Bleaching method of low-concentration paper pulp

Abstract

The invention discloses a bleaching method of low-concentration paper pulp, which comprises the following steps: 1) adjusting the pH value of the low-concentration paper pulp to 2.0-4.0, adding a viscosity protective agent, transferring into a low-pressure ozone adding device, and introducing normal-pressure ozone to obtain ozone-containing low-concentration paper pulp; and 2) transferring the ozone-containing low-concentration paper pulp into a turbulent flow type high-shear mixer for fully mixing the paper pulp and ozone, then transferring into an up-flow ozone reaction tower for bleaching reaction, and then transferring into a steam-water separator for steam-water separation to obtain bleached paper pulp and residual ozone. The bleaching method of the low-concentration paper pulp has the advantages of simplicity and convenience in operation, simple equipment, good bleaching effect, high ozone utilization efficiency, small influence on the viscosity of the paper pulp and the like, is suitable for bleaching paper pulp formed by cooking wood and grass raw materials and paper pulp subjected to oxygen delignification treatment, and is suitable for large-scale industrial application.

Description

Technical Field

[0001] The invention relates to the technical field of pulping and papermaking, and in particular to a method for bleaching low-concentration pulp. Background Art

[0002] Currently, pulp bleaching methods can be broadly divided into two major systems: chlorine-containing bleaching systems and oxygen-containing bleaching systems. Chlorine-containing bleaching systems use bleaching agents including chlorine gas, hypochlorite, and chlorine dioxide. Because these agents contain chlorine, they produce a large amount of organic chlorides (especially dioxins) that are harmful to the environment and human health during the pulp bleaching process. Oxygen-containing bleaching systems use bleaching agents including oxygen, ozone, and hydrogen peroxide. These agents do not contain chlorine and do not produce substances harmful to the environment and human health during the pulp bleaching process. Compared with chlorine-containing bleaching systems, oxygen-containing bleaching systems have broader application prospects.

[0003] The ozone bleaching method for pulp has the advantages of high efficiency, low cost, strong delignification selectivity, and environmental friendliness, and has become one of the indispensable key processes in the pulp chlorine-free bleaching process. At present, the ozone bleaching method for pulp is mainly divided into the following two modes according to the pulp concentration: 1) High-concentration pulp (pulp concentration: 30wt%~45wt%) ozone bleaching method: first use a high-concentration pulp dewatering machine to dehydrate the pulp to make its concentration reach 30wt%~45wt%, and then use a high-concentration pulp disperser to disperse the pulp into a fluffy state, and then transport it to a special ozone reactor to react with ozone. The advantage of this method is that the reaction time is very short (the pulp concentration is high, the water film layer on the surface of the pulp fiber is thin, and ozone can easily react with the pulp). Fiber reaction), but there are also disadvantages such as large damage to pulp fibers, large decrease in pulp viscosity, and easy fire accidents. In addition, the high-consistency pulp dewatering machine, high-consistency pulp disperser and ozone reactor used are complex in structure, expensive and difficult to obtain; 2) Medium-consistency pulp (pulp concentration: 10wt% to 15wt%) ozone bleaching method: first use a medium-consistency pulp pump to transport the pulp to the medium-consistency pulp ozone feeder, and then add high-pressure ozone (the surface pressure of ozone is 8kg / cm 2 ~12kg / cm 2), then enters a specially designed medium-consistency pulp ozone mixer for mixing, and then transfers to a medium-consistency pulp ozone bleaching reactor for reaction, followed by washing and dehydration. This method has the advantage of a shorter reaction time (the pulp concentration is relatively high, and the water film on the pulp surface is relatively thin, allowing the ozone to react well under the dual effects of high pressure and the specially designed ozone mixer). However, it also has the disadvantages of high equipment costs (the atmospheric ozone generated by the ozone generator must be pressurized by an ozone pressurizer before being added to the medium-consistency pulp ozone feeder, which is expensive to manufacture and maintain; the medium-consistency pulp ozone feeder and medium-consistency pulp ozone mixer are also expensive) and a significant decrease in pulp viscosity. Furthermore, there is currently a lack of ozone bleaching methods suitable for low-consistency pulp (pulp concentration: 2wt% to 6wt%).

[0004] Therefore, it is of great significance to develop a low-concentration pulp bleaching method with simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency and little effect on the viscosity of the pulp. Summary of the Invention

[0005] The object of the present invention is to provide a method for bleaching low-concentration pulp.

[0006] The technical solution adopted by the present invention is:

[0007] A method for bleaching low-consistency pulp comprises the following steps:

[0008] 1) The pH value of the low-concentration pulp is adjusted to 2.0-4.0, a viscosity protecting agent is added, and the pulp is transferred to a low-pressure ozone feeder, and then atmospheric pressure ozone is introduced to obtain low-concentration ozone-containing pulp;

[0009] 2) The ozone-containing low-concentration pulp is transferred to a turbulent high-shear mixer for thorough mixing of the pulp and ozone, then transferred to an upflow ozone reaction tower for bleaching reaction, and then transferred to a steam-water separator for steam-water separation to obtain bleached pulp and residual ozone.

[0010] Preferably, the low-concentration pulp in step 1) is at least one of kraft coniferous pulp, kraft broadleaf pulp, kraft bamboo pulp, kraft wheat straw pulp, kraft reed pulp, sulfite reed pulp, kraft bagasse pulp, oxygen-delignified kraft coniferous pulp, oxygen-delignified kraft broadleaf pulp, oxygen-delignified kraft bamboo pulp, oxygen-delignified kraft wheat straw pulp, oxygen-delignified kraft reed pulp, oxygen-delignified sulfite reed pulp, and oxygen-delignified kraft bagasse pulp.

[0011] Preferably, the concentration of the low-concentration pulp in step 1) is 2 wt% to 6 wt%.

[0012] Preferably, the alkali content of the low-consistency pulp in step 1) is no more than 0.4 g / L in terms of NaOH.

[0013] Preferably, the regulator used to adjust the pH value of the low-concentration pulp in step 1) is sulfuric acid.

[0014] Preferably, the viscosity protecting agent in step 1) is chitosan and carboxymethyl cellulose.

[0015] Preferably, the mass ratio of chitosan to carboxymethyl cellulose is 1.8-2.2:1.

[0016] Preferably, the amount of the viscosity protecting agent in step 1) is 0.3% to 1.0% by weight of the absolute dry pulp.

[0017] Preferably, the low-pressure ozone injector in step 1) is a Venturi ejector.

[0018] Preferably, the low-pressure ozone injector in step 1) is a single one or multiple ones connected in series.

[0019] Preferably, the pulp fluid inlet surface pressure of the low-pressure ozone injector in step 1) is ≥3.0 kgf / cm 2 .

[0020] Preferably, the mass concentration of the atmospheric pressure ozone in step 1) is 100g / Nm 3 ~150g / Nm 3 .

[0021] Preferably, the atmospheric pressure ozone in step 1) is generated by an atmospheric pressure ozone generator, and the inlet surface pressure of the atmospheric pressure ozone generator is 1.0 kgf / cm 2 ~1.2kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 ~1.1kgf / cm 2 .

[0022] Preferably, the atmospheric pressure ozone generator is a glass tube type atmospheric pressure ozone generator or a ceramic tube type atmospheric pressure ozone generator.

[0023] Preferably, the inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%.

[0024] Preferably, the amount of atmospheric pressure ozone used in step 1) is 5 kg / t absolute dry pulp to 20 kg / t absolute dry pulp.

[0025] Preferably, the design rotation speed of the turbulent high shear mixer in step 2) is 1200 r / min to 3000 r / min.

[0026] Preferably, the turbulent high shear mixer in step 2) is a single unit or multiple units connected in series.

[0027] Preferably, the turbulent high shear mixer in step 2) is made of SS316L stainless steel.

[0028] Preferably, the top of the upflow ozone reaction tower in step 2) is an inverted cone structure, and the angle between the side of the cone and the horizontal direction is 30° to 45°.

[0029] Preferably, the lower portion of the upflow ozone reaction tower in step 2) is a cone structure, and the angle between the side of the cone and the horizontal direction is 45° to 60°.

[0030] Preferably, the diameter of the upflow ozone reaction tower in step 2) is 1000 mm to 5000 mm.

[0031] Preferably, the upflow ozone reaction tower in step 2) is made of SS316L stainless steel.

[0032] Preferably, the residence time of the ozone-containing low-concentration pulp in step 2) in the upflow ozone reaction tower is 10 min to 60 min.

[0033] Preferably, the residual ozone in step 2) is passed into an ozone destroyer for decomposition or into an ozone recovery device for recovery and reuse.

[0034] Preferably, the ozone destroyer is a heating type ozone destroyer or a packing type ozone destroyer.

[0035] Preferably, the steam-water separator in step 2) is made of SS316L stainless steel.

[0036] The beneficial effects of the present invention are as follows: the low-concentration pulp bleaching method of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp. It is suitable for bleaching pulp formed by cooking wood and grass raw materials and pulp that has undergone oxygen delignification treatment, and is suitable for large-scale industrial application.

[0037] Specifically:

[0038] 1) The ozone utilization rate of the low-concentration pulp bleaching method of the present invention is high, and the effective utilization rate of ozone can reach more than 99.5%;

[0039] 2) The low-consistency pulp bleaching method of the present invention uses simple equipment, and the equipment cost is only 1 / 50 to 1 / 80 of the high / medium-consistency pulp ozone bleaching method;

[0040] 3) The low-consistency pulp bleaching method of the present invention is to fully mix the low-consistency pulp with ozone to react. The instantaneous reaction concentration of ozone is low, which greatly reduces the destructive effect of ozone on the viscosity of the pulp. The effect on the viscosity of the pulp is small, and the degree of pulp viscosity reduction is greatly reduced compared with the high / medium consistency pulp ozone bleaching method;

[0041] 4) The low-consistency pulp bleaching method of the present invention also utilizes a viscosity-protecting agent. With the addition of this viscosity-protecting agent, the impact of the bleaching process on pulp viscosity is further reduced. Therefore, the ozone dosage can be significantly increased without excessively damaging the pulp viscosity, which is not possible with high- / medium-consistency pulp ozone bleaching systems.

[0042] 5) The low-concentration pulp bleaching method of the present invention is not only applicable to newly built kraft pulp bleaching projects, but also to kraft pulp bleaching technical improvement projects;

[0043] 6) The low-concentration pulp bleaching method of the present invention is not only applicable to natural pulp formed by cooking, but also to pulp treated with oxygen delignification;

[0044] 7) The low-concentration pulp bleaching method of the present invention can be used with a chlorine dioxide bleaching agent to form a light-ECF (Light-ECF) bleaching production process, or can be used with an oxygen / hydrogen peroxide chlorine-free bleaching agent to form a total chlorine-free (TCF) bleaching production process;

[0045] 8) The low-concentration pulp bleaching method of the present invention can reuse more than 90% of the wastewater to the previous process, and the water consumption is very small, and the water consumption per ton of pulp is less than 3m3. 3 the following;

[0046] 9) The low-concentration pulp bleaching method of the present invention has the advantages of simple operation, high safety, low operating energy consumption, low equipment maintenance cost, etc., and is suitable for large-scale industrial application. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 The figure is a process flow chart of the low-concentration pulp bleaching method of the present invention. DETAILED DESCRIPTION

[0048] The present invention will be further explained and illustrated below with reference to specific embodiments.

[0049] Example 1:

[0050] A method for bleaching low-concentration pulp (process flow chart as shown in FIG. Figure 1 As shown), the steps are as follows:

[0051] 1) Kraft bagasse pulp (made by cooking, washing, screening and purification, with a hardness Kappa value of 14.8, a viscosity of 950 mL / g, a brightness of 31.6% ISO, and an alkali content of 0.1 g / L as NaOH) is added to a paper machine, and then the concentration is adjusted to 2.0 wt% with recycled white water, and then sulfuric acid is added to adjust the pH value to 2.0-3.0, and then 0.3% of the weight of the absolute dry pulp is added. The viscosity protectant is composed of chitosan and carboxymethyl cellulose in a mass ratio of 2:1, and then pumped into a low-pressure ozone feeder, which is a Venturi ejector, and the surface pressure of the pulp fluid inlet is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 120g / Nm 3 , the dosage is 6kg / t absolute dry pulp, and low concentration pulp containing ozone is obtained;

[0052] 2) Pumping the ozone-containing low-concentration pulp into a turbulent high-shear mixer to fully mix the pulp and ozone, the rotation speed of the turbulent high-shear mixer is 1450 r / min, and the material of the turbulent high-shear mixer is SS316L stainless steel, and then pumping it into an upflow ozone reaction tower for bleaching reaction, the top of the upflow ozone reaction tower is an inverted cone structure, the angle between the side of the cone and the horizontal direction is 30° to 45°, the lower part of the upflow ozone reaction tower is a cone structure, the angle between the side of the cone and the horizontal direction is 45° to 60°, the diameter of the upflow ozone reaction tower is 1000 mm to 5000 mm, and the material is SS316L stainless steel. The pulp stays in the upflow ozone reaction tower for 10 minutes, and then transferred to a steam-water separator for steam-water separation, the steam-water separator is made of SS316L stainless steel, to obtain bleached pulp and residual ozone, and the residual ozone is passed into an ozone recovery device for recycling.

[0053] Performance testing:

[0054] The bleached pulp in this embodiment was added to a vacuum pulp washer for washing and concentration, and then tested. The hardness Kappa value of the bleached pulp was 8.8, the viscosity was 870 mL / g, the brightness was 52.8% ISO, the lignin removal rate was 40.5%, the brightness increase was 21.2% ISO, and the viscosity reduction rate was 8.4%. This shows that the low-concentration pulp bleaching method of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp.

[0055] Example 2:

[0056] A method for bleaching low-concentration pulp (process flow chart as shown in FIG. Figure 1 As shown), the steps are as follows:

[0057] 1) Sulfate bamboo pulp (made by cooking, washing, screening and purification, with a hardness Kappa value of 18.5, a viscosity of 1180 mL / g, a brightness of 26.5% ISO, and an alkali content of 0.15 g / L calculated as NaOH) is added to a paper machine, and then the concentration is adjusted to 2.5 wt% with recycled white water, and then sulfuric acid is added to adjust the pH value to 2.0-3.0, and then a viscosity protectant is added at 0.5% of the absolute dry pulp weight, the viscosity protectant being composed of chitosan and carboxymethyl cellulose in a mass ratio of 2:1, and then pumped into a low-pressure ozone feeder, which is a Venturi ejector, and the pulp fluid inlet surface pressure is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 150g / Nm 3 , the dosage is 8kg / t absolute dry pulp, and low concentration pulp containing ozone is obtained;

[0058] 2) Pumping the ozone-containing low-concentration pulp into a turbulent high-shear mixer to fully mix the pulp and ozone, the rotation speed of the turbulent high-shear mixer is 1450 r / min, and the material of the turbulent high-shear mixer is SS316L stainless steel, and then pumping it into an upflow ozone reaction tower for bleaching reaction, the top of the upflow ozone reaction tower is an inverted cone structure, the angle between the side of the cone and the horizontal direction is 30° to 45°, the lower part of the upflow ozone reaction tower is a cone structure, the angle between the side of the cone and the horizontal direction is 45° to 60°, the diameter of the upflow ozone reaction tower is 1000 mm to 5000 mm, and the material is SS316L stainless steel. The pulp stays in the upflow ozone reaction tower for 15 minutes, and then transferred to a steam-water separator for steam-water separation, the steam-water separator is made of SS316L stainless steel, to obtain bleached pulp and residual ozone, and the residual ozone is passed into an ozone recovery device for recycling.

[0059] Performance testing:

[0060] The bleached pulp in this embodiment was added to a vacuum pulp washer for washing and concentration, and then tested. The hardness Kappa value of the bleached pulp was 10.5, the viscosity was 1060 mL / g, the brightness was 43.5% ISO, the lignin removal rate was 43.2%, the brightness increase was 17.0% ISO, and the viscosity decrease rate was 10.2%. It shows that the low-concentration pulp bleaching method of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp.

[0061] Example 3:

[0062] A method for bleaching low-concentration pulp (process flow chart as shown in FIG. Figure 1 As shown), the steps are as follows:

[0063] 1) Sulfate eucalyptus pulp (made by cooking, washing, screening and purification, with a hardness Kappa value of 17.2, a viscosity of 1020 mL / g, a brightness of 29.3% ISO, and an alkali content of 0.2 g / L calculated as NaOH) is added to a paper machine, and then the concentration is adjusted to 4.0 wt% with recycled white water, and then sulfuric acid is added to adjust the pH value to 2.0-3.0, and then 0.3% of the weight of the absolute dry pulp is added. The viscosity protectant is composed of chitosan and carboxymethyl cellulose in a mass ratio of 2:1, and then pumped into a low-pressure ozone feeder, which is a Venturi ejector, and the pulp fluid inlet surface pressure is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 120g / Nm 3 , the dosage is 6kg / t absolute dry pulp, and low concentration pulp containing ozone is obtained;

[0064] 2) Pumping the ozone-containing low-concentration pulp into a turbulent high-shear mixer to fully mix the pulp and ozone, the rotation speed of the turbulent high-shear mixer is 1450 r / min, and the material of the turbulent high-shear mixer is SS316L stainless steel, and then pumping it into an upflow ozone reaction tower for bleaching reaction, the top of the upflow ozone reaction tower is an inverted cone structure, the angle between the side of the cone and the horizontal direction is 30° to 45°, the lower part of the upflow ozone reaction tower is a cone structure, the angle between the side of the cone and the horizontal direction is 45° to 60°, the diameter of the upflow ozone reaction tower is 1000 mm to 5000 mm, and the material is SS316L stainless steel. The pulp stays in the upflow ozone reaction tower for 10 minutes, and then transferred to a steam-water separator for steam-water separation, the steam-water separator is made of SS316L stainless steel, to obtain bleached pulp and residual ozone, and the residual ozone is passed into an ozone recovery device for recycling.

[0065] Performance testing:

[0066] The bleached pulp in this embodiment was added to a vacuum pulp washer for washing and concentration, and then tested. The hardness Kappa value of the bleached pulp was 10.3, the viscosity was 930 mL / g, the brightness was 48.5% ISO, the lignin removal rate was 40.1%, the brightness increase was 19.2% ISO, and the viscosity reduction rate was 8.8%. This shows that the low-concentration pulp bleaching method of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp.

[0067] Example 4:

[0068] A method for bleaching low-concentration pulp (process flow chart as shown in FIG. Figure 1 As shown), the steps are as follows:

[0069] 1) Add oxygen delignified kraft masson pine pulp (hardness Kappa value of 17.9, viscosity of 1050 mL / g, brightness of 31.5% ISO, alkali content of 0.4 g / L calculated as NaOH) to the paper machine, adjust the concentration to 3.0 wt% with recycled white water, add sulfuric acid to adjust the pH to 2.5-3.0, add 0.3% of the weight of the absolute dry pulp viscosity protectant, the viscosity protectant is composed of chitosan and carboxymethyl cellulose in a mass ratio of 2:1, and then pump it into a low-pressure ozone feeder, which is a Venturi ejector, and the pulp fluid inlet surface pressure is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2, outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 150g / Nm 3 , the dosage is 6kg / t absolute dry pulp, and low concentration pulp containing ozone is obtained;

[0070] 2) Pumping the ozone-containing low-concentration pulp into a turbulent high-shear mixer to fully mix the pulp and ozone, the rotation speed of the turbulent high-shear mixer is 1450 r / min, and the material of the turbulent high-shear mixer is SS316L stainless steel, and then pumping it into an upflow ozone reaction tower for bleaching reaction, the top of the upflow ozone reaction tower is an inverted cone structure, the angle between the side of the cone and the horizontal direction is 30° to 45°, the lower part of the upflow ozone reaction tower is a cone structure, the angle between the side of the cone and the horizontal direction is 45° to 60°, the diameter of the upflow ozone reaction tower is 1000 mm to 5000 mm, and the material is SS316L stainless steel. The pulp stays in the upflow ozone reaction tower for 15 minutes, and then transferred to a steam-water separator for steam-water separation, the steam-water separator is made of SS316L stainless steel, to obtain bleached pulp and residual ozone, and the residual ozone is passed into an ozone recovery device for recycling.

[0071] Note:

[0072] The preparation method of oxygen delignified kraft masson pine pulp in this embodiment is as follows:

[0073] 1) Kraft masson pine pulp (produced by cooking, washing, screening and purification, with a hardness Kappa value of 28.5, a viscosity of 1230 mL / g, a brightness of 20.8% ISO, an alkali content of 0.3 g / L calculated as NaOH, and a concentration of 12 wt%) is transferred to a medium-consistency pulp mixer using a medium-consistency pump, and 2.0% by weight of NaOH and 0.2% by weight of MgSO4 based on the absolute dry pulp weight are added through the medium-consistency pump riser, and oxygen is then introduced at a rate of 25 kg / t of absolute dry pulp to obtain oxygen-containing pulp;

[0074] 2) Pump the oxygen-containing pulp into the oxygen bleaching reaction tower, and control the top pressure of the oxygen bleaching reaction tower at 4.0kgf / cm 2 ~5.0kgf / cm 2 The pulp stays in the oxygen bleaching reaction tower for 60 minutes, and then is sprayed into the spray pot, and then transported by a low-concentration pulp pump to two vacuum pulp washers in series for washing and concentration to obtain oxygen delignified sulfate Masson pine pulp.

[0075] Performance testing:

[0076] The bleached pulp in this embodiment was added to a vacuum pulp washer for washing and concentration, and then tested. The hardness Kappa value of the bleached pulp was 12.3, the viscosity was 970 mL / g, the brightness was 54.8% ISO, the lignin removal rate was 31.3%, the brightness increase was 23.3% ISO, and the viscosity reduction rate was 7.6%. This shows that the low-concentration pulp bleaching method of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp.

[0077] Example 5:

[0078] A method for bleaching low-concentration pulp (process flow chart as shown in FIG. Figure 1 As shown), the steps are as follows:

[0079] 1) Kraft reed pulp (made by cooking, washing, screening and purification, with a hardness Kappa value of 18.9, a viscosity of 1070 mL / g, a brightness of 26.5% ISO, and an alkali content of 0.2 g / L calculated as NaOH) is added to the paper machine, and then the concentration is adjusted to 2.5wt% with recycled white water, and then sulfuric acid is added to adjust the pH value to 2.5-3.0, and then 0.5% of the weight of the absolute dry pulp is added with a viscosity protectant composed of chitosan and carboxymethyl cellulose in a mass ratio of 2:1. Then, the pulp is pumped into the 1# low-pressure ozone injector, which is a Venturi ejector, and the surface pressure of the pulp fluid inlet is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 120g / Nm 3 , the dosage is 5kg / t absolute dry pulp, and low-concentration pulp I containing ozone is obtained;

[0080] 2) The ozone-containing low-concentration pulp I is pumped into the 1# turbulent high-shear mixer to fully mix the pulp and ozone. The rotation speed of the turbulent high-shear mixer is 1450 r / min. The material of the turbulent high-shear mixer is SS316L stainless steel. Then, the pulp is pumped into the 2# low-pressure ozone feeder. The low-pressure ozone feeder is a Venturi ejector. The surface pressure of the pulp fluid inlet is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 120g / Nm 3 , the dosage is 5kg / t absolute dry pulp, and low-concentration pulp II containing ozone is obtained;

[0081] 3) The ozone-containing low-concentration pulp II is pumped into the 2# turbulent high-shear mixer to fully mix the pulp and ozone. The rotation speed of the turbulent high-shear mixer is 1450r / min. The material of the turbulent high-shear mixer is SS316L stainless steel. Then, the pulp is pumped into the upflow ozone reaction tower for bleaching reaction. The top of the upflow ozone reaction tower is an inverted cone structure. The angle between the side of the cone and the horizontal direction is 30° to 45°. The lower part of the upflow ozone reaction tower is 10000rpm. It has a cone structure, and the angle between the side of the cone and the horizontal direction is 45° to 60°. The diameter of the upflow ozone reaction tower is 1000mm to 5000mm, and the material is SS316L stainless steel. The residence time of the pulp in the upflow ozone reaction tower is 30 minutes, and then it is transferred to the steam-water separator for steam-water separation. The steam-water separator is made of SS316L stainless steel to obtain bleached pulp and residual ozone, and the residual ozone is passed into the ozone recovery device for recycling and reuse.

[0082] Performance testing:

[0083] The bleached pulp in this embodiment was added to a vacuum pulp washer for washing and concentration, and then tested. The hardness Kappa value of the bleached pulp was 7.5, the viscosity was 920 mL / g, the brightness was 54.5% ISO, the lignin removal rate was 60.3%, the brightness increase was 28.0% ISO, and the viscosity reduction rate was 14.0%. It shows that the bleaching method of low-concentration pulp of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp.

[0084] Example 6:

[0085] A method for bleaching low-concentration pulp (process flow chart as shown in FIG. Figure 1 As shown), the steps are as follows:

[0086] 1) Sulfate bamboo pulp (made by cooking, washing, screening and purification, with a hardness Kappa value of 18.5, a viscosity of 1180 mL / g, a brightness of 26.5% ISO, and an alkali content of 0.15 g / L calculated as NaOH) is added to the paper machine, and the concentration is adjusted to 3.0 wt% with recycled white water, and the pH value is adjusted to 2.0-2.5 by adding sulfuric acid, and then a viscosity protectant is added at 0.7% by weight of the absolute dry pulp, the viscosity protectant being composed of chitosan and carboxymethyl cellulose in a mass ratio of 2:1, and then pumped into the 1# low-pressure ozone feeder, which is a Venturi ejector, and the surface pressure of the pulp fluid inlet is ≥3.0 kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 120g / Nm 3 The dosage is 7kg / t absolute dry pulp, and then pumped into the 2# low-pressure ozone injector. The low-pressure ozone injector is a Venturi ejector. The pulp fluid inlet surface pressure is ≥3.0kgf / cm 2 Then, atmospheric pressure ozone generated by a glass tube atmospheric pressure ozone generator is introduced. The inlet gas source of the atmospheric pressure ozone generator is oxygen with a concentration of ≥92wt%. The inlet surface pressure of the atmospheric pressure ozone generator is 1.0kgf / cm 2 ~1.1kgf / cm 2 , outlet surface pressure is 1.0kgf / cm 2 The mass concentration of ozone at atmospheric pressure is 120g / Nm 3 , the dosage is 7kg / t absolute dry pulp, and low concentration pulp containing ozone is obtained;

[0087] 2) Pumping the ozone-containing low-concentration pulp into a turbulent high-shear mixer to fully mix the pulp and ozone, the rotation speed of the turbulent high-shear mixer is 1450 r / min, and the material of the turbulent high-shear mixer is SS316L stainless steel, and then pumping it into an upflow ozone reaction tower for bleaching reaction, the top of the upflow ozone reaction tower is an inverted cone structure, the angle between the side of the cone and the horizontal direction is 30° to 45°, the lower part of the upflow ozone reaction tower is a cone structure, the angle between the side of the cone and the horizontal direction is 45° to 60°, the diameter of the upflow ozone reaction tower is 1000 mm to 5000 mm, and the material of the upflow ozone reaction tower is SS316L stainless steel. The pulp stays in the upflow ozone reaction tower for 40 minutes, and then transferred to a steam-water separator for steam-water separation, the steam-water separator is made of SS316L stainless steel, to obtain bleached pulp and residual ozone, and the residual ozone is passed into an ozone recovery device for recycling and reuse.

[0088] Performance testing:

[0089] The bleached pulp in this embodiment was added to a vacuum pulp washer for washing and concentration, and then tested. The hardness Kappa value of the bleached pulp was 6.8, the viscosity was 980 mL / g, the brightness was 57.3% ISO, the lignin removal rate was 63.2%, the brightness increase was 30.8% ISO, and the viscosity reduction rate was 16.9%. This shows that the low-concentration pulp bleaching method of the present invention has the advantages of simple operation, simple equipment, good bleaching effect, high ozone utilization efficiency, and little effect on the viscosity of the pulp.

[0090] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for bleaching low-concentration pulp, characterized in that: The following steps are involved: 1) The pH value of the low-concentration pulp is adjusted to 2.0-4.0, a viscosity protecting agent is added, and the pulp is transferred to a low-pressure ozone feeder, and then atmospheric pressure ozone is introduced to obtain low-concentration ozone-containing pulp; 2) The ozone-containing low-concentration pulp is transferred to a turbulent high-shear mixer for thorough mixing of the pulp and ozone, then transferred to an upflow ozone reaction tower for bleaching reaction, and then transferred to a steam-water separator for steam-water separation to obtain bleached pulp and residual ozone.

2. The method for bleaching low-concentration pulp according to claim 1, characterized in that: The low-concentration pulp in step 1) is at least one of kraft coniferous pulp, kraft broadleaf pulp, kraft bamboo pulp, kraft wheat straw pulp, kraft reed pulp, sulfite reed pulp, kraft bagasse pulp, oxygen-delignified kraft coniferous pulp, oxygen-delignified kraft broadleaf pulp, oxygen-delignified kraft bamboo pulp, oxygen-delignified kraft wheat straw pulp, oxygen-delignified kraft reed pulp, oxygen-delignified sulfite reed pulp, and oxygen-delignified kraft bagasse pulp; the concentration of the low-concentration pulp in step 1) is 2wt% to 6wt%.

3. The method for bleaching low-concentration pulp according to claim 1 or 2, characterized in that: In step 1), the alkali content of the low-concentration pulp is no more than 0.4 g / L in terms of NaOH.

4. The method for bleaching low-concentration pulp according to claim 1 or 2, characterized in that: The viscosity protecting agent in step 1) is chitosan and carboxymethyl cellulose; the amount of the viscosity protecting agent in step 1) is 0.3% to 1.0% of the weight of the absolute dry pulp.

5. The method for bleaching low-concentration pulp according to claim 1 or 2, characterized in that: Step 1) The mass concentration of atmospheric ozone is 100g / Nm 3 ~150g / Nm 3 .

6. The method for bleaching low-concentration pulp according to claim 1 or 2, characterized in that: In step 1), the dosage of atmospheric pressure ozone is 5kg / t absolute dry pulp to 20kg / t absolute dry pulp.

7. The method for bleaching low-consistency pulp according to claim 1, wherein: Step 2) The design rotation speed of the turbulent high shear mixer is 1200 r / min to 3000 r / min.

8. The method for bleaching low-consistency pulp according to claim 1 or 7, characterized in that: The top of the upflow ozone reaction tower in step 2) is an inverted cone structure, and the angle between the side of the cone and the horizontal direction is 30° to 45°; the lower part of the upflow ozone reaction tower in step 2) is a cone structure, and the angle between the side of the cone and the horizontal direction is 45° to 60°.

9. The method for bleaching low-consistency pulp according to claim 1 or 7, characterized in that: In step 2), the residence time of the ozone-containing low-concentration pulp in the upflow ozone reaction tower is 10 to 60 minutes.

10. The method for bleaching low-consistency pulp according to claim 1 or 7, characterized in that: In step 2), the residual ozone is passed into an ozone destroyer for decomposition or into an ozone recovery device for recycling.

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