Separation system and separation method for silicon nitride recovery liquid

Abstract

The invention provides a separation system and a separation method for silicon nitride recovery liquid. The separation system comprises a raw material supply unit and a separation unit. Wherein the raw material supply unit comprises a raw material supply storage tank, silicon nitride recovery liquid is stored in the raw material supply storage tank, and the silicon nitride recovery liquid comprises ammonium chloride, liquid ammonia and methylbenzene. The separation unit comprises a flash evaporation device, a rectification device and a solid-liquid separation device; the flash evaporation device is used for carrying out flash evaporation separation on the silicon nitride recovery liquid to obtain ammonia-toluene mixed gas and ammonium chloride slurry; the rectification device is used for separating the ammonia-toluene mixed gas to obtain liquid ammonia and toluene; the solid-liquid separation device is used for carrying out solid-liquid separation on the ammonium chloride slurry to obtain an ammonium chloride solid. According to the separation system, methylbenzene and liquid ammonia with high purity can be obtained through separation, the technological process is simple, the number of devices is small, the operation cost is low, the automation degree is high, and continuous production can be achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of separation and recovery of ammonium chloride, toluene and liquid ammonia, and in particular to a separation system and method for silicon nitride recovery liquid. BACKGROUND

[0002] In the process of producing silicon nitride from silicon tetrachloride, liquid ammonia and toluene, the main components in the liquid after separation of silicon nitride are ammonium chloride, liquid ammonia and toluene. These three components are separated and recovered, among which ammonium chloride can be sold, and liquid ammonia and toluene can be recycled and used for the preparation of silicon nitride.

[0003] In the past separation and recovery process, ammonium chloride and liquid ammonia are mutually soluble, but both have a very small mutual solubility with toluene. A method of low-temperature freezing is used to freeze the mixture including ammonium chloride, liquid ammonia and toluene to a sufficiently low temperature to precipitate a large amount of ammonium chloride. After the ammonium chloride is separated by filtration, the remaining saturated liquid is separated into liquid ammonia and toluene by static separation, and then enters a liquid ammonia rectification tower and a toluene rectification tower respectively for rectification. The rectified ammonia and toluene are recycled. However, in this method, the separation of ammonium chloride, the static separation of liquid ammonia and toluene are all intermittent operations, and the liquid ammonia and toluene separated by static separation also need to be rectified separately, resulting in a complex process, a large number of equipment, high labor intensity, and the risk of material leakage and harm to the human body. In addition, since a small amount of ammonium chloride is still dissolved in the liquid ammonia during low-temperature freezing, the ammonium chloride cannot be completely separated, resulting in low purity of the recovered liquid ammonia and toluene, and the residual ammonium chloride can block the rectification tower pipeline or equipment after long-term operation. The low-purity liquid ammonia and toluene recovered also affect the product quality of silicon nitride during recycling. In addition, the method of low-temperature freezing to precipitate ammonium chloride requires a large amount of refrigerant, resulting in high operating costs.

[0004] Therefore, the present application is proposed. SUMMARY

[0005] The main purpose of the present application is to provide a separation system and method for silicon nitride recovery liquid, to solve the problems of low purity of liquid ammonia and toluene, easy blocking of the device, high energy consumption and high operating costs in the separation of ammonium chloride, liquid ammonia and toluene from silicon nitride recovery liquid in the prior art.

[0006] In order to achieve the above object, according to one aspect of the present application, a separation system of silicon nitride recovery liquid is provided, which comprises: a raw material supply unit comprising a raw material supply tank, the raw material supply tank storing silicon nitride recovery liquid, the silicon nitride recovery liquid comprising ammonium chloride, liquid ammonia and toluene; a separation unit comprising a flash device, a rectification device and a solid-liquid separation device, the flash device being used for flash separation of the silicon nitride recovery liquid to obtain ammonia-toluene mixed gas and ammonium chloride slurry, the rectification device being used for separation of the ammonia-toluene mixed gas to obtain liquid ammonia and toluene, and the solid-liquid separation device being used for solid-liquid separation of the ammonium chloride slurry to obtain ammonium chloride solid.

[0007] Further, the flash device comprises a first flash tank and a second flash tank connected in sequence; the inlet of the first flash tank is connected with the outlet of the raw material supply tank, the first flash tank has a first slurry outlet and a first gas outlet, the first slurry outlet is connected with the inlet of the second flash tank, and the second flash tank has a second slurry outlet and a second gas outlet.

[0008] Further, the flash device further comprises a first heater and a second heater; the first heater is arranged on the pipeline between the raw material supply tank and the first flash tank; and the second heater is arranged on the pipeline between the first flash tank and the second flash tank.

[0009] Further, the rectification device comprises a rectification column; the rectification column comprises a first rectification inlet and a second rectification inlet, the first rectification inlet is connected with the first gas outlet, and the second rectification inlet is connected with the second gas outlet; the rectification column further comprises a first rectification gas outlet and a first rectification liquid outlet, the first rectification gas outlet is used for discharging ammonia gas, and the first rectification liquid outlet is used for discharging toluene.

[0010] Further, the rectification column is a sieve tray column or a float valve column.

[0011] Further, the rectification column is provided with a first condenser at the top, the first condenser being used for condensing ammonia gas into liquid ammonia.

[0012] Further, a second condenser is further arranged on the pipeline between the second flash tank and the rectification column.

[0013] Further, the solid-liquid separation device comprises a filter, the inlet of the filter is connected with the second slurry outlet, the filter has a filtrate outlet and a solid outlet, the filtrate outlet is connected with the inlet of the first heater, and the solid outlet is used for discharging ammonium chloride solid.

[0014] Further, the separation system further comprises a drying device, the drying device comprising a dryer, the inlet of the dryer is connected with the solid outlet, and the outlet of the dryer is used for discharging ammonium chloride solid.

[0015] According to another aspect of the present application, a separation method of silicon nitride recovery liquid is provided, the separation method comprising the following steps: step S1, performing flash separation on the silicon nitride recovery liquid to obtain ammonia-toluene mixed gas and ammonium chloride slurry; step S2, performing rectification separation on the ammonia-toluene mixed gas to obtain liquid ammonia and toluene; and step S3, performing solid-liquid separation on the ammonium chloride slurry to obtain ammonium chloride solid.

[0016] Further, step S1 comprises: step S11, performing primary flash separation on the silicon nitride recovery liquid to obtain first ammonia-toluene mixed gas and first ammonium chloride slurry; and step S12, performing secondary flash separation on the first ammonium chloride slurry to obtain second ammonia-toluene mixed gas and second ammonium chloride slurry.

[0017] Further, the pressure of the primary flash separation is 0-0.8 MPa.

[0018] Further, the pressure of the secondary flash separation is -0.02-0.1 MPa.

[0019] Further, before the primary flash separation, the silicon nitride recovery liquid further comprises first pre-heating treatment.

[0020] Further, the temperature of the first pre-heating treatment is 70-110°C.

[0021] Further, before the secondary flash separation, the first ammonium chloride slurry further comprises second pre-heating treatment.

[0022] Further, the temperature of the second pre-heating treatment is 60-110°C.

[0023] Further, step S2 comprises: performing rectification separation on the first ammonia-toluene mixed gas and the second ammonia-toluene mixed gas respectively.

[0024] Further, in step S2, the overhead temperature of the rectification column is -32.7-13.7°C, the overhead pressure of the rectification column is 0-0.6 MPa, and the bottom temperature of the rectification column is 96.7-196.2°C.

[0025] Further, the ammonia gas obtained by the rectification separation is subjected to first condensation treatment to convert it into liquid ammonia; the temperature of the first condensation treatment is -33.1-14.1°C, and the pressure is 0-0.6 MPa.

[0026] Further, before the rectification separation of the second ammonia-toluene mixed gas, the second ammonia-toluene mixed gas is subjected to second condensation treatment, and the temperature of the second condensation treatment is 40-90°C.

[0027] Further, step S3 comprises: performing solid-liquid separation on the second ammonium chloride slurry to obtain ammonium chloride solid and filtrate.

[0028] Further, the filtrate is returned to step S1 to continue the flash separation.

[0029] Further, the solid-liquid separation method is filtration.

[0030] Further, the separation method further comprises step S4: drying the ammonium chloride solid.

[0031] Further, the drying temperature is 60-110℃.

[0032] According to the separation system of the silicon nitride recovery liquid provided by the application, the outlet of the raw material supply tank is connected with the inlet of the flash device, the silicon nitride recovery liquid is subjected to flash separation in the flash device to obtain ammonia-toluene mixed gas and ammonium chloride slurry. The ammonia-toluene mixed gas is subjected to rectification separation in the rectification device to obtain liquid ammonia and toluene; the ammonium chloride slurry is subjected to solid-liquid separation in the solid-liquid separation device to obtain ammonium chloride solid. The separation system provided by the application not only has high purity of the toluene and liquid ammonia obtained by separation, but also has simple process flow, small number of equipment, low running cost, high automation degree and can realize continuous production. BRIEF DESCRIPTION OF DRAWINGS

[0033] The drawings accompanying the specification of this application form a part thereof, serve to provide further understanding of the application, and together with the description of the exemplary embodiments of the application given below, make an explanation of the application, and do not constitute an improper limitation of the application. In the drawings:

[0034] Figure 1 A flow diagram of the separation system of the silicon nitride recovery liquid provided by some embodiments of the application is shown.

[0035] Among them, the above drawings include the following reference signs:

[0036] 10, raw material supply tank; 21, first heater; 22, first flash tank; 23, second heater; 24, second flash tank; 31, rectification tower; 32, second condenser; 40, filter; 50, dryer. DETAILED DESCRIPTION

[0037] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0038] As analyzed in the background art of the present application, the existing separation of ammonium chloride, liquid ammonia and toluene in the silicon nitride recovery liquid has low purity of the obtained liquid ammonia and toluene, and the device is easy to be blocked, has large energy consumption and high running cost. In order to solve this problem, the present application provides a separation system and method of silicon nitride recovery liquid.

[0039] In one typical embodiment of the present application, a separation system of silicon nitride recovery liquid is provided, as shown in the figure, the recovery device comprises a raw material supply unit and a separation unit; the raw material supply unit comprises a raw material supply tank 10, which stores the silicon nitride recovery liquid, the silicon nitride recovery liquid comprises ammonium chloride, liquid ammonia and toluene; the separation unit comprises a flash device, a rectification device and a solid-liquid separation device; the flash device is used for flash separation of the silicon nitride recovery liquid to obtain ammonia-toluene mixed gas and ammonium chloride slurry, the rectification device is used for separation of the ammonia-toluene mixed gas to obtain liquid ammonia and toluene; the solid-liquid separation device is used for solid-liquid separation of the ammonium chloride slurry to obtain ammonium chloride solid. Figure 1

[0040] The silicon nitride recovery liquid in the present application is derived from the waste liquid obtained after solid-liquid separation of silicon nitride in the process of preparing silicon nitride.

[0041] According to the separation system of silicon nitride recovery liquid provided by the present application, the outlet of the raw material supply tank is connected with the inlet of the flash device, the silicon nitride recovery liquid is flash separated in the flash device to obtain ammonia-toluene mixed gas and ammonium chloride slurry. The ammonia-toluene mixed gas enters the rectification device for rectification separation to obtain liquid ammonia and toluene; the ammonium chloride slurry enters the solid-liquid separation device for solid-liquid separation to obtain ammonium chloride solid. The separation system provided by the present application not only has high purity of toluene and liquid ammonia obtained by separation, but also has simple process flow, less equipment quantity, low running cost, high automation degree and can realize continuous production.

[0042] In some embodiments, the flash device comprises a first flash tank 22 and a second flash tank 24 connected in sequence; wherein the inlet of the first flash tank 22 is connected with the outlet of the raw material supply tank 10, so as to facilitate the first flash of the silicon nitride recovery liquid in the first flash tank 22 to obtain first ammonia-toluene mixed gas and first ammonium chloride slurry. In the first flash process, a large amount of liquid ammonia and part of toluene in the silicon nitride recovery liquid are flash separated into gas state, and after the evaporation of the liquid ammonia, the ammonium chloride gradually precipitates into solid. The first flash tank 22 has a first slurry outlet and a first gas outlet, the first slurry outlet is connected with the inlet of the second flash tank 24, so as to facilitate the second flash of the first ammonium chloride slurry discharged from the first slurry outlet in the second flash tank 24 to obtain second ammonia-toluene mixed gas and second ammonium chloride slurry, the main component of the second ammonia-toluene mixed gas is toluene, and only a small amount of ammonia gas is contained. The second flash tank 24 has a second slurry outlet and a second gas outlet.

[0043] ​In some embodiments, the flash device further comprises a first heater 21 and a second heater 23. The first heater 21 is arranged on the pipeline connecting the raw material supply tank 10 and the first flash tank 22 to facilitate the first preheating of the silicon nitride recovery liquid, thereby further facilitating the flash separation of liquid ammonia and toluene in the silicon nitride recovery liquid. The second heater 23 is arranged on the pipeline connecting the first flash tank 22 and the second flash tank 24 to facilitate the second preheating of the first ammonium chloride slurry discharged from the first flash tank 22, thereby further facilitating the flash separation of toluene in the first ammonium chloride slurry.

[0044] In order to further pass the silicon nitride recovery liquid into the first flash tank 22, a first pump (not shown in the figure) is preferably arranged on the pipeline connecting the raw material supply tank 10 and the first flash tank 22, and more preferably the first pump is arranged on the pipeline connecting the raw material supply tank 10 and the first heater 21 to further transport the silicon nitride recovery liquid into the first heater 21 for preheating before the first flash.

[0045] In some embodiments, the rectification device comprises a rectification column 31. The rectification column 31 comprises a first rectification inlet and a second rectification inlet. The first rectification inlet is connected with the first gas outlet, and the second rectification inlet is connected with the second gas outlet to rectify and separate the first ammonia-toluene mixed gas and the second ammonia-toluene mixed gas respectively to obtain toluene and ammonia gas. The rectification column 31 further comprises a first rectification gas outlet and a first rectification liquid outlet. The first rectification gas outlet is used to discharge ammonia gas, and the first rectification liquid outlet is used to discharge toluene.

[0046] The rectification column 31 has a first condenser (not shown in the figure) at the top. The first condenser is used to condense ammonia gas into liquid ammonia. The obtained liquid ammonia after condensation is returned to the upstream for the production of silicon nitride for recycling, except for meeting the reflux. The toluene obtained by rectification at the bottom of the column is also returned to the upstream for the production of silicon nitride for recycling after cooling.

[0047] In order to further separate ammonia gas and toluene more fully, a second condenser 32 is preferably arranged on the pipeline connecting the second flash tank 24 and the rectification column 31, so that the second ammonia-toluene mixed gas is condensed into a second mixed liquid before being introduced into the rectification column 31 for rectification.

[0048] In order to facilitate the transportation of the second mixed liquid to the rectification column 31, a second pump (not shown in the figure) is preferably arranged on the pipeline connecting the second condenser 32 and the rectification column 31.

[0049] In order to further improve the purity of ammonia gas and toluene, the rectification column 31 is preferably a sieve tray column or a float valve column to prevent the long-term accumulation of trace amounts of ammonium chloride solids carried by the feed to block the internals.

[0050] In some embodiments, the solid-liquid separation device comprises a filter 40, an inlet of the filter 40 being connected with the second slurry outlet to separate the second ammonium chloride slurry discharged from the second slurry outlet, the filter 40 having a filtrate outlet and a solid outlet, the ammonium chloride solid being discharged from the solid outlet after the solid-liquid separation, and the filtrate after the removal of the ammonium chloride being discharged from the filtrate outlet.

[0051] In order to further extract the ammonium chloride, liquid ammonia and toluene remaining in the separated filtrate, the filtrate outlet is preferably connected with the flash device, and more preferably connected with the inlet of the first heater 21. In order to facilitate the delivery of the filtrate into the first heater 21, a third pump (not shown in the figure) is preferably arranged on the pipeline connecting the filter 40 and the first heater 21.

[0052] In some embodiments, the separation system further comprises a drying device, the drying device comprising a dryer 50, an inlet of the dryer 50 being connected with the solid outlet to facilitate the drying of the ammonium chloride solid obtained after the solid-liquid separation, the dried ammonium chloride solid being discharged from the outlet of the dryer 50. The dried ammonium chloride solid is packaged as an ammonium chloride product for sale.

[0053] In another typical embodiment of the present application, a separation method of a silicon nitride recovery liquid is provided, the separation method comprising the following steps: step S1, flash separation of the silicon nitride recovery liquid to obtain ammonia-toluene mixed gas and ammonium chloride slurry; step S2, rectification separation of the ammonia-toluene mixed gas to obtain liquid ammonia and toluene; and step S3, solid-liquid separation of the ammonium chloride slurry to obtain ammonium chloride solid.

[0054] The silicon nitride recovery liquid is subjected to flash treatment in the present application, so that the liquid ammonia and toluene in the silicon nitride recovery liquid are flash separated to form ammonia-toluene mixed gas, and the ammonium chloride is precipitated to form solid due to supersaturation, and forms ammonium chloride slurry with the remaining liquid. Subsequently, the ammonia-toluene mixed gas is subjected to rectification treatment to separate the toluene and ammonia gas, and the ammonia gas is condensed to form liquid ammonia. The ammonium chloride slurry is subjected to solid-liquid separation to obtain ammonium chloride solid. The method of flash separation followed by rectification in the present application not only makes the obtained toluene and liquid ammonia have high purity, but also has simple process flow, less equipment, low running cost, high automation degree and can realize continuous production.

[0055] In some embodiments, step S11, first flash separation of the silicon nitride recovery liquid to obtain first ammonia-toluene mixed gas and first ammonium chloride slurry; and step S12, second flash separation of the first ammonium chloride slurry to obtain second ammonia-toluene mixed gas and second ammonium chloride slurry.

[0056] The silicon nitride recovery liquid is introduced into the first flash tank 22, and a large amount of liquid ammonia and part of the toluene in the silicon nitride recovery liquid are separated by first-stage flash evaporation into a gaseous state to obtain first ammonia-toluene mixed gas, and after the liquid ammonia evaporates, ammonium chloride gradually precipitates into a solid to obtain first ammonium chloride slurry. The first ammonium chloride slurry is then introduced into the second flash tank 24 to continue second-stage flash evaporation, so that the toluene is separated by flash evaporation, and a small amount of ammonia gas is also included, to obtain second ammonia-toluene mixed gas, and the ammonium chloride solid forms second ammonium chloride slurry with the remaining liquid.

[0057] In some embodiments, the pressure of the first-stage flash evaporation is 0-0.8 MPa. A pressure value in the above range can promote more complete flash evaporation of the liquid ammonia and toluene into a gaseous state, and the energy consumption is low. When the pressure is too large, the energy consumption will increase.

[0058] In some embodiments, the pressure of the second-stage flash evaporation is -0.02-0.1 MPa. A pressure value in the above range can further promote the flash evaporation of the toluene, and the energy consumption is low. When the pressure is too large, the energy consumption will increase.

[0059] In order to further improve the flash evaporation separation efficiency of the toluene and liquid ammonia in the silicon nitride recovery liquid in the first flash tank 22, it is preferable that the silicon nitride recovery liquid is subjected to first preheating treatment before first-stage flash evaporation. More preferably, the temperature of the first preheating treatment is 70-110°C.

[0060] In order to further separate more toluene from the first ammonium chloride slurry by flash evaporation, it is preferable that the first ammonium chloride slurry is subjected to second preheating treatment before second-stage flash evaporation. More preferably, the temperature of the second preheating treatment is 60-110°C.

[0061] In some embodiments, step S2 includes separately subjecting the first ammonia-toluene mixed gas and the second ammonia-toluene mixed gas to rectification separation to obtain toluene and ammonia gas.

[0062] In order to further improve the purity of the separated toluene and ammonia gas, it is preferable that the overhead temperature of the rectification column 31 is -32.7-13.7°C, the overhead pressure of the rectification column 31 is 0-0.6 MPa, and the column bottom temperature of the rectification column 31 is 96.7-196.2°C.

[0063] In order to facilitate the condensation of the ammonia gas at the top of the tower into liquid ammonia, the ammonia gas obtained by the rectification separation is preferably subjected to a first condensation treatment, and part of the obtained liquid ammonia is used for reflux, and the remaining liquid ammonia is returned to the upstream for the production of silicon nitride as liquid ammonia product for recycling. Further preferably, the temperature of the first condensation treatment is -33.1-14.1°C, and the pressure is 0-0.6 MPa. In the separation method provided in the present application, only the first condenser at the top of the rectification tower 31 needs a small amount of refrigerant for the first condensation treatment, so the operation cost is relatively low.

[0064] Similarly, the toluene obtained from the tower kettle of the rectification tower 31 is also returned to the upstream for the production of silicon nitride for recycling after being cooled.

[0065] In order to further promote the separation of ammonia gas and toluene, the second ammonia-toluene mixed gas is preferably subjected to a second condensation treatment before being subjected to the rectification separation, so that the second ammonia-toluene mixed gas is condensed into a second mixed liquid before being introduced into the rectification tower 31 for the rectification separation. Further preferably, the temperature of the second condensation treatment is 40-90°C.

[0066] In the present application, the liquid ammonia and toluene in the silicon nitride recovery liquid are first separated by two-stage flash evaporation, and the ammonia-toluene mixed gas is obtained, and the ammonium chloride is supersaturated and precipitated, and then the liquid ammonia and toluene are separated by the method of rectification, so that the purity of the obtained liquid ammonia and toluene is relatively high, and when the liquid ammonia and toluene are recycled to the upstream process, the pipeline will not be blocked due to the early reaction of the materials, and the quality of the silicon nitride product prepared by recycling to the upstream is better.

[0067] In some embodiments, step S3 comprises subjecting the second ammonium chloride slurry to solid-liquid separation to obtain ammonium chloride solids and a filtrate. In order to further recover the residual ammonium chloride, toluene and liquid ammonia in the filtrate, the filtrate is preferably returned to step S1 for further flash separation.

[0068] In the present application, the form of solid-liquid separation is not specifically limited, and the commonly used solid-liquid separation methods in the art can be used, such as centrifugation, filtration, etc. In order to facilitate operation, the solid-liquid separation method is preferably filtration. In order to improve the filtration efficiency, the filtration precision is preferably ≤1-5 μm.

[0069] In order to further facilitate the sale of the ammonium chloride solids obtained by the solid-liquid separation, the separation method further comprises step S4: the ammonium chloride solids are subjected to a drying treatment. In order to further improve the drying rate, the temperature of the drying treatment is preferably 60-110°C.

[0070] The beneficial effects of the present application will be further illustrated in the following examples.

[0071] Example 1

[0072] The present embodiment provides a separation system for silicon nitride recovery liquid, which comprises steps S1-S4 as follows:Figure 1 As shown, the separation system includes: a raw material supply storage tank 10, a first heater 21, a first flash tank 22, a second heater 23, a second flash tank 24, a distillation column 31, a second condenser 32, a filter 40, and a dryer 50.

[0073] The inlet of the first flash tank 22 is connected to the outlet of the raw material supply tank 10, which stores silicon nitride recovery liquid, including ammonium chloride, liquid ammonia, and toluene. A first heater 21 is installed on the pipeline connecting the raw material supply tank 10 and the first flash tank 22 to preheat the silicon nitride recovery liquid before it is introduced into the first flash tank 22. A first pump is installed on the pipeline connecting the raw material supply tank 10 and the first heater 21. Figure 1 (Not shown), the silicon nitride recovery liquid is pressurized by a first pump and transported to a first heater 21. The first flash tank 22 has a first slurry outlet and a first gas outlet. The first slurry outlet is connected to the inlet of the second flash tank 24. A second heater 23 is installed on the pipeline connecting the first flash tank 22 and the second flash tank 24 to preheat the first ammonium chloride slurry discharged from the first slurry outlet before it is introduced into the second flash tank 24.

[0074] The second flash tank 24 has a second slurry outlet and a second gas outlet. The distillation column 31 includes a first distillation inlet and a second distillation inlet. The first distillation inlet is connected to the first gas outlet, and the second distillation inlet is connected to the second gas outlet. The first distillation inlet is higher than the second distillation inlet. A second condenser 32 is installed on the pipeline connecting the second flash tank 24 and the distillation column 31 so that the second ammonia-toluene mixture separated from the second flash tank 24 is condensed into a second mixed liquid before being introduced into the distillation column 31. A second pump is installed on the pipeline connecting the second condenser 32 and the distillation column 31. Figure 1 (Not shown), the second mixed liquid obtained after condensation is pressurized by a second pump and conveyed to distillation column 31. Distillation column 31 also includes a first distillation gas outlet and a first distillation liquid outlet. The first distillation gas outlet is used to discharge ammonia gas, and the first distillation liquid outlet is used to discharge toluene. The top of distillation column 31 has a first condenser ( Figure 1 (Not shown), the first condenser is used to condense ammonia gas into liquid ammonia.

[0075] The second slurry outlet of the second flash tank 24 is connected to the inlet of the filter 40. The filter 40 has a filtrate outlet and a solids outlet. The solids outlet is connected to the inlet of the dryer 50 to facilitate the drying of the ammonium chloride solids obtained after filtration. The outlet of the dryer 50 is used to discharge the ammonium chloride solids. A third pump is installed on the pipeline connecting the filter 40 and the first heater 21 to transport the filtrate obtained after filtration to the first heater 21 for further flash separation.

[0076] Example 2

[0077] This embodiment provides a separation method of silicon nitride recovery liquid, which uses the separation system in Example 1, and specifically includes the following steps:

[0078] (1) The silicon nitride recovery liquid obtained after the solid-liquid separation of silicon nitride in the silicon nitride preparation process has a pressure of 1 MPa (the components and contents are shown in Table 1), the silicon nitride recovery liquid at 1 MPa is first subjected to first preheating treatment to 90°C, and then is introduced into the first flash tank 22 to perform first-stage flash evaporation under a pressure of 0.3 MPa, to obtain first ammonia-toluene mixed gas and first ammonium chloride slurry;

[0079] (2) The first ammonium chloride slurry is subjected to second preheating treatment to 90°C, and then is introduced into the second flash tank 24 to perform second-stage flash evaporation under a pressure of 0.02 MPa, to obtain second ammonia-toluene mixed gas and second ammonium chloride slurry;

[0080] (3) The first ammonia-toluene mixed gas is introduced into the rectifying column 31, and the second ammonia-toluene mixed gas is also introduced into the rectifying column 31 after being cooled to a second mixed liquid at 50°C, and the first ammonia-toluene mixed gas and the second mixed liquid are subjected to rectification separation, wherein the rectifying column 31 has a column top pressure of 0.25 MPa, a column top temperature of -4.8°C, a column bottom temperature of 160°C, and a column bottom pressure of 0.27 MPa. After the rectification separation, ammonia gas is obtained at the column top, toluene is obtained at the column bottom, the ammonia gas at the column top is condensed to liquid ammonia at -15°C, and the remaining liquid ammonia is returned to the upstream for use in the production of silicon nitride, and the toluene at the column bottom is also returned to the upstream for use in the production of silicon nitride.

[0081] (4) The second ammonium chloride slurry is introduced into the filter 40, and after filtration, ammonium chloride solids and a filtrate are obtained, the ammonium chloride solids are dried at 90°C and sold, and the filtrate is returned to step (1) for continuous flash separation.

[0082] Table 1

[0083] Component Liquid ammonia Toluene Ammonium chloride Component content (wt%) 65％ 31％ 4％

[0084] Example 3

[0085] The difference between this embodiment and Example 2 is that the pressure of the first flash tank 22 is 0 MPa, and the pressure of the second flash tank 24 is 0.1 MPa.

[0086] Example 4

[0087] The difference between this embodiment and Example 2 is that the pressure of the first flash tank 22 is 0.8 MPa, and the pressure of the second flash tank 24 is -0.02 MPa.

[0088] Test example

[0089] The purity of the toluene, liquid ammonia and ammonium chloride separated from the above-mentioned examples 2-4 was tested respectively, and the results are shown in Table 2.

[0090] Table 2

[0091] Purity of toluene (%) Purity of liquid ammonia (%) Purity of ammonium chloride (%) Example 2 >99.9％ >99.999％ >70％ Example 3 >99.9％ >99.999％ >70％ Example 4 >99.9％ >99.999％ >70％

[0092] From the above description, it can be seen that the above-mentioned examples of the present application achieve the following technical effects:

[0093] The separation system of the silicon nitride recovery liquid provided by the present application connects the outlet of the raw material supply tank with the inlet of the flash device, and the silicon nitride recovery liquid is subjected to flash separation in the flash device to obtain ammonia-toluene mixed gas and ammonium chloride slurry. The ammonia-toluene mixed gas is subjected to rectification separation in the rectification device to obtain liquid ammonia and toluene; and the ammonium chloride slurry is subjected to solid-liquid separation in the solid-liquid separation device to obtain ammonium chloride solid. The separation system provided by the present application not only has high purity of the separated toluene and liquid ammonia, but also has simple process flow, less equipment quantity, low running cost, high automation degree and can realize continuous production.

[0094] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A separation system for silicon nitride recovery liquid, characterized in that, The separation system includes: The raw material supply unit includes a raw material supply storage tank (10), which stores silicon nitride recovery liquid, which includes ammonium chloride, liquid ammonia and toluene; The separation unit includes a flash evaporation device, a distillation device, and a solid-liquid separation device. The flash evaporation device is used to flash separate the silicon nitride recovery liquid to obtain an ammonia-toluene mixture and an ammonium chloride slurry. The distillation device is used to separate the ammonia-toluene mixture to obtain liquid ammonia and toluene. The solid-liquid separation device is used to perform solid-liquid separation on the ammonium chloride slurry to obtain solid ammonium chloride.

2. The separation system according to claim 1, characterized in that, The flash evaporation device includes a first flash tank (22) and a second flash tank (24) connected in sequence; The inlet of the first flash tank (22) is connected to the outlet of the raw material supply storage tank (10). The first flash tank (22) has a first slurry outlet and a first gas outlet. The first slurry outlet is connected to the inlet of the second flash tank (24). The second flash tank (24) has a second slurry outlet and a second gas outlet. Preferably, the flash evaporation device further includes a first heater (21) and a second heater (23); the first heater (21) is disposed on the pipeline between the raw material supply storage tank (10) and the first flash tank (22); the second heater (23) is disposed on the pipeline between the first flash tank (22) and the second flash tank (24).

3. The separation system according to claim 2, characterized in that, The distillation apparatus includes a distillation column (31); The distillation column (31) includes a first distillation inlet and a second distillation inlet, the first distillation inlet being connected to the first gas outlet and the second distillation inlet being connected to the second gas outlet; the distillation column (31) also includes a first distillation gas outlet and a first distillation liquid outlet, the first distillation gas outlet being used to discharge ammonia and the first distillation liquid outlet being used to discharge toluene; Preferably, the distillation column (31) is a sieve tray column or a valve column; Preferably, the distillation column (31) is provided with a first condenser at the top, which is used to condense ammonia gas into liquid ammonia; Preferably, a second condenser (32) is also provided on the pipeline connecting the second flash tank (24) and the distillation column (31).

4. The separation system according to claim 2, characterized in that, The solid-liquid separation device includes a filter (40). The inlet of the filter (40) is connected to the outlet of the second slurry. The filter (40) has a filtrate outlet and a solid outlet. The filtrate outlet is connected to the inlet of the first heater (21). The solid outlet is used to discharge ammonium chloride solid.

5. The separation system according to claim 4, characterized in that, The separation system further includes a drying device, which includes a dryer (50), the inlet of which is connected to the solid outlet, and the outlet of which is used to discharge ammonium chloride solid.

6. A method for separating silicon nitride recovery liquid, characterized in that, The separation method includes the following steps: Step S1: Flash evaporation separation is performed on the silicon nitride recovery liquid to obtain ammonia-toluene mixed gas and ammonium chloride slurry; Step S2: The ammonia-toluene mixture is distilled to separate liquid ammonia and toluene. Step S3: The ammonium chloride slurry is subjected to solid-liquid separation to obtain ammonium chloride solid.

7. The separation method according to claim 6, characterized in that, Step S1 includes: Step S11: The silicon nitride recovery liquid is subjected to primary flash evaporation to obtain a first ammonia-toluene mixed gas and a first ammonium chloride slurry; Step S12: The first ammonium chloride slurry is subjected to two-stage flash evaporation to obtain a second ammonia-toluene mixture and a second ammonium chloride slurry; Preferably, the pressure of the first-stage flash evaporation is 0–0.8 MPa; Preferably, the pressure of the secondary flash evaporation is -0.02 to 0.1 MPa; Preferably, before the silicon nitride recovery liquid undergoes the first flash evaporation, the silicon nitride recovery liquid is further subjected to a first preheating treatment; more preferably, the temperature of the first preheating treatment is 70-110°C. Preferably, before the first ammonium chloride slurry undergoes the second flash evaporation, the first ammonium chloride slurry is further subjected to a second preheating treatment. More preferably, the temperature of the second preheating treatment is 60-110°C.

8. The separation method according to claim 7, characterized in that, Step S2 includes: performing the distillation separation on the first ammonia-toluene mixture and the second ammonia-toluene mixture, respectively. Preferably, in step S2, the top temperature of the distillation column (31) is -32.7 to 13.7°C, the top pressure of the distillation column (31) is 0 to 0.6 MPa, and the bottom temperature of the distillation column (31) is 96.7 to 196.2°C. Preferably, the ammonia gas obtained from distillation is subjected to a first condensation treatment to convert it into liquid ammonia; the temperature of the first condensation treatment is -33.1 to 14.1°C and the pressure is 0 to 0.6 MPa; Preferably, before the second ammonia-toluene mixture undergoes the distillation separation, the second ammonia-toluene mixture is further subjected to a second condensation treatment at a temperature of 40–90°C.

9. The separation method according to claim 7, characterized in that, Step S3 includes: performing solid-liquid separation on the second ammonium chloride slurry to obtain ammonium chloride solid and filtrate; Preferably, the filtrate is returned to step S1 to continue the flash evaporation separation; Preferably, the solid-liquid separation method is filtration.

10. The separation method according to claim 9, characterized in that, The separation method further includes step S4: drying the ammonium chloride solid; Preferably, the drying temperature is 60–110°C.

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