A production line for recycling fluorine-containing tail gas from lithium hexafluorophosphate

Abstract

The present application relates to the technical field of chemical tail gas treatment, and specifically discloses a production line for recycling fluorine-containing tail gas of lithium hexafluorophosphate, which includes a tail gas treatment unit and a wastewater treatment unit. The tail gas treatment unit adopts a "two-stage ammonia washing + one-stage water washing" method for treatment, and the washing liquid is recycled to save the amount of washing liquid used; the washing circulating liquid generated in the tail gas treatment unit is treated by the wastewater treatment unit, and the wastewater treatment unit adopts an MVR evaporation system to treat the wastewater. The hot steam and distilled water generated in the MVR evaporation system are recycled to save the amount of resources used for wastewater treatment.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical tail gas treatment, and in particular to a production line for recycling fluorine-containing tail gas of lithium hexafluorophosphate. Background Art

[0002] The industrial production of lithium hexafluorophosphate primarily uses the hydrofluoric acid solvent method, which involves dissolving lithium halide in anhydrous hydrogen fluoride and then introducing high-purity PF5 gas to react, producing lithium hexafluorophosphate crystals. The resulting product is then separated and dried. This process produces large amounts of off-gases containing HF, HCl, and PF5. HF is a toxic gas with a pungent odor; HCl is a colorless gas with a pungent odor; and PF5 is a colorless, foul-smelling gas at room temperature and pressure that is highly irritating to the skin, eyes, and mucous membranes. All three off-gases are hazardous to both the environment and humans and must be treated before discharge. The main methods for treating tail gas from lithium hexafluorophosphate process are as follows: (1) designing a new tail gas recovery device, which contains a heat exchanger, a dust collector, and a water scrubber, wherein the heat exchanger includes a heat exchanger outer shell and a heat exchanger inner shell, the heat exchanger inner shell is provided with an exhaust gas inlet and an exhaust gas outlet, the outer shell and the inner shell are filled with coolant, the dust collector exhaust gas outlet is connected, the water scrubber is provided with a water scrubber inlet and a water scrubber outlet, the water scrubber inlet is connected to the dust collector, and the separation efficiency is high; however, this type of equipment is widely used, and the water scrubber is connected to an electrostatic precipitator, the corona electrode wire of which needs to pass through 45 to 48 kV DC high voltage electricity, which has a high risk factor.

[0003] (2) Using organic solvents such as carbon tetrachloride or benzene to absorb phosphorus pentafluoride; using chloride salt solution to absorb hydrogen fluoride in the tail gas, the reaction generates fluoride salt precipitation; using water to absorb hydrogen chloride in multiple stages to prepare industrial-grade hydrochloric acid; using tail gas in a comprehensive way to improve separation efficiency, but the quality of the resulting fluorinated byproduct is low, difficult to remove using organic solvents, and has a certain degree of toxicity.

[0004] (3) Chlorosulfonic acid is used to absorb hydrogen fluoride in the tail gas, and hydrogen chloride in the tail gas is absorbed by sulfur trioxide to generate chlorosulfonic acid. The treatment device is a primary absorption tower, and circulating liquid chlorosulfonic acid is used as a reaction medium to absorb HF. The secondary absorption tower is a reaction container for sulfur trioxide and HCl. The produced chlorosulfonic acid and fluorosulfonic acid are important chemical raw materials, realizing the resource utilization of mixed acid; however, this method introduces chlorosulfonic acid and sulfur trioxide as strong oxidants, which will corrode metals when reacting with metals in humid air, reduce the utilization rate of equipment, and are toxic in case of leakage.

[0005] In summary, among the existing methods for treating fluorine-containing tail gas from lithium hexafluorophosphate, method 1 has the problem of high energy consumption of equipment and greater safety hazards; method 2 has the problem of difficult to handle by-products and toxic solvents; method 3 has the problem of large equipment damage caused by the reagents used. Summary of the Invention

[0006] The purpose of the present invention is to provide a production line for recycling fluorine-containing tail gas of lithium hexafluorophosphate to solve the problems of high risk factor, great environmental pollution and large energy loss in recycling fluorine-containing tail gas of lithium hexafluorophosphate.

[0007] In order to solve the above problems, the technical solutions provided are as follows:

[0008] A production line for recycling fluorine-containing tail gas of lithium hexafluorophosphate comprises a tail gas treatment unit and a wastewater treatment unit. The tail gas treatment unit comprises a primary alkali washing tower, a secondary alkali washing tower and a water washing tower. The tops of the primary alkali washing tower and the secondary alkali washing tower are provided with tail gas inlets, and the bottoms of the primary alkali washing tower and the secondary alkali washing tower are provided with tail gas outlets. A buffer tank is provided at the bottom of the water washing tower, and a tail gas inlet is provided at the top of the buffer tank. Water accumulates in the buffer tank. An exhaust fan is provided at the top of the water washing tank. Ammonia water spraying devices are provided in the primary alkali washing tower and the secondary alkali washing tower. Washing liquid generated by washing the tail gas is pumped into the wastewater treatment unit.

[0009] The basic principle of the above technical solution is that the tail gas of the lithium hexafluorophosphate device enters the first-level alkali washing tower from the top of the first-level alkali washing tower, and the HF, HCl and PF5 in the tail gas are mainly washed with ammonia water, wherein HF and HCl are neutralized with ammonia water to generate water, PF5 reacts with water to generate phosphoric acid and hydrofluoric acid, and phosphoric acid and hydrofluoric acid react with ammonia water to generate ammonium phosphate and NH4F. In this process, PF5 is completely hydrolyzed, and 79% of HF and about 90% of HCl in the tail gas can be removed at this time; the HF and HCl waste gas that is not completely absorbed by the ammonia water in the first-level alkali washing tower enters the second-level alkali washing tower through the tower of the second-level alkali washing tower, and is further washed with ammonia water. Absorption, wherein HF, HCl react with ammonia water to generate NH4F and NH4Cl, HF and HCl are almost removed, and the washing liquid is returned to the primary alkali washing tower from the bottom of the tower. The washing circulating liquid produced in the primary alkali washing tower is discharged into the wastewater treatment unit. After two-stage alkali washing, HF, HCl and PF5 in the tail gas are basically absorbed. The waste gas after the two-stage alkali washing is entered into the water washing tower and washed with tap water to remove a small amount of NH3 entrained in the waste gas. The washing liquid is returned to the secondary alkali washing tower. After the above steps, HF≤9mg / m3 and HCl≤100mg / m3 in the tail gas can meet the emission standards and are discharged to the outside from the top of the water washing tower.

[0010] The beneficial effects of the above technical solution are:

[0011] 1. Compared with the existing new tail gas recovery equipment in which the water washing tower is connected to the high-risk electrostatic demister, this technical solution sets up a two-stage alkaline washing to first absorb a large amount of HF and HCl gases in the tail gas. The water washing tower does not need to be connected to the electrostatic demister, and the whole process is highly safe.

[0012] 2. Compared with the existing use of organic solvents to remove tail gas, this technical solution adopts a "two-stage ammonia washing + one-stage water washing" method to treat tail gas. First, two alkaline washes are used to wash and absorb HF, HCl, and PF5 in the tail gas. The tail gas and the absorption liquid are absorbed in countercurrent. The washing liquid of the second alkaline wash is used as the absorption liquid of the first wash. This can not only greatly save ammonia consumption, but also ensure that the recovery rate of hydrogen chloride in the tail gas is greater than 98%. After the alkaline wash removes HF, HCl, and PF5, the small amount of NH3 in the tail gas is then absorbed and washed away with water, ensuring that the tail gas meets the emission standards. The wastewater generated by the tail gas treatment is treated in a dedicated wastewater treatment unit;

[0013] 3. Compared with the existing method of using chlorosulfonic acid to treat tail gas, this technical solution uses ammonia water which does not have strong oxidizing properties, causes less loss to the equipment, and extends the service life of the equipment.

[0014] Preferred solution 1: As a further optimization of the basic solution, the wastewater treatment unit is sequentially connected to a reaction tank, a first centrifuge, an MVR evaporation system, and a mother liquor tank. The washing circulating liquid is discharged into the reaction tank and a calcium chloride solution is used for impurity removal reaction. The reaction liquid in the reaction tank is discharged into the first centrifuge for solid-liquid separation. The liquid after solid-liquid separation is discharged into the MVR evaporation system. The solid after solid-liquid separation is washed with two stages of water. The first-stage mother liquor produced by the first-stage water washing is also discharged into the MVR evaporation system, and the second-stage mother liquor produced by the second-stage water washing is discharged into the mother liquor tank; the MVR evaporation system includes a buffer tank, an MVR evaporator, and a separator connected in sequence. The buffer tank is used to merge the liquid separated by the first centrifuge and the mother liquor after the separated solid is washed with water. The liquid in the buffer tank is pumped to the MVR evaporator for evaporation and crystallization to form a concentrated liquid. The mother liquor tank is used to store the mother liquor after the concentrated liquid is separated by solid-liquid separation.

[0015] The beneficial effects of the preferred solution 1 are as follows: the main components of the washing circulating liquid are ammonium phosphate, NH4F, and H4Cl. Calcium chloride is used to defluorinate and dephosphorize the washing circulating liquid, effectively removing fluorine and calcium from the circulating liquid without introducing other ions. After solid-liquid separation, the calcium fluoride and calcium phosphate are subjected to two-stage water washing, and the chloride ion content is ≤100 ppm. The final solid water content of the calcium fluoride and calcium phosphate is ≤15%, and the wastewater is directly sent to a waste residue treatment unit. MVR evaporation and concentration are used for the salt-containing wastewater. Compared with the traditional process of directly centrifuging the evaporated and concentrated liquid, the temperature is still relatively high, and the solubility of ammonium chloride increases with increasing temperature, so that the centrifuged material still contains a large amount of ammonium salt. The separation effect after MVR evaporation is significant and the energy consumption is low.

[0016] Preferred solution 2: As a further optimization of preferred solution 1, the top of the water washing tower is connected to an exhaust fan, and the air outlet end of the exhaust fan is connected to a chimney.

[0017] The beneficial effect of the preferred solution 2 is that the exhaust gas treated in the water washing tower can be quickly extracted and discharged to the outside through the chimney by the exhaust fan.

[0018] Preferred solution three: As a further optimization of preferred solution two, a pH regulating tank is provided between the first centrifuge and the buffer tank, and the pH regulating tank is used to regulate the liquid separated by the first centrifuge.

[0019] The beneficial effect of the preferred solution three is that the liquid used for solid-liquid separation in the first centrifuge contains ammonia water, and its pH is adjusted to 2-4 to be acidic, thereby preventing ammonia from volatilizing during evaporation, concentration and crystallization in the MVR evaporator, causing air pollution.

[0020] Preferred Option 4: As a further optimization of Preferred Option 3, a preheater is provided between the buffer tank and the MVR evaporator in the MVR evaporation system, a thickener is connected after the separator, a second centrifuge is connected after the thickener, the liquid outlet end of the second centrifuge is connected to the mother liquid tank, and the solid outlet end of the second centrifuge is connected to a dryer; the liquid in the buffer tank is preheated by the preheater and then discharged into the MVR evaporator for evaporation, concentration and crystallization, and then enters the separator for solid-liquid separation. After separation, it enters the thickener for concentration and then centrifuges. The solid at the solid outlet end of the centrifuge is discharged into the dryer for drying and packaging, and the liquid at the liquid outlet end of the centrifuge is discharged into the mother liquid tank for storage.

[0021] The beneficial effects of the preferred solution 4 are: the preheater preheats the liquid entering the MVR evaporator for evaporation and concentration to increase the activity of the liquid, and the thickener increases the concentration of the liquid separated by the separator, making it easier for the subsequent centrifuge to separate more thoroughly.

[0022] Preferred Option 5: As a further optimization of Preferred Option 4, the MVR evaporator outlet is connected to a compressor. The secondary steam generated at the MVR evaporator outlet is compressed by the compressor and its temperature is increased. The secondary steam with increased temperature flows back into the MVR evaporator as a heat source for the evaporator.

[0023] The beneficial effect of the preferred scheme 5 is that the secondary steam that originally needs to be condensed by cooling water in the evaporator is compressed by the compressor to increase its pressure and saturation temperature, increase its enthalpy value, and then sent to the evaporator heater as a heat source to heat the feed liquid, so that the latent heat of the secondary steam is utilized, which is equivalent to 10-effect evaporation. Compared with traditional multi-effect evaporation, it can save about 70% of standard coal. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of a production line according to an embodiment of the present invention;

[0025] Figure 2 Schematic diagram of the process flow of an embodiment of the present invention.

[0026] The figure marks in the drawings of the specification include: a primary alkali washing tower 11, a secondary alkali washing tower 12, a water washing tower 13, a fan 14, a chimney 15, a reaction tank 21, a first centrifuge 22, a pH adjustment tank 23, an MVR evaporation system 24 (framed by a dotted line), a mother liquor tank 25, a dryer 26, a compressor 27, a buffer tank 241, a preheater 242, an MVR evaporator 243, a separator 244, a thickener 245, and a second centrifuge 246. DETAILED DESCRIPTION

[0027] The following is further described in detail through specific implementation methods:

[0028] The embodiment is basically as shown in the attached Figure 1 As shown:

[0029] A production line for recycling fluorine-containing tail gas of lithium hexafluorophosphate, such as Figure 1 As shown, the exhaust gas treatment unit consists of a tail gas treatment unit and a wastewater treatment unit. The tail gas treatment unit is connected to the primary alkali scrubber 11, the secondary alkali scrubber 12, and the water scrubber 13 from left to right. The top of the water scrubber 13 is connected to an exhaust fan 14, and the air outlet of the exhaust fan 14 is connected to a chimney 15. The bottom end of the primary alkali scrubber 11 is connected to the wastewater treatment unit.

[0030] The wastewater treatment unit is connected to the reaction tank 21, the first centrifuge 22, the pH adjustment tank 23, the MVR evaporation system ( Figure 1 The MVR evaporation system includes a buffer tank 241, a preheater 242, an MVR evaporator 243, a separator 244, a thickener 245, and a second centrifuge 246, which are connected from left to right. The liquid outlet of the second centrifuge 246 is connected to the mother liquid tank 25, the solid outlet of the second centrifuge 246 is connected to the dryer 26, and the air outlet of the MVR evaporator 243 is connected to the compressor 27.

[0031] The specific implementation process is as follows:

[0032] like Figure 2 As shown, the tail gas of the lithium hexafluorophosphate device is preheated to 20℃ and passed through a spray tower (not part of this production line, Figure 1 The top of the first-stage alkali washing tower enters the first-stage alkali washing tower, in which HF, HCl, and PF5 in the tail gas are mainly washed with 10% ammonia water. During this process, PF5 is completely hydrolyzed, and 79% of HF and about 90% of HCl in the tail gas can be removed at this time;

[0033] The HF and HCl waste gas that has not been completely absorbed by the ammonia water in the primary alkali washing tower enters the secondary alkali washing tower through the top of the secondary alkali washing tower, and is further absorbed by 10% ammonia water. HF and HCl are basically removed. The washing liquid in the secondary alkali washing tower is returned to the primary alkali washing tower from the bottom of the tower. The washing circulating liquid produced in the primary alkali washing tower is discharged into the wastewater treatment unit. After two stages of alkali washing, HF, HCl and PF5 in the tail gas are basically absorbed.

[0034] After the two-stage alkali washing is completed, the waste gas enters the water washing tower and is washed with tap water to remove a small amount of NH3 entrained in the waste gas. The washing liquid is returned to the secondary alkali washing tower. After the above steps, the HF in the tail gas can be made ≤ 9 mg / m3 and the HCl ≤ 100 mg / m3, meeting the emission standards. The exhaust fan quickly extracts the treated tail gas in the water washing tower and discharges it to the outside through the chimney.

[0035] The chemical reaction equations involved in the above tail gas treatment process are as follows:

[0036] 1. HF + NH3·H2O = NH4F + H2O

[0037] 2. HCl + NH HO = NH Cl + HO

[0038] 3. PF5+4H2O=H3PO4+5HF

[0039] 4. H3PO4+3NH HO=(NH4)3PO4+3H O

[0040] From the above reaction equation, we know that the main components of the washing circulating liquid are (NH4)3PO4, NH4F, and NH4Cl. The washing circulating liquid is pumped into the reaction tank through a diaphragm pump, and 60% calcium chloride slurry is added to the reaction tank to carry out fluorine and phosphorus removal reaction. 3- 、F - Remove.

[0041] The chemical reaction equations involved in the above-mentioned washing circulating fluid treatment process are as follows:

[0042] 1. 2(NH4)3PO4+3CaCl2→Ca3(PO4)2+6NH4Cl

[0043] 2. CaCl2+2NH4F→CaF2+2NH3.H2O

[0044] The reaction liquid in the reaction tank is discharged into the first centrifuge for solid-liquid separation. According to the above reaction equation, the liquid separated by the first centrifuge contains NH3.H2O, which is discharged into the pH adjustment tank to adjust the pH to 3 to avoid NH3 volatilization during subsequent evaporation, causing air pollution; the solid separated by the first centrifuge (containing CaP2 and Ca3(PO4)2) is washed in the water tank ( Figure 1 (not shown) distilled water is used for two-stage water washing, and the liquid with a pH of 3 is discharged into the buffer tank. The liquid in the buffer tank is pumped into the preheater and preheated to above 60°C. The preheating is to enhance the activity of the liquid. The preheated liquid is pumped into the MVR evaporator for evaporation, concentration and crystallization. The distilled water generated by the MVR evaporator during use is recovered for two-stage water washing, evaporation and concentration, and cooling and crystallization. The solid-liquid mixture after cooling and crystallization enters the separator for solid-liquid separation. The separated liquid enters the thickener to concentrate and increase its concentration. The separated solid is discharged into the dryer for drying and packaging. The concentrated and concentrated liquid is centrifuged in the second centrifuge. The ammonium chloride solid at the solid outlet of the second centrifuge is also discharged into the dryer for drying and packaging. The ammonium chloride meets the industrial-grade qualified product standards in the specification "GB / T2946-2018" and can be sold. The liquid at the liquid outlet of the second centrifuge is discharged into the mother liquor tank for storage.

[0045] The first-stage mother liquor produced by the first-stage water washing is also discharged into the buffer tank and treated together with the liquid with a pH of 3 according to the above steps.

[0046] The secondary mother liquor produced by the second stage water washing is discharged into the mother liquor tank, and the produced CaF2.Ca3(PO4)2 is recovered and sold.

[0047] This process production line uses a "two-stage dilute ammonia washing + one-stage water washing" method to treat tail gas. First, two alkaline washes are used to wash and absorb HF, HCl, and PF5 in the tail gas. The tail gas and the absorption liquid are countercurrently absorbed. The washing liquid from the second-stage alkaline washing is used as the absorption liquid for the first-stage washing. This can greatly save ammonia consumption and ensure that the recovery rate of hydrogen chloride in the tail gas is greater than 98%. After the alkaline washing removes HF, HCl, and PF5, water is used to absorb and wash away a small amount of NH3 in the tail gas to ensure that the tail gas meets emission standards.

[0048] The above is only an embodiment of the present invention, and the common knowledge such as the specific structure and characteristics of the scheme is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A production line for recycling fluorine-containing tail gas from lithium hexafluorophosphate, comprising a tail gas treatment unit and a wastewater treatment unit, characterized in that: The tail gas treatment unit comprises a primary alkali washing tower, a secondary alkali washing tower and a water washing tower, wherein the top of the primary alkali washing tower and the secondary alkali washing tower are provided with a tail gas inlet, and the bottom of the primary alkali washing tower and the secondary alkali washing tower are provided with a tail gas outlet; A buffer tank is provided at the bottom of the tower, an exhaust gas inlet is provided at the top of the buffer tank, water accumulates in the buffer tank, an exhaust fan is provided at the top of the water washing tank, an ammonia water spraying device is provided in the first-stage alkali washing tower and the second-stage alkali washing tower, the washing liquid generated by washing the exhaust gas is pumped into the wastewater treatment unit, the washing liquid in the second-stage alkali washing tower is returned to the first-stage alkali washing tower from the bottom of the tower, and the washing circulating liquid produced in the first-stage alkali washing tower is discharged into the wastewater treatment unit; The wastewater treatment unit is sequentially connected to a reaction tank, a first centrifuge, an MVR evaporation system, and a mother liquor tank. The washing circulating liquid is discharged into the reaction tank and a calcium chloride solution is used for impurity removal reaction. The reaction liquid in the reaction tank is discharged into the first centrifuge for solid-liquid separation. The liquid after solid-liquid separation is discharged into the MVR evaporation system. The solid after solid-liquid separation is washed with two stages of water. The first-stage mother liquor generated by the first-stage water washing is also discharged into the MVR evaporation system, and the second-stage mother liquor generated by the second-stage water washing is discharged into the mother liquor tank; the MVR evaporation system includes a buffer tank, an MVR evaporator, and a separator connected in sequence from left to right. The buffer tank is used to merge the liquid separated by the first centrifuge and the mother liquor after the separated solid is washed with water. The liquid in the buffer tank is pumped to the MVR evaporator for evaporation and crystallization to form a concentrated liquid. The mother liquor tank is used to store the mother liquor after the concentrated liquid is subjected to solid-liquid separation; A pH regulating tank is provided between the first centrifuge and the buffer tank, and the pH regulating tank is used to regulate the liquid separated by the first centrifuge; In the MVR evaporation system, a preheater is provided between the buffer tank and the MVR evaporator, a thickener is connected after the separator, a second centrifuge is connected after the thickener, a liquid outlet end of the second centrifuge is connected to a mother liquid tank, and a solid outlet end of the second centrifuge is connected to a dryer; the liquid in the buffer tank is preheated by the preheater and then discharged into the MVR evaporator for evaporation, concentration and crystallization, and then enters the separator for solid-liquid separation. After separation, it enters the thickener for concentration and then centrifuges. The solid at the solid outlet end of the centrifuge is discharged into the dryer for drying and packaging, and the liquid at the liquid outlet end of the centrifuge is discharged into the mother liquid tank for storage.

2. A production line for recycling fluorine-containing tail gas from lithium hexafluorophosphate according to claim 1, characterized in that: The top of the water washing tower is connected to an exhaust fan, and the air outlet end of the exhaust fan is connected to a chimney.

3. The production line for recycling fluorine-containing tail gas of lithium hexafluorophosphate according to claim 1, characterized in that: The air outlet of the MVR evaporator is connected to a compressor. The secondary steam generated at the air outlet of the MVR evaporator is compressed by the compressor and its temperature is increased. The secondary steam with increased temperature flows back to the MVR evaporator as a heat source for the evaporator.

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