Nanofiltration treatment method of impregnation liquid for viscose fiber production

Abstract

The invention relates to a viscose fiber production impregnation liquid nanofiltration treatment method, which comprises the following steps: sequentially adding a reducing agent and a complexing agent into an impregnation liquid, carrying out nanofiltration treatment through a nanofiltration membrane, and collecting a permeate as a recovered alkali liquid. According to the method, the reducing agent sodium sulfite is added firstly to reduce the cobalt ions from trivalent to divalent, meanwhile, the sodium sulfite consumes free oxygen in feed liquid, divalent cobalt is in a dissolved state, the next-step reaction efficiency is improved, then EDTA is added to form a complex with a cobalt ion compound, the complex loses the catalytic action, and the cobalt ions are separated from the cobalt ions. And a protection effect on the nanofiltration membrane and related rubber sealing elements is achieved. The service life of the nanofiltration membrane can be prolonged to more than 2 years.

Description

TECHNICAL FIELD

[0001] The application relates to a viscose fiber production impregnation liquid nanofiltration treatment method and relates to the technical field of sewage treatment. BACKGROUND

[0002] Currently, the main raw material of the viscose fiber production industry is changed from the previous dissolving pulp to pulp. In the production process, first, alkali solution is soaked to remove impurities such as waxes and lipids other than fibers, and then the fibers are recovered through filtration. The impregnation liquid produced by filtration contains impurities and a very high sodium hydroxide content, usually more than 200 grams per liter. The temperature of the impregnation liquid is higher than 50 DEG C. In the existing process, an alkali-resistant nanofiltration membrane is used to recover the alkali solution. The alkali solution can pass through the nanofiltration membrane, and the related lipids, sugars and waxes are concentrated by the nanofiltration membrane and used to prepare xylose. The alkali solution is reused and enters the impregnation process again.

[0003] In order to reduce the degree of polymerization of the liquid, a certain amount of cobalt chloride is usually added in the current impregnation process. Cobalt chloride acts as a catalyst to break the carboxyl group of organic matter, thereby achieving the purpose of reducing the degree of polymerization of the material. Cobalt chloride enters the liquid from divalent to trivalent and participates in the catalytic reaction. Trivalent cobalt finally enters the impregnation liquid. The alkali-resistant nanofiltration membrane is made of sulfonated polyether sulfone material. Trivalent cobalt has a serious destructive effect on the nanofiltration membrane during nanofiltration treatment. At a temperature of 50 DEG C, the catalytic oxidation effect of trivalent cobalt is 2-3 times higher than that at room temperature, especially in the case of high alkali concentration and high oxygen content. This destructive effect is devastating, and the service life of the membrane module is less than 5 months. The high replacement cost of the nanofiltration membrane limits the application of nanofiltration treatment of viscose fiber impregnation liquid. SUMMARY

[0004] The purpose of the present application is to solve the problem of short service life of nanofiltration membrane in nanofiltration treatment of viscose fiber production impregnation liquid, and to provide a viscose fiber production impregnation liquid nanofiltration treatment method. A viscose fiber production impregnation liquid nanofiltration treatment method, the impregnation liquid is sequentially added with a reducing agent and a complexing agent, and then subjected to nanofiltration treatment through a nanofiltration membrane. The permeate is collected as recovered alkali solution.

[0005] Further, the amount of the reducing agent added is 5-20 PPM / L; and the amount of the complexing agent added is 2-20 PPM / L.

[0006] Further, the reducing agent is sulfite, such as sodium sulfite and sodium bisulfite.

[0007] Further, the reducing agent is sodium sulfite.

[0008] Further, the complexing agent is EDTA.

[0009] Furthermore, the nanofiltration process includes a high-pressure concentration stage and a low-pressure cleaning stage. In the high-pressure concentration stage, the amount of reducing agent added is 2-10 PPM / L, and a portion of the permeate is collected as the cleaning solution for the low-pressure cleaning stage. In the low-pressure cleaning stage, the cleaning solution and the impregnation solution are mixed to circulate and clean the nanofiltration membrane. The permeate is used as the recovered alkali solution, and the circulating solution is used as the feed solution and mixed with the impregnation solution.

[0010] Furthermore, the cleaning solution and the impregnation solution are mixed at a volume ratio of 1:1-2.

[0011] Furthermore, a primary feed pipe and a secondary feed pipe are sequentially installed along the flow direction at the inlet end of the nanofiltration treatment system. Both the primary and secondary feed pipes are equipped with a pressure pump and a flow valve. Sodium sulfite solution is added to the primary feed pipe through the pressure pump, and EDTA solution is added to the secondary feed pipe through the pressure pump.

[0012] This invention employs a method of first adding sodium sulfite as a reducing agent to reduce cobalt ions from trivalent to divalent. Simultaneously, sodium sulfite consumes free oxygen in the feed solution, leaving the divalent cobalt in a dissolved state, thus improving the efficiency of the next reaction step. Then, EDTA is added to form a complex with the cobalt ion compound. This complex loses its catalytic effect, protecting the nanofiltration membrane and related rubber seals. This invention can extend the lifespan of nanofiltration membranes to more than two years. Attached Figure Description

[0013] Figure 1 Anatomical image of a membrane module being replaced.

[0014] Figure 2 Enlarged image of the membrane surface during membrane module replacement.

[0015] Figure 3 Image of the permeate from a membrane pressure staining experiment. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0017] Taking a viscose fiber manufacturer as an example, in the alkaline impregnation process, cobalt chloride catalyst is added at a ratio of 0.005 wt%, resulting in an impregnation solution with a sodium hydroxide content of 200 g / L and a cobalt content of 5 mg / L, at a temperature of 55°C. Nanofiltration treatment uses a sulfonated polyethersulfone nanofiltration membrane. During nanofiltration, the operating pressure is 1–1.5 MPa, and the cleaning pressure is 0.3–0.5 MPa. After 10 hours of high-pressure operation, it is cleaned for 1 hour using alkaline solution circulation. Before the implementation of this invention, the service life of the membrane module was only 5 months. After the membrane structure is damaged, the permeate color darkens, and the organic matter rejection rate decreases.Figure 1 and Figure 2 Dissection of the replaced membrane module revealed corrosion and damage on the membrane surface. A pressure staining test was performed on samples of the membrane. Figure 3 The obvious color of the permeate indicates a significant disruption in the retention rate.

[0018] Prior to the implementation of this invention, the solutions listed in Comparative Examples 1-2 were used to attempt to solve the problem of nanofiltration membrane corrosion and damage.

[0019] In the following comparative examples and embodiments, the reducing agent and complexing agent are added in solution form via a pressure pump into the feed pipe. The reducing agent is a 10% (w / w) sodium sulfite solution, and the complexing agent is a 17% (w / w) EDTA solution. Specifically, a primary feed pipe and a secondary feed pipe are sequentially installed along the flow direction at the inlet end of the nanofiltration system. Both the primary and secondary feed pipes are equipped with pressure pumps and flow valves. Sodium sulfite solution is added to the primary feed pipe via the pressure pump, and EDTA solution is added to the secondary feed pipe via the pressure pump.

[0020] Comparative Example 1 Since the membrane structure damage was caused by cobalt oxidation, the first consideration was adding a cobalt complexing agent to the impregnation solution. EDTA is a commonly used cobalt complexing agent that can form a stable complex with cobalt metal ions, thus deactivating them. Adding 5–20 PPM / L of EDTA to the impregnation solution before nanofiltration was performed. Using this approach, the nanofiltration system still exhibited problems such as a darkening of the permeate color and a decrease in organic matter rejection rate after approximately 5 months of operation. It is speculated that the addition of the complexing agent did not form a stable complex with the cobalt metal ions, thus failing to achieve the desired effect.

[0021] Comparative Example 2 Further analysis revealed that cobalt ions in the impregnation solution existed in the trivalent form. Trivalent cobalt exhibits significant catalytic oxidation under high dissolved oxygen conditions. Adding a reducing agent to the impregnation solution was considered, as it could both reduce trivalent cobalt to divalent cobalt and remove dissolved oxygen. A sodium sulfite reducing agent of 2–20 PPM / L was added to the impregnation solution before nanofiltration. Using this method, the membrane module lifespan was extended to 7 months. However, after approximately 7 months of operation, the nanofiltration system exhibited problems such as a darkening of the permeate color and a decrease in organic matter rejection rate. This indicates that the addition of the reducing agent alleviated the oxidation problem of the nanofiltration membrane by trivalent cobalt, but still did not achieve the expected results. It is speculated that the high dissolved oxygen content in the impregnation solution meant that adding a small amount of reducing agent could not completely solve the catalytic oxidation problem of trivalent cobalt. Furthermore, adding too much reducing agent would increase costs and exacerbate the scaling problem of the nanofiltration membrane.

[0022] Example 1 The present invention employs a method where 5–20 PPM / L of sodium sulfite reducing agent is first added to the impregnation solution, followed by 2–20 PPM / L of EDTA complexing agent. Since the impregnation solution is strongly alkaline, the added cobalt chloride is converted to trivalent cobalt. Trivalent cobalt actually exists as a solid Go(OH)3 suspended in the impregnation solution, and directly adding the complexing agent cannot form a complex with Go(OH)3. However, after adding a small amount of reducing agent, the trivalent cobalt is reduced to divalent cobalt, becoming dissolved and able to form a stable complex with the complexing agent, thus losing its catalytic activity. Using this method, the service life of the membrane module is extended to 2 years.

[0023] Example 2 This embodiment is an improvement on Embodiment 1. The addition of sodium sulfite increases the sulfate ion concentration in the impregnation solution, leading to sulfate ion enrichment during the recycling of the alkali solution, which increases scaling on the nanofiltration membrane. The reduction effect of divalent cobalt and dissolved oxygen consumption in the impregnation solution are related to the amount of sodium sulfite added. Considering the nanofiltration system's operating scheme, trivalent cobalt has a more significant destructive effect on the nanofiltration membrane during the high-pressure concentration stage. This embodiment maintains the sodium sulfite addition at 2–10 PPM / L during the high-pressure concentration stage to ensure effective reduction of trivalent cobalt and minimize dissolved oxygen consumption, facilitating complete complexation of cobalt ions after the subsequent addition of EDTA. Unreacted sodium sulfite enters the permeate. A portion of the permeate from the high-pressure concentration stage is collected as the washing solution for the cleaning stage. This washing solution is mixed with the raw material impregnation solution at a ratio of 1:1-2 and used to circulate and clean the nanofiltration membrane. The circulating liquid generated during the cleaning process is still used as the raw material solution in the raw material tank, and the generated permeate is used as the recycled alkali solution in the alkali solution tank. This solution reduces the amount of reducing agent added during the high-pressure concentration stage, and allows for the reuse of some of the residual reducing agent in the alkaline solution during the cleaning stage, ensuring effective reduction of trivalent cobalt in the impregnation solution and minimizing dissolved oxygen consumption. Simultaneously, the reduced total amount of reducing agent decreases the enrichment of sulfate ions in the impregnation solution, thus mitigating scaling issues on the nanofiltration membrane.

Claims

1. A nanofiltration treatment method for impregnating liquor in viscose fiber production, characterized in that, After adding reducing agent and complexing agent to the impregnation solution in sequence, it is then subjected to nanofiltration through a nanofiltration membrane, and the permeate is collected as a recovered alkali solution.

2. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 1, characterized in that, The amount of reducing agent added is 5–20 PPM / L; the amount of complexing agent added is 2–20 PPM / L.

3. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 1, characterized in that, The reducing agent is sulfite.

4. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 1, characterized in that, The reducing agent is sodium sulfite.

5. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 1, characterized in that, The complexing agent is EDTA.

6. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 1, characterized in that, The nanofiltration process includes a high-pressure concentration stage and a low-pressure cleaning stage. The amount of reducing agent added in the high-pressure concentration stage is 2 to 10 PPM / L, and a portion of the permeate is collected as the cleaning solution for the low-pressure cleaning stage. In the low-pressure cleaning stage, the cleaning solution and impregnation solution are mixed to circulate and clean the nanofiltration membrane. The permeate is used as the recovered alkali solution, and the circulating solution is used as the feed solution and mixed with the impregnation solution.

7. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 6, characterized in that, The cleaning solution and the impregnation solution are mixed at a volume ratio of 1:1-2.

8. The nanofiltration treatment method for viscose fiber production impregnation solution according to claim 6, characterized in that, A primary feed pipe and a secondary feed pipe are sequentially installed along the flow direction at the inlet end of the nanofiltration treatment system. Both the primary and secondary feed pipes are equipped with a pressure pump and a flow valve. Sodium sulfite solution is added to the primary feed pipe through the pressure pump, and EDTA solution is added to the secondary feed pipe through the pressure pump.

Images