A method for recovering DMAC solvent and lithium chloride in aramid production waste liquid

Abstract

The invention relates to a method for recovering DMAC solvent and lithium chloride in aramid production waste liquid, belongs to the technical field of waste liquid treatment, and solves at least one of the problems that DMAC solvent and lithium chloride in aramid production waste liquid cannot be recovered simultaneously in the prior art, environmental pollution is caused during the recovery process, and the energy consumption of the recovery process is high. The invention provides a method for recovering DMAC solvent and lithium chloride in aramid production waste liquid, which realizes the simultaneous recovery of DMAC solvent and lithium chloride in aramid production waste liquid through flocculation precipitation, azeotropic distillation, distillation + evaporation crystallization and other process sections in sequence, and the azeotropic agent can be recycled; the above method has low energy consumption, does not introduce highly toxic extractants and other substances, is environmentally friendly and has high separation and recovery efficiency, and the total recovery rate of DMAC can reach 97%, and the total recovery rate of lithium chloride can reach 95%.

Description

Technical Field

[0001] The invention relates to the technical field of waste liquid treatment, in particular to a method for recovering DMAC solvent and lithium chloride in aramid production waste liquid. Background Art

[0002] N,N-dimethylacetamide, abbreviated as DMAC, is a highly polar colorless transparent liquid that can dissolve a variety of compounds and is completely miscible with water, ether, ketones, esters, etc. It has the characteristics of high thermal stability, not easy to hydrolyze, low corrosiveness, and low toxicity. It is often used as a raw material for synthetic fibers and an excellent polar solvent for organic synthesis.

[0003] In the process of fiber forming, a large amount of waste liquid will be produced, wherein the waste liquid (by mass percentage) contains dimethylacetamide (25%-40%), lithium chloride (0.2-2%), oligomer (0.5%-1.5%), and the rest is water. DMAC is used in large quantities in the aramid spinning industry, and its market price is high. It has certain toxicity again. It will have a great impact on the water quality environment when discharged into the water body during production, use and disposal, endangering people's health and causing waste simultaneously. Lithium chloride is an inorganic salt, and if it is not recycled and directly discharged, it will also harm the environment and the animals and plants in the environment. Recycling is beneficial to the environment and production cycle and improves product quality. Therefore, the recovery and treatment of DMAC-LiCl waste liquid in aramid spinning production has important significance in terms of protecting the environment and reducing the production cost of enterprises.

[0004] At present, the methods for recovering DMAC-LiCl waste liquid in aramid production mainly include extraction distillation and evaporation distillation. Solution extraction is an operation method that uses the principle of different solubility of solutes in immiscible solvents to transfer solutes from a solvent with low solubility to another solvent with high solubility, and then separates them by distillation. Tsinghua University and the State Key Laboratory once simulated chloroform as an extractant to treat DMAC wastewater. The disadvantage of this method is that the extractant is introduced, the extractant is highly toxic, and the extractant is easy to cause secondary pollution to the environment; at the same time, in the extraction recovery, no recovery method for lithium chloride is provided. Evaporation distillation is to separate and recover DMAC and LiCl by evaporating all the waste liquid and then distilling it. This recovery of dimethylacetamide in high-concentration wastewater such as 70-90% does not consume much energy, but in low-concentration DMAC such as 50% or lower, if the distillation method is used, its energy consumption is huge.

[0005] There is no public report on a method for recovering DMAC solvent and lithium chloride from aramid production waste liquid at the same time, so it is very necessary to develop a method for recovering DMAC solvent and lithium chloride from aramid production waste liquid with low energy consumption, low toxicity and high efficiency. Summary of the invention

[0006] In view of the above analysis, the embodiment of the present invention aims to provide a method for recovering DMAC solvent and lithium chloride in aramid production waste liquid, so as to solve at least one of the problems in the prior art that DMAC solvent and lithium chloride in aramid production waste liquid cannot be recovered simultaneously, environmental pollution is caused during the recovery process, and the energy consumption of the recovery process is high.

[0007] The present invention provides a method for recovering DMAC solvent and lithium chloride in aramid production waste liquid, comprising the following specific steps:

[0008] S1: Separate oligomers and mechanical impurities from waste liquid through flocculation and filtration processes to obtain filter residue and filtrate;

[0009] S2: adding an azeotropic agent to the filtrate, conveying the filtrate to a primary azeotropic distillation tower, removing water from the waste liquid through an azeotropic distillation process, and obtaining a tower top distillate D1 containing the azeotropic agent and water and a tower bottom mixed liquid F1 containing the azeotropic agent, DMAC, and lithium chloride;

[0010] S3: The bottom mixed liquid F1 is subjected to primary distillation to obtain a top distillate D3 containing an azeotropic agent and DMAC and a bottom mixed liquid F3 containing DMAC and lithium chloride:

[0011] S4: transporting the mixed liquid F3 from the bottom of the tower to an evaporator to obtain DMAC vapor and concentrated lithium chloride solution;

[0012] S5: performing secondary distillation on the tower top distillate D3 and DMAC vapor to obtain tower top distillate D4, i.e., a high-concentration entrainer;

[0013] The gas phase discharge near the bottom of the tower is obtained, and high-purity DMAC is obtained after condensation;

[0014] S6: The concentrated lithium chloride solution is transported to a crystallization kettle, cooled and crystallized, and the lithium chloride product is obtained after filtration.

[0015] Specifically, the composition of the aramid production waste liquid is DMAC: 25% to 40% by mass, lithium chloride: 0.2% to 2% by mass, oligomer: 0.5% to 1.5% by mass, and the rest is water.

[0016] Preferably, the azeotropic agent is isobutanol.

[0017] Preferably, the mass ratio of the entrainer to the filtrate is 0.5 to 1:1.

[0018] Specifically, in step S1, a pH regulator needs to be added before flocculation to adjust the pH value of the waste liquid to be neutral to weakly alkaline, and the pH regulator is LiOH.

[0019] Specifically, the operating parameters of the primary azeotropic distillation tower in step S2 are: normal pressure, tower top temperature of 89.9-95° C., tower bottom temperature of 100-110° C., number of tower plates of 12-18, and reflux ratio of 0.5-5.

[0020] Specifically, a secondary azeotropic distillation tower is provided after the primary azeotropic distillation tower for further separation and recovery of the entrainer;

[0021] The operating parameters of the secondary azeotropic distillation tower are: normal pressure, tower top temperature 89-92°C, tower bottom temperature 99-105°C, number of tower plates 5-15, reflux ratio 0.1-0.5; a second condenser is provided behind the top discharge port of the secondary azeotropic distillation tower, and the cooling water temperature is 25-35°C.

[0022] Specifically, the primary distillation described in step S3 is carried out in a primary distillation tower, and the operating parameters of the tower are: vacuum degree 0.06-0.09Mpa, tower top temperature 60-90°C, tower bottom temperature 115-135°C, tower plate number 5-15, and reflux ratio 0.3-1.

[0023] Specifically, the secondary distillation described in step S5 is carried out in a secondary distillation tower, and the operating parameters of the tower are: vacuum degree 0.06-0.09Mpa, tower top temperature 60-80°C, tower bottom temperature 110-130°C, tower plate number 12-30, and reflux ratio 1-5.

[0024] Specifically, the specific parameters of the evaporation process in step S4 are that the evaporator temperature is 100-135° C. and the evaporation pressure is -0.1-0 MPa.

[0025] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0026] 1. The recovery method provided by the present invention has high recovery efficiency and good product purity; and can realize the simultaneous recovery of organic solvent DMAC and lithium chloride.

[0027] The present invention first uses a flocculant to act on oligomers in wastewater, so that oligomers flocculate and precipitate, and then filters and separates oligomers and mechanical impurities, preventing oligomers and mechanical impurities from precipitating, accumulating and clogging equipment when the wastewater is further distilled, which can greatly improve the distillation effect; it also prevents oligomers from entering the concentration and crystallization treatment unit, affecting the purity of the crystallized salt, and also prevents oligomers from entering the final distillation tower kettle, resulting in a large amount of tower kettle residues that need to be treated in the end, and the cost is greatly increased. An azeotropic agent is introduced into the system to reduce the boiling point of the material. At the same time, only water and the azeotropic agent are required to be distilled out of the tower according to the azeotropic ratio in the material to be treated, and all materials do not need to reach the boiling point distillation, which directly reduces energy consumption and improves the processing efficiency. Finally, in the refining stage, it is mainly the separation of isobutanol and DMAC. The boiling points of the two materials are very different, and the separation is easy, the separation effect is good, and the product purity is high; a salt solution concentration and crystallization treatment unit is added to the system, and the solvent and salt are recovered at the same time, and the recovery of the salt uses a crystallization operation method to ensure the high purity of the final product.

[0028] The total recovery rate of DMAC can reach 97%, and the total recovery rate of lithium chloride can reach 95%; wherein the detection purity of DMAC is ≥99.99%, the contents of water and alcohol are respectively <200ppm, the chromaticity is ≤3°, the acetic acid content is ≤500ppm, and the dimethylamine is ≤500ppm.

[0029] 2. The recovery method provided by the present invention does not introduce an extractant, and the entrainer used has low toxicity and can be recycled, with a low risk of secondary pollution and is environmentally friendly.

[0030] Among the extraction and distillation routes that are currently more commonly used, chloroform with good extraction effect is often used. However, chloroform is harmful to both the human body and the environment. First, it is highly toxic and has certain teratogenic and mutagenic effects on humans and animals. It is a substance on the list of Class 2B carcinogens and a suspected human carcinogen. Secondly, it is a substance on the list of toxic and harmful water pollutants. It has an accumulation effect in groundwater, which will pollute drinking water, food and vegetables, and damage the respiratory system of sensitive aquatic organisms. It is also easy to make poisonous chemicals. In the residual phase of the extraction operation, there will be 100 to 5000 ppm of chloroform residues, which is harmful to humans or causes secondary pollution to the environment. The entrainer used in the present invention, such as isobutanol, has very low toxicity and can even be used as an edible flavoring. At the same time, it is relatively easy to be biodegraded.

[0031] In the evaporation-distillation process in the prior art, the wastewater needs to be completely evaporated to separate the high-boiling-point substances (oligomers, salts and other impurities) and low-boiling-point substances (mainly water and solvents) dissolved in the wastewater. Because the boiling point of the solvent is high, and the latent heat of evaporation of water is large and the content is also high, the evaporation time is long, the energy consumption is large, and the material will also decompose into small molecular acetic acid and dimethylamine after a long period of heating, which reduces the quality and yield of the product.

[0032] Most of the entrainers that can be used in the present invention are alcohols containing 3 to 6 carbon atoms, and there are also a few benzenes and ethylene dichloride, preferably isobutyl alcohol. The boiling point is between 80 and 140 degrees, and the stability is good under normal conditions. Their azeotropic points with water are all lower than the boiling point of water, about 70 to 95 degrees, and the azeotropic ratio is high (water content is 15 to 50%). During operation, it is not necessary to use evaporation, and only the water or alcohol in the mixture needs to be taken out by azeotropy. At the same time, after azeotropy with water, the intermiscibility between the entrainer itself and water is low, which is conducive to subsequent separation and reuse (such as simple separation by an oil-water separator), and the entrainer is continuously recycled, thereby reducing energy consumption and improving the recovery rate.

[0033] Taking the above aspects into consideration, the azeotropic agent that can be used in the present invention has a higher azeotropic ratio, a lower azeotropic point, and relatively low energy consumption. In addition, they have low solubility in water at 40 to 60 degrees, can be quickly separated by stratification, have good separation effect, and have low toxicity. Isobutanol, which has low environmental impact, is preferred.

[0034] 3. The recovery method provided by the present invention has reasonable process settings and significantly reduces energy consumption.

[0035] In the process of the present invention, a flocculant is first used to flocculate and filter the oligomers in the waste liquid to separate the oligomers, thereby preventing the oligomers from clogging in the recovery equipment, such as the filling material of the distillation tower, and improving the azeotropic separation effect and separation efficiency. In addition, the cost of processing the final distillation residue is reduced. If the oligomers are left in the final distillation tower as the bottom residue, the amount of the bottom residue will be increased many times. These aspects can significantly reduce the operating energy consumption and cost.

[0036] After the oligomers are removed, the present invention takes the simple materials into recycling, and the first stage azeotropic separation solvent and salt are extracted from the bottom of the tower; the tower top material is separated by an oil-water separator, and the entrainer is recovered, and the material containing a small amount of entrainer is further separated from water and the entrainer by a simple azeotropic step, and then the entrainer is recovered; and the solvent and salt are simply evaporated, and the tower bottom concentrate enters the crystallization to recover the salt; and the evaporated material is rectified to obtain a high-purity solvent product, and the entrainer is recovered at the same time. The whole process is efficient, reasonable and energy-saving.

[0037] 4. The relevant reagents and devices / equipment of the recovery method provided by the present invention are easily available and can be combined or appropriately modified on the existing basis to meet the implementation requirements, which is conducive to large-scale production / processing and wide promotion.

[0038] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0040] Figure 1 The present invention is a process flow chart of the method for recovering organic solvent DMAC and lithium chloride from aramid production waste liquid.

[0041] Reference numerals:

[0042] 1-pH regulator; 2-flocculating agent; 3-waste liquid; 4-stirring tank; 5-first filter; 6-mixer; 7-primary azeotropic distillation tower; 8-reboiler; 9-first condenser; 10-first oil-water separator; 11-first reboiler; 12-secondary azeotropic distillation tower; 13-second condenser; 14-second oil-water separator; 15-second reboiler; 16-primary distillation tower; 17-third condenser; 18-fourth condenser; 1 9-fifth condenser; 20-evaporator; 21-secondary distillation tower; 22-sixth condenser; 23-crystallizer; 24-third reboiler; 25-organic solvent DMAC storage tank; 26-first delivery pump; 27-second filter; 28-DMAC / lithium chloride mixed storage tank; 29-second delivery pump; 30-entrainer (isobutanol) storage tank; 31-third delivery pump; 32-oligomer filter residue; 33-bottom waste liquid; 34-filter residue. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0044] The invention discloses a method for recovering an organic solvent DMAC and lithium chloride in aramid production waste liquid. The recovery method comprises three process sections, namely, a flocculation precipitation process section, an azeotropic distillation process section, and a "distillation + evaporation crystallization" process section, and finally obtains pure high-concentration organic solvent and lithium chloride crystals.

[0045] The present invention provides a method for recovering DMAC solvent and lithium chloride in aramid production waste liquid, comprising the following specific steps:

[0046] S1: Separate oligomers and mechanical impurities from waste liquid through flocculation and filtration processes to obtain filter residue and filtrate;

[0047] S2: adding an azeotropic agent to the filtrate, conveying the filtrate to a primary azeotropic distillation tower, removing water from the waste liquid through an azeotropic distillation process, and obtaining a tower top distillate D1 containing the azeotropic agent and water and a tower bottom mixed liquid F1 containing the azeotropic agent, DMAC, and lithium chloride;

[0048] S3: The bottom mixed liquid F1 is subjected to primary distillation to obtain a top distillate D3 containing an azeotropic agent and DMAC and a bottom mixed liquid F3 containing DMAC and lithium chloride:

[0049] S4: transporting the mixed liquid F3 from the bottom of the tower to an evaporator to obtain DMAC vapor and concentrated lithium chloride solution;

[0050] S5: performing secondary distillation on the tower top distillate D3 and DMAC vapor to obtain tower top distillate D4, i.e., a high-concentration entrainer;

[0051] The gas phase discharge near the bottom of the tower is obtained, and high-purity DMAC is obtained after condensation;

[0052] S6: The concentrated lithium chloride solution is transported to a crystallization kettle, cooled and crystallized, and the lithium chloride product is obtained after filtration.

[0053] Specifically, the composition of the aramid production waste liquid is DMAC: 25% to 40% by mass, lithium chloride: 0.2% to 2% by mass, oligomer: 0.5% to 1.5% by mass, and the rest is water.

[0054] Specifically, the flocculant used in the flocculation process in S1 is one or more of polyaluminum sulfate, aluminum sulfate, polyaluminum chloride, aluminum chloride, polyferric sulfate, and polyacrylamide, preferably polyaluminum sulfate and polyaluminum chloride. The amount of flocculant added is 0.01% to 0.1% of the total mass of the waste liquid, preferably 0.01 to 0.02%.

[0055] The above flocculant has a good flocculation effect on this waste liquid system. After the waste liquid is neutralized and the pH is adjusted, more oligomers are precipitated from the solution to form very small suspended particles. The added flocculant quickly disperses in the solution and adsorbs or interacts with these suspended particles, so that these suspended particles can quickly aggregate into large particles or floccules. These floccules also have good filterability and can stay on the filter cake to achieve a good separation effect. When the amount of flocculant added is less than 0.01%, the flocculation time is long, the flocculent particles obtained are not large, and suspended matter is still found in the filtrate after filtration; if the amount of flocculant added is greater than 0.1%, the flocculation effect is somewhat reduced.

[0056] Exemplarily, the flocculation process can be carried out in a stirring tank, preferably a corrosion-resistant stainless steel stirring tank or a PP plastic stirring tank, with a stirring speed of 50 to 150 rpm.

[0057] Specifically, in S2, an azeotropic agent is added to mix with the waste liquid (filtrate), and the heating temperature rises to the azeotropic point. The steam formed by the azeotropic agent and water in the azeotropic ratio comes out from the top of the tower, and then is cooled into a liquid in the condenser. The liquid enters the stratifier. Since the azeotropic agent and water are not miscible (the azeotropic structure automatically collapses under low temperature conditions), the stratification is performed in the stratifier; the upper layer is an azeotropic agent containing a small amount of water, and the lower layer is water containing a small amount of azeotropic agent. The upper layer can be refluxed into the tower to enter azeotropy with the water in the system again and then take water out, and the lower layer of water can be discharged after further simple azeotropy, and the azeotropic agent of simple azeotropy in the secondary azeotropic distillation tower can also be recycled and reused. Compared with the extraction-distillation process, the present invention does not use an extractant that is highly toxic and harmful to humans and the environment. Compared with the evaporation-distillation process, the present invention does not require all the evaporation of wastewater, and only requires the azeotropic agent to take the water out of the tower at a lower azeotropic point. The material operation time is short, thereby reducing energy consumption and improving product quality.

[0058] Furthermore, the azeotropic agent is one or more of isobutyl alcohol, n-butyl alcohol, isopentanol, n-pentanol, xylene and ethylene dichloride.

[0059] Preferably, the azeotropic agent is isobutanol.

[0060] The selected entrainer has a moderate boiling point, stable physical properties within the operating temperature range, and less decomposition over a long period of time; the material has low toxicity and is less harmful to people and the environment; and at the same time, the azeotropic point is low, the water content in the azeotropic composition is high, the mutual solubility of the entrainer and water is not high, and it is easy to separate. Among them, isobutanol has low toxicity, a stable azeotropic system and a low boiling point (89.9℃), and is the best choice considering the separation effect and economic benefits.

[0061] In actual implementation, if multiple entrainers are mixed and used, it may cause the azeotropic system to be unstable or the boiling point of the azeotrope to fluctuate, which is not conducive to stable operation and separation and recovery effects. It is preferred to use one entrainer for azeotropic distillation.

[0062] Preferably, the mass ratio of the entrainer to the filtrate is 0.5 to 1: 1. Too low an entrainer to waste liquid ratio cannot fully azeotropize with the water in the waste liquid and carry the water out of the tower top for separation; too high a ratio will increase the burden on the tower and increase energy consumption.

[0063] Specifically, in step S1, a pH regulator needs to be added before flocculation to adjust the pH value of the waste liquid to neutral to weak alkalinity, and the pH regulator is LiOH. During the synthesis of aramid polymers, an acidic by-product, hydrogen chloride, is produced. In subsequent treatments, some are neutralized with alkali, and some are not neutralized or not completely neutralized. Therefore, the waste liquid generated in production will be neutral or acidic. In order to reduce the impact (corrosion) on equipment or processes, these waste liquids need to be treated with alkaline substances before treatment to make the waste liquid neutral or weakly alkaline. The pH regulator is an alkaline compound corresponding to alkaline earth metal chloride (i.e., LiOH) to avoid introducing new ions / impurities into the waste liquid.

[0064] Specifically, the operating parameters of the primary azeotropic distillation tower in step S2 are: normal pressure, tower top temperature of 89.9-95° C., tower bottom temperature of 100-110° C., number of tower plates of 12-18, and reflux ratio of 0.5-5.

[0065] The selection of the number of plates and the reflux ratio is mainly based on whether the tower can meet the material separation effect. Secondly, after meeting the material separation effect, the energy consumption should be reduced as much as possible. At the same time, the investment in equipment should not be too high. Experiments show that the number of plates of the first-stage azeotropic tower can meet the requirements when it is more than 12, but the reflux required when there are 12 plates is relatively high, reaching more than 5; when the number of plates is 15 or more, or even 18 plates, the reflux ratio is lower.

[0066] Specifically, a secondary azeotropic distillation tower is provided after the primary azeotropic distillation tower for further separation and recovery of the entrainer;

[0067] The operating parameters of the secondary azeotropic distillation tower are: normal pressure, tower top temperature 89-92℃, tower bottom temperature 99-105℃, tower plate number 5-15, reflux ratio 0.1-0.5; a second condenser is provided after the top outlet of the secondary azeotropic distillation tower, and the cooling water temperature is 25-35℃. The secondary azeotropic distillation tower is mainly used to separate a small amount of entrainer in the water layer and recover it to the entrainer storage tank. Because the content of the entrainer required for azeotropy is not high, the burden on the tower is not great; more than 5 tower plates can meet the requirements, and when the tower plates are 7 or more, or even 10 tower plates, the reflux ratio is lower.

[0068] In actual operation, a multi-stage azeotropic distillation tower may be provided, but the economic efficiency is poor and the effect of improving the recovery of the entrainer is limited.

[0069] The mixed gas (tower top distillate D1) azeotropically distilled from the top of the primary azeotropic distillation tower is liquefied into a liquid state after passing through a condenser and enters an oil-water separator. Since the entrainer and water are immiscible (the azeotropic system automatically disintegrates under low temperature conditions), the mixture is phase-separated into two liquid layers in the oil-water separator. The aqueous solution layer with a trace of entrainer and the entrainer layer containing a small amount of water, the entrainer layer is refluxed to the primary azeotropic distillation tower for recycling. The aqueous solution layer with a trace of entrainer is extracted and enters the secondary azeotropic distillation tower, and is further separated by azeotropic distillation. The secondary azeotropic distillation tower top distillate D2 is condensed and liquefied and enters the water layer of the oil-water separator, and then discharged as waste liquid (tower bottom discharge liquid F2) in the tower bottom. The entrainer can be extracted to the entrainer storage tank for recycling. The recycling of the entrainer reduces the production / processing cost and reduces the discharge of waste liquid.

[0070] Specifically, the primary distillation described in step S3 is carried out in a primary distillation tower, and the operating parameters of the tower are: vacuum degree 0.06-0.09Mpa, tower top temperature 60-90°C, tower bottom temperature 115-135°C, tower plate number 5-15, and reflux ratio 0.3-1.

[0071] Selecting negative pressure operating conditions can reduce the boiling point of materials and reduce operating costs. At the same time, different operating temperatures can be selected according to different material types, salt content, etc. to reduce the decomposition of the recovery solvent and improve the yield and quality of material recovery.

[0072] Specifically, the secondary distillation described in step S5 is carried out in a secondary distillation tower, and the operating parameters of the tower are: vacuum degree 0.06-0.09Mpa, tower top temperature 60-80°C, tower bottom temperature 110-130°C, tower plate number 12-30, and reflux ratio 1-5.

[0073] The secondary distillation tower is the main equipment for separating the azeotropic agent and the solvent, and has high requirements. The number of plates needs to be more than 12 to achieve the separation requirements. When the number of plates is more than 15 or 20, the reflux energy is relatively small. Use negative pressure operation to reduce energy.

[0074] Specifically, the specific parameters of the evaporation process in step S4 are that the evaporator temperature is 100-135°C, the evaporation pressure is -0.1-0Mpa; a condenser is provided at the rear end of the evaporator, the cooling water temperature is 25-35°C, and the temperature of the organic solvent DMAC after cooling is 40-60°C.

[0075] Selecting negative pressure operating conditions can reduce the boiling point of materials and reduce operating costs. At the same time, different operating temperatures are selected according to different material types and salt content to reduce the decomposition of recovery solvents and improve the yield and quality of material recovery. The material is mainly condensed from steam into liquid by the main cooling water, and then enters the secondary distillation tower; the liquid temperature is kept at a high temperature or even directly steam fed; the main control is to stabilize the temperature and flow of the material.

[0076] Specifically, the small amount of filtrate obtained in step S6 is an organic solvent solution containing lithium chloride and some impurities, which can be returned to the evaporator to re-evaporate and recover the organic solution, or can be taken out for harmless treatment.

[0077] Example 1

[0078] The waste liquid composition is calculated by mass percentage: DMAC: 25% to 40%, lithium chloride: 0.2% to 2%, oligomer: 0.5% to 1.5%, and the rest is water.

[0079] Step 1: Add the aramid production waste liquid into the stirring tank at a stirring speed of 100 rpm. First, add a small amount of LiOH to the aramid production waste liquid in the stirring tank until the pH meter test is neutral, then add 0.015% aluminum sulfate while stirring, and the flocculant concentrates the oligomers in the waste liquid to flocculate, and filter to obtain the oligomer residue, which needs to be harmlessly treated. The filtrate is a mixed solution of DMAC-LiCl-water.

[0080] Step 2: After DMAC-LiCl-water mixed solution passes through a mixer at a flow rate of 100kg / h and isobutyl alcohol at a flow rate of 80kg / h, a primary azeotropic distillation tower 7 is added to the middle of the primary azeotropic distillation tower 7; the primary azeotropic distillation tower 7 has 15 theoretical plates; the tower bottom temperature is maintained at 100 degrees, and the tower top temperature is maintained at 90 degrees. The isobutyl alcohol and water in the tower form an azeotrope, which is distilled from the top of the tower, condensed by the condenser 9 and enters the oil-water separator 10, where the water and isobutyl alcohol are separated, and the upper layer is isobutyl alcohol containing a small amount of water, which is refluxed to the primary azeotropic distillation tower 7 at a flow rate of 208kg / h; the lower layer is transported to the secondary azeotropic distillation tower 12 for further separation; isobutyl alcohol-DMAC-LiCl containing a small amount of water is obtained at the bottom of the tower, the pressure is normal pressure, and the reflux ratio of the primary azeotropic distillation tower is 2.8.

[0081] Step 3: In step 2, the temperature of the top steam of the primary azeotropic distillation tower 7 is 60°C after condensation and liquefaction, and enters the oil-water separator 10, and the upper layer refluxes to the primary azeotropic distillation tower 7; the lower layer is water containing 5% isobutanol, which is transported to the secondary azeotropic distillation tower 12 with a flow rate of 70.9kg / h; after entering the secondary azeotropic distillation tower 12, isobutanol and water are azeotropically distilled from the top of the tower again, and after condensation, they enter the oil-water separator 14 to further separate isobutanol and water. The upper layer liquid is isobutanol containing 10% water, which can be extracted and placed in the isobutanol storage tank with a flow rate of 4.6kg / h; the lower layer is water containing 200ppm of alcohol, which refluxes to the secondary azeotropic distillation tower 12 with a flow rate of 0.86kg / h. The bottom of the secondary azeotropic distillation tower 12 is water containing 10 to 100ppm of alcohol and DMAC, which can be directly discharged to the wastewater treatment station. The secondary azeotropic distillation tower 12 has 9 plates, a top temperature of 90°C, a bottom temperature of 100°C, a reflux ratio of 0.19, and a tower pressure of normal pressure.

[0082] Step 4: The mixed liquid in the bottom of the first azeotropic distillation tower 7 in step 2 is extracted and transported to the primary distillation tower 16. The composition of the mixed liquid is as follows by mass percentage: isobutanol: 70.4%, DMAC: 23.8%, water: 3.1%, LiCl: 2.6%; the flow rate is 113.5 kg / h;

[0083] After separation in the primary distillation tower 16, the top distillate is water-isobutanol-DMAC, with a composition of 83.6% isobutanol, 12.6% DMAC, 3.75% water, and 0% LiCl, with a flow rate of 94.9 kg / h, and is transported to the secondary distillation tower 21;

[0084] The bottom of the primary distillation tower 16 is a trace isobutanol-DMAC-LiCl solution, with isobutanol of 3.2%, DMAC of 80.6%, water of 0.0004%, LiCl of 16.1%, and a flow rate of 18.61 kg / h. The top temperature of the primary distillation tower 16 is 60-75°C, the bottom temperature is 112-121°C, the number of plates is 8, the reflux ratio is 0.5, and the vacuum degree is 0.08Mpa.

[0085] Step 5: The top distillate D3 of the primary distillation tower 16 in step 4 is isobutanol-water-DMAC, which is liquefied through a condenser and enters the secondary distillation tower 21; the trace isobutanol-DMAC-LiCl solution (mixed liquid F3 in the bottom of the primary distillation tower 16) is extracted and input into an evaporator for evaporation, and the resulting steam can directly enter the secondary distillation tower 21 or enter the secondary distillation tower 21 after condensation;

[0086] The composition of liquefied steam is: 5.9% isobutanol, 94.1% DMAC, 0.007% water, 0% LiCl, and the flow rate is 10.11 kg / h. The residual material from the evaporator enters the crystallization kettle.

[0087] Step 6: In step 4 and step 5, the top distillate D3 of the primary distillation tower 16 and the evaporated steam of the evaporator 29 enter the secondary distillation tower 21 for separation. The top discharge of the secondary distillation tower 21 (top distillate D4) is high-concentration isobutanol, with isobutanol of 94.1%, DMAC of 1.68%, water of 4.18%, LiCl of 0%, and a flow rate of 85.0 kg / h. The isobutanol storage tank is extracted and stored, and then transported to the inlet of the primary azeotropic distillation tower 7 by pump 31 to form the recycling of the isobutanol entrainer. The vapor phase discharge near the bottom of the tower is DMAC. After condensation, a high-purity DMAC product is obtained, which should be ≧99.99%, with water and alcohol contents of <200ppm, chroma ≤3°, acetic acid content ≤500ppm, dimethylamine ≤500ppm, and a flow rate of 18.82kg / h. It is stored in a DMAC product storage tank and can be returned to the workshop for direct use; the tower bottom discharge is tower bottom waste, which needs to be harmlessly treated.

[0088] The top temperature of the secondary distillation tower 21 is 70-80°C, the bottom temperature is 110-130°C, the number of plates is 25, the reflux ratio is 3.0, and the vacuum degree is 0.08Mpa.

[0089] Step 7: The concentrated solution in the evaporator in step 4 is a concentrated DMAC-LiCl solution, which can be transported to a glass-lined crystallization kettle with a stirrer and a screw discharger, and the jacket cooling water is introduced while stirring to cool to room temperature for crystallization. The crystals (i.e., high-purity lithium chloride) are sealed and squeezed from the crystallization kettle using a screw discharger to a filter to obtain a filtrate, with a flow rate of 6.48 kg / h, of which DMAC is 91.5%, lithium chloride is 7.98%, and the rest is impurities. The filtrate can be re-entered into the evaporator to evaporate DMAC.

[0090] The crystals in the sealed filter are used to recover lithium chloride, with a yield of 2.52 kg / h. These crystals can also be dissolved by adding qualified DMAC solvent, filtered and stored in DMAC-LiCl storage tanks, and then transported to the workshop for use.

[0091] The total recovery rate of DMAC is 97%, and the total recovery rate of lithium chloride is 95%. The detection purity of DMAC is ≥99.99%, the content of water and alcohol is respectively <200ppm, the chromaticity is ≤3°, the acetic acid content is ≤500ppm, and the dimethylamine is ≤500ppm.

[0092] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for recovering DMAC solvent and lithium chloride in aramid fiber production wastewater, characterized in that: The specific steps include: S1: Separate oligomers and mechanical impurities from waste liquid through flocculation and filtration processes to obtain filter residue and filtrate; S2: adding an azeotropic agent to the filtrate, conveying the filtrate to a primary azeotropic distillation tower, removing water from the waste liquid through an azeotropic distillation process, and obtaining a tower top distillate D1 containing the azeotropic agent and water and a tower bottom mixed liquid F1 containing the azeotropic agent, DMAC, and lithium chloride; S3: The bottom mixed liquid F1 is subjected to primary distillation to obtain a top distillate D3 containing an azeotropic agent and DMAC and a bottom mixed liquid F3 containing DMAC and lithium chloride: S4: transporting the mixed liquid F3 from the bottom of the tower to an evaporator to obtain DMAC vapor and concentrated lithium chloride solution; S5: performing secondary distillation on the tower top distillate D3 and DMAC vapor to obtain tower top distillate D4, i.e., a high-concentration entrainer; The gas phase discharge near the bottom of the tower is obtained, and high-purity DMAC is obtained after condensation; S6: conveying the concentrated lithium chloride solution to a crystallization kettle, cooling and crystallizing, and filtering to obtain a lithium chloride product; The azeotropic agent is one or more of isobutyl alcohol, n-butyl alcohol, isopentanol, n-pentanol, xylene and ethylene dichloride; N,N-dimethylacetamide is abbreviated as DMAC.

2. The recycling method according to claim 1, characterized in that: The composition of the aramid production waste liquid is DMAC: 25% to 40% by mass, lithium chloride: 0.2% to 2% by mass, oligomer: 0.5% to 1.5% by mass, and the rest is water.

3. The recycling method according to claim 1, characterized in that: The azeotropic agent is isobutanol.

4. The recycling method according to claim 1, characterized in that: The mass ratio of the azeotropic agent to the filtrate is 0.5 to 1:

1.

5. The recycling method according to claim 1, characterized in that: In step S1, a pH regulator needs to be added before flocculation to adjust the pH value of the waste liquid to be neutral to weakly alkaline, and the pH regulator is LiOH.

6. The recycling method according to claim 1, characterized in that: The operating parameters of the primary azeotropic distillation tower in step S2 are: normal pressure, tower top temperature at 89.9-95° C., tower bottom temperature at 100-110° C., number of tower plates at 12-18, and reflux ratio at 0.5-5.

7. The recycling method according to claim 1, characterized in that: A secondary azeotropic distillation tower is also provided after the primary azeotropic distillation tower for further separation and recovery of the entrainer; The operating parameters of the secondary azeotropic distillation tower are: normal pressure, tower top temperature 89-92°C, tower bottom temperature 99-105°C, number of tower plates 5-15, reflux ratio 0.1-0.5; a second condenser is provided behind the top discharge port of the secondary azeotropic distillation tower, and the cooling water temperature is 25-35°C.

8. The recycling method according to claim 1, characterized in that: The primary distillation described in step S3 is carried out in a primary distillation tower, and the operating parameters of the tower are: vacuum degree 0.06-0.09Mpa, tower top temperature 60-90°C, tower bottom temperature 115-135°C, tower plate number 5-15, and reflux ratio 0.3-1.

9. The recycling method according to claim 1, characterized in that: The secondary distillation described in step S5 is carried out in a secondary distillation tower, and the operating parameters of the tower are: vacuum degree 0.06-0.09Mpa, tower top temperature 60-80°C, tower bottom temperature 110-130°C, tower plate number 12-30, and reflux ratio 1-5.

10. The recycling method according to claim 1, characterized in that: The specific parameters of the evaporation process in step S4 are that the evaporator temperature is 100-135° C. and the evaporation pressure is -0.1-0 MPa.

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