Method for recycling NMP (N-Methyl Pyrrolidone) in cleaning waste liquid of lithium battery positive electrode pulping equipment

Abstract

The invention discloses a method for recycling NMP (N-Methyl Pyrrolidone) in cleaning waste liquid of lithium battery positive electrode pulping equipment. The method comprises the following steps: step S10, collecting the cleaning waste liquid; s20, pumping the cleaning waste liquid into a rake dryer, and evaporating and concentrating the cleaning waste liquid to obtain high-concentration PVDF waste liquid and NMP recovery liquid; step S30, adding 25-35 parts of pure water into 100 parts of the high-concentration PVDF waste liquid, and stirring for a preset period of time to obtain a first mixed liquid; s40, carrying out centrifugal separation on the first mixed solution to obtain a second mixed solution and a filter cake; step S50, squeezing the filter cake to obtain a third mixed solution and solid wastes; and step S60, pumping the second mixed solution and the third mixed solution into the rake dryer. According to the method, the NMP can be effectively recovered from the cleaning waste liquid of the positive electrode pulping equipment at low cost, and industrial operation is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of NMP recovery, and in particular to a method for recovering NMP from waste liquid from washing of lithium battery positive electrode pulping equipment. Background Art

[0002] N-Methylpyrrolidone (NMP) is a highly polar advanced solvent with excellent performance and is non-toxic and harmless. It is widely used in the fields of pesticides, medicine, electronic materials, and new energy manufacturing. Currently, in the field of new energy manufacturing, NMP is widely used as an excellent solvent in the production process of positive electrode sheets using PVDF (polyvinylidene fluoride) as a binder.

[0003] NMP is used as a solvent for the positive electrode conductive slurry of lithium-ion batteries. During the production of positive electrode sheets, due to equipment failure and production model switching, NMP is used to clean the slurry equipment, filters, pipelines, etc. to avoid the residual slurry during the model switching process affecting the quality of the subsequent slurry. The main component of the waste liquid is NMP, containing a small amount of SP (acetylene black), lithium nickel cobalt manganese oxide, and PVDF, and the solution has good fluidity.

[0004] The particle size of the powder in the conductive slurry is Dmax≤20µm. The proportion of powder in the conductive slurry varies greatly, and it contains adhesives. When filtering, the slurry will agglomerate on the filter screen, making it impossible to separate NMP and powder. In addition, the small pore size increases the difficulty of filtering. At present, many companies treat it as hazardous waste, which seriously wastes resources. Therefore, how to recover NMP solvent from cleaning waste liquid has become an urgent problem that needs to be solved.

[0005] CN102544627A discloses a method for recovering N-methylpyrrolidone in battery electrode materials, including: a. taking waste of lithium battery electrode materials, adding deionized water thereto, stirring, and obtaining a waste mixture; the waste contains N-methylpyrrolidone; b. adding a flocculant solution to the waste mixture, stirring, and obtaining a to-be-separated substance; the flocculant solution is an aqueous solution of a flocculant, and the flocculant is selected from one or more of polyaluminium chloride, calcium chloride, ferrous sulfate, and polyacrylamide; c. filtering the to-be-separated substance to separate a crude liquid containing N-methylpyrrolidone; d. fractionating the crude liquid to obtain N-methylpyrrolidone. This method requires a large amount of water to dissolve NMP in waste lithium batteries, and a specific flocculant must be added, which is costly, increases impurity components, increases the efficiency and difficulty of distillation separation, and the output of NMP trace elements has a certain impact on the quality of the reused NMP.

[0006] The method described in CN113480464B uses filter press to effectively separate the powder and NMP solvent in the conductive slurry according to the properties of the non-Newtonian fluid of the conductive slurry; then the powder remaining in the filtrate is further removed by vacuum flash evaporation to prevent part of the powder from entering the distillation tower with the solution and completely adhering to the inner wall of the distillation tower, causing damage to the equipment; then water and NMP are separated by vacuum distillation to obtain a high-purity NMP solvent. However, due to the small particle size of the powder particles and the adhesive contained in the slurry, it is actually impossible to effectively filter. When the flash evaporation process is used, the slurry adheres to the inner wall of the equipment, affecting the heat exchange efficiency. Summary of the invention

[0007] In view of the deficiencies in the prior art, an object of this specification is to provide a method for recovering NMP from the cleaning waste liquid of lithium battery positive electrode pulping equipment, which can recover NMP from the cleaning waste liquid of positive electrode pulping equipment at low cost and effectively, and is convenient for industrial operation.

[0008] To achieve the above purpose, the present specification provides a method for recovering NMP in the waste liquid of lithium battery positive electrode pulping equipment, comprising the following steps: Step S10: collecting cleaning waste liquid; Step S20: pumping the cleaning waste liquid into a rake dryer to evaporate and concentrate the cleaning waste liquid to obtain a high-concentration PVDF waste liquid and NMP recovery liquid; Step S30: adding 25-35 parts of pure water to 100 parts of the high-concentration PVDF waste liquid and stirring for a predetermined time to obtain a first mixed liquid; Step S40: centrifuging the first mixed solution to obtain a second mixed solution and a filter cake; Step S50: squeezing the filter cake to obtain a third mixed liquid and solid waste; Step S60: pumping the second mixed liquid and the third mixed liquid into the rake dryer.

[0009] According to the influence of NMP under different environments and due to the instability of batches, in order to achieve the highest efficiency and maximize the precipitation of PVDF in high-concentration PVDF waste liquid, the preferred solution of the present invention is that the amount of pure water finally added to the high-concentration PVDF waste liquid is preferably 25%-35% of the volume of the high-concentration PVDF waste liquid; as a more preferred solution, the amount of pure water added is 25%-32%, 28%-32%, etc. of the volume of the high-concentration PVDF waste liquid, and the most preferred point values ​​are, for example, 28%, 29%, 30%, 31%, 32%, etc. As a preferred embodiment, the solute of the high-concentration PVDF waste liquid is PVDF, and the solvent is pure NMP or the solvent is NMP and contains a trace amount of water.

[0010] The concentration of PVDF in the high-concentration PVDF waste liquid is 5% to 10% (mass ratio), and the weight ratio of PVDF:NMP is preferably 0.5:9.5-1:9. In the present invention, it can be understood that no solid PVDF is precipitated at room temperature. The concentration should be close to the saturation concentration of PVDF in NMP or the saturation concentration that can be achieved in theory; of course, the PVDF concentration in the high-concentration PVDF waste liquid in the present invention can also adopt a certain proportion of the saturation concentration at room temperature, such as more than 50% of the current protection concentration, more than 60%, more than 70%, more than 80%, more than 90%, more than 95% and other values.

[0011] As a preferred implementation, in step S20, the amount of the high-concentration PVDF waste liquid is controlled to be 20% to 30% of the feed amount of the rake dryer.

[0012] As a preferred implementation, in step S20, the liquid level in the rake dryer is controlled at 85%.

[0013] As a preferred implementation, in step S20, the vacuum degree of the rake dryer is controlled to be -90 kPa to -92 kPa, the steam pressure is controlled to be 0.55 mPa to 0.65 mPa, and the operating temperature is controlled to be 140°C to 160°C.

[0014] As a preferred embodiment, in the step S20, the cleaning waste liquid, the second mixed liquid and the third mixed liquid are evaporated and concentrated, and the obtained NMP recovery liquid is sent to a distillation system for distillation to obtain qualified NMP.

[0015] As a preferred implementation, in the step S30, the high-concentration PVDF waste liquid is first poured into the analytical reaction tank; and then stirred and reacted for 30 minutes.

[0016] As a preferred implementation, in the step S40, a centrifuge is used to centrifuge the first mixed solution; and in the step S50, a press is used to press the filter cake.

[0017] As a preferred implementation, the method further includes step S70: treating the solid waste as hazardous waste. Beneficial Effects

[0018] Since the cleaning waste liquid contains a small amount of water, its PVDF concentration is low. If the method of directly adding water to reduce the NMP concentration is adopted, the PVDF precipitation effect is poor. Therefore, the method for recovering NMP in the cleaning waste liquid of the lithium battery positive electrode pulping equipment provided in this embodiment is to use a rake dryer to evaporate and concentrate the cleaning waste liquid first to obtain a high-concentration PVDF waste liquid and NMP recovery liquid; then, taking advantage of the characteristics that PVDF is insoluble and will precipitate in the NMP solution with a water content of 10% to 40%, 25-35 parts of pure water are added to 100 parts of high-concentration PVDF waste liquid to reduce the NMP concentration, so that PVDF is precipitated in the form of floccules in the largest amount, and the second mixed liquid and the third mixed liquid are collected after centrifugal separation and filter cake pressing, and then the second mixed liquid and the third mixed liquid are pumped into the rake dryer to evaporate and recover NMP.

[0019] During the centrifugal separation process, the filter cake has a liquid content of 40% to 60% due to the blockage of the filter channel of the filter cake itself. Further pressing is required to separate the solid and liquid, so as to efficiently recover the NMP solution.

[0020] The method provided in this embodiment only requires the addition of pure water and the use of a rake dryer, so the cost is low. The NMP recovery liquid is distilled to obtain qualified NMP, that is, the method can effectively recover NMP from the cleaning waste liquid of the positive electrode pulping equipment, which is convenient for industrial operation.

[0021] With reference to the following description and the accompanying drawings, the specific embodiments of the present invention are disclosed in detail, indicating the manner in which the principles of the present invention can be adopted. It should be understood that the embodiments of the present invention are not limited in scope thereby.

[0022] Features described and / or illustrated with respect to one embodiment may be used in the same or similar manner in one or more other embodiments, combined with features in other embodiments, or substituted for features in other embodiments.

[0023] It should be emphasized that the term “include / comprises” when used herein refers to the presence of features, integers, steps or components, but does not exclude the presence or addition of one or more other features, integers, steps or components. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without paying creative labor.

[0025] Figure 1A flowchart of the steps of a method for recovering NMP from a waste liquid from a lithium battery positive electrode pulping device provided in this embodiment; Figure 2 A flow chart of an embodiment of a method for recovering NMP from a waste liquid from a lithium battery positive electrode pulping device provided in this embodiment; Figure 3a~Figure 3d This is the result of the PVDF analysis experiment on high-concentration cleaning wastewater. DETAILED DESCRIPTION

[0026] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0027] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be another element centered thereon. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be another element centered thereon at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only and do not represent the only implementation method.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0029] In the present invention, low-concentration cleaning waste liquid refers to: when switching models, pure NMP needs to be injected into the pulping equipment, and the pump circulates to clean the pulping equipment and pipelines to generate cleaning waste liquid containing PVDF, lithium nickel cobalt manganese oxide, and graphite. The contents of PVDF, lithium nickel cobalt manganese oxide, and graphite in the cleaning waste liquid generated each time are different. During the cleaning and packaging process, NMP absorbs moisture in the air, and the generated waste liquid contains a small amount of water. Therefore, the cleaning waste liquid is a low-concentration cleaning waste liquid; the small amount of water can be understood as a small amount of water adsorbed by NMP due to exposure to the air. The proportion of this small amount of water in the mixed solution is quite considerable. In the subsequent negative pressure distillation, this part of water will basically all be evaporated, and the content of the trace amount of water mentioned in the subsequent steps is much lower than the content of this small amount of water.

[0030] In the present invention, the high-concentration PVDF waste liquid is preferably a high-concentration PVDF waste liquid after passing through a rake dryer (or evaporator). The solute in the cleaning waste liquid initially obtained from the production line is PVDF, and the solvent is a mixture of NMP and a small amount of water, and its specifications are not uniform. Due to the overall low concentration, in most cases, in actual waste liquid treatment, a negative pressure distillation (140°C~160°C) is performed, wherein the boiling point of NMP at standard atmospheric pressure is 202°C, and the boiling point of water at standard atmospheric pressure is 100°C. Although the mixture of NMP and water will form an azeotrope, it is actually found in the present invention that the water content in the high-concentration PVDF waste liquid after a distillation is sharply reduced, and the high-concentration PVDF waste liquid can basically be considered as a pure NMP solvent, and the solute is PVDF.

[0031] To judge the actual high-concentration PVDF waste liquid, use a beaker to sample and add a small amount (2% to 5% by mass or volume) of pure water. If obvious PVDF precipitation is seen, it is judged to be the high-concentration PVDF waste liquid defined in the present invention; if no obvious PVDF precipitation is seen, it is judged not to be a high-concentration PVDF waste liquid, and the subsequent process is still a single distillation.

[0032] In addition, the present invention also creatively defines the concentration of high-concentration PVDF waste liquid. Since the present invention requires low-concentration PVDF waste liquid to be evaporated in a rake dryer to obtain high-concentration PVDF waste liquid, but if PVDF is precipitated in advance and adsorbed in the rake dryer, the entire device will be very difficult to clean and handle. The present invention chooses to stop evaporation when PVDF is close to a high concentration, and discharge the high-concentration PVDF waste liquid from the rake dryer after the temperature drops to a lower temperature, such as below 50°C or cools to room temperature. When it is below 50°C (for example, taking the 50°C point value) or cooled to room temperature, it is ensured that PVDF in the high-concentration PVDF waste liquid does not precipitate.

[0033] The high-concentration PVDF waste liquid treated by the present invention has PVDF as the solute, pure NMP or nearly pure NMP as the solvent, and contains trace amounts of water, such as water content of less than 2%, preferably less than 0.5%, and optimally a tracer concentration, or the present invention recognizes in practice that the high-concentration PVDF waste liquid is a pure NMP solution, because water is removed during the distillation process. "High concentration" can be understood as close to the saturation concentration of PVDF in NMP or the saturation concentration that can be achieved in theory; "high concentration" can also be understood as if evaporation and concentration continue, PVDF solids will precipitate, and the solution at this time is obviously already at a "high concentration".

[0034] Specifically, the concentration of PVDF in the high-concentration PVDF waste liquid is 5% to 10% (mass ratio), and the weight ratio of PVDF:NMP is preferably 0.5:9.5-1:9. In the present invention, it can be understood that no solid PVDF is precipitated at room temperature. The concentration should be close to the saturation concentration of PVDF in NMP or the saturation concentration that can be achieved in theory. Of course, the PVDF concentration in the high-concentration PVDF waste liquid in the present invention can also adopt a certain proportion of the saturation concentration at room temperature, such as more than 50% of the current protection concentration, more than 60%, more than 70%, more than 80%, more than 90%, more than 95% and other values.

[0035] See also Figure 1 The present application provides a method for recovering NMP from a waste liquid from a lithium battery positive electrode pulping device, comprising the following steps (step S10 to step S60): Step S10: Collecting cleaning waste liquid.

[0036] Specifically, the cleaning waste liquid is the cleaning waste liquid of lithium battery positive electrode pulping equipment, which is a low-concentration cleaning waste liquid.

[0037] Step S20: pumping the cleaning waste liquid into a rake dryer to evaporate and concentrate the cleaning waste liquid to obtain high-concentration PVDF waste liquid and NMP recovery liquid.

[0038] Specifically, the cleaning waste liquid is pumped to the rake dryer, the liquid level in the rake dryer is controlled at 85%, stirring is started, and the feed amount is recorded. Controlling the feed liquid level of the rake dryer at 85% can keep the equipment with sufficient evaporation and vaporization space. Recording the feed amount can be used for material balance records. The feed amount of the rake dryer is equal to the sum of the NMP evaporation condensate amount and the evaporation residual discharge amount, that is, the feed amount of the rake dryer is equal to the sum of the NMP recovery liquid amount and the high-concentration PVDF waste liquid amount.

[0039] In step S20, after the heating steam of the rake dryer is turned on, the NMP in the cleaning waste liquid is vaporized and cooled by the condenser to obtain the NMP recovery liquid. The vacuum degree of the rake dryer is controlled to be -90kPa to -92kPa, the steam pressure is 0.55mPa to 0.65mPa, and the operating temperature is 140℃ to 160℃.

[0040] In step S20, the amount of concentrated liquid (high-concentration PVDF waste liquid) is controlled to be 20% to 30% of the feed amount of the rake dryer. The evaporation equipment in the embodiment of the present application uses a rake dryer, which is suitable for slurry, paste, and powder materials. The concentration ratio needs to be controlled well, and the preferred concentration ratio is 5:1. The operating temperature of negative pressure distillation is 140°C to 160°C. If the concentration multiple is too large, the concentration of PVDF will exceed the saturation concentration of the current temperature or approach the saturation concentration (if it is close to the saturation concentration at 150°C, during the cooling process, due to the decrease in concentration, PVDF will precipitate during the cooling process), and then PVDF precipitates and adheres to the surface of the rake dryer, seriously affecting the normal operation of the rake dryer.

[0041] Step S30: adding 25-35 parts of pure water to 100 parts of high-concentration PVDF waste liquid and stirring for a predetermined time to obtain a first mixed liquid.

[0042] As an experiment, take 800ml of high-concentration cleaning waste liquid, dilute it with pure water at a ratio of 10%, 20%, 25%, 28%, 29%, 30%, 31%, 32%, 35%, and observe the precipitation of PVDF. Among them, 10%, 20%, 25%, 28%, 29%, 30%, 31%, 32%, 35% means that the amount of pure water accounts for 10%, 20%, 25%, 28%, 29%, 30%, 31%, 32%, 35% of the total amount of high-concentration cleaning waste liquid, that is, the amount of pure water added is 80ml, 160ml, 200ml, 224ml, 232ml, 240ml, 248ml, 256ml, 280ml respectively. Figure 3a~Figure 3d As shown, it is the result diagram of PVDF analysis when the amount of pure water accounts for 10%, 20%, 25% and 30% of the total amount of high-concentration cleaning waste liquid respectively. Figure 3a~Figure 3d In the figure, the glass on the left is the high-concentration cleaning waste liquid after adding pure water, and the black solid substance on the right is the precipitated PVDF. When the amount of pure water is above 30%, for example, 35% pure water, no additional PVDF precipitation can be observed (not provided with the attached figure).

[0043] Based on the above experimental results, the precipitation of PVDF in high-concentration cleaning wastewater after adding different amounts of pure water is summarized as shown in Table 1 below. Figure 3a As shown, when 10% pure water is added, a small amount of PVDF is precipitated. Figure 3b As shown in the figure, the amount of PVDF precipitated increases when 20% pure water is added. Figure 3c As shown in the figure, when 25% pure water is added, a large amount of PVDF is precipitated. When 28% pure water is added, PVDF continues to precipitate. When 29% pure water is added, PVDF continues to precipitate, and the amount of precipitation begins to decrease. Figure 3dAs shown in the figure, when 30% pure water is added, PVDF continues to precipitate, and the amount of precipitation begins to decrease. When 31% pure water is added, PVDF no longer precipitates. When 32% pure water is added, PVDF no longer precipitates. When 35% pure water is added, PVDF no longer precipitates. It is concluded that when the amount of pure water added is 30% of the total amount of high-concentration cleaning waste liquid, PVDF precipitates in the largest amount.

[0044] Table 1 Precipitation of PVDF in high-concentration cleaning wastewater after adding different amounts of pure water As another preferred embodiment, according to different high-concentration PVDF waste liquids, pure water can be continuously added to the continuously stirred high-concentration PVDF waste liquid according to actual conditions, and PVDF has no obvious precipitation after stirring for a period of time, and this amount can be used to determine the amount of pure water added. According to the influence of NMP under different environments, and due to the instability of batches, in order to achieve the highest efficiency and maximize the precipitation of PVDF in high-concentration PVDF waste liquid, the preferred embodiment of the present invention is that the amount of pure water finally added to the high-concentration PVDF waste liquid is preferably 25%-35% of the volume of the high-concentration PVDF waste liquid; as a more preferred embodiment, the amount of pure water added is 25%-32% of the volume of the high-concentration PVDF waste liquid, and the most preferred point values ​​are, for example, 28%, 29%, 30%, 31%, etc.

[0045] 25%-35% pure water can ensure the maximum precipitation of PVDF. It can be seen that at the critical point of 25%, PVDF precipitates in large quantities, and then the precipitation continues and gradually decreases. Finally, the precipitation stops when it reaches 30%. Pure water exceeding 30% basically does not cause additional PVDF precipitation. Therefore, the present invention believes that pure water at a ratio of 25%-35% can achieve the experimental purpose of efficiently precipitating PVDF in the present invention. Of course, a better choice can be 25%-32%, and the most preferred point values ​​are, for example, 28%, 29%, 30%, 31%, etc.

[0046] The embodiment of the present application adds 25%-35% pure water to the waste liquid sample to reduce the NMP concentration, so that the PVDF is precipitated in the form of floccules in the largest amount, which significantly improves the recovery efficiency and reduces the workload of the subsequent recovery steps. The predetermined stirring time can be 30 minutes.

[0047] In step S30, the high-concentration PVDF waste liquid can be first poured into the analytical reaction tank to provide a reaction site for the precipitation reaction of PCVF. 30% of the amount of high-concentration PVDF waste liquid is added to the analytical reaction tank and stirred, and the reaction is carried out for 30 minutes after stirring, which can ensure that PVDF is effectively precipitated without wasting recovery time.

[0048] In other embodiments, pure water may be introduced into the analytical reaction tank first, the stirring function may be turned on, and then the high-concentration PVDF waste liquid may be slowly added.

[0049] Step S40: centrifugally separating the first mixed solution to obtain a second mixed solution and a filter cake.

[0050] In step S40, the first mixed solution is centrifuged using a centrifuge. Specifically, the centrifuge is started, the first mixed solution is slowly added, and centrifuged. The filter cake mainly contains PVDF.

[0051] Step S50: squeezing the filter cake to obtain a third mixed liquid and solid waste.

[0052] Specifically, in step S50, the filter cake is squeezed by a squeezer to further separate the solid and liquid. During the centrifugal separation process of step S40, since the filter channel of the filter cake itself is blocked, the liquid content of the filter cake is 40% to 60%, and further squeezing is required for solid-liquid separation, so as to efficiently recover the NMP solution.

[0053] Step S60: pumping the second mixed liquid and the third mixed liquid into the rake dryer.

[0054] Specifically, the second mixed liquid and the third mixed liquid enter step S20, so that the NMP in the second mixed liquid and the third mixed liquid is recovered. Therefore, in step S20, the feed of the rake dryer includes the cleaning waste liquid, the second mixed liquid and the third mixed liquid. The rake dryer evaporates and concentrates the cleaning waste liquid, the second mixed liquid and the third mixed liquid, and the obtained NMP recovered liquid is sent to the distillation system for distillation to obtain qualified NMP. The existing distillation system can be used to distill the NMP recovered liquid, and the obtained qualified NMP can be reused for production.

[0055] The method for recovering NMP from the waste liquid from washing of lithium battery positive electrode pulping equipment provided in the embodiment of the present application may further include step S70: treating the solid waste as hazardous waste.

[0056] Among them, the main component of solid waste is PVDF, and it also contains a small amount of lithium nickel cobalt manganese oxide and SP.

[0057] Since the cleaning waste liquid contains a small amount of water, its PVDF concentration is low. If the method of directly adding water to reduce the NMP concentration is adopted, the PVDF precipitation effect is poor. Therefore, the method for recovering NMP in the cleaning waste liquid of the lithium battery positive electrode pulping equipment provided in this embodiment is to use a rake dryer to evaporate and concentrate the cleaning waste liquid first to obtain a high-concentration PVDF waste liquid and NMP recovery liquid; then, taking advantage of the characteristics that PVDF is insoluble and will precipitate in the NMP solution with a water content of 10% to 40%, 25-35 parts of pure water are added to 100 parts of high-concentration PVDF waste liquid to reduce the NMP concentration, so that PVDF is precipitated in the form of floccules in the largest amount, and the second mixed liquid and the third mixed liquid are collected after centrifugal separation and filter cake pressing, and then the second mixed liquid and the third mixed liquid are pumped into the rake dryer to evaporate and recover NMP.

[0058] During the centrifugal separation process, the filter cake has a liquid content of 40% to 60% due to the blockage of the filter channel of the filter cake itself. Further pressing is required to separate the solid and liquid, so as to efficiently recover the NMP solution.

[0059] The method provided in this embodiment only requires the addition of pure water and the use of a rake dryer, so the cost is low. The NMP recovery liquid is distilled to obtain qualified NMP, that is, the method can effectively recover NMP from the cleaning waste liquid of the positive electrode pulping equipment, which is convenient for industrial operation.

[0060] In a specific application scenario, the method for recovering NMP in the waste liquid of the lithium battery positive electrode pulping equipment provided in the embodiment of the present application is as follows: Figure 2 As shown, it includes the following processes: First, the waste liquid from cleaning the positive electrode pulping equipment is recovered and collected.

[0061] Then pump the cleaning waste liquid to the rake dryer, control the liquid level at 85%, start stirring, and keep a record of feeding; turn on the heating steam, open the NMP condenser circulating cooling water inlet and outlet valves; control the vacuum degree of the rake dryer to -90kPa~-92kPa, and the steam pressure to 0.55mPa~0.65mPa; evaporate and recover NMP, and control the remaining concentrated liquid (i.e., high-concentration PVDF waste liquid) to 20~30% of the feed amount.

[0062] Add 30% pure water to the analysis reaction tank, start stirring, slowly add high-concentration PVDF waste liquid, and perform PVDF analysis. Stir the reaction for 30 minutes to obtain the first mixed solution; start the centrifuge, slowly add the first mixed solution, and centrifuge to obtain the second mixed solution and filter cake; the filter cake is collected and sent to the slag press for further solid-liquid separation to obtain the third mixed solution and solid waste. The second mixed solution and the third mixed solution are collected and pumped to the rake dryer, and evaporated and concentrated together with the cleaning waste liquid. The NMP recovery liquid evaporated from the rake dryer is sent to the distillation system for distillation to obtain qualified NMP; the solid waste is treated as hazardous waste.

[0063] It should be noted that, in the description of this specification, the terms "first", "second", etc. are only used for descriptive purposes and to distinguish similar objects. There is no order of precedence between the two, and they cannot be understood as indicating or implying relative importance. In addition, in the description of this specification, unless otherwise specified, the meaning of "plurality" is two or more.

[0064] Any numerical value cited herein includes all values ​​of lower and upper values ​​that increase by one unit from the lower limit to the upper limit, and there is at least a two-unit interval between any lower value and any higher value. For example, if the value of the quantity of a component or a process variable (such as temperature, pressure, time, etc.) is set forth to be from 1 to 90, preferably from 20 to 80, and more preferably from 30 to 70, the purpose is to illustrate that values ​​such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 are also explicitly listed in the specification. For values ​​less than 1, one unit is appropriately considered to be 0.0001, 0.001, 0.01, 0.1. These are merely examples that are intended to be clearly expressed, and it can be considered that all possible combinations of numerical values ​​listed between the lowest value and the highest value are clearly set forth in the specification in a similar manner.

[0065] Unless otherwise indicated, all ranges include the endpoints and all numbers between the endpoints. "About" or "approximately" used with a range applies to both endpoints of the range. Thus, "about 20 to 30" is intended to cover "about 20 to about 30", including at least the specified endpoints.

[0066] All articles and references disclosed, including patent applications and publications, are incorporated herein by reference for all purposes. The term "consisting essentially of..." to describe a combination should include the identified elements, ingredients, parts or steps and other elements, ingredients, parts or steps that do not substantially affect the basic novel characteristics of the combination. The use of the terms "comprising" or "including" to describe a combination of elements, ingredients, parts or steps herein also contemplates embodiments that consist essentially of these elements, ingredients, parts or steps. By using the term "may", it is intended to indicate that any attribute described that "may" include is optional.

[0067] Multiple elements, ingredients, parts or steps can be provided by a single integrated element, ingredient, part or step. Alternatively, a single integrated element, ingredient, part or step can be divided into separate multiple elements, ingredients, parts or steps. The disclosure "one" or "an" used to describe an element, ingredient, part or step is not intended to exclude other elements, ingredients, parts or steps.

[0068] It should be understood that the above description is for illustration and not for limitation. Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but should be determined with reference to the appended claims and the full scope of equivalents possessed by such claims. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the preceding claims is not intended to be a waiver of the subject matter, nor should it be considered that the inventors did not consider the subject matter to be part of the disclosed inventive subject matter.

Claims

1. A method for recovering NMP from waste liquid from lithium battery positive electrode pulping equipment, characterized in that: The following steps are involved: step S10: collecting cleaning waste liquid; Step S20: pumping the cleaning waste liquid into a rake dryer to evaporate and concentrate the cleaning waste liquid to obtain a high-concentration PVDF waste liquid and NMP recovery liquid; Step S30: adding 25-35 parts of pure water to 100 parts of the high-concentration PVDF waste liquid and stirring for a predetermined time to obtain a first mixed liquid; Step S40: centrifuging the first mixed solution to obtain a second mixed solution and a filter cake; Step S50: squeezing the filter cake to obtain a third mixed liquid and solid waste; Step S60: pumping the second mixed liquid and the third mixed liquid into the rake dryer.

2. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: The solute of the high-concentration PVDF waste liquid is PVDF, and the solvent is pure NMP or the solvent is NMP and contains a trace amount of water.

3. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In the step S30, pure water is continuously added to the continuously stirred high-concentration PVDF waste liquid until no obvious precipitation of PVDF occurs, and finally the amount of pure water added to the high-concentration PVDF waste liquid is 25%-35% of the volume of the high-concentration PVDF waste liquid.

4. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In the step S20, the amount of the high-concentration PVDF waste liquid is controlled to be 20% to 30% of the feed amount of the rake dryer.

5. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In step S20, the liquid level in the rake dryer is controlled to be 85%.

6. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In step S20, the vacuum degree of the rake dryer is controlled to be -90 kPa to -92 kPa, the steam pressure is controlled to be 0.55 mPa to 0.65 mPa, and the operating temperature is controlled to be 140°C to 160°C.

7. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In the step S20, the cleaning waste liquid, the second mixed liquid and the third mixed liquid are evaporated and concentrated, and the obtained NMP recovery liquid is sent to a distillation system for distillation to obtain qualified NMP.

8. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In the step S30, the high-concentration PVDF waste liquid is first poured into the analytical reaction tank; and then stirred and reacted for 30 minutes.

9. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: In the step S40, the first mixed solution is centrifuged using a centrifuge; and in the step S50, the filter cake is squeezed using a press.

10. The method for recovering NMP in the waste liquid from the lithium battery positive electrode pulping equipment according to claim 1, characterized in that: The method further includes step S70: treating the solid waste as hazardous waste.

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