A membrane separation treatment method for papermaking and linter processing black liquor

Abstract

The present application relates to a kind of membrane separation treatment methods of papermaking and cotton linter processing black liquor, comprising the following steps: 1) papermaking black liquor or cotton linter processing black liquor is coarsely filtered, and refined black liquor is obtained;2) refined black liquor is concentrated by multiple reverse osmosis through nanofiltration membrane, and concentrated dealkalization black liquor and permeate are obtained, part of permeate is used as circulating washing liquid, and the rest of permeate is used as recovered pure alkali liquor.When the total flow of permeate is not less than 90% of initial flow during reverse osmosis concentration, maintain high pressure condition to concentrate refined black liquor, when the total flow of permeate is less than 90% of initial flow, use circulating washing liquid to wash nanofiltration membrane under low pressure condition, until the total flow of permeate recovers to more than 90% of initial flow, then restore to high pressure condition to concentrate refined black liquor.This method can prolong the operation cycle of membrane treatment black liquor, prolong the service life of membrane, reduce the operation cost, and is beneficial to the industrialization popularization of membrane method treatment black liquor.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to a membrane separation treatment method for black liquor from papermaking and cotton linter processing. Background Technology

[0002] The papermaking and cotton linter processing industries generate a large amount of black liquor. Black liquor is a cooking black liquor with high organic matter and high alkali content. The organic matter includes humic acid, fulvic acid, fulvic acid-like substances, lignin and other substances, making it an ideal raw material for biomass fertilizer. The alkali in the black liquor can also be recycled.

[0003] The following are some of the practical methods for utilizing black liquor resources:

[0004] The causticization process for alkali recovery is currently a relatively effective and thorough method for solving black liquor problems. It includes key steps such as black liquor concentration, concentrated black liquor incineration, causticization of ash to recover caustic soda, and white mud disposal, achieving a caustic soda recovery rate of approximately 90%. However, the evaporation, concentration, and incineration of high-pH black liquor place stringent requirements on equipment materials, resulting in high technical demands and equipment investment. The process is energy-intensive, and the utilization channels for white mud are limited, requiring enterprises to subsidize downstream companies for resource utilization. Currently, it is only applicable to large paper mills and other enterprises with high black liquor processing volumes, and is not suitable for small paper mills. Furthermore, this method has a very low utilization rate of organic matter in the black liquor.

[0005] The evaporation drying method involves first concentrating black liquor to over 50% through multi-effect evaporation, and then spray drying to obtain granular or powdered sodium lignin humate, which is used as a fertilizer raw material. During spray drying, most of the free alkali reacts with carbon dioxide in the hot air to form sodium carbonate. The resulting sodium lignin humate has a high sodium content, and excessive use as a fertilizer raw material can easily lead to soil salinization, making it difficult to promote on a large scale in the market.

[0006] In addition to these, acid precipitation, coagulation sedimentation, and membrane treatment technologies have also been extensively studied. Acid precipitation has the advantage of low equipment investment, making it suitable for small and medium-sized paper mills. However, it requires equipment with extremely strong acid corrosion resistance, thus limiting its widespread application in actual production.

[0007] Due to the complexity of the black liquor composition, only ordinary Fe and Al coagulants can be used, which still produces sludge (white mud), making subsequent production difficult. Therefore, traditional coagulation and sedimentation technology is not very effective in treating papermaking black liquor.

[0008] Membrane separation offers advantages such as high efficiency, stability, and resource utilization, making it a promising method for black liquor treatment. However, membrane treatment of black liquor faces technical challenges, including the susceptibility of nanofiltration membranes to fouling. The membrane's single-cycle cleaning is short, rarely exceeding 8 hours, resulting in low operating rates. Furthermore, the membrane's lifespan is short, rarely exceeding 3 months, necessitating frequent replacements. Alkali-resistant nanofiltration membranes are also expensive, leading to high operating costs. These issues have hindered the large-scale industrial application of membrane-based black liquor treatment.

[0009] In addition, existing technologies for treating black liquor using membrane methods also suffer from problems such as low concentration of organic matter and high alkali content in the concentrate, making it impossible to directly recycle the concentrate. Summary of the Invention

[0010] To address the aforementioned technical problems, this invention provides a membrane separation treatment method for black liquor from papermaking and cotton linter processing. This method solves the problems of low membrane separation efficiency and short membrane life caused by membrane fouling, and increases the organic matter concentration of the concentrated black liquor. The specific solution is as follows:

[0011] A membrane separation treatment method for black liquor from papermaking and cotton linter processing includes the following steps:

[0012] 1) Coarsely filter the black liquor from papermaking or cotton linter processing to obtain refined black liquor;

[0013] 2) The refined black liquor is concentrated through multi-stage reverse osmosis via nanofiltration membrane to obtain permeate and concentrated dealkalized black liquor. Part of the permeate is used as circulating washing liquid, and the rest is used as recycled alkali solution.

[0014] During reverse osmosis concentration, when the total flow rate of the permeate is not less than 90% of the initial flow rate, high pressure conditions are maintained to concentrate the refined black liquor. When the total flow rate of the permeate is less than 90% of the initial flow rate, the nanofiltration membrane is forward washed with circulating washing liquid under low pressure conditions until the total flow rate of the permeate recovers to more than 90% of the initial flow rate, and then the high pressure conditions are restored to concentrate the refined black liquor.

[0015] Furthermore, in step 2), the pressure under high-pressure conditions is 1.5–4.5 MPa, and the pressure under low-pressure conditions is 0.35–0.55 MPa. High-pressure concentration can increase the concentration of organic matter in the concentrate and improve the alkali recovery rate, resulting in a low alkali content in the concentrate.

[0016] Furthermore, in step 1), a polymerization inhibitor is added to the refined black liquor at a concentration of 2–20 mg / L. The polymerization inhibitor is at least one of polyaspartic acid with a molecular weight of 2000 and polysuccinic acid. By adding the polymerization inhibitor, the precipitation of black liquor fouling factors on the membrane can be prevented, thereby reducing the degree of membrane fouling.

[0017] Furthermore, in step 2), after concentration, the nanofiltration membrane is cleaned with refined black liquor under low pressure, and then cleaned with alkaline solution under low pressure.

[0018] Furthermore, in step 2), the nanofiltration system includes a feed tank, a first-stage membrane unit, a second-stage membrane unit, and a third-stage membrane unit connected in sequence; the feed tank has a feed pump and a high-pressure pump at its outlet; the feed end of each membrane unit is connected to a circulation pump; it also includes a cleaning tank, the permeate pipes of each membrane unit are connected to the cleaning tank through branches, and the outlet of the cleaning tank is connected in parallel with the outlet of the feed tank; the permeate pipes of each membrane unit are equipped with flow meters.

[0019] Furthermore, a security filter is connected between the feed pump and the high-pressure pump.

[0020] Furthermore, the membrane devices at each stage are spiral wound nanofiltration membranes or disc nanofiltration membranes.

[0021] Furthermore, the feed ends of the second-stage and third-stage membrane units are connected to the feed end of the first-stage membrane unit via feed branches, and the concentrate pipes of the second-stage and third-stage membrane units are connected to the feed end of the first-stage membrane unit via discharge branches; the feed branches are connected before the discharge branches; when the permeate flow rate of the second-stage or third-stage membrane unit is lower than 90% of the initial flow rate, and the total permeate flow rate is not lower than 90% of the initial flow rate, the second-stage or third-stage membrane unit is switched to the feed and discharge branches, and high-pressure conditions are maintained to concentrate the refined black liquor.

[0022] The advantages of this invention are that it enables membrane treatment of black liquor from papermaking and cotton linter processing. This invention pre-treats the black liquor before membrane treatment and adopts a special operating mode, enabling the membrane to operate efficiently for long periods, with a single operating cycle of more than 36 hours, a high operating rate, and a membrane lifespan of more than one year, significantly reducing membrane replacement costs and enabling the industrial application of membrane treatment for black liquor.

[0023] In addition, the organic matter content of the black liquor concentrate produced by this invention during long-term operation can reach more than 100g / L. The treated concentrate has a low alkali content and a sodium salt content of less than 5%, with an actual minimum of 3%. It can be used to produce humic acid fertilizer with a salt content that meets national fertilizer standards, reducing carbon emissions from alkali furnace combustion and improving the recycling value of biomass resources. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the nanofiltration system in Example 1.

[0025] Figure 2 This is a schematic diagram of the nanofiltration system in Example 2. Detailed Implementation

[0026] The present invention will now be clearly described in conjunction with specific embodiments. This description is merely illustrative and is not intended to limit the scope of the invention. Any modifications, equivalent substitutions, or improvements made by those skilled in the art based on the embodiments of the present invention without inventive effort to obtain all other embodiments should be included within the scope of protection of the present invention.

[0027] Example

[0028] Example 1

[0029] This embodiment illustrates the method of the present invention. The black liquor used is cooking black liquor from a paper mill. The black liquor test data is shown in the table below:

[0030] Item Water-soluble Organic matter % Humic acid % Fulvic acid % Moisture % PH value Test result Easily soluble 59.36 5.12 20.04 10.11 13.29

[0031] When using traditional tubular nanofiltration membranes for direct nanofiltration, clogging occurs after approximately 2 hours of operation, and membrane flux becomes too low after no more than 8 hours of continuous operation, requiring shutdown and cleaning. The single-operation cycle is short. The organic matter content of the concentrate does not exceed 40 g / L, and the alkali recovery rate is low. The humic acid produced from the treated black liquor concentrate has a salt content of 25-30%, exceeding national standards.

[0032] The processing method in this embodiment includes two processes: the preparation of refined black liquor and the nanofiltration concentration of refined black liquor.

[0033] The preparation of refined black liquor involves heating the black liquor to 55–110°C using steam, electricity, or waste heat. The black liquor, with a temperature not lower than 60°C, is then coarsely filtered through a plate and frame filter or a microfiltration filter. After cooling, refined black liquor is obtained. Coarse filtration removes particulate matter from the black liquor, reducing membrane fouling during subsequent nanofiltration. Adding 2000 molecular weight polyaspartic acid and polysuccinic acid to the refined black liquor after coarse filtration prevents fouling factors from precipitating onto the membrane, further reducing membrane fouling during nanofiltration and extending the single-cycle operation.

[0034] Nanofiltration concentration of refined black liquor involves using multi-stage nanofiltration membranes to concentrate the refined black liquor at approximately 50°C through reverse osmosis. The pressure of the reverse osmosis concentration is controlled between 1.5 and 4.5 MPa. After concentration, the refined black liquor yields permeate and concentrated dealkalized black liquor. Part of the permeate is used as circulating washing liquid, and the remainder is used to recover the purified alkali.

[0035] During reverse osmosis concentration, when the permeate flow rate is not less than 90% of the initial flow rate, high pressure conditions are maintained to concentrate the refined black liquor. When the permeate flow rate is less than 90% of the initial flow rate, the nanofiltration membrane is circulated and forward washed under low pressure conditions with cleaning solution until the permeate flow rate recovers to more than 90% of the initial flow rate, and then the high pressure conditions are restored to concentrate the refined black liquor.

[0036] like Figure 1 This embodiment uses a nanofiltration system comprising a feed tank T07, a first-stage membrane unit NF01, a second-stage membrane unit NF02, and a third-stage membrane unit NF03 connected in sequence. The feed tank's outlet is connected to a feed pump P101 and a high-pressure pump P102. Each of the first, second, and third-stage membrane units has a circulation pump connected to its inlet, namely, a first-stage circulation pump P103, a second-stage circulation pump P106, and a third-stage circulation pump P105, respectively. A cleaning tank T06 is also included, with its outlet connected in parallel to the feed tank's outlet. The permeate pipes of each membrane unit are connected to a permeate main pipe, which in turn connects to a caustic soda tank. A branch line connects to the cleaning tank. Each membrane unit's permeate pipe is equipped with a flow meter. A security filter F101A / B is connected between the feed pump and the high-pressure pump.

[0037] The feed tank is used to store refined black liquor, and the washing tank is used to store circulating washing liquid or cleaning solution. The washing tank is equipped with a steam heating jacket.

[0038] In this embodiment, the nanofiltration membrane is either a spiral wound nanofiltration membrane or a disc nanofiltration membrane, and the applicable pH range is 0–14. Using a disc nanofiltration membrane is beneficial for further increasing the organic matter concentration of the concentrate.

[0039] As a key technical means of this invention, the reverse osmosis concentration operation includes a low-concentration replacement cycle + high-concentration operation mode and a low-concentration replacement + alkaline cleaning mode. Using this method, the membrane's service life can be extended to more than one year.

[0040] The implementation of the low-concentration replacement circulation + high-concentration operation mode is as follows: When operating high-concentration feed liquid (alkali and organic matter ratio is less than 1:3), membrane fouling is prone to occur due to increased viscosity and decreased flow rate, resulting in a cleaning cycle of less than 8 hours, which seriously affects production capacity.

[0041] After 3-4 hours of operation, when the permeate flow rate drops below 90% of the initial value under stable operating conditions, switch to raw material cleaning mode for 15-20 minutes. During this time, the permeate pipeline continues to deliver purified liquid to the purified liquid tank, while the concentrate pipeline fully opens the concentrate bypass valve to return to the raw material tank. Subsequently, depending on the circulating pressure differential, gradually increase the circulation pump to 100% frequency for cleaning. The high-pressure pump maintains low-pressure operation to reduce production fluctuations. Once the circulating pressure differential drops back to the initial value, restore the original operating mode.

[0042] Using a circulating pump to flush the nanofiltration membrane module with the permeate as the circulating washing liquid can accelerate the flow velocity on the membrane surface. The tangential flow velocity on the membrane surface is greater than 15 cm / s, and the actual flow velocity reaches more than 45 cm / s. This fully destroys the polarization layer on the membrane surface, causing a significant reduction in the polarization layer thickness. This makes the membrane less prone to fouling, less likely to form soap crystals, and less likely to accumulate alkali-soluble waxes, thus greatly extending the single operation cycle and the lifetime of the membrane.

[0043] Based on actual test results, the system can maintain a clean liquid permeation rate of no less than 90% of the initial value within a 28-hour continuous operation cycle including material cleaning, essentially achieving non-stop cleaning operation. Within a 36-hour operating cycle, the clean liquid permeation rate is no less than 85% of the initial value.

[0044] The low-concentration replacement + alkaline cleaning mode is implemented as follows: When shutting down before cleaning at the end of operation, a dilute working solution (refined black liquor) is first used to replace the black liquor in the system under low pressure using a plug flow method. Then, a cleaning solution (alkaline solution with a concentration of 120-200 g / L) is used to replace the dilute black liquor in the system under low pressure using a plug flow method. This improves the cleanliness of the cleaning solution, enhances the cleaning effect, and extends the membrane's efficiency and lifespan.

[0045] In this scheme, the operating and cleaning temperature is 40~65℃, the cleaning pressure is 0.35~0.55MPa, and the operating pressure is 1.5~4.5MPa.

[0046] Using this method, the organic matter content of the concentrate reaches 100–140 g / L, and the sodium salt content in the concentrate is 3–5 wt%. Some experimental data are as follows:

[0047]

[0048] Example 2

[0049] This embodiment is an improvement based on Embodiment 1, such as... Figure 2 Before the feed ends of the second and third membrane units are connected to the feed end of the first membrane unit via feed branch L01, the concentrate pipes of the second and third membrane units are connected to the feed end of the first membrane unit via discharge branch L02. The feed branch is connected before the discharge branch. Each main line and branch is equipped with a valve, which can be used to control the switching between the main line and the branch.

[0050] During operation, it was found that the second and third stage membrane units were more prone to fouling than the first stage membrane unit. This was manifested in the faster decrease in permeate flow rate in the second and third stage membrane units, especially the third stage. Analysis revealed that after passing through the first stage membrane unit, the black liquor concentration increased, the flow rate decreased, and the flow velocity decreased, making it easier for fouling to form on the membrane surface when entering the second and third stage membrane units.

[0051] Based on this, and in conjunction with the nanofiltration system of this embodiment, when the permeate flow rate of the second or third stage membrane unit is lower than 90% of the initial flow rate, and the total permeate flow rate is not lower than 90% of the initial flow rate, the second or third stage membrane unit is switched to the feed branch and the discharge branch, and high pressure conditions are maintained to concentrate the refined black liquor. In other words, when the second or third stage membrane unit becomes fouled, but the first stage membrane unit does not show significant fouling, the feed sequence of the fouled second or third stage membrane unit is switched to before the first stage membrane unit. Using relatively low-concentration, high-flow-rate black liquor to flush the membrane surface can alleviate the fouling rate of the second or third stage membrane unit, extending the high-concentration operating time from 3-4 hours to 5-6 hours. Then, switching to low-concentration replacement circulation further extends the single operating cycle to over 40 hours, improving the operating rate of the membrane treatment system.

Claims

1. A membrane separation treatment method for black liquor from papermaking and cotton linter processing, characterized in that, Includes the following steps: 1) Coarsely filter the black liquor from papermaking or cotton linter processing to obtain refined black liquor; 2) The refined black liquor is concentrated through multi-stage reverse osmosis via nanofiltration membrane to obtain permeate and concentrated dealkalized black liquor. Part of the permeate is used as circulating washing liquid, and the rest is used as recycled alkali solution. During reverse osmosis concentration, when the total flow rate of the permeate is not less than 90% of the initial flow rate, high pressure conditions are maintained to concentrate the refined black liquor. When the total flow rate of the permeate is less than 90% of the initial flow rate, the nanofiltration membrane is forward washed with circulating washing liquid under low pressure conditions until the total flow rate of the permeate recovers to more than 90% of the initial flow rate, and then the high pressure conditions are restored to concentrate the refined black liquor. In step 2), the pressure under high pressure conditions is 1.5~4.5MPa, and the pressure under low pressure conditions is 0.35~0.55MPa; In step 2), the nanofiltration system includes a feed tank, a first-stage membrane unit, a second-stage membrane unit, and a third-stage membrane unit connected in sequence; the feed tank has a feed pump and a high-pressure pump at its outlet; each membrane unit's feed end is connected to a circulation pump; it also includes a cleaning tank, with the permeate pipes of each membrane unit connected to the cleaning tank via branches, and the outlet of the cleaning tank being connected in parallel with the outlet of the feed tank; each membrane unit's permeate pipe is equipped with a flow meter. The feed ends of the second-stage and third-stage membrane units are connected to the feed end of the first-stage membrane unit via feed branches. The concentrate pipes of the second-stage and third-stage membrane units are connected to the feed end of the first-stage membrane unit via discharge branches. The feed branches are connected before the discharge branches. When the permeate flow rate of the second-stage or third-stage membrane unit is lower than 90% of the initial flow rate, and the total permeate flow rate is not lower than 90% of the initial flow rate, the second-stage or third-stage membrane unit is switched to the feed and discharge branches, and high-pressure conditions are maintained to concentrate the refined black liquor.

2. The membrane separation treatment method for black liquor from papermaking and cotton linter processing according to claim 1, characterized in that: In step 1), a polymerization inhibitor is added to the refined black liquor at a concentration of 2-20 mg / L. The polymerization inhibitor is at least one of polyaspartic acid with a molecular weight of 2000 and polysuccinic acid.

3. The membrane separation treatment method for black liquor from papermaking and cotton linter processing according to claim 1, characterized in that: In step 2), after concentration, the nanofiltration membrane is cleaned with refined black liquor under low pressure, and then cleaned with alkaline solution under low pressure.

4. The membrane separation treatment method for black liquor from papermaking and cotton linter processing according to claim 1, characterized in that: A safety filter is connected between the feed pump and the high-pressure pump.

5. The membrane separation treatment method for black liquor from papermaking and cotton linter processing according to claim 1, characterized in that: Each stage of the membrane device is a spiral wound nanofiltration membrane or a disc nanofiltration membrane.

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