Purification method and purification device for nmp

Abstract

The present invention relates to a method and apparatus for purifying NMP. In particular, according to one embodiment of the present invention, the method for purifying NMP purifies a gas mixture (A) containing NMP separated by a sludge separator before performing an NMP purification step, thereby improving process workability, NMP recovery rate and purity. Furthermore, a mixture containing sludge separated by the sludge separator is injected into a filtrate recovery apparatus and subjected to heat treatment, thereby effectively removing components having a higher boiling point than NMP in the mixture containing sludge, and a recycling step using an impurity tank is included to allow components having a higher boiling point than the recycled NMP to serve as a transfer material of waste NMP, thereby facilitating use in a process for purifying NMP, and further improving process workability.

Description

Technical Field

[0001] This invention relates to a method and apparatus for purifying waste NMP. Background Technology

[0002] In recent years, with the accelerated development of technologies for portable devices such as mobile phones and cameras, interest in secondary batteries as an energy source has been increasing. Among such secondary batteries, research on their recycling continues as the demand for lithium secondary batteries, which exhibit high energy density and operating potential, long cycle life, and low self-discharge rate, is rapidly increasing.

[0003] N-Methyl-2-pyrrolidone (NMP) is an organic solvent with chemical stability, low volatility, and the ability to dissolve a variety of substances. Therefore, it is widely used as a solvent in industries such as petrochemicals, plastics, and pharmaceuticals, and is also used in the manufacturing process of secondary batteries.

[0004] Specifically, lithium-ion secondary batteries consist of a positive electrode and a negative electrode. The positive electrode is manufactured by applying an electrode material comprising a lithium compound (such as lithium cobalt oxide or lithium manganese oxide), a binder (such as polyvinylidene fluoride), and N-methyl-2-pyrrolidone (NMP) as a solvent to a substrate and then calcining the electrode material. The negative electrode is manufactured by applying an electrode material comprising compounds containing carbon, titanium, etc., a binder (such as polyvinylidene fluoride), and water as a solvent to a substrate and then calcining the electrode material. As mentioned above, NMP is used not only as a major material for manufacturing the positive electrode in the electrode manufacturing process but also in the cleaning and drying steps, resulting in significant emissions. Therefore, research on technologies for its efficient recovery continues.

[0005] For example, Korean Patent No. 10-2050990 discloses a system for separating and recovering NMP from NMP gas by making NMP-containing gas with water that has absorbed NMP through gas-liquid contact, whereby NMP in the NMP-containing gas is absorbed by water.

[0006] [Existing Technical Documents] [Patent Literature] Patent Document 1: Korean Patent No. 10-2050990 Summary of the Invention

[0007] Technical issues Therefore, the present invention aims to provide a method and apparatus for purifying NMP, which can improve the recovery rate of NMP from waste NMP and provide high-purity NMP.

[0008] Technical solutions to the problem According to one embodiment of the present invention, a method for purifying NMP includes: (1) feeding waste NMP from a feed tank to a sludge separator; (2) discharging a gaseous mixture (A) containing NMP separated in the sludge separator through the top of the sludge separator; and discharging a mixture containing sludge through the bottom of the sludge separator; (3) feeding the discharged gaseous mixture (A) containing NMP to a first distillation column for first purification; (4) discharging a water-containing overhead product separated by the first purification to the outside of the system through the top of the first distillation column; and discharging a first bottom product through the bottom of the first distillation column; and (5) feeding the discharged first bottom product to a second distillation column for further purification. Second purification; (6) Discharge the purified NMP separated by the second purification to the outside of the system through the top of the second distillation column; and discharge the second bottom product through the bottom of the second distillation column; (7) Feed the mixture containing sludge discharged in step (2) to the filtrate recovery unit for heat treatment; (8) Discharge the third bottom product separated by the heat treatment to the outside of the system through the bottom of the filtrate recovery unit; and discharge the gas mixture (B) containing NMP through the top of the filtrate recovery unit; (9) Feed the second bottom product discharged in step (6) and the gas mixture (B) containing NMP discharged in step (8) to the impurity tank and mix them; and (10) Circulate the mixture from the impurity tank to the feed tank.

[0009] An apparatus for purifying NMP according to another embodiment of the present invention comprises: a feed tank for supplying waste NMP; a sludge separator having a sludge filter and a sludge separation plate for discharging a gaseous mixture (A) containing NMP separated from the supplied waste NMP through its top and a mixture containing sludge through its bottom; a first distillation column for performing a first purification on the gaseous mixture (A) containing NMP, thereby discharging a water-containing overhead product separated by the first purification through its top to the outside of the system and discharging a first bottom product through its bottom; and a second distillation column for distilling the first bottom product. A second purification process, which discharges purified NMP separated by the second purification process to the outside of the system through its top and discharges a second bottom product through its bottom; a filtrate recovery unit, which heat-treats the sludge-containing mixture discharged from the sludge separator, discharges a third bottom product separated by the heat treatment to the outside of the system through its bottom and discharges a gaseous mixture (B) containing NMP through its top; and an impurity tank, which mixes the second bottom product discharged from the second distillation column and the gaseous mixture (B) containing NMP discharged from the filtrate recovery unit and feeds the mixture into the feed tank.

[0010] Advantages of the present invention The method for purifying NMP according to one embodiment of the present invention can improve the recovery rate of NMP from waste NMP, thereby providing high-purity NMP.

[0011] Specifically, once NMP has been used as a raw material for lithium-ion batteries, the recycled waste NMP may contain a large amount of sludge. Transporting such waste NMP containing such a large amount of sludge during the recycling process is not easy, as it may clog the delivery pipes and may require separate measures for its transport.

[0012] In a method for purifying NMP according to one embodiment of the invention, the NMP-containing gaseous mixture (A) separated in a sludge separator is purified before the NMP purification step, thereby improving processability and the recovery and purity of NMP. In particular, since the sludge separator includes a sludge filter and a sludge separation plate, large quantities of sludge contained in waste NMP can be effectively separated without special control of process conditions.

[0013] Furthermore, when the sludge-containing mixture separated in the sludge separator is fed into the filtrate recovery unit and subjected to heat treatment therein, components with boiling points higher than NMP contained in the sludge-containing mixture can be effectively removed.

[0014] Furthermore, the second bottom product discharged in the second purification step and the NMP-containing gas mixture (B) discharged from the filtrate recovery unit are fed into the impurity tank, then mixed, and the mixture is recycled to the feed tank. Therefore, not only can the recovery rate and purity of NMP be improved, but the use of components with boiling points higher than those of the recycled NMP as transport materials for waste NMP promotes their use in the NMP purification process, thereby improving processability.

[0015] Furthermore, the recovery rate of NMP can be further improved by heat-treating the mixture in the impurity tank and feeding it into the second distillation column. Attached Figure Description

[0016] Figure 1 A flowchart illustrating an NMP purification method according to one embodiment of the present invention is shown schematically.

[0017] Figure 2 An example of an NMP purification apparatus according to another embodiment of the present invention is shown. Detailed Implementation

[0018] The present invention will be described in detail below. The present invention is not limited to the disclosure given below, but can be modified in various ways as long as the essential points of the invention remain unchanged.

[0019] Throughout this specification, when a component is referred to as "including" an element, it should be understood that, unless otherwise specifically stated, other elements may be included, rather than excluded.

[0020] Unless otherwise stated, all figures and expressions relating to the amounts of components, reaction conditions, etc., used herein should be understood to be modified by the term “about”.

[0021] In this specification, when it is mentioned that an element is formed "on" or "below" another element, it means not only that an element is formed directly "on" or "below" another element, but also that an element is formed indirectly on or below another element, with other elements inserted between them.

[0022] Throughout this specification, the terms first, second, etc., are used to describe various components. However, these components should not be limited by these terms. These terms are used only for the purpose of distinguishing one component from another.

[0023] NMP purification method According to one embodiment of the present invention, a method for purifying NMP includes: (1) feeding waste NMP from a feed tank to a sludge separator; (2) discharging a gaseous mixture (A) containing NMP separated in the sludge separator through the top of the sludge separator; and discharging a mixture containing sludge through the bottom of the sludge separator; (3) feeding the discharged gaseous mixture (A) containing NMP to a first distillation column for first purification; (4) discharging a water-containing overhead product separated by the first purification to the outside of the system through the top of the first distillation column; and discharging a first bottom product through the bottom of the first distillation column; and (5) feeding the discharged first bottom product to a second distillation column for further purification. Second purification; (6) Discharge the purified NMP separated by the second purification to the outside of the system through the top of the second distillation column; and discharge the second bottom product through the bottom of the second distillation column; (7) Feed the mixture containing sludge discharged in step (2) to the filtrate recovery unit for heat treatment; (8) Discharge the third bottom product separated by the heat treatment to the outside of the system through the bottom of the filtrate recovery unit; and discharge the gas mixture (B) containing NMP through the top of the filtrate recovery unit; (9) Feed the second bottom product discharged in step (6) and the gas mixture (B) containing NMP discharged in step (8) to the impurity tank and mix them; and (10) Circulate the mixture from the impurity tank to the feed tank.

[0024] Figure 1 An NMP purification method according to one embodiment of the present invention is illustrated schematically. Figure 2 An example of an NMP purification apparatus according to another embodiment of the present invention is shown. Specifically, Figure 2 An NMP purification apparatus consisting of a feed tank (RT), a sludge separator (W), a first distillation column (D1), a second distillation column (D2), a filtrate recovery unit (R), and an impurity tank (PT) is shown.

[0025] In the following text, reference will be made to Figure 1 and Figure 2 A method for purifying NMP according to one embodiment of the present invention is described.

[0026] Step (1): Feed waste NMP from the feed tank to the sludge separator. A method for purifying NMP according to one embodiment of the present invention includes the step of feeding waste NMP from a feed tank to a sludge separator.

[0027] Waste NMP can be recycled once it has been used in the manufacturing process of lithium-ion batteries, but it is not limited to this. For example, waste NMP can be recycled once it has been used as a raw material for lithium-ion batteries and may contain a large amount of sludge.

[0028] Waste NMP may contain 70% by weight or more of NMP. For example, based on the total weight of the waste NMP, the NMP content in the waste NMP may be 72% by weight or more, 75% by weight or more, 78% by weight or more, 80% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more.

[0029] In addition, waste NMP may contain water. For example, based on the total weight of the waste NMP, the water content in the waste NMP may be 30% by weight or less, 25% by weight or less, 20% by weight or less, or 15% by weight or less.

[0030] The sludge separator may include a sludge filter and a sludge separation plate. Specifically, the sludge filter may be in the form of a mesh screen located on the upper inner side of the sludge separator. Furthermore, the sludge separation plate may be positioned below the sludge filter and may be positioned where waste NMP is fed into the sludge separator. The sludge separation plate is used to prevent the waste NMP fluid fed into the sludge separator from directly contacting the sludge filter, and it may have an "U" shape, but is not limited to this. The sludge separation plate blocks the waste NMP fluid fed into the sludge separator to primarily filter the sludge, thereby not only removing sludge with high efficiency but also extending the life of the sludge filter.

[0031] When waste NMP passes through the sludge separation plate and sludge filter, the gas mixture (A) containing NMP can be discharged through the top of the sludge separator, and a large amount of sludge contained in the waste NMP can be effectively separated by the sludge separation plate and sludge filter.

[0032] Since the method for purifying NMP according to one embodiment of the present invention uses a sludge separator with the above-described technical features, it can easily separate large amounts of sludge contained in waste NMP without special control of process conditions.

[0033] Furthermore, the method may further include step (1-1) of preheating the waste NMP to a temperature of 60°C to 100°C before feeding it to the sludge separator in step (1). For example, step (1-1) may be a step of preheating the waste NMP to a temperature of 62°C to 100°C or 65°C to 95°C. The preheated waste NMP may be fed to the sludge separator.

[0034] Step (2): Discharge the NMP-containing sludge separated in the sludge separator through the top of the sludge separator. The gas mixture (A) is discharged through the bottom of the sludge separator. A method for purifying NMP according to one embodiment of the present invention includes the following steps: discharging a gaseous mixture (A) containing NMP separated in the sludge separator through the top of the sludge separator; and discharging a mixture containing sludge through the bottom of the sludge separator.

[0035] Specifically, in step (2), the gas mixture (A) containing NMP and the mixture containing sludge, which are separated by a sludge separator, are effectively separated, thereby removing a large amount of sludge contained in waste NMP to improve the processability of recycled NMP and the recovery rate and purity of NMP.

[0036] Step (3): The discharged gas mixture (A) containing NMP is fed into the first distillation column for the first purification. change A method for purifying NMP according to one embodiment of the present invention includes the step of feeding an discharged gas mixture (A) containing NMP into a first distillation column for a first purification.

[0037] In step (3), the discharged gas mixture (A) containing NMP can be fed into the middle or upper part of the first distillation column. Specifically, it is preferable to feed the gas mixture (A) containing NMP into the middle or upper part of the first distillation column for more efficient first purification, but it is not limited thereto.

[0038] The first purification in step (3) can be carried out at a temperature of 50°C to 100°C and a pressure of 100 Torr to 130 Torr. For example, the first purification can be carried out at a temperature of 52°C to 85°C or 55°C to 75°C and a pressure of 105 Torr to 130 Torr or 110 Torr to 125 Torr. When the temperature and pressure conditions of the first purification meet the above ranges, water contained in the gas mixture (A) containing NMP can be effectively removed.

[0039] Step (4): Pass the water-containing top of the first purification separation through the top of the first distillation column. The product is discharged to the outside of the system; and the bottom product of the first distillation column is discharged through the bottom of the first distillation column. According to one embodiment of the present invention, a method for purifying NMP includes the following steps: discharging an aqueous overhead product separated by a first purification process to the outside of the system through the top of a first distillation column; and discharging a first bottom product through the bottom of the first distillation column.

[0040] Specifically, in step (4), the water-containing top product is separated from the NMP-containing gas mixture (A) in the first purification step (3) and discharged to the outside of the system, which reduces the water content and thus increases the purity of NMP.

[0041] The water content in the first bottom product can be 0.01% or less. For example, the water content in the first bottom product can be 0.008% or less, 0.006% or less, or 0.005% or less.

[0042] Furthermore, the first bottoms product may contain 99.9% by weight or more of NMP. For example, based on the total weight of the first bottoms product, the NMP content in the first bottoms product may be 99.92% by weight or more, 99.94% by weight or more, or 99.95% by weight or more. Since most of the water in the waste NMP is effectively removed in the above-described first purification, the purity of NMP in the first bottoms product can be further improved.

[0043] Step (5): Feed the discharged product from the first distillation column to the second distillation column for secondary purification. According to one embodiment of the present invention, a method for purifying NMP includes the step of feeding the discharged first bottom product into a second distillation column for a second purification.

[0044] The purity of NMP can be improved through a second purification. Specifically, the waste NMP may contain NMP as well as components with boiling points higher than NMP. Since the first bottom product is purified in step (5), water has already been removed from the first bottom product in the first purification, and components with boiling points higher than NMP boiling points can be effectively removed, thereby further improving the purity of NMP.

[0045] In step (5), the bottom product of the first column can be fed into the middle or lower part of the second distillation column. Specifically, it is preferable to feed the bottom product of the first column into the middle or lower part of the second distillation column to perform the second purification more effectively, but it is not limited thereto.

[0046] The second purification can be carried out at a temperature of 120°C to 150°C and a pressure of 50 Torr to 90 Torr. For example, the second purification can be carried out at a temperature of 122°C to 148°C or 126°C to 145°C and a pressure of 55 Torr to 85 Torr or 65 Torr to 80 Torr. When the temperature and pressure conditions of the second purification meet the above ranges, components with boiling points higher than NMP and contained in the bottom product of the first column can be effectively removed.

[0047] Step (6): The purified NMP separated by the second purification process is discharged through the top of the second distillation column. The product is discharged to the outside of the system; and the bottom product of the second distillation column is discharged through the bottom of the second distillation column. According to one embodiment of the present invention, a method for purifying NMP includes the following steps: discharging purified NMP separated by a second purification process to the outside of the system through the top of a second distillation column; and discharging the bottom product of the second distillation column through the bottom of the second distillation column.

[0048] Specifically, in step (6), purified NMP can be obtained because the second bottom product containing components with boiling points higher than the boiling point of NMP is separated from the first bottom product and discharged through the second purification in step (5).

[0049] The second bottoms product may contain components with a boiling point higher than that of NMP in an amount of 1.0% by weight or less. For example, based on the total weight of the second bottoms product, the content of components with a boiling point higher than that of NMP in the second bottoms product may be 0.8% by weight or less, 0.6% by weight or less, or 0.4% by weight or less.

[0050] The second bottom product may contain 99% by weight or more NMP. For example, based on the total weight of the second bottom product, the NMP content in the second bottom product may be 99.2% by weight or more, 99.4% by weight or more, 99.5% by weight or more, or 99.6% by weight or more.

[0051] Purified NMP can have a purity of 99.9% or higher. For example, the purity of purified NMP can be 99.92% or higher, 99.94% or higher, or 99.95% or higher.

[0052] Step (7): Feed the sludge-containing mixture discharged in step (2) into the filtrate recovery unit for processing. Heat treatment According to one embodiment of the present invention, a method for purifying NMP includes feeding the mixture containing sludge discharged in step (2) into a filtrate recovery device for heat treatment.

[0053] Specifically, the sludge-containing mixture separated in the sludge separator may also contain NMP. Therefore, when the sludge-containing mixture is fed to a filtrate recovery unit and subjected to heat treatment therein, the NMP contained in the sludge-containing mixture can be separated and recycled in gaseous form.

[0054] In step (7), heat treatment can be performed at a temperature of 110°C to 140°C and a pressure of 60 Torr to 100 Torr, with stirring at 20 rpm to 80 rpm. For example, heat treatment can be performed at a temperature of 115°C to 140°C, 125°C to 138°C, or 128°C to 135°C and a pressure of 65 Torr to 95 Torr or 75 Torr to 85 Torr, with stirring at 25 rpm to 70 rpm or 30 rpm to 60 rpm. As the heat treatment conditions meet the above ranges, the recovery rate of NMP can be further improved.

[0055] Furthermore, the pressure of the filtrate recovery device can be controlled by a vacuum pump connected to the filtrate recovery device, and more specifically, by a vacuum pump connected to a cooler connected to the filtrate recovery device. It is important to control the pressure of the filtrate recovery device to produce the desired effect of the present invention; therefore, the pressure of the filtrate recovery device can be controlled by a vacuum pump.

[0056] Step (8): The third bottom product separated by the heat treatment is passed through the bottom of the filtrate recovery device. It is discharged to the outside of the system; and a gaseous mixture containing NMP (B) is discharged through the top of the filtrate recovery device. A method for purifying NMP according to one embodiment of the present invention includes the following steps: discharging a third bottom product separated by heat treatment to the outside of the system through the bottom of a filtrate recovery device; and discharging a gaseous mixture (B) containing NMP through the top of the filtrate recovery device.

[0057] Specifically, in step (8), the third bottom product and the gas mixture (B) containing NMP can be separated by heat treatment in step (7), and the gas mixture (B) containing NMP can be recycled, thereby improving processability and NMP recovery rate.

[0058] The gas mixture (B) containing NMP may contain 1.5% by weight or less of a component with a boiling point higher than that of NMP. For example, based on the total weight of the gas mixture (B) containing NMP, the content of the component with a boiling point higher than that of NMP in the gas mixture (B) containing NMP may be 1.2% by weight or less, 1.0% by weight or less, 0.8% by weight or less, 0.6% by weight or less, or 0.5% by weight or less.

[0059] Step (9): Dispose of the second bottom product discharged in step (6) and the NMP-containing gas discharged in step (8). The bulk mixture (B) is fed into the impurity tank and mixed. According to one embodiment of the present invention, a method for purifying NMP includes feeding a second bottom product discharged in step (6) and a gas mixture (B) containing NMP discharged in step (8) into an impurity tank and mixing them.

[0060] The mixture in the impurity tank may contain 1.5% by weight or less of a component with a boiling point higher than that of NMP. For example, based on the total weight of the mixture in the impurity tank, the content of a component with a boiling point higher than that of NMP in the mixture in the impurity tank may be 1.0% by weight or less, 0.8% by weight or less, 0.6% by weight or less, or 0.5% by weight or less.

[0061] According to another embodiment of the present invention, the method may further include step (9-1): cooling the gas mixture (B) containing NMP discharged in step (9) to 45°C or lower, and then feeding it into the impurity tank.

[0062] The gas mixture (B) containing NMP discharged in step (9) can be fed into the impurity tank without further treatment and then recycled. However, to promote mixing with the second bottom product discharged in step (6), a further step (9-1) can be performed: before it is fed into the impurity tank, it can be cooled to 42°C or lower, 40°C or lower, or 35°C or lower using a cooler.

[0063] Step (10): Circulate the mixture from the impurity tank to the raw material tank. A method for purifying NMP according to one embodiment of the present invention includes the step of circulating a mixture from an impurity tank to a feed tank.

[0064] Specifically, recycling the mixture in the impurity tank during its feeding to the feed tank can improve the recovery and purity of NMP. In particular, the use of components with boiling points higher than NMP as transport materials for waste NMP facilitates their application in the NMP purification process, resulting in improved processability.

[0065] According to another embodiment of the invention, the method may further include the step (11) of heat-treating the mixture in the impurity tank to feed it to the second distillation column. Specifically, the mixture in the impurity tank may be fed to the feed tank for recycling, and it may be heat-treated and then fed to the second distillation column for recycling, thereby further improving the recovery rate of NMP.

[0066] In step (11), the heat treatment can be performed at 100°C to 130°C. For example, the heat treatment in step (11) can be performed at a temperature of 105°C to 130°C or 115°C to 130°C.

[0067] Furthermore, the mixture from the heat-treated impurity tank in step (11) can be fed into the middle or lower part of the second distillation column. Specifically, the gas mixture can preferably be fed into the middle or lower part of the second distillation column for more efficient purification, but is not limited thereto.

[0068] More specifically, the position where the heat-treated mixture from the impurity tank in step (11) is fed into the second distillation column can be the same as or lower than the position where the bottom product from the first column is fed into the second distillation column in step (5). Since the position where the heat-treated mixture from the impurity tank in step (11) is fed into the second distillation column is the same as or lower than the position where the bottom product from the first column is fed into the second distillation column in step (5), NMP can be purified more efficiently. In particular, since high-purity NMP as the final product is discharged to the outside of the system through the top of the second distillation column, the mixture of the bottom product from the first column and the heat-treated mixture from the impurity tank must be fed into the middle or lower position of the second distillation column to improve the efficiency of NMP purification. Since the feed position of the heat-treated mixture from the impurity tank with low NMP content is set relatively lower than the feed position of the bottom product from the first column, more efficient purification can be achieved.

[0069] NMP purification device An apparatus for purifying NMP according to another embodiment of the present invention comprises: a feed tank for supplying waste NMP; a sludge separator having a sludge filter and a sludge separation plate for discharging a gaseous mixture (A) containing NMP separated from the supplied waste NMP through its top and a mixture containing sludge through its bottom; a first distillation column for performing a first purification on the gaseous mixture (A) containing NMP, thereby discharging a water-containing overhead product separated by the first purification through its top to the outside of the system and discharging a first bottom product through its bottom; and a second distillation column for distilling the first bottom product. A second purification process, which discharges purified NMP separated by the second purification process to the outside of the system through its top and discharges a second bottom product through its bottom; a filtrate recovery unit, which heat-treats the sludge-containing mixture discharged from the sludge separator, discharges a third bottom product separated by the heat treatment to the outside of the system through its bottom and discharges a gaseous mixture (B) containing NMP through its top; and an impurity tank, which mixes the second bottom product discharged from the second distillation column and the gaseous mixture (B) containing NMP discharged from the filtrate recovery unit and feeds the mixture into the feed tank.

[0070] Figure 2 An example of an NMP purification apparatus according to another embodiment of the present invention is shown. Specifically, Figure 2 An NMP purification apparatus consisting of a feed tank (RT), a sludge separator (W), a first distillation column (D1), a second distillation column (D2), a filtrate recovery unit (R), and an impurity tank (PT) is shown.

[0071] First, waste NMP is supplied to the sludge separator (W) (X-1) via the feed tank (RT). This supply can be made by the first pump (P1). In this case, the waste NMP can be preheated by the preheater (B1).

[0072] The sludge separator (W) may include a sludge filter (W1) disposed on its upper inner side and a sludge separation plate (W2) disposed on its upper inner side at a position lower than the sludge filter.

[0073] The NMP-containing gas mixture (A), separated from waste NMP or preheated waste NMP by the sludge filter (W1) and sludge separation plate (W2) in the sludge separator (W), is discharged from the top of the sludge separator (T-1) and fed to the first distillation column (D1) (X-2) via the third pump (P3), and discharged from the bottom of the sludge separator as a sludge-containing mixture (Y-1). The discharged sludge-containing mixture can be transferred to the filtrate recovery unit (R) (X-3) via the second pump (P2) and then recycled back to the sludge separator (W). In this case, the sludge-containing mixture can be preheated by the heater (B2) of the sludge separator before being sent back to the sludge separator (W).

[0074] The first distillation column (D1) performs a first purification by distilling the feed gas mixture (A) containing NMP. The water-containing overhead product separated by the first purification is discharged to the outside of the system through the top of the first distillation column (D1) (T-2), and the first bottom product (Y-2) is discharged through the bottom of the first distillation column (D1). The discharged first bottom product can be transferred to the second distillation column (D2) (X-3) by a fourth pump (P4) and can be recycled back to the first distillation column (D1) (S-2). In this case, the first bottom product can be preheated by the first reboiler (B3) before being sent back to the first distillation column (D1).

[0075] The second distillation column (D2) performs a second purification by distilling the feed from the bottom product of the first column. The purified NMP separated by the second purification is discharged to the outside of the system through the top of the second distillation column (D2) (T-3), and the second bottom product is discharged through the bottom of the second distillation column (D2) (Y-3). The purified NMP discharged from the system can be stored in a storage tank (ST). Furthermore, the discharged second bottom product can be transferred to an impurity tank (PT) (X-4) via a fifth pump (P5) and recycled back to the second distillation column (D2) (S-3). In this case, the second bottom product can be preheated by a second reboiler (B4) before being sent back to the second distillation column (D2).

[0076] The filtrate recovery unit (R) heat-treats the feed mixture containing sludge. In this case, the pressure can be controlled by a vacuum pump (A). The third bottom product separated by heat treatment is discharged to the outside of the system (Y-4) through the bottom of the filtrate recovery unit, and a gaseous mixture (B) containing NMP (T-5) is discharged through the top of the filtrate recovery unit and fed to the impurity tank (PT) (X-5) by a sixth pump (P6). In this case, the discharged NMP-containing gaseous mixture (B) can be cooled by a cooler (C1) before being fed to the impurity tank (PT).

[0077] In the impurity tank (PT), the second bottom product discharged from the second distillation column (D2) and the NMP-containing gas mixture (B) discharged from the filtrate recovery unit (R) are mixed by stirring, and then the mixture is recycled to the feed tank (RT) (S-5). In this case, a heat treatment device can be provided to heat treat the mixture in the impurity tank and feed it to the second distillation column (D2) (S-4).

[0078] Figure Labels D1, D2: First distillation column and second distillation column P1 to P6: Pumps 1 to 6 B1: Preheater B2: Heater for sludge separator B3: First Reboiler B4: Second Reboiler W: Sludge separator W1: Sludge Filter W2: Sludge Separation Plate R: Filtrate recovery device RT: Raw material tank PT: Impurity tank ST: Storage tank T-1 to T-5, Y-1 to Y-4: Discharge procedures S-1 to S-5: Cyclic Steps X-1 to X-5: Feeding Steps A: Vacuum pump

Claims

1. A method for purifying NMP, comprising: (1) Feed waste NMP from the raw material tank to the sludge separator; (2) The gas mixture (A) containing NMP separated in the sludge separator is discharged through the top of the sludge separator; and the mixture containing sludge is discharged through the bottom of the sludge separator; (3) The discharged gas mixture (A) containing NMP is fed into the first distillation column for first purification; (4) The water-containing overhead product separated by the first purification is discharged to the outside of the system through the top of the first distillation column; and the first bottom product is discharged through the bottom of the first distillation column. (5) The product discharged from the bottom of the first column is fed into the second distillation column for second purification; (6) The purified NMP separated by the second purification is discharged to the outside of the system through the top of the second distillation column; and the bottom product of the second column is discharged through the bottom of the second distillation column. (7) The mixture containing sludge discharged in step (2) is fed into a filtrate recovery unit for heat treatment; (8) The third bottom product separated by the heat treatment is discharged to the outside of the system through the bottom of the filtrate recovery device; and a gas mixture (B) containing NMP is discharged through the top of the filtrate recovery device. (9) The second bottom product discharged in step (6) and the NMP-containing gas mixture (B) discharged in step (8) are fed into the impurity tank and mixed; and (10) The mixture from the impurity tank is recycled to the raw material tank.

2. The method for purifying NMP according to claim 1, wherein the sludge separator comprises a sludge filter and a sludge separation plate.

3. The method for purifying NMP according to claim 1, further comprising step (1-1) of preheating the waste NMP to a temperature of 60°C to 100°C before feeding the waste NMP into the sludge separator in step (1).

4. The method for purifying NMP according to claim 1, wherein the mixture in the impurity tank contains 1.5% by weight or less of a component with a boiling point higher than that of NMP.

5. The method for purifying NMP according to claim 1, further comprising the step (11) of heat-treating the mixture in the impurity tank to feed it into the second distillation column.

6. The method for purifying NMP according to claim 5, wherein in step (11), the heat treatment is performed at 100°C to 130°C.

7. The method for purifying NMP according to claim 5, wherein the mixture from the heat-treated impurity tank in step (11) is fed into the second distillation column at the same or lower position as the bottom product of the first column is fed into the second distillation column in step (5).

8. The method for purifying NMP according to claim 1, wherein the pressure of the filtrate recovery device is controlled by a vacuum pump connected to the filtrate recovery device.

9. The method for purifying NMP according to claim 1, further comprising (9-1) cooling the NMP-containing gas mixture (B) discharged in step (9) to 45°C or lower before feeding it into the impurity tank.

10. The method for purifying NMP according to claim 1, wherein the gas mixture (B) containing NMP contains 1.5% by weight or less of a component with a boiling point higher than that of NMP.

11. The method for purifying NMP according to claim 1, wherein the waste NMP contains 70% by weight or higher of NMP.

12. The method for purifying NMP according to claim 1, wherein in step (3), the discharged gas mixture (A) containing NMP is fed into the middle or upper part of the first distillation column.

13. The method for purifying NMP according to claim 1, wherein the first bottom product contains 99.9% by weight or higher of NMP.

14. The method for purifying NMP according to claim 1, wherein the water content in the first bottom product is 0.01% or less.

15. The method for purifying NMP according to claim 1, wherein in step (5), the first bottom product is fed into the middle or lower part of the second distillation column.

16. The method for purifying NMP according to claim 1, wherein the second bottom product contains 99% by weight or more of NMP.

17. The method for purifying NMP according to claim 1, wherein the first purification in step (3) is carried out at a temperature of 50°C to 100°C and a pressure of 100 Torr to 130 Torr, and the second purification in step (5) is carried out at a temperature of 120°C to 150°C and a pressure of 50 Torr to 90 Torr.

18. The method for purifying NMP according to claim 1, wherein the purified NMP has a purity of 99.9% or higher.

19. An apparatus for purifying NMP, comprising: Raw material tanks, used to supply waste NMP; A sludge separator having a sludge filter and a sludge separation plate, which discharges a gaseous mixture (A) containing NMP separated from the supplied waste NMP through its top and a mixture containing sludge through its bottom; A first distillation column is used to perform a first purification on the gas mixture (A) containing NMP, thereby discharging a water-containing overhead product separated by the first purification to the outside of the system through its top and a first bottom product through its bottom. The second distillation column is used to perform a second purification on the bottom product of the first column by distillation, thereby discharging the purified NMP separated by the second purification to the outside of the system through its top and discharging the second bottom product through its bottom. A filtrate recovery unit is used to heat-treat the sludge-containing mixture discharged from the sludge separator, thereby discharging the third bottom product separated by the heat treatment to the outside of the system through its bottom and discharging a gaseous mixture (B) containing NMP through its top; and An impurity tank is used to mix the second bottom product discharged from the second distillation column and the NMP-containing gas mixture (B) discharged from the filtrate recovery unit, and to feed the mixture into the feed tank.

20. The apparatus for purifying NMP according to claim 19, further comprising a vacuum pump for controlling the pressure of the filtrate recovery apparatus.

21. The apparatus for purifying NMP according to claim 19, further comprising a heat treatment device for heat-treating the mixture in the impurity tank and feeding it into the second distillation column.

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