Method for cooperatively promoting precipitation and sedimentation of silicon in non-wood pulp alkali recovered green liquor by Ca-Al

Abstract

The invention belongs to the field of pulping and papermaking, and particularly relates to a method for promoting precipitation and sedimentation of silicon in non-wood pulp alkali recovery green liquor through Ca-Al synergism. The method comprises the following steps: 1, adding lime into the non-wood-pulp alkali recovery green liquor, carrying out a pre-causticization reaction, and ensuring that the use amount of the lime is 0.5-15% of the total causticized lime amount; secondly, aluminum salt is added into the green liquor treated in the first step for a silicon removal reaction, the molar ratio of the dosage of the aluminum salt to the silicon content of the green liquor is 0.1-2, and after the specified reaction time is reached, standing and settling are conducted. Through the Ca-Al synergistic effect, the settling speed of precipitated silicon is greatly increased, the settling height is remarkably reduced, the problems that the silicon removal rate is low by using a lime pre-causticization method alone and silicon particles are difficult to settle by using an aluminum salt method alone are solved, and an effective method is provided for industrial application of green liquor silicon removal; the problem of serious environmental pollution caused by discharge of a large amount of white mud is solved.

Description

Technical Field

[0001] The present invention belongs to the field of pulp and paper making, and particularly relates to a method for promoting the precipitation and sedimentation of silicon in green liquor during the alkali recovery of non-wood pulp by the synergistic action of Ca and Al. Background Art

[0002] China has a long history of non-wood fiber pulp and paper making. Non-wood fiber raw materials mainly include wheat straw, rice straw, bamboo, reed, bagasse, etc. Different from wood, non-wood fiber raw materials contain a high proportion of silicon, which has a serious impact on the operation of pulp making and alkali recovery production, namely "silicon interference". Therefore, in order to make full use of non-wood fiber raw materials, the "silicon interference" problem must be solved.

[0003] In non-wood fiber raw materials, silicon mainly exists in the forms of SiO 2 , silicic acid, alkaline silicate, etc. In response to the "silicon interference" problem, a large number of studies have been carried out at home and abroad. For example: (1) The carbon dioxide method for silicon removal. Due to the high content of silicate in black liquor, carbon dioxide gas is introduced into black liquor or green liquor in the alkali recovery process, and it can react with silicate to form silicic acid precipitate, so as to achieve the purpose of silicon removal. Research shows that with the addition of carbon dioxide gas, when the pH value is below about 10, the silicon removal rate measured after reacting at 50-100 °C for 1 h is as high as over 97%, but this method will almost consume all the effective alkali in the green liquor, and the precipitated silicon particles are fine and difficult to settle. (2) The two-step causticization method for silicon removal, that is, a part of the lime required for causticization is added to the green liquor, and Ca 2+ reacts with the silicate in the green liquor to form calcium silicate precipitate and precipitate out from the green liquor. Research shows that for high-temperature pre-causticization, when the amount of pre-causticization lime accounts for 30% of the total amount of causticization lime, the silicon removal rate of the green liquor is only about 50%. When pre-causticization is carried out at a low temperature below 40 °C, the silicon removal rate can reach 80%-90%, but low-temperature pre-causticization will waste a huge amount of heat energy. (3) The method of removing silicon by aluminum and magnesium compounds. An appropriate amount of aluminum compound is added to the green liquor to achieve the purpose of silicon removal, but some aluminum will remain in the green liquor after silicon removal, and the influence of aluminum on the green liquor can be reduced by adding magnesium sulfate. Research shows that when the molar ratio of Al 2 O 3 usage to the silicon content in the green liquor is 0.35 and the reaction is carried out at 100 °C for 1 h, the silicon removal rate in the white mud is only about 54.77%, which cannot meet the requirements of white mud calcined lime, and the precipitated silicon particles are fine and difficult to settle. Thus, it can be seen that the above methods all have various problems, especially the formed silicon particles are fine and difficult to settle, which is not conducive to separation from the green liquor and difficult to apply in actual production. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for promoting the precipitation and sedimentation of silicon in the green liquor of non-wood pulp alkali recovery by the synergistic action of Ca and Al. This method has a high silicon removal rate, a fast sedimentation speed, and a low sedimentation height, and will solve the problems of low silicon removal rate, fine silicon particles, and difficult sedimentation encountered in the current silicon removal of non-wood pulp alkali recovery green liquor.

[0005] The present invention includes the following steps:

[0006] The first step: Add lime to the non-wood pulp alkali recovery green liquor for pre-causticization reaction. The dosage of the lime is 0.5% - 15% of the total causticization lime amount, the reaction temperature is 50 - 100 °C, the stirring speed is 50 - 2000 rpm, and the reaction time is 5 - 60 min;

[0007] The second step: Add an aluminum salt to the green liquor after the first-step treatment for silicon removal reaction. The molar ratio of the dosage of the aluminum salt to the silicon content in the green liquor is 0.1 - 2, the reaction temperature is 50 - 100 °C, the stirring speed is 50 - 2000 rpm, the reaction time is 0 - 30 min, and after the reaction, let it stand for sedimentation;

[0008] Among them, the lime is a solid powder with a mesh number of 20 - 2000 mesh, and the aluminum salt is selected from one or more of aluminum sulfate, aluminum chloride, polyaluminum chloride, aluminum hydroxide, or aluminum oxide.

[0009] Optionally, in the first step, the optimal dosage of the lime is 5% - 10% of the total causticization lime amount, and the optimal reaction time is 10 - 20 min.

[0010] Optionally, in the second step, the molar ratio of the dosage of the aluminum salt to the silicon content in the green liquor is 0.4 - 1.2, and the reaction time is 5 - 20 min.

[0011] Optionally, the reaction temperatures of both the first step and the second step are 80 - 95 °C, and the stirring speeds are both 100 - 1000 rpm.

[0012] Optionally, the aluminum salt is aluminum sulfate or polyaluminum chloride.

[0013] Optionally, the mesh number of the lime is 100 - 1000 mesh.

[0014] Optionally, the non-wood pulp is one or more of wheat straw, rice straw, reed, bamboo, or cotton stalk.

[0015] Optionally, in the first step, the lime is added slowly to avoid side reactions caused by local supersaturation.

[0016] Optionally, in the second step, after standing for sedimentation in a 250 mL graduated cylinder for 1 - 3 hours, the sedimentation height can be lower than 20 ml.

[0017] Optionally, the method further includes filtering the settled green liquor after static settlement to further separate the precipitated silicon particles.

[0018] Advantages of the present invention:

[0019] (1) Through the Ca-Al synergistic effect, the aluminosilicate precipitated after adding the aluminum salt adsorbs and grows on the surface of the composite particles formed by calcium carbonate and calcium silicate, greatly improving the sedimentation rate of the precipitated silicon, significantly reducing the sedimentation height, making the precipitated silicon easy to separate from the green liquor, solving the problems of low silicon removal rate in the lime pre-causticization method alone and difficult sedimentation of silicon particles in the aluminum salt method alone, and providing an effective method for the industrial application of green liquor silicon removal.

[0020] (2) Greatly reducing the silicon content in the green liquor after silicon removal, so that the calcium carbonate in the white mud after causticization can be reused, solving the serious environmental pollution problem caused by the large amount of white mud discharge.

[0021] (3) Improving the clarity of the recovered lye.

[0022] (4) Significantly increasing the dryness of the causticized white mud, increasing the amount of effective alkali recovered, and reducing the production cost. Description of the drawings

[0023] Figure 1 Effect of Ca-Al synergistic aluminum salt reaction time on sedimentation height;

[0024] Figure 2 Effect of Ca-Al synergistic lime addition amount on sedimentation height and silicon removal rate (molar ratio of aluminum salt to silicon in green liquor is 0.882:1);

[0025] Figure 3 Effect of Ca-Al synergistic aluminum salt addition amount on sedimentation height and silicon removal rate (lime addition amount is 10%). Detailed implementation manners

[0026] The following will describe in detail the specific implementation manners of the present application with reference to the accompanying drawings. It should be understood that the specific implementation manners described herein are only used to illustrate and explain the present disclosure, and are not used to limit the present application.

[0027] The method of the present application is specifically as follows:

[0028] The first step: Add lime to the non-wood pulp alkali recovery green liquor to carry out the pre-causticization reaction. The amount of lime used is 0.5% - 15% of the total causticization lime amount. The lime is slowly added to the green liquor, the reaction temperature is 50 - 100°C, the stirring speed is 50 - 2000 rpm, and the reaction time is 5 - 60 min. Among them, the optimal lime addition amount is 5% - 10%, and the optimal reaction time is 10 - 20 min.

[0029] Step 2: Add aluminum salt into the green liquor after the treatment in Step 1 for silicon removal reaction. The molar ratio of the dosage of aluminum salt to the silicon content in the green liquor is 0.1 - 2, the reaction temperature is 50 - 100 °C, the stirring speed is 50 - 2000 rpm, and the reaction time is 0 - 30 min. After reaching the specified reaction time, let it stand for sedimentation.

[0030] The non-wood pulp alkali recovery green liquor described in this application refers to the alkali recovery green liquor from non-wood fiber raw material pulping.

[0031] Principle of the method of this application:

[0032] Reaction equation:

[0033] CaO + H 2 O → Ca(OH) 2 (1)

[0034] Ca(OH) 2 + Na 2 CO 3 → CaCO 3 + 2NaOH (2)

[0035] Ca(OH) 2 + Na 2 SiO 3 + H 2 O → CaO·SiO 2 ·H 2 O + 2NaOH (3)

[0036] Powdery lime is added into the green liquor. As shown in reaction formula (1), the lime first reacts with water to generate calcium hydroxide. Then, the calcium hydroxide reacts with sodium carbonate and sodium silicate in the green liquor simultaneously to generate calcium carbonate and calcium silicate hydrate (see reaction formulas 2 and 3). Since the lime is in powder form, that is, in fine particle form, the generated calcium carbonate and calcium silicate hydrate exist in one particle simultaneously. After reacting for a certain time, most of the lime has become fine particles composed of calcium carbonate and calcium silicate hydrate. Since the main component of the green liquor is sodium carbonate, the main component in the particle is calcium carbonate. Then, aluminum salt is added. The aluminum salt reacts with sodium silicate to generate aluminosilicate. Since aluminosilicate is insoluble in water, it precipitates from the green liquor. In addition, since there are already fine particles composed of calcium carbonate and calcium silicate hydrate in the green liquor, these particles become crystal nuclei. Under appropriate stirring, the precipitated aluminosilicate adsorbs and grows on the surface of the crystal nuclei. Since calcium carbonate has a relatively high density and hydrophobicity, the generated particles are easy to sediment, thus promoting the sedimentation separation of silicon from the green liquor.

[0037] Examples:

[0038] I. Examples 1 to 3 below illustrate the influence of adding only aluminum salt on the silicon precipitation and sedimentation performance of the green liquor

[0039] Example 1

[0040] (1) Take 200 mL to 500 mL of green liquor into a beaker, heat the green liquor to 90 °C in a water bath, add aluminum sulfate to the green liquor for silicon removal reaction, the molar ratio of aluminum sulfate to silicon in the green liquor is 0.882:1, the reaction temperature is 90 °C, the stirring speed is 200 rpm, and the reaction time is 1 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0041] (2) Test: Let it stand for 1 h, the sedimentation height is 115 mL, and the silicon removal rate is 92.67%.

[0042] Example 2

[0043] (1) Take 200 mL to 500 mL of green liquor into a beaker, heat the green liquor to 90 °C in a water bath, add aluminum sulfate to the green liquor for silicon removal reaction, the molar ratio of aluminum sulfate to silicon in the green liquor is 0.882:1, the reaction temperature is 90 °C, the stirring speed is 200 rpm, and the reaction time is 5 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0044] (2) Test: Let it stand for 1 h, the sedimentation height is 102 mL, and the silicon removal rate is 92.82%.

[0045] Example 3

[0046] (1) Take 200 mL to 500 mL of green liquor into a beaker, heat the green liquor to 90 °C in a water bath, add aluminum sulfate to the green liquor for silicon removal reaction, the molar ratio of aluminum sulfate to silicon in the green liquor is 0.882:1, the reaction temperature is 90 °C, the stirring speed is 200 rpm, and the reaction time is 60 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0047] (2) Test: Let it stand for 1 h, the sedimentation height is 90 mL, and the silicon removal rate is 95.97%.

[0048] Conclusion 1:

[0049] Based on Examples 1 to 3, it is concluded that when only adding aluminum salt without adding lime, the silicon removal rate of green liquor reaches more than 90%, but the sedimentation height is only about 100 mL, and it is difficult to sediment the precipitated silicon. See Figure 1 .

[0050] II. Examples 4 to 6 are the influence of the combined use of Ca - Al and the dosage of aluminum salt on the silicon precipitation and sedimentation performance of green liquor

[0051] Example 4

[0052] (1) First step: Take 200 mL to 500 mL of green liquor in a beaker, heat the green liquor to 90 °C using a water bath, add lime to the non-wood pulp alkali recovery green liquor to carry out a pre-causticization reaction, and the amount of lime used is 10% of the total causticization lime amount. The lime is slowly added to the green liquor, the reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min.

[0053] (2) Second step: Add aluminum sulfate to the green liquor treated in the first step for silicon removal. The molar ratio of aluminum sulfate to silicon in the green liquor is 0.235:1. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0054] (3) Test: Let it stand for 1 h, the sedimentation height is 10 mL, and the silicon removal rate is 49.59%.

[0055] Example 5

[0056] (1) First step: Take 200 mL to 500 mL of green liquor in a beaker, heat the green liquor to 90 °C using a water bath, add lime to the non-wood pulp alkali recovery green liquor to carry out a pre-causticization reaction, and the amount of lime used is 10% of the total causticization lime amount. The lime is slowly added to the green liquor, the reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min.

[0057] (2) Second step: Add aluminum sulfate to the green liquor treated in the first step for silicon removal. The molar ratio of aluminum sulfate to silicon in the green liquor is 0.412:1. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0058] (3) Test: Let it stand for 1 h, the sedimentation height is 16 mL, and the silicon removal rate is 76.36%.

[0059] Example 6

[0060] (1) First step: Take 200 mL to 500 mL of green liquor in a beaker, heat the green liquor to 90 °C using a water bath, add lime to the non-wood pulp alkali recovery green liquor to carry out a pre-causticization reaction, and the amount of lime used is 10% of the total causticization lime amount. The lime is slowly added to the green liquor, the reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min.

[0061] (2) Second step: Add aluminum sulfate to the green liquor treated in the first step for silicon removal. The molar ratio of aluminum sulfate to silicon in the green liquor is 0.882:1. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0062] (3) Test: Stand still for 1 h, the sedimentation height is 18 mL, and the silicon removal rate is 92.61%.

[0063] Conclusion 2:

[0064] Based on Examples 4 to 6, it is obtained that with the Ca-Al cooperation, as the dosage of aluminum salt increases, the silicon removal rate of green liquor increases significantly, and the sedimentation height of the precipitated silicon drops below 20 mL, which proves that the aluminum salt reacts with the silicate in the green liquor to precipitate, and the precipitated silicon is adsorbed on the surface of the calcium carbonate in the generated white mud, thus producing good sedimentation. It can be seen that Ca-Al has a good synergistic effect, which not only precipitates silicon in the green liquor but also makes the silicon have good sedimentation. See Figure 1 and Figure 3 .

[0065] III. Examples 7 to 9 are about the influence of the Ca-Al cooperation and the dosage of lime on the silicon precipitation and sedimentation performance of green liquor

[0066] Example 7

[0067] (1) The first step: Take 200 mL of green liquor into a 500 mL beaker, heat the green liquor to 90 °C with a water bath, add lime to the non-wood pulp alkali recovery green liquor, and a pre-causticization reaction occurs. The dosage of lime is 2% of the total causticization lime amount. The lime is slowly added to the green liquor, the reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min.

[0068] (2) The second step: Add aluminum sulfate to the green liquor treated in the first step for silicon removal. The molar ratio of the dosage of aluminum salt to the silicon content in the green liquor is 0.882:1. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and stand still for sedimentation.

[0069] (3) Test: Stand still for 1 h, the sedimentation height is 42 mL, and the silicon removal rate is 92.34%.

[0070] Example 8

[0071] (1) The first step: Take 200 mL of green liquor into a 500 mL beaker, heat the green liquor to 90 °C with a water bath, add lime to the non-wood pulp alkali recovery green liquor, and a pre-causticization reaction occurs. The dosage of lime is 5% of the total causticization lime amount. The lime is slowly added to the green liquor, the reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min.

[0072] (2) Second step: Add aluminum sulfate to the green liquor after the first-step treatment for silicon removal. The molar ratio of aluminum sulfate to silicon in the green liquor is 0.882:1. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0073] (3) Test: Let it stand for 1 h, the sedimentation height is 28 mL, and the silicon removal rate is 92.94%.

[0074] Example 9

[0075] (1) First step: Take 200 mL of green liquor into a 500 mL beaker, heat the green liquor to 90 °C in a water bath, add lime to the non-wood pulp alkali recovery green liquor to carry out a pre-causticization reaction, and the lime dosage is 7% of the total causticization lime amount. The lime is slowly added to the green liquor. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min.

[0076] (2) Second step: Add aluminum sulfate to the green liquor after the first-step treatment for silicon removal. The molar ratio of aluminum sulfate to silicon in the green liquor is 0.882:1. The reaction temperature is 90 °C, the stirring speed is 500 rpm, and the reaction time is 20 min. After reaching the specified reaction time, pour it into a 250 mL graduated cylinder and let it stand for sedimentation.

[0077] (3) Test: Let it stand for 1 h, the sedimentation height is 22 mL, and the silicon removal rate is 96.32%.

[0078] Conclusion Three:

[0079] Based on Examples 7 to 9, it can be concluded that as the lime addition amount increases, the silicon removal rate of the green liquor increases, while the sedimentation height of the precipitated silicon decreases significantly, which proves that the added lime effectively adsorbs the precipitated silicon in the green liquor. Thus, it can be seen that the Ca-Al synergy significantly improves the sedimentation performance of the precipitated silicon, as shown in Figure 1 and Figure 2 .

[0080] The above are only the preferred embodiments of the present invention, and it is not a limitation of the invention in other forms. Any person skilled in the art may use the disclosed technical content to make changes or equivalent changes to equivalent embodiments, but as long as it does not depart from the technical content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for Ca-Al synergistically promoting silicon precipitation and sedimentation in non-wood pulp alkali recovery green liquor, characterized in that: The following steps are involved: Step 1: adding lime to the non-wood pulp alkali recovery green liquor to carry out a pre-causticizing reaction, wherein the amount of lime used is 0.5% to 15% of the total causticizing lime amount, the reaction temperature is 50 to 100° C., the stirring speed is 50 to 2000 rpm, and the reaction time is 5 to 60 minutes; Step 2: adding aluminum salt to the green liquor treated in the first step to carry out a silicon removal reaction, wherein the molar ratio of the aluminum salt to the silicon content in the green liquor is 0.1-2, the reaction temperature is 50-100°C, the stirring speed is 50-2000rpm, the reaction time is 0-30min, and the reaction is allowed to settle after the reaction is completed; Wherein, the lime is a solid powder with a mesh size of 20 to 2000 meshes, and the aluminum salt is selected from one or more of aluminum sulfate, aluminum chloride, polyaluminum chloride, aluminum hydroxide or aluminum oxide.

2. The method of Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1, characterized in that: In the first step, the optimal amount of lime added is 5% to 10% of the total causticizing lime amount, and the optimal reaction time is 10 to 20 minutes.

3. The method of Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1, characterized in that: In the second step, the molar ratio of the amount of aluminum salt to the silicon content in the green liquor is 0.4-1.2, and the reaction time is 5-20 minutes.

4. The method for Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1, 2 or 3, characterized in that: The reaction temperatures of the first step and the second step are both 80-95° C., and the stirring speeds are both 100-1000 rpm.

5. The method of Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1, characterized in that: The aluminum salt is aluminum sulfate or polyaluminum chloride.

6. The method of Ca-Al synergistically promoting silicon precipitation and sedimentation in non-wood pulp alkali recovery green liquor according to claim 5, characterized in that: The mesh number of the lime is 100 to 1000 meshes.

7. The method of Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1, characterized in that: The non-wood pulp is one or more of wheat straw, rice straw, reed, bamboo or cotton stalk.

8. The method for Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1, 5 or 7, characterized in that: In the first step, lime is added slowly to avoid side reactions caused by local oversaturation.

9. The method of Ca-Al synergistically promoting silicon precipitation and sedimentation in non-wood pulp alkali recovery green liquor according to claim 4, characterized in that: In the second step, after the sedimentation time is 1 to 3 hours in a 250 mL measuring cylinder, the sedimentation height may be less than 20 ml.

10. The method for Ca-Al synergistically promoting silicon precipitation and sedimentation of non-wood pulp alkali recovery green liquor according to claim 1 or 9, characterized in that: The method further comprises filtering the settled green liquor after static sedimentation to further separate the precipitated silicon particles.

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