Method for removing metal ions in dissolving pulp and dissolving pulp

Abstract

The invention belongs to the technical field of dissolving pulp preparation, and discloses a method for removing metal ions in dissolving pulp and the dissolving pulp. According to the invention, a specifically prepared organic acid pickling solution is adopted to carry out pickling complexing treatment on the slurry. After treatment, the removal effect of metal ions in the slurry is remarkable, and the following index requirements are met: the content of iron ions is less than or equal to 5mg / kg, the content of calcium ions is less than or equal to 30mg / kg, the content of magnesium ions is less than or equal to 20mg / kg, and the content of copper ions is less than or equal to 5mg / kg. The method particularly has an excellent removal effect on firmly combined iron ions, the highest removal rate reaches 89.9%, and the content of the iron ions is reduced from higher than 45 mg / kg to lower than 5 mg / kg, so that the high-purity slurry is obtained. The obtained slurry is beneficial to subsequent dissolution and spinning processes, and the pickling process has slight influence on the polymerization degree of the slurry and can be maintained within a reasonable range.

Description

Technical Field

[0001] This invention relates to the field of dissolving pulp preparation technology, and in particular to a method for removing metal ions from dissolving pulp and a dissolving pulp. Background Technology

[0002] Dissolving pulp is a special chemical pulp with high-purity α-cellulose, widely used in the production of cellulose derivatives such as cellulose acetate, cellulose nitrate, viscose fiber, rayon, methylcellulose, and carboxymethylcellulose. During its preparation, especially when used in the manufacture of lyocell fiber, the requirements for ash content and metal ion content (such as iron, calcium, magnesium, and copper) are extremely strict, typically requiring a total ash content of less than 0.1%.

[0003] Ash is the mineral residue remaining after plant fiber raw materials are burned in a muffle furnace at 575°C or 725°C. Its main components include silicon dioxide, iron, copper, calcium, magnesium, potassium, and other metal ions. In dissolving pulp, ash and metal ions (such as iron and copper) not only interfere with the bleaching process but are also difficult to dissolve in NMMO, affecting spinning quality. Furthermore, iron and copper ions catalyze the decomposition of NMMO during dissolution, reducing the stability of the solution system and potentially leading to thermal explosions in severe cases. Currently, methods for removing ash and metal ions mainly include acid washing, chelating agent treatment, and acid washing combined with chelating treatment (patent application 201710597684.1). These methods rely on the H+ in the acid. + Ion replacement of metal ions and the formation of chelates with metal ions by chelating agents to achieve a shielding effect. Commonly used acids include inorganic acids such as sulfuric acid, hydrochloric acid, and phosphoric acid, and commonly used chelating agents include EDTA, DTPA, and SMHP. However, traditional inorganic acid treatment sacrifices the degree of polymerization of the slurry (patent application 201710650004.8), while conventional organic acids such as formic acid and acetic acid are difficult to reduce the metal ion content to the required level (patent application 201810574230.7). Iron ions are the most stubbornly bound metal ions, and traditional removal methods only achieve a removal rate of about 30% for total iron; even with increased acid dosage, the removal rate only reaches 60-70%, making it difficult to reduce the content to below 5 mg / kg (patent applications 202310832056.2, 201310320265.5).

[0004] To address the challenges of ash and metal ion removal, it is crucial to develop a method that efficiently removes metal ions while maintaining the degree of polymerization of the slurry within a suitable range. Summary of the Invention

[0005] The purpose of this invention is to provide a method for removing metal ions from dissolved pulp and a dissolved pulp, so as to solve the above-mentioned problems existing in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for removing metal ions from dissolved pulp, comprising the following steps: (1) Prepare pickling solution using organic acids; (2) The pickling solution and slurry are mixed and pickled together, and the mixture is kneaded regularly during the pickling and complexing process; (3) After the pickling and complexation are completed, remove the pickling solution, wash the slurry, and obtain the dissolved slurry; The organic acid in step (1) includes one or more of citric acid, oxalic acid, phytic acid, malic acid, lactic acid, and tartaric acid.

[0007] Furthermore, in the method, when the organic acid in step (1) includes two types, the mass ratio of the two organic acids is 1~3:1~3; Alternatively, when the organic acid in step (1) includes three types, the mass ratio of the three organic acids is 1~2:1~2:1~2.

[0008] Furthermore, in the method, the mass ratio of organic acid in the pickling solution to the dry basis of the slurry in step (2) is 1~20:100.

[0009] Furthermore, in the method, the organic acid in step (1) includes a mixture of phytic acid, oxalic acid and citric acid; or, the organic acid in step (1) includes a mixture of tartaric acid and lactic acid. The mass ratio of organic acid in the pickling solution to the dry basis of the slurry in step (2) is 8~10:100.

[0010] Furthermore, in the method, the slurry in step (2) is bleached bamboo pulp after being screened through a 50-200 mesh screen.

[0011] Furthermore, in the method, the concentration of the slurry after mixing the pickling solution and the slurry in step (2) is 1~6wt%.

[0012] Furthermore, in the method, the conditions for acid washing complexation in step (2) include: acid washing temperature of 40~100℃ and acid washing time of 20~180min.

[0013] Furthermore, in the method, the kneading in step (2) specifically refers to kneading every 5 to 10 minutes during the acid washing and complexing process.

[0014] Furthermore, in the method, the washing conditions in step (3) include: washing with deionized water, and washing 10 to 20 times.

[0015] The present invention also provides a dissolving slurry.

[0016] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: H + The pickling solution, a mixture of phytic acid, oxalic acid, and citric acid, displaces metal ions bound to the slurry. This solution has a low pH and numerous binding sites on the phytic, citric, and oxalic acids. The displaced metal ions are then complexed by the organic acids, preventing them from re-binding to the slurry and achieving a shielding effect. After this organic acid pickling and complexation process, the removal of metal ions from the slurry is significantly improved, meeting the following requirements: iron ion content ≤ 5 mg / kg, calcium ion content ≤ 30 mg / kg, magnesium ion content ≤ 20 mg / kg, and copper ion content ≤ 5 mg / kg. This method is particularly effective at removing firmly bound iron ions, achieving a maximum removal rate of 89.9%, reducing the iron ion content from a high initial level (>45 mg / kg) to below 5 mg / kg, thus obtaining a high-purity slurry. Detailed Implementation

[0017] This invention provides a method for removing metal ions from dissolved pulp, comprising the following steps: (1) Prepare pickling solution using organic acids; (2) The pickling solution and slurry are mixed and pickled together, and the mixture is kneaded regularly during the pickling and complexing process; (3) After the pickling and complexation are completed, remove the pickling solution, wash the slurry, and obtain the dissolved slurry; The organic acid in step (1) includes one or more of citric acid, oxalic acid, phytic acid, malic acid, lactic acid, and tartaric acid.

[0018] In this invention, the organic acid in step (1) preferably includes a variety of citric acid, oxalic acid, phytic acid, malic acid, lactic acid, and tartaric acid, more preferably includes a variety of citric acid, oxalic acid, phytic acid, lactic acid, and tartaric acid, and more preferably is a mixture of phytic acid, oxalic acid, and citric acid or a mixture of tartaric acid and lactic acid.

[0019] In this invention, when the organic acid in step (1) includes two types, the mass ratio of the two organic acids is preferably 1~3:1~3, more preferably 2~3:1~2, and even more preferably 3:1; Alternatively, when the organic acid in step (1) includes three types, the mass ratio of the three organic acids is preferably 1~2:1~2:1~2, more preferably 1.5~2:1~1.5:1~1.5, and even more preferably 2:1:1.

[0020] In the present invention, in a preferred embodiment, when the organic acid in step (1) preferably includes multiple types, the mass ratio of phytic acid, oxalic acid and citric acid is 2:1:1; and the mass ratio of tartaric acid and lactic acid is 3:1.

[0021] In this invention, the mass ratio of organic acid in the pickling solution to dry slurry in step (2) is preferably 1~20:100, more preferably 8~15:100, and even more preferably 8~10:100.

[0022] In this invention, the pulp in step (2) is preferably bleached bamboo pulp after being screened through a 50-200 mesh screen, more preferably 150-200 mesh, and even more preferably 200 mesh.

[0023] In this invention, the source of the bleached bamboo pulp is not limited, and any product or method well known in the art can be used.

[0024] In this invention, the screening device is preferably a Bauer-McNett fiber sieve.

[0025] In this invention, the slurry concentration after mixing the pickling solution and slurry in step (2) is preferably 1~6wt%, more preferably 3~5wt%, and even more preferably 4wt%.

[0026] In this invention, the conditions for acid washing complexation in step (2) include: the acid washing temperature is preferably 40~100℃, more preferably 60~100℃, and even more preferably 80℃; the acid washing time is preferably 20~180min, more preferably 120~180min, and even more preferably 150min.

[0027] In this invention, the kneading in step (2) is preferably performed every 5 to 10 minutes during the acid washing and complexing process, more preferably every 5 to 8 minutes, and even more preferably every 5 minutes.

[0028] In this invention, the mixing of pickling solution and slurry, the pickling complexation, and the kneading in step (2) are preferably carried out in a polytetrafluoroethylene bag.

[0029] In this invention, the washing conditions in step (3) include: preferably washing with deionized water; the number of washing cycles is preferably 10 to 20 times, more preferably 10 to 15 times, and even more preferably 10 times.

[0030] The present invention also provides a dissolving slurry.

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] In the embodiments, reagents or instruments whose manufacturers are not specified are considered to be conventional products that can be purchased commercially.

[0033] The bleached bamboo pulp was supplied by a company in Yibin.

[0034] Example 1

[0035] This embodiment provides a method for removing metal ions from dissolved pulp, including the following steps: (1) Citric acid was selected as the pickling agent, and a pickling solution with an acid dosage of 1 wt% of the dry basis of the slurry was prepared; (2) Bleached bamboo pulp after screening with a 200-mesh Bauer-McNett fiber sieve was mixed with pickling solution and pulp at a mixed pulp concentration of 4wt% in a polytetrafluoroethylene bag and reacted at 60℃ for 60min, kneading once every 5min to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing liquid and wash it 10 times with deionized water to obtain the dissolving slurry.

[0036] Example 2

[0037] Based on Example 1, only the type of pickling agent was changed to oxalic acid, while all other conditions remained exactly the same as in Example 1, resulting in Example 2.

[0038] Example 3

[0039] Based on Example 1, only the type of pickling agent was changed to phytic acid, while all other conditions were exactly the same as in Example 1, resulting in Example 3.

[0040] Example 4

[0041] Based on Example 1, only the type of pickling agent was changed to malic acid, while all other conditions remained exactly the same as in Example 1, resulting in Example 4.

[0042] Example 5

[0043] Based on Example 1, only the type of pickling agent was changed to lactic acid, while all other conditions remained exactly the same as in Example 1, resulting in Example 5.

[0044] Example 6

[0045] Based on Example 1, only the type of pickling agent was changed to tartaric acid, while all other conditions remained exactly the same as in Example 1, resulting in Example 6.

[0046] Test methods

[0047] Determination of metal ion content: First, ash content was prepared according to GB / T 742-2018 "Determination of Ash Content in Papermaking Raw Materials, Pulp, Paper and Paperboard Residues". Then, the crucible containing the ash content was placed on a 90℃ heating plate, and 5 mL of 6 mol / L hydrochloric acid solution was added and heated until nearly dry. This acid addition and heating operation was repeated twice. After the third acid addition, the heating was continued for 5 min. After that, the crucible was removed, and the solution was quantitatively transferred to a 50 mL volumetric flask. The crucible was washed several times with deionized water, and the washing liquid was transferred to the volumetric flask and diluted to 50 mL. Finally, an appropriate amount of the diluted acid solution was taken and tested using the inductively coupled plasma optical emission spectrometry (ICP-OES) standard curve to calculate the metal ion content.

[0048] Degree of polymerization test: The intrinsic viscosity η of the pulp was determined according to GB / T 1548-2016 "Determination of intrinsic viscosity value in copper ethylenediamine solution of pulp", and converted into degree of polymerization according to Formula 1.

[0049] Formula 1

[0050] The above-described testing methods were used to test the dissolving slurries of Examples 1-6 before treatment, and the results are shown in Table 1.

[0051] Table 1. Test results of dissolving slurry in Examples 1-6 before treatment

[0052] Example 7

[0053] (1) Select a mixed acid solution of oxalic acid and citric acid as pickling agent. The mass ratio of oxalic acid to citric acid is 1:3. Prepare a pickling solution with an acid dosage of 10wt% of the dry basis of the slurry. (2) After sieving with a 200-mesh Bauer-McNett fiber sieve, the bleached bamboo pulp was mixed with the pickling solution and pulp at a mixed pulp concentration of 1wt% in a polytetrafluoroethylene bag and reacted at 40℃ for 20 minutes. The mixture was kneaded every 5 minutes to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing liquid and wash it 10 times with deionized water to obtain the dissolving slurry.

[0054] Example 8

[0055] Based on Example 7, only the amount of acid was changed to 20 wt% of the dry slurry, while other conditions were exactly the same as in Example 4, resulting in Example 8.

[0056] Example 9

[0057] (1) Select a mixture of phytic acid and citric acid as pickling agent. The mass ratio of phytic acid to citric acid is 2:2. Prepare a pickling solution with an acid dosage of 10wt% of the dry basis of the slurry. (2) Bleached bamboo pulp after screening with a 200-mesh Bauer-McNett fiber sieve was mixed with pickling solution and pulp in a polytetrafluoroethylene bag at a mixed pulp concentration of 6wt%, and reacted at 100℃ for 180 min, kneading once every 10 min to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing solution and wash it 20 times with deionized water to obtain the dissolving slurry.

[0058] The dissolving slurries of Examples 7-9 were tested using the above test methods, and the results are shown in Table 2.

[0059] Table 2. Test results of dissolving slurry in Examples 7-9 before treatment

[0060] Example 10

[0061] (1) Select a mixture of tartaric acid and lactic acid as pickling agent, with a mass ratio of tartaric acid to lactic acid of 3:1, and prepare a pickling solution with an acid dosage of 10wt% of the dry basis of the slurry. (2) Bleached bamboo pulp after screening with a 200-mesh Bauer-McNett fiber sieve was mixed with pickling solution and pulp at a mixed pulp concentration of 4wt% in a polytetrafluoroethylene bag and reacted at 70℃ for 120min. The mixture was kneaded every 5min to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing liquid and wash it 10 times with deionized water to obtain the dissolving slurry.

[0062] Example 11

[0063] (1) Select a mixture of oxalic acid, tartaric acid and malic acid as pickling agent. The mass ratio of oxalic acid, tartaric acid and malic acid is 1:1:2. Prepare a pickling solution with an acid dosage of 20wt% of the dry basis of the slurry. (2) Bleached bamboo pulp after screening with a 200-mesh Bauer-McNett fiber sieve was mixed with pickling solution and pulp in a polytetrafluoroethylene bag at a pulp concentration of 4wt% after mixing. The mixture was reacted at 80℃ for 120 minutes, and kneaded every 5 minutes to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing liquid and wash it 10 times with deionized water to obtain the dissolving slurry.

[0064] Example 12

[0065] (1) Select a mixture of oxalic acid, citric acid and malic acid as pickling agent. The mass ratio of oxalic acid, citric acid and malic acid is 1:2:1. Prepare a pickling solution with an acid dosage of 10wt% of the dry basis of the slurry. (2) Bleached bamboo pulp after screening with a 200-mesh Bauer-McNett fiber sieve was mixed with pickling solution and pulp at a mixed pulp concentration of 5wt% in a polytetrafluoroethylene bag and reacted at 80℃ for 180min, kneading once every 5min to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing liquid and wash it 10 times with deionized water to obtain the dissolving slurry.

[0066] Example 13

[0067] Based on Example 12, only the reaction time was changed to 150 min, while all other conditions remained exactly the same as in Example 12, resulting in Example 13.

[0068] Example 14

[0069] (1) Select a mixture of phytic acid, oxalic acid and citric acid as pickling agent. The mass ratio of phytic acid, oxalic acid and citric acid is 2:1:1. Prepare a pickling solution with an acid dosage of 8wt% of the dry basis of the slurry. (2) Bleached bamboo pulp after screening with a 200-mesh Bauer-McNett fiber sieve was mixed with pickling solution and pulp in a polytetrafluoroethylene bag at a mixed pulp concentration of 3wt%, and reacted at 80℃ for 150 min, kneading once every 5 min to ensure uniform reaction. (3) After the reaction is complete, squeeze out the acid washing liquid and wash it 10 times with deionized water to obtain the dissolving slurry.

[0070] The dissolving slurries of Examples 10-14 were tested using the above test methods, and the results are shown in Table 3.

[0071] Table 3. Test results of dissolving slurry in Examples 10-14 before treatment

[0072] Comparative Example 1

[0073] This comparative example is based on Example 1, except that the type of pickling agent in step (1) is changed to inorganic acid sulfuric acid, while the other raw materials and process conditions remain unchanged.

[0074] Comparative Example 2

[0075] This comparative example is based on Example 1, except that the type of pickling agent in step (1) is changed to an inorganic acid salt, while the other raw materials and process conditions remain unchanged.

[0076] Comparative Example 3

[0077] This comparative example is based on Example 10, except that the pickling agent in step (1) is changed to inorganic acid sulfuric acid and compounded with 0.5 wt% DTPA, while the other raw materials and process conditions remain unchanged.

[0078] Comparative Example 4

[0079] This comparative example is based on Example 14, except that the type of pickling agent in step (1) is changed to an inorganic acid salt, while the other raw materials and process conditions remain unchanged.

[0080] The dissolving slurries of Comparative Examples 1 to 4 were tested using the above test method, and the results are shown in Table 4.

[0081] Table 4. Test results of dissolving slurry before treatment and in comparative examples 1-4

[0082] In summary, a specially formulated organic acid pickling solution was used to perform acid pickling and complexation treatment on the slurry. After treatment, the removal of metal ions from the slurry was significantly improved, meeting the following requirements: iron ion content ≤ 5 mg / kg, calcium ion content ≤ 30 mg / kg, magnesium ion content ≤ 20 mg / kg, and copper ion content ≤ 5 mg / kg. This method is particularly effective in removing firmly bound iron ions, achieving a maximum removal rate of 89.9%, reducing the iron ion content from above 45 mg / kg to below 5 mg / kg, thus obtaining a high-purity slurry. The resulting slurry is beneficial for subsequent dissolution and spinning processes, and the acid pickling process has a slight impact on the degree of polymerization of the slurry, maintaining it within a reasonable range.

[0083] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for removing metal ions from dissolved pulp, characterized in that, Includes the following steps: (1) Prepare pickling solution using organic acids; (2) The pickling solution and slurry are mixed and pickled together, and the mixture is kneaded regularly during the pickling and complexing process; (3) After the pickling and complexation are completed, remove the pickling solution, wash the slurry, and obtain the dissolved slurry; The organic acid in step (1) includes one or more of citric acid, oxalic acid, phytic acid, malic acid, lactic acid, and tartaric acid.

2. The method according to claim 1, characterized in that, When the organic acid in step (1) includes two types, the mass ratio of the two organic acids is 1~3:1~3; Alternatively, when the organic acid in step (1) includes three types, the mass ratio of the three organic acids is 1~2:1~2:1~2.

3. The method according to claim 1 or 2, characterized in that, The mass ratio of organic acid in the pickling solution to dry slurry in step (2) is 1~20:

100.

4. The method according to claim 3, characterized in that, The organic acid in step (1) includes a mixture of phytic acid, oxalic acid and citric acid; or, the organic acid in step (1) includes a mixture of tartaric acid and lactic acid. The mass ratio of organic acid in the pickling solution to the dry basis of the slurry in step (2) is 8~10:

100.

5. The method according to claim 1, characterized in that, The pulp in step (2) is bleached bamboo pulp after being screened through a 50-200 mesh screen.

6. The method according to claim 3, characterized in that, The concentration of the slurry after mixing the pickling solution and the slurry in step (2) is 1~6wt%.

7. The method according to claim 6, characterized in that, The conditions for acid pickling complexation in step (2) include: acid pickling temperature of 40~100℃ and acid pickling time of 20~180min.

8. The method according to claim 7, characterized in that, The kneading in step (2) specifically refers to kneading every 5 to 10 minutes during the acid washing and complexing process.

9. The method according to claim 1, characterized in that, The washing conditions in step (3) include: washing with deionized water, and washing 10 to 20 times.

10. A dissolving slurry obtained by the method according to any one of claims 1 to 9.

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