A system and method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate

Abstract

The present invention provides a system and method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. The system includes a coagulation and precipitation system, a filtration system, an adsorption and desorption system, a calcium and magnesium removal system, a boron removal system, a concentration system, an evaporation system, and a lithium precipitation system connected in sequence. The calcium and magnesium removal system includes a first-stage reverse osmosis concentration unit, a first-stage nanofiltration calcium and magnesium removal unit, a second-stage reverse osmosis concentration unit, a multi-stage nanofiltration calcium and magnesium removal unit, and a calcium and magnesium ion exchange unit connected in sequence. The boron removal system includes a first-stage nanofiltration boron removal unit, a multi-stage nanofiltration boron removal unit, and a boron ion exchange unit connected in sequence. After the lithium precipitation system, there are also a precision filtration system and a water washing and drying system connected in sequence. By adopting the process of the present invention, the lithium recovery rate is improved, and the product is battery-grade lithium carbonate, greatly improving the resource utilization and recovery rate of the lithium extraction system, reducing the amount of externally added water and the consumption of sodium carbonate in the process of the lithium precipitation system.

Description

Technical Field

[0001] The present invention belongs to the field of lithium extraction from salt lakes, and particularly relates to a system and method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Background Art

[0002] Lithium is the most important energy metal and an indispensable strategic resource in modern industry, playing an important role in industries such as battery chemistry, glass ceramics, aviation metals, nuclear industry, lubricating greases, and refrigerants. Under the global climate ambition, low-carbon and carbon reduction have become the global consensus and trend; especially in recent years, with the explosive development of lithium batteries, the global lithium consumption has been in a rapid expansion state. At present, most domestic lithium salts are extracted from ores. However, with the continuous reduction of high-grade lithium ores and the continuous increase in the cost of lithium extraction from ores, since salt lakes are rich in a large amount of lithium elements, lithium extraction from salt lakes has obvious resource and cost advantages compared with lithium extraction from ores. Therefore, extracting lithium from salt lakes has become an inevitable trend in lithium resource development.

[0003] The key to lithium extraction from salt lakes is to enrich lithium ions from salt lake brine at a relatively low cost and precipitate them into lithium carbonate or lithium hydroxide, etc., and produce additional high-value products. There are two main influencing factors for lithium extraction from salt lakes: one is the lithium content in the salt lake. The lower the lithium content, the longer the treatment process flow, the greater the evaporation after treatment, and the relatively higher the cost; the other is the proportional relationship of various mineral ions in the salt lake, especially the magnesium-lithium ratio and boron-lithium ratio. Generally, the smaller the magnesium-lithium ratio and boron-lithium ratio, the better. At present, for directly extracting lithium from raw salt lake brine and producing lithium carbonate, the main treatment methods include: solar evaporation method, solar pond method, solvent extraction method, precipitation method, calcination method, adsorption method, membrane method, electrodialysis method, etc.

[0004] The solar evaporation method and the solar pond method are applicable to salt lake brines with a small magnesium-lithium ratio, and their efficiency is low and the lithium recovery rate is low; the extraction agent used in the solvent extraction method is an organic solvent. The use of a large amount of organic solvents will not only seriously corrode the equipment, but also cause pollution to the surrounding environment; the electrodialysis method has high energy consumption and low water recovery rate. The calcination method uses lithium-containing brine after potassium and boron extraction as raw material, evaporates water to obtain lithium-containing magnesium chloride tetrahydrate, sprays and dries and calcines to obtain lithium-containing magnesium oxide, adds water to wash and filter to leach lithium, uses lime milk to remove impurities such as calcium and magnesium, evaporates and concentrates the solution to about 2% Li content, and adds soda ash to precipitate lithium carbonate. The lithium recovery rate is about 90%. It has high energy consumption and pollutes the environment.

[0005] With the large-scale utilization of lithium resources, providing a method for extracting lithium from salt lake brine with low production cost, small environmental pollution, low energy consumption, high lithium recovery rate, and simultaneously extracting and producing additional high-value other products from salt lakes to achieve the comprehensive utilization of salt lake resources has become a technical problem that urgently needs to be solved by researchers in this field. Summary of the Invention

[0006] The object of the present invention is to provide a system and method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. By using the process of the lithium extraction system from salt lake brine of the present invention to prepare lithium carbonate, the lithium recovery rate is improved and the product is battery-grade lithium carbonate, greatly improving the resource utilization and recovery rate of the lithium extraction system, reducing the amount of external water used and the consumption of sodium carbonate in the lithium precipitation system treatment process.

[0007] To achieve the object of this invention, the following technical solutions are adopted in the present invention:

[0008] In the first aspect, the present invention provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, which system includes a coagulation and precipitation system, a filtration system, an adsorption and desorption system, a calcium and magnesium removal system, a boron removal system, a concentration system, an evaporation system, and a lithium precipitation system connected in sequence;

[0009] The calcium and magnesium removal system includes a first-stage reverse osmosis concentration unit, a first-stage nanofiltration calcium and magnesium removal unit, a second-stage reverse osmosis concentration unit, a multi-stage nanofiltration calcium and magnesium removal unit, and a calcium and magnesium ion exchange unit connected in sequence;

[0010] The water outlet of the adsorption and desorption system is connected to the water inlet of the first-stage reverse osmosis concentration unit;

[0011] The concentrated water outlet of the multi-stage nanofiltration calcium and magnesium removal unit is connected to the water inlet of the first-stage reverse osmosis concentration unit;

[0012] The boron removal system includes a first-stage nanofiltration boron removal unit, a multi-stage nanofiltration boron removal unit, and a boron ion exchange unit connected in sequence;

[0013] The water production outlet of the calcium and magnesium ion exchange unit is connected to the water inlet of the first-stage nanofiltration boron removal unit;

[0014] The concentrated water outlet of the multi-stage nanofiltration boron removal unit is connected to the water inlet of the first-stage nanofiltration boron removal unit;

[0015] The water production of the first-stage reverse osmosis concentration unit, the water production of the second-stage reverse osmosis concentration unit, the water production of the concentration system, and the distilled water of the evaporation system are all treated by a third-stage reverse osmosis concentration unit and then recycled;

[0016] The lithium precipitation system includes a sodium carbonate dosing unit and a lithium precipitation reaction unit; the lithium precipitation system is used to produce battery-grade lithium carbonate.

[0017] The present invention further optimizes the lithium extraction system. The various systems form an overall part-internal circulation whole, greatly improving the resource utilization and recovery rate of the lithium extraction system, reducing the consumption of sodium carbonate in the lithium precipitation system, saving system water and electricity, reducing energy consumption and material consumption, reducing environmental pollution, maximizing the comprehensive utilization of salt lake brine, and achieving good economic, social and environmental benefits.

[0018] As a preferred technical solution of the present invention, the coagulation sedimentation system includes a coagulation reaction unit, a flocculation reaction unit and a sedimentation unit connected in sequence.

[0019] Preferably, the filtration system includes a filtration unit, an ultrafiltration unit, a first impurity removal nanofiltration unit and a second impurity removal nanofiltration unit connected in sequence.

[0020] Preferably, the sludge outlet of the sedimentation unit and the backwash water outlet of the filtration unit are both connected to the dehydration system.

[0021] Preferably, the filtrate outlet of the dehydration system is connected to the water inlet of the coagulation reaction unit.

[0022] Preferably, the concentrated water outlet of the second impurity removal nanofiltration unit is connected to the water inlet of the first impurity removal nanofiltration unit.

[0023] Preferably, the first impurity removal nanofiltration unit includes an acid addition device, a security filtration device and a nanofiltration membrane device connected in sequence.

[0024] Preferably, the adsorption and desorption system includes an adsorption device and a desorption device.

[0025] As a preferred technical solution of the present invention, the calcium and magnesium removal system further includes a three-stage reverse osmosis concentration unit and a second-stage nanofiltration calcium and magnesium removal unit.

[0026] Preferably, the concentrated water outlet of the first-stage nanofiltration calcium and magnesium removal unit is connected to the water inlet of the second-stage nanofiltration calcium and magnesium removal unit.

[0027] Preferably, the water production outlet of the second-stage nanofiltration calcium and magnesium removal unit is connected to the water inlet of the second-stage reverse osmosis concentration unit.

[0028] Preferably, the concentrated water outlet of the three-stage reverse osmosis concentration unit is connected to the water inlet of the first-stage nanofiltration calcium and magnesium removal unit.

[0029] Preferably, the first-stage reverse osmosis concentration unit includes an acid addition device, a security filtration device and a reverse osmosis device connected in sequence.

[0030] Preferably, the first-stage nanofiltration calcium and magnesium removal unit includes an acid addition device, a security filtration device and a nanofiltration membrane device connected in sequence.

[0031] In the present invention, the produced water treated by the three-stage reverse osmosis concentration unit can be used for the water for the adsorption and desorption system of the lithium extraction system, the water for membrane dialysis, the top feed water for the calcium and magnesium ion removal ion exchange unit and the boron ion removal ion exchange unit, the water for acid and base dilution, the low-pressure flushing water for the membrane system and the chemical cleaning water, the water for preparing medicines in the lithium extraction system, etc.

[0032] Preferably, the calcium and magnesium ion removal ion exchange unit includes an alkali adding device, an ion exchange device and a resin regeneration device connected in sequence.

[0033] In the present invention, by setting the combination of reverse osmosis, nanofiltration and ion exchange, the calcium and magnesium ions in the solution can be completely removed, and the produced water can be recycled for the lithium extraction system.

[0034] As a preferred technical solution of the present invention, the boron removal system further includes a first-stage reverse osmosis boron removal unit, a second-stage reverse osmosis boron removal unit and a third-stage reverse osmosis boron removal unit connected in sequence.

[0035] Preferably, the concentrated water outlet of the first-stage nanofiltration boron removal unit is connected to the water inlet of the first-stage reverse osmosis boron removal unit.

[0036] Preferably, the concentrated water of the third-stage reverse osmosis boron removal unit is used to prepare borax.

[0037] Preferably, the boron removal system further includes a second-stage reverse osmosis boron removal unit, the concentrated water outlet of which is connected to the water inlet of the first-stage nanofiltration boron removal unit, and the produced water outlet of which is connected to the water inlet of the second-stage reverse osmosis boron removal unit.

[0038] Preferably, the concentrated water outlets of the first-stage reverse osmosis boron removal unit and the second-stage reverse osmosis boron removal unit are both connected to the water inlet of the second-stage reverse osmosis boron removal unit.

[0039] Preferably, the first-stage nanofiltration boron removal unit includes an alkali adding device, a security filtration device and a nanofiltration membrane device connected in sequence.

[0040] Preferably, the first-stage reverse osmosis boron removal unit includes an acid adding device, a dialysis device, a filtration device and a reverse osmosis device, wherein the dialysis inlet flow rate in the dialysis device is 3-15 times the inlet flow rate of the first-stage reverse osmosis boron removal unit, and a multi-stage fractional dialysis method is adopted.

[0041] Preferably, the third-stage reverse osmosis boron removal unit includes an alkali adding device, a security filtration device and a reverse osmosis device connected in sequence.

[0042] Preferably, the multi-stage nanofiltration boron removal unit includes an alkali adding device, a security filtration device and at least a two-stage nanofiltration membrane device connected in sequence.

[0043] Preferably, the boron removal ion exchange unit includes an alkali adding device, an ion exchange device, and a resin regeneration device connected in sequence.

[0044] In the present invention, by setting a combination of nanofiltration, reverse osmosis, and ion exchange, boron ions in the solution can be completely removed, and the boron recovery rate is increased.

[0045] As a preferred technical solution of the present invention, the concentration system includes a high-pressure reverse osmosis unit or an electrodialysis unit.

[0046] Preferably, the high-pressure reverse osmosis unit is a high-pressure spiral wound reverse osmosis membrane module or a DTRO disk tube membrane module.

[0047] As a preferred technical solution of the present invention, a precision filtration system and a water washing and drying system are further included after the lithium precipitation system, which are connected in sequence.

[0048] Preferably, the system further includes a lithium precipitation mother liquor recycling system.

[0049] Preferably, the lithium precipitation mother liquor recycling system includes a heat exchange unit, a first-stage nanofiltration carbonate recovery unit, and a second-stage nanofiltration carbonate recovery unit connected in sequence.

[0050] Preferably, the mother liquor outlet of the lithium precipitation system is connected to the water inlet of the heat exchange unit.

[0051] Preferably, the concentrated water outlets of the first-stage nanofiltration carbonate recovery unit and the second-stage nanofiltration carbonate recovery unit are both connected to the water inlet of the lithium precipitation system.

[0052] Preferably, the water produced by the second-stage nanofiltration carbonate recovery unit is recycled to the first-stage reverse osmosis concentration unit.

[0053] In the present invention, by setting a carbonate recovery unit, the consumption of sodium carbonate in the lithium precipitation system treatment process is reduced.

[0054] In a second aspect, the present invention provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. The method uses the system described in the first aspect and includes the following steps:

[0055] (1) The salt lake brine is sequentially subjected to a precipitation reaction in a coagulation and precipitation system, a filtration treatment in a filtration system, and an adsorption and desorption treatment in an adsorption and desorption system to obtain a lithium chloride solution;

[0056] (2) The lithium chloride solution described in step (1) is sequentially passed through a first-stage reverse osmosis concentration unit, a first-stage nanofiltration calcium and magnesium removal unit, a second-stage reverse osmosis concentration unit, a multi-stage nanofiltration calcium and magnesium removal unit, and a calcium and magnesium ion exchange unit to remove calcium and magnesium ions;

[0057] (3) The lithium chloride solution after removing calcium and magnesium ions in step (2) is successively treated through a primary nanofiltration boron removal unit, a multi-stage nanofiltration boron removal unit, and a boron removal ion exchange unit to remove boron ions.

[0058] (4) The lithium chloride solution after removing boron ions in step (3) is successively concentrated through a concentration system and evaporated through an evaporation system. Finally, after adding sodium carbonate through a sodium carbonate dosing unit and reacting in a lithium precipitation reaction unit, battery-grade lithium carbonate is obtained.

[0059] Using the system provided by the present invention, lithium elements can be extracted from salt lake brine, and the prepared battery-grade lithium carbonate meets the standards specified in YS / T 582-2013.

[0060] As a preferred technical solution of the present invention, in step (1), the content of lithium ions in the salt lake brine is 600-1200 ppm, the content of magnesium ions is 2000-6000 ppm, the content of sodium ions is 25000-50000 ppm, the content of calcium ions is ≤500 ppm, the content of potassium ions is ≤8000 ppm, the content of carbonate ions is ≤400 ppm, the content of sulfate ions is 10000-20000 ppm, and the content of boron ions is ≤600 ppm.

[0061] Preferably, the filtration rate of the filtration treatment in step (1) is 5-15 m / h.

[0062] Preferably, the adsorbent for the adsorption and desorption treatment in step (1) includes any one of aluminum-based lithium adsorbents, manganese-based lithium adsorbents, or titanium-based lithium adsorbents.

[0063] Preferably, before the lithium chloride solution enters the primary reverse osmosis concentration unit and the primary nanofiltration calcium and magnesium removal unit in step (2), the pH is adjusted to 3-6.

[0064] Preferably, before the lithium chloride solution enters the calcium and magnesium ion exchange unit in step (2), the pH is adjusted to 7-10.

[0065] As a preferred technical solution of the present invention, before the lithium chloride solution enters the primary nanofiltration boron removal unit and the multi-stage nanofiltration boron removal unit in step (3), the pH is adjusted to 9.2-11.

[0066] Preferably, before the lithium chloride solution enters the boron removal ion exchange unit in step (3), the pH is adjusted to 9.2-10.

[0067] Preferably, the mass ratio of sodium carbonate to lithium chloride in step (4) is (1-1.2):1.

[0068] Preferably, the pH of the lithium chloride solution after adding sodium carbonate in step (4) is 9-12.

[0069] Preferably, the temperature of the reaction in step (4) is 80-99 °C.

[0070] As a preferred technical solution of the present invention, after the reaction in the lithium precipitation reaction unit in step (4), a lithium carbonate solution and a lithium precipitation mother liquor are obtained.

[0071] Preferably, the lithium carbonate solution is sequentially treated by a precision filtration system and a water washing and drying system to obtain battery-grade lithium carbonate.

[0072] Preferably, the lithium precipitation mother liquor is sequentially separated and recovered of carbonate ions and lithium chloride solution through heat exchange by a heat exchange unit, a first-stage nanofiltration carbonate recovery unit, and a second-stage nanofiltration carbonate recovery unit.

[0073] Preferably, the temperature after heat exchange is 5-40 °C.

[0074] Compared with the prior art, the present invention has the following beneficial effects:

[0075] The present invention deeply treats low-magnesium-lithium ratio salt lake brine. Through the comprehensive application of various processes and the regulation of pH, lithium elements are maximally extracted from the salt lake brine to produce battery-grade lithium carbonate, and boron elements are extracted to produce borax; carbonate ions are extracted from the mother liquor of the lithium precipitation system and recycled to the lithium precipitation system, reducing the consumption of sodium carbonate in the production process of the lithium precipitation system; the water resources in the salt lake brine are maximally recovered and recycled to the lithium extraction system, reducing the consumption of external water, and realizing the comprehensive development and utilization of the deep resources of the salt lake brine. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 It is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate provided in Example 1;

[0077] Figure 2 It is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate provided in Example 2;

[0078] Figure 3 It is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate provided in Example 3;

[0079] Figure 4 It is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate provided in Example 4;

[0080] Figure 5 It is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate provided in Example 5;

[0081] Among them, 10 - coagulation sedimentation system, 20 - filtration unit, 30 - ultrafiltration unit, 40 - primary impurity removal nanofiltration unit, 50 - secondary impurity removal nanofiltration unit, 60 - adsorption and desorption system, 70 - primary reverse osmosis concentration unit, 80 - primary nanofiltration calcium and magnesium removal unit, 90 - secondary reverse osmosis concentration unit, 100 - multi - stage nanofiltration calcium and magnesium removal unit, 110 - calcium and magnesium removal ion exchange unit, 120 - primary nanofiltration boron removal unit, 130 - multi - stage nanofiltration boron removal unit, 140 - boron removal ion exchange unit, 150 - concentration system, 160 - evaporation system, 170 - lithium precipitation system, 180 - precision filtration system, 190 - water washing and drying system, 11 - dehydration system, 81 - secondary nanofiltration calcium and magnesium removal unit, 82 - tertiary reverse osmosis concentration unit, 121 - primary reverse osmosis boron removal unit, 122 - secondary reverse osmosis boron removal unit, 123 - tertiary reverse osmosis boron removal unit, 124 - evaporation and crystallization device, 125 - secondary reverse osmosis boron removal unit, 171 - heat exchange unit, 172 - primary nanofiltration carbonate recovery unit, and 173 - secondary nanofiltration carbonate recovery unit. Detailed implementation manners

[0082] The technical solution of the present invention will be further described below through specific implementation manners. Those skilled in the art should understand that the embodiments are only to help understand the present invention and should not be regarded as specific limitations to the present invention.

[0083] The present invention provides a system for extracting lithium from salt lake brine and preparing battery - grade lithium carbonate. The system includes a coagulation sedimentation system, a filtration system, an adsorption and desorption system, a calcium and magnesium removal system, a boron removal system, a concentration system, an evaporation system, and a lithium precipitation system connected in sequence;

[0084] The calcium and magnesium removal system includes a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multi - stage nanofiltration calcium and magnesium removal unit, and a calcium and magnesium removal ion exchange unit connected in sequence;

[0085] The water outlet of the adsorption and desorption system is connected to the water inlet of the primary reverse osmosis concentration unit;

[0086] The concentrated water outlet of the multi - stage nanofiltration calcium and magnesium removal unit is connected to the water inlet of the primary reverse osmosis concentration unit;

[0087] The boron removal system includes a primary nanofiltration boron removal unit, a multi - stage nanofiltration boron removal unit, and a boron removal ion exchange unit connected in sequence;

[0088] The water production outlet of the calcium and magnesium removal ion exchange unit is connected to the water inlet of the primary nanofiltration boron removal unit;

[0089] The concentrated water outlet of the multi - stage nanofiltration boron removal unit is connected to the water inlet of the primary nanofiltration boron removal unit;

[0090] The product water of the primary reverse osmosis concentration unit, the product water of the secondary reverse osmosis concentration unit, the product water of the concentration system, and the distilled water of the evaporation system are all recycled after being treated by the tertiary reverse osmosis concentration unit;

[0091] The lithium precipitation system includes a sodium carbonate dosing unit and a lithium precipitation reaction unit; the lithium precipitation system is used to produce battery-grade lithium carbonate.

[0092] In the present invention, the coagulation and sedimentation system includes a coagulation reaction unit, a flocculation reaction unit, and a sedimentation unit connected in sequence.

[0093] In the present invention, a bactericide dosing device is provided in front of the coagulation reaction unit; the coagulation reaction unit includes a coagulant dosing device; the flocculation reaction unit includes a flocculant dosing device.

[0094] In the present invention, the sedimentation unit includes any one of an inclined plate sedimentation unit, a horizontal flow sedimentation unit, or a vertical flow sedimentation unit.

[0095] In the present invention, the filtration system includes a filtration unit, an ultrafiltration unit, a primary impurity removal nanofiltration unit, and a secondary impurity removal nanofiltration unit connected in sequence.

[0096] In the present invention, the filtration unit includes any one or a combination of at least two of a V-type filter, a quartz sand filtration device, a multi-media filtration device, or a variable gap filtration device.

[0097] In the present invention, the ultrafiltration membrane in the ultrafiltration unit includes any one of polysulfone, polyvinylidene fluoride, polyvinyl chloride, or ceramic membrane.

[0098] In the present invention, the pore size of the ultrafiltration membrane is 0.001 - 0.02 μm, for example, it can be 0.001 μm, 0.005 μm, 0.01 μm, 0.015 μm, or 0.02 μm, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0099] In the present invention, the ultrafiltration unit further includes a pre-filtering device, and the filtering device includes any one of a bag filter, a pleated filter cartridge filter, or a self-cleaning filter, and its filtration accuracy is 50 - 200 μm.

[0100] In the present invention, the sludge outlet of the sedimentation unit and the backwash water outlet of the filtration unit are both connected to the dewatering system.

[0101] In the present invention, the filtrate outlet of the dewatering system is connected to the water inlet of the coagulation reaction unit.

[0102] In the present invention, the dewatering system includes a coagulation reaction, a dosing system, and a dehydrator system connected in sequence.

[0103] In the present invention, the concentrated water outlet of the secondary impurity removal nanofiltration unit is connected to the water inlet of the primary impurity removal nanofiltration unit.

[0104] In the present invention, the primary impurity removal nanofiltration unit includes an acid adding device, a security filtration device, and a nanofiltration membrane device that are connected in sequence.

[0105] In the present invention, the secondary impurity removal nanofiltration unit includes a security filtration device and a nanofiltration membrane device that are connected in sequence.

[0106] In the present invention, the adsorption and desorption system includes an adsorption device and a desorption device.

[0107] In the present invention, the adsorption device includes a continuous rotary disc ion exchange device or a valve array ion exchange device.

[0108] In the present invention, the calcium and magnesium removal system further includes a tertiary reverse osmosis concentration unit and a secondary nanofiltration calcium and magnesium removal unit.

[0109] In the present invention, the concentrated water outlet of the primary nanofiltration calcium and magnesium removal unit is connected to the water inlet of the secondary nanofiltration calcium and magnesium removal unit; the water production outlet of the secondary nanofiltration calcium and magnesium removal unit is connected to the water inlet of the secondary reverse osmosis concentration unit; the concentrated water of the secondary nanofiltration calcium and magnesium removal unit is discharged; the concentrated water outlet of the tertiary reverse osmosis concentration unit is connected to the water inlet of the primary nanofiltration calcium and magnesium removal unit.

[0110] In the present invention, the primary reverse osmosis concentration unit includes an acid adding device, a security filtration device, and a reverse osmosis device that are connected in sequence.

[0111] In the present invention, the primary nanofiltration calcium and magnesium removal unit includes an acid adding device, a security filtration device, and a nanofiltration membrane device that are connected in sequence.

[0112] In the present invention, the secondary nanofiltration calcium and magnesium removal unit includes a nanofiltration dialysis device, a security filtration device, and a nanofiltration membrane device that are connected in sequence.

[0113] In the present invention, the dialysis water of the nanofiltration dialysis device uses pure water with low salt content, and the conductivity of the pure water is ≤100 us / cm; the dialysis inlet flow rate is 3-6 times the inlet flow rate of the secondary nanofiltration, and a multi-stage fractional dialysis method is adopted.

[0114] In the present invention, the secondary reverse osmosis concentration unit includes a security filtration device and a reverse osmosis device that are connected in sequence.

[0115] In the present invention, the multi-stage nanofiltration unit for calcium and magnesium removal includes a security filtration device and at least a two-stage nanofiltration membrane device connected in sequence. The product water of the previous-stage nanofiltration membrane device enters the next-stage nanofiltration membrane device for further treatment; the product water of the last stage of the multi-stage nanofiltration membrane device enters the calcium and magnesium ion exchange unit for treatment; the concentrated water of each stage of the multi-stage nanofiltration membrane device enters the first-stage reverse osmosis concentration unit for treatment.

[0116] In the present invention, the three-stage reverse osmosis concentration unit includes a security filtration device and a reverse osmosis device connected in sequence.

[0117] In the present invention, the calcium and magnesium ion exchange unit includes an alkali addition device, an ion exchange device, and a resin regeneration device connected in sequence.

[0118] In the present invention, the resin used in the resin regeneration device includes any one of gel-type ion exchange resin, macroporous ion exchange resin, carrier-type ion exchange resin, or chelating resin.

[0119] In the present invention, the boron removal system further includes a first-stage reverse osmosis boron removal unit, a second-stage reverse osmosis boron removal unit, and a third-stage reverse osmosis boron removal unit connected in sequence; the concentrated water outlet of the first-stage nanofiltration boron removal unit is connected to the water inlet of the first-stage reverse osmosis boron removal unit; the concentrated water of the third-stage reverse osmosis boron removal unit is used to prepare borax; the concentrated water outlet of the third-stage reverse osmosis boron removal unit is connected to an evaporation device, and the product water of the third-stage reverse osmosis boron removal unit and the distilled water of the evaporation device are both treated by the third-stage reverse osmosis concentration unit and then recycled.

[0120] In the present invention, the evaporation device includes any one or a combination of at least two of a single-effect evaporation device, a multi-effect evaporation device, a low-temperature vacuum evaporation device, a high-temperature vacuum evaporation device, and a crystallizer.

[0121] In the present invention, the boron removal system further includes a second-stage reverse osmosis boron removal unit, whose concentrated water outlet is connected to the water inlet of the first-stage nanofiltration boron removal unit, and whose product water outlet is connected to the water inlet of the second-stage reverse osmosis boron removal unit; the concentrated water outlets of the first-stage reverse osmosis boron removal unit and the second-stage reverse osmosis boron removal unit are both connected to the water inlet of the second-stage reverse osmosis boron removal unit.

[0122] In the present invention, the first-stage nanofiltration boron removal unit includes an alkali addition device, a security filtration device, and a nanofiltration membrane device connected in sequence.

[0123] In the present invention, the first-stage reverse osmosis boron removal unit includes an acid addition device, a dialysis device, a filtration device, and a reverse osmosis device connected in sequence. Among them, the dialysis inlet flow rate in the dialysis device is 3-15 times the inlet flow rate of the first-stage reverse osmosis boron removal unit, and a multi-stage hierarchical dialysis method is adopted.

[0124] In the present invention, the two-stage reverse osmosis boron removal unit includes a security filtration device and a reverse osmosis device connected in sequence.

[0125] In the present invention, the secondary reverse osmosis boron removal unit includes a security filtration device and a reverse osmosis device connected in sequence.

[0126] In the present invention, the three-stage reverse osmosis boron removal unit includes an alkali addition device, a security filtration device and a reverse osmosis device connected in sequence.

[0127] In the present invention, the multi-stage nanofiltration boron removal unit includes an alkali addition device, a security filtration device and a nanofiltration membrane device with at least two stages connected in sequence. The water produced by the previous stage of the nanofiltration membrane device enters the next stage of the nanofiltration membrane device for further treatment; the water produced by the final stage of the multi-stage nanofiltration membrane device enters the boron removal ion exchange unit for treatment; the concentrated water of each stage of the multi-stage nanofiltration membrane device enters the first-stage boron removal nanofiltration unit for treatment.

[0128] In the present invention, the boron removal ion exchange unit includes an alkali addition device, an ion exchange device and a resin regeneration device connected in sequence.

[0129] In the present invention, the concentration system includes a high-pressure reverse osmosis unit or an electrodialysis unit; the high-pressure reverse osmosis unit is a high-pressure spiral wound reverse osmosis membrane module or a DTRO disk tube membrane module.

[0130] In the present invention, a security filtration device is provided in front of the concentration system.

[0131] In the present invention, the evaporation system includes any one of a single-effect evaporation device, a multi-effect evaporation device, a low-temperature vacuum evaporation device or a high-temperature vacuum evaporation device.

[0132] In the present invention, after the lithium precipitation system, there is also a precision filtration system and a water washing and drying system connected in sequence.

[0133] In the present invention, the system also includes a lithium precipitation mother liquor recycling system.

[0134] In the present invention, the lithium precipitation mother liquor recycling system includes a heat exchange unit, a first-stage nanofiltration carbonate recovery unit and a second-stage nanofiltration carbonate recovery unit connected in sequence; the mother liquor outlet of the lithium precipitation system is connected to the water inlet of the heat exchange unit; the concentrated water outlets of the first-stage nanofiltration carbonate recovery unit and the second-stage nanofiltration carbonate recovery unit are both connected to the water inlet of the lithium precipitation system; the water produced by the second-stage nanofiltration carbonate recovery unit is recycled to the first-stage reverse osmosis concentration unit.

[0135] In the present invention, the heat exchange unit includes a plate heat exchange device or a tubular heat exchange device.

[0136] In the present invention, the primary nanofiltration carbonate recovery unit includes a dialysis device, a security filtration device, and a nanofiltration membrane device connected in sequence. Among them, the dialysis inlet water flow rate of the dialysis device is 1-5 times the inlet water flow rate of the primary nanofiltration carbonate recovery unit, and a multi-stage hierarchical dialysis method is adopted; the dialysis water uses pure water with low salt content, and the pure water conductivity ≤ 100 us / cm.

[0137] In the present invention, the secondary nanofiltration carbonate recovery unit includes a security filtration device and a nanofiltration membrane device connected in sequence.

[0138] In the present invention, by setting up the carbonate recovery unit, the consumption of sodium carbonate in the lithium precipitation system treatment process is reduced.

[0139] In the present invention, the pore diameters of the nanofiltration membranes in the nanofiltration membrane device are all 1-2 nm. For example, they can be 1 nm, 1.2 nm, 1.4 nm, 1.6 nm, 1.8 nm, 2 nm, etc., but are not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0140] In the present invention, the filtration accuracies of the security filtration devices (precision filtration devices) are all 1-5 μm. For example, they can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, etc., but are not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0141] The present invention also provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. The method uses the system described in the first aspect and specifically includes the following steps:

[0142] (1) Perform precipitation treatment on the salt lake brine in the coagulation reaction unit, flocculation reaction unit, and precipitation unit to reduce suspended solids (SS), solid particles, and some organic substances in the water;

[0143] In the present invention, the lithium ion content in the brine of the salt lake in step (1) is 600 - 1200 ppm, for example, it can be 600 ppm, 800 ppm, 1000 ppm, 1200 ppm, etc.; the magnesium ion content is 2000 - 6000 ppm, for example, it can be 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, etc.; the sodium ion content is 25000 - 50000 ppm, for example, it can be 25000 ppm, 30000 ppm, 35000 ppm, 40000 ppm, 45000 ppm, 50000 ppm, etc.; the calcium ion content ≤ 500 ppm, for example, it can be 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, etc.; the potassium ion content ≤ 8000 ppm, for example, it can be 1000 ppm, 3000 ppm, 5000 ppm, 7000 ppm, 8000 ppm, etc.; the carbonate content ≤ 400 ppm, for example, it can be 100 ppm, 200 ppm, 300 ppm, 400 ppm, etc.; the sulfate content is 10000 - 20000 ppm, for example, it can be 10000 ppm, 12000 ppm, 14000 ppm, 18000 ppm, 20000 ppm, etc., but it is not limited to the listed values, and other unlisted values within this value range are equally applicable;

[0144] In the present invention, the coagulant in the coagulation reaction unit in step (1) includes any one of polyaluminum chloride, polyferric sulfate or polyacrylamide; the bactericide is an oxidizing bactericide or a non - oxidizing bactericide;

[0145] (2) Feed the water body obtained in step (1) into a filtration unit for filtration treatment to further reduce the SS in the water;

[0146] (3) Feed the water body obtained in step (2) into an ultrafiltration unit for filtration treatment to deeply remove the SS and colloids in the water;

[0147] (4) Feed the water body obtained in step (3) into a primary impurity - removing nanofiltration unit and a secondary impurity - removing nanofiltration unit in sequence for filtration and impurity - removing treatment to further remove divalent ions in the water;

[0148] In the present invention, the pH is adjusted to 3 - 5 before the water body enters the primary impurity - removing nanofiltration unit;

[0149] In the present invention, the filtration rate of the filtration treatment is 5 to 15 m / h. For example, it can be 5 m / h, 7 m / h, 9 m / h, 13 m / h, 15 m / h, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable;

[0150] (5) Feed the water body obtained in step (4) into the adsorption and desorption system for adsorption and desorption treatment, adsorb lithium chloride in the salt lake brine, and separate and remove other impurity components to obtain a lithium chloride solution by desorption;

[0151] In the present invention, the adsorbent in the adsorption and desorption treatment includes any one of aluminum-based lithium adsorbents, manganese-based lithium adsorbents, or titanium-based lithium adsorbents; the desorbing solution can be a normal temperature solution or a desorbing solution with a temperature of 20 - 40 °C. When using an aluminum-based lithium adsorbent, pure water is used for desorption in the desorption device, and the conductivity of the pure water ≤ 100 us / cm; when using a manganese-based or titanium-based lithium adsorbent, an acid solution is used for desorption in the desorption device, and the desorption device is equipped with a heating device or a heat exchange device;

[0152] (6) Feed the lithium chloride solution obtained in step (5) into the first-stage reverse osmosis concentration unit for concentration;

[0153] In the present invention, before the lithium chloride solution enters the first-stage reverse osmosis concentration unit, the pH is adjusted to 3 - 6. For example, it can be 3, 3.5, 4, 4.5, 5, 5.5, or 6, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable;

[0154] (7) Feed the concentrated water obtained in step (6) into the first-stage nanofiltration calcium and magnesium removal unit for calcium and magnesium removal treatment to reduce divalent ions such as calcium and magnesium in the solution;

[0155] In the present invention, before the concentrated water enters the first-stage nanofiltration calcium and magnesium removal unit, the pH is adjusted to 3 - 6. For example, it can be 3, 3.5, 4, 4.5, 5, 5.5, or 6, etc., but is not limited to the listed values. Other unlisted values within this numerical range are equally applicable.

[0156] (8) Feed the concentrated water obtained in step (7) into the second-stage nanofiltration calcium and magnesium removal unit for calcium and magnesium removal treatment to further reduce divalent ions such as calcium and magnesium in the concentrated water solution, recover the lithium chloride solution, and return it to the second-stage reverse osmosis concentration unit for treatment;

[0157] (9) Feed the produced water obtained in step (8) into the second-stage reverse osmosis concentration unit for concentration;

[0158] (10) Feed the concentrated water obtained in step (9) into the multi-stage nanofiltration calcium and magnesium removal unit for calcium and magnesium removal treatment to further reduce divalent ions such as calcium and magnesium in the solution;

[0159] (11) Feed the product water obtained in step (10) into a calcium and magnesium ion exchange unit for calcium and magnesium removal to completely remove calcium and magnesium ions in the solution;

[0160] In the present invention, before the product water enters the calcium and magnesium ion exchange unit, the pH is adjusted to 7-10. For example, it can be 7, 8, 9, 10, etc., but is not limited to the listed values. Other unlisted values within this range are also applicable.

[0161] (12) Feed the product water obtained in step (11) into a primary nanofiltration boron removal unit for boron removal to reduce boron ions in the solution;

[0162] In the present invention, before the product water enters the primary nanofiltration boron removal unit and the multi-stage nanofiltration boron removal unit, the pH is adjusted to 9.2-11. For example, it can be 9.2, 9.5, 10, 10.5, 11, etc., but is not limited to the listed values. Other unlisted values within this range are also applicable.

[0163] (13) Feed the concentrated water obtained in step (12) into a primary reverse osmosis boron removal unit for treatment to recover boron ions in the solution;

[0164] In the present invention, before the concentrated water enters the primary reverse osmosis boron removal unit, the pH is adjusted to 3-6. For example, it can be 3, 3.5, 4, 4.5, 5, 5.5, 6, etc., but is not limited to the listed values. Other unlisted values within this range are also applicable.

[0165] (14) Feed the concentrated water obtained in step (13) into a secondary reverse osmosis boron removal unit for treatment to further recover boron ions in the solution;

[0166] (15) Feed the product water obtained in step (13) into a secondary reverse osmosis boron removal unit for treatment; further recover boron ions in the solution;

[0167] (16) Feed the product water obtained in step (15) into a tertiary reverse osmosis boron removal unit for treatment; further recover boron ions in the solution;

[0168] In the present invention, before the product water enters the tertiary reverse osmosis boron removal unit, the pH is adjusted to 9.2-11. For example, it can be 9.2, 9.4, 9.8, 10, 10.4, 10.8, 11, etc., but is not limited to the listed values. Other unlisted values within this range are also applicable.

[0169] (17) Feed the concentrated water obtained in step (16) into an evaporation system for evaporation crystallization treatment to prepare borax;

[0170] (18) Feed the product water obtained in step (12) into a multi-stage nanofiltration boron removal unit for boron removal to further reduce boron ions in the solution;

[0171] (19) Feed the product water obtained in step (18) into a boron removal ion exchange unit for boron removal treatment to completely remove boron ions in the solution;

[0172] In the present invention, before the product water enters the boron removal ion exchange unit, the pH is adjusted to 9.2 - 10. For example, it can be 9.2, 9.5 or 10, etc., but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0173] (20) Feed the product water obtained in step (19) into a concentration system for concentration treatment to further concentrate the lithium chloride solution;

[0174] (21) Feed the concentrated water obtained in step (20) into an evaporation unit for concentration treatment to further concentrate the lithium chloride solution;

[0175] In the present invention, the temperature of the low - temperature evaporator used in the evaporation unit is 35 - 55°C, and the temperature of the high - temperature evaporator is 85 - 99°C.

[0176] (22) After adding sodium carbonate through a sodium carbonate dosing unit to the concentrated water obtained in step (21) and reacting in a lithium precipitation reaction unit, lithium carbonate precipitate and lithium precipitation mother liquor are obtained;

[0177] In the present invention, the mass ratio of sodium carbonate to lithium chloride in the concentrated water is (1 - 1.2):1. For example, it can be 1:1, 1.1:1, 1.2:1, etc., but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0178] In the present invention, the pH of the concentrated water (lithium chloride solution) after adding sodium carbonate is 9 - 12. For example, it can be 9, 9.5, 10, 10.5, 11, 11.5 or 12, etc., but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0179] In the present invention, the temperature of the reaction is 80 - 99°C. For example, it can be 80°C, 84°C, 88°C, 90°C, 94°C or 99°C, etc., but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0180] (23) Feed the lithium precipitation mother liquor obtained in step (22) into a heat exchange unit for heat exchange treatment to reduce the temperature of the lithium precipitation mother liquor;

[0181] In the present invention, the temperature after the heat exchange treatment is 5 - 40°C. For example, it can be 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C or 40°C, etc., but is not limited to the listed values. Other unlisted values within this range are equally applicable.

[0182] (24) Feed the lithium precipitation mother liquor obtained in step (23) into the first-stage nanofiltration carbonate recovery unit for first-stage nanofiltration carbonate recovery treatment to separate and recover carbonate ions and lithium chloride solution in the solution;

[0183] (25) Feed the produced water obtained in step (24) into the second-stage nanofiltration carbonate recovery unit for second-stage nanofiltration carbonate recovery treatment to further separate and recover carbonate ions and lithium chloride solution in the solution;

[0184] (26) Feed the lithium carbonate precipitation solution obtained in step (22) into the precision filtration system for precision filtration treatment to further remove impurities in the lithium carbonate precipitation solution;

[0185] (27) Feed the lithium carbonate precipitation solution obtained in step (26) into the water washing and drying system for water washing treatment and drying treatment in sequence to obtain battery-grade lithium carbonate product.

[0186] In the present invention, the water washing is as follows: wash the lithium carbonate precipitation with pure water for 3-5 times. The pure water is ultrapure water or distilled water, and the conductivity ≤ 10 us / cm.

[0187] In the present invention, the temperature of the drying is 70-360 °C.

[0188] The numerical ranges described in the present invention not only include the above-listed point values, but also include any point values between the above numerical ranges not listed. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the ranges.

[0189] Example 1

[0190] This example provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. The structural schematic diagram is as Figure 1 shown; the system includes a coagulation and precipitation system 10, a filtration system, an adsorption and desorption system 60, a calcium and magnesium removal system, a boron removal system, a concentration system 150, an evaporation system 160, and a lithium precipitation system 170 connected in sequence;

[0191] The coagulation and precipitation system 10 includes a coagulation reaction unit, a flocculation reaction unit, and a precipitation unit connected in sequence; the sludge outlet of the precipitation unit and the backwash water outlet of the filtration unit 20 are both connected to the dehydration system 11; the filtrate outlet of the dehydration system 11 is connected to the water inlet of the coagulation reaction unit;

[0192] A bactericide dosing device is arranged in front of the coagulation reaction unit; the coagulation reaction unit is a coagulant dosing device; the flocculation reaction unit is a flocculant dosing device; the precipitation unit is an inclined plate precipitation unit; the dehydration system 11 includes a coagulation reaction, a dosing system, and a dewatering machine system connected in sequence;

[0193] The filtration system includes a filtration unit 20, an ultrafiltration unit 30, a primary impurity removal nanofiltration unit 40, and a secondary impurity removal nanofiltration unit 50 that are connected in sequence; the concentrated water outlet of the secondary impurity removal nanofiltration unit 50 is connected to the water inlet of the primary impurity removal nanofiltration unit 40;

[0194] The filtration unit 20 is a quartz sand filtration device; the ultrafiltration unit 30 includes a filtration device and an ultrafiltration membrane that are connected in sequence. The filtration device is a folded filter element type filter with a filtration accuracy of 100 μm; the material of the ultrafiltration membrane is polyvinylidene fluoride, and the pore size of the ultrafiltration membrane is 0.005 μm; the primary impurity removal nanofiltration unit 40 includes an acid addition device, a security filtration device, and a nanofiltration membrane device that are connected in sequence; the secondary impurity removal nanofiltration unit 50 includes a security filtration device and a nanofiltration membrane device that are connected in sequence;

[0195] The adsorption and desorption system 60 includes an adsorption device and a desorption device; the adsorption device is a valve array type ion exchange device;

[0196] The calcium and magnesium removal system includes a primary reverse osmosis concentration unit 70, a primary nanofiltration calcium and magnesium removal unit 80, a secondary reverse osmosis concentration unit 90, a multi-stage nanofiltration calcium and magnesium removal unit 100, and a calcium and magnesium ion exchange unit 110 that are connected in sequence; the water outlet of the adsorption and desorption system is connected to the water inlet of the primary reverse osmosis concentration unit 70; the concentrated water outlet of the multi-stage nanofiltration calcium and magnesium removal unit 100 is connected to the water inlet of the primary reverse osmosis concentration unit 70;

[0197] The primary reverse osmosis concentration unit 70 includes an acid addition device, a security filtration device, and a reverse osmosis device that are connected in sequence; the primary nanofiltration calcium and magnesium removal unit 80 includes an acid addition device, a security filtration device, and a nanofiltration membrane device that are connected in sequence; the secondary reverse osmosis concentration unit 90 includes a security filtration device and a reverse osmosis device that are connected in sequence; the multi-stage nanofiltration calcium and magnesium removal unit 100 includes a security filtration device and at least a two-stage nanofiltration membrane device. The water produced by the previous stage nanofiltration membrane device enters the next stage nanofiltration membrane device for further treatment; the water produced by the final stage of the multi-stage nanofiltration membrane device enters the calcium and magnesium ion exchange unit for treatment; the concentrated water of each stage of the multi-stage nanofiltration membrane device enters the primary reverse osmosis concentration unit for treatment; the calcium and magnesium ion exchange unit 110 includes an alkali addition device, an ion exchange device, and a resin regeneration device; the resin used in the resin regeneration device is a gel type ion exchange resin;

[0198] The boron removal system includes a primary nanofiltration boron removal unit 120, a multi-stage nanofiltration boron removal unit 130, and a boron removal ion exchange unit 140 that are connected in sequence; the concentrated water outlet of the multi-stage nanofiltration boron removal unit 130 is connected to the water inlet of the primary nanofiltration boron removal unit 120;

[0199] The primary nanofiltration boron removal unit 120 includes a caustic adding device, a security filtration device, and a nanofiltration membrane device connected in sequence; the multi-stage nanofiltration boron removal unit 130 includes a caustic adding device, a security filtration device, and at least a two-stage nanofiltration membrane device, and the water produced by the previous-stage nanofiltration membrane device enters the next-stage nanofiltration membrane device for further treatment; the water produced by the final stage of the multi-stage nanofiltration membrane device enters the boron removal ion exchange unit for treatment; the concentrated water of each stage of the multi-stage nanofiltration membrane device enters the primary boron removal nanofiltration unit for treatment; the boron removal ion exchange unit 140 includes a caustic adding device, an ion exchange device, and a resin regeneration device connected in sequence;

[0200] The concentration system 150 is a high-pressure reverse osmosis unit, and the evaporation system 160 is a multi-effect evaporation device; the high-pressure reverse osmosis unit is a high-pressure spiral-wound reverse osmosis membrane module, and a security filtration device is provided in front of the high-pressure reverse osmosis unit;

[0201] The water produced by the primary reverse osmosis concentration unit 70, the water produced by the secondary reverse osmosis concentration unit 90, the water produced by the high-pressure reverse osmosis unit, and the distilled water of the multi-effect evaporation device are all treated by the tertiary reverse osmosis concentration unit 82 and then recycled;

[0202] The lithium precipitation system 170 includes a sodium carbonate dosing unit and a lithium precipitation reaction unit; the lithium precipitation system 170 is used for producing battery-grade lithium carbonate;

[0203] After the lithium precipitation system 170, there is also a precision filtration system 180 and a water washing and drying system 190 connected in sequence;

[0204] The pore size of the nanofiltration membranes in the nanofiltration membrane device is all 1 nm; the filtration accuracy of the security filtration device (precision filtration device) is all 3 μm.

[0205] Example 2

[0206] This example provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and its structural schematic diagram is as shown in Figure 2 shown; on the basis of the system provided in Example 1, the following units are further included:

[0207] The calcium and magnesium removal system further includes a tertiary reverse osmosis concentration unit 82 and a secondary nanofiltration calcium and magnesium removal unit 81; the concentrated water outlet of the primary nanofiltration calcium and magnesium removal unit 70 is connected to the water inlet of the secondary nanofiltration calcium and magnesium removal unit 81; the water production outlet of the secondary nanofiltration calcium and magnesium removal unit 81 is connected to the water inlet of the secondary reverse osmosis concentration unit 90; the concentrated water of the secondary nanofiltration calcium and magnesium removal unit 81 is discharged; the concentrated water outlet of the tertiary reverse osmosis concentration unit 82 is connected to the water inlet of the primary nanofiltration calcium and magnesium removal unit 80;

[0208] The two-stage nanofiltration calcium and magnesium removal unit 81 includes a nanofiltration dialysis device, a security filtration device, and a nanofiltration membrane device connected in sequence. The dialysis water of the nanofiltration dialysis device uses pure water with low salt content, the pure water conductivity ≤ 100 us / cm, the dialysis inlet water flow rate is 5 times the two-stage nanofiltration inlet water flow rate, and a multi-stage hierarchical dialysis method is adopted; the three-stage reverse osmosis concentration unit 82 includes a security filtration device and a reverse osmosis device connected in sequence.

[0209] Example 3

[0210] This embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and its structural schematic diagram is as Figure 3 shown; on the basis of the system provided in Example 1, the following units are further included:

[0211] The boron removal system further includes a first-stage reverse osmosis boron removal unit 121, a second-stage reverse osmosis boron removal unit 122, and a third-stage reverse osmosis boron removal unit 123 connected in sequence; the concentrated water outlet of the first-stage nanofiltration boron removal unit 120 is connected to the water inlet of the first-stage reverse osmosis boron removal unit 121; the concentrated water of the third-stage reverse osmosis boron removal unit 123 is used to prepare borax; the concentrated water outlet of the third-stage reverse osmosis boron removal unit 123 is connected to the evaporation and crystallization device 124, and the produced water of the third-stage reverse osmosis boron removal unit 123 and the distilled water of the evaporation and crystallization device 124 are both treated by the three-stage reverse osmosis concentration unit 82 and then recycled; the boron removal system further includes a second-stage reverse osmosis boron removal unit 125, the concentrated water outlet of which is connected to the water inlet of the first-stage nanofiltration boron removal unit 120, and the produced water outlet of which is connected to the water inlet of the second-stage reverse osmosis boron removal unit 122; the concentrated water outlets of the first-stage reverse osmosis boron removal unit 121 and the second-stage reverse osmosis boron removal unit 123 are both connected to the water inlet of the second-stage reverse osmosis boron removal unit 125;

[0212] The first-stage reverse osmosis boron removal unit 121 includes an acid addition device, a dialysis device, a filtration device, and a reverse osmosis device connected in sequence. Among them, the dialysis inlet water flow rate in the dialysis device is 3-15 times the water inlet flow rate of the first-stage reverse osmosis boron removal unit, and a multi-stage hierarchical dialysis method is adopted; the second-stage reverse osmosis boron removal unit 122 includes a security filtration device and a reverse osmosis device connected in sequence; the third-stage reverse osmosis boron removal unit 123 includes an alkali addition device, a security filtration device, and a reverse osmosis device connected in sequence; the second-stage reverse osmosis boron removal unit 125 includes a security filtration device and a reverse osmosis device connected in sequence.

[0213] Example 4

[0214] This embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and its structural schematic diagram is as Figure 4 shown; on the basis of the system provided in Example 1, the following units are further included:

[0215] The system further includes a mother liquor recycling system for lithium precipitation; the mother liquor recycling system for lithium precipitation includes a heat exchange unit 171, a first-stage nanofiltration carbonate recovery unit 172, and a second-stage nanofiltration carbonate recovery unit 173 that are connected in sequence; the mother liquor outlet of the lithium precipitation system 170 is connected to the water inlet of the heat exchange unit 171; the concentrated water outlets of the first-stage nanofiltration carbonate recovery unit 172 and the second-stage nanofiltration carbonate recovery unit 173 are both connected to the water inlet of the lithium precipitation system 170; the produced water of the second-stage nanofiltration carbonate recovery unit 173 is recycled to the first-stage reverse osmosis concentration unit 70;

[0216] The heat exchange unit 171 is a plate heat exchanger; the first-stage nanofiltration carbonate recovery unit 172 includes a dialysis device, a security filtration device, and a nanofiltration membrane device that are connected in sequence. Among them, the dialysis inlet flow rate of the dialysis device is 3 times the inlet flow rate of the first-stage nanofiltration carbonate recovery unit, and a multi-stage hierarchical dialysis method is adopted. The dialysis water uses pure water with low salt content, and the conductivity of the pure water is ≤100 us / cm; the second-stage nanofiltration carbonate recovery unit 173 includes a security filtration device and a nanofiltration membrane device that are connected in sequence.

[0217] Example 5

[0218] This example provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. The structural schematic diagram is as Figure 5 shown; on the basis of the system provided in Example 1, the following units are further included:

[0219] The calcium and magnesium removal system further includes a third-stage reverse osmosis concentration unit 82 and a second-stage nanofiltration calcium and magnesium removal unit 81; the concentrated water outlet of the first-stage nanofiltration calcium and magnesium removal unit 70 is connected to the water inlet of the second-stage nanofiltration calcium and magnesium removal unit 81; the produced water outlet of the second-stage nanofiltration calcium and magnesium removal unit 81 is connected to the water inlet of the second-stage reverse osmosis concentration unit 90; the concentrated water of the second-stage nanofiltration calcium and magnesium removal unit 81 is discharged; the concentrated water outlet of the third-stage reverse osmosis concentration unit 82 is connected to the water inlet of the first-stage nanofiltration calcium and magnesium removal unit 80;

[0220] The second-stage nanofiltration calcium and magnesium removal unit 81 includes a nanofiltration dialysis device, a security filtration device, and a nanofiltration membrane device that are connected in sequence. The dialysis water of the nanofiltration dialysis device uses pure water with low salt content, and the dialysis inlet flow rate is 5 times the inlet flow rate of the second-stage nanofiltration; the third-stage reverse osmosis concentration unit 82 includes a security filtration device and a reverse osmosis device that are connected in sequence.

[0221] The boron removal system further includes a first-stage reverse osmosis boron removal unit 121, a second-stage reverse osmosis boron removal unit 122, and a third-stage reverse osmosis boron removal unit 123 that are connected in sequence; the concentrated water outlet of the first-stage nanofiltration boron removal unit 120 is connected to the water inlet of the first-stage reverse osmosis boron removal unit 121; the concentrated water of the third-stage reverse osmosis boron removal unit 123 is used to prepare borax; the concentrated water outlet of the third-stage reverse osmosis boron removal unit 123 is connected to an evaporation and crystallization device 124, and the produced water of the third-stage reverse osmosis boron removal unit 123 and the distilled water of the evaporation and crystallization device 124 are both treated by a third-stage reverse osmosis concentration unit 82 and then recycled; the boron removal system further includes a second-stage reverse osmosis boron removal unit 125, whose concentrated water outlet is connected to the water inlet of the first-stage nanofiltration boron removal unit 120, and whose produced water outlet is connected to the water inlet of the second-stage reverse osmosis boron removal unit 122; the concentrated water outlets of the first-stage reverse osmosis boron removal unit 121 and the second-stage reverse osmosis boron removal unit 123 are both connected to the water inlet of the second-stage reverse osmosis boron removal unit 125;

[0222] The first-stage reverse osmosis boron removal unit 121 includes an acid addition device, a dialysis device, a filtration device, and a reverse osmosis device that are connected in sequence, wherein the dialysis inlet water flow rate in the dialysis device is 3-15 times the inlet water flow rate of the first-stage reverse osmosis boron removal unit; the second-stage reverse osmosis boron removal unit 122 includes a security filtration device and a reverse osmosis device that are connected in sequence; the third-stage reverse osmosis boron removal unit 123 includes an alkali addition device, a security filtration device, and a reverse osmosis device that are connected in sequence; the second-stage reverse osmosis boron removal unit 125 includes a security filtration device and a reverse osmosis device that are connected in sequence;

[0223] The system further includes a lithium precipitation mother liquor recycling system; the lithium precipitation mother liquor recycling system includes a heat exchange unit 171, a first-stage nanofiltration carbonate recovery unit 172, and a second-stage nanofiltration carbonate recovery unit 173 that are connected in sequence; the mother liquor outlet of the lithium precipitation system 170 is connected to the water inlet of the heat exchange unit 171; the concentrated water outlets of the first-stage nanofiltration carbonate recovery unit 172 and the second-stage nanofiltration carbonate recovery unit 173 are both connected to the water inlet of the lithium precipitation system 170; the produced water of the second-stage nanofiltration carbonate recovery unit 173 is recycled to the first-stage reverse osmosis concentration unit 70;

[0224] The heat exchange unit 171 is a plate heat exchange device; the first-stage nanofiltration carbonate recovery unit 172 includes a dialysis device, a security filtration device, and a nanofiltration membrane device that are connected in sequence, wherein the dialysis inlet water flow rate of the dialysis device is 1-5 times the inlet water flow rate of the first-stage nanofiltration carbonate recovery unit; the second-stage nanofiltration carbonate recovery unit 173 includes a security filtration device and a nanofiltration membrane device that are connected in sequence.

[0225] Example 6

[0226] This embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Except that no acid addition device is provided in the first-stage reverse osmosis concentration unit 70 and the first-stage nanofiltration calcium and magnesium removal unit 80 in the calcium and magnesium removal system, and no alkali addition device is provided in the calcium and magnesium ion exchange unit 110, other conditions are the same as those in Embodiment 1.

[0227] Embodiment 7

[0228] This embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Except that no alkali addition device is provided in the first-stage nanofiltration boron removal unit 120, the multi-stage nanofiltration boron removal unit 130 and the boron removal ion exchange unit 140 in the boron removal system, other conditions are the same as those in Embodiment 1.

[0229] Application Example 1

[0230] In this application example, the lithium ion content in the salt lake brine is 1000 ppm, the magnesium ion content is 4000 ppm, the sodium ion content is 40000 ppm, the calcium ion content is 300 ppm, the potassium ion content is 6000 ppm, the carbonate content is 200 ppm, the sulfate content is 15000 ppm, and the boron ion content is 200 ppm;

[0231] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Using the system provided in Embodiment 1, the method includes the following steps:

[0232] (1) The salt lake brine sequentially passes through a coagulation reaction unit, a flocculation reaction unit and a precipitation unit for precipitation reaction. The dosage of the coagulant in the coagulation reaction unit is 100 mg / L; the dosage of the flocculant in the flocculation reaction unit is 5 mg / L; then it sequentially passes through a filtration unit 20, an ultrafiltration unit 30, a first-stage impurity removal nanofiltration unit 40 and a second-stage impurity removal nanofiltration unit 50 for filtration and impurity removal. The pH of the salt lake brine is adjusted to 4 before entering the first-stage impurity removal nanofiltration unit 40, and then it is subjected to adsorption and desorption treatment by an adsorption and desorption system 60. The adsorbent is an aluminum-based lithium adsorbent and the desorbent is pure water to obtain a lithium chloride solution;

[0233] (2) The lithium chloride solution in step (1) is concentrated by a first-stage reverse osmosis concentration unit 70. The concentrated water obtained is filtered by a first-stage nanofiltration calcium and magnesium removal unit 80 to remove calcium and magnesium ions. The produced water obtained is concentrated by a second-stage reverse osmosis concentration unit 90. The concentrated water obtained is passed through a multi-stage nanofiltration calcium and magnesium removal unit 100 to remove calcium and magnesium ions. The produced water obtained is passed through a calcium and magnesium ion exchange unit 110 to completely remove calcium and magnesium ions to obtain a lithium chloride solution with calcium and magnesium ions removed;

[0234] Among them, the pH of the lithium chloride solution is adjusted to 4 before entering the first-stage reverse osmosis concentration unit 70 and the first-stage nanofiltration calcium and magnesium removal unit 80, and the pH is adjusted to 8 before the produced water enters the calcium and magnesium ion exchange unit 110;

[0235] (3) The lithium chloride solution described in step (2) is successively passed through the first-stage nanofiltration boron removal unit 120, the multi-stage nanofiltration boron removal unit 130 and the boron removal ion exchange unit 140 to remove boron ions, and the lithium chloride solution with boron ions removed is obtained; the pH of the lithium chloride solution is adjusted to 9.5 before entering the first-stage nanofiltration boron removal unit 120 and the multi-stage nanofiltration boron removal unit 130; the pH is adjusted to 10 before entering the boron removal ion exchange unit 140;

[0236] (4) The lithium chloride solution described in step (3) is successively concentrated by the high-pressure reverse osmosis unit, evaporated by the multi-effect evaporation device at 97 °C, and finally sodium carbonate is added by the sodium carbonate dosing unit. The mass ratio of sodium carbonate to lithium chloride is 1.3:1, the pH of the solution is adjusted to 10, and the temperature is raised to 90 °C in the lithium precipitation reaction unit for reaction. The obtained lithium carbonate precipitation solution is treated by the precision filtration system 180 and the water washing and drying system 190 to obtain battery-grade lithium carbonate;

[0237] In this application example, the lithium recovery rate of the system is 54%, and the purity of the battery-grade lithium carbonate is 99.61%.

[0238] Application Example 2

[0239] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Using the system provided in Application Example 2, except that step (2) is adjusted to "the lithium chloride solution described in step (1) is concentrated by the first-stage reverse osmosis concentration unit 70, and the concentrated water obtained is successively passed through the first-stage nanofiltration calcium and magnesium removal unit 80 and the second-stage nanofiltration calcium and magnesium removal unit 81 to remove calcium and magnesium ions. The produced water obtained is concentrated by the second-stage reverse osmosis concentration unit 90, and the concentrated water obtained is passed through the multi-stage nanofiltration calcium and magnesium removal unit 100 to remove calcium and magnesium ions. The produced water obtained is passed through the calcium and magnesium ion exchange unit 110 to completely remove calcium and magnesium ions, and the lithium chloride solution with calcium and magnesium ions removed is obtained", other conditions are the same as those in Application Example 1.

[0240] In this application example, the lithium recovery rate of the system is 70%, and the purity of the battery-grade lithium carbonate is 99.61%.

[0241] Application Example 3

[0242] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, using the system provided in Application Example 3. In this method, except that the concentrated water obtained from the first-stage nanofiltration boron removal unit 120 in step (3) is sequentially subjected to boron removal and concentration through the first-stage reverse osmosis boron removal unit 121, the second-stage reverse osmosis boron removal unit 125, the second-stage reverse osmosis boron removal unit 122, and the third-stage reverse osmosis boron removal unit 123, and then borax is obtained by evaporation in the evaporation crystallization device 124; the pH is adjusted to 4 before the concentrated water enters the first-stage reverse osmosis boron removal unit 121; the pH is adjusted to 10 before the produced water enters the third-stage reverse osmosis boron removal unit 123, and other conditions are the same as those in Application Example 1.

[0243] In this application example, the lithium recovery rate of the system is 62%, the purity of battery-grade lithium carbonate is 99.61%, and after the adsorption and desorption process: the boron recovery rate is 78%.

[0244] In this application example, by adding a first-stage reverse osmosis boron removal unit, a second-stage reverse osmosis boron removal unit, a third-stage reverse osmosis boron removal unit, and a second-stage reverse osmosis boron removal unit, the boron recovery rate is improved.

[0245] Application Example 4

[0246] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, using the system provided in Application Example 4. In this method, except that the mother liquor after the lithium precipitation reaction in the lithium precipitation reaction unit in step (4) is heated by the heat exchange unit 171 to a temperature of 20°C, and then sequentially passes through the first-stage nanofiltration carbonate recovery unit 172 and the second-stage nanofiltration carbonate recovery unit 173 to separate and recover carbonate ions and lithium chloride solution in the solution, other conditions are the same as those in Application Example 1.

[0247] In this application example, the lithium recovery rate of the system is 64%, and the purity of battery-grade lithium carbonate is 99.61%.

[0248] In this application example, by adding a mother liquor recycling system for lithium precipitation, the consumption of sodium carbonate in the production process of the lithium precipitation system is reduced.

[0249] Application Example 5

[0250] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, using the system provided in Application Example 5. The salt lake brine is the same as that in Application Example 1. The method includes the following steps:

[0251] (1) The brine from the salt lake undergoes precipitation reactions in a coagulation reaction unit, a flocculation reaction unit, and a precipitation unit in sequence. The dosage of the coagulant in the coagulation reaction unit is 100 mg / L; the dosage of the flocculant in the flocculation reaction unit is 5 mg / L; then it undergoes filtration and impurity removal in a filtration unit 20, an ultrafiltration unit 30, a primary impurity removal nanofiltration unit 40, and a secondary impurity removal nanofiltration unit 50 in sequence. Before the brine from the salt lake enters the primary impurity removal nanofiltration unit 40, the pH is adjusted to 4, and then it undergoes adsorption and desorption treatment in an adsorption and desorption system 60. The adsorbent is an aluminum-based lithium adsorbent, and the desorbent is pure water to obtain a lithium chloride solution;

[0252] (2) The lithium chloride solution obtained in step (1) is concentrated by a primary reverse osmosis concentration unit 70. The concentrated water obtained is sequentially passed through a primary nanofiltration unit for calcium and magnesium removal 80 and a secondary nanofiltration unit for calcium and magnesium removal 81 to remove calcium and magnesium ions. The produced water obtained is concentrated by a secondary reverse osmosis concentration unit 90. The concentrated water obtained is passed through a multi-stage nanofiltration unit for calcium and magnesium removal 100 to remove calcium and magnesium ions. The produced water obtained is passed through a calcium and magnesium ion exchange unit 110 to completely remove calcium and magnesium ions to obtain a lithium chloride solution with calcium and magnesium ions removed;

[0253] Among them, before the lithium chloride solution enters the primary reverse osmosis concentration unit 70 and the primary nanofiltration unit for calcium and magnesium removal 80, the pH is adjusted to 4. Before the produced water enters the calcium and magnesium ion exchange unit 110, the pH is adjusted to 8;

[0254] (3) The lithium chloride solution obtained in step (2) is sequentially passed through a primary nanofiltration unit for boron removal 120, a multi-stage nanofiltration unit for boron removal 130, and a boron ion exchange unit 140 for treatment to remove boron ions to obtain a lithium chloride solution with boron ions removed; before the lithium chloride solution enters the primary nanofiltration unit for boron removal 120 and the multi-stage nanofiltration unit for boron removal 130, the pH is adjusted to 9.5; before entering the boron ion exchange unit 140, the pH is adjusted to 10;

[0255] Among them, the concentrated water obtained by the primary nanofiltration unit for boron removal 120 is passed through a primary reverse osmosis unit for boron removal 121, a secondary reverse osmosis unit for boron removal 125, a secondary reverse osmosis unit for boron removal 122, and a tertiary reverse osmosis unit for boron removal 123 for boron removal and concentration, and then borax is obtained by evaporation in an evaporation crystallization device 124; before the concentrated water enters the primary reverse osmosis unit for boron removal 121, the pH is adjusted to 4; before the produced water enters the tertiary reverse osmosis unit for boron removal 123, the pH is adjusted to 10;

[0256] (4) The lithium chloride solution described in step (3) is successively concentrated by a high-pressure reverse osmosis unit, evaporated by a multiple-effect evaporation device at 97 °C, and finally sodium carbonate is added through a sodium carbonate dosing unit. The mass ratio of sodium carbonate to lithium chloride is 1.3:1, the pH of the solution is adjusted to 10, and the temperature is raised to 90 °C for reaction in a lithium precipitation reaction unit. The obtained lithium carbonate precipitation solution is treated by a precision filtration system 180 and a water washing and drying system 190 to obtain battery-grade lithium carbonate;

[0257] Among them, the temperature of the lithium precipitation mother liquor obtained after the reaction in the lithium precipitation reaction unit is 20 °C after heat exchange by a heat exchange unit 171, and the carbonate ions in the solution are separated and recovered successively by a primary nanofiltration carbonate recovery unit 172 and a secondary nanofiltration carbonate recovery unit 173.

[0258] In this example, the lithium recovery rate of the system is 90%, the purity of battery-grade lithium carbonate is 99.61%, and after the adsorption and desorption process: the boron recovery rate is 96%.

[0259] Application Example 6

[0260] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Using the system provided in Application Example 6, except for not regulating the pH of the lithium chloride solution in step (2), other conditions are the same as those in Application Example 1.

[0261] In this example, the lithium recovery rate of the system is 45%, and the purity of lithium carbonate is 99.2%.

[0262] By regulating the acidity and alkalinity of the lithium chloride solution in the calcium and magnesium removal system in this application, not only can the removal rate of calcium and magnesium ions be improved, ensuring the stable operation of the system, but also it is beneficial to improve the lithium recovery rate and the purity of lithium carbonate.

[0263] Application Example 7

[0264] This application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate. Using the system provided in Application Example 7, except for not regulating the pH of the lithium chloride solution in step (3), other conditions are the same as those in Application Example 1.

[0265] In this example, the lithium recovery rate of the system is 54%, and the purity of lithium carbonate is 99%.

[0266] By regulating the acidity and alkalinity of the lithium chloride solution in the boron removal system in this application, not only can the removal rate and recovery rate of boron ions be improved, ensuring the stable operation of the system, but also it is beneficial to improve the lithium recovery rate and the purity of lithium carbonate.

[0267] The applicant declares that the detailed structural features of the present invention are illustrated by the above embodiments, but the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent replacement of the components selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, characterized in that, The system includes a coagulation and sedimentation system, a filtration system, an adsorption and desorption system, a calcium and magnesium removal system, a boron removal system, a concentration system, an evaporation system, and a lithium precipitation system, which are connected in sequence; The filtration system includes a filtration unit, an ultrafiltration unit, a first impurity removal nanofiltration unit, and a second impurity removal nanofiltration unit, which are connected in sequence; The calcium and magnesium removal system includes a first reverse osmosis concentration unit, a first nanofiltration calcium and magnesium removal unit, a second reverse osmosis concentration unit, a multi-stage nanofiltration calcium and magnesium removal unit, and a calcium and magnesium ion exchange unit, which are connected in sequence; The calcium and magnesium removal system further includes a third reverse osmosis concentration unit and a second-stage nanofiltration calcium and magnesium removal unit; The concentrated water outlet of the first nanofiltration calcium and magnesium removal unit is connected to the water inlet of the second-stage nanofiltration calcium and magnesium removal unit; The water production outlet of the second-stage nanofiltration calcium and magnesium removal unit is connected to the water inlet of the second reverse osmosis concentration unit; The concentrated water outlet of the third reverse osmosis concentration unit is connected to the water inlet of the first nanofiltration calcium and magnesium removal unit; The water outlet of the adsorption and desorption system is connected to the water inlet of the first reverse osmosis concentration unit; The concentrated water outlet of the multi-stage nanofiltration calcium and magnesium removal unit is connected to the water inlet of the first reverse osmosis concentration unit; The boron removal system includes a first nanofiltration boron removal unit, a multi-stage nanofiltration boron removal unit, and a boron ion exchange unit, which are connected in sequence; The boron removal system further includes a first reverse osmosis boron removal unit, a second reverse osmosis boron removal unit, and a third reverse osmosis boron removal unit, which are connected in sequence; The concentrated water outlet of the first nanofiltration boron removal unit is connected to the water inlet of the first reverse osmosis boron removal unit; The concentrated water of the third reverse osmosis boron removal unit is used to prepare borax; The boron removal system further includes a second-stage reverse osmosis boron removal unit, whose concentrated water outlet is connected to the water inlet of the first nanofiltration boron removal unit, and whose water production outlet is connected to the water inlet of the second reverse osmosis boron removal unit; The concentrated water outlets of the first reverse osmosis boron removal unit and the second reverse osmosis boron removal unit are both connected to the water inlet of the second-stage reverse osmosis boron removal unit; The water production outlet of the calcium and magnesium ion exchange unit is connected to the water inlet of the first nanofiltration boron removal unit; The concentrated water outlet of the multi-stage nanofiltration boron removal unit is connected to the water inlet of the first nanofiltration boron removal unit; The water production of the first reverse osmosis concentration unit, the water production of the second reverse osmosis concentration unit, the water production of the concentration system, and the distilled water of the evaporation system are all treated by the third reverse osmosis concentration unit and then recycled; The lithium precipitation system includes a sodium carbonate dosing unit and a lithium precipitation reaction unit; the lithium precipitation system is used to produce battery-grade lithium carbonate; The system further includes a lithium precipitation mother liquor recycling system; The lithium precipitation mother liquor recycling system includes a heat exchange unit, a first nanofiltration carbonate recovery unit, and a second nanofiltration carbonate recovery unit, which are connected in sequence; The mother liquor outlet of the lithium precipitation system is connected to the water inlet of the heat exchange unit; The concentrated water outlets of the first nanofiltration carbonate recovery unit and the second nanofiltration carbonate recovery unit are both connected to the water inlet of the lithium precipitation system; The water production of the second nanofiltration carbonate recovery unit is recycled to the first reverse osmosis concentration unit.

2. The system according to claim 1, wherein The coagulation and sedimentation system includes a coagulation reaction unit, a flocculation reaction unit, and a sedimentation unit, which are connected in sequence.

3. The system according to claim 2, wherein The sludge outlet of the precipitation unit and the backwash water outlet of the filtration unit are both connected to a dewatering system.

4. The system according to claim 3, characterized in that The filtrate outlet of the dewatering system is connected to the water inlet of the coagulation reaction unit.

5. The system according to claim 1, wherein The concentrated water outlet of the secondary impurity removal nanofiltration unit is connected to the water inlet of the primary impurity removal nanofiltration unit.

6. The system according to claim 1, wherein The primary impurity removal nanofiltration unit includes an acid adding device, a security filtration device, and a nanofiltration membrane device connected in sequence.

7. The system according to claim 1, wherein The adsorption and desorption system includes an adsorption device and a desorption device.

8. The system according to claim 1, characterized in that The primary reverse osmosis concentration unit includes an acid adding device, a security filtration device, and a reverse osmosis device connected in sequence.

9. The system according to claim 1, wherein The primary nanofiltration unit for removing calcium and magnesium includes an acid adding device, a security filtration device, and a nanofiltration membrane device connected in sequence.

10. The system according to claim 1, characterized in that, The calcium and magnesium ion exchange unit for removal includes an alkali adding device, an ion exchange device, and a resin regeneration device connected in sequence.

11. The system according to claim 1, wherein The primary nanofiltration unit for removing boron includes an alkali adding device, a security filtration device, and a nanofiltration membrane device connected in sequence.

12. The system according to claim 1, wherein The primary reverse osmosis unit for removing boron includes an acid adding device, a dialysis device, a security filtration device, and a reverse osmosis device connected in sequence.

13. The system according to claim 1, characterized in that The tertiary reverse osmosis unit for removing boron includes an alkali adding device, a security filtration device, and a reverse osmosis device connected in sequence.

14. The system according to claim 1, characterized in that, The multi-stage nanofiltration unit for removing boron includes an alkali adding device, a security filtration device, and at least a two-stage nanofiltration membrane device connected in sequence.

15. The system according to claim 1, wherein The boron ion exchange unit for removal includes an alkali adding device, an ion exchange device, and a resin regeneration device connected in sequence.

16. The system according to claim 1, wherein The concentration system includes a high-pressure reverse osmosis unit or an electrodialysis unit.

17. The system according to claim 16, wherein The high-pressure reverse osmosis unit is a high-pressure spiral wound reverse osmosis membrane module or a DTRO (Disk Tube Reverse Osmosis) membrane module.

18. The system according to claim 1, wherein, After the lithium precipitation system, there is also a precision filtration system and a water washing and drying system connected in sequence.

19. A method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, characterized in that, The method uses the system according to any one of claims 1-18, and includes the following steps: (1) The salt lake brine is sequentially subjected to a precipitation reaction in a coagulation and precipitation system, a filtration treatment in a filtration system, and an adsorption and desorption treatment in an adsorption and desorption system to obtain a lithium chloride solution; (2) The lithium chloride solution obtained in step (1) is sequentially passed through a primary reverse osmosis concentration unit, a primary nanofiltration unit for removing calcium and magnesium, a secondary reverse osmosis concentration unit, a multi-stage nanofiltration unit for removing calcium and magnesium, and a calcium and magnesium ion exchange unit for removal to remove calcium and magnesium ions; (3) The lithium chloride solution after removing calcium and magnesium ions in step (2) is sequentially passed through a primary nanofiltration unit for removing boron, a multi-stage nanofiltration unit for removing boron, and a boron ion exchange unit for removal to remove boron ions; (4) The lithium chloride solution after removing boron ions in step (3) is sequentially subjected to a concentration treatment in a concentration system, an evaporation treatment in an evaporation system, and finally, after adding sodium carbonate in a sodium carbonate dosing unit and reacting in a lithium precipitation reaction unit, battery-grade lithium carbonate is obtained.

20. The method according to claim 19, wherein In the salt lake brine in step (1), the lithium ion content is 600-1200 ppm, the magnesium ion content is 2000-6000 ppm, the sodium ion content is 25000-50000 ppm, the calcium ion content ≤ 500 ppm, the potassium ion content ≤ 8000 ppm, the carbonate ion content ≤ 400 ppm, the sulfate ion content is 10000-20000 ppm, and the boron ion content ≤ 600 ppm.

21. The method according to claim 19, wherein The filtration rate of the filtration treatment described in step (1) is 5 - 15 m / h.

22. The method according to claim 19, wherein The adsorbent for the adsorption and desorption treatment described in step (1) includes any one of aluminum-based lithium adsorbents, manganese-based lithium adsorbents, or titanium-based lithium adsorbents.

23. The method according to claim 19, wherein Before the lithium chloride solution described in step (2) enters the first-stage reverse osmosis concentration unit and the first-stage nanofiltration calcium and magnesium removal unit, the pH is adjusted to 3 - 6.

24. The method according to claim 19, wherein Before the lithium chloride solution described in step (2) enters the calcium and magnesium removal ion exchange unit, the pH is adjusted to 7 - 10.

25. The method according to claim 19, characterized in that, Before the lithium chloride solution described in step (3) enters the first-stage nanofiltration boron removal unit and the multi-stage nanofiltration boron removal unit, the pH is adjusted to 9.2 - 11.

26. The method according to claim 19, wherein Before the lithium chloride solution described in step (3) enters the boron removal ion exchange unit, the pH is adjusted to 9.2 - 10.

27. The method according to claim 19, characterized in that, The mass ratio of sodium carbonate to lithium chloride described in step (4) is (1 - 1.2):

1.

28. The method according to claim 19, wherein The pH of the lithium chloride solution after adding sodium carbonate in step (4) is 9 - 12.

29. The method according to claim 19, characterized in that, The temperature of the reaction described in step (4) is 80 - 99 °C.

30. The method according to claim 19, wherein After the reaction in the lithium precipitation reaction unit described in step (4), a lithium carbonate solution and a lithium precipitation mother liquor are obtained.

31. The method according to claim 30, wherein The lithium carbonate solution is successively treated by a precision filtration system and a water washing and drying system to obtain battery-grade lithium carbonate.

32. The method according to claim 30, characterized in that, The lithium precipitation mother liquor is successively heat-exchanged by a heat exchange unit, and the carbonate ions and lithium chloride solution separated and recovered by the first-stage nanofiltration carbonate recovery unit and the second-stage nanofiltration carbonate recovery unit.

33. The method according to claim 32, wherein The temperature after heat exchange is 5 - 40 °C.

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