Chlorine-free bleaching method for paper pulp

Abstract

The invention discloses a chlorine-free bleaching method for paper pulp, which comprises the following steps: 1) performing ozone bleaching to obtain ozone bleached paper pulp; 2) hydrogen peroxide bleaching to obtain hydrogen peroxide bleached paper pulp; (3) carrying out chelating treatment to obtain chelated paper pulp; and 4) bleaching with hydrogen peroxide to obtain bleached pulp. The chlorine-free bleaching method for the paper pulp has the advantages of being low in temperature, normal in pressure, low in energy consumption, low in cost, efficient, suitable for various paper pulp systems, environmentally friendly and the like, the bleached paper pulp is high in whiteness, high in viscosity, low in kappa number and free of AOX, and the chlorine-free bleaching method for the paper pulp is suitable for large-scale industrial application.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pulp and papermaking, in particular to a paper pulp chlorine-free bleaching method. BACKGROUND

[0002] The conventional Light-ECF bleaching method (O-Z-D1-E p -D2 process: oxygen delignification-extraction-ozone bleaching-extraction-chlorine dioxide bleaching) and the traditional ECF bleaching method (O-D1-E p -D2 process: oxygen delignification-chlorine dioxide bleaching-extraction-chlorine dioxide bleaching) are the more common ECF (elemental chlorine-free) bleaching methods at present, but both of these two process ECF bleaching methods have obvious defects and cannot completely meet the actual application requirements, specifically as follows: 1) high temperature and high pressure requirement: the O section needs to be carried out under the condition of temperature ≥ 90℃ and pressure ≥ 0.5MPa, which not only has high energy consumption but also high equipment investment; 2) efficiency limitation: O2 in the O section mainly reacts with non-phenolic lignin, and the treatment effect on phenolic lignin is poor, and the residual phenolic lignin will affect the pulp brightness; 3) complex process and large water consumption: it involves multi-stage extraction and section switching, and the system occupies a wide area, and the water consumption is as high as 25m 3 / t~30m 3 / t; 4) high ClO2 dosage and heavy environmental burden: the dosage of ClO2 in the D section is ≥ 4%, and the dosage of AOX is 0.2kg / t~0.8kg / t, which has great pressure on wastewater treatment.

[0003] Therefore, it is of great significance to develop a paper pulp chlorine-free bleaching method with low temperature, normal pressure, low energy consumption, low cost, high efficiency and green environmental protection. SUMMARY

[0004] The purpose of the present application is to provide a paper pulp chlorine-free bleaching method.

[0005] The technical scheme adopted by the present application is as follows:

[0006] A paper pulp chlorine-free bleaching method comprises the following steps:

[0007] 1) ozone bleaching: concentrating the paper pulp, then transferring it into an ozone reactor to introduce ozone for ozone bleaching, and then washing the paper pulp to obtain the ozone-bleached paper pulp;

[0008] 2) hydrogen peroxide bleaching: concentrating the ozone-bleached paper pulp, then adding hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate, and then transferring it into a hydrogen peroxide bleaching tower for hydrogen peroxide bleaching, and then washing the paper pulp to obtain the hydrogen peroxide-bleached paper pulp;

[0009] 3) chelating treatment: concentrating the hydrogen peroxide bleached pulp, adding a chelating agent to perform chelating treatment, and then washing the pulp to obtain the chelating treated pulp;

[0010] 4) hydrogen peroxide bleaching: concentrating the chelating treated pulp, adding hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate, and then transferring to a hydrogen peroxide bleaching tower to perform hydrogen peroxide bleaching, and then washing the pulp to obtain the bleached pulp.

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

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

[0013] Preferably, the operation of adjusting the pH value of the pulp to 1.5 to 2.5 is further included between the concentration and the ozone bleaching in step 1).

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

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

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

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

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

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

[0022] Preferably, the hydrogen peroxide bleaching in step 2) is performed at a temperature of 80°C to 90°C, and the hydrogen peroxide bleaching time is 70min to 110min.

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

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

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

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

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

[0028] Diethylenetriamine pentaacetic acid: 0.1% to 0.3%;

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

[0030] Trisodium citrate: 0.3% to 0.7%;

[0031] Tween 80: 0.1% to 0.3%;

[0032] Water: the balance.

[0033] Note: Ethylenediaminetetraacetic acid, diethylenetriamine pentaacetic acid, and carboxymethyl chitosan can chelate metal ions such as Fe 3+ , Mn 2+ , thereby preventing the decomposition of H2O2; trisodium citrate can play an auxiliary chelating role and can improve chelating stability; Tween 80 (non-ionic surfactant) can improve dispersibility and permeability; and water (solvent) can ensure that ethylenediaminetetraacetic acid, diethylenetriamine pentaacetic acid, carboxymethyl chitosan, trisodium citrate, and Tween 80 are fully dissolved and stable.

[0034] Preferably, the chelating treatment in step 3) is carried out at a temperature of 50°C to 60°C, and the chelating treatment time is 50min to 70min.

[0035] Preferably, the concentration of the chelating treated pulp after concentration in step 4) is 8wt% to 12wt%.

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

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

[0038] Preferably, the amount of sodium hydroxide used in step 4) is 1.0% to 1.5% of the absolute dry weight of the chelation-treated pulp.

[0039] Preferably, the amount of sodium hydroxide used in step 4) is 1.0% to 1.5% of the absolute dry weight of the chelation-treated pulp.

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

[0041] The pulp chlorine-free bleaching method of the present application has the advantages of low temperature, normal pressure, low energy consumption, low cost, high efficiency, suitability for various pulp systems, green environmental protection, and the like, and the bleached pulp has high whiteness, high viscosity, low Kappa number, and no AOX. The method is suitable for large-scale industrial application.

[0042] Specifically:

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

[0044] 2) The pulp chlorine-free bleaching method of the present application uses ozone instead of oxygen as the first-stage 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). The ozone can efficiently oxidize phenolic lignin and non-phenolic lignin under acidic conditions, improve the delignification rate, and also remove hexenuronic acid and metal ions, thereby avoiding the consumption of oxidizing agents and inhibiting the subsequent bleaching process.

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

[0046] 4) The pulp chlorine-free bleaching method of the present application has a chelation section between the two-stage hydrogen peroxide bleaching. The chelation of metal ions in the pulp inhibits the decomposition of P2-stage hydrogen peroxide, ensures the activity and stability of the P2-stage bleaching agent, and significantly improves the bleaching effect of the P2 stage.

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

[0048] 6) The pulp bleaching method of the present application has the advantages of high brightness, high viscosity, low Kappa number, no AOX (adsorbable organic halogen), and no chlorination by-products of O3 and H2O2, and good pulp quality. DETAILED DESCRIPTION

[0049] The present application will be further explained and described with reference to the following specific examples.

[0050] Example 1

[0051] A pulp bleaching method, comprising the following steps:

[0052] 1) Ozone bleaching (Z section): The kraft eucalyptus pulp is concentrated to a concentration of 2 wt%, and the pH value of the kraft eucalyptus pulp is adjusted to 2.0, then it is transferred to an ozone reactor, ozone is introduced at normal temperature and pressure, the amount of ozone introduced is 1.6% of the absolute dry weight of the kraft eucalyptus pulp, the stirring rate is controlled at 800 rpm for ozone bleaching for 15 min, then the kraft eucalyptus pulp is washed with water to remove residual ozone and dissolved lignin products, and ozone-bleached kraft eucalyptus pulp is obtained;

[0053] 2) Hydrogen peroxide bleaching (P1 section): The ozone-bleached kraft eucalyptus pulp is concentrated to a concentration of 10 wt%, then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate are sequentially added, the amount of hydrogen peroxide introduced is 2.5% of the absolute dry weight of the ozone-bleached kraft eucalyptus pulp, the amount of sodium hydroxide used is 1.8% of the absolute dry weight of the ozone-bleached kraft eucalyptus pulp, the amount of sodium silicate used is 1.5% of the absolute dry weight of the ozone-bleached kraft eucalyptus pulp, the amount of magnesium sulfate used is 0.2% of the absolute dry weight of the ozone-bleached kraft eucalyptus pulp, then it is transferred to a hydrogen peroxide bleaching tower, water vapor is introduced to heat to 90°C for constant temperature hydrogen peroxide bleaching for 90 min, then the kraft eucalyptus pulp is washed with water, and hydrogen peroxide-bleached kraft eucalyptus pulp is obtained;

[0054] 3) Chelation treatment (Q section): The hydrogen peroxide-bleached kraft eucalyptus pulp is concentrated to a concentration of 10 wt%, then a chelating agent (consisting 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 used is 1.0% of the absolute dry weight of the hydrogen peroxide-bleached kraft eucalyptus pulp, then it is transferred to a reactor, water vapor is introduced to heat to 50°C for constant temperature chelation treatment for 60 min, then the kraft eucalyptus pulp is washed with water, and chelation-treated kraft eucalyptus pulp is obtained;

[0055] 4) Hydrogen peroxide bleaching (P2 section): The chelating treated kraft eucalyptus pulp was concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate were added, the amount of hydrogen peroxide was 1.5% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of sodium hydroxide was 1.0% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of sodium silicate was 1.5% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of magnesium sulfate was 0.2% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, and then it was transferred to a hydrogen peroxide bleaching tower, and then water vapor was introduced to heat to 90°C constant temperature for 120 min of hydrogen peroxide bleaching, and then the kraft eucalyptus pulp was washed with water to obtain bleached kraft eucalyptus pulp.

[0056] Example 2:

[0057] A paper pulp chlorine-free bleaching method, the steps are as follows:

[0058] 1) Ozone bleaching (Z section): The kraft eucalyptus pulp (same as example 1) was concentrated to a concentration of 2 wt%, and the pH value of the kraft eucalyptus pulp was adjusted to 2.0, and then it was transferred to an ozone reactor, and ozone was introduced at normal temperature and pressure, the amount of ozone was 1.8% of the absolute dry weight of the kraft eucalyptus pulp, and then the stirring rate was controlled at 800 rpm for 15 min of ozone bleaching, and then the kraft eucalyptus pulp was washed with water (to remove residual ozone and dissolved lignin products) to obtain ozone bleached kraft eucalyptus pulp;

[0059] 2) Hydrogen peroxide bleaching (P1 section): The ozone bleached kraft eucalyptus pulp was concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate were added in turn, the amount of hydrogen peroxide was 2.5% of the absolute dry weight of the ozone bleached kraft eucalyptus pulp, the amount of sodium hydroxide was 1.8% of the absolute dry weight of the ozone bleached kraft eucalyptus pulp, the amount of sodium silicate was 2.0% of the absolute dry weight of the ozone bleached kraft eucalyptus pulp, the amount of magnesium sulfate was 0.3% of the absolute dry weight of the ozone bleached kraft eucalyptus pulp, and then it was transferred to a hydrogen peroxide bleaching tower, and then water vapor was introduced to heat to 90°C constant temperature for 90 min of hydrogen peroxide bleaching, and then the kraft eucalyptus pulp was washed with water to obtain hydrogen peroxide bleached kraft eucalyptus pulp;

[0060] 3) Chelating treatment (Q section): The hydrogen peroxide bleached kraft eucalyptus pulp is concentrated to a concentration of 10 wt%, and then a chelating agent (consisting of the following weight percentage components: 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 absolute dry weight of the hydrogen peroxide bleached kraft eucalyptus pulp, then it is transferred into a reactor, and then water vapor is introduced to heat to 50°C constant temperature for 60 min of chelating treatment, and then the kraft eucalyptus pulp is washed with water to obtain a chelating treated kraft eucalyptus pulp;

[0061] 4) Hydrogen peroxide bleaching (P2 section): The chelating treated kraft eucalyptus pulp is concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, magnesium sulfate are added, the amount of hydrogen peroxide is 1.5% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of sodium hydroxide is 1.0% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of sodium silicate is 2.0% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of magnesium sulfate is 0.3% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, then it is transferred into a hydrogen peroxide bleaching tower, and then water vapor is introduced to heat to 90°C constant temperature for 120 min of hydrogen peroxide bleaching, and then the kraft eucalyptus pulp is washed with water to obtain a bleached kraft eucalyptus pulp.

[0062] Example 3:

[0063] A paper pulp chlorine-free bleaching method, the steps are as follows:

[0064] 1) Ozone bleaching (Z section): The kraft bamboo pulp is concentrated to a concentration of 2 wt%, and the pH value of the kraft bamboo pulp is adjusted to 2.0, then it is transferred into an ozone reactor, ozone is introduced at normal temperature and pressure, the amount of ozone is 1.8% of the absolute dry weight of the kraft bamboo pulp, then the stirring rate is controlled at 800 rpm for 15 min of ozone bleaching, and then the kraft bamboo pulp is washed with water (to remove residual ozone and dissolved lignin products) to obtain an ozone bleached kraft bamboo pulp;

[0065] 2) Hydrogen peroxide bleaching (P1 section): The ozone bleached kraft bamboo pulp is concentrated to a concentration of 10 wt%, then hydrogen peroxide, sodium hydroxide, sodium silicate and magnesium sulfate are added in turn, the amount of hydrogen peroxide is 2.5% of the absolute dry weight of the ozone bleached kraft bamboo pulp, the amount of sodium hydroxide is 1.8% of the absolute dry weight of the ozone bleached kraft bamboo pulp, the amount of sodium silicate is 2.0% of the absolute dry weight of the ozone bleached kraft bamboo pulp, the amount of magnesium sulfate is 0.3% of the absolute dry weight of the ozone bleached kraft bamboo pulp, then it is transferred to a hydrogen peroxide bleaching tower, and water vapor is introduced to heat to 90°C constant temperature for 90 minutes of hydrogen peroxide bleaching, then the kraft bamboo pulp is washed with water to obtain hydrogen peroxide bleached kraft bamboo pulp;

[0066] 3) Chelation treatment (Q section): The hydrogen peroxide bleached kraft bamboo pulp is concentrated to a concentration of 10 wt%, then a chelating agent (consisting of the following weight percentage components: 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 absolute dry weight of the hydrogen peroxide bleached kraft bamboo pulp, then it is transferred to a reactor, and water vapor is introduced to heat to 50°C constant temperature for 60 minutes of chelation treatment, then the kraft bamboo pulp is washed with water to obtain chelation treated kraft bamboo pulp;

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

[0068] Example 4:

[0069] A method for chlorine-free bleaching of pulp, the steps are as follows:

[0070] 1) Ozone bleaching (Z section): The Kraft pine pulp was concentrated to a concentration of 2 wt%, and the pH of the Kraft pine pulp was adjusted to 2.0, and then was transferred to an ozone reactor, and ozone was introduced at normal temperature and pressure, and the amount of ozone introduced was 1.8% of the absolute dry weight of the Kraft pine pulp, and then the stirring rate was controlled to 800 rpm for ozone bleaching for 15 min, and then the Kraft pine pulp was washed with water (to remove residual ozone and dissolved lignin products) to obtain ozone-bleached Kraft pine pulp;

[0071] 2) Hydrogen peroxide bleaching (P1 section): The ozone-bleached Kraft pine pulp was concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate were sequentially added, the amount of hydrogen peroxide introduced was 2.5% of the absolute dry weight of the ozone-bleached Kraft pine pulp, the amount of sodium hydroxide used was 1.8% of the absolute dry weight of the ozone-bleached Kraft pine pulp, the amount of sodium silicate used was 2.0% of the absolute dry weight of the ozone-bleached Kraft pine pulp, the amount of magnesium sulfate used was 0.3% of the absolute dry weight of the ozone-bleached Kraft pine pulp, and then was transferred to a hydrogen peroxide bleaching tower, and then water vapor was introduced to heat to 90°C for constant temperature hydrogen peroxide bleaching for 90 min, and then the Kraft pine pulp was washed with water to obtain hydrogen peroxide-bleached Kraft pine pulp;

[0072] 3) Chelation treatment (Q section): The hydrogen peroxide-bleached Kraft pine pulp was concentrated to a concentration of 10 wt%, and then a chelating agent (consisting 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) was added, the amount of chelating agent used was 1.0% of the absolute dry weight of the hydrogen peroxide-bleached Kraft pine pulp, and then was transferred to a reactor, and then water vapor was introduced to heat to 50°C for constant temperature chelation treatment for 60 min, and then the Kraft pine pulp was washed with water to obtain chelation-treated Kraft pine pulp;

[0073] 4) Hydrogen peroxide bleaching (P2 section): The chelation-treated Kraft pine pulp was concentrated to a concentration of 10 wt%, and then hydrogen peroxide, sodium hydroxide, sodium silicate, and magnesium sulfate were added, the amount of hydrogen peroxide introduced was 1.5% of the absolute dry weight of the chelation-treated Kraft pine pulp, the amount of sodium hydroxide used was 1.0% of the absolute dry weight of the chelation-treated Kraft pine pulp, the amount of sodium silicate used was 2.0% of the absolute dry weight of the chelation-treated Kraft pine pulp, the amount of magnesium sulfate used was 0.3% of the absolute dry weight of the chelation-treated Kraft pine pulp, and then was transferred to a hydrogen peroxide bleaching tower, and then water vapor was introduced to heat to 90°C for constant temperature hydrogen peroxide bleaching for 120 min, and then the Kraft pine pulp was washed with water to obtain bleached Kraft pine pulp.

[0074] Comparative Example 1: (O-Z-D1-E p -D2 procedure)

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

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

[0077] 2) ozone bleaching (Z stage): the oxygen delignification kraft eucalyptus pulp was concentrated to a concentration of 3 wt%, and the pH of the oxygen delignification kraft eucalyptus pulp was adjusted to 2.0, then it was transferred to an ozone reactor, ozone was introduced at room temperature and atmospheric pressure, the amount of ozone introduced was 1.0% of the absolute dry weight of the oxygen delignification kraft eucalyptus pulp, then the stirring rate was controlled at 800 rpm for 15 min of ozone bleaching, then the kraft eucalyptus pulp was washed with water (to remove residual ozone and degradation products) to obtain ozone bleached kraft eucalyptus pulp;

[0078] 3) chlorine dioxide bleaching (D1 stage): the ozone bleached kraft eucalyptus pulp was concentrated to a concentration of 10 wt%, then chlorine dioxide was added, the amount of chlorine dioxide was 1.0% of the absolute dry weight of the ozone bleached kraft eucalyptus pulp, then it was transferred to a bleaching tower, heated to 70°C for constant temperature chlorine dioxide bleaching for 60 min, then the kraft eucalyptus pulp was washed with water to obtain chlorine dioxide bleached kraft eucalyptus pulp;

[0079] 4) hydrogen peroxide enhanced alkaline extraction bleaching (E p stage): the chlorine dioxide bleached kraft eucalyptus pulp was concentrated to a concentration of 10 wt%, then hydrogen peroxide and sodium hydroxide were added, the amount of hydrogen peroxide introduced was 1.0% of the absolute dry weight of the chlorine dioxide bleached kraft eucalyptus pulp, the amount of sodium hydroxide was 1.0% of the absolute dry weight of the chlorine dioxide bleached kraft eucalyptus pulp, then it was transferred to a reaction tower, heated to 90°C for constant temperature hydrogen peroxide bleaching for 60 min, then the kraft eucalyptus pulp was washed with water to obtain hydrogen peroxide bleached kraft eucalyptus pulp;

[0080] 5)Chlorine dioxide bleaching (D2 stage): The hydrogen peroxide bleached kraft eucalyptus pulp was concentrated to a consistency of 10 wt%, chlorine dioxide, the amount of chlorine dioxide was 0.4% of the oven dry weight of the hydrogen peroxide bleached kraft eucalyptus pulp, and was transferred to a bleaching tower, heated to 70°C constant temperature for 60 min of chlorine dioxide bleaching, and then washed with water to obtain bleached kraft eucalyptus pulp.

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

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

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

[0084] 2) Chlorine dioxide bleaching (D1 stage): The oxygen delignified kraft eucalyptus pulp was concentrated to a consistency of 10 wt%, chlorine dioxide was added, the amount of chlorine dioxide was 1.0% of the oven dry weight of the oxygen delignified kraft eucalyptus pulp, and was transferred to a bleaching tower, heated to 70°C constant temperature for 60 min of chlorine dioxide bleaching, and then washed with water to obtain chlorine dioxide bleached kraft eucalyptus pulp;

[0085] 3) Hydrogen peroxide enhanced alkali extraction bleaching (E p stage): The chlorine dioxide bleached kraft eucalyptus pulp was concentrated to a consistency of 10 wt%, hydrogen peroxide and sodium hydroxide were added, the amount of hydrogen peroxide was 1.0% of the oven dry weight of the chlorine dioxide bleached kraft eucalyptus pulp, the amount of sodium hydroxide was 1.0% of the oven dry weight of the chlorine dioxide bleached kraft eucalyptus pulp, and was transferred to a reaction tower, heated to 90°C constant temperature for 60 min of hydrogen peroxide bleaching, and then washed with water to obtain hydrogen peroxide bleached kraft eucalyptus pulp;

[0086] 4) Chlorine dioxide bleaching (D2 stage): The hydrogen peroxide bleached kraft eucalyptus pulp was concentrated to a consistency of 10 wt%, chlorine dioxide, the amount of chlorine dioxide was 0.5% of the oven dry weight of the hydrogen peroxide bleached kraft eucalyptus pulp, and was transferred to a bleaching tower, heated to 70°C constant temperature for 60 min of chlorine dioxide bleaching, and then washed with water to obtain bleached kraft eucalyptus pulp.

[0087] Comparative Example 3: (chelating agent in Q section is EDTA; single chelating agent)

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

[0089] 1) Chelating treatment (Q section): the hydrogen peroxide bleached kraft eucalyptus pulp (same as Example 2) was concentrated to a concentration of 10 wt%, then EDTA was added, the amount of chelating agent was 1.0% of the absolute dry weight of the hydrogen peroxide bleached kraft eucalyptus pulp, then it was transferred to a reactor, and then water vapor was introduced to heat to 50°C constant temperature for 60 min of chelating treatment, then the kraft eucalyptus pulp was washed with water to obtain chelating treated kraft eucalyptus pulp;

[0090] 2) hydrogen peroxide bleaching (P2 section): the chelating treated kraft eucalyptus pulp was concentrated to a concentration of 10 wt%, then hydrogen peroxide, sodium hydroxide, sodium silicate, magnesium sulfate were added, the amount of hydrogen peroxide was 1.5% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of sodium hydroxide was 1.0% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of sodium silicate was 2.0% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, the amount of magnesium sulfate was 0.3% of the absolute dry weight of the chelating treated kraft eucalyptus pulp, then it was transferred to a hydrogen peroxide bleaching tower, then water vapor was introduced to heat to 90°C constant temperature for 120 min of hydrogen peroxide bleaching, then the kraft eucalyptus pulp was washed with water to obtain bleached kraft eucalyptus pulp.

[0091] Performance test:

[0092] 1) The ozone bleaching effect of the ozone bleached kraft eucalyptus pulp in Example 1, the ozone bleached kraft eucalyptus pulp in Example 2, the ozone bleached kraft bamboo pulp in Example 3, the ozone bleached kraft pine pulp in Example 4 and the ozone bleached kraft eucalyptus pulp in Comparative Example 1 is shown in the following table:

[0093] Table 1 Comparison of bleaching effect

[0094]

[0095] Note:

[0096] The calculation formula of delignification selectivity is: delignification selectivity = (K0-K1) / (V0-V1), wherein V0 represents the initial viscosity value, V1 represents the viscosity value after bleaching, K0 represents the initial Kappa number, and K1 represents the Kappa number after bleaching.

[0097] As shown in Table 1, compared with the traditional bleaching method, the whiteness of the ozone bleached kraft eucalyptus pulp is increased by more than 30%, the kappa number is significantly reduced, the viscosity loss is relatively small, and the ozone bleaching method of the present application has excellent bleaching effect on kraft bamboo pulp and kraft pine pulp, which shows that the ozone bleaching method of the present application effectively promotes the removal of phenolic lignin and non-phenolic lignin, and removes hexenuronic acid and metal ions, protects the fiber structure, ensures the physical properties of the pulp, and embodies high oxidation selectivity and process stability.

[0098] 2) The performance indicators of the bleached kraft eucalyptus pulp in Example 2 and Comparative Example 3 are shown in the following table:

[0099] Table 2 Comparison of performance indicators of bleached kraft eucalyptus pulp

[0100] Test Index Ozone dosage (%) Whiteness / ISO Kappa number Viscosity (mL / g) Delignification selectivity 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 further improve the whiteness of the pulp, effectively reduce the viscosity loss while maintaining a low kappa number, and improve the delignification selectivity, which shows that the composite chelating agent in Example 2 more effectively protects the fiber structure during the bleaching process, and enhances the removal efficiency of lignin, which has the significant advantages of environmental protection, high efficiency and good protection of pulp quality.

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

[0103] Table 3 Comparison of performance indicators of bleached kraft eucalyptus pulp

[0104]

[0105] Note:

[0106] The formula for calculating the viscosity reduction rate is as follows: viscosity reduction rate (%) = (V0-V1) / V0*100%, wherein 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 (Z-P1-Q-P2 process) of the present application has the following significant advantages compared with the conventional Light-ECF bleaching (O-Z-D1-E p -D2 process) and the traditional ECF bleaching method (O-D1-E p -D2 process):

[0109] a) Higher brightness: The brightness of the bleached kraft eucalyptus pulp in Example 2 is as high as 85.64, which is 1.35 units higher than that of the bleached kraft eucalyptus pulp in Comparative Example 1 (brightness of 84.29);

[0110] b) Lower fiber damage: The viscosity of the bleached kraft eucalyptus pulp in Example 1 is still 733 mL / g, and the viscosity reduction rate is only 22.68%, which is 12.24% lower than that of the bleached kraft eucalyptus pulp in Comparative Example 1 (viscosity as low as 617 mL / g, and the viscosity reduction rate reaches 34.92%);

[0111] c) Higher delignification efficiency: The delignification selectivity of the bleached kraft eucalyptus pulp in Example 1 is 0.0647, which is 24.4% higher than that of the bleached kraft eucalyptus pulp in Comparative Example 2 (delignification selectivity of 0.0520), indicating that the chlorine-free bleaching process of the present application can better protect the fiber strength while efficiently removing lignin;

[0112] In summary, the chlorine-free bleaching method (Z-P1-Q-P2 process) of the present application realizes the unity of high brightness, low fiber damage and high delignification selectivity through ozone-hydrogen peroxide collaborative bleaching and chelation treatment, and the comprehensive performance is superior to that of the conventional Light-ECF bleaching method and the traditional ECF bleaching method.

[0113] 4) Z-P1-Q-P2 process (Examples 1-4), O-Z-D1-E p -D2 process (Comparative Example 1) and O-D1-E p -D2 process (Comparative Example 2) are shown in the following table:

[0114] Table 4 Comparison of bleaching agent / chelating agent costs

[0115]

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

[0117] As can be seen from Table 4, the cost of the main bleaching agent (≤335 yuan / ton of pulp) of the chlorine-free bleaching method (Z-P1-Q-P2 process) of the present application is at least 32% lower than that of the conventional Light-ECF bleaching method (≥395 yuan / ton of pulp) and the traditional ECF bleaching method (520 yuan / ton of pulp), realizing the unity of high bleaching performance and low cost.

[0118] 5) Z-P1-Q-P2 process (Example 2), O-Z-D1-E p- D2 procedure (comparative example 1) and O-D1-E p The energy consumption (per ton of absolute dry pulp) of the D2 procedure (comparative example 2) is shown in the following table:

[0119] Table 5 Energy consumption comparison table

[0120]

[0121]

[0122] Note: The starting temperature of all bleaching procedures is 25℃; the starting temperature of the lower section is 50℃ for the high temperature section (≥70℃).

[0123] As can be seen from Table 5, the energy consumption cost of the chlorine-free bleaching method (Z-P1-Q-P2 procedure) of the present application is only 488.91 yuan / ton of pulp, which is 27.6% lower than that of the conventional Light-ECF bleaching method (675.13 yuan / ton of pulp) and the traditional ECF bleaching method (675.13 yuan / ton of pulp).

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

[0125] The above examples are preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A method for chlorine-free bleaching of pulp, characterized in that: The following steps are involved: 1) Ozone bleaching: The pulp is concentrated and then transferred to an ozone reactor to be bleached by introducing ozone, and then the pulp is washed to obtain ozone-bleached pulp; 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, and then the pulp is washed to obtain hydrogen peroxide-bleached pulp; 3) Chelating treatment: The pulp bleached with hydrogen peroxide is concentrated, a chelating agent is added for chelating treatment, and the pulp is washed to obtain chelated pulp; 4) Hydrogen peroxide bleaching: The chelated 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, and then the pulp is washed to obtain bleached pulp.

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

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

4. The method for chlorine-free bleaching of pulp according to claim 1 or 2, characterized in that: The ozone bleaching in step 1) is carried out at room temperature, and the ozone bleaching time is 10 minutes to 50 minutes.

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

6. The method for chlorine-free bleaching of pulp according to claim 1 or 5, characterized in that: Step 2) The hydrogen peroxide bleaching is carried out at a temperature of 80° C. to 90° C., and the hydrogen peroxide bleaching time is 70 min to 110 min.

7. The method for chlorine-free bleaching of pulp according to claim 1, characterized in that: The amount of the chelating agent in step 3) is 0.8% to 1.2% of the absolute dry weight of the pulp bleached with hydrogen peroxide; the chelating agent in step 3) is at least one of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, carboxymethyl chitosan, trisodium citrate, and Tween 80.

8. The method for chlorine-free bleaching of pulp according to claim 1 or 7, characterized in that: Step 3) The chelation treatment is carried out at a temperature of 50° C. to 60° C., and the chelation treatment time is 50 min to 70 min.

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

10. The chlorine-free bleaching method for pulp according to claim 1 or 9, characterized in that: Step 4) The hydrogen peroxide bleaching is carried out at a temperature of 80° C. to 90° C., and the hydrogen peroxide bleaching time is 100 min to 150 min.

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