Method and device for purifying mixed waste acid

Abstract

The invention relates to the technical field of waste acid treatment, and provides a method and a device for purifying mixed waste acid. According to the method, nitric acid-hydrofluoric acid mixed acid (namely mixed waste acid) used in semiconductor or photovoltaic production is purified, firstly, a precipitator is added into the mixed waste acid, fluosilicic acid in the mixed waste acid is converted into fluosilicate precipitate for separation, and then purified mixed acid is obtained through light component removal rectification and rectification purification; the purity of the obtained purified mixed acid reaches G4 grade of SEMI international standard grade, and the use requirements of the semiconductor industry are met. According to the method, the cyclic utilization of the mixed acid is realized, the generation of three wastes is greatly reduced, the method is energy-saving and environment-friendly, the cost is low, a by-product containing fluosilicate can be obtained during purification, the generation of solid wastes is avoided, the environmental pressure is reduced, and the economic benefit is good.

Description

Technical Field

[0001] The invention relates to the technical field of waste acid treatment, and in particular to a method and device for purifying mixed waste acid. Background Art

[0002] In the semiconductor manufacturing process, a mixture of nitric acid (HNO3) and hydrofluoric acid (HF) is widely used in the etching process of silicon wafers. It is used to form insulating regions between adjacent transistors in single-crystal silicon etching, and to form gates and local connections in polycrystalline silicon etching. The chemical reaction process in the etching process is as follows: first, nitric acid oxidizes the silicon on the surface to form a thin layer of silicon dioxide, then hydrofluoric acid reacts with silicon dioxide to dissolve the silicon dioxide and expose the silicon underneath, then the silicon is oxidized by nitric acid, and then the oxide is etched away by hydrofluoric acid, and the above reaction is repeated to achieve etching.

[0003] However, the used mixed acid contains a large amount of impurities such as fluorosilicic acid and metal ions, which will affect its effect of reuse and may even affect the performance of semiconductors. Usually, the used mixed acid can only be neutralized as waste liquid to achieve the purpose of harmlessness. However, neutralization of waste acid requires a large amount of alkali, which is costly, and a large amount of solid waste will be generated during the neutralization process, which puts great pressure on the environment.

[0004] At present, the treatment and recovery of mixed waste acid of nitric acid and hydrofluoric acid has always been a hot topic of research in this field. Common recovery methods include solvent extraction and ion exchange, among which the solvent extraction method uses solvents to selectively extract different components in the waste acid to achieve the purpose of separation and recovery. The ion exchange method removes metal ions or other harmful ions in the waste acid by adsorbing and releasing ions through ion exchange resins. However, the purity of nitric acid and hydrofluoric acid treated by the above method is not high and cannot meet the needs of the semiconductor manufacturing industry. Therefore, how to effectively remove impurities in the mixed waste acid of nitric acid and hydrofluoric acid and improve its purity is a problem that needs to be urgently solved in the current semiconductor manufacturing industry. Summary of the invention

[0005] In view of this, the present invention provides a method and device for purifying mixed waste acid. The purification method provided by the present invention can effectively remove impurities in the mixed waste acid, and the purity of the obtained purified mixed acid reaches the G4 level of the SEMI international standard grade, which meets the use requirements of the semiconductor industry, realizes the recycling of nitric acid and hydrofluoric acid, reduces the generation of three wastes, and has low purification cost.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: A method for purifying mixed waste acid comprises the following steps: The mixed waste acid is subjected to a first solid-liquid separation, the obtained first separated liquid is mixed with a precipitant, and then a second solid-liquid separation is performed to obtain a by-product containing fluorosilicate and a second separated liquid; the components of the mixed waste acid include hydrofluoric acid, nitric acid, fluorosilicic acid and metal ions; the mass fraction of hydrofluoric acid in the mixed waste acid is 1% to 10%, the mass fraction of nitric acid is 20% to 50%, the mass fraction of fluorosilicic acid is ≤5%, and the total content of metal ions is ≥2000ppb; the precipitant is a soluble nitrate; The second separated liquid is subjected to light-removal rectification, and the obtained bottom product is subjected to a third solid-liquid separation to obtain a by-product containing fluorosilicate and a third separated liquid; The third separated liquid is distilled and purified to obtain a purified mixed acid; the purity of the purified mixed acid is G4 grade.

[0007] Preferably, the soluble nitrate includes one or more of alkali metal nitrate and barium nitrate; the molar ratio of the soluble nitrate to the fluorosilicic acid in the mixed waste acid is 1-4:1; and the mixing time is 4-24 hours.

[0008] Preferably, the conditions for the light removal distillation include: a tower top temperature of 70-105°C, a tower temperature of 95-120°C, a tower bottom temperature of 100-140°C, a pressure of 5-20 kPa, and a feed temperature of 40-90°C; The distillation purification conditions include: a tower top temperature of 100-125° C., a tower bottom temperature of 105-140° C., and a pressure of 5-20 kPa.

[0009] Preferably, the first solid-liquid separation is filtration, and the filtration accuracy is 0.1 μm~10 μm; After the distillation and purification, the purified liquid obtained from the top of the tower is subjected to precision filtration; the filtration accuracy of the precision filtration is 10nm~500nm.

[0010] Preferably, the mass fraction of nitric acid in the purified mixed acid is 35% to 70%, and the mass fraction of hydrofluoric acid is 1% to 5%.

[0011] Preferably, non-condensable gas is also produced during the light removal distillation and distillation purification; the non-condensable gas is absorbed by alkali spraying.

[0012] The present invention also provides a device used in the purification method described in the above scheme, comprising a buffer tank; the buffer tank is provided with a mixed waste acid inlet and a precipitant inlet; A light-removal tower; the feed port of the light-removal tower is connected to the liquid outlet of the buffer tank; a first condenser is provided on the top of the light-removal tower, the liquid outlet of the first condenser is connected to a reflux tank, and the outlet of the reflux tank is connected to the top of the light-removal tower; the bottom outlet of the light-removal tower is connected to the kettle of the light-removal tower; the kettle of the light-removal tower is provided with a circulation port, a light-removal liquid outlet and a by-product outlet; A purification tower; the feed inlet of the purification tower is connected to the light liquid outlet of the light liquid removal tower kettle; the top of the purification tower is connected to the second condenser, the liquid outlet of the second condenser is connected to a stabilization tank, the stabilization tank is provided with a reflux port and a product outlet, the reflux port is connected to the top of the purification tower; the bottom outlet of the purification tower is connected to the purification tower kettle; the purification tower kettle is provided with a circulation port, a kettle residue outlet and a by-product outlet.

[0013] Preferably, the device further comprises a product container; the inlet of the product container is connected to the product outlet of the stabilization tank; The device further comprises a tail gas absorption device; the inlet of the tail gas absorption device is respectively connected with the gas outlet of the first condenser, the gas outlet of the second condenser and the gas outlet of the product container.

[0014] Preferably, a preheater is provided on the pipeline connecting the buffer tank and the lightness removal tower; The circulation port of the tower kettle of the light removal tower is connected to the bottom of the dehydrogenation tower, and a first reboiler is arranged on the connected pipeline; The circulation port of the purification tower kettle is connected to the bottom of the purification tower, and a filter and a second reboiler are arranged on the connected pipeline; The liquid outlet of the product container is communicated with the precision filter, and the outlet of the precision filter is communicated with the product container.

[0015] Preferably, the lightness removal tower, the first condenser, the lightness removal tower kettle, the first reboiler, the purification tower, the second condenser, the purification tower kettle and the second reboiler are made of fluorine-lined steel; the fluorine-lined steel includes a steel substrate and a fluorine-containing plastic lined on the surface of the steel substrate; The number of plates of the lightness removal tower is 5 to 25, and the number of plates of the purification tower is 5 to 25; The diameters of the lightness removal tower kettle and the purification tower kettle are independently 300-10000 mm, and the heights are independently 6000-10000 mm.

[0016] The invention provides a method for purifying a mixed waste acid, comprising the following steps: performing a first solid-liquid separation on the mixed waste acid, mixing the obtained first separated liquid and a precipitant, and then performing a second solid-liquid separation to obtain a by-product containing fluorosilicate and a second separated liquid; the components of the mixed waste acid include hydrofluoric acid, nitric acid, fluorosilicic acid and metal ions; the mass fraction of hydrofluoric acid in the mixed waste acid is 1% to 10%, the mass fraction of nitric acid is 20% to 50%, the mass fraction of fluorosilicic acid is ≤5%, and the total content of metal ions is ≥2000ppb; the precipitant is a soluble nitrate; the second separated liquid is subjected to light removal distillation, and the obtained bottom product is subjected to a third solid-liquid separation to obtain a by-product containing fluorosilicate and a third separated liquid; the third separated liquid is subjected to distillation and purification to obtain a purified mixed acid; the purity of the purified mixed acid is G4 grade. The present invention first performs a first solid-liquid separation on the mixed waste acid, then adds a precipitant to form a byproduct containing fluorine silicate by the fluorosilicic acid therein, and then separates the byproduct by the second solid-liquid separation, and removes the light components (mainly water) therein through the light removal distillation of the remaining second separated liquid, and at the same time, precipitates part of the byproduct containing fluorine silicate through the light removal distillation, and separates the byproduct by the third solid-liquid separation, and the remaining third separated liquid is purified by distillation to obtain a purified mixed acid. The method provided by the present invention can effectively remove impurities in the mixed waste acid, and the purity of the obtained purified mixed acid reaches the G4 level of the SEMI international standard grade, which meets the use requirements of the semiconductor industry, realizes the recycling of the mixed acid, greatly reduces the generation of three wastes, and saves energy and is environmentally friendly. At the same time, the present invention collects the fluorosilicic acid in the mixed waste acid in the form of a byproduct, avoids the generation of solid waste, reduces environmental pressure, and improves economic benefits; in addition, the purification method provided by the present invention does not need to consume a large amount of alkali, and the processing cost is low.

[0017] The present invention also provides a device used in the purification method described in the above scheme. The device provided by the present invention has a simple structure and can meet the purification requirements; further, the present invention also limits the material of the core equipment, so that the device of the present invention is suitable for the strong corrosive environment of hydrofluoric acid and nitric acid, and is suitable for the high temperature environment of distillation. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the structure of a purification device for purifying waste acid provided by the present invention; Figure 1 In: 1-buffer tank, 2-delightening tower, 2-1-first condenser, 2-2-reflux tank, 2-3-delightening tower kettle, 2-4-first reboiler, 3-purification tower, 3-1-second condenser, 3-2-stabilization tank, 3-3-purification tower kettle, 3-4-filter, 3-5-second reboiler, 4-product container, 5-preheater, 6-precision filter, 7-tail gas absorption device. DETAILED DESCRIPTION

[0019] The present invention provides a method for purifying mixed waste acid, comprising the following steps: The mixed waste acid is subjected to a first solid-liquid separation, the obtained first separated liquid is mixed with a precipitant, and then a second solid-liquid separation is performed to obtain a by-product containing fluorosilicate and a second separated liquid; the components of the mixed waste acid include hydrofluoric acid, nitric acid, fluorosilicic acid and metal ions; the mass fraction of hydrofluoric acid in the mixed waste acid is 1% to 10%, the mass fraction of nitric acid is 20% to 50%, the mass fraction of fluorosilicic acid is ≤5%, and the content of metal ions is ≥2000ppb; the precipitant is a soluble nitrate; The second separated liquid is subjected to light-removal rectification, and the obtained bottom product is subjected to a third solid-liquid separation to obtain a by-product containing fluorine silicate and a third separated liquid; The third separated liquid is distilled and purified to obtain a purified mixed acid; the purity of the purified mixed acid is G4 grade; the purity of the purified mixed acid is G4 grade.

[0020] The invention performs a first solid-liquid separation on the mixed waste acid, mixes the obtained first separated liquid with a precipitant, and then performs a second solid-liquid separation to obtain a by-product and a second separated liquid. In the present invention, the components of the mixed waste acid include hydrofluoric acid, nitric acid, fluorosilicic acid and metal ions; the mass fraction of hydrofluoric acid in the mixed waste acid is 1% to 10%, specifically 1%, 2%, 2.88%, 5%, 8% or 10%, the mass fraction of nitric acid is 20% to 50%, specifically 20%, 25%, 30%, 40%, 40.15% or 50%, the mass fraction of fluorosilicic acid is ≤5%, specifically greater than 0 and less than or equal to 5%, more specifically 1%, 2%, 3%, 4% or 5%; the content of metal ions in the mixed waste acid is ≥2000ppb; the metal ions include one or more of Fe ions, Cu ions, Mn ions, Li ions, Be ions, Na ions, Ca ions, Sr ions, Zr ions, Nb ions, Mo ions, Cd ions, Ag ions, Sn ions, Sb ions, Ba ions, Pt ions, Pb ions, Mg ions, Ca ions, Ti ions and Cr ions. In a specific embodiment of the present invention, the mixed waste acid is a nitric acid-hydrofluoric acid mixed acid used in semiconductor production or photovoltaic production.

[0021] In the present invention, the first solid-liquid separation is preferably filtration, and the filtration accuracy of the filtration is preferably 0.1 μm to 10 μm, specifically 0.5 μm, 5 μm, 8 μm or 10 μm.

[0022] In the present invention, the precipitant is a soluble nitrate; the soluble nitrate preferably includes one or more of alkali metal nitrate and barium nitrate; the alkali metal nitrate is preferably one or both of sodium nitrate and potassium nitrate; the precipitant is preferably used in the form of an aqueous solution of the precipitant; the mass fraction of the aqueous solution of the precipitant is preferably 20% to 40%; the molar ratio of the soluble nitrate to the fluorosilicic acid in the mixed waste acid is preferably 1 to 4:1, specifically 1:1, 2:1, 3:1 or 4:1; the mixing time is preferably 4 to 24 hours, specifically 4 hours, 10 hours, 15 hours, 20 hours or 24 hours; the soluble nitrate reacts with the fluorosilicic acid in the mixed waste acid to generate a fluorosilicate precipitate, and the fluorosilicate precipitate is separated from the liquid by the second solid-liquid separation, that is, a fluorosilicate byproduct (recorded as the first part of the byproduct) and a second separated liquid are obtained. The second solid-liquid separation method is preferably filtration, and the present invention has no special requirements for the filtration accuracy of the second solid-liquid separation, and the effective separation of the fluorosilicate precipitate can be achieved.

[0023] After obtaining the second separated liquid, the present invention performs light-removal distillation on the second separated liquid, and performs a third solid-liquid separation on the obtained bottom product to obtain a by-product containing fluorine silicate and a third separated liquid. In the present invention, the conditions for light-removal distillation preferably include: a tower top temperature of 70 to 105°C, specifically 70°C, 80°C, 90°C, 100°C or 105°C; a tower temperature of 95 to 120°C, specifically 95°C, 100°C, 110°C or 120°C; a bottom temperature of 100 to 140°C, specifically 100°C, 120°C, 130°C or 140°C; a pressure of 5 to 20 kPa, specifically 5 kPa, 10 kPa, 15 kPa or 20 kPa, and a feed temperature of 40 to 90°C, specifically 40°C, 60°C, 80°C or 90°C. In a specific embodiment of the present invention, the second separated liquid is preheated and then subjected to light fractionation. The light components in the second separated liquid are removed by light fractionation. The light components are water. Fluorosilicate precipitates are precipitated after light fractionation. The fluorosilicate precipitates are separated from the liquid by a third solid-liquid separation, thereby obtaining a fluorosilicate-containing byproduct (recorded as the second part of the byproduct) and a third separated liquid. The third solid-liquid separation is preferably filtration. The present invention has no special requirements for the filtration accuracy of the third solid-liquid separation. It is sufficient to achieve effective separation of the fluorosilicate precipitate. After obtaining the third separated liquid, the present invention performs rectification and purification on the third separated liquid to obtain a purified mixed acid. In the present invention, the conditions for the rectification and purification preferably include: a tower top temperature of 100-125°C, specifically 100°C, 110°C, 120°C or 125°C, a tower bottom temperature of 105-140°C, specifically 105°C, 120°C, 130°C or 140°C, and a pressure of 5-20kPa, specifically 5kPa, 10kPa, 15kPa or 20kPa.

[0024] In the present invention, after the distillation purification, it is preferred that the purified liquid obtained from the top of the tower is subjected to precision filtration; the filtration accuracy of the precision filtration is preferably 10nm~500nm, specifically 10nm, 100nm, 200nm, 300nm or 500nm.

[0025] In the present invention, the mass fraction of nitric acid in the purified mixed acid is 35% to 70%, the mass fraction of hydrofluoric acid is 1% to 5%, and the content of fluorosilicic acid is ≤10ppm; the purity of the purified mixed acid is G4 grade; in a specific embodiment of the present invention, it is preferred to adjust the conditions of distillation purification according to the mass fractions of nitric acid and hydrofluoric acid in the target purified mixed acid to obtain a purified mixed acid with a concentration that meets the requirements. In a specific embodiment of the present invention, after obtaining the purified mixed acid, it is preferred to use semiconductor-grade nitric acid or hydrofluoric acid to adjust the content of nitric acid or hydrofluoric acid in the purified mixed acid to return to the original content and realize the recycling of the mixed acid.

[0026] In the present invention, a fluorosilicate precipitate is precipitated in the kettle liquid remaining after the distillation and purification. The present invention preferably separates the precipitated fluorosilicate precipitate to obtain a by-product containing fluorosilicate (recorded as the third part of by-product), and the remaining heavy components are removed as kettle residue.

[0027] In the present invention, the first part of by-products, the second part of by-products and the third part of by-products are preferably refined; the refining method preferably includes: washing the by-products with water and then purifying them; the present invention removes the acid and soluble salt in the by-products by washing with water; the purification is preferably recrystallization or acid leaching.

[0028] In the present invention, non-condensable gas is also generated during the light removal distillation and distillation purification; the non-condensable gas is preferably absorbed by alkali spraying.

[0029] The present invention also provides a device used in the purification method described in the above scheme, and the structural schematic diagram of the device is as follows Figure 1 As shown below, combined Figure 1 Provide detailed explanation.

[0030] The device provided by the present invention comprises a buffer tank 1; the buffer tank 1 is provided with a mixed waste acid inlet and a precipitant inlet; the mixed waste acid is preferably filtered before being introduced into the buffer tank 1; a stirring device is provided in the buffer tank 1; the buffer tank 1 is provided with a liquid outlet and a solid outlet, the liquid outlet is preferably provided on the side of the buffer tank 1, and the solid outlet is preferably provided at the bottom of the buffer tank 1; in a specific embodiment of the present invention, the filtered mixed waste acid and the precipitant are respectively pumped into the buffer tank 1 through a feed pump, the fluorosilicate precipitate generated by the reaction is discharged from the solid outlet, and the remaining liquid is discharged from the liquid outlet.

[0031] The device provided by the present invention comprises a lightness removal tower 2, and the feed port of the lightness removal tower 2 is connected to the liquid outlet of the buffer tank 1. In the present invention, a preheater 5 is preferably provided on the pipeline connecting the buffer tank 1 and the lightness removal tower 2; in a specific embodiment of the present invention, the liquid flowing out of the buffer tank is pumped into the preheater 5 by a transfer pump for preheating, and then enters the lightness removal tower 2.

[0032] In the present invention, a first condenser 2-1 is provided at the top of the lightness removal tower 2, and the liquid outlet of the first condenser 2-1 is connected to the reflux tank 2-2, and the outlet of the reflux tank 2-2 is connected to the top of the lightness removal tower 2; specifically, the outlet of the reflux tank 2-2 is connected to a three-way valve, and the three-way valve is provided with three ports, one of which is connected to the reflux tank 2-2, the second port is connected to the top of the lightness removal tower 2, and the third port is used as a wastewater outlet. In a specific embodiment of the present invention, the liquid flowing out of the reflux tank 2-2 is pumped into the three-way valve through a transfer pump, part of which flows back into the lightness removal tower 2, and part of which is discharged as wastewater.

[0033] In the present invention, the bottom outlet of the lightness removal tower 2 is connected with the tower kettle 2-3 of the lightness removal tower; the tower kettle 2-3 of the lightness removal tower is provided with a circulation port, a lightness removal liquid outlet and a by-product outlet; the circulation port of the tower kettle 2-3 of the lightness removal tower is connected with the bottom of the lightness removal tower 2, and a first reboiler 2-4 is provided on the connected pipeline; in a specific embodiment of the present invention, the liquid in the lightness removal tower 2 enters the tower kettle 2-3 of the lightness removal tower, and is then pumped into the first reboiler 2-4 by a circulation pump through the circulation port, and returns to the lightness removal tower 2 after heating; the by-product outlet of the tower kettle 2-3 of the lightness removal tower is preferably arranged at the bottom for discharging fluorosilicate precipitate, and the lightness removal liquid outlet of the tower kettle 2-3 of the lightness removal tower is used for discharging the liquid after lightness removal distillation.

[0034] The device provided by the present invention includes a purification tower 3; the feed port of the purification tower 3 is connected to the light liquid outlet of the light liquid removal tower kettle 2-3; the top of the purification tower 3 is connected to the second condenser 3-1, the liquid outlet of the second condenser 3-1 is connected to the stabilization tank 3-2, the stabilization tank 3-2 is provided with a reflux port and a product outlet, the reflux port is connected to the top of the purification tower 3; specifically, the stabilization tank 3-2 is connected to a three-way valve, the three-way valve is provided with three ports, one of which is connected to the stabilization tank 3-2, the second port is connected to the top of the purification tower 3, and the third port is used as a product outlet. In a specific embodiment of the present invention, the liquid flowing out of the stabilization tank 3-2 is pumped into the three-way valve through a transfer pump, part of which is refluxed into the purification tower 2, and part of which enters the product container 4 as a product.

[0035] In the present invention, the bottom of the purification tower 3 is connected with the outlet purification tower kettle 3-3; the purification tower kettle 3-3 is provided with a circulation port, a kettle residue outlet and a by-product outlet; the circulation port of the purification tower kettle 3-3 is connected with the bottom inlet of the purification tower 3, and a filter 3-4 and a second reboiler 3-5 are provided on the connected pipeline; in a specific embodiment of the present invention, the liquid in the purification tower 3 enters the purification tower kettle 3-3, and then is pumped into the filter 3-4 through the circulation port by a circulation pump for filtration, and the by-product containing fluorosilicate produced by filtration is sent to the refining process, and the filtrate enters the second reboiler 3-5, and returns to the purification tower 3 after heating; the by-product outlet of the purification tower kettle 3-3 is preferably arranged at the bottom for discharging fluorosilicate precipitate.

[0036] The device provided by the present invention preferably further comprises a product container 4, the inlet of the product container 4 being connected to the product outlet of the stabilization tank 3-2. In the present invention, the liquid outlet of the product container 4 is preferably also connected to a precision filter 6, the outlet of the precision filter 6 being connected to the product container 4; in a specific embodiment of the present invention, the liquid in the product container 4 enters the precision filter 6 for filtration, and then returns to the product container 4; in a specific embodiment of the present invention, the product container 4 is a product tank.

[0037] In the present invention, the device also includes a tail gas absorption device 7; the inlet of the tail gas absorption device 7 is respectively connected to the gas outlet of the first condenser 2-1, the gas outlet of the second condenser 3-1 and the gas outlet of the product container 4; the tail gas absorption device 7 is preferably an alkali spraying device for absorbing non-condensable gas.

[0038] In the present invention, the lightness removal tower 2, the first condenser 2-1, the lightness removal tower kettle 2-3, the first reboiler 2-5, the purification tower 3, the second condenser 3-1, the purification tower kettle 3-3 and the second reboiler 3-6 are made of steel lining with fluorine; the steel lining with fluorine preferably includes a steel substrate and a fluorine-containing plastic lined on the surface of the steel substrate; the steel substrate is preferably carbon steel or stainless steel, and the fluorine-containing plastic is preferably one of soluble polytetrafluoroethylene (PFA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFF, PCTFE), perfluoroethylene propylene copolymer (FEP), and ethylene-chlorotrifluoroethylene copolymer (ECTFE).

[0039] In the present invention, the lightness removal tower 2 and the purification tower 3 are preferably distillation towers.

[0040] In the present invention, the number of plates of the lightness removal tower 2 is preferably 5-25, and the number of plates of the purification tower 3 is preferably 5-25.

[0041] In the present invention, the diameters of the light removal tower kettle 2-3 and the purification tower kettle 3-3 are independently preferably 300~10000mm, specifically 300mm, 500mm, 1000mm, 3000mm, 5000mm or 10000mm, and the heights are independently preferably 6000~10000mm, specifically 6000mm, 8000mm or 10000mm.

[0042] Combine the following Figure 1 The purification method of the present invention is described as follows: the filtered mixed waste acid and precipitant are passed into a buffer tank 1, and the by-product containing fluorine silicate is separated from the bottom of the buffer tank 1. The liquid flowing out of the buffer tank 1 is pumped into a preheater 5, and after preheating, it enters a light removal tower 2 for light removal distillation. The product in the bottom of the light removal tower 2 is subjected to solid-liquid separation. The by-product containing fluorine silicate is separated and discharged from the bottom of the tower bottom 2-3 of the light removal tower. The remaining light removal liquid is pumped into a purification tower 3 for distillation and purification. The distillation gas phase is condensed by the second condenser 3-1 and then enters a stabilization tank 3-2. Part of the liquid in the stabilization tank 3-2 is refluxed, and part of it enters a product container 4. The liquid in the product container 4 is filtered through a precision filter 6 to obtain a purified mixed acid; the liquid flowing out of the circulation port of the purification tower bottom 3-3 enters the filter 3-4 for filtration, and the filtrate is returned to the purification tower 3. At the same time, the by-product containing fluorine silicate is filtered to generate. The by-product containing fluorine silicate is discharged from the bottom of the purification tower bottom 3-3, and the remainder is discharged as a bottom residue.

[0043] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions 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 are within the scope of protection of the present invention.

[0044] Example 1 The mixed waste acid in this embodiment is a nitric acid-hydrofluoric acid mixed acid used in semiconductor production, wherein the contents of nitric acid and hydrofluoric acid, and the contents of various metal ions are shown in Table 1.

[0045] use Figure 1 The device in the embodiment purifies the mixed waste acid. The materials of the light removal tower 2, the first condenser 2-1, the light removal tower kettle 2-3, the first reboiler 2-5, the purification tower 3, the second condenser 3-1, the purification tower kettle 3-3 and the second reboiler 3-6 are all carbon steel lined with polytetrafluoroethylene. The diameter of the light removal tower kettle 2-3 is 1000 mm and the height is 8000 mm. The diameter of the purification tower kettle 3-3 is 1000 mm and the height is 8000 mm. The specific purification steps are as follows: The mixed waste acid is filtered with a filter bag with an accuracy of 1 μm, and then passed into a buffer tank 1. At the same time, a sodium nitrate solution is added to the buffer tank 1, wherein the amount of sodium nitrate is twice the molar amount of fluorosilicic acid in the mixed waste acid. The mixed waste acid and the sodium nitrate solution are stirred in the buffer tank for 12 hours, and a by-product containing sodium fluoride silicate is separated from the bottom of the buffer tank. The by-product is sent to the refining process, and the liquid is pumped into the preheater 5 through a transfer pump, and after preheating, it enters the light removal tower 2 for light removal distillation. The operating conditions of the light removal distillation are: pressure of 10 kPa, tower top temperature of 85°C, tower temperature of 105°C, and tower bottom temperature of 115°C; the by-product containing sodium fluoride silicate produced by the light removal distillation is separated and discharged from the bottom of the tower bottom 2-3 of the light removal tower, and sent to the refining process. The light removal liquid after light removal distillation is The transfer pump is pumped into the purification tower 3 for distillation and purification. The conditions for distillation and purification are: pressure of 10kpa, tower top temperature of 115℃, tower bottom temperature of 130℃. The distillation gas phase is condensed by the second condenser 3-1 and enters the stabilization tank 3-2. The liquid in the stabilization tank 3-2 is partially refluxed, and partially enters the product container 4. The liquid in the product container 4 is filtered through a precision filter 6 (with a filtration accuracy of 100nm) to obtain a purified mixed acid; the liquid flowing out of the circulation port of the purification tower bottom 3-3 enters the filter 3-4 for filtration, and the filtrate returns to the purification tower 3. The by-product containing fluorine silicate produced by filtration is sent to the refining process. The by-product containing fluorine silicate discharged from the bottom of the purification tower bottom 3-3 is sent to the refining process, and the remainder is discharged as the bottom residue.

[0046] The contents of nitric acid, hydrofluoric acid and metal ions in the purified mixed acid obtained in this example are shown in Table 1.

[0047] Table 1 Nitric acid, hydrofluoric acid and metal ion content of mixed waste acid before and after purification

[0048] In Table 1: ND means not detected.

[0049] According to the data in Table 1, it can be seen that after the mixed waste acid is purified, the contents of HF and nitric acid in the purified mixed acid are significantly increased, and various metal impurities are effectively removed. The purity of the purified mixed acid meets the requirements of semiconductor G4 level.

[0050] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for purifying mixed waste acid, characterized in that: The following steps are involved: The mixed waste acid is subjected to a first solid-liquid separation, the obtained first separated liquid is mixed with a precipitant, and then a second solid-liquid separation is performed to obtain a by-product containing fluorosilicate and a second separated liquid; the components of the mixed waste acid include hydrofluoric acid, nitric acid, fluorosilicic acid and metal ions; the mass fraction of hydrofluoric acid in the mixed waste acid is 1% to 10%, the mass fraction of nitric acid is 20% to 50%, the mass fraction of fluorosilicic acid is ≤5%, and the total content of metal ions is ≥2000ppb; the precipitant is a soluble nitrate; The second separated liquid is subjected to light-removal distillation, and the obtained bottom product is subjected to a third solid-liquid separation to obtain a by-product containing fluorosilicate and a third separated liquid; The third separated liquid is distilled and purified to obtain a purified mixed acid; the purity of the purified mixed acid is G4 grade.

2. The purification method according to claim 1, characterized in that The soluble nitrate includes one or more of alkali metal nitrate and barium nitrate; the molar ratio of the soluble nitrate to the fluorosilicic acid in the mixed waste acid is 1-4:1; and the mixing time is 4-24 hours.

3. The purification method according to claim 1, characterized in that The conditions of the light removal distillation include: the tower top temperature is 70-105°C, the tower temperature is 95-120°C, the tower bottom temperature is 100-140°C, the pressure is 5-20 kPa, and the feed temperature is 40-90°C; The distillation purification conditions include: a tower top temperature of 100-125° C., a tower bottom temperature of 105-140° C., and a pressure of 5-20 kPa.

4. The purification method according to claim 1, characterized in that The first solid-liquid separation is filtration, and the filtration accuracy is 0.1 μm to 10 μm; After the distillation and purification, the purified liquid obtained from the top of the tower is subjected to precision filtration; the filtration accuracy of the precision filtration is 10nm~500nm.

5. The purification method according to claim 1 or 4, characterized in that: The mass fraction of nitric acid in the purified mixed acid is 35% to 70%, and the mass fraction of hydrofluoric acid is 1% to 5%.

6. The purification method according to claim 1, characterized in that: The light removal distillation and distillation purification also produce non-condensable gas; the non-condensable gas is absorbed by alkali spraying.

7. The device used in the purification method according to any one of claims 1 to 6, characterized in that: The invention comprises a buffer tank (1); the buffer tank (1) is provided with a mixed waste acid inlet and a precipitant inlet; A lightness removal tower (2); the feed port of the lightness removal tower (2) is connected to the liquid outlet of the buffer tank (1); a first condenser (2-1) is provided at the top of the lightness removal tower (2); the liquid outlet of the first condenser (2-1) is connected to a reflux tank (2-2); the outlet of the reflux tank (2-2) is connected to the top of the lightness removal tower (2); the bottom outlet of the lightness removal tower (2) is connected to a tower kettle (2-3) of the lightness removal tower; the tower kettle (2-3) of the lightness removal tower is provided with a circulation port, a lightness removal liquid outlet and a by-product outlet; A purification tower (3); the feed inlet of the purification tower (3) is connected to the light liquid outlet of the light liquid removal tower kettle (2-3); the top of the purification tower (3) is connected to the second condenser (3-1), the liquid outlet of the second condenser (3-1) is connected to the stabilization tank (3-2), the stabilization tank (3-2) is provided with a reflux port and a product outlet, the reflux port is connected to the top of the purification tower (3); the bottom outlet of the purification tower (3) is connected to the purification tower kettle (3-3); the purification tower kettle (3-3) is provided with a circulation port, a kettle residue outlet and a by-product outlet.

8. The device according to claim 7, characterized in that The device further comprises a product container (4); an inlet of the product container (4) is connected to a product outlet of the stabilization tank (3-2); The device further comprises a tail gas absorption device (7); the inlet of the tail gas absorption device (7) is respectively connected to the gas outlet of the first condenser (2-1), the gas outlet of the second condenser (3-1) and the gas outlet of the product container (4).

9. The device according to claim 8, characterized in that A preheater (5) is provided on the pipeline connecting the buffer tank (1) and the light removal tower (2); The circulation port of the light-removal tower kettle (2-3) is connected to the bottom of the dehydrogenation tower (2), and a first reboiler (2-4) is arranged on the connected pipeline; The circulation port of the purification tower kettle (3-3) is connected to the bottom of the purification tower (3), and a filter (3-4) and a second reboiler (3-5) are arranged on the connected pipeline; The liquid outlet of the product container (4) is in communication with the precision filter (6), and the outlet of the precision filter (6) is in communication with the product container (4).

10. The device according to claim 9, characterized in that The lightness removal tower (2), the first condenser (2-1), the lightness removal tower kettle (2-3), the first reboiler (2-4), the purification tower (3), the second condenser (3-1), the purification tower kettle (3-3) and the second reboiler (3-5) are made of fluorine-lined steel; the fluorine-lined steel comprises a steel substrate and a fluorine-containing plastic lined on the surface of the steel substrate; The number of plates of the light removal tower (2) is 5 to 25, and the number of plates of the purification tower (3) is 5 to 25; The diameters of the light removal tower kettle (2-3) and the purification tower kettle (3-3) are independently 300-10000 mm, and the heights are independently 6000-10000 mm.

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