A preparation method of bamboo natural color pulp fiber and fiber

Abstract

The invention discloses a method for preparing bamboo natural color pulp fiber, comprising: (1) cutting, boiling, washing, screening and purifying bamboo to obtain fine pulp; (2) subjecting the fine pulp in (1) to multiple stages of alkalization, pressing, aging, yellowing, dissolving, filtering and spinning to obtain bamboo natural color pulp fiber; the multiple stages of alkalization include a first stage of alkalization and several subsequent stages of alkalization, the subsequent stage of alkalization uses an alkali solution to countercurrently alkali wash the fine pulp after the first stage of alkalization, and the alkali solution used in the first stage of alkalization includes an alkali agent and a catalyst. The fiber preparation method provided by the invention saves bleaching and papermaking processes, greatly simplifies the production process, and reduces production costs.

Description

Technical Field

[0001] The invention belongs to the technical field of regenerated cellulose fibers, and in particular relates to a preparation method of bamboo natural color pulp fibers and the fibers. Background Art

[0002] Bamboo has the characteristics of fast growth, and it can be used as a forest in three years, so it has the advantage of sustainable development. Bamboo pulp fiber not only has the excellent characteristics of regenerated cellulose, such as excellent moisture absorption and air permeability, easy dyeing, and low static electricity. It also has natural antibacterial and antibacterial functions. Therefore, bamboo is a new type of cellulose raw material worthy of promotion.

[0003] However, in the prior art, there are still many deficiencies in manufacturing cellulose fibers from bamboo:

[0004] 1. The production of bamboo pulp fiber from bamboo usually adopts the common production process, including material preparation, cooking, screening, purification, bleaching, pulping, and then soaking, pressing, crushing, aging, yellowing, dissolving, filtering, and spinning. The production process is cumbersome and the cycle is long;

[0005] 2. In the prior art, the bleaching agent used in the bleaching process reacts violently, which causes certain damage to the structural function of the bamboo itself;

[0006] 3. In the process of preparing bamboo cellulose fibers by the prior art, bamboo pulp fibers prepared by conventional processes have low dry and wet strength and poor physical properties.

[0007] Chinese patent application number 201610944538.7 discloses a method for preparing natural color bamboo cellulose fiber using natural color bamboo fiber pulp. The method uses oxygen-alkali cooking to pulp bamboo and then uses conventional processes to produce bamboo natural color fiber. Before cooking, pretreatment, fiber combing, hot water washing and enzyme treatment are carried out, which has the technical problem of a long and complicated process flow.

[0008] In view of this, the present invention is proposed. Summary of the invention

[0009] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for preparing bamboo pulp fiber, which saves the bleaching and papermaking steps, improves the alkalization process, and adopts a multi-stage countercurrent alkalization method to fully depolymerize cellulose.

[0010] In order to solve the above technical problems, the basic concept of the technical solution adopted by the present invention is:

[0011] The present invention provides a method for preparing bamboo color pulp fiber, comprising:

[0012] (1) The bamboo is cut, steamed, washed, screened and purified to obtain fine pulp;

[0013] (2) subjecting the fine pulp in (1) to multiple stages of alkalization, pressing, aging, yellowing, dissolving, filtering, and spinning to obtain bamboo natural color pulp fiber;

[0014] The multi-stage alkalization pressing includes a first stage alkalization pressing and several subsequent alkalization pressings. The alkali liquor produced by the subsequent alkalization pressing is drained into a first stage alkalization pressing process to realize countercurrent alkali washing of the fine pulp. The alkali liquor used in the first stage alkalization pressing includes an alkali agent and a catalyst.

[0015] In the above scheme, in the traditional regenerated cellulose fiber preparation process, the fibers are often washed with water after alkalization to remove impurities in the fibers. However, due to the high alkali concentration during alkalization, the alkali concentration varies greatly when directly washed with water, making it difficult to wash out the semi-fibers and impurities inside the cellulose, which will affect the washing effect. In order to solve the above problem, the present invention uses countercurrent alkali liquid to wash the first-stage alkalized squeezed product with alkali liquid, which can better wash out the semi-fibers and impurities, and the final water washing reduces the alkali amount and temperature, in order to better meet the production process requirements during yellowing.

[0016] A further solution of the present invention is: the concentration of sodium hydroxide in the alkali solution of the first alkali pressing is 160-260 g / L, and the alkali solution comes from the added fresh alkali solution and the alkali solution obtained by the latter alkali pressing; the added amount of the catalyst is 200-600 ppm, preferably 300-400 ppm; the catalyst is selected from cobalt-containing compounds, and the cobalt-containing compound is selected from at least one of cobalt chloride and cobalt sulfate, preferably cobalt chloride.

[0017] A further solution of the present invention is that the multi-stage alkalization pressing includes a first stage alkalization pressing, a second stage alkalization pressing and a third stage alkalization pressing, the alkali liquor produced by the third stage alkalization pressing is reversely supplied to the second stage alkalization pressing process, and the alkali liquor produced by the second stage alkalization pressing is reversely supplied to the first stage alkalization pressing.

[0018] In the above scheme, the cellulose content in bamboo is generally 40-50%, the pentose content is 20-30%, and the lignin content is about 30%. After bamboo is steamed, its lignin content is significantly reduced, and the pentose content also decreases to a certain extent. At the same time, cellulose is also damaged to a certain extent, and a certain amount of hemicellulose is also contained in the fiber. Under this premise, the main purpose of adopting multi-stage alkalization and squeezing is to make cellulose fully swell, further dissolve hemicellulose while purifying cellulose, so as to improve the combination of cellulose and alkali and reduce the side reaction in the subsequent yellowing reaction.

[0019] In the above scheme, the multi-stage countercurrent alkalization pressing process avoids the waste of alkali solution after each stage of pressing operation, so that it can be reversely supplied to the previous stage of alkalization pressing process, which can save water supply costs by reducing the supply of fresh alkali solution; on the other hand, the multi-stage countercurrent alkalization pressing can further increase the bound alkali in the cellulose fine pulp. The multi-stage countercurrent alkalization process can be carried out continuously in a specific device, or a plurality of independent alkalization pressing devices can be used in combination. The first, second and third stages of alkalization pressing are arranged along the direction of material flow, wherein the alkali solution of the first stage of alkalization pressing process comes from the supplemented fresh alkali solution and the alkali solution obtained by the second stage of alkalization pressing, the alkali solution of the second stage of alkalization pressing process comes from the supplemented fresh alkali solution and the alkali solution obtained by the third stage of alkalization pressing, and the alkali solution of the third stage of alkalization pressing process all comes from fresh alkali solution. Therefore, the alkali solution after each stage of pressing is driven by a certain power to flow back to the previous stage of alkalization process in the opposite direction of the material flow direction, thereby realizing multi-stage countercurrent alkalization.

[0020] A further scheme of the present invention is: the temperature of the first alkali pressing and the second alkali pressing is 50-100°C, the time is 30-90 min, and the alkali concentration in the alkali solution used is the same; preferably, the temperature of the first alkali pressing and the second alkali pressing is 60-80°C, the time is 45-60 min, and the concentration of the alkali solution used is 200-220 g / L.

[0021] A further solution of the present invention is: the temperature of the three-stage alkalization is 30-60°C, and the concentration of the alkali solution used is 140-200 g / L; preferably, the temperature of the three-stage alkalization is 40-50°C, and the concentration of the alkali solution used is 160-180 g / L.

[0022] A further solution of the present invention is: in step (2), the cellulose fine pulp after multi-stage alkalization and pressing has a methyl cellulose content of 30-32%, a viscosity of 5-6 mPa·s, an alkali content of 18-22%, a combined alkali content of 12-17%, and a semi-fiber content in the alkali solution after three-stage alkalization and pressing of no more than 16 g / L.

[0023] In the above scheme, the bound alkali of alkali cellulose in the traditional process is generally in the range of 8-10%. The present invention adopts multi-stage countercurrent alkalization treatment to make the bound alkali content in the prepared cellulose higher. The higher the bound alkali in the cellulose, the more conducive it is to the subsequent yellowing in the positive reaction direction, so that carbon disulfide and cellulose react to generate more cellulose sulfonate, reducing the occurrence of side reactions.

[0024] A further embodiment of the present invention is as follows: in step (2), the amount of carbon disulfide added in the xantholysis step is 30-35% of the methyl cellulose content in the cellulose fine pulp.

[0025] In the above scheme, in the yellowing process, carbon disulfide not only reacts with alkali cellulose, but also reacts with free alkali to produce toxic gas hydrogen sulfide. In order to avoid the generation of excess harmful substances, the present invention further increases the bound alkali content through a multi-stage countercurrent alkalization process, which not only reduces the amount of carbon disulfide added, but also reduces the emission of harmful gas hydrogen sulfide.

[0026] A further solution of the present invention is: in step (1), the steaming comprises steaming the cut bamboo with alkali solution, and the viscosity of the solution after steaming is controlled to be 6 to 15 mPa·s, preferably 8 to 10 mPa·s; more preferably, the steaming adopts a pre-hydrolysis sulfate method.

[0027] In the above scheme, the fiber is bleached in the traditional process mainly to improve the whiteness of the fiber and further adjust the degree of polymerization of the fiber to meet the production needs. In the scheme provided by the present invention, since the bleaching process is omitted, in order to ensure that the degree of polymerization of the fiber meets the production requirements of viscose fiber, the viscosity after cooking needs to be adjusted to a specific range.

[0028] A further solution of the present invention is that the bamboo material in step (1) is selected from one or more of Phyllostachys chinensis, Phyllostachys pubescens, and Phyllostachys chinensis.

[0029] The preparation method provided by the present invention specifically comprises the following steps:

[0030] (1) cutting, steaming, washing, screening and purifying bamboo to obtain fine pulp, wherein the viscosity of the solution after steaming is 6 to 15 mPa·s;

[0031] (2) subjecting the fine pulp in (1) to a multi-stage alkalization treatment, wherein the multi-stage alkalization treatment includes a first-stage alkalization pressing, a second-stage alkalization pressing and a third-stage alkalization pressing, wherein the concentration of sodium hydroxide in the alkali solution of the first-stage alkalization pressing is 160 to 260 g / L, the amount of catalyst added is 200 to 600 ppm, and no catalyst is added in the second-stage alkalization pressing and the third-stage alkalization pressing; the alkali solution concentration used in the first-stage alkalization pressing and the second-stage alkalization pressing is the same, the alkalization temperature is 50 to 100° C., and the time is 30 to 90 min; the alkali solution of the third-stage alkalization pressing is 100 to 200 g / L, and the catalyst added is 200 to 600 ppm. The concentration of the alkali solution is 140-200 g / L, and the alkalization temperature is 30-60°C; the cellulose fine pulp obtained by multi-stage alkalization pressing has a methylcellulose content of 30-32%, a viscosity of 5-6 mPa·s, an alkali content of 18-22%, a combined alkali content of 12-17%, and a semi-fiber content of no more than 16 g / L in the alkali solution after three-stage alkalization pressing; subsequently, the cellulose fine pulp is aged, yellowed, dissolved, filtered, and spun to obtain bamboo natural color pulp fiber; the amount of carbon disulfide added in the yellowing step is 30-35%.

[0032] The present invention also provides a bamboo natural color pulp fiber prepared by the preparation method as described above, wherein the bamboo natural color pulp fiber is khaki in color, has a dry breaking strength of not less than 2.4 cN / dtex, a dry strength variation coefficient of not more than 14.0%, a wet breaking strength of not less than 1.3 cN / dtex, and a whiteness of 15 to 30%.

[0033] After adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art:

[0034] 1. Compared with conventional production processes, the present invention omits the bleaching and papermaking processes, combines alkali treatment and impregnation through multi-stage alkalization, shortens the process flow, and reduces production costs;

[0035] 2. In the preparation method provided by the present invention, the binding alkali of cellulose is high, which reduces the amount of carbon disulfide used in the yellowing process and greatly reduces the emission of harmful gas hydrogen sulfide;

[0036] 3. The preparation method provided by the present invention adopts alkaline solution for countercurrent washing, which can reuse the alkaline solution, reduce water consumption, and reduce production costs;

[0037] 4. In the preparation method provided by the present invention, the fibers are not bleached, so the degree of damage to the cellulose is reduced and the mechanical strength of the fiber products is improved.

[0038] The specific embodiments of the present invention are described in further detail below. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below. The following embodiments are used to illustrate the present invention but are not used to limit the scope of the present invention.

[0040] Example 1

[0041] In this embodiment, bamboo and moso bamboo are used as bamboo materials, and the following method is used to prepare bamboo pulp fiber:

[0042] (1) cutting, steaming, washing, screening and purifying bamboo to obtain fine pulp, wherein the viscosity of the solution after steaming is 8 mPa·s;

[0043] (2) subjecting the fine pulp in (1) to a multi-stage alkalization treatment, wherein the multi-stage alkalization treatment includes a first-stage alkalization pressing, a second-stage alkalization pressing and a third-stage alkalization pressing, wherein the concentration of sodium hydroxide in the alkali solution of the first-stage alkalization pressing is 210 g / L, the amount of catalyst cobalt chloride added is 400 ppm, and no catalyst is added in the second-stage alkalization pressing and the third-stage alkalization pressing; the alkali solution concentrations used in the first-stage alkalization pressing and the second-stage alkalization pressing are the same, the first-stage alkalization temperature is 80° C., the time is 45 min, and the second-stage alkalization temperature is 60° C., The time is 45 minutes; the alkali solution concentration used in the three-stage alkalization pressing is 180g / L, and the alkalization temperature is 40°C; the cellulose fine pulp obtained by multi-stage alkalization pressing has a methyl cellulose content of 30%, a viscosity of 5mPa·s, an alkali content of 20%, a combined alkali content of 15%, and a semi-fiber content in the alkali solution after the three-stage alkalization pressing is not more than 15g / L; subsequently, the cellulose fine pulp is aged, yellowed, dissolved, filtered, and spun to obtain bamboo natural color pulp fiber; the amount of carbon disulfide added in the yellowing step is 31%.

[0044] Example 2

[0045] In this embodiment, moso bamboo is used as bamboo material, and bamboo pulp fiber is prepared by the following method:

[0046] (1) cutting, steaming, washing, screening and purifying bamboo to obtain fine pulp, wherein the viscosity of the solution after steaming is 10 mPa·s;

[0047] (2) subjecting the fine pulp in (1) to a multi-stage alkalization treatment, wherein the multi-stage alkalization treatment includes a first-stage alkalization pressing, a second-stage alkalization pressing and a third-stage alkalization pressing, wherein the concentration of sodium hydroxide in the alkali solution of the first-stage alkalization pressing is 180 g / L, the amount of cobalt chloride catalyst added is 300 ppm, and no catalyst is added in the second-stage alkalization pressing and the third-stage alkalization pressing; the alkali solution concentrations used in the first-stage alkalization pressing and the second-stage alkalization pressing are the same, the first-stage alkalization temperature is 60° C., the time is 60 min, and the second-stage alkalization temperature is 80° C., The time is 60 minutes; the alkali solution concentration used in the three-stage alkalization pressing is 160g / L, and the alkalization temperature is 50°C; the cellulose fine pulp obtained by multi-stage alkalization pressing has a methyl cellulose content of 31%, a viscosity of 6mPa·s, an alkali content of 18%, a combined alkali content of 12%, and a semi-fiber content in the alkali solution after the three-stage alkalization pressing is not more than 16g / L; subsequently, the cellulose fine pulp is aged, yellowed, dissolved, filtered, and spun to obtain bamboo natural color pulp fiber; the amount of carbon disulfide added in the yellowing step is 35%.

[0048] Example 3

[0049] In this embodiment, green bamboo is used as bamboo material, and bamboo pulp fiber is prepared by the following method:

[0050] (1) cutting, steaming, washing, screening and purifying bamboo to obtain fine pulp, wherein the viscosity of the solution after steaming is 6 mPa·s;

[0051] (2) subjecting the fine pulp in (1) to a multi-stage alkalization treatment, wherein the multi-stage alkalization treatment includes a first-stage alkalization pressing, a second-stage alkalization pressing and a third-stage alkalization pressing, wherein the concentration of sodium hydroxide in the alkali solution of the first-stage alkalization pressing is 160 g / L, the amount of catalyst cobalt sulfate added is 600 ppm, and no catalyst is added in the second-stage alkalization pressing and the third-stage alkalization pressing; the alkali solution concentrations used in the first-stage alkalization pressing and the second-stage alkalization pressing are the same, the first-stage alkalization temperature is 100° C., the time is 30 min, and the second-stage alkalization temperature is 50° C., The time is 90 minutes; the alkali solution concentration used in the three-stage alkalization pressing is 140g / L, and the alkalization temperature is 60°C; the cellulose fine pulp obtained by the multi-stage alkalization pressing has a methyl cellulose content of 32%, a viscosity of 6mPa·s, an alkali content of 20%, a combined alkali content of 16%, and a semi-fiber content in the alkali solution after the three-stage alkalization pressing is not more than 14g / L; subsequently, the cellulose fine pulp is aged, yellowed, dissolved, filtered, and spun to obtain bamboo natural color pulp fiber; the amount of carbon disulfide added in the yellowing step is 32%.

[0052] Example 4

[0053] In this embodiment, moso bamboo and bamboo of green bamboo are used as bamboo materials, and bamboo pulp fiber is prepared by the following method:

[0054] (1) cutting, steaming, washing, screening and purifying bamboo to obtain fine pulp, wherein the viscosity of the solution after steaming is 15 mPa·s;

[0055] (2) subjecting the fine pulp in (1) to a multi-stage alkalization treatment, wherein the multi-stage alkalization treatment includes a first-stage alkalization pressing, a second-stage alkalization pressing and a third-stage alkalization pressing, wherein the concentration of sodium hydroxide in the alkali solution of the first-stage alkalization pressing is 260 g / L, the amount of cobalt sulfate catalyst added is 200 ppm, and no catalyst is added in the second-stage alkalization pressing and the third-stage alkalization pressing; the alkali solution concentrations used in the first-stage alkalization pressing and the second-stage alkalization pressing are the same, the first-stage alkalization temperature is 50° C., the time is 90 min, and the second-stage alkalization temperature is 100° C., The time is 30 minutes; the alkali solution concentration used in the three-stage alkali pressing is 200g / L, and the alkalization temperature is 30°C; the cellulose fine pulp obtained by the multi-stage alkalization pressing has a methyl cellulose content of 31%, a viscosity of 5mPa·s, an alkali content of 22%, a combined alkali content of 17%, and a semi-fiber content in the alkali solution after the three-stage alkalization pressing is not more than 13g / L; subsequently, the cellulose fine pulp is aged, yellowed, dissolved, filtered, and spun to obtain bamboo natural color pulp fiber; the amount of carbon disulfide added in the yellowing step is 30%.

[0056] Example 5

[0057] In this embodiment, moso bamboo and cycad bamboo are used as bamboo materials, and the following method is used to prepare bamboo pulp fiber:

[0058] (1) cutting, steaming, washing, screening and purifying bamboo to obtain fine pulp, wherein the viscosity of the solution after steaming is 9 mPa·s;

[0059] (2) subjecting the fine pulp in (1) to a multi-stage alkalization treatment, wherein the multi-stage alkalization treatment includes a first-stage alkalization pressing, a second-stage alkalization pressing and a third-stage alkalization pressing, wherein the concentration of sodium hydroxide in the alkali solution of the first-stage alkalization pressing is 210 g / L, the amount of catalyst cobalt chloride added is 350 ppm, and no catalyst is added in the second-stage alkalization pressing and the third-stage alkalization pressing; the alkali solution concentrations used in the first-stage alkalization pressing and the second-stage alkalization pressing are the same, the first-stage alkalization temperature is 75° C., the time is 50 min, and the second-stage alkalization temperature is 70° C., The time is 50 minutes; the alkali solution concentration used in the three-stage alkali pressing is 170g / L, and the alkalization temperature is 45°C; the cellulose fine pulp obtained by multi-stage alkalization pressing has a methyl cellulose content of 31%, a viscosity of 5mPa·s, an alkali content of 19%, a combined alkali content of 16%, and a semi-fiber content in the alkali solution after the three-stage alkalization pressing is not more than 13g / L; subsequently, the cellulose fine pulp is aged, yellowed, dissolved, filtered, and spun to obtain bamboo natural color pulp fiber; the amount of carbon disulfide added in the yellowing step is 30%.

[0060] Comparative Example 1

[0061] This comparative example is based on Example 5, and the three-stage alkalization pressing in step (1) is adjusted to two-stage alkalization pressing, that is, the third alkalization pressing step is removed and the first two alkalization pressing processes are retained. The semi-fiber content in the alkali solution after alkalization pressing is greater than 18 g / L. The other implementation methods of this comparative example are the same as Example 5.

[0062] Comparative Example 2

[0063] This comparative example is based on Example 5, and the alkalization and squeezing in step (1) is adjusted to a single process, and the specific step (2) is adjusted as follows: the fine pulp in (1) is alkalized and then squeezed, the concentration of sodium hydroxide in the alkali solution used for alkalization is 210g / L, the amount of cobalt chloride catalyst added is 350ppm, the alkalization temperature is 75°C, the time is 130min (the time here includes the total alkalization time of multiple alkalizations in Example 5), and then washed with water; the semi-fiber content in the alkali solution after alkalization and squeezing is greater than 20g / L;

[0064] The other implementations of this comparative example are the same as those of Example 5.

[0065] Comparative Example 3

[0066] This comparative example is based on Example 5, except that the catalyst added in step (1) is removed. The other implementation methods of this comparative example are the same as those of Example 5.

[0067] Experimental Example 1

[0068] In this experimental example, the dry breaking strength, dry strength variation coefficient, wet breaking strength and whiteness of the regenerated cellulose fibers prepared in Examples 1 to 5 and Comparative Examples 1 to 3 were measured, and the results are shown in the following table:

[0069]

[0070] It can be seen from the above table that the regenerated cellulose fibers provided in Examples 1 to 5 of the present invention are treated with multi-stage countercurrent alkalization and pressing, which reduces the waste of pressing alkali solution in the process and increases the bound alkali in the cellulose fine pulp. On the one hand, the bound alkali of the prepared alkali cellulose is higher than the bound alkali level of normal alkali cellulose, which is more conducive to the reaction of carbon disulfide and alkali cellulose in the subsequent yellowing process, reduces side reactions and improves the uniformity of the prepared fibers, making them have good mechanical properties. On the other hand, impurities and semi-fibers inside the fibers are effectively washed out, making the intermediate products more conducive to the needs of subsequent processes.

[0071] Among them, Comparative Example 1 is based on Example 5, and the third stage of the three-stage alkalization pressing is deleted and adjusted to two-stage alkalization pressing. Due to the lack of alkali solution reflux and sufficient alkalization process of the three-stage alkalization pressing, the degradation of hemicellulose is adversely affected, and the subsequent yellowing reaction also produces more side reactions, affecting the uniformity and related mechanical properties of the fiber. Although the fiber obtained in Comparative Example 1 has certain mechanical properties, it still cannot meet the higher product requirements. Comparative Example 2 does not adopt a multi-stage alkalization pressing method, but uses the method in the prior art to wash the cellulose after alkalization pressing. Due to the large variation range of alkali concentration, the hemicellulose and impurities inside the cellulose are not easy to wash out, which affects the subsequent process, making the performance of the fiber obtained further weaker than that of Comparative Example 1; while Comparative Example 3 does not add a catalyst, and the performance parameters of the obtained fiber are far inferior to those of Example 5, indicating that the catalyst plays a more important role in adjusting the fiber performance.

[0072] The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment, it is not used to limit the present invention. Any technician familiar with this patent can make some changes or modify the technical contents suggested above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the solution of the present invention.

Claims

1. A method for preparing bamboo pulp fiber, characterized in that: include: (1) bamboo is cut, boiled, washed, screened and purified to obtain fine pulp, wherein the boiling comprises boiling the cut bamboo with alkali solution, and the viscosity of the solution after boiling is controlled to be 6 to 15 mPa·s; (2) subjecting the fine pulp in (1) to multiple stages of alkalization, pressing, aging, yellowing, dissolving, filtering, and spinning to obtain bamboo natural color pulp fiber; The multi-stage alkalization pressing comprises a first stage alkalization pressing, a second stage alkalization pressing and a third stage alkalization pressing, the alkali solution produced by the third stage alkalization pressing is reversely supplied to the second stage alkalization pressing process, and the alkali solution produced by the second stage alkalization pressing is reversely supplied to the first stage alkalization pressing to realize countercurrent alkali washing of the fine pulp, and the alkali solution used in the first stage alkalization pressing comprises an alkali agent and a catalyst; The concentration of sodium hydroxide in the alkali solution of the first alkali pressing is 160-260 g / L, the amount of catalyst added is 200-600 ppm, and no catalyst is added in the second alkali pressing and the third alkali pressing; the concentration of the alkali solution used in the first alkali pressing and the second alkali pressing is the same, the temperature of the alkali pressing is 50-100° C., and the time is 30-90 min; the concentration of the alkali solution used in the third alkali pressing is 140-200 g / L, and the temperature of the alkali pressing is 30-60° C.; After the multi-stage alkalization and squeezing treatment, the cellulose fine pulp has a cellulose alpha content of 30-32%, a viscosity of 5-6 mPa·s, and an alkali content of 18-22%. The combined alkali content of the cellulose fine pulp after the three-stage alkalization and pressing treatment is 12-17%, and the semi-fiber content in the alkali solution after the three-stage alkalization and pressing treatment is not more than 16g / L.

2. The method for preparing bamboo pulp fiber according to claim 1, characterized in that: The alkali solution of the first-stage alkalization pressing comes from the added fresh alkali solution and the alkali solution obtained from the second-stage alkalization pressing; the catalyst is selected from cobalt-containing compounds, and the cobalt-containing compound is selected from at least one of cobalt chloride and cobalt sulfate.

3. The method for preparing bamboo pulp fiber according to claim 2, characterized in that: The added amount of the catalyst is 300-400 ppm.

4. The method for preparing bamboo pulp fiber according to claim 2, characterized in that: The catalyst is cobalt chloride.

5. The method for preparing bamboo pulp fiber according to any one of claims 1 to 4, characterized in that: The temperature of the first-stage alkalization pressing and the second-stage alkalization pressing is 60-80°C, the time is 45-60min, and the concentration of the alkali solution used is 200-220g / L.

6. The method for preparing bamboo pulp fiber according to any one of claims 1 to 4, characterized in that: The temperature of the three-stage alkalization pressing is 40-50°C, and the concentration of the alkali solution used is 160-180g / L.

7. The method for preparing bamboo pulp fiber according to any one of claims 1 to 4, characterized in that: In step (2), the amount of carbon disulfide added in the yellowing step is 30-35% of the methyl cellulose content in the cellulose fine pulp.

8. The method for preparing bamboo pulp fiber according to any one of claims 1 to 4, characterized in that: The viscosity of the solution after cooking is controlled to be 8-10 mPa·s.

9. The method for preparing bamboo pulp fiber according to any one of claims 1 to 4, characterized in that: The cooking adopts a pre-hydrolysis sulfate method.

10. The method for preparing bamboo pulp fiber according to any one of claims 1 to 4, characterized in that: The bamboo material described in step (1) is selected from one or more of Phyllostachys edulis, Phyllostachys moso and Phyllostachys chinensis.

11. A bamboo pulp fiber prepared by the preparation method according to any one of claims 1 to 10, characterized in that: The bamboo pulp fiber is khaki in color, has a dry breaking strength of not less than 2.4 cN / dtex, a dry strength variation coefficient of not more than 14.0%, a wet breaking strength of not less than 1.3 cN / dtex, and a whiteness of 15-30%.

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