Method for treating lithium-containing wastewater generated in acid leaching carbonization lithium precipitation process

Abstract

The invention discloses a method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process, which comprises the following steps of: mixing a carbonization lithium precipitation mother solution generated in a carbonization lithium precipitation section with carbonization lithium precipitation washing water, preheating, evaporating until lithium carbonate is separated out, dehydrating the material to obtain solid lithium carbonate and a secondary lithium mother solution, adding concentrated sulfuric acid into the secondary lithium mother solution, and then adding the concentrated sulfuric acid into the secondary lithium mother solution; then adding sodium hydroxide to alkalescence to obtain sodium-separated raw water, evaporating the sodium-separated raw water until sodium sulfate is separated out, performing solid-liquid separation to obtain solid sodium sulfate and sodium-separated mother liquor, freezing and crystallizing the sodium-separated mother liquor until sodium sulfate crystals are separated out, and separating to obtain sodium sulfate crystals and frozen centrifugal mother liquor. According to the treatment method, secondary output of lithium carbonate in industrial grade and even battery grade product forms can be achieved, sodium sulfate is output in a larger proportion in an anhydrous sodium sulphate product form, and meanwhile consumption of sulfuric acid and sodium hydroxide is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of new energy battery recycling, and in particular relates to a method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process. Background Art

[0002] At present, in the new energy industry, lithium carbonate is widely used as a battery material. With the development of circular economy models such as reduction, reuse, and resource utilization, the production and recycling of lithium-ion batteries has become an important part of the current new energy industry chain. At present, there are two main types of processes for the production and recycling of lithium-ion batteries: pyrolysis and hydrolysis. Among them, the pyrolysis method uses high-temperature sintering in a rotary kiln as the core. This process has large tail gas emissions, high energy consumption, and low product purity. Therefore, the current production and recycling of lithium-ion batteries generally adopts a wet method, especially the acid leaching technology of the sulfuric acid system, and lithium carbonate is produced as the core product. In this type of wet process, a large amount of lithium-containing wastewater is generated. The reasonable resource recovery and classification treatment of this wastewater is particularly important for the operation efficiency and cost control of the process system. The common process for this wastewater is to first adjust the acid with sulfuric acid, convert the lithium carbonate in the wastewater into lithium sulfate, enter the evaporation system for concentration, produce sodium sulfate for sale, and produce lithium sulfate enriched liquid to return to the front-end workshop for secondary use.

[0003] Chinese patent application documents CN 109095481 A and disclose a comprehensive recovery method for lithium iron phosphate waste powder, and Chinese patent application document CN 108075202 A discloses a comprehensive recovery method for lithium iron phosphate positive electrode materials. At present, the common method for acid leaching carbonization precipitation to produce and recover lithium-ion battery materials is that the wastewater treatment process after carbonization precipitation is direct acidification, pH adjustment and then evaporation of sodium sulfate. It has the following shortcomings: (1) Neither of them considers the specific salt separation process of lithium carbonate products and sodium sulfate byproducts dissolved in the mother liquor of carbonization precipitation and the efficient recovery method of products. (2) The mother liquor and washing water after carbonization precipitation have a large total water volume (usually, 1 ton of lithium carbonate product corresponds to 20-25m³ of wastewater). This raw water is directly adjusted in acid and pH, and the amount of sulfuric acid and sodium hydroxide is large. In addition, the concentrated sulfuric acid contains impurities, which are brought into the system in large quantities, increasing the cost of impurity removal and being unfavorable for stable operation of the process. (3) Lithium carbonate is directly added with acid to adjust pH, and then sodium sulfate and lithium sulfate mixed salt is evaporated and concentrated. When the lithium in the system reaches a certain concentration, further concentration needs to be stopped to avoid lithium sulfate salt precipitation and the formation of mixed salt discharge system together with sodium sulfate salt. In the above-mentioned process invention, the lithium carbonate in the raw water of carbonization precipitation is not further salted and recovered, and the total amount of lithium in the wastewater system is relatively high. Because further concentration needs to be stopped when the lithium concentration in the evaporation system reaches a certain concentration, the amount of concentrated liquid (sodium precipitation mother liquor) after concentration is larger. This sodium precipitation mother liquor needs to enter the freezing section to further remove sodium sulfate (the above-mentioned patent application document only considers the process of producing post-crystallization liquid in CN 109095481 A, and is designed as a calcium and magnesium removal process. It does not consider that the post-liquid does not remove sodium sulfate, which affects the smooth progress of the previous process and the quality of carbonization precipitation). If the amount of sodium precipitation mother liquor is large, the burden of the freezing section is large, the energy consumption is high, the output of sodium sulfate decahydrate is high, and the amount of reverse dissolution treatment is larger. (4) Under the process flow in patent application document CN 109095481A, the lithium in the wastewater has been repeatedly reacting and circulating in this process system, and the export is only at the front-end carbonization and lithium precipitation step, which is not conducive to cost reduction and efficiency improvement. Patent application document CN 108075202A does not consider the recovery of lithium carbonate in wastewater. Summary of the invention In order to overcome the problems in the prior art, the present invention provides a method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process, which realizes efficient and high-purity salt separation and recovery of lithium carbonate and sodium sulfate. It can realize secondary output of lithium carbonate in the form of industrial grade or even battery grade products, and a larger proportion of sodium sulfate is produced in the form of glauber salt products, while reducing the consumption of sulfuric acid and sodium hydroxide.

[0004] In order to solve the above technical problems, the technical solution proposed by the present invention is: The present invention provides a method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process, comprising the following steps: S1. Mixing the lithium carbide precipitation mother liquor and the lithium carbide precipitation washing water produced in the lithium carbide precipitation section to obtain raw water.

[0005] S2. Preheat the raw water and evaporate it until lithium carbonate is precipitated. When the sodium sulfate in the solution is close to saturation, the system balance is maintained. The material is dehydrated to obtain solid lithium carbonate and secondary lithium mother liquor.

[0006] S3. Add concentrated sulfuric acid to the secondary lithium mother liquor to convert the carbonate in the solution into sulfate, and then add sodium hydroxide to adjust to weak alkalinity to obtain sodium precipitation raw water.

[0007] S4, evaporating the sodium precipitation raw water until sodium sulfate is precipitated, maintaining the system balance, and dehydrating the material to obtain solid sodium sulfate and sodium precipitation mother liquor.

[0008] S5, freezing and crystallizing the sodium precipitation mother liquor until sodium sulfate crystals are precipitated, and obtaining sodium sulfate decahydrate crystals and frozen centrifugal mother liquor after separation, and returning the frozen centrifugal mother liquor to the front end of carbonization lithium precipitation, and re-performing lithium precipitation operation.

[0009] In the lithium carbonization process, two types of lithium-containing water bodies are produced. The first is the lithium carbonization mother liquor, and the second is the lithium carbonization wash water. The volume ratio is about 1:1, and the salt components are mainly sodium sulfate, lithium carbonate, and sodium carbonate. In the present invention, the lithium carbonate in the mother liquor and the wash water is saturated, and its solubility is relatively low, about 13g / L. After the mother liquor is mixed with the wash water, the sodium sulfate concentration is about 150-180g / L. Sodium carbonate is added in excess (generally exceeding 1%-3%) during the lithium carbonization process and remains. Because of its high solubility, its influence can be ignored in this salt separation step.

[0010] The mother liquor is mixed with the washing water to obtain raw water, which is preheated and evaporated. Since the lithium carbonate salt in the raw water is in a saturated solution state, the lithium carbonate salt is continuously precipitated during the heating and evaporation process, and the sodium sulfate salt is continuously concentrated. During the evaporation process, when it is judged that the sodium sulfate solution is close to saturation, the material is discharged and dehydrated, and solid lithium carbonate and a centrifuged solution (secondary lithium mother liquor) are obtained after solid-liquid separation. Concentrated sulfuric acid is added to the secondary lithium mother liquor for acid adjustment. The sulfuric acid reacts with the lithium carbonate and sodium carbonate in the mother liquor to convert the carbonate into lithium sulfate and sodium sulfate, so that the pH of the solution after acid adjustment is 5-6, and then sodium hydroxide is added to neutralize the excess sulfuric acid so that it can be evaporated under weak alkalinity. The pH is about 8-10, and the obtained solution is the raw water for sodium precipitation. The raw water for sodium precipitation is preheated and then evaporated. Since the sodium sulfate in the raw water for sodium precipitation is close to a saturated solution state, sodium sulfate salt is continuously precipitated during the evaporation process, and lithium sulfate salt is continuously concentrated. When the solid-liquid volume ratio reaches 15-20%, the material is discharged and dehydrated. After solid-liquid separation, solid sodium sulfate and sodium precipitation mother liquor are obtained. The solid sodium sulfate salt is collected as an industrial-grade glauber powder product through subsequent drying and packaging equipment. Its control indicators are moisture content <0.5% and lithium content <0.1%. Lithium sulfate has a high solubility and generally does not precipitate in large quantities. The obtained sodium precipitation mother liquor is frozen and crystallized until sodium sulfate crystals are precipitated, most of which are sodium sulfate decahydrate. After freezing, centrifugal dehydration is performed to obtain sodium sulfate decahydrate crystals and frozen centrifugal mother liquor. The sodium sulfate decahydrate crystals enter the anti-dissolution system for anti-dissolution. The sodium sulfate concentration in the frozen centrifugal mother liquor will be <30g / L, and the lithium sulfate concentration will be >110g / L. This frozen centrifugal mother liquor is returned to the front end of carbonization lithium precipitation, and the lithium precipitation operation is repeated. The treatment of lithium-containing wastewater is now completed.

[0011] As an optional embodiment, in the treatment method provided by the present invention, Na 2 SO 4 150~180g / L, Na 2 CO 3 2~9g / L, Li 2 CO 3 It is 11-13g / L, and NaCl is 10-100mg / L.

[0012] As an optional embodiment, in the treatment method provided by the present invention, in step S2, when the density of the solution is 1.20-1.22 g / cm³, it is judged that the sodium sulfate in the solution is close to saturation.

[0013] In the present invention, due to the presence of sodium carbonate, lithium carbonate and other components, the saturated solubility of sodium sulfate under this working condition is about 380g / L, and the density of the mixed solution is 1.20-1.22g / cm³. Therefore, it can be judged that the sodium sulfate solution is close to saturation based on the solution density.

[0014] As an optional embodiment, in the treatment method provided by the present invention, in step S2, another judgment method is to maintain the system feed and evaporation rate balance when the sodium ion concentration in the solution is 120-122 g / L.

[0015] In the present invention, after the temperature is raised, it is a continuous steady-state process, evaporating while feeding, and after reaching the concentration, feeding, evaporating and discharging are performed at the same time.

[0016] As an optional embodiment, in the treatment method provided by the present invention, in step S2, the raw water is evaporated at 83-87°C; in step S4, the sodium precipitation raw water is evaporated at 88-92°C.

[0017] In the present invention, the raw water is evaporated at 83-87°C, which can effectively reduce the scaling phenomenon during the evaporation and crystallization of lithium carbonate, and the low temperature can also save energy. The raw water for sodium analysis is evaporated at 88-92°C, which is conducive to reducing energy consumption.

[0018] As an optional embodiment, in the treatment method provided by the present invention, in step S2, the material is further concentrated and then centrifuged and dehydrated, and the solid lithium carbonate obtained is washed and dried to a purity of ≥99.6%. The lithium carbonate salt product can be directly recovered in this step.

[0019] As an optional embodiment, in the treatment method provided by the present invention, in step S4, after evaporation to a solid-liquid volume ratio of 15-20% (specifically, the volume ratio after sampling and clarification with a measuring cup), the system feed and evaporation rate are balanced, and then the material is further concentrated and centrifuged for dehydration, and the solid sodium sulfate is dried and used as industrial sodium sulfate, wherein the lithium content of the industrial sodium sulfate is less than 0.1%.

[0020] As an optional embodiment, in the treatment method provided by the present invention, in step S4, when the concentration of lithium sulfate in the sodium precipitation mother liquor reaches 110-125 g / L, part of the analyzed sodium mother liquor is refluxed to the evaporation system, and the remaining part is subjected to freeze crystallization treatment.

[0021] In the present invention, the sodium precipitation raw water is evaporated at a temperature of 88 to 92° C. to a solid-liquid ratio of 15 to 20%, sodium sulfate is precipitated, and a sodium precipitation mother liquor is obtained. When the concentration of lithium sulfate in the sodium precipitation mother liquor exceeds 125 g / L, the lithium content in the sodium sulfate salt obtained by evaporation and crystallization may exceed the standard. Therefore, when it is detected that the concentration of lithium sulfate in the sodium precipitation mother liquor reaches 110 to 125 g / L, part of the sodium precipitation mother liquor is discharged from the system and enters the freezing section to reduce the lithium concentration in the evaporation system.

[0022] As an optional embodiment, in the processing method provided by the present invention, in step S5, the temperature of the freezing crystallization is -5°C to 5°C.

[0023] As an optional embodiment, in the processing method provided by the present invention, in step S5, the freezing crystallization time is 1.5 to 3 hours.

[0024] As an optional embodiment, in the treatment method provided by the present invention, in step S5, the frozen centrifugal mother liquor is dechlorinated by a nanofiltration membrane and then returned to the front end of the carbonization lithium precipitation, and the lithium precipitation operation is performed again, and the sodium chloride concentrated water produced by the nanofiltration membrane dechlorination is dried and evaporated.

[0025] In the lithium carbonate preparation and recovery process system, it is inevitable that trace amounts of other impurity ions are introduced, such as sodium carbonate added in the process of carbonization and precipitation of lithium. Due to the preparation process of sodium carbonate, sodium chloride is inevitably present. Therefore, in this application, a nanofiltration membrane device is introduced at the rear end of the refrigerated centrifuge. By detecting the chloride ion content in the refrigerated centrifuge mother liquor, the nanofiltration membrane device is regularly introduced for dechlorination operation, and the generated sodium chloride concentrated water can be dried and evaporated by an evaporator.

[0026] As an optional embodiment, in the treatment method provided by the present invention, in step S2, an evaporation system in which a two-stage evaporator is connected in series and an external circulation form of a forced circulation pump is used to evaporate lithium carbonate during the evaporation process; in the evaporation system, the evaporator, the crystal separator, the circulation pipeline and the discharge pipeline are mirror plated or polished; and an ultrasonic generator is provided in the evaporator, the crystal separator and the circulation pipeline.

[0027] As an optional embodiment, in the processing method provided by the present invention, Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention classifies and recycles lithium-containing wastewater after the lithium-ion battery material carbonization and lithium precipitation process in a reasonable resource manner, and lithium carbonate is secondary output in the form of a battery-grade product, while a larger proportion of sodium sulfate is output in the form of a glauber powder product, thereby achieving efficient and high-purity recovery of lithium carbonate and sodium sulfate.

[0028] (2) In the present invention, the total amount of wastewater is reduced by more than 50% after the first evaporation, and more than 70% of lithium carbonate salt has been precipitated after the first evaporation. Therefore, the total consumption of sulfuric acid and sodium hydroxide in the acid adjustment process is reduced, which reduces the consumption of sulfuric acid and sodium hydroxide and reduces material costs and operating costs.

[0029] (3) In the present invention, lithium carbonate is first evaporated to directly produce lithium carbonate products. The evaporation cost is less than 4,000 yuan. The obtained lithium carbonate products can directly generate higher economic benefits, avoiding the loss of lithium being carried away in sodium sulfate caused by the method of directly separating lithium sulfate and sodium sulfate in the prior art.

[0030] (4) Since lithium carbonate is very easy to scale during evaporation and crystallization, the lithium carbonate industry currently mainly evaporates and crystallizes lithium sulfate or concentrates lithium carbonate solution. In the present invention, a two-stage evaporator is used in series and an evaporation system in the form of a forced circulation pump is used. In the evaporation system, the evaporator, crystallizer, circulation pipeline and discharge pipeline are mirror plated or polished, and an ultrasonic generator is provided to directly evaporate lithium carbonate salt, which can effectively prevent scaling. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 The figure is a process flow chart of the treatment process of lithium-containing wastewater in the present invention. DETAILED DESCRIPTION

[0033] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0034] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0035] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0036] Example 1 A method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process, the process flow is as follows Figure 1 As shown, the following steps are included: (1) In a lithium iron phosphate battery recycling system, after the carbonization reaction, the Na in the raw water composed of the mother liquor and the washing water is detected. + The concentration is 60.3 g / L, Li + The concentration is 2.46 g / L, SO 4 2- The concentration is 121.7 g / L, CO 3 2- The concentration is 13.2 g / L, Cl - 30ppm, that is, Na in raw water 2 SO 4About 180g / L, residual Na 2 CO 3 About 4.6g / L, Li 2 CO 3 It is about 13g / L, and NaCl is about 60mg / L.

[0037] (2) Take 2 portions of original water, 1 L each.

[0038] (3) Take one portion of the raw water and evaporate it by negative pressure heating according to the process of the present invention. After heating, a small amount of white precipitate begins to form. The evaporation temperature is controlled at 84°C and the vacuum degree is about 46KPa. During the evaporation process, the density of the solution is tested. When it reaches 1.20g / cm³, the heating is stopped, the solution is filtered and dried while keeping warm. At this time, the residual liquid volume is about 500ml, and about 8.9g of lithium carbonate solid is obtained. + The concentration is 1.55 g / L (i.e. Li 2 CO 3 8.2 g / L), Na + The concentration is about 121g / L (Na 2 SO 4 About 360g / L, Na 2 CO 3 is 9.2g / L).

[0039] (4) Pour the residual liquid (i.e., secondary lithium mother liquor) into a beaker and add H 2 SO 4 React with lithium carbonate and residual sodium carbonate, and stir thoroughly to adjust the pH value to 5-6, produce lithium sulfate and sodium sulfate, consume 8g of sulfuric acid, and the volume after acid adjustment is about 505ml. At this time, the concentration of sodium sulfate is about 370g / / L.

[0040] (5) Add caustic soda flakes to adjust the pH to 9 (trace amount, 0.02 g) to obtain 505 ml of sodium precipitation raw water.

[0041] (6) Pour the 505 ml solution into the flask again for secondary negative pressure heating evaporation concentration at 88 degrees Celsius and a vacuum degree of about 36 kPa until Li + The concentration is 14g / L (i.e. Li 2 SO 4 110g / L) Stop concentration, keep warm, filter and dry. At this time, the amount of residual liquid (sodium mother liquor) is about 56ml. Detect Na in the residual liquid + The concentration was about 129.6 g / L, and about 164 g of sodium sulfate solid was obtained. The Li content was detected. + About 0.02%.

[0042] (7) The mother liquor was placed in a freezer and kept at 0°C. After 2 hours, it was taken out and filtered to obtain 52 g of mirabilite and 27 ml of filtrate. + The concentration was detected to be 29g / L, Na + The concentration detected was 1.6g / L.

[0043] Example 2 A method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process, the process flow is as follows Figure 1 As shown, the following steps are included: (1) In a lithium iron phosphate battery recycling system, after the carbonization reaction, the Na in the raw water composed of the mother liquor and the washing water is detected. + The concentration is 49.5g / L, Li + The concentration is 2.46 g / L, SO 4 2- The concentration is 103.3 g / L, CO 3 2- The concentration is 11.7 g / L, Cl - 6ppm, that is, Na 2 SO 4 About 150g / L, residual Na 2 CO 3 About 2.1g / L, Li 2 CO 3 It is about 13g / L, and NaCl is about 10mg / L.

[0044] (2) Take 2 portions of original water, 1 L each.

[0045] (3) Take one portion of the raw water and evaporate it by negative pressure heating according to the process of the present invention. After heating, a small amount of white precipitate begins to form. The evaporation temperature is controlled at 84°C (vacuum degree is about 48KPa). During the evaporation process, the solution density is tested. When it reaches 1.22g / cm³, the heating is stopped, the solution is filtered and dried by heat preservation. At this time, the residual liquid volume is about 405ml, and about 9.7g of lithium carbonate solid is obtained. + The concentration is 1.55 g / L (i.e. Li 2 CO 3 8.2 g / L), Na + The concentration is about 122g / L (Na 2 SO 4 About 370g / L, Na 2 CO 3 is 5.2g / L).

[0046] (4) Pour the residual liquid (i.e., secondary lithium mother liquor) into a beaker and add H 2 SO 4React with lithium carbonate and residual sodium carbonate, and stir thoroughly to adjust the pH value to 5-6, produce lithium sulfate and sodium sulfate, consume 6.5g of sulfuric acid, and the volume after acid adjustment is about 409ml. At this time, the concentration of sodium sulfate is about 373g / / L.

[0047] (5) Add caustic soda flakes to adjust the pH to 10 (trace amount) to obtain 409 ml of sodium precipitation raw water.

[0048] (6) Pour the 409 ml solution into the flask again for secondary negative pressure heating evaporation concentration at 92 degrees Celsius and a vacuum degree of about 26 kPa until Li + The concentration is 16g / L (i.e. Li 2 SO 4 The concentration was stopped, the temperature was kept high and filtered to dryness. At this time, the amount of the residual liquid (sodium mother liquor) was about 27 ml. The Na + The concentration is about 129.6g / L, and about 142g of sodium sulfate solid is obtained. The salt is tested to contain Li + About 0.08%.

[0049] (7) The mother liquor was placed in a freezer and kept at 5°C. After 3 hours, it was taken out and filtered to obtain 23 g of mirabilite and 15 ml of filtrate. + The concentration was detected to be 28.8 g / L, Na + The concentration detected was 2.3g / L.

[0050] Example 3 A method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process, the process flow is as follows Figure 1 As shown, the following steps are included: (1) In a lithium iron phosphate battery recycling system, after the carbonization reaction, the Na in the raw water composed of the mother liquor and the washing water is detected. + The concentration is 54.4 g / L, Li + The concentration is 2.08 g / L, SO 4 2- The concentration is 105.5 g / L, CO 3 2- The concentration is 14g / L, Cl - is 12ppm, that is, Na 2 SO 4 About 156g / L, residual Na 2 CO 3 About 9g / L, Li 2 CO 3 It is about 11g / L, and NaCl is about 20mg / L.

[0051] (2) Take 2 portions of original water, 1 L each.

[0052] (3) Take one portion of the raw water and evaporate it by negative pressure heating according to the process of the present invention. After heating, a small amount of white precipitate begins to form. The evaporation temperature is controlled at 87°C (vacuum degree is about 39KPa). During the evaporation process, the solution density is tested. When it reaches 1.22g / cm³, the heating is stopped, the solution is filtered and dried by heat preservation. At this time, the residual liquid volume is about 422ml, and about 7.6g of lithium carbonate solid is obtained. + The concentration is 1.51 g / L (i.e. Li 2 CO 3 8g / L), Na + The concentration is about 129g / L (Na 2 SO 4 About 370g / L, Na 2 CO 3 is 21.3g / L).

[0053] (4) Pour the residual liquid (i.e., secondary lithium mother liquor) into a beaker and add H 2 SO 4 React with lithium carbonate and residual sodium carbonate, and stir thoroughly to adjust the pH value to 5-6, produce lithium sulfate and sodium sulfate, consume 13g of sulfuric acid, and the volume after acid adjustment is about 429ml. At this time, the concentration of sodium sulfate is about 398g / / L.

[0054] (5) Add caustic soda flakes to adjust the pH to 9 (trace amount) to obtain 429 ml of sodium precipitation raw water.

[0055] (6) Pour the 429 ml solution into the flask again for secondary negative pressure heating evaporation concentration at 90 degrees Celsius and a vacuum degree of about 31 kPa until Li + The concentration is 15g / L (i.e. Li 2 SO 4 The concentration was stopped, the temperature was kept high and filtered to dryness. At this time, the amount of the residual liquid (sodium mother liquor) was about 43 ml. The Na + The concentration is about 129.6g / L, and about 153.5g of sodium sulfate solid is obtained. The salt is tested to contain Li + About 0.06%.

[0056] (7) The mother liquor was placed in a freezer and kept at -5°C. After 1.5 hours, it was taken out and filtered to obtain 39 g of mirabilite and 21 ml of filtrate. + The concentration was detected to be 30.7 g / L, Na + The concentration detected was 1.6g / L.

[0057] In summary, the lithium carbonate concentration / saturation concentration in the raw water is greater than the sodium sulfate concentration / saturation concentration, which means that it is feasible to evaporate and collect lithium carbonate salt first. The following is a further introduction to the equipment used in the process of treating lithium-containing wastewater based on a production example and a comparative example: (1) A lithium iron phosphate battery recycling production line uses acid leaching carbonization to precipitate lithium. Its carbonization and precipitation section uses 4 lithium precipitation reactors. Each reactor produces about 10 cubic meters of raw water and 10 cubic meters of wash water. Each device precipitates lithium 3 times, producing 60m³ of water. That is, the mother liquor and raw water after carbonization and precipitation of lithium in this production line constitute its wastewater for 240 hours / day. + The concentration is 60.5g / L, Li + The concentration is 2.46 g / L, SO 4 2- The concentration is 121.7 g / L, CO 3 2- The concentration is 13.2 g / L, Cl - 350ppm, that is, Na 2 SO 4 About 180g / L, residual Na 2 CO 3 About 4.6g / L, Li 2 CO 3 It is about 13g / L, and NaCl is about 0.6g / L.

[0058] (2) Preheating 1: The mixed wastewater (hereinafter referred to as raw water) is transported to the condensate (i.e. condensate produced by evaporation) preheating plate heat exchanger through a feed pump (flow rate 10m³ / h) for primary preheating, and then enters the steam preheating shell and tube heat exchanger. The temperature after preheating is interlocked with the raw steam regulating valve to control the temperature of the wastewater after heat exchange, i.e. the feed temperature, to 85°C, to meet the bubble point feed requirements.

[0059] (3) MVR evaporation 1: This system is designed as an evaporation system with two-stage evaporators in series and a forced circulation pump external circulation. After preheating 1, the wastewater enters the upper part of the crystal separator, and the raw water flows out from the middle pipe of the crystal separator and enters the top of the first-stage tube-and-tube evaporator. The area of ​​the first-stage heater is 200 m2. The inside of the tube is the wastewater solution, and the outside of the tube is the secondary steam (raw steam during the startup period). After passing through the first-stage heater, it flows out from the bottom and enters the forced circulation pump on the bottom pipe. After being pressurized by the forced circulation pump, it enters the bottom of the second-stage tube-and-tube evaporator. The area of ​​the second-stage heater is 300 m2. The inside of the tube is the wastewater solution, and the outside of the tube is the secondary steam (raw steam during the startup period). After passing through the second-stage heater, it flows out from the top and returns to the upper part of the crystal separator. The steam generated after entering the crystal separator rises, and the liquid sinks. The rising steam is extracted from the top of the crystal separator and enters the centrifugal steam compressor after passing through the steam scrubber. After compression, the temperature of this steam rises by 16°C to 101°C, which is called secondary steam. This secondary steam is discharged from the compressor outlet and enters the shell side of the first and second stage tube-and-tube evaporator for evaporation and heating. After condensation, it is discharged from the bottom of the shell side of the first and second stage tube-and-tube evaporator, which is called steam condensate. The amount of this condensate is related to the evaporation capacity of this MVR evaporation 1 system, and the designed evaporation rate is 5m³ / h. The condensate is transported by the condensate discharge pump, and after heat exchange with the feed raw water, it is discharged from the system and reused at the front end of the carbonized lithium precipitator.

[0060] (4) Centrifugation: After the wastewater is evaporated and concentrated, white crystals are precipitated and concentrated at the bottom of the crystallizer. They flow out from the bottom pipe of the crystallizer and enter the circulating discharge pump for transportation. A pipeline density meter is installed on the discharge pipeline. After passing through the density meter, it is divided into two pipelines, one returning to the bottom of the crystallizer and the other to the double-stage thickener. When the pipeline density meter shows 1.20-1.22 g / cm³, the sodium sulfate concentration is about 355-380 g / L. The circulation valve is closed to 30%, the discharge valve is opened to 50%, and the material is continuously discharged to the double-stage thickener. After being concentrated by the thickener, the supernatant (secondary lithium mother liquor) overflows and is discharged from the top of the thickener side, with a flow rate of 5m³ / h. The crystalline solid is enriched at the bottom of the thickener, and the thickening kettle bottom valve is continuously opened at a small opening to discharge the material to the vertical scraper centrifuge (using the PZ1250 model). After centrifugation, lithium carbonate wet salt is obtained, which is washed with water and mixed with the primary carbonization precipitation lithium wet salt, and dried with a disk dryer, and its lithium carbonate content is ≥99.6%. In this evaporation and centrifugation section, the evaporator, crystal separator, circulation pipeline and discharge pipeline and other parts in contact with the liquid are mirror plated or polished, with a minimum requirement of Ra0.4; the evaporator, crystal separator and circulation pipeline are all equipped with ultrasonic generators.

[0061] (5) Acid adjustment: The secondary lithium mother liquor flow rate is 5m³ / h. The mother liquor is collected and stored in a mother liquor tank with stirring and insulation. It is transferred to the acid adjustment tank A / B through the acid adjustment feed pump. The acid adjustment processing capacity of a single tank is 20m³. 560Kg of 98% concentrated sulfuric acid is evenly injected into the single tank to react with lithium carbonate and sodium carbonate for acid adjustment. The acid adjustment tank is equipped with a stirring drive device. By designing a mixing and defoaming agitator, the reaction is stable and efficient. The single batch reaction time is short and controlled at about 45 minutes. After the reaction is completed, the solution pH is 5-6.

[0062] (6) pH adjustment: The acid-adjusted liquid is stored in the middle tank and transported to the pH adjustment tank A / B through the pH adjustment feed pump. The processing capacity of a single tank is 20m³. 10 kg of sodium hydroxide flakes are added to the single tank. The pH value change is observed. After stabilization, its value is not less than 9. The pH adjustment tank is designed and equipped with a stirring drive device. By designing a high-efficiency mixing multi-layer agitator, the single batch adjustment time is ensured to be less than 30 minutes.

[0063] (7) Preheating 2: The pH-adjusted liquid (hereinafter referred to as sodium precipitation raw water) is transported to the condensate (i.e., condensate produced by evaporation) preheating plate heat exchanger through a feed pump for primary preheating, and then enters the steam preheating shell-and-tube heat exchanger. The temperature after preheating is interlocked with the raw steam regulating valve to control the temperature of the wastewater after heat exchange, i.e., the feed temperature, to 90°C, to meet the bubble point feed requirement.

[0064] (8) MVR evaporation 2: This system is designed as an evaporation system with two-stage evaporators in series and a forced circulation pump external circulation. After preheating 2, the wastewater enters the upper part of the crystal separator, and the raw water flows out from the middle pipe of the crystal separator and enters the top of the first-stage tube-and-tube evaporator. The area of ​​the first-stage heater is 220 m2. The wastewater solution is inside the tube and the secondary steam (raw steam during the startup period) is outside the tube. After passing through the first-stage heater, it flows out from the bottom and enters the forced circulation pump on the bottom pipe. After being pressurized by the forced circulation pump, it enters the bottom of the second-stage tube-and-tube evaporator. The area of ​​the second-stage heater is 330 m2. The wastewater solution is inside the tube and the secondary steam (raw steam during the startup period) is outside the tube. After passing through the second-stage heater, it flows out from the top and returns to the upper part of the crystal separator. The steam generated after entering the crystal separator rises and the liquid sinks. The rising steam is extracted from the top of the crystal separator and enters the centrifugal steam compressor after passing through the steam scrubber. After compression, the temperature of this steam rises by 16°C to 106°C, which is called secondary steam. This secondary steam is discharged from the compressor outlet and enters the shell side of the first and second stage tube-in-shell evaporator for evaporation and heating. After condensation, it is discharged from the bottom of the shell side of the first and second stage tube-in-shell evaporator, which is called steam condensate. The amount of this condensate is related to the evaporation capacity of the MVR evaporation 2 system. The maximum evaporation rate designed in this embodiment is 5.5m³ / h. The condensate is transported by the condensate discharge pump, and after heat exchange with the feed raw water, it is discharged from the system and reused at the front end of the carbonized lithium precipitate or used for cleaning.

[0065] (9) Centrifugation: After the wastewater is evaporated and concentrated, white crystals are precipitated and concentrated at the bottom of the crystallizer. They flow out from the bottom pipe of the crystallizer and enter the circulating discharge pump for transportation. A sampling port is set on the discharge pipe, which is divided into two pipelines, one returning to the bottom of the crystallizer and the other to the double-stage thickener. When the solid-liquid ratio reaches 15% to 20% during regular sampling, the circulation valve is closed to 30%, the discharge valve is opened to 50%, and the discharge is continuously discharged to the thickener. After concentration by the thickener, the supernatant (sodium mother liquor) overflows from the top of the thickener side and is discharged into the mother liquor tank. The crystalline solid is concentrated at the bottom of the thickener. The thickening kettle bottom valve is continuously opened with a small opening to discharge the material to the centrifuge (HR500 double-stage piston pusher centrifuge is used in the design). Sodium sulfate wet salt is obtained after centrifugation. It is transported to the vibrating fluidized bed for drying and packaging by ton package machine. A water curtain dust removal system is set up. The sodium sulfate salt output is 1680kg / h. It is sampled regularly for testing. The quality control indicators are water content <0.5% and lithium content <0.1%. After centrifugation, the mother liquor is discharged into the mother liquor tank and transported by the sodium precipitation mother liquor circulation discharge pump. It is divided into two pipelines, one returning to the system and the other discharging to the rear end refrigeration system. A flow meter is set on the discharge pipeline for measurement. The average discharge speed is 0.45m³ / h. In fact, it is intermittently discharged according to the discharge situation, and the average discharge volume is controlled to be 10.8m³ / day.

[0066] (10) Frozen crystallization: The sodium precipitation mother liquor is heat exchanged with the MVR evaporator 2 sodium precipitation raw water preheating plate through the sodium precipitation mother liquor circulating discharge pump. The temperature of the sodium precipitation mother liquor is reduced to 50°C, and then enters the mother liquor transfer tank. It is transported to the circulating water heat exchange cooler through the frozen crystallization feed pump and cooled to 35-38°C (not too low to avoid the precipitation of sodium sulfate), and then enters the frozen crystallization kettle A / B. The processing capacity of a single kettle is 5m³. Frozen crystallization is carried out. The cold source is a 50kW variable frequency refrigeration unit with an adjustable refrigeration temperature of -5°C to 5°C. The inner wall of the frozen crystallization kettle is polished and is designed as a jacketed crystallization kettle with stirring. The single kettle frozen crystallization time is controlled at 3h. When the freezing time reaches the requirement, no more crystals are precipitated in the kettle, and the frozen crystallization is completed.

[0067] (11) Centrifugation: After the freezing crystallization is completed, the mixed slurry in the freezing crystallization kettle is transported to a high-level double-stage piston pusher centrifuge (HR400 double pusher) through a discharge pump for centrifugal dehydration to obtain sodium sulfate decahydrate. The average output of sodium sulfate decahydrate is 380kg / h, and a single freezing crystallization kettle produces 4200kg of sodium sulfate decahydrate. The average amount of mother liquor after freezing is 0.235m³ / h, and a single freezing crystallization kettle produces about 2.6m³ of mother liquor. The Li + The concentration is 26g / L, Na + The concentration is 14.6 g / L. The sodium sulfate decahydrate is reversely dissolved and enters the MVR evaporation 2 system again, and the mother liquor is transported to the front end of the carbonization lithium precipitation for reuse, or optionally enters the back-end nanofiltration membrane system.

[0068] (12) Nanofiltration membrane: The main process of carbonization lithium precipitation section uses sodium carbonate for lithium precipitation operation. Because the sodium carbonate preparation process cannot avoid the residual sodium chloride salt, it brings in chloride ion impurities. Since this recovery is a zero-emission process, the chloride ions will be enriched. In this embodiment, the concentration of chloride ions in the mother liquor after freezing reaches 1280ppm, and the flow rate is 5.64m³ / day. This embodiment is equipped with a set of single-stage nanofiltration membranes, which can be selected for high-pressure filtration. The interception rate is a low value of 90%, and the sodium chloride concentrated water produced is 30% of the inlet water, that is, when the frozen mother liquor passes through the membrane system, the sodium chloride concentrated water produced is about 1.7m³ / day, and the dilute water and wash water are transported to the front-end carbonization lithium precipitation section.

[0069] (13) Evaporation of impure salts: The sodium chloride concentrate produced by the nanofiltration membrane is stored in the concentrate intermediate tank, and is transported by a delivery pump to the non-condensable gas of the MVR system through a plate heat exchanger for preheating, and then to a jacketed impure salt evaporator with stirring for drying and evaporation. In this embodiment, the heat source in the jacket of the impure salt evaporator is raw steam, and the reactor is connected to the compressor inlet in the front-end MVR process. The steam generated is pumped into the compressor for reuse. At the same time, the compressor can extract a certain vacuum to make its evaporation temperature around 90°C, thereby improving the evaporation efficiency. After continuous concentration and drying, slurry is generated, and a sample is taken to measure its solid-liquid ratio. When it reaches 20%, it is discharged from the bottom of the evaporator into a centrifuge for centrifugal dehydration, and the impure salt is packaged and collected, and the mother liquor continues to return to the impure salt evaporator for evaporation.

[0070] Comparative Example 1 In a lithium iron phosphate battery recycling system, after the carbonization reaction, the Na in the raw water composed of the mother liquor and the washing water is detected. + The concentration is 60.3 g / L, Li + The concentration is 2.46 g / L, SO 4 2- The concentration is 121.7 g / L, CO 3 2- The concentration is 13.2 g / L, Cl - 30ppm, that is, Na in raw water 2 SO 4 About 180g / L, residual Na 2 CO 3 About 4.6g / L, Li 2 CO 3 It is about 13g / L, and NaCl is about 60mg / L.

[0071] (1) As a comparison with Example 1, another portion of the raw water was taken and added with H according to the common process, i.e., acid adjustment followed by evaporation. 2 SO 4 It reacts with lithium carbonate and residual sodium carbonate, and is stirred thoroughly to adjust the pH value to 5, producing lithium sulfate and sodium sulfate, consuming 22 g of sulfuric acid, and the volume of the solution is 1022 ml.

[0072] (2) Add sodium carbonate flakes to adjust the pH to 9 (trace amount, 0.04 g).

[0073] (3) Pour the solution into a flask and evaporate it at room temperature until Li + The concentration is 14g / L (i.e. Li 2 SO 4 110g / L) Stop concentration, keep warm, filter and dry. At this time, the residual liquid volume is about 175ml. Detect Na in the residual liquid + The concentration is about 129.6g / L, and about 110g of sodium sulfate solid is obtained. The Li content is detected. + About 0.02%.

[0074] (4) The solution was placed in a freezer and maintained at 0°C. After 1.5 hours, it was taken out and filtered to obtain 148 g of sodium sulfate decahydrate (glauber's salt). + The concentration detected was 26g / L.

[0075] Based on the above, we can know that taking 1L of wastewater from lithium iron phosphate battery after carbonization and precipitation of lithium, ignoring the influence of trace impurities and experimental errors, the results are as follows: The conventional process recovers 110g of sodium sulfate, produces 175ml of residual liquid, enters the freezing crystallization section, consumes 22g of sulfuric acid, consumes 0.04g of sodium hydroxide flakes, and produces 133g of Glauber's salt to dissolve.

[0076] The process of the present invention recovers 8.9g of lithium carbonate and 164g of sodium sulfate, generates 56ml of residual liquid to enter the freezing crystallization section, consumes 8g of sulfuric acid, consumes 0.02g of sodium hydroxide flakes, and generates 52g of mirabilite to dissolve.

[0077] Therefore, it can be seen that the present invention has high recovery value, can directly recover 68% of lithium carbonate in wastewater, the by-product sodium sulfate is 1.5 times that of the conventional process, the raw material consumption is low, the sulfuric acid consumption is about 36% of the conventional process, the alkali consumption is about 50% of the conventional process, and the by-product Glauber's salt that needs to be re-dissolved is only 39% of the conventional process.

[0078] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims

1. A method for treating lithium-containing wastewater generated in an acid leaching carbonization lithium precipitation process, characterized in that: The following steps are involved: S1, mixing the lithium carbide precipitation mother liquor and the lithium carbide precipitation washing water produced in the lithium carbide precipitation section to obtain raw water; S2, preheating the raw water and evaporating it until lithium carbonate is precipitated, maintaining the system balance after the sodium sulfate in the solution is close to saturation, and dehydrating the material to obtain solid lithium carbonate and secondary lithium mother liquor; S3, adding concentrated sulfuric acid to the secondary lithium mother liquor to convert the carbonate in the solution into sulfate, and then adding sodium hydroxide to adjust to weak alkalinity to obtain sodium precipitation raw water; S4, evaporating the sodium precipitation raw water until sodium sulfate is precipitated, maintaining the system balance, and dehydrating the material to obtain solid sodium sulfate and sodium precipitation mother liquor; S5, freezing and crystallizing the sodium precipitation mother liquor until sodium sulfate crystals are precipitated, and obtaining sodium sulfate crystals and frozen centrifugal mother liquor after separation, and returning the frozen centrifugal mother liquor to the front end of carbonization lithium precipitation, and re-performing lithium precipitation operation.

2. The method for treating lithium-containing wastewater produced in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S1, the raw water contains Na2SO4 of 150-180 g / L, Na2CO3 of 2-9 g / L, Li2CO3 of 11-13 g / L, and NaCl of 10-100 mg / L.

3. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S2, the solution density is 1.20-1.22 g / cm³, which is the discharge salt concentration, and the sodium sulfate concentration is 360-370 g / L.

4. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S2, the raw water is evaporated at 83-87°C; in step S4, the sodium precipitation raw water is evaporated at 88-92°C.

5. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S4, after evaporation to a solid-liquid volume ratio of 15-20%, the system feed and evaporation rate are kept balanced, and then the material is further concentrated and centrifuged for dehydration, and the solid sodium sulfate is dried and used as industrial sodium sulfate, wherein the lithium content of the industrial sodium sulfate is less than 0.1%.

6. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S4, when the concentration of lithium sulfate in the sodium precipitation mother liquor reaches 110-125 g / L, part of the sodium precipitation mother liquor is refluxed to the evaporation system, and the remaining part is subjected to freeze crystallization treatment.

7. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S5, the temperature of the freeze crystallization is -5°C to 5°C.

8. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S5, the freezing crystallization time is 1.5 to 3 hours.

9. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S5, the mother liquor of the frozen centrifuge is dechlorinated by the nanofiltration membrane and then returned to the front end of the carbonization lithium precipitation, and the lithium precipitation operation is repeated. The sodium chloride concentrated water produced by the dechlorination of the nanofiltration membrane is dried and evaporated.

10. The method for treating lithium-containing wastewater generated in the acid leaching carbonization lithium precipitation process according to claim 1, characterized in that: In step S2, during the evaporation process, an evaporation system in which a two-stage evaporator is connected in series and a forced circulation pump is used for external circulation is used to evaporate lithium carbonate; in the evaporation system, the evaporator, the crystal separator, the circulation pipeline and the discharge pipeline are mirror plated or polished; and an ultrasonic generator is provided in the evaporator, the crystal separator and the circulation pipeline.

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