Production process of dissolving pulp

Abstract

The invention particularly relates to a dissolving pulp production process which comprises the following steps: washing and soaking wood chips or cotton linters, and naturally draining to obtain a pretreated raw material; adding adjusting water into the pretreated raw material, then adding sodium sulfite, mixing and stirring, heating and preserving heat, and extracting pre-hydrolysis waste liquid to obtain semi-slurry; the ratio of the absolute dry mass of the pretreated raw materials to the volume of the adjusting water is 1: (5-8); in terms of SO2, the use amount of sodium sulfite is 2%-4% of the absolute dry mass of the pretreated raw material; the heating and heat preservation temperature ranges from 150 DEG C to 160 DEG C, and the heat preservation time ranges from 90 min to 120 min; carrying out sulfate cooking on the semi-slurry to prepare coarse slurry and cooking black liquor; the coarse pulp is bleached, pulped and sieved by adopting an OD0EOPD1 section sequence, and the dissolving pulp is prepared. According to the method, the quality of the dissolving pulp is remarkably improved by adopting sodium sulfite to prehydrolyze the dissolving pulp, the high-end application requirements of cellulose acetate, Lyocell fibers and the like can be met, the vacuum pulp washing link of semi-pulp after prehydrolysis is omitted, the pulp wastage rate is reduced, the MVR evaporator utilizes secondary steam compression reuse, and the water resource utilization rate is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pulp and papermaking, and particularly relates to a production process of dissolving pulp. BACKGROUND

[0002] As a core raw material for producing regenerated cellulose fiber and cellulose derivatives (such as cellulose acetate and nitrocellulose), the quality (such as alpha-cellulose content, lignin residual amount and reaction performance) of dissolving pulp directly determines the performance of downstream products. At present, the mainstream production process of dissolving pulp in the industry is steam pre-hydrolysis-sulfate cooking-blaning process. This process relies on high-temperature steam (160-180 DEG C) to pre-hydrolyze raw materials (such as wood chips and cotton linters) to remove part of hemicellulose and lignin, and then deeply removes lignin through sulfate cooking, and finally gets dissolving pulp through bleaching.

[0003] However, the existing steam pre-hydrolysis-sulfate process has the following technical defects:

[0004] Limited quality of dissolving pulp: steam pre-hydrolysis only removes hemicellulose and lignin through physical high-temperature hydrolysis, and the removal efficiency is low (hemicellulose removal rate 30%-50%, lignin removal rate 10%-20%), which leads to high load of subsequent cooking and bleaching, the alpha-cellulose content of finished dissolving pulp is usually only 94.0%-95.5%, the lignin residual amount is 0.2%-0.3%, and the cellulose molecular chain is prone to random degradation due to high temperature, the copper ammonia viscosity is low (35-50 mPa·s), and the reaction performance (Fock reaction value 75%-85%) is difficult to meet the demand of high-end application;

[0005] Serious energy consumption and water resource waste: steam pre-hydrolysis consumes a large amount of high-temperature steam, and when the pre-hydrolysis waste liquid (solid content 5%-8%) is directly sent to the alkali recovery system, it needs to be concentrated by evaporation due to low concentration, which is high in energy consumption; at the same time, the process needs to wash the semi-pulp after pre-hydrolysis separately, which increases the consumption of washing water, and the water resource reuse rate is low (<30%);

[0006] High chemical consumption: after steam pre-hydrolysis, there are many residual impurities, and the sulfate cooking needs higher active alkali dosage (22%-26% to absolute dry raw material), and the consumption of chemicals such as chlorine dioxide (ClO2) and sodium hydroxide (NaOH) in the bleaching stage also increases significantly, which leads to high production cost;

[0007] Low process efficiency: the semi-pulp after pre-hydrolysis needs to be transferred to a vacuum washing machine for washing, which increases the process complexity and pulp loss rate (usually 2%-3%), and needs to increase the investment of washing equipment.

[0008] In order to solve the above problems, it is urgent to develop a dissolving pulp production process with simplified process, improved quality, reduced energy consumption and chemical consumption. SUMMARY

[0009] The purpose of this invention is to provide a dissolving slurry production process that simplifies the process, improves quality, and reduces energy and chemical consumption.

[0010] To achieve the above objectives, the present invention provides the following technical solution:

[0011] This invention proposes a dissolving slurry production process, comprising the following steps:

[0012] S1. Take wood chips or short cotton linters, rinse and soak them, and drain them naturally to obtain pre-treated raw materials;

[0013] S2, add conditioning water to the pretreatment raw material, then add sodium sulfite, mix and stir, heat and keep warm, extract the pre-hydrolysis waste liquid to obtain a semi-slurry; the volume ratio of the oven-dry mass of the pretreatment raw material to the conditioning water is 1:5-8; the amount of sodium sulfite, calculated as SO2, is 2%-4% of the oven-dry mass of the pretreatment raw material; the heating and holding temperature is 150-160℃, and the holding time is 90-120min;

[0014] SO3 in sodium sulfite 2- It undergoes a sulfonation reaction with the lignin in the raw material to produce water-soluble sulfonated lignin; at the same time, SO3 2- Catalyzing the glycosidic bond cleavage of hemicellulose achieves selective removal of 50%-70% hemicellulose and 30%-40% lignin; a mild hydrolysis temperature (150-160℃) reduces cellulose molecular chain breakage and preserves the degree of cellulose polymerization; sodium sulfite pre-hydrolysis achieves lignin sulfonation and efficient removal of hemicellulose; the finished dissolving pulp has an α-cellulose content of 96.5%-98.0% and a lignin residue of <0.15%; the mild pre-hydrolysis temperature (150-160℃) reduces cellulose degradation, and the cuprammonium viscosity reaches 45-60 mPa·s;

[0015] S3, the semi-slurry is subjected to sulfate cooking to obtain coarse slurry and cooking black liquor;

[0016] S4. The coarse pulp is bleached, pulped, and sieved using the ODOEOPD1 step sequence to obtain the dissolving pulp.

[0017] In step S2, the pre-hydrolyzed waste liquid extracted is sent to a circulating MVR evaporator for MVR concentration to obtain concentrated waste liquid (solid content 30%-35%) and MVR condensate (containing SO2 50-200 mg / L). NaOH is added to the MVR condensate to adjust the pH to 1.5-3.0, and sodium sulfite is added at the same time to make the total SO32- concentration in the MVR condensate reach 2-5 g / L, which is the recycled conditioning water. The recycled conditioning water is then returned to step S2 for use as conditioning water.

[0018] In the above technical solution, adding NaOH to adjust the pH to 1.5-3.0 is to neutralize free SO2 and convert it into sulfite.

[0019] Furthermore, the pre-hydrolysis process in step S2 is performed by monitoring the pH in real time using an online pH meter to maintain the pH between 1.5 and 3.0.

[0020] Furthermore, step S2 is carried out in a sulfate cooking pot; the sulfate cooking pot also serves as a pre-hydrolysis reaction vessel, and is made of acid and alkali resistant Hastelloy C-276 or 316L stainless steel, which is suitable for the acidic environment of pre-hydrolysis and the alkaline environment of subsequent cooking.

[0021] Furthermore, in step S3, the cooking conditions are as follows: the ratio of the oven-dry mass of the semi-slurry to the volume of the cooking liquid is 1:3-5; the cooking temperature is 165-175℃; the holding time is 120-150 min; the active alkali, calculated as NaOH, is used at 18%-22% of the oven-dry mass of the semi-slurry; and the amount of sodium sulfide (Na2S) is 2%-3% of the oven-dry mass of the semi-slurry.

[0022] Furthermore, in step S4, the bleaching process conditions for the OD0EOPD1 segment are as follows:

[0023] O-stage: reaction temperature 100-110℃, NaOH dosage 2%-3% of the oven-dry weight of the crude pulp, oxygen pressure 0.5-0.6MPa, holding time 60-90min, removing 30%-40% of residual lignin;

[0024] D0 stage: reaction temperature 60-70℃, ClO2 dosage is 0.3%-0.5% of the oven-dry mass of the crude slurry.

[0025] The system pH is 2.0-3.0, the incubation time is 45-60 min, and the chromophores of oxidized lignin are activated.

[0026] EOP section: reaction temperature 80-90℃, NaOH dosage is 1%-2% of the oven-dry weight of the crude pulp, H2O2 dosage is 0.5%-0.8% of the oven-dry weight of the crude pulp, heat preservation time 60-80min, deep removal of lignin and protection of cellulose;

[0027] D1 stage: reaction temperature 70-80℃, ClO2 dosage is 0.2%-0.4% of the oven-dry mass of the coarse pulp, system pH 3.0-4.0, heat preservation time 60-90min, to obtain bleached pulp with a whiteness of 88%-92%.

[0028] Furthermore, the beating degree of the pulp in step S4 is 30-40°SR.

[0029] Furthermore, in step S4, the filtration is performed using a pressure screen with a sieve opening of 0.15 mm.

[0030] Furthermore, the operating conditions for MVR concentration are: vacuum degree -0.08 to -0.09 MPa, evaporation temperature 60-80℃, and feed flow rate 1.5-2.5 m / s.

[0031] Further, liquid alkali is added to the concentrated waste liquid, stirred, and the pH is adjusted to 9.0-10.0; the concentrated waste liquid after pH adjustment is mixed with the cooking black liquor in step S3 to obtain mixed black liquor; the mixed black liquor is evaporated, burned, and subjected to a causticization reaction to generate NaOH. The NaOH generated by causticization can be used to supplement the active alkali in the sulfate cooking in step S3 and to adjust the pH of the concentrated waste liquid in step S2, so as to realize the recycling of NaOH;

[0032] In the above technical solution, the pH is adjusted to 9.0-10.0 to match the pH range of 12-14 of the cooking black liquor in step S3, so that the concentrated waste liquid and the cooking black liquor do not separate or neutralize and form scale after mixing.

[0033] Furthermore, the specific operational steps for evaporation, combustion, and causticization reaction to generate NaOH are as follows: The mixed black liquor is concentrated to a solid content of 65-70% through a multi-effect evaporator, and then fed into an alkali furnace for combustion at 800-1000℃. The recovered heat energy is used for heating in the dissolving slurry production process, and the melt is dissolved to obtain green liquor. Lime milk is added to the green liquor to carry out a causticization reaction to generate NaOH.

[0034] Compared with the existing steam pre-hydrolysis-sulfate process, the quality of the dissolving pulp of this invention is significantly improved: sodium sulfite pre-hydrolysis achieves efficient lignin sulfonation and hemicellulose removal, with the finished dissolving pulp having an α-cellulose content of 96.5%-98.0% (94.0%-95.5% in the traditional process) and a lignin residue of <0.15% (0.2%-0.3% in the traditional process); the mild pre-hydrolysis temperature (150-160℃) reduces cellulose degradation, and the cuprammonium viscosity reaches 45-60 mPa·s (35-50 mPa·s in the traditional process); sulfonation introduces sulfonic acid groups on the cellulose surface, with a Fock reaction value of 85%-92% (75%-85% in the traditional process), resulting in better reaction performance and meeting the needs of high-end applications such as cellulose acetate and Lyocell fiber;

[0035] The process flow is significantly simplified: the sulfate cooking pot doubles as the pre-hydrolysis reactor, eliminating the need for additional dedicated pre-hydrolysis equipment and reducing equipment investment; the vacuum washing step of the semi-solid pulp after pre-hydrolysis is eliminated, and the waste liquid is directly extracted and then alkali is added for cooking, reducing the number of processes by 1-2, and reducing the pulp loss rate from 2%-3% to <1%, while also avoiding the consumption of washing water (the traditional process consumes about 5-8 cubic meters of washing water).3 / ton of pulp);

[0036] Reduced energy and water consumption: MVR evaporators utilize secondary steam compression and reuse, with an energy consumption of only 40-60 kWh per ton of water evaporation, saving 60%-70% more energy than traditional multi-effect evaporators; MVR condensate reuse rate is 60%-80%, fresh water consumption is reduced by more than 50%, and water resource utilization rate is increased to more than 65%.

[0037] Reduced chemical consumption: Sodium sulfite pre-hydrolysis reduces cooking load, and the amount of active alkali used is reduced by 15%-20% compared to traditional processes; ClO2 and NaOH usage in the bleaching stage is reduced by 20%-35%; condensate is reused to recover 30%-40% of unreacted SO2, and the amount of fresh sodium sulfite used is reduced by 30%; the alkali recovery system realizes NaOH recycling, resulting in a 10%-15% reduction in total chemical consumption compared to traditional processes and an 8%-12% reduction in overall costs;

[0038] Excellent environmental friendliness and compatibility: The pre-hydrolyzed waste liquid is concentrated by MVR and then mixed with black liquor for traditional alkali recovery, eliminating the need for additional waste liquid treatment equipment and ensuring compatibility with existing production lines; SO2, Na + The system achieves recycling through condensate reuse and alkali recovery, resulting in no secondary pollutant emissions and meeting green production requirements.

[0039] Instruction manual illustrations

[0040] Figure 1 This is a block flow diagram of the dissolving slurry production process of the present invention. Arrows indicate the flow direction of materials / chemicals. Detailed Implementation

[0041] The technical solution of the present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0042] Example 1

[0043] Step 1: Raw material pretreatment

[0044] Eucalyptus chips (1000 kg oven-dry weight, specifications: length 25-35 mm, thickness 3-4 mm) were selected and screened by a vibrating screen to remove debris (particle size <3 mm, removal amount 50 kg) and bark; rinsed twice with fresh water (800 L of water each time), soaked for 30 min and then drained naturally to obtain 945 kg of pretreated eucalyptus chips (780 kg oven-dry weight, no obvious mud or sand on the surface).

[0045] Step 2: Pre-hydrolysis of sodium sulfite

[0046] The pretreated eucalyptus chips were fed into a 10m³ sulfate digester made of Hastelloy C-276 alloy. 3 Add 6240L of conditioning water (water and recycled conditioning water from step 5) (liquid ratio 1:8); add sodium sulfite (industrial grade, 31.2kg as SO2, 4% of the oven-dry raw material), stir for 10min to mix evenly; heat to 155℃ with steam, keep warm for 100min, and maintain pH at 2.0-2.5 with online pH meter monitoring to complete pre-hydrolysis, obtaining semi-slurry and pre-hydrolyzed waste liquid.

[0047] Step 3: Transfer of pre-hydrolysis waste liquid and sulfate cooking

[0048] The pre-hydrolyzed waste liquid (7.2% solids, pH 2.3) is transported to a 316L stainless steel waste liquid storage tank (8m³) via an acid-resistant drainage pipe at the bottom of the cooker. 3 To prevent acid corrosion, cooking liquor (117 kg of active alkali NaOH, representing 18% of the oven-dry weight of the half-slurry; 13 kg of Na₂S, representing 2% of the oven-dry weight of the half-slurry) is added to the remaining semi-slurry in the pot, controlling the liquor ratio at 1:4. The mixture is heated to 170℃ and held for 130 min to complete sulfate cooking, yielding a coarse pulp (600 kg oven-dry weight) and cooking black liquor (pH 13.0, solid content 18%). The cooking black liquor is temporarily stored in a black liquor tank (5 m³ / s). 3 ).

[0049] Step 4: Bleaching of coarse pulp

[0050] The coarse pulp was treated using the OD0EOPD1 sequence:

[0051] Section O: 105℃, 15kg NaOH (2.5% of oven-dry coarse pulp), oxygen pressure 0.55MPa, heat treatment for 75min;

[0052] D0 section: 65℃, ClO2 2.4kg (0.4% of oven-dry coarse pulp), pH 2.5, heat treatment for 50min;

[0053] EOP section: 85℃, NaOH 9kg (1.5% of oven-dry coarse pulp), H2O2 4.2kg (0.7% of oven-dry coarse pulp), hold for 70min;

[0054] D1 stage: 75℃, ClO2 1.8kg (0.3% of oven-dry coarse pulp), pH 3.5, heat treatment for 75min;

[0055] The bleached pulp (brightness 90.5% ISO) was obtained, pulped to 35°SR by a pulper, and impurities were removed by pressure screening through a 0.15mm sieve to obtain 580kg of dissolving pulp product.

[0056] Step 5: MVR concentration of pre-hydrolyzed waste liquid and condensate conditioning and reuse

[0057] MVR Concentration: The pre-hydrolyzed waste liquid in the waste liquid storage tank is sent to a forced circulation MVR evaporator (model MVR-10, heat exchange area 100m²). 2 The vacuum degree was controlled at -0.085MPa, the evaporation temperature at 70℃, and the liquid flow rate at 2.0m / s. After evaporation for 4 hours, 816L of concentrated waste liquid (solid content 35%, pH 2.2) and 5984L of MVR condensate (SO2 content 120mg / L) were obtained.

[0058] Concentrated waste liquor treatment: Add liquid alkali to the concentrated waste liquor, stir to adjust the pH to 9.5, and then transport it to the black liquor tank to mix with the cooking black liquor to obtain mixed black liquor;

[0059] Condensate water conditioning: The SO2 content in the MVR condensate water was determined by ion chromatography. NaOH was added to the MVR condensate water to adjust the pH to 2.2; simultaneously, fresh sodium sulfite was added to ensure adequate total SO3 levels. 2- When the concentration reaches 3.5 g / L, recycled conditioning water is obtained, which can be reused as conditioning water in step 2 to participate in pre-hydrolysis.

[0060] Step 6: Alkali recovery system processing

[0061] The mixed black liquor is fed into the alkali recovery system, concentrated to a solid content of 68% by multi-effect evaporation, and then fed into the alkali furnace (900℃) for combustion, generating 1.2t of steam, which can be used for process heating; the melt is dissolved to obtain green liquor, which is then causticized with lime milk (12% Ca(OH)2) to obtain regenerated NaOH; the regenerated NaOH can be used for sulfate cooking in step 3 and pH adjustment of the concentrated waste liquid in step 5.

[0062] Example 2

[0063] Step 1: Raw material pretreatment

[0064] Pine chips (1000 kg oven-dry weight, specifications: length 30-40 mm, thickness 4-5 mm) were selected and screened by a vibrating screen to remove debris (particle size <3 mm, removal amount 45 kg) and bark; they were rinsed once with process recycled water (from the clean water before MVR condensate adjustment) (water consumption 700 L), and allowed to drain naturally to obtain 940 kg of pretreated pine chips (780.2 kg oven-dry weight).

[0065] Step 2: Pre-hydrolysis of sodium sulfite

[0066] The pretreated pine chips were fed into a 316L stainless steel sulfate digester (8m³ capacity). 3Add 3901L of adjusting water (liquid ratio 1:5); add sodium sulfite (15.6kg as SO2, which is 2% of the oven-dry raw material), stir and mix evenly; heat to 160℃, keep warm for 120min, maintain pH 2.5-3.0, and obtain pre-hydrolyzed waste liquid (solid content 6.8%) and semi-slurry.

[0067] Step 3: Transfer of pre-hydrolysis waste liquid and sulfate cooking

[0068] The pre-hydrolyzed waste liquid was pumped into a waste liquid storage tank; cooking liquor (149.6 kg of active alkali NaOH, which is 22% of the oven-dry weight of the half-slurry; 13 kg of Na2S, which is 2% of the oven-dry weight of the half-slurry) was injected into the half-slurry in the pot, and the liquor ratio was controlled at 1:5; the mixture was heated to 175℃ and kept at that temperature for 150 min to obtain coarse pulp (590 kg oven-dry weight) and cooking black liquor (pH 13.2).

[0069] Step 4: Bleaching of coarse pulp

[0070] Using the ODOEOPD1 sequence, the total amount of ClO2 was adjusted to 0.8% (octane-dry coarse pulp), and other conditions were the same as in Example 1, to obtain bleached pulp (brightness 91.2% ISO). After beating and impurity removal, 575 kg of dissolving pulp (α-cellulose content 97.2%, Fock reaction value 88.5%) was obtained.

[0071] Steps 5-6: Same as in Example 1, finally obtaining reclaimed conditioning water and regenerated NaOH.

[0072] Comparative test

[0073] Based on the eucalyptus raw material of Example 1, the conventional steam pre-hydrolysis sulfate process (control group) was used. The specific differences in the control group are as follows:

[0074] Pre-hydrolysis: using saturated steam at 175℃, holding for 120 minutes, without adding sodium sulfite;

[0075] Semi-slurry treatment: After pre-hydrolysis, the semi-slurry is transferred to a vacuum washer and washed three times with 75℃ hot water (water consumption 12m³). 3 ), to remove attached waste liquid;

[0076] Waste liquid treatment: Pre-hydrolyzed waste liquid is directly sent to multi-effect evaporation concentration (without MVR), and the condensate is not reused;

[0077] The other steps (cooking, bleaching, alkali recovery) are the same as in Example 1.

[0078] The performance of the dissolving slurry and the chemical consumption of Example 1 and the control group are shown in Tables 1 and 2 below.

[0079] Table 1: Comparison of dissolving pulp properties

[0080]

[0081]

[0082] Table 2: Comparison of Chemical Consumption and Energy Consumption (per ton of oven-dry dissolving slurry)

[0083]

[0084] While specific embodiments of the present invention have been described above, those skilled in the art should understand that the specific embodiments described are merely illustrative and not intended to limit the scope of the invention. Modifications and variations made by those skilled in the art in accordance with the spirit of the invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A dissolving slurry production process, characterized in that: Includes the following steps: S1. Take wood chips or short cotton linters, rinse and soak them, and drain them naturally to obtain pre-treated raw materials; S2, add conditioning water to the pretreatment raw material, then add sodium sulfite, mix and stir, heat and keep warm, extract the pre-hydrolysis waste liquid to obtain a semi-slurry; the volume ratio of the oven-dry mass of the pretreatment raw material to the conditioning water is 1:5-8; the amount of sodium sulfite, calculated as SO2, is 2%-4% of the oven-dry mass of the pretreatment raw material; the heating and holding temperature is 150-160℃, and the holding time is 90-120min; S3, the semi-slurry is subjected to sulfate cooking to obtain coarse slurry and cooking black liquor; S4. The coarse pulp is bleached, pulped, and sieved using the ODOEOPD1 step sequence to obtain the dissolving pulp. In step S2, the pre-hydrolyzed waste liquid extracted is sent to a circulating MVR evaporator for MVR concentration to obtain concentrated waste liquid and MVR condensate. NaOH is added to the MVR condensate to adjust the pH to 1.5-3.0, and sodium sulfite is added at the same time to make the total SO32- concentration in the MVR condensate reach 2-5 g / L, which is the recycled conditioning water. The recycled conditioning water is then returned to step S2 for use as conditioning water.

2. The dissolving slurry production process according to claim 1, characterized in that: The pre-hydrolysis process in step S2 involves real-time pH monitoring using an online pH meter to maintain the pH between 1.5 and 3.

0.

3. The dissolving slurry production process according to claim 1, characterized in that: In step S3, the cooking conditions are as follows: the ratio of the oven-dry mass of the semi-slurry to the volume of the cooking liquid is 1:3-5; the cooking temperature is 165-175℃; the holding time is 120-150 min; the active alkali is calculated as NaOH and the dosage is 18%-22% of the oven-dry mass of the semi-slurry; the dosage of sodium sulfide (Na2S) is 2%-3% of the oven-dry mass of the semi-slurry.

4. The dissolving slurry production process according to claim 1, characterized in that: In step S4, the bleaching process conditions for the OD0EOPD1 segment are as follows: O-stage: reaction temperature 100-110℃, NaOH dosage 2%-3% of the oven-dry weight of the crude pulp, oxygen pressure 0.5-0.6MPa, holding time 60-90min, removing 30%-40% of residual lignin; D0 section: reaction temperature 60-70℃, ClO2 dosage is 0.3%-0.5% of the oven-dry mass of the crude pulp, system pH 2.0-3.0, holding time 45-60 min, oxidized lignin chromophores; EOP section: reaction temperature 80-90℃, NaOH dosage is 1%-2% of the oven-dry weight of the crude pulp, H2O2 dosage is 0.5%-0.8% of the oven-dry weight of the crude pulp, heat preservation time 60-80min, deep removal of lignin and protection of cellulose; D1 stage: reaction temperature 70-80℃, ClO2 dosage is 0.2%-0.4% of the oven-dry mass of the coarse pulp, system pH 3.0-4.0, heat preservation time 60-90min, to obtain bleached pulp with a whiteness of 88%-92%.

5. The dissolving slurry production process according to claim 1, characterized in that: The beating degree of the pulp in step S4 is 30-40°SR.

6. The dissolving slurry production process according to claim 1, characterized in that: In step S4, the filtration is performed using a pressure screen with a sieve opening of 0.15 mm.

7. The dissolving slurry production process according to claim 1, characterized in that: The operating conditions for MVR concentration are: vacuum degree -0.08 to -0.09 MPa, evaporation temperature 60-80℃, and feed flow rate 1.5-2.5 m / s.

8. The dissolving slurry production process according to claim 1, characterized in that: Liquid alkali is added to the concentrated waste liquid, stirred, and the pH is adjusted to 9.0-10.

0. The concentrated waste liquid after pH adjustment is mixed with the cooking black liquor in step S3 to obtain mixed black liquor. The mixed black liquor is evaporated, burned, and subjected to a causticization reaction to generate NaOH. The NaOH generated by causticization can be used to supplement the active alkali in the sulfate cooking in step S3 and to adjust the pH of the concentrated waste liquid in step S2, so as to realize the recycling of NaOH.

9. The dissolving slurry production process according to claim 8, characterized in that: The specific operation steps for evaporation, combustion, and causticization reaction to generate NaOH are as follows: The mixed black liquor is concentrated to a solid content of 65-70% through a multi-effect evaporator, and then fed into an alkali furnace for combustion at 800-1000℃. The recovered heat energy is used for heating in the dissolving slurry production process, and the melt is dissolved to obtain green liquor. Lime milk is added to the green liquor to carry out a causticization reaction to generate NaOH.

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