent is added for chelation treatment, and then the pulp is washed to obtain chelated pulp.

[0010] 4) Hydrogen peroxide bleaching: The chelated pulp is concentrated, then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate are added, and then it is transferred to the hydrogen peroxide bleaching tower for hydrogen peroxide bleaching. The pulp is then washed to obtain bleached pulp.

[0011] Preferably, the pulp in step 1) is at least one of sulfate eucalyptus pulp, sulfate bamboo pulp, sulfate pine pulp, sulfate reed pulp, sulfate bagasse pulp, sulfate wheat straw pulp, and caustic soda-anthraquinone eucalyptus pulp.

[0012] Preferably, the pulp in step 1) has a concentration of 1.5 wt% to 2.5 wt% after concentration.

[0013] Preferably, step 1) further includes adjusting the pH of the pulp to 1.5 to 2.5 between concentration and ozone bleaching.

[0014] Preferably, the amount of ozone introduced in step 1) is 1.5% to 2.0% of the oven-dry weight of the pulp.

[0015] Preferably, the ozone bleaching in step 1) is carried out at room temperature, and the ozone bleaching time is 10 min to 50 min.

[0016] Preferably, the ozone bleaching in step 1) is carried out under a stirring rate of 500 rpm to 1000 rpm.

[0017] Preferably, the concentration of the ozone-bleached pulp in step 2) is 8wt% to 12wt% after concentration.

[0018] Preferably, the amount of hydrogen peroxide introduced in step 2) is 2.0% to 3.0% of the oven-dry weight of the ozone-bleached pulp.

[0019] Preferably, the amount of sodium hydroxide used in step 2) is 1.5% to 2.0% of the oven-dry weight of the ozone-bleached pulp.

[0020] Preferably, the amount of sodium silicate used in step 2) is 1.5% to 2.0% of the oven-dry weight of the pulp after ozone bleaching.

[0021] Preferably, the amount of magnesium sulfate used in step 2) is 0.1% to 0.5% of the oven-dry weight of the ozone-bleached pulp.

[0022] Preferably, the hydrogen peroxide bleaching in step 2) is carried out at a temperature of 80℃~90℃ and the hydrogen peroxide bleaching time is 70min~110min.

[0023] Preferably, the concentration of the hydrogen peroxide-bleached pulp in step 3) is 8wt% to 12wt% after concentration.

[0024] Preferably, the amount of chelating agent used in step 3) is 0.8% to 1.2% of the oven-dry weight of the hydrogen peroxide-bleached pulp.

[0025] Preferably, the chelating agent in step 3) is at least one of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), carboxymethyl chitosan, trisodium citrate, and Tween 80.

[0026] More preferably, the chelating agent in step 3) comprises the following components by weight percentage:

[0027] Ethylenediaminetetraacetic acid: 0.2%–0.5%;

[0028] Diethylenetriaminepentaacetic acid: 0.1%–0.3%;

[0029] Carboxymethyl chitosan: 0.3%–0.7%;

[0030] Trisodium citrate: 0.3%–0.7%;

[0031] Tween 80: 0.1%–0.3%;

[0032] Water: Balance.

[0033] Note: ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and carboxymethyl chitosan can chelate Fe. 3+ Mn 2+ The presence of metal ions helps prevent the decomposition of H2O2; trisodium citrate assists in chelation and enhances chelation stability; Tween 80 (a nonionic surfactant) improves dispersibility and permeability; and water (the solvent) ensures the full dissolution and stability of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, carboxymethyl chitosan, trisodium citrate, and Tween 80.

[0034] Preferably, the chelation treatment in step 3) is carried out at a temperature of 50℃ to 60℃ for a time of 50 min to 70 min.

[0035] Preferably, the pulp subjected to chelation treatment in step 4) has a concentration of 8wt% to 12wt% after concentration.

[0036] Preferably, the amount of hydrogen peroxide introduced in step 4) is 1.0% to 2.0% of the oven-dry weight of the chelated pulp.

[0037] Preferably, the amount of sodium hydroxide used in step 4) is 1.0% to 1.5% of the oven-dry weight of the chelated pulp.

[0038] Preferably, the amount of sodium silicate used in step 4) is 1.5% to 2.0% of the oven-dry weight of the chelated pulp.

[0039] Preferably, the amount of magnesium sulfate used in step 4) is 0.1% to 0.5% of the oven-dry weight of the chelated pulp.

[0040] Preferably, the hydrogen peroxide bleaching in step 4) is carried out at a temperature of 80℃~90℃ and the hydrogen peroxide bleaching time is 100min~150min.

[0041] The beneficial effects of this invention are: the chlorine-free pulp bleaching method of this invention has the advantages of low temperature, normal pressure, low energy consumption, low cost, high efficiency, applicability to various pulp systems, and green environmental protection. Moreover, the bleached pulp has high whiteness, high viscosity, low kappa value, and no AOX. This method is suitable for large-scale industrial application.

[0042] Specifically:

[0043] 1) The chlorine-free pulp bleaching method of the present invention can be widely applied to various plant fiber systems such as bamboo pulp and hardwood pulp, and exhibits good raw material adaptability and bleaching stability;

[0044] 2) The chlorine-free pulp bleaching method of the present invention uses ozone instead of oxygen as the first stage of bleaching (the main lignin oxidation can be completed within 10 min to 50 min, and the reaction rate is much higher than that of O2). Under acidic conditions, it can efficiently oxidize phenolic lignin and non-phenolic lignin, improve the delignification rate, and at the same time remove hexenuronic acid and metal ions, avoiding their consumption of oxidant and inhibition of subsequent bleaching processes.

[0045] 3) The chlorine-free pulp bleaching method of the present invention utilizes two stages of hydrogen peroxide (high oxidation potential, superior to chlorine and chlorine dioxide, which can efficiently oxidize lignin benzene rings and side chains) bleaching to further improve the whiteness and viscosity of pulp and significantly reduce the environmental pollution load;

[0046] 4) The chlorine-free pulp bleaching method of the present invention has a chelation section between the two hydrogen peroxide bleaching stages. By chelating metal ions in the pulp, the decomposition of hydrogen peroxide in the P2 stage is inhibited, ensuring the activity and stability of the bleaching agent in the P2 stage and significantly improving the bleaching effect of the P2 stage.

[0047] 5) The entire process of the chlorine-free pulp bleaching method of the present invention does not require the use of chlorine dioxide and does not require high-temperature and high-pressure equipment (significantly reducing steam / electricity consumption), thus realizing low-temperature, normal-pressure, low-energy-consumption, and low-cost pulp bleaching.

[0048] 6) The pulp obtained by the chlorine-free bleaching method of the present invention has high whiteness, high viscosity, low kappa value, no AOX (adsorbable organic halides; O3 and H2O2 will not produce chlorination byproducts), and good pulp quality. Detailed Implementation

[0049] The present invention will be further explained and described below with reference to specific embodiments.

[0050] Example 1:

[0051] A chlorine-free bleaching method for pulp, comprising the following steps:

[0052] 1) Ozone bleaching (Z stage): The sulfate eucalyptus pulp is concentrated to a concentration of 2wt% and the pH value of the sulfate eucalyptus pulp is adjusted to 2.0. Then it is transferred to an ozone reactor and ozone is introduced at room temperature and pressure. The ozone introduction rate is 1.6% of the oven-dry weight of the sulfate eucalyptus pulp. The stirring speed is controlled at 800 rpm for 15 minutes for ozone bleaching. Then the sulfate eucalyptus pulp is washed with water (to remove residual ozone and dissolved lignin products) to obtain ozone-bleached sulfate eucalyptus pulp.

[0053] 2) Hydrogen peroxide bleaching (P1 stage): The ozone-bleached sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%. Then, hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added sequentially. The amount of hydrogen peroxide introduced is 2.5% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp, the amount of sodium hydroxide is 1.8% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp, the amount of sodium silicate is 1.5% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp, and the amount of magnesium sulfate is 0.2% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, where steam is introduced to heat it to 90°C and maintain the temperature for 90 minutes for hydrogen peroxide bleaching. The sulfate eucalyptus pulp is then washed with water to obtain hydrogen peroxide-bleached sulfate eucalyptus pulp.

[0054] 3) Chelation treatment (Q stage): Hydrogen peroxide bleached eucalyptus sulfate pulp is concentrated to a concentration of 10 wt%, and then a chelating agent (composed of the following components by weight percentage: EDTA: 0.4%; DTPA: 0.2%; carboxymethyl chitosan: 0.5%; trisodium citrate: 0.5%; Tween 80: 0.2%; water: balance) is added. The amount of chelating agent is 1.0% of the oven-dry weight of the hydrogen peroxide bleached eucalyptus sulfate pulp. The pulp is then transferred to a reactor, and steam is introduced to heat it to 50°C for 60 minutes for chelation treatment. The eucalyptus sulfate pulp is then washed with water to obtain chelated eucalyptus sulfate pulp.

[0055] 4) Hydrogen peroxide bleaching (P2 stage): The chelated sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added. The amount of hydrogen peroxide introduced is 1.5% of the oven-dry weight of the chelated sulfate eucalyptus pulp, the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chelated sulfate eucalyptus pulp, the amount of sodium silicate is 1.5% of the oven-dry weight of the chelated sulfate eucalyptus pulp, and the amount of magnesium sulfate is 0.2% of the oven-dry weight of the chelated sulfate eucalyptus pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, and steam is introduced to heat it to 90°C for 120 minutes for hydrogen peroxide bleaching. The eucalyptus pulp is then washed with water to obtain bleached sulfate eucalyptus pulp.

[0056] Example 2:

[0057] A chlorine-free bleaching method for pulp, comprising the following steps:

[0058] 1) Ozone bleaching (Z stage): The sulfate eucalyptus pulp (same as in Example 1) was concentrated to a concentration of 2wt%, and the pH value of the sulfate eucalyptus pulp was adjusted to 2.0. Then it was transferred to an ozone reactor, and ozone was introduced at room temperature and pressure. The amount of ozone introduced was 1.8% of the oven-dry weight of the sulfate eucalyptus pulp. The stirring speed was controlled at 800 rpm for 15 minutes for ozone bleaching. Then the sulfate eucalyptus pulp was washed with water (to remove residual ozone and dissolved lignin products) to obtain ozone-bleached sulfate eucalyptus pulp.

[0059] 2) Hydrogen peroxide bleaching (P1 stage): The ozone-bleached sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%. Then, hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added sequentially. The amount of hydrogen peroxide introduced is 2.5% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp, the amount of sodium hydroxide is 1.8% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, where steam is introduced to heat it to 90°C and maintain the temperature for 90 minutes for hydrogen peroxide bleaching. Finally, the sulfate eucalyptus pulp is washed with water to obtain hydrogen peroxide-bleached sulfate eucalyptus pulp.

[0060] 3) Chelation treatment (Q stage): Hydrogen peroxide bleached eucalyptus sulfate pulp is concentrated to a concentration of 10 wt%, and then a chelating agent (composed of the following components by weight percentage: EDTA: 0.4%; DTPA: 0.2%; carboxymethyl chitosan: 0.5%; trisodium citrate: 0.5%; Tween 80: 0.2%; water: balance) is added. The amount of chelating agent is 1.0% of the oven-dry weight of the hydrogen peroxide bleached eucalyptus sulfate pulp. The pulp is then transferred to a reactor, and steam is introduced to heat it to 50°C for 60 minutes for chelation treatment. The eucalyptus sulfate pulp is then washed with water to obtain chelated eucalyptus sulfate pulp.

[0061] 4) Hydrogen peroxide bleaching (P2 stage): The chelated sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added. The amount of hydrogen peroxide introduced is 1.5% of the oven-dry weight of the chelated sulfate eucalyptus pulp, the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chelated sulfate eucalyptus pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the chelated sulfate eucalyptus pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the chelated sulfate eucalyptus pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, and steam is introduced to heat it to 90°C for 120 minutes for hydrogen peroxide bleaching. The eucalyptus pulp is then washed with water to obtain bleached sulfate eucalyptus pulp.

[0062] Example 3:

[0063] A chlorine-free bleaching method for pulp, comprising the following steps:

[0064] 1) Ozone bleaching (Z stage): The sulfate bamboo pulp is concentrated to a concentration of 2wt% and the pH value of the sulfate bamboo pulp is adjusted to 2.0. Then it is transferred to an ozone reactor and ozone is introduced at room temperature and pressure. The ozone introduction rate is 1.8% of the oven-dry weight of the sulfate bamboo pulp. The stirring speed is controlled at 800 rpm for 15 minutes for ozone bleaching. Then the sulfate bamboo pulp is washed with water (to remove residual ozone and dissolved lignin products) to obtain ozone-bleached sulfate bamboo pulp.

[0065] 2) Hydrogen peroxide bleaching (P1 section): The ozone-bleached sulfate bamboo pulp is concentrated to a concentration of 10 wt%. Then, hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added sequentially. The amount of hydrogen peroxide introduced is 2.5% of the oven-dry weight of the ozone-bleached sulfate bamboo pulp, the amount of sodium hydroxide is 1.8% of the oven-dry weight of the ozone-bleached sulfate bamboo pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the ozone-bleached sulfate bamboo pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the ozone-bleached sulfate bamboo pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, where steam is introduced to heat it to 90°C and maintain the temperature for 90 minutes for hydrogen peroxide bleaching. Finally, the sulfate bamboo pulp is washed with water to obtain hydrogen peroxide-bleached sulfate bamboo pulp.

[0066] 3) Chelation treatment (Q stage): The hydrogen peroxide-bleached sulfate bamboo pulp is concentrated to a concentration of 10 wt%, and then a chelating agent (composed of the following components by weight percentage: EDTA: 0.4%; DTPA: 0.2%; carboxymethyl chitosan: 0.5%; trisodium citrate: 0.5%; Tween 80: 0.2%; water: balance) is added. The amount of chelating agent is 1.0% of the oven-dry weight of the hydrogen peroxide-bleached sulfate bamboo pulp. The pulp is then transferred to a reactor, and steam is introduced to heat it to 50°C for 60 minutes for chelation treatment. The sulfate bamboo pulp is then washed with water to obtain chelated sulfate bamboo pulp.

[0067] 4) Hydrogen peroxide bleaching (P2 stage): The chelated sulfate bamboo pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added. The amount of hydrogen peroxide introduced is 1.5% of the oven-dry weight of the chelated sulfate bamboo pulp, the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chelated sulfate bamboo pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the chelated sulfate bamboo pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the chelated sulfate bamboo pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, and steam is introduced to heat it to 90°C for 120 minutes for hydrogen peroxide bleaching. The sulfate bamboo pulp is then washed with water to obtain bleached sulfate bamboo pulp.

[0068] Example 4:

[0069] A chlorine-free bleaching method for pulp, comprising the following steps:

[0070] 1) Ozone bleaching (Z stage): The sulfate pine pulp is concentrated to a concentration of 2wt% and the pH value of the sulfate pine pulp is adjusted to 2.0. Then it is transferred to an ozone reactor and ozone is introduced at room temperature and pressure. The ozone introduction rate is 1.8% of the oven-dry weight of the sulfate pine pulp. The stirring speed is controlled at 800 rpm for 15 minutes for ozone bleaching. Then the sulfate pine pulp is washed with water (to remove residual ozone and dissolved lignin products) to obtain ozone-bleached sulfate pine pulp.

[0071] 2) Hydrogen peroxide bleaching (P1 stage): The ozone-bleached sulfate pine pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added sequentially. The amount of hydrogen peroxide introduced is 2.5% of the oven-dry weight of the ozone-bleached sulfate pine pulp, the amount of sodium hydroxide is 1.8% of the oven-dry weight of the ozone-bleached sulfate pine pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the ozone-bleached sulfate pine pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the ozone-bleached sulfate pine pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, and steam is introduced to heat it to 90°C for 90 minutes for hydrogen peroxide bleaching. The sulfate pine pulp is then washed with water to obtain hydrogen peroxide-bleached sulfate pine pulp.

[0072] 3) Chelation treatment (Q stage): The hydrogen peroxide-bleached sulfate pine pulp is concentrated to a concentration of 10 wt%, and then a chelating agent (composed of the following components by weight percentage: EDTA: 0.4%; DTPA: 0.2%; carboxymethyl chitosan: 0.5%; trisodium citrate: 0.5%; Tween 80: 0.2%; water: balance) is added. The amount of chelating agent is 1.0% of the oven-dry weight of the hydrogen peroxide-bleached sulfate pine pulp. The pulp is then transferred to a reactor, and steam is introduced to heat it to 50°C for 60 minutes for chelation treatment. The sulfate pine pulp is then washed with water to obtain chelated sulfate pine pulp.

[0073] 4) Hydrogen peroxide bleaching (P2 stage): The chelated sulfate pine pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added. The amount of hydrogen peroxide introduced is 1.5% of the oven-dry weight of the chelated sulfate pine pulp, the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chelated sulfate pine pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the chelated sulfate pine pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the chelated sulfate pine pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, and steam is introduced to heat it to 90°C for 120 minutes for hydrogen peroxide bleaching. The sulfate pine pulp is then washed with water to obtain bleached sulfate pine pulp.

[0074] Comparative Example 1: (OZ-D1-E) p -D2 process)

[0075] A pulp bleaching method, the steps of which are as follows:

[0076] 1) Oxy-delignification (O stage): The sulfate eucalyptus pulp (same as in Example 1) was concentrated to a concentration of 10 wt%, and then sodium hydroxide and magnesium sulfate were added. The amount of sodium hydroxide was 2.0% of the oven-dry weight of the sulfate eucalyptus pulp, and the amount of magnesium sulfate was 0.5% of the oven-dry weight of the sulfate eucalyptus pulp. The pulp was then transferred to an oxygen reactor, oxygen was introduced and the oxygen pressure was maintained at 0.5 MPa, and then heated to 90°C for 60 min for oxygen delignification. The sulfate eucalyptus pulp was then washed with water to obtain oxygen-delignified sulfate eucalyptus pulp.

[0077] 2) Ozone bleaching (Z stage): The oxygen delignified sulfate eucalyptus pulp is concentrated to a concentration of 3wt%, and the pH value of the oxygen delignified sulfate eucalyptus pulp is adjusted to 2.0. Then it is transferred to an ozone reactor, and ozone is introduced at room temperature and pressure. The ozone introduction rate is 1.0% of the oven-dry weight of the oxygen delignified sulfate eucalyptus pulp. The stirring speed is controlled at 800 rpm for 15 minutes for ozone bleaching. Then the sulfate eucalyptus pulp is washed with water (to remove residual ozone and degradation products) to obtain ozone-bleached sulfate eucalyptus pulp.

[0078] 3) Chlorine dioxide bleaching (D1 stage): The ozone-bleached sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%, and then chlorine dioxide is added. The amount of chlorine dioxide is 1.0% of the oven-dry weight of the ozone-bleached sulfate eucalyptus pulp. The pulp is then transferred to a bleaching tower and heated to 70°C for 60 minutes for chlorine dioxide bleaching. The sulfate eucalyptus pulp is then washed with water to obtain chlorine dioxide bleached sulfate eucalyptus pulp.

[0079] 4) Hydrogen peroxide-enhanced alkaline extraction bleaching (E p Section 1: The chlorine dioxide bleached eucalyptus sulfate pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide and sodium hydroxide are added. The amount of hydrogen peroxide introduced is 1.0% of the oven-dry weight of the chlorine dioxide bleached eucalyptus sulfate pulp, and the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chlorine dioxide bleached eucalyptus sulfate pulp. The pulp is then transferred to a reaction tower and heated to 90°C for 60 minutes for hydrogen peroxide bleaching. The eucalyptus sulfate pulp is then washed with water to obtain hydrogen peroxide bleached eucalyptus sulfate pulp.

[0080] 5) Chlorine dioxide bleaching (D2 stage): The hydrogen peroxide-bleached sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%. Chlorine dioxide is added at a rate of 0.4% of the oven-dry weight of the hydrogen peroxide-bleached sulfate eucalyptus pulp. The pulp is then transferred to a bleaching tower and heated to 70°C for 60 minutes for chlorine dioxide bleaching. The sulfate eucalyptus pulp is then washed with water to obtain bleached sulfate eucalyptus pulp.

[0081] Comparative Example 2: (O-D1-E) p -D2 process)

[0082] A pulp bleaching method, the steps of which are as follows:

[0083] 1) Oxy-delignification (O stage): The sulfate eucalyptus pulp (same as in Example 1) was concentrated to a concentration of 10 wt%, and then sodium hydroxide and magnesium sulfate were added. The amount of sodium hydroxide was 2.0% of the oven-dry weight of the sulfate eucalyptus pulp, and the amount of magnesium sulfate was 0.5% of the oven-dry weight of the sulfate eucalyptus pulp. The pulp was then transferred to an oxygen reactor, oxygen was introduced and the oxygen pressure was maintained at 0.5 MPa, and then heated to 90°C for 60 min for oxygen delignification. The sulfate eucalyptus pulp was then washed with water to obtain oxygen-delignified sulfate eucalyptus pulp.

[0084] 2) Chlorine dioxide bleaching (D1 stage): The oxygen delignified sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%, and then chlorine dioxide is added. The amount of chlorine dioxide is 1.0% of the oven-dry weight of the oxygen delignified sulfate eucalyptus pulp. The pulp is then transferred to a bleaching tower and heated to 70°C for 60 minutes for chlorine dioxide bleaching. The sulfate eucalyptus pulp is then washed with water to obtain chlorine dioxide bleached sulfate eucalyptus pulp.

[0085] 3) Hydrogen peroxide-enhanced alkaline extraction bleaching (E p Section 1: The chlorine dioxide bleached eucalyptus sulfate pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide and sodium hydroxide are added. The amount of hydrogen peroxide introduced is 1.0% of the oven-dry weight of the chlorine dioxide bleached eucalyptus sulfate pulp, and the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chlorine dioxide bleached eucalyptus sulfate pulp. The pulp is then transferred to a reaction tower and heated to 90°C for 60 minutes for hydrogen peroxide bleaching. The eucalyptus sulfate pulp is then washed with water to obtain hydrogen peroxide bleached eucalyptus sulfate pulp.

[0086] 4) Chlorine dioxide bleaching (D2 stage): The hydrogen peroxide-bleached sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%. Chlorine dioxide is added at a rate of 0.5% of the oven-dry weight of the hydrogen peroxide-bleached sulfate eucalyptus pulp. The pulp is then transferred to a bleaching tower and heated to 70°C for 60 minutes for chlorine dioxide bleaching. The sulfate eucalyptus pulp is then washed with water to obtain bleached sulfate eucalyptus pulp.

[0087] Comparative Example 3: (The chelating agent for segment Q is EDTA; a single chelating agent)

[0088] A pulp bleaching method, the steps of which are as follows:

[0089] 1) Chelation treatment (Q stage): Hydrogen peroxide bleached eucalyptus sulfate pulp (same as in Example 2) is concentrated to a concentration of 10 wt%, and then chelating agent EDTA is added. The amount of chelating agent is 1.0% of the oven-dry weight of hydrogen peroxide bleached eucalyptus sulfate pulp. The pulp is then transferred to a reactor, and steam is introduced to heat it to 50°C for 60 min for chelation treatment. The eucalyptus sulfate pulp is then washed with water to obtain chelated eucalyptus sulfate pulp.

[0090] 2) Hydrogen peroxide bleaching (P2 stage): The chelated sulfate eucalyptus pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate are added. The amount of hydrogen peroxide introduced is 1.5% of the oven-dry weight of the chelated sulfate eucalyptus pulp, the amount of sodium hydroxide is 1.0% of the oven-dry weight of the chelated sulfate eucalyptus pulp, the amount of sodium silicate is 2.0% of the oven-dry weight of the chelated sulfate eucalyptus pulp, and the amount of magnesium sulfate is 0.3% of the oven-dry weight of the chelated sulfate eucalyptus pulp. The pulp is then transferred to a hydrogen peroxide bleaching tower, and steam is introduced to heat it to 90°C for 120 minutes for hydrogen peroxide bleaching. The eucalyptus pulp is then washed with water to obtain bleached sulfate eucalyptus pulp.

[0091] Performance testing:

[0092] 1) The ozone bleaching effects of ozone-bleached eucalyptus sulfate pulp in Example 1, ozone-bleached eucalyptus sulfate pulp in Example 2, ozone-bleached bamboo sulfate pulp in Example 3, ozone-bleached pine sulfate pulp in Example 4, and ozone-bleached eucalyptus sulfate pulp in Comparative Example 1 are compared in the table below:

[0093] Table 1 shows a comparison of bleaching effects.

[0094]

[0095] Note:

[0096] The formula for calculating delignification selectivity is: delignification selectivity = (K0-K1) / (V0-V1), where V0 represents the initial viscosity value, V1 represents the viscosity value after bleaching, K0 represents the initial kappa value, and K1 represents the kappa value after bleaching.

[0097] As shown in Table 1, compared with the traditional bleaching method, the ozone bleaching method of the present invention improves the brightness of sulfate eucalyptus pulp by more than 30%, significantly reduces the kappa value, and has relatively small viscosity loss. Moreover, the ozone bleaching method of the present invention has excellent bleaching effect on both sulfate bamboo pulp and sulfate pine pulp, indicating that the ozone bleaching method of the present invention effectively promotes the removal of phenolic lignin and non-phenolic lignin, while removing hexenuronic acid and metal ions, protecting the fiber structure, ensuring the physical properties of the pulp, and demonstrating high oxidation selectivity and process stability.

[0098] 2) The performance indicators of bleached sulfate eucalyptus pulp in Example 2 and Comparative Example 3 are compared in the table below:

[0099] Table 2 Comparison of Performance Indicators of Bleached Sulfate Eucalyptus Pulp

[0100] Example 2 1.8 85.64 2.2 698 0.0560 Comparative Example 3 1.8 84.26 2.2 687 0.0536

[0101] As shown in Table 2, compared with the single chelating agent EDTA in Comparative Example 3, the composite chelating agent in Example 2 can not only further improve the brightness of the pulp, but also effectively reduce viscosity loss while maintaining a low kappa value. The delignification selectivity is also improved. This indicates that the composite chelating agent in Example 2 more effectively protects the fiber structure during the bleaching process and enhances the lignin removal efficiency. It has significant advantages of being environmentally friendly, highly efficient, and providing good protection for pulp quality.

[0102] 3) The performance indicators of bleached sulfate eucalyptus pulp in Examples 1, 2, Comparative Example 1, and Comparative Example 2 are compared in the following table:

[0103] Table 3 Comparison of Performance Indicators of Bleached Sulfate Eucalyptus Pulp

[0104]

[0105] Note:

[0106] The formula for calculating the viscosity reduction rate is as follows: Viscosity reduction rate (%) = (V0-V1) / V0×100%, where V0 represents the initial viscosity value and V1 represents the viscosity value after bleaching.

[0107] As shown in Table 3:

[0108] The chlorine-free bleaching method of the present invention (Z-P1-Q-P2 process) differs from conventional Light-ECF bleaching (OZ-D1-E) p -D2 process) and traditional ECF bleaching method (O-D1-E) p Compared to the D2 process, it has the following significant advantages:

[0109] a) Bleached sulfate eucalyptus pulp has higher whiteness: The bleached sulfate eucalyptus pulp in Example 2 has a whiteness of up to 85.64, which is 1.35 units higher than that of the bleached sulfate eucalyptus pulp in Comparative Example 1 (whiteness of 84.29).

[0110] b) Lower fiber damage: The viscosity of the bleached sulfate eucalyptus pulp in Example 1 was still 733 mL / g, with a viscosity reduction rate of only 22.68%, which reduced fiber damage by 12.24% compared with the bleached sulfate eucalyptus pulp in Comparative Example 1 (viscosity as low as 617 mL / g, with a viscosity reduction rate of 34.92%).

[0111] c) Superior delignification efficiency: The delignification selectivity of the bleached sulfate eucalyptus pulp in Example 1 was 0.0647, which was 24.4% higher than that of the bleached sulfate eucalyptus pulp in Comparative Example 2 (delignification selectivity was 0.0520). This indicates that the chlorine-free bleaching process of the present invention can better protect fiber strength while efficiently removing lignin.

[0112] In summary, the chlorine-free bleaching method (Z-P1-Q-P2 process) of the present invention achieves a balance of high whiteness, low fiber damage, and high delignification selectivity through ozone-hydrogen peroxide synergistic bleaching and chelation treatment. Its overall performance is superior to conventional Light-ECF bleaching methods and traditional ECF bleaching methods.

[0113] 4) Z-P1-Q-P2 process (Examples 1-4), OZ-D1-E p -D2 process (Comparative Example 1) and O-D1-E p The cost comparison of bleaching agent / chelating agent in process D2 (Comparative Example 2) is shown in the table below:

[0114] Table 4. Cost Comparison of Bleaching Agents and Chelating Agents

[0115]

[0116] Note: The price of chlorine dioxide is approximately 13 yuan / kg, the price of ozone is approximately 7 yuan / kg, the price of hydrogen peroxide is approximately 3.5 yuan / kg, and the price of chelating agents is approximately 2 yuan / kg.

[0117] As shown in Table 4, the cost of the main bleaching agent in the chlorine-free bleaching method (Z-P1-Q-P2 process) of the present invention (≤335 yuan / ton of pulp) is reduced by at least 32% compared with the conventional Light-ECF bleaching method (≥395 yuan / ton of pulp) and the traditional ECF bleaching method (520 yuan / ton of pulp), thus achieving a balance between high bleaching performance and low cost.

[0118] 5) Z-P1-Q-P2 process (Example 2), OZ-D1-E p-D2 process (Comparative Example 1) and O-D1-E p The energy consumption (per ton of oven-dry pulp) of process D2 (Comparative Example 2) is compared in the table below:

[0119] Table 5 Energy Consumption Comparison Table

[0120]

[0121]

[0122] Note: The starting temperature for the first stage of all bleaching processes is 25℃; if the first stage is at a high temperature (≥70℃), the starting temperature for the next stage is 50℃.

[0123] As shown in Table 5, the energy consumption cost of the chlorine-free bleaching method (Z-P1-Q-P2 process) of the present invention is only RMB 488.91 per ton of pulp, which is 27.6% lower than that of the conventional Light-ECF bleaching method (RMB 675.13 per ton of pulp) and the traditional ECF bleaching method (RMB 675.13 per ton of pulp).

[0124] In summary, the chlorine-free pulp bleaching method (Z-P1-Q-P2 process) of the present invention not only achieves efficient and gentle pulp bleaching, but also has advantages in cost control and energy consumption optimization, providing a feasible technical path for high-performance and environmentally friendly pulp bleaching.

[0125] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for chlorine-free bleaching of pulp, characterized in that, Includes the following steps: 1) Ozone bleaching: The pulp is concentrated and then transferred to an ozone reactor to be bleached with ozone. The pulp is then washed to obtain ozone-bleached pulp. 2) Hydrogen peroxide bleaching: The ozone-bleached pulp is concentrated, then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate are added, and then it is transferred to the hydrogen peroxide bleaching tower for hydrogen peroxide bleaching. The pulp is then washed to obtain hydrogen peroxide-bleached pulp. 3) Chelation treatment: The hydrogen peroxide bleached pulp is concentrated, then a chelating agent is added for chelation treatment, and then the pulp is washed to obtain chelated pulp. 4) Hydrogen peroxide bleaching: The chelated pulp is concentrated, then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate are added, and then it is transferred to the hydrogen peroxide bleaching tower for hydrogen peroxide bleaching. The pulp is then washed to obtain bleached pulp. Step 1) The ozone bleaching is carried out at room temperature for 10 to 50 minutes. Step 2) The hydrogen peroxide bleaching is carried out at a temperature of 80℃~90℃ for a time of 70min~110min. Step 3) The chelating agent comprises the following components in weight percentage: ethylenediaminetetraacetic acid: 0.2%–0.5%; diethylenetriaminepentaacetic acid: 0.1%–0.3%; Carboxymethyl chitosan: 0.3%–0.7%; trisodium citrate: 0.3%–0.7%; Tween 80: 0.1%–0.3%; Water: Balance; Step 3) The chelation treatment is carried out at a temperature of 50℃~60℃ for a time of 50min~70min; Step 4) The hydrogen peroxide bleaching is carried out at a temperature of 80℃~90℃ for a time of 100min~150min.

2. The method for chlorine-free bleaching of pulp according to claim 1, characterized in that: Step 1) The pulp is at least one of sulfate eucalyptus pulp, sulfate bamboo pulp, sulfate pine pulp, sulfate reed pulp, sulfate bagasse pulp, sulfate wheat straw pulp, and caustic soda-anthraquinone eucalyptus pulp; Step 1) The pulp concentration after concentration is 1.5wt% to 2.5wt%; Step 2) The ozone-bleached pulp concentration after concentration is 8wt% to 12wt%; Step 3) The hydrogen peroxide-bleached pulp concentration after concentration is 8wt% to 12wt%; Step 4) The chelated pulp concentration after concentration is 8wt% to 12wt%.

3. The method for chlorine-free bleaching of pulp according to claim 1 or 2, characterized in that: Step 1) The amount of ozone introduced is 1.5% to 2.0% of the oven-dry weight of the pulp.

4. The method for chlorine-free bleaching of pulp according to claim 1, characterized in that: Step 2) The amount of hydrogen peroxide introduced is 2.0% to 3.0% of the oven-dry weight of the ozone-bleached pulp; Step 2) The amount of sodium hydroxide used is 1.5% to 2.0% of the oven-dry weight of the ozone-bleached pulp; Step 2) The amount of sodium silicate used is 1.5% to 2.0% of the oven-dry weight of the ozone-bleached pulp; Step 2) The amount of magnesium sulfate used is 0.1% to 0.5% of the oven-dry weight of the ozone-bleached pulp.

5. The method for chlorine-free bleaching of pulp according to claim 1, characterized in that: Step 3) The amount of chelating agent used is 0.8% to 1.2% of the oven-dry weight of the hydrogen peroxide-bleached pulp.

6. The method for chlorine-free bleaching of pulp according to claim 1, characterized in that: Step 4) The amount of hydrogen peroxide introduced is 1.0% to 2.0% of the oven-dry weight of the chelated pulp; Step 4) The amount of sodium hydroxide used is 1.0% to 1.5% of the oven-dry weight of the chelated pulp; Step 4) The amount of sodium silicate used is 1.5% to 2.0% of the oven-dry weight of the chelated pulp; Step 4) The amount of magnesium sulfate used is 0.1% to 0.5% of the oven-dry weight of the chelated pulp.

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