Metatitanic acid bleaching method

Abstract

The invention belongs to the field of titanium dioxide production by a sulfuric acid method, and particularly relates to a metatitanic acid bleaching method. The invention provides a bleaching method of metatitanic acid. The bleaching method comprises the following steps: S1, preparing 280-350g / L of metatitanic acid slurry; s2, adding sulfuric acid, heating while stirring, and heating the system to 60-70 DEG C; s3, adding a Ti < 3 + > solution and sodium formaldehyde sulfoxylate, stirring and preserving heat, and reducing Fe < 3 + > in the metatitanic acid slurry into Fe < 2 + >; wherein the mass ratio of Ti < 3 + > to sodium formaldehyde sulfoxylate in S3 is (3.5-8): (8-9). Through the synergistic effect of two reducing substances, namely Ti < 3 + > and sodium formaldehyde sulfoxylate, the bleaching effect is good, the use amount of sulfuric acid in the system is greatly reduced, and the production energy consumption is reduced.

Description

Technical Field

[0001] The invention belongs to the field of sulfuric acid process titanium dioxide production, and specifically relates to a metatitanic acid bleaching method. Background Art

[0002] Titanium dioxide (TiO2, commonly known as titanium dioxide) is a white inorganic pigment with excellent performance, which is widely used in coatings, plastics, papermaking, inks, cosmetics, food additives, electronics and other fields. There are two main production processes for titanium dioxide: sulfuric acid method and chlorination method. At present, the domestic production process is mainly sulfuric acid method. The production of titanium dioxide by sulfuric acid method mainly includes four parts: preparation of titanium liquid, preparation and purification of metatitanic acid, preparation of TiO2 and intermediate crushing, and post-treatment of TiO2. Titanium concentrate is prepared into qualified hydrolyzed titanium liquid through titanium liquid, and then hydrolyzed to generate metatitanic acid. The hydrolyzed metatitanic acid is purified to remove impurities, and seed crystals and salt treatment agents are added. It is calcined at high temperature in a rotary kiln to prepare TiO2 titanium dioxide with pigment properties. The primary product needs to be depolymerized by dry crushing, added with dispersant and desalted water for pulping and dispersion, and then wet ground to prepare TiO2 slurry, and finally produced by post-treatment to produce qualified titanium dioxide.

[0003] The content of impurity elements is the main factor affecting the whiteness and other properties of titanium dioxide. The iron content in the production process of sulfuric acid titanium dioxide is a representative element of impurities. Titanic acid purification is the key link to remove iron impurities, and the bleaching station is the key process to remove iron impurities with titanic acid.

[0004] The sulfuric acid process for producing titanium dioxide is mainly based on the bleaching of trivalent titanium. 3+ To achieve bleaching effect by reducing the titanium, sulfuric acid and aluminum powder must be added to prepare trivalent titanium, and then the trivalent titanium solution is added to metatitanic acid. 3+ It can adsorb on the surface of metatitanic acid at low acidity and can react with Fe adsorbed on hydrated titanium dioxide. 2+ ions are replaced, and the replaced Fe 2+ Ions are also due to Ti 3+ The existence of Fe 2+ It will not be oxidized to Fe 3+ , thereby maintaining the ionic state and being continuously carried away by the washing water, the purpose of purifying titanic acid can be achieved.

[0005] Sodium formaldehyde sulfoxylate has a strong reducing property at high temperature and has a good bleaching effect. Its high reducing property can be used to reduce the Fe 3+ Reduction to Fe 2+ The presence of residual reducing agent ensures that the replaced Fe 2+ Will not be oxidized to Fe 3+, thus ensuring that Fe 2+ The prior art CN 103318953 B discloses a titanium dioxide bleaching method, wherein a metatitanic acid slurry is taken and sodium bisulfite formaldehyde is added to the slurry as a reducing agent; a stirring device is added to stir the slurry, and during the stirring process, the sodium bisulfite formaldehyde decomposes to produce a reducing factor, which converts the Fe 3+ Reduction to Fe 2+ The fully reduced slurry is filtered and washed with deionized water to obtain bleached metatitanic acid. The prior art examines the 3+ All of them are replaced by reducing agents to bleach titanium dioxide, but Ti 3+ All replacement with reducing agents may result in higher iron content.

[0006] Therefore, it is urgent to develop a titanic acid bleaching method that can reduce the consumption and cost by reducing the amount of trivalent titanium solution and sulfuric acid. Summary of the invention

[0007] The technical problem to be solved by the present invention is: a metatitanic acid bleaching method which can reduce the consumption and cost by reducing the usage of trivalent titanium solution and sulfuric acid.

[0008] In order to solve the above problems, the present invention provides a bleaching method of metatitanic acid, comprising the following steps: S1. Dilute the titanic acid slurry to 280-350 g / L; S2, add sulfuric acid, and heat the system to 60-70°C while stirring; S3. Add Ti 3+ solution and sodium formaldehyde sulfoxylate, stirring and keeping warm, the Fe 3+ Reduction to Fe 2+ ; Among them, S3 Ti 3+ The mass ratio of formaldehyde and sodium bisulfite is (3.5~8):(8~9).

[0009] In some embodiments, in S1, the titanate slurry is diluted to 280-285 g / L.

[0010] In some embodiments, in S2, the concentration of sulfuric acid is 16~60g / L.

[0011] In some embodiments, in S2, the concentration of sulfuric acid is 17~25g / L.

[0012] In some embodiments, in S2, the system is heated to 60-65°C.

[0013] In some embodiments, the Ti3+ The mass ratio of sodium formaldehyde sulfoxylate to the total titanium in the slurry is: 0.35~0.8: 0.8~0.9: 440~500.

[0014] In some embodiments, in S2, the stirring time is 20 to 60 minutes.

[0015] In some embodiments, in S3, the stirring and insulation time is 100-130 min.

[0016] In some embodiments, in S3, the titanate slurry Fe 3+ Reduction to Fe 2+ In the system after 3+ The concentration is 0.5~1.5g / L.

[0017] In some embodiments, the step further includes S4: vacuum filtering and washing the bleached pulp.

[0018] Beneficial effects of the present invention: Existing technology uses Ti 3+ The bleaching effect is achieved by reducing the Ti 3+ It can adsorb on the surface of metatitanic acid at low acidity and can react with Fe adsorbed on hydrated titanium dioxide. 2+ Ions are replaced to bleach metatitanic acid. In the present invention, Ti is added simultaneously under acidic conditions. 3+ solution and sodium formaldehyde sulfoxylate to remove Fe 3+ Reduction to Fe 2+ , through Ti 3+ The synergistic effect of two reducing substances, sodium formaldehyde sulfoxylate, and the following four beneficial effects are achieved under the conditions of a specific concentration of metatitanic acid slurry and a specific reaction temperature: 1. The method of the present invention has a good bleaching effect, and the mass percentage of iron in the slurry after bleaching is ≤0.0050%; 2. The method of the present invention can greatly save the amount of sulfuric acid used in the system, and compared with the traditional method, it can save more than 50% of sulfuric acid; 3. The method of the present invention reduces Ti by nearly 50% 3+ Solution dosage; 4. The method of the present invention reduces the temperature required for the bleaching reaction, and the reaction can be carried out at only 60~70℃, saving production energy consumption.

[0019] The present invention summarizes various aspects of the embodiments and should not be used to limit the claims. Other embodiments are conceivable based on the technology described herein, which will be obvious to those skilled in the art after studying the following drawings and detailed descriptions, and these embodiments are intended to be included within the scope of the present invention.

[0020] Those skilled in the art will understand and appreciate these and other aspects, objects and features of the present invention after studying the following specification, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] For a more complete understanding of embodiments of the present invention, reference should be made to the embodiments illustrated in more detail in the accompanying drawings and described below by way of example, in which: Figure 1 Shown is a flow chart of the steps of the bleaching method of titanic acid in some embodiments of the present invention. DETAILED DESCRIPTION

[0022] Embodiments of the present invention are described below. However, it should be understood that the disclosed embodiments are merely examples, and other embodiments may take various alternative forms. The drawings are not necessarily drawn to scale; certain features may be exaggerated or minimized to show the details of a particular component. Therefore, the specific structural and functional details disclosed herein should not be interpreted as limiting, but merely as a representative basis for teaching those skilled in the art to use the present invention in various ways. As will be understood by those skilled in the art, the various features shown and described with reference to any one of the figures may be combined with the features shown in one or more other figures to produce embodiments that are not explicitly shown or described. The combination of features shown provides representative embodiments for typical applications. However, various combinations and modifications of features consistent with the teachings of the present invention may be desirable for certain specific applications or embodiments.

[0023] Furthermore, in the present invention, the terms "comprises," "includes," or any other variations thereof are intended to cover non-exclusive inclusions, such that a process or method comprising a series of elements includes not only those elements, but may also include elements not explicitly listed or inherent to these processes or methods.

[0024] One or more embodiments of the present invention will be described below in conjunction with the accompanying drawings. The flowchart illustrates the process performed by the system according to the present invention. It is understood that the execution of the flowchart does not need to be performed in sequence, one or more steps may be omitted, one or more execution steps may be added, and one or more steps may be performed in sequence or in reverse order, and even in some embodiments, one or more steps may be performed simultaneously.

[0025] The present invention provides a bleaching method of metatitanic acid, such as Figure 1 As shown, the following steps are included: S1. Dilute the titanic acid slurry to 280-350 g / L; S2, add sulfuric acid, and heat the system to 60-70°C while stirring; S3. Add Ti 3+solution and sodium formaldehyde sulfoxylate, stirring and keeping warm, the Fe 3+ Reduction to Fe 2+ ; Among them, S3 Ti 3+ The mass ratio of formaldehyde and sodium bisulfite is (3.5~8):(8~9).

[0026] In the present invention, Ti is added to the metatitanic acid slurry system under acidic conditions. 3+ solution and sodium formaldehyde sulfoxylate to remove Fe 3+ Reduction to Fe 2+ , through Ti 3+ The synergistic effect of two reducing substances, formaldehyde and sodium sulfoxylate, has achieved good bleaching effect. The mass percentage of iron in the pulp after bleaching is ≤0.0050%; the amount of sulfuric acid used in the system can be greatly saved. Compared with the traditional method, it can save more than 50% of sulfuric acid; reduce Ti by nearly 50% 3+ The amount of solution used; the temperature required for the bleaching reaction is reduced to only 60~70℃, which saves production energy consumption.

[0027] In some embodiments, in S1, the titanate slurry is diluted to 280-285 g / L.

[0028] In the production process, in order to ensure the stability of the product, the concentration range of the metatitanic acid slurry is generally limited. If the concentration is too high, it is not suitable for the specifications of the industrial production container (Φ4000×5000, V=62m 3 ), and it will also affect the bleaching effect and energy waste, and will also affect the product bleaching effect. Too low a concentration will reduce production efficiency. The present invention limits the dilution of the metatitanic acid slurry to 280-285 g / L to maximize the balance between production quality and production efficiency.

[0029] In some embodiments, in S2, the concentration of sulfuric acid is 16~60g / L.

[0030] In this field, the role of sulfuric acid is to provide an acidic environment to ensure that Ti 3+ and sodium formaldehyde sulfoxylate, while ensuring the Fe 2+ Ionic state, no precipitation will form. If the sulfuric acid concentration is low, it will affect the Fe 2+ Ion state, forming precipitation or high-valent iron, losing the bleaching effect. In production, it is necessary to ensure that the sulfuric acid concentration in the system is 20~60g / L to make Ti 3+ and sodium formaldehyde sulfoxylate to reduce Fe to the maximum extent 3+ .

[0031] In some embodiments, in S2, the concentration of sulfuric acid is 17~25g / L.

[0032] In the present invention, in order to better achieve the bleaching effect, the concentration of S2 sulfuric acid is limited to 30-50 g / L. 3+ The synergistic effect of sodium formaldehyde sulfoxylate as a reducing agent greatly reduces the amount of sulfuric acid used in the bleaching process. Under the conditions of the specific titanic acid slurry of the present invention, the sulfuric acid concentration in the S2 system is controlled at 30-50 g / L, which can save the amount of sulfuric acid and better play a bleaching role.

[0033] In some embodiments, in S2, the system is heated to 60-65°C.

[0034] In the art, the higher the temperature of the reduction reaction system, the more conducive it is to the reaction. The higher the temperature, the faster the decomposition of sodium formaldehyde sulfoxylate. However, if the temperature is too high, the decomposition rate of sodium formaldehyde sulfoxylate may be too fast, and the decomposed reducing factor will not have time to participate in the reduction reaction, thus causing Fe in the slurry to 3+ In addition, in industrial production, the energy consumption increases after the temperature rises, and the production cost will increase significantly. In order to reduce the production cost, the present invention uses Ti 3+ The synergistic effect with sodium formaldehyde sulfoxylate makes the reduced hydrogen decomposed by sodium formaldehyde sulfoxylate at 60~65℃ meet the production needs, saving production energy consumption, while taking into account production efficiency and reducing production costs.

[0035] In some embodiments, the Ti 3+ The mass ratio of sodium formaldehyde sulfoxylate to the total titanium in the slurry is: 0.35~0.8: 0.8~0.9: 440~500.

[0036] The present invention defines a specific Ti 3+ The mass ratio of sodium formaldehyde sulfoxylate to the mass ratio of the total titanium in the slurry takes into account both the bleaching effect and the saving of sulfuric acid in the system.

[0037] In some embodiments, in S2, the stirring time is 20 to 60 minutes.

[0038] In the present invention, the stirring frequency of S2 is 15-45 rpm / min. In some specific embodiments, the stirring frequency of S2 is 15-20 rpm / min, and the stirring time is 20-30 min.

[0039] In some embodiments, in S3, the stirring and insulation time is 100-130 min.

[0040] In the present invention, the stirring frequency of S3 is 15-45 rpm / min. In some specific embodiments, the stirring frequency of S3 is 15-20 rpm / min.

[0041] In some embodiments, in S3, the titanate slurry Fe 3+ Reduction to Fe 2+ In the system after 3+ The concentration is 0.5~1.5g / L.

[0042] In S3 of the present invention, the metatitanic acid slurry Fe 3+ Reduction to Fe 2+ In the system after 3+ When the concentration is 0.5~1.5g / L, it represents the reducing substances (Ti 3+ and sodium formaldehyde sulfoxylate) is added in an appropriate amount. If it is lower than this range, it proves that the reducing substance is insufficiently added. If it is higher than this range, it proves that the reducing substance is wasted.

[0043] In some embodiments, the step further comprises S4: vacuum filtering and washing the bleached pulp. In some embodiments, the vacuum degree of the vacuum filtration is 0.008-0.009 MPa.

[0044] The scheme of the present invention will be explained below in conjunction with specific embodiments. Those skilled in the art will understand that the following embodiments are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. If no specific techniques or conditions are specified in the embodiments, the techniques or conditions described in the prior art (such as literature, technical manuals) in the art or the disclosed product or method instructions shall be followed.

[0045] In the present invention, the configuration includes steps 1) and 2) in embodiments 1 to 4.

[0046] In the present invention, the 98% sulfuric acid is common sulfuric acid on the market with a mass fraction of 98%.

[0047] Example 1 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3 L of 300-330 g / L metatitanic acid slurry into a 5 L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 280~285g / L; S2: S2-1) Slowly add 35-40 ml of 98% sulfuric acid and stir for 30 minutes (stirring speed is 15-20 rpm / min); S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 60-65°C and stir for 30 minutes (stirring speed is 15-20 rpm / min); S3: Add 70~90g / L Ti successively 3+ Solution 7~8ml, sodium formaldehyde sulfoxylate 0.8~0.9g, stir (stirring speed is 15~20rpm / min) and keep warm for 120min. (At this time, Ti 3+ The concentration is 0.9g / L); S4: The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0048] Example 2 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3 L of 300-330 g / L metatitanic acid slurry into a 5 L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 280~285g / L; S2: S2-1) Slowly add 35-40 ml of 98% sulfuric acid and stir for 30 minutes (stirring speed is 15-20 rpm / min).

[0049] S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 60~65℃ and stir for 30min. (Stirring speed is 15~20rpm / min); S3: 5) Add 70~90g / L Ti successively 3+ Solution 5~6ml, sodium formaldehyde sulfoxylate 0.8~0.9g, stir (stirring speed is 15~20rpm / min) and keep warm for 120min. (At this time, Ti 3+ The concentration is 0.7g / L); S4: The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0050] Example 3 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3L of 350-400g / L metatitanic acid slurry into a 5L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 300~350g / L; S2: S2-1) Slowly add 35-40 ml of 98% sulfuric acid and stir for 20 minutes. (Stirring speed is 15-20 rpm / min); S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 60-70°C and stir for 30 minutes (stirring speed is 15-20 rpm / min); S3: 5) Add 70~90g / L Ti successively 3+ Solution 7~8ml, sodium formaldehyde sulfoxylate 0.8~0.9g, stir (stirring speed is 15~20rpm / min) and keep warm for 120min. (At this time, Ti 3+ The concentration is 0.8g / L); S4: The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0051] Example 4 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3 L of 300-330 g / L metatitanic acid slurry into a 5 L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 280~285g / L; S2: S2-1) Slowly add 35-40 ml of 98% sulfuric acid and stir for 30 minutes. (Stirring speed is 15-20 rpm / min); S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 85°C and stir for 30 minutes (stirring speed is 15~20rpm / min); S3: Add 70~90g / L Ti successively 3+ Solution 7~8ml, sodium formaldehyde sulfoxylate 0.8~0.9g, stir (stirring speed is 15~20rpm / min) and keep warm for 120min. (At this time, Ti 3+ The concentration is 1.0g / L) S4: The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0052] Comparative Example 1 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3 L of 300-330 g / L metatitanic acid slurry into a 5 L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 280~285g / L; S2: S2-1) Slowly add 80~85ml of 98% sulfuric acid and stir for 30 minutes. (Stirring speed is 15~20rpm / min) S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 60-65°C and stir for 30 minutes (stirring speed is 15-20 rpm / min).

[0053] S3: 5) Add 70~90g / L Ti 3+ Solution 17~18ml, stir (stirring speed is 15~20rpm / min) and keep warm for 120min. 3+ The concentration is 1.4g / L); S4: The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0054] Comparative Example 2 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3 L of 300-330 g / L metatitanic acid slurry into a 5 L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 280~285g / L; S2: S2-1) Slowly add 35-40 ml of 98% sulfuric acid and stir for 30 minutes (stirring speed is 15-20 rpm / min); S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 60-65°C and stir for 30 minutes (stirring speed is 15-20 rpm / min); S3: 5) Add 1.9-2.0 g of sodium formaldehyde sulfoxylate, stir and keep warm for 120 min. 3+ The concentration is 0.5g / L); S4: 6) The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0055] Comparative Example 3 A bleaching method of metatitanic acid comprises the following steps: S1: S1-1) Add 3 L of 300-330 g / L metatitanic acid slurry into a 5 L round-bottom flask and start stirring; S1-2) Add 0.4~0.5L of desalted water to dilute the titanate slurry to 280~285g / L; S2: S2-1) Slowly add 35-40 ml of 98% sulfuric acid and stir for 30 minutes. (Stirring speed is 15-20 rpm / min); S2-2) Turn on the electric heating jacket to raise the temperature of the titanate slurry to 60-65°C and stir for 30 minutes (stirring speed is 15-20 rpm / min); S3: Add 70~90g / L Ti successively 3+ Solution 7~8ml, sodium formaldehyde sulfoxylate 0.4~0.5g, stir (stirring speed is 15~20rpm / min) and keep warm for 120min. (At this time, Ti 3+ The concentration is 0.9g / L) S4: The bleached pulp is vacuum filtered and washed with a vacuum degree of 0.008~0.009Mpa.

[0056] The metatitanic acid slurry prepared in the above Examples 1 to 4 and Comparative Examples 1 to 3 was washed and then tested for iron content. The results are shown in Table 1 below.

[0057] Table 1 Comparison test results

[0058] From the results in Table 1, it can be concluded that the iron content of the titanate slurry obtained by the method of the present invention after washing is ≤0.0023%, which is relatively low. At the same time, the method of the present invention can reduce the amount of trivalent titanium solution and sulfuric acid. Compared with the traditional method, it can save more than 50% of sulfuric acid and reduce nearly 50% of Ti 3+ The method of the present invention reduces the temperature required for the bleaching reaction, and the reaction can be carried out at only 60-70°C, which can reduce consumption and cost.

[0059] In summary, the method of the present invention can achieve the required iron content, which is relatively low, and can also reduce the amount of trivalent titanium solution and sulfuric acid, thereby reducing consumption and cost.

[0060] The present invention is intended to illustrate how to use the disclosed technology and various embodiments, and is not intended to limit the scope and spirit to which it is actually directed and equivalent. Moreover, the above description is not intended to be exhaustive of all possibilities or to limit the scope of protection to the precise form disclosed. According to the above teachings, changes and variations are possible. The selected and illustrated embodiments provide the best description of the principles of the technology and its practical application, and enable those skilled in the art to use the disclosed technology for various changes in various conceivable specific applications. Therefore, without substantially departing from the spirit and principles of the technology described herein, various changes and modifications made to the above embodiments are intended to be included in the scope of the present invention.

Claims

1. A bleaching method of metatitanic acid, It is characterized in that The following steps are involved: S1. Prepare the titanate slurry to 280-350 g / L; S2, add sulfuric acid, and heat the system to 60-70°C while stirring; S3. Add Ti 3+ solution and sodium formaldehyde sulfoxylate, stirring and keeping warm, the Fe 3+ Reduction to Fe 2+ ; Among them, S3 Ti 3+ The mass ratio of formaldehyde and sodium bisulfite is (3.5~8):(8~9).

2. The method according to claim 1, It is characterized in that In S1, the titanic acid slurry is diluted to 280-285 g / L.

3. The method according to claim 1, It is characterized in that In the S2, the concentration of sulfuric acid is 16-60 g / L.

4. The method according to claim 1, It is characterized in that In the S2, the sulfuric acid concentration is 17-25 g / L.

5. The method according to claim 1, It is characterized in that In S2, the system is heated to 60-65°C.

6. The method according to claim 1, It is characterized in that The Ti 3+ The mass ratio of sodium formaldehyde sulfoxylate to the total titanium in the slurry is: 0.35~0.8: 0.8~0.9: 440~500.

7. The method according to claim 1, It is characterized in that In S2, the stirring time is 20 to 60 minutes.

8. The method according to claim 1, It is characterized in that In S3, the stirring and heat preservation time is 100-130 min.

9. The method according to claim 1, It is characterized in that In S3, the titanate slurry Fe 3+ Reduction to Fe 2+ In the system after 3+ The concentration is 0.5~1.5g / L.

10. The method according to claim 1, It is characterized in that The step further includes S4: vacuum filtering and washing the bleached pulp.

Images