CN115784503A - System and method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate - Google Patents

Abstract

The invention provides a system and a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, wherein the system comprises a coagulating sedimentation system, a filtering 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 sequentially connected; the calcium and magnesium removal system comprises a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multistage nanofiltration calcium and magnesium removal unit and a calcium and magnesium removal ion exchange unit which are connected in sequence; the boron removal system comprises a primary nanofiltration boron removal unit, a multi-stage nanofiltration boron removal unit and a boron removal ion exchange unit which are sequentially connected; the lithium precipitation system is also provided with a precise filtering system and a water washing and drying system which are connected in sequence. By adopting the process, the recovery rate of lithium is improved, the product is battery-grade lithium carbonate, the resource utilization and recovery rate of a lithium extraction system are greatly improved, and the consumption of added water and sodium carbonate in the treatment process of a lithium precipitation system are reduced.

Description

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

Technical Field

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

Background

Lithium is the most important energy metal and an indispensable strategic resource in modern industry, and plays an important role in battery chemistry, glass ceramics, aviation metal, nuclear industry, lubricating grease, refrigerant and other industries. Under the ambitious condition of global climate, the low carbon and carbon reduction become global consensus and trend; especially, the explosive development of lithium batteries in recent years has led to a rapid expansion of the global lithium consumption. At present, lithium salt is mostly extracted from ore in China, but with the continuous reduction of high-grade lithium ore and the continuous improvement of the cost of extracting lithium from ore, because the salt lake is rich in a large amount of lithium elements, the method for extracting lithium from the salt lake has obvious resource and cost advantages compared with the method for extracting lithium from ore. Therefore, extraction of lithium from salt lakes has become a necessary trend in the development of lithium resources.

The key point of the lithium extraction from the salt lake is that lithium ions are enriched from the salt lake brine in a low-cost mode and are separated out to lithium carbonate or lithium hydroxide and the like, and a high-value added product is produced. The main influencing factors of the lithium extraction in the salt lake are two: firstly, the lower the lithium content is, the longer the treatment process flow is, the larger the evaporation capacity after treatment is, and the higher the cost is; secondly, the ion proportion relation of various minerals in the salt lake, particularly the magnesium-lithium ratio and the boron-lithium ratio, wherein the smaller the magnesium-lithium ratio and the boron-lithium ratio is, the better the magnesium-lithium ratio and the boron-lithium ratio are. At present, the main treatment methods for directly extracting lithium from salt lake raw brine and producing lithium carbonate comprise: tedding, solar pond, solvent extraction, precipitation, calcination, adsorption, membrane, electrodialysis, etc.

The tedding method and the solar pond method are suitable for salt lake brine with small magnesium-lithium ratio, and have low efficiency and low lithium yield; the extractant used in the solvent extraction method is an organic solvent, and the use of a large amount of organic solvent not only seriously corrodes equipment, but also pollutes the surrounding environment; the electrodialysis method has high energy consumption and low water yield. The calcining method is characterized in that lithium-containing brine after potassium extraction and boron extraction is used as a raw material, water is evaporated to obtain lithium-containing magnesium chloride tetrahydrate, spray drying and calcining are adopted to obtain lithium-containing magnesium oxide, water is added to wash and filter to leach lithium, lime milk is used to remove impurities such as calcium, magnesium and the like, the solution is evaporated and concentrated until the content of Li is about 2%, sodium carbonate is added to precipitate lithium carbonate, the lithium yield is about 90%, the energy consumption is high, and the environment is polluted.

With the large utilization of lithium resources, the method for extracting lithium from salt lake brine is provided, which has the advantages of low production cost, small environmental pollution, low energy consumption and high lithium recovery rate, and simultaneously extracts and produces other products with additional high value from salt lakes, so that the comprehensive utilization of the salt lake resources is achieved, and the technical problem to be solved by researchers in the field is urgently needed.

Disclosure of Invention

The invention aims to provide a system and a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate.

In order to achieve the purpose, the invention adopts the following technical scheme:

in a first aspect, the invention provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, which comprises a coagulating sedimentation system, a filtering 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 deposition system which are sequentially connected;

the calcium and magnesium removal system comprises a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multistage nanofiltration calcium and magnesium removal unit and a calcium and magnesium removal ion exchange unit which are connected in sequence;

the water outlet of the adsorption and desorption system is connected with the water inlet of the first-stage reverse osmosis concentration unit;

a concentrated water outlet of the multi-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the first-stage reverse osmosis concentration unit;

the boron removal system comprises a primary nanofiltration boron removal unit, a multi-stage nanofiltration boron removal unit and a boron removal ion exchange unit which are sequentially connected;

the water outlet of the calcium and magnesium removal ion exchange unit is connected with the water inlet of the primary nanofiltration boron removal unit;

a concentrated water outlet of the multi-stage nano-filtration boron removal unit is connected with a water inlet of the first-stage nano-filtration boron removal unit;

the produced water of the first-stage reverse osmosis concentration unit, the produced water of the second-stage reverse osmosis concentration unit, the produced water of the concentration system and the distilled water of the evaporation system are all treated by the third-stage reverse osmosis concentration unit and then recycled;

the lithium precipitation system comprises a sodium carbonate adding unit and a lithium precipitation reaction unit; the lithium precipitation system is used for producing battery-grade lithium carbonate.

The invention further optimizes the lithium extraction system, and each system is a part of internal circulation, thereby greatly improving the resource utilization and recovery rate of the lithium extraction system, reducing the consumption of sodium carbonate in the treatment process of the lithium precipitation system, saving the water and electricity consumption of the system, reducing the energy consumption and material consumption, reducing the pollution to the environment, maximally improving the resource comprehensive utilization of the salt lake brine, and achieving better economic benefit, social benefit and environmental benefit.

As a preferable technical scheme of the invention, the coagulating sedimentation system comprises a coagulating reaction unit, a flocculation reaction unit and a sedimentation unit which are connected in sequence.

Preferably, the filtration system comprises a filtration unit, an ultrafiltration unit, a primary impurity removal nanofiltration unit and a secondary impurity removal nanofiltration unit which are connected in sequence.

Preferably, the sludge outlet of the sedimentation unit and the backwash water outlet of the filtration unit are both connected to a dewatering system.

Preferably, a filtrate outlet of the dewatering system is connected with a water inlet of the coagulation reaction unit.

Preferably, a concentrated water outlet of the secondary impurity-removing nanofiltration unit is connected with a water inlet of the primary impurity-removing nanofiltration unit.

Preferably, the primary impurity removal nanofiltration unit comprises an acid adding device, a security filtration device and a nanofiltration membrane device which are connected in sequence.

Preferably, the adsorption and desorption system comprises an adsorption device and a desorption device.

As the preferable technical scheme of the invention, the calcium and magnesium removal system also comprises a three-stage reverse osmosis concentration unit and a two-stage nanofiltration calcium and magnesium removal unit.

Preferably, a concentrated water outlet of the first-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the second-stage nanofiltration calcium and magnesium removal unit.

Preferably, the water outlet of the second-stage nanofiltration calcium and magnesium removal unit is connected with the water inlet of the second-stage reverse osmosis concentration unit.

Preferably, a concentrated water outlet of the third-stage reverse osmosis concentration unit is connected with a water inlet of the first-stage nanofiltration calcium and magnesium removal unit.

Preferably, the first-stage reverse osmosis concentration unit comprises an acid adding device, a safety filter device and a reverse osmosis device which are connected in sequence.

Preferably, the first-stage nanofiltration calcium and magnesium removal unit comprises an acid adding device, a safety filter device and a nanofiltration membrane device which are connected in sequence.

The produced water treated by the three-stage reverse osmosis concentration unit can be used for adsorption analysis system water, membrane dialysis water, calcium and magnesium removal ion exchange unit and boron removal ion exchange unit material-ejecting water, acid-base dilution water, membrane system low-pressure flushing water, chemical cleaning water, lithium extraction system dispensing water and the like of a lithium extraction system.

Preferably, the calcium and magnesium removal ion exchange unit comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence.

In the invention, by adopting a mode of combining reverse osmosis, nanofiltration and ion exchange, calcium and magnesium ions in the solution can be thoroughly removed, and the produced water can be reused in a lithium extraction system.

As the preferable technical scheme of the invention, the boron removal system also comprises 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 which are sequentially connected.

Preferably, a concentrated water outlet of the primary nano-filtration boron removal unit is connected with a water inlet of the primary reverse osmosis boron removal unit.

Preferably, the concentrated water of the three-stage reverse osmosis boron removal unit is used for preparing borax.

Preferably, the boron removal system further comprises a second-stage reverse osmosis boron removal unit, wherein a concentrated water outlet of the second-stage reverse osmosis boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit, and a produced water outlet of the second-stage reverse osmosis boron removal unit is connected with a water inlet of the second-stage reverse osmosis boron removal unit.

Preferably, concentrated water outlets of the first-stage reverse osmosis boron removal unit and the second-stage reverse osmosis boron removal unit are connected with a water inlet of the second-stage reverse osmosis boron removal unit.

Preferably, the primary nanofiltration boron removal unit comprises an alkali adding device, a safety filtration device and a nanofiltration membrane device which are connected in sequence.

Preferably, the first-stage reverse osmosis boron removal unit comprises an acid adding device, a dialysis device, a filtering device and a reverse osmosis device which are sequentially connected, wherein the dialysis water inlet flow in the dialysis device is 3-15 times of the water inlet flow of the first-stage reverse osmosis boron removal unit, and a multi-stage grading dialysis mode is adopted.

Preferably, the three-stage reverse osmosis boron removal unit comprises an alkali adding device, a safety filter device and a reverse osmosis device which are connected in sequence.

Preferably, the multistage nanofiltration boron removal unit comprises an alkali adding device, a safety filter device and at least two stages of nanofiltration membrane devices which are connected in sequence.

Preferably, the boron-removing ion exchange unit comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence.

According to the invention, the boron ions in the solution can be thoroughly removed by combining nanofiltration, reverse osmosis and ion exchange, and the boron yield is improved.

As a preferred embodiment of the present invention, the concentration system comprises a high pressure reverse osmosis unit or an electrodialysis unit.

Preferably, the high-pressure reverse osmosis unit is a high-pressure roll-type reverse osmosis membrane module or a DTRO butterfly tube type membrane module.

As a preferable technical scheme of the invention, the lithium precipitation system further comprises a precise filtering system and a water washing and drying system which are sequentially connected.

Preferably, the system further comprises a lithium precipitation mother liquor recycling system.

Preferably, the lithium precipitation mother liquor recycling system comprises a heat exchange unit, a primary nanofiltration carbonate recovery unit and a secondary nanofiltration carbonate recovery unit which are sequentially connected.

Preferably, a mother liquor outlet of the lithium precipitation system is connected with a water inlet of the heat exchange unit.

Preferably, concentrated water outlets of the primary nanofiltration carbonate recovery unit and the secondary nanofiltration carbonate recovery unit are both connected with a water inlet of the lithium precipitation system.

Preferably, the produced water of the secondary nanofiltration carbonate recovery unit is recycled to the primary reverse osmosis concentration unit.

According to the invention, the consumption of sodium carbonate in the treatment process of the lithium precipitation system is reduced by arranging the carbonate recovery unit.

In a second aspect, the present invention provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, wherein the method employs the system of the first aspect, and includes the following steps:

(1) Carrying out precipitation reaction on salt lake brine sequentially through a coagulating sedimentation system, and carrying out filtration treatment on a filtration system and adsorption and desorption treatment on an adsorption and desorption system to obtain a lithium chloride solution;

(2) Sequentially passing the lithium chloride solution obtained in the step (1) through a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multistage nanofiltration calcium and magnesium removal unit and a calcium and magnesium removal ion exchange unit to remove calcium and magnesium ions;

(3) Sequentially treating the lithium chloride solution with calcium and magnesium ions removed in the step (2) by a primary nanofiltration boron removal unit, a multistage nanofiltration boron removal unit and a boron ion removal exchange unit to remove boron ions;

(4) And (4) sequentially carrying out concentration treatment on the lithium chloride solution from which the boron ions are removed in the step (3) by a concentration system and evaporation treatment by an evaporation system, and finally adding sodium carbonate by a sodium carbonate adding unit and reacting by a lithium precipitation reaction unit to obtain the battery-grade lithium carbonate.

By adopting the system provided by the invention, lithium elements can be extracted from salt lake brine, and the prepared battery-grade lithium carbonate meets YS/T582-2013 standard regulation.

As a preferable technical scheme of the invention, in the salt lake brine obtained in the step (1), the content of lithium ions 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 less than or equal to 500ppm, the content of potassium ions is less than or equal to 8000ppm, the content of carbonate is less than or equal to 400ppm, the content of sulfate radicals is 10000-20000 ppm, and the content of boron ions is less than or equal to 600ppm.

Preferably, the filtration speed of the filtration treatment in the step (1) is 5-15 m/h.

Preferably, the adsorbent in the adsorption and desorption treatment in step (1) includes any one of an aluminum-based lithium adsorbent, a manganese-based lithium adsorbent, and a titanium-based lithium adsorbent.

Preferably, the pH of the lithium chloride solution obtained in the step (2) is adjusted to 3-6 before entering the first-stage reverse osmosis concentration unit and the first-stage nanofiltration calcium and magnesium removal unit.

Preferably, the pH of the lithium chloride solution in the step (2) is adjusted to 7-10 before entering the calcium and magnesium removal ion exchange unit.

As a preferable technical scheme of the invention, the pH of the lithium chloride solution obtained in the step (3) is adjusted to 9.2-11 before entering the primary boron nanofiltration unit and the multistage boron nanofiltration unit.

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

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

Preferably, the pH of the lithium chloride solution after the sodium carbonate is added in the step (4) is 9 to 12.

Preferably, the temperature of the reaction in step (4) is 80 to 99 ℃.

As a preferable technical scheme of the invention, a lithium carbonate solution and a lithium deposition mother solution are obtained after the reaction of the lithium deposition reaction unit in the step (4).

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.

Preferably, the lithium precipitation mother liquor is subjected to heat exchange by a heat exchange unit, a primary nanofiltration carbonate recovery unit and a secondary nanofiltration carbonate recovery unit in sequence to separate recovered carbonate ions and a lithium chloride solution.

Preferably, the temperature after heat exchange is 5-40 ℃.

Compared with the prior art, the invention has the following beneficial effects:

the method carries out advanced treatment on the salt lake brine with low magnesium-lithium ratio, extracts lithium element from the salt lake brine to prepare battery-grade lithium carbonate and extracts boron element to prepare borax to the maximum extent through the comprehensive application of each process and the regulation and control of pH; carbonate is extracted from the mother liquor of the lithium precipitation system and is reused in the lithium precipitation system, so that the consumption of sodium carbonate in the production process of the lithium precipitation system is reduced; the water resource in the salt lake brine is recycled to be used in the lithium extraction system to the maximum extent, the consumption of external water is reduced, and the comprehensive development and utilization of the salt lake brine deep resource are realized.

Drawings

Fig. 1 is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate according to example 1;

fig. 2 is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate according to example 2;

fig. 3 is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate according to example 3;

fig. 4 is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate according to example 4;

fig. 5 is a schematic structural diagram of a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate according to example 5;

the system comprises a 10-coagulating sedimentation system, a 20-filtering unit, a 30-ultrafiltration unit, a 40-first-stage impurity removal and nanofiltration unit, a 50-second-stage impurity removal and nanofiltration unit, a 60-adsorption and desorption system, a 70-first-stage reverse osmosis concentration unit, an 80-first-stage nanofiltration calcium and magnesium removal unit, a 90-second-stage reverse osmosis concentration unit, a 100-multistage nanofiltration calcium and magnesium removal unit, a 110-calcium and magnesium removal ion exchange unit, a 120-first-stage nanofiltration boron removal unit, a 130-multistage nanofiltration boron removal unit, a 140-boron removal ion exchange unit, a 150-concentration system, a 160-evaporation system, a 170-lithium deposition system, a 180-precision filtration system, a 190-water washing and drying system, an 11-dehydration system, an 81-second-stage nanofiltration calcium and magnesium removal unit, an 82-third-stage reverse osmosis concentration unit, a 121-first-stage reverse osmosis boron removal unit, a 122-second-stage reverse osmosis boron removal unit, a 123-third-stage reverse osmosis boron removal unit, a 124-evaporation and crystallization device, a 125-second-stage boron removal unit, a 171-first-stage reverse osmosis heat exchange unit, a 172-nanofiltration carbonate recovery unit and a 173-second-reverse osmosis carbonate recovery unit.

Detailed Description

The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the examples are only for the understanding of the present invention and should not be construed as the specific limitation of the present invention.

The invention provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, which comprises a coagulating sedimentation system, a filtering 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 sequentially connected;

the calcium and magnesium removal system comprises a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multistage nanofiltration calcium and magnesium removal unit and a calcium and magnesium removal ion exchange unit which are sequentially connected;

the water outlet of the adsorption and desorption system is connected with the water inlet of the first-stage reverse osmosis concentration unit;

a concentrated water outlet of the multi-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the first-stage reverse osmosis concentration unit;

the boron removal system comprises a primary nanofiltration boron removal unit, a multistage nanofiltration boron removal unit and a boron removal ion exchange unit which are sequentially connected;

the water outlet of the calcium and magnesium removal ion exchange unit is connected with the water inlet of the primary nanofiltration boron removal unit;

a concentrated water outlet of the multi-stage nanofiltration boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit;

the produced water of the first-stage reverse osmosis concentration unit, the produced water of the second-stage reverse osmosis concentration unit, the produced water of the concentration system and the distilled water of the evaporation system are all treated by the third-stage reverse osmosis concentration unit and then recycled;

the lithium precipitation system comprises a sodium carbonate adding unit and a lithium precipitation reaction unit; the lithium precipitation system is used for producing battery-grade lithium carbonate.

In the invention, the coagulating sedimentation system comprises a coagulating reaction unit, a flocculation reaction unit and a sedimentation unit which are connected in sequence.

In the invention, a bactericide adding device is arranged in front of the coagulation reaction unit; the coagulation reaction unit comprises a coagulant adding device; the flocculation reaction unit comprises a flocculating agent feeding device.

In the invention, the precipitation unit comprises any one of an inclined plate precipitation unit, a horizontal flow precipitation unit or a vertical flow precipitation unit.

The filtering system comprises a filtering unit, an ultrafiltration unit, a primary impurity removal and nanofiltration unit and a secondary impurity removal and nanofiltration unit which are connected in sequence.

In the invention, the filtering unit comprises any one of a V-shaped filtering pool, a quartz sand filtering device, a multi-medium filtering device or a variable-gap filtering device or a combination of at least two of the filtering pools.

In the invention, the ultrafiltration membrane in the ultrafiltration unit comprises any one of polysulfone, polyvinylidene fluoride, polyvinyl chloride or ceramic membrane.

In the present invention, the pore diameter of the ultrafiltration membrane is 0.001 to 0.02. Mu.m, and may be, for example, 0.001. Mu.m, 0.005. Mu.m, 0.01. Mu.m, 0.015. Mu.m, or 0.02. Mu.m, but is not limited to the above-mentioned values, and other values not mentioned in the above-mentioned range are also applicable.

In the invention, the ultrafiltration unit also comprises a preposed filtering device, wherein the filtering device comprises any one of a bag filter, a folding filter element type filter or a self-cleaning filter, and the filtering precision is 50-200 μm.

In the invention, the sludge outlet of the precipitation unit and the backwashing water outlet of the filtering unit are both connected with a dewatering system.

In the invention, a filtrate outlet of the dehydration system is connected with a water inlet of the coagulation reaction unit.

In the invention, the dehydration system comprises a coagulation reaction system, a dosing system and a dehydrator system which are connected in sequence.

In the invention, a concentrated water outlet of the secondary impurity removal nanofiltration unit is connected with a water inlet of the primary impurity removal nanofiltration unit.

In the invention, the primary impurity removal nanofiltration unit comprises an acid adding device, a security filtration device and a nanofiltration membrane device which are connected in sequence.

In the invention, the secondary impurity removal nanofiltration unit comprises a security filtration device and a nanofiltration membrane device which are connected in sequence.

In the invention, the adsorption and analysis system comprises an adsorption device and an analysis device.

In the invention, the adsorption device comprises a continuous turntable ion exchange device or a valve array type ion exchange device.

In the invention, the calcium and magnesium removal system also comprises a three-stage reverse osmosis concentration unit and a two-stage nanofiltration calcium and magnesium removal unit.

In the invention, a concentrated water outlet of the first-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the second-stage nanofiltration calcium and magnesium removal unit; the water outlet of the second-stage nanofiltration calcium and magnesium removal unit is connected with the water inlet of the second-stage reverse osmosis concentration unit; concentrated water of the two-section nanofiltration calcium and magnesium removal unit is discharged; and a concentrated water outlet of the third-stage reverse osmosis concentration unit is connected with a water inlet of the first-stage nanofiltration calcium and magnesium removal unit.

In the invention, the first-stage reverse osmosis concentration unit comprises an acid adding device, a safety filter device and a reverse osmosis device which are connected in sequence.

In the invention, the primary nanofiltration calcium and magnesium removal unit comprises an acid adding device, a safety filter device and a nanofiltration membrane device which are sequentially connected.

In the invention, the two-stage nanofiltration calcium and magnesium removal unit comprises a nanofiltration dialysis device, a safety filtration device and a nanofiltration membrane device which are connected in sequence.

In the invention, the dialyzing water of the nanofiltration dialysis device is low-salt pure water, and the conductivity of the pure water is less than or equal to 100us/cm; the dialysis water inflow is 3-6 times of the two-stage nanofiltration water inflow, and a multi-stage grading dialysis mode is adopted.

In the invention, the secondary reverse osmosis concentration unit comprises a security filter device and a reverse osmosis device which are connected in sequence.

In the invention, the multi-stage nanofiltration calcium and magnesium removal unit comprises a security filter device and at least two stages of nanofiltration membrane devices which are sequentially connected, and the produced water of the previous stage of nanofiltration membrane device enters the next stage of nanofiltration membrane device for further treatment; the final stage of produced water of the multi-stage nanofiltration membrane device enters a calcium and magnesium ion exchange unit for treatment; concentrated water of each stage of the multi-stage nanofiltration membrane device enters a first-stage reverse osmosis concentration unit for treatment.

In the invention, the three-stage reverse osmosis concentration unit comprises a safety filter device and a reverse osmosis device which are connected in sequence.

In the invention, the calcium and magnesium removal ion exchange unit comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence.

In the present invention, the resin used in the resin regeneration device includes any one of a gel-type ion exchange resin, a macroporous-type ion exchange resin, a carrier-type ion exchange resin, and a chelate resin.

In the invention, the boron removal system also comprises a primary reverse osmosis boron removal unit, a secondary reverse osmosis boron removal unit and a tertiary reverse osmosis boron removal unit which are connected in sequence; a concentrated water outlet of the primary nanofiltration boron removal unit is connected with a water inlet of the primary reverse osmosis boron removal unit; concentrated water of the three-stage reverse osmosis boron removal unit is used for preparing borax; the concentrated water outlet of the three-stage reverse osmosis boron removal unit is connected with the evaporation plant, and the produced water of the three-stage reverse osmosis boron removal unit and the distilled water of the evaporation plant are recycled after being treated by the three-stage reverse osmosis concentration unit.

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

In the invention, the boron removal system also comprises a two-stage reverse osmosis boron removal unit, wherein a concentrated water outlet of the two-stage reverse osmosis boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit, and a produced water outlet of the two-stage reverse osmosis boron removal unit is connected with a water inlet of the second-stage reverse osmosis boron removal unit; and concentrated water outlets of the first-stage reverse osmosis boron removal unit and the second-stage reverse osmosis boron removal unit are connected with a water inlet of the second-stage reverse osmosis boron removal unit.

In the invention, the primary nanofiltration boron removal unit comprises an alkali adding device, a security filtering device and a nanofiltration membrane device which are sequentially connected.

In the invention, the primary reverse osmosis boron removal unit comprises an acid adding device, a dialysis device, a filtering device and a reverse osmosis device which are sequentially connected, wherein the dialysis water inlet flow in the dialysis device is 3-15 times of the water inlet flow of the primary reverse osmosis boron removal unit, and a multi-stage grading dialysis mode is adopted.

In the invention, the two-stage reverse osmosis boron removal unit comprises a safety filter device and a reverse osmosis device which are connected in sequence.

In the invention, the secondary reverse osmosis boron removal unit comprises a security filter device and a reverse osmosis device which are connected in sequence.

In the invention, the three-stage reverse osmosis boron removal unit comprises an alkali adding device, a safety filter device and a reverse osmosis device which are sequentially connected.

In the invention, the multistage nanofiltration boron removal unit comprises an alkali adding device, a safety filter device and at least two stages of nanofiltration membrane devices which are sequentially connected, and the produced water of the previous stage of nanofiltration membrane device enters the next stage of nanofiltration membrane device for further treatment; the final stage of produced water of the multi-stage nanofiltration membrane device enters a boron-removing ion exchange unit for treatment; and the concentrated water of each stage of the multi-stage nanofiltration membrane device enters a first-stage boron removal nanofiltration unit for treatment.

In the invention, the boron-removing ion exchange unit comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence.

In the present invention, the concentration system comprises a high pressure reverse osmosis unit or an electrodialysis unit; the high-pressure reverse osmosis unit is a high-pressure roll type reverse osmosis membrane module or a DTRO butterfly tube type membrane module.

In the invention, a security filter device is arranged in front of the concentration system.

In the invention, the evaporation system comprises 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.

In the invention, the lithium precipitation system is followed by a precise filtering system and a water washing and drying system which are connected in sequence.

In the invention, the system also comprises a lithium precipitation mother liquor recycling system.

According to the invention, the lithium precipitation mother liquor recycling system comprises a heat exchange unit, a primary nanofiltration carbonate recovery unit and a secondary nanofiltration carbonate recovery unit which are sequentially connected; a mother liquor outlet of the lithium precipitation system is connected with a water inlet of the heat exchange unit; concentrated water outlets of the primary nanofiltration carbonate recovery unit and the secondary nanofiltration carbonate recovery unit are connected with a water inlet of the lithium precipitation system; and the produced water of the secondary nanofiltration carbonate recovery unit is recycled to the primary reverse osmosis concentration unit.

In the invention, the heat exchange unit comprises a plate type heat exchange device or a tubular type heat exchange device.

According to the invention, the primary nanofiltration carbonate recovery unit comprises a dialysis device, a security filtration device and a nanofiltration membrane device which are sequentially connected, wherein the dialysis water inlet flow of the dialysis device is 1-5 times of the water inlet flow of the primary nanofiltration carbonate recovery unit, and a multi-stage grading dialysis mode is adopted; the water for dialysis is pure water with low salt content, and the conductivity of the pure water is less than or equal to 100us/cm.

In the invention, the secondary nanofiltration carbonate recovery unit comprises a security filtration device and a nanofiltration membrane device which are connected in sequence.

According to the invention, the consumption of sodium carbonate in the treatment process of the lithium precipitation system is reduced by arranging the carbonate recovery unit.

In the present invention, the pore diameter of the nanofiltration membrane in the nanofiltration membrane apparatus is 1 to 2nm, and may be, for example, 1nm, 1.2nm, 1.4nm, 1.6nm, 1.8nm, or 2nm, but is not limited to the above numerical values, and other numerical values not listed in the numerical value range are also applicable.

In the present invention, the filtration precision of the security filter (microfiltration device) is 1 to 5 μm, and may be, for example, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm, but is not limited to the numerical values listed above, and other numerical values not listed above within the numerical value range are also applicable.

The invention also provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, which adopts the system of the first aspect and specifically comprises the following steps:

(1) Performing precipitation treatment on the salt lake brine in a coagulation reaction unit, a flocculation reaction unit and a precipitation unit to reduce suspended matters (SS), solid particles and partial organic matters in the water;

in the invention, the content of lithium ions in the salt lake brine in the step (1) is 600-1200 ppm, for example, 600ppm, 800ppm, 1000ppm or 1200 ppm; the magnesium ion content is 2000 to 6000ppm, for example 2000ppm, 3000ppm, 4000ppm, 5000ppm or 6000 ppm; the sodium ion content is 25000 to 50000ppm, and may be 25000ppm, 30000ppm, 35000ppm, 40000ppm, 45000ppm, 50000ppm, or the like, for example; the calcium ion content is 500ppm or less, and may be, for example, 100ppm, 200ppm, 300ppm, 400ppm or 500 ppm; the potassium ion content is 8000ppm or less, for example 1000ppm, 3000ppm, 5000ppm, 7000ppm or 8000 ppm; the carbonate content is 400ppm or less, and may be, for example, 100ppm, 200ppm, 300ppm or 400 ppm; the sulfate group content is 10000 to 20000ppm, for example, 10000ppm, 12000ppm, 14000ppm, 18000ppm or 20000 ppm; the boron ion content is 600ppm or less, and may be, for example, 200ppm, 300ppm, 400ppm or 600ppm, but is not limited to the recited values, and other values not recited within the range of the recited values are also applicable;

in the invention, the coagulant in the coagulation reaction unit in the step (1) comprises any one of polyaluminium chloride, polyferric sulfate or polyacrylamide; the bactericide is an oxidizing bactericide or a non-oxidizing bactericide;

(2) The water body obtained in the step (1) enters a filtering unit for filtering treatment, and SS in the water is further reduced;

(3) Filtering the water body obtained in the step (2) in an ultrafiltration unit to deeply remove SS and colloid in the water;

(4) Sequentially feeding the water obtained in the step (3) into a primary impurity removal and nanofiltration unit and a secondary impurity removal and nanofiltration unit for filtration and impurity removal treatment, and further removing divalent ions in the water;

in the invention, before the water enters a primary impurity removal nanofiltration unit, the pH value of the water is adjusted to 3-5;

in the present invention, the filtration rate of the filtration treatment is 5 to 15m/h, and may be, for example, 5m/h, 7m/h, 9m/h, 13m/h or 15m/h, but is not limited to the above-mentioned values, and other values not shown in the above-mentioned value range are also applicable;

(5) Allowing the water obtained in the step (4) to enter an adsorption and desorption system for adsorption and desorption treatment, adsorbing lithium chloride in salt lake brine, separating and removing other impurity components, and performing desorption to obtain a lithium chloride solution;

in the present invention, the adsorbent in the adsorption and desorption treatment includes any one of an aluminum-based lithium adsorbent, a manganese-based lithium adsorbent, or a titanium-based lithium adsorbent; the solution can be normal temperature solution or 20-40 deg.C solution. When the aluminum lithium adsorbent is adopted, the analysis device adopts pure water for analysis, and the conductivity of the pure water is less than or equal to 100us/cm; when the manganese-based or titanium-based lithium adsorbent is adopted, the desorption device adopts acid liquor for desorption, and is provided with a heating device or a heat exchange device;

(6) Concentrating the lithium chloride solution obtained in the step (5) in a first-stage reverse osmosis concentration unit;

in the present invention, the pH of the lithium chloride solution is adjusted to 3 to 6, for example, 3, 3.5, 4, 4.5, 5, 5.5 or 6 before entering the first reverse osmosis concentration unit, but is not limited to the values listed, and other values not listed in the range of the values are also applicable;

(7) Enabling the concentrated water obtained in the step (6) to enter a first-stage nanofiltration calcium and magnesium removal unit for calcium and magnesium removal treatment, and reducing divalent ions such as calcium and magnesium in the solution;

in the present invention, the pH of the concentrated water is adjusted to 3 to 6, for example, 3, 3.5, 4, 4.5, 5, 5.5 or 6, before entering the first stage of the unit for removing calcium and magnesium by nanofiltration, but the pH is not limited to the above-mentioned values, and other values not listed in the above-mentioned range are also applicable.

(8) The concentrated water obtained in the step (7) enters a two-stage nanofiltration calcium and magnesium removal unit for calcium and magnesium removal treatment, divalent ions such as calcium and magnesium in the concentrated water solution are further reduced, and a lithium chloride solution is recovered and returned to a two-stage reverse osmosis concentration unit for treatment;

(9) Concentrating the produced water obtained in the step (8) in a secondary reverse osmosis concentration unit;

(10) Enabling the concentrated water obtained in the step (9) to enter a multi-stage nanofiltration calcium and magnesium removal unit for calcium and magnesium removal treatment, and further reducing divalent ions such as calcium and magnesium in the solution;

(11) The produced water obtained in the step (10) enters a calcium and magnesium removal ion exchange unit for calcium and magnesium removal treatment, so that calcium and magnesium ions in the solution are completely removed;

in the present invention, the pH of the produced water is adjusted to 7 to 10, for example, 7, 8, 9 or 10 before it is introduced into the calcium-magnesium removal ion exchange unit, but the pH is not limited to the values listed, and other values not listed in the range of the pH are also applicable.

(12) The produced water obtained in the step (11) enters a primary nanofiltration boron removal unit for boron removal treatment, so that boron ions in the solution are reduced;

in the present invention, the pH of the product water is adjusted to 9.2 to 11, for example, 9.2, 9.5, 10, 10.5 or 11, before the product water enters the first-stage boron nanofiltration unit and the multi-stage boron nanofiltration unit, but the pH is not limited to the above-mentioned values, and other values not shown in the above-mentioned range are also applicable.

(13) Treating the concentrated water obtained in the step (12) in a first-stage reverse osmosis boron removal unit, and recovering boron ions in the solution;

in the present invention, the pH of the concentrated water is adjusted to 3 to 6, for example, 3, 3.5, 4, 4.5, 5, 5.5 or 6 before it is introduced into the first reverse osmosis boron removal unit, but the pH is not limited to the above-mentioned values, and other values not shown in the above-mentioned range are also applicable.

(14) Treating the concentrated water obtained in the step (13) in a two-stage reverse osmosis boron removal unit, and further recovering boron ions in the solution;

(15) Treating the produced water obtained in the step (13) in a secondary reverse osmosis boron removal unit; further recovering boron ions in the solution;

(16) Treating the produced water obtained in the step (15) in a three-stage reverse osmosis boron removal unit; further recovering boron ions in the solution;

in the present invention, the pH of the produced water is adjusted to 9.2 to 11, for example, 9.2, 9.4, 9.8, 10, 10.4, 10.8 or 11 before it is introduced into the three-stage reverse osmosis boron removal unit, but the pH is not limited to the values listed above, and other values not listed above in the range of the values are also applicable.

(17) Enabling the concentrated water obtained in the step (16) to enter an evaporation system for evaporation crystallization treatment to prepare borax;

(18) The produced water obtained in the step (12) enters a multi-stage nanofiltration boron removal unit for boron removal treatment, so that boron ions in the solution are further reduced;

(19) The produced water obtained in the step (18) enters a boron removal ion exchange unit for boron removal treatment, so that boron ions in the solution are completely removed;

in the present invention, the pH of the produced water is adjusted to 9.2 to 10, for example, 9.2, 9.5 or 10 before it is introduced into the boron-removing ion exchange unit, but the pH is not limited to the values listed, and other values not listed in the range of the values are also applicable.

(20) Concentrating the produced water obtained in the step (19) in a concentration system, and further concentrating the lithium chloride solution;

(21) Concentrated water obtained in the step (20) enters an evaporation unit for concentration treatment, and lithium chloride solution is further concentrated;

in the invention, the temperature of the low-temperature evaporator used in the evaporation unit is 35-55 ℃, and the temperature of the high-temperature evaporator is 85-99 ℃.

(22) Adding sodium carbonate into the concentrated water obtained in the step (21) through a sodium carbonate adding unit and reacting with a lithium precipitation reaction unit to obtain lithium carbonate precipitation and lithium precipitation mother liquor;

in the present invention, the mass ratio of the sodium carbonate to the lithium chloride in the concentrated water is (1-1.2): 1, and for example, 1, 1.1.

In the present invention, the pH of the concentrated water (lithium chloride solution) after adding sodium carbonate may be 9 to 12, for example, 9, 9.5, 10, 10.5, 11, 11.5 or 12, but is not limited to the above-mentioned values, and other values not shown in the above-mentioned range are also applicable.

In the present invention, the reaction temperature is 80 to 99 ℃, for example, 80 ℃, 84 ℃, 88 ℃, 90 ℃, 94 ℃ or 99 ℃, but the reaction temperature is not limited to the recited values, and other values not recited in the numerical range are also applicable.

(23) The lithium precipitation mother liquor obtained in the step (22) enters a heat exchange unit for heat exchange treatment, and the temperature of the lithium precipitation mother liquor is reduced;

in the present invention, the temperature after the heat exchange treatment is 5 to 40 ℃ and may be, for example, 5 ℃,10 ℃, 15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃ or 40 ℃, but is not limited to the values listed, and other values not listed in the range of the values are also applicable.

(24) Enabling the lithium precipitation mother liquor obtained in the step (23) to enter a primary nanofiltration carbonate recovery unit for primary nanofiltration carbonate recovery treatment, and separating and recovering carbonate ions and a lithium chloride solution in the solution;

(25) The produced water obtained in the step (24) enters a secondary nanofiltration carbonate recovery unit to be subjected to secondary nanofiltration carbonate recovery treatment, and carbonate ions and a lithium chloride solution in the recovered solution are further separated;

(26) Enabling the lithium carbonate precipitation solution obtained in the step (22) to enter a precise filtering system for precise filtering treatment, and further removing impurities in the lithium carbonate precipitation solution;

(27) And (5) enabling the lithium carbonate precipitation solution obtained in the step (26) to enter a washing and drying system to be sequentially subjected to washing treatment and drying treatment, and obtaining a battery-grade lithium carbonate product.

In the invention, the water washing comprises the following steps: and (3) washing the lithium carbonate precipitate for 3-5 times by using pure water, wherein the pure water is ultrapure water or distilled water, and the conductivity is less than or equal to 10us/cm.

In the invention, the drying temperature is 70-360 ℃.

The recitation of numerical ranges herein includes not only the above-recited values, but also any values between any of the above-recited numerical ranges not recited, and for brevity and clarity, is not intended to be exhaustive of the specific values encompassed within the range.

Example 1

The embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and a schematic structural diagram of the system is shown in fig. 1; the system comprises a coagulating sedimentation system 10, a filtering 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 which are connected in sequence;

the coagulation sedimentation system 10 comprises a coagulation reaction unit, a flocculation reaction unit and a sedimentation unit which are connected in sequence; the sludge outlet of the precipitation unit and the backwashing water outlet of the filtering unit 20 are both connected with a dewatering system 11; a filtrate outlet of the dehydration system 11 is connected with a water inlet of the coagulation reaction unit;

a bactericide adding device is arranged in front of the coagulation reaction unit; the coagulation reaction unit is a coagulant adding device; the flocculation reaction unit is a flocculating agent adding device; the precipitation unit is an inclined plate precipitation unit; the dehydration system 11 comprises a coagulation reaction system, a dosing system and a dehydrator system which are connected in sequence;

the filtration system comprises a filtration unit 20, an ultrafiltration unit 30, a primary impurity removal and nanofiltration unit 40 and a secondary impurity removal and nanofiltration unit 50 which are connected in sequence; a concentrated water outlet of the secondary impurity-removing nanofiltration unit 50 is connected with a water inlet of the primary impurity-removing nanofiltration unit 40;

the filtering unit 20 is a quartz sand filtering device; the ultrafiltration unit 30 comprises a filter device and an ultrafiltration membrane which are connected in sequence, wherein the filter device is a folding filter element type filter, and the filtering precision of the filter device is 100 mu m; the ultrafiltration membrane is made of polyvinylidene fluoride, and the aperture of the ultrafiltration membrane is 0.005 mu m; the primary impurity removal nanofiltration unit 40 comprises an acid adding device, a security filtration device and a nanofiltration membrane device which are connected in sequence; the secondary impurity removal nanofiltration unit 50 comprises a security filtration device and a nanofiltration membrane device which are connected in sequence;

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;

the calcium and magnesium removal system comprises a primary reverse osmosis concentration unit 70, a primary nanofiltration calcium and magnesium removal unit 80, a secondary reverse osmosis concentration unit 90, a multistage nanofiltration calcium and magnesium removal unit 100 and a calcium and magnesium removal ion exchange unit 110 which are sequentially connected; the water outlet of the adsorption and desorption system is connected with the water inlet of the first-stage reverse osmosis concentration unit 70; a concentrated water outlet of the multistage nanofiltration calcium and magnesium removal unit 100 is connected with a water inlet of the first-stage reverse osmosis concentration unit 70;

the first-stage reverse osmosis concentration unit 70 comprises an acid adding device, a safety filter device and a reverse osmosis device which are connected in sequence; the first-stage nanofiltration calcium and magnesium removal unit 80 comprises an acid adding device, a security filtering device and a nanofiltration membrane device which are connected in sequence; the second-stage reverse osmosis concentration unit 90 comprises a security filter device and a reverse osmosis device which are connected in sequence; the multistage nanofiltration calcium and magnesium removal unit 100 comprises a security filtration device and at least two stages of nanofiltration membrane devices which are sequentially connected, and water produced by the previous stage of nanofiltration membrane device enters the next stage of nanofiltration membrane device for further treatment; the final stage of produced water of the multi-stage nanofiltration membrane device enters a calcium and magnesium removal ion exchange unit for treatment; concentrated water of each stage of the multi-stage nanofiltration membrane device enters a first-stage reverse osmosis concentration unit for treatment; the calcium and magnesium removal ion exchange unit 110 comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence; the resin adopted by the resin regeneration device is gel type ion exchange resin;

the boron removal system comprises a primary nanofiltration boron removal unit 120, a multistage nanofiltration boron removal unit 130 and a boron removal ion exchange unit 140 which are connected in sequence; a concentrated water outlet of the multi-stage nano-filtration boron removal unit 130 is connected with a water inlet of the first-stage nano-filtration boron removal unit 120;

the primary nanofiltration boron removal unit 120 comprises an alkali adding device, a security filtering device and a nanofiltration membrane device which are connected in sequence; the multistage nanofiltration boron removal unit 130 comprises an alkali adding device, a safety filter device and at least two stages of nanofiltration membrane devices which are sequentially connected, and the produced water of the previous stage of nanofiltration membrane device enters the next stage of nanofiltration membrane device for further treatment; the final first-stage produced water of the multi-stage nanofiltration membrane device enters a boron-removing ion exchange unit for treatment; concentrated water of each stage of the multi-stage nanofiltration membrane device enters a first-stage boron removal nanofiltration unit for treatment; the boron removal ion exchange unit 140 comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence;

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 roll type reverse osmosis membrane module, and a safety filter device is arranged in front of the high-pressure reverse osmosis unit;

the produced water of the first-stage reverse osmosis concentration unit 70, the produced water of the second-stage reverse osmosis concentration unit 90, the produced water of the high-pressure reverse osmosis unit and the distilled water of the multi-effect evaporation device are treated by the third-stage reverse osmosis concentration unit 82 and then recycled;

the lithium precipitation system 170 comprises a sodium carbonate adding unit and a lithium precipitation reaction unit; the lithium precipitation system 170 is used to produce battery grade lithium carbonate;

the lithium precipitation system 170 is further provided with a precise filtering system 180 and a water washing and drying system 190 which are sequentially connected;

the aperture of the nanofiltration membrane in the nanofiltration membrane device is 1nm; the filtering precision of the security filter device (precise filter device) is 3 μm.

Example 2

The embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and a schematic structural diagram of the system is shown in fig. 2; the system provided by the embodiment 1 further comprises the following units:

the calcium and magnesium removal system also comprises a three-stage reverse osmosis concentration unit 82 and a two-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 with the water inlet of the second-stage nanofiltration calcium and magnesium removal unit 81; the water production outlet of the two-stage nanofiltration calcium and magnesium removal unit 81 is connected with the water inlet of the two-stage reverse osmosis concentration unit 90; concentrated water of the two-stage nanofiltration calcium and magnesium removal unit 81 is discharged; a concentrated water outlet of the third-stage reverse osmosis concentration unit 82 is connected with a water inlet of the first-stage nanofiltration calcium and magnesium removal unit 80;

the two-stage nanofiltration calcium and magnesium removal unit 81 comprises a nanofiltration dialysis device, a security filtration device and a nanofiltration membrane device which are sequentially connected, wherein the dialysis water of the nanofiltration dialysis device is low-salt pure water, the conductivity of the pure water is less than or equal to 100us/cm, the dialysis water inflow is 5 times of the two-stage nanofiltration water inflow, and a multi-stage grading dialysis mode is adopted; the third-stage reverse osmosis concentration unit 82 comprises a security filtration device and a reverse osmosis device which are connected in sequence.

Example 3

The embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and a schematic structural diagram of the system is shown in fig. 3; the system provided by the embodiment 1 further comprises the following units:

the boron removal system also comprises a primary reverse osmosis boron removal unit 121, a secondary reverse osmosis boron removal unit 122 and a tertiary reverse osmosis boron removal unit 123 which are sequentially connected; the concentrated water outlet of the primary nano-filtration boron removal unit 120 is connected with the water inlet of the primary reverse osmosis boron removal unit 121; the concentrated water of the three-stage reverse osmosis boron removal unit 123 is used for preparing borax; the concentrated water outlet of the three-stage reverse osmosis boron removal unit 123 is connected with the evaporative crystallization device 124, and the produced water of the three-stage reverse osmosis boron removal unit 123 and the distilled water of the evaporative crystallization device 124 are treated by the three-stage reverse osmosis concentration unit 82 and then recycled; the boron removal system also comprises a second-stage reverse osmosis boron removal unit 125, wherein a concentrated water outlet of the second-stage reverse osmosis boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit 120, and a produced water outlet 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 connected with the water inlet of the second-stage reverse osmosis boron removal unit 125;

the primary reverse osmosis boron removal unit 121 comprises an acid adding device, a dialysis device, a filtering device and a reverse osmosis device which are sequentially connected, wherein the dialysis water inlet flow in the dialysis device is 3-15 times of the water inlet flow of the primary reverse osmosis boron removal unit, and a multi-stage grading dialysis mode is adopted; the secondary reverse osmosis boron removal unit 122 comprises a security filtration device and a reverse osmosis device which are connected in sequence; the three-stage reverse osmosis boron removal unit 123 comprises an alkali adding device, a safety filter device and a reverse osmosis device which are connected in sequence; the two-stage reverse osmosis boron removal unit 125 comprises a security filtration device and a reverse osmosis device which are connected in sequence.

Example 4

The embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and a schematic structural diagram of the system is shown in fig. 4; the system provided by the embodiment 1 further comprises the following units:

the system also comprises a lithium precipitation mother liquor recycling system; the lithium precipitation mother liquor recycling system comprises a heat exchange unit 171, a primary nanofiltration carbonate recovery unit 172 and a secondary nanofiltration carbonate recovery unit 173 which are sequentially connected; a mother liquor outlet of the lithium precipitation system 170 is connected with a water inlet of the heat exchange unit 171; concentrated water outlets of the primary nanofiltration carbonate recovery unit 172 and the secondary nanofiltration carbonate recovery unit 173 are connected with a water inlet of the lithium precipitation system 170; the produced water of the secondary nanofiltration carbonate recovery unit 173 is recycled to the primary reverse osmosis concentration unit 70;

the heat exchange unit 171 is a plate type heat exchange device; the primary nanofiltration carbonate recovery unit 172 comprises a dialysis device, a security filtration device and a nanofiltration membrane device which are sequentially connected, wherein the dialysis water inflow of the dialysis device is 3 times of the water inflow of the primary nanofiltration carbonate recovery unit, a multi-stage fractional dialysis mode is adopted, the dialysis water is pure water with low salt content, and the conductivity of the pure water is less than or equal to 100us/cm; the secondary nanofiltration carbonate recovery unit 173 comprises a security filtration device and a nanofiltration membrane device which are connected in sequence.

Example 5

The embodiment provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and a schematic structural diagram of the system is shown in fig. 5; the system provided by the embodiment 1 further comprises the following units:

the calcium and magnesium removal system also comprises a three-stage reverse osmosis concentration unit 82 and a two-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 with the water inlet of the second-stage nanofiltration calcium and magnesium removal unit 81; the water production outlet of the second-stage nanofiltration calcium and magnesium removal unit 81 is connected with the water inlet of the second-stage reverse osmosis concentration unit 90; concentrated water of the two-stage nanofiltration calcium and magnesium removal unit 81 is discharged; a concentrated water outlet of the third-stage reverse osmosis concentration unit 82 is connected with a water inlet of the first-stage nanofiltration calcium and magnesium removal unit 80;

the second-stage nanofiltration calcium and magnesium removal unit 81 comprises a nanofiltration dialysis device, a security filtration device and a nanofiltration membrane device which are connected in sequence, wherein the dialysis water of the nanofiltration dialysis device is pure water with low salinity, and the dialysis water inflow flow is 5 times of that of the second-stage nanofiltration water inflow; the third-stage reverse osmosis concentration unit 82 comprises a security filtration device and a reverse osmosis device which are connected in sequence;

the boron removal system also comprises a primary reverse osmosis boron removal unit 121, a secondary reverse osmosis boron removal unit 122 and a tertiary reverse osmosis boron removal unit 123 which are sequentially connected; the concentrated water outlet of the primary nano-filtration boron removal unit 120 is connected with the water inlet of the primary reverse osmosis boron removal unit 121; the concentrated water of the three-stage reverse osmosis boron removal unit 123 is used for preparing borax; the concentrated water outlet of the three-stage reverse osmosis boron removal unit 123 is connected with the evaporative crystallization device 124, and the produced water of the three-stage reverse osmosis boron removal unit 123 and the distilled water of the evaporative crystallization device 124 are treated by the three-stage reverse osmosis concentration unit 82 and then recycled; the boron removal system also comprises a second-stage reverse osmosis boron removal unit 125, wherein a concentrated water outlet of the second-stage reverse osmosis boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit 120, and a produced water outlet 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 connected with the water inlet of the second-stage reverse osmosis boron removal unit 125;

the primary reverse osmosis boron removal unit 121 comprises an acid adding device, a dialysis device, a filtering device and a reverse osmosis device which are sequentially connected, wherein the dialysis water inlet flow in the dialysis device is 3-15 times of the water inlet flow of the primary reverse osmosis boron removal unit; the secondary reverse osmosis boron removal unit 122 comprises a security filter device and a reverse osmosis device which are connected in sequence; the three-stage reverse osmosis boron removal unit 123 comprises an alkali adding device, a safety filter device and a reverse osmosis device which are connected in sequence; the two-stage reverse osmosis boron removal unit 125 comprises a security filter device and a reverse osmosis device which are connected in sequence;

the system also comprises a lithium precipitation mother liquor recycling system; the lithium precipitation mother liquor recycling system comprises a heat exchange unit 171, a primary nanofiltration carbonate recovery unit 172 and a secondary nanofiltration carbonate recovery unit 173 which are sequentially connected; a mother liquor outlet of the lithium precipitation system 170 is connected with a water inlet of the heat exchange unit 171; concentrated water outlets of the primary nanofiltration carbonate recovery unit 172 and the secondary nanofiltration carbonate recovery unit 173 are connected with a water inlet of the lithium precipitation system 170; the produced water of the secondary nanofiltration carbonate recovery unit 173 is recycled to the primary reverse osmosis concentration unit 70;

the heat exchange unit 171 is a plate type heat exchange device; the primary nanofiltration carbonate recovery unit 172 comprises a dialysis device, a security filtration device and a nanofiltration membrane device which are sequentially connected, wherein the dialysis water inlet flow of the dialysis device is 1-5 times of the water inlet flow of the primary nanofiltration carbonate recovery unit; the secondary nanofiltration carbonate recovery unit 173 includes a security filtration device and a nanofiltration membrane device connected in sequence.

Example 6

The present embodiment provides a system for extracting lithium from salt lake brine and preparing battery-level lithium carbonate, except that neither the first-stage reverse osmosis concentration unit 70 nor the first-stage nanofiltration calcium and magnesium removal unit 80 in the calcium and magnesium removal system is provided with an acid adding device, and except that the calcium and magnesium ion exchange unit 110 is provided with no alkali adding device, the other conditions are the same as those in example 1.

Example 7

This example provides a system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, except that in the boron removal system, no alkali adding device is provided for each of the first-stage nanofiltration boron removal unit 120, the multistage nanofiltration boron removal unit 130, and the boron removal ion exchange unit 140, the other conditions are the same as those in example 1.

Application example 1

In the application example, the salt lake brine has the lithium ion content of 1000ppm, the magnesium ion content of 4000ppm, the sodium ion content of 40000ppm, the calcium ion content of 300ppm, the potassium ion content of 6000ppm, the carbonate content of 200ppm, the sulfate content of 15000ppm and the boron ion content of 200ppm;

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

(1) The salt lake brine sequentially passes through a coagulation reaction unit, a flocculation reaction unit and a precipitation unit to carry out precipitation reaction, wherein the coagulant adding amount in the coagulation reaction unit is 100mg/L; the adding amount of the flocculating agent in the flocculation reaction unit is 5mg/L; then, filtering and impurity removing are carried out sequentially through a filtering unit 20, an ultrafiltration unit 30, a primary impurity removing and nanofiltration unit 40 and a secondary impurity removing and nanofiltration unit 50, the pH of the salt lake brine is adjusted to be 4 before the salt lake brine enters the primary impurity removing and nanofiltration unit 40, then, adsorption and desorption treatment is carried out through an adsorption and desorption system 60, an adsorbent is an aluminum lithium adsorbent, and a desorption agent is pure water, so that a lithium chloride solution is obtained;

(2) Concentrating the lithium chloride solution obtained in the step (1) by a primary reverse osmosis concentration unit 70, filtering the obtained concentrated water by a primary nanofiltration calcium and magnesium removal unit 80 to remove calcium and magnesium ions, concentrating the obtained produced water by a secondary reverse osmosis concentration unit 90, removing the calcium and magnesium ions from the obtained concentrated water by a multistage nanofiltration calcium and magnesium removal unit 100, and completely removing the calcium and magnesium ions from the obtained produced water by a calcium and magnesium removal ion exchange unit 110 to obtain the lithium chloride solution with the calcium and magnesium ions removed;

wherein, 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 of the produced water is adjusted to 8 before entering the calcium and magnesium removal ion exchange unit 110;

(3) Sequentially treating the lithium chloride solution obtained in the step (2) by a primary nanofiltration boron removal unit 120, a multistage nanofiltration boron removal unit 130 and a boron ion removal exchange unit 140 to remove boron ions, so as to obtain a lithium chloride solution from which the boron ions are removed; the pH of the lithium chloride solution is adjusted to 9.5 before entering the primary nanofiltration boron removal unit 120 and the multistage nanofiltration boron removal unit 130; the pH is adjusted to 10 before entering the boron removal ion exchange unit 140;

(4) Sequentially carrying out concentration treatment on the lithium chloride solution obtained in the step (3) through a high-pressure reverse osmosis unit, carrying out evaporation treatment at 97 ℃ through a multiple-effect evaporation device, finally adding sodium carbonate through a sodium carbonate adding unit, wherein the mass ratio of the sodium carbonate to the lithium chloride is 1.3, adjusting the pH of the solution to 10, heating to 90 ℃ in a lithium precipitation reaction unit for reaction, and treating the obtained lithium carbonate precipitation solution through a precise filtering system 180 and a washing and drying system 190 to obtain battery-grade lithium carbonate;

in the present application example, the yield of lithium in the system was 54%, and the purity of the battery grade lithium carbonate was 99.61%.

Application example 2

The application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and the system provided by the application example 2 is applied, except that the step (2) is adjusted to be that the lithium chloride solution obtained in the step (1) is concentrated by a first-stage reverse osmosis concentration unit 70, the obtained concentrated water is sequentially subjected to first-stage nanofiltration calcium and magnesium removal unit 80 and a second-stage nanofiltration calcium and magnesium removal unit 81 to remove calcium and magnesium ions, the obtained produced water is concentrated by a second-stage reverse osmosis concentration unit 90, the calcium and magnesium ions in the obtained concentrated water are removed by a multi-stage nanofiltration calcium and magnesium removal unit 100, the calcium and magnesium ions in the obtained produced water are completely removed by a calcium and magnesium ion removal exchange unit 110 to obtain a lithium chloride solution from which the calcium and magnesium ions are removed, and other conditions are the same as those in the application example 1.

In the present application example, the yield of lithium in the system was 70%, and the purity of the battery-grade lithium carbonate was 99.61%.

Application example 3

The application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and the system provided by the application example 3 is applied, wherein the method comprises the steps of sequentially removing boron from concentrated water obtained by a primary nanofiltration boron removal unit 120 in the step (3) through a primary reverse osmosis boron removal unit 121, a secondary reverse osmosis boron removal unit 125, a secondary reverse osmosis boron removal unit 122 and a tertiary reverse osmosis boron removal unit 123, concentrating the concentrated water, and evaporating the concentrated water through an evaporation crystallization device 124 to obtain borax; the pH of the concentrated water is adjusted to 4 before the concentrated water enters the first-stage reverse osmosis boron removal unit 121; the pH of the produced water was adjusted to 10 before entering the three-stage reverse osmosis boron removal unit 123, and the other conditions were the same as in application example 1.

In the application example, the yield of lithium in the system is 62%, the purity of battery-grade lithium carbonate is 99.61%, and after the adsorption and desorption process: the yield of boron was 78%.

According to the application example, the recovery rate of boron is improved by additionally arranging the first-stage reverse osmosis boron removal unit, the second-stage reverse osmosis boron removal unit, the third-stage reverse osmosis boron removal unit and the second-stage reverse osmosis boron removal unit.

Application example 4

The application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and the system provided by the application example 4 is applied, except that the temperature of lithium precipitation mother liquor obtained after the reaction of the lithium precipitation reaction unit in the step (4) is 20 ℃ after the heat exchange of the lithium precipitation mother liquor by the heat exchange unit 171, carbonate ions and a lithium chloride solution in the solution are separated and recovered by the primary nanofiltration carbonate recovery unit 172 and the secondary nanofiltration carbonate recovery unit 173 in sequence, and other conditions are the same as those in the application example 1.

The yield of lithium in the system in this application example was 64%, and the purity of battery grade lithium carbonate was 99.61%.

According to the application example, the consumption of sodium carbonate in the production process of the lithium precipitation system is reduced by additionally arranging the lithium precipitation mother liquor recycling system.

Application example 5

The application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and the system provided in application example 5 is applied, wherein the salt lake brine is the same as in application example 1, and the method comprises the following steps:

(1) The salt lake brine sequentially passes through a coagulation reaction unit, a flocculation reaction unit and a precipitation unit to carry out precipitation reaction, wherein the coagulant adding amount in the coagulation reaction unit is 100mg/L; the adding amount of the flocculating agent in the flocculation reaction unit is 5mg/L; then, sequentially passing through a filtering unit 20, an ultrafiltration unit 30, a primary impurity removal nanofiltration unit 40 and a secondary impurity removal nanofiltration unit 50 to remove impurities, adjusting the pH of the salt lake brine to 4 before entering the primary impurity removal nanofiltration unit 40, and then performing adsorption and desorption treatment by an adsorption and desorption system 60, wherein the adsorbent is an aluminum-based lithium adsorbent, and the desorption agent is pure water, so as to obtain a lithium chloride solution;

(2) Concentrating the lithium chloride solution obtained in the step (1) by a first-stage reverse osmosis concentration unit 70, sequentially removing calcium and magnesium ions from the obtained concentrated water by a first-stage nanofiltration calcium and magnesium removal unit 80 and a second-stage nanofiltration calcium and magnesium removal unit 81, concentrating the obtained produced water by a second-stage reverse osmosis concentration unit 90, removing calcium and magnesium ions from the obtained concentrated water by a multi-stage nanofiltration calcium and magnesium removal unit 100, and completely removing calcium and magnesium ions from the obtained produced water by a calcium and magnesium removal ion exchange unit 110 to obtain the lithium chloride solution with calcium and magnesium ions removed;

wherein, 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 of the produced water is adjusted to 8 before entering the calcium and magnesium removal ion exchange unit 110;

(3) Sequentially treating the lithium chloride solution obtained in the step (2) by a primary nanofiltration boron removal unit 120, a multistage nanofiltration boron removal unit 130 and a boron removal ion exchange unit 140 to remove boron ions, so as to obtain a lithium chloride solution with the boron ions removed; the pH of the lithium chloride solution is adjusted to 9.5 before entering the primary nanofiltration boron removal unit 120 and the multistage nanofiltration boron removal unit 130; the pH is adjusted to 10 before entering the boron removal ion exchange unit 140;

wherein, the concentrated water obtained by the first-stage nanofiltration boron removal unit 120 is subjected to boron removal by a first-stage reverse osmosis boron removal unit 121, a second-stage reverse osmosis boron removal unit 125, a second-stage reverse osmosis boron removal unit 122 and a third-stage reverse osmosis boron removal unit 123, and concentrated, and then evaporated by an evaporation crystallization device 124 to obtain borax; the pH of the concentrated water is adjusted to 4 before the concentrated water enters the first-stage reverse osmosis boron removal unit 121; adjusting the pH value of the produced water to 10 before the produced water enters a three-stage reverse osmosis boron removal unit 123;

(4) Sequentially carrying out concentration treatment on the lithium chloride solution obtained in the step (3) through a high-pressure reverse osmosis unit, carrying out evaporation treatment at 97 ℃ through a multi-effect evaporation device, finally adding sodium carbonate through a sodium carbonate adding unit, wherein the mass ratio of the sodium carbonate to the lithium chloride is 1.3, adjusting the pH value of the solution to 10, heating the solution to 90 ℃ in a lithium precipitation reaction unit for reaction, and treating the obtained lithium carbonate precipitation solution through a precise filtering system 180 and a washing and drying system 190 to obtain battery-grade lithium carbonate;

the temperature of the lithium precipitation mother liquor obtained after the reaction of the lithium precipitation reaction unit is 20 ℃ after heat exchange of the heat exchange unit 171, and carbonate ions in the solution are separated and recovered through the primary nanofiltration carbonate recovery unit 172 and the secondary nanofiltration carbonate recovery unit 173 in sequence.

In this example, the yield of lithium in the system is 90%, the purity of battery-grade lithium carbonate is 99.61%, and after the adsorption and desorption process: the yield of boron was 96%.

Application example 6

The application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and the system provided in application example 6 is the same as in application example 1 except that the pH of the lithium chloride solution in step (2) is not controlled.

The yield of lithium in the system in this example was 45%, and the purity of lithium carbonate was 99.2%.

According to the method, the pH value of the lithium chloride solution in the calcium and magnesium removing system is regulated, so that the removal rate of calcium and magnesium ions can be improved, the stable operation of the system is ensured, and the improvement of the lithium yield and the lithium carbonate purity are facilitated.

Application example 7

The application example provides a method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, and the system provided by the application example 7 is the same as the application example 1 except that the pH value of the lithium chloride solution in the step (3) is not controlled.

The yield of lithium in the system of this example was 54%, and the purity of lithium carbonate was 99%.

This application removes the acid-base nature of lithium chloride solution in the boron system through regulation and control, not only can improve boron ion's clearance and yield, has guaranteed the steady operation of system, still is favorable to improving the yield of lithium and the purity of lithium carbonate.

The applicant declares that the present invention illustrates the detailed structural features of the present invention through 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 be implemented depending on the above detailed structural features. It should be understood by those skilled in the art that any modifications, equivalent substitutions of selected elements of the present invention, additions of auxiliary elements, selection of specific forms, etc., are intended to fall within the scope and disclosure of the present invention.

Claims (10)

1. A system for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate is characterized by comprising a coagulating sedimentation system, a filtering 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 sequentially connected;

the calcium and magnesium removal system comprises a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multistage nanofiltration calcium and magnesium removal unit and a calcium and magnesium removal ion exchange unit which are sequentially connected;

the water outlet of the adsorption and desorption system is connected with the water inlet of the first-stage reverse osmosis concentration unit;

a concentrated water outlet of the multi-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the first-stage reverse osmosis concentration unit;

the boron removal system comprises a primary nanofiltration boron removal unit, a multistage nanofiltration boron removal unit and a boron removal ion exchange unit which are sequentially connected;

the water outlet of the calcium and magnesium removal ion exchange unit is connected with the water inlet of the primary nanofiltration boron removal unit;

a concentrated water outlet of the multi-stage nanofiltration boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit;

the produced water of the first-stage reverse osmosis concentration unit, the produced water of the second-stage reverse osmosis concentration unit, the produced water of the concentration system and the distilled water of the evaporation system are all treated by the third-stage reverse osmosis concentration unit and then recycled;

the lithium precipitation system comprises a sodium carbonate adding unit and a lithium precipitation reaction unit; the lithium precipitation system is used for producing battery-grade lithium carbonate.

2. The system of claim 1, wherein the coagulation and sedimentation system comprises a coagulation reaction unit, a flocculation reaction unit and a sedimentation unit which are connected in sequence;

preferably, the filtration system comprises a filtration unit, an ultrafiltration unit, a primary impurity removal nanofiltration unit and a secondary impurity removal nanofiltration unit which are connected in sequence;

preferably, the sludge outlet of the sedimentation unit and the backwash water outlet of the filtration unit are both connected with a dewatering system;

preferably, a filtrate outlet of the dewatering system is connected with a water inlet of the coagulation reaction unit;

preferably, a concentrated water outlet of the secondary impurity removal and nanofiltration unit is connected with a water inlet of the primary impurity removal and nanofiltration unit;

preferably, the primary impurity removal nanofiltration unit comprises an acid adding device, a security filtration device and a nanofiltration membrane device which are connected in sequence;

preferably, the adsorption and desorption system comprises an adsorption device and a desorption device.

3. The system of claim 1 or 2, wherein the calcium and magnesium removal system further comprises a three-stage reverse osmosis concentration unit and a two-stage nanofiltration calcium and magnesium removal unit;

preferably, a concentrated water outlet of the first-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the second-stage nanofiltration calcium and magnesium removal unit;

preferably, a water outlet of the second-stage nanofiltration calcium and magnesium removal unit is connected with a water inlet of the second-stage reverse osmosis concentration unit;

preferably, a concentrated water outlet of the third-stage reverse osmosis concentration unit is connected with a water inlet of the first-stage nanofiltration calcium and magnesium removal unit;

preferably, the first-stage reverse osmosis concentration unit comprises an acid adding device, a safety filter device and a reverse osmosis device which are connected in sequence;

preferably, the first-stage nanofiltration calcium and magnesium removal unit comprises an acid adding device, a safety filter device and a nanofiltration membrane device which are connected in sequence;

preferably, the calcium and magnesium removal ion exchange unit comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence.

4. The system of any one of claims 1 to 3, wherein the boron removal system further comprises a primary reverse osmosis boron removal unit, a secondary reverse osmosis boron removal unit and a tertiary reverse osmosis boron removal unit connected in sequence;

preferably, a concentrated water outlet of the primary nanofiltration boron removal unit is connected with a water inlet of the primary reverse osmosis boron removal unit;

preferably, the concentrated water of the three-stage reverse osmosis boron removal unit is used for preparing borax;

preferably, the boron removal system further comprises a second-stage reverse osmosis boron removal unit, a concentrated water outlet of the second-stage reverse osmosis boron removal unit is connected with a water inlet of the first-stage nanofiltration boron removal unit, and a produced water outlet of the second-stage reverse osmosis boron removal unit is connected with a water inlet of the second-stage reverse osmosis boron removal unit;

preferably, concentrated water outlets of the first-stage reverse osmosis boron removal unit and the second-stage reverse osmosis boron removal unit are both connected with a water inlet of the second-stage reverse osmosis boron removal unit;

preferably, the primary nanofiltration boron removal unit comprises an alkali adding device, a safety filter device and a nanofiltration membrane device which are connected in sequence;

preferably, the first-stage reverse osmosis boron removal unit comprises an acid adding device, a dialysis device, a safety filter device and a reverse osmosis device which are connected in sequence;

preferably, the three-stage reverse osmosis boron removal unit comprises an alkali adding device, a safety filter device and a reverse osmosis device which are connected in sequence;

preferably, the multistage nanofiltration boron removal unit comprises an alkali adding device, a safety filter device and at least two stages of nanofiltration membrane devices which are connected in sequence;

preferably, the boron-removing ion exchange unit comprises an alkali adding device, an ion exchange device and a resin regeneration device which are connected in sequence.

5. The system of any one of claims 1-4, wherein the concentration system comprises a high pressure reverse osmosis unit or an electrodialysis unit;

preferably, the high-pressure reverse osmosis unit is a high-pressure roll-type reverse osmosis membrane module or a DTRO butterfly tube type membrane module.

6. The system according to any one of claims 1 to 5, further comprising a precision filtration system and a water washing and drying system connected in sequence after the lithium precipitation system;

preferably, the system also comprises a lithium precipitation mother liquor recycling system;

preferably, the lithium precipitation mother liquor recycling system comprises a heat exchange unit, a primary nanofiltration carbonate recovery unit and a secondary nanofiltration carbonate recovery unit which are sequentially connected;

preferably, a mother liquor outlet of the lithium precipitation system is connected with a water inlet of the heat exchange unit;

preferably, concentrated water outlets of the primary nanofiltration carbonate recovery unit and the secondary nanofiltration carbonate recovery unit are both connected with a water inlet of the lithium precipitation system;

preferably, the produced water of the secondary nanofiltration carbonate recovery unit is recycled to the primary reverse osmosis concentration unit.

7. A method for extracting lithium from salt lake brine and preparing battery-grade lithium carbonate, which is characterized by adopting the system of any one of claims 1-6, and comprises the following steps:

(1) Carrying out precipitation reaction on salt lake brine sequentially through a coagulating sedimentation system, and carrying out filtration treatment on a filtration system and adsorption and desorption treatment on an adsorption and desorption system to obtain a lithium chloride solution;

(2) Sequentially passing the lithium chloride solution obtained in the step (1) through a primary reverse osmosis concentration unit, a primary nanofiltration calcium and magnesium removal unit, a secondary reverse osmosis concentration unit, a multistage nanofiltration calcium and magnesium removal unit and a calcium and magnesium removal ion exchange unit to remove calcium and magnesium ions;

(3) Sequentially treating the lithium chloride solution with calcium and magnesium ions removed in the step (2) by a primary nanofiltration boron removal unit, a multistage nanofiltration boron removal unit and a boron ion removal exchange unit to remove boron ions;

(4) And (4) sequentially carrying out concentration treatment on the lithium chloride solution from which the boron ions are removed in the step (3) by a concentration system and evaporation treatment by an evaporation system, and finally adding sodium carbonate by a sodium carbonate adding unit and reacting by a lithium precipitation reaction unit to obtain the battery-grade lithium carbonate.

8. The method according to claim 7, wherein the content of lithium ions in the salt lake brine in the step (1) 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 less than or equal to 500ppm, the content of potassium ions is less than or equal to 8000ppm, the content of carbonate is less than or equal to 400ppm, the content of sulfate radicals is 10000-20000 ppm, and the content of boron ions is less than or equal to 600ppm;

preferably, the filtering speed of the filtering treatment in the step (1) is 5-15 m/h;

preferably, the adsorbent for adsorption and desorption treatment in step (1) comprises any one of an aluminum-based lithium adsorbent, a manganese-based lithium adsorbent or a titanium-based lithium adsorbent;

preferably, before the lithium chloride solution in the step (2) enters a first-stage reverse osmosis concentration unit and a first-stage nanofiltration calcium and magnesium removal unit, the pH value is adjusted to 3-6;

preferably, the pH of the lithium chloride solution in the step (2) is adjusted to 7-10 before entering the calcium and magnesium removal ion exchange unit.

9. The method according to claim 7 or 8, wherein the lithium chloride solution in the step (3) is adjusted to pH 9.2-11 before entering the primary nano-filtration boron removal unit and the multi-stage nano-filtration boron removal unit;

preferably, the pH of the lithium chloride solution in the step (3) is adjusted to 9.2-10 before entering the boron-removing ion exchange unit;

preferably, the mass ratio of the sodium carbonate to the lithium chloride in the step (4) is (1-1.2): 1;

preferably, the pH value of the lithium chloride solution after the sodium carbonate is added in the step (4) is 9-12;

preferably, the temperature of the reaction in the step (4) is 80-99 ℃.

10. The method according to any one of claims 7 to 9, wherein a lithium carbonate solution and a lithium precipitation mother liquor are obtained after the reaction of the lithium precipitation reaction unit in the step (4);

preferably, the lithium carbonate solution is sequentially treated by a precision filtration system and a washing and drying system to obtain battery-grade lithium carbonate;

preferably, the lithium precipitation mother liquor is subjected to heat exchange by a heat exchange unit, a primary nanofiltration carbonate recovery unit and a secondary nanofiltration carbonate recovery unit in sequence to separate recovered carbonate ions and a lithium chloride solution;

preferably, the temperature after heat exchange is 5-40 ℃.

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