Method and device for producing potassium fertilizer as by-product in lithium production from salt lake brine

Abstract

The present application relates to the technical field of potassium-rich salt lake production of potassium fertilizer, and is a method and device for producing potassium fertilizer as by-product in the production of lithium from salt lake brine. The former is carried out according to the following method: the brine from a salt lake is first subjected to a nanofiltration unit to remove divalent impurity ions therein, obtaining purified unsaturated brine and nanofiltration concentrated water, the nanofiltration concentrated water is returned to the salt lake for recycling, the purified unsaturated brine is sent to a potassium extraction unit, and after dissolution, evaporation and concentration, and vacuum potassium extraction, potassium chloride products and potassium extraction brine are obtained, the potassium extraction brine is sent to an evaporation unit, and after evaporation, crystallization and separation, high-lithium brine and sodium-potassium mixed salt are obtained, the sodium-potassium mixed salt is returned to the potassium extraction unit again, and potassium chloride products are recovered. The latter includes a nanofiltration unit, a potassium extraction unit and an evaporation unit. The present application uses the principle of by-product potassium chloride in the production of lithium carbonate products from salt lake resources, achieving the purpose of producing potassium fertilizer products, and has the advantages of high potassium extraction efficiency, good environmental compatibility, low energy consumption and low investment.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of producing potassium fertilizer from potassium-rich salt lakes, and is a method and device for producing potassium fertilizer as a byproduct in the production of lithium from salt lake brine. BACKGROUND

[0002] Potassium fertilizer is an essential resource for food production. Due to resource endowments, the self-sufficiency rate is relatively low. However, due to the rapid growth of the population, the per capita arable land continues to decline, making the demand for potassium fertilizer more urgent. China is the largest potassium fertilizer demand country in the world and also the largest potassium fertilizer importer in the world. China's lithium resources are mainly distributed in the salt lakes of the Qinghai-Tibet Plateau. These resources are characterized by a wide variety of types, and most of them contain a large amount of potassium, sodium, boron, and other elements in addition to lithium resources. In the current lithium extraction process from salt lakes, more attention is paid to the resource utilization of lithium elements, and the existing co-production of potassium fertilizer process has problems to varying degrees.

[0003] Chinese patent document CN103508462A discloses a method for comprehensive utilization of potassium, boron, and lithium in carbonate-type salt lake brine. The method uses acidification extraction to extract boron and flotation to extract potassium, etc., to produce lithium carbonate while resourcefully utilizing boron and potassium resources. However, the flotation method requires the addition of organic materials such as flotation collectors and frothers, which may damage the fragile environmental carrying capacity of the salt lake area.

[0004] Chinese patent document CN108275703A discloses a process for producing lithium carbonate and potassium co-production using lithium-containing nanofiltration product water. This process, which is aimed at potassium-rich magnesium sulfate sub-type salt lakes, first concentrates and removes impurities from the nanofiltration product water after nanofiltration separates magnesium sulfate, and then obtains industrial-grade potassium chloride and industrial-grade lithium carbonate products through low-temperature cooling crystallization and lithium precipitation crystallization, respectively. This process can well achieve lithium and potassium co-production without adding any external chemical reagents. However, before entering the cooling crystallization for potassium extraction, the brine needs to be concentrated by high evaporation to reach the sodium-potassium co-saturation point. High concentration will cause a significant increase in lithium ion content, further deviating the sodium-potassium co-saturation point from the theoretical three-phase equilibrium point, thereby reducing the potassium ion content. Under the same potassium extraction recovery rate, this process requires a larger amount of brine for cooling crystallization, higher investment, and greater energy consumption.

[0005] Chinese patent document CN114538475A discloses a potassium chloride production system and production method, which mentions using a saturated sodium chloride solution mixed with a potassium-containing mixed salt in solid phase and heated for dissolution, and obtaining potassium chloride products through cooling crystallization. This method utilizes the different solubility characteristics of potassium chloride and sodium chloride at high temperatures to achieve sodium-potassium displacement and recover potassium resources. However, it requires a large and even multiple-stage hot dissolving tank to ensure the dissolution time, and a large thickener to separate fine sodium chloride.

[0006] Therefore, it is still a technical challenge to develop a process technology for producing by-product potassium fertilizer that has good connection with lithium extraction technology, high potassium extraction efficiency, good environmental affinity, low energy consumption and low investment, and realize the comprehensive application of potassium-rich salt lakes. Summary of the Invention

[0007] The present invention provides a method and device for producing by-product potash fertilizer in the process of lithium extraction from salt lake brine, which overcomes the shortcomings of the above-mentioned prior art and can effectively solve the problems of low potassium recovery rate and high investment in the production of potash fertilizer in potassium-rich salt lakes.

[0008] One of the technical solutions of the present invention is achieved through the following measures: a method for producing by-product potassium fertilizer in the lithium extraction production of salt lake brine, which is carried out according to the following method: in the first step, the brine from the salt lake is first removed from the divalent impurity ions in the nanofiltration unit to obtain purified unsaturated brine and nanofiltration concentrated water, the nanofiltration concentrated water is returned to the salt lake for recycling, and the purified unsaturated brine is sent to the potassium extraction unit; in the second step, the purified unsaturated brine is dissolved, evaporated and concentrated, and vacuum-extracted potassium in the potassium extraction unit to obtain potassium chloride product and potassium-extracted brine; in the third step, the potassium-extracted brine is sent to the evaporation unit, and after evaporation, crystallization and separation, high-lithium brine and sodium-potassium mixed salt are obtained, the high-lithium brine is sent to the downstream process, and the sodium-potassium mixed salt is returned to the potassium extraction unit to be recovered to obtain the potassium chloride product.

[0009] The following is a further optimization and / or improvement of one of the above-mentioned technical solutions:

[0010] In the above-mentioned second step, the potassium extraction unit includes a dissolution process, an evaporation concentration process and a vacuum potassium extraction process.

[0011] In the above dissolution process, the purified unsaturated brine is heated to 70°C to 80°C in a brine preheater, and then mixed with the sodium-potassium mixed salt obtained from the evaporation unit and dissolved to obtain potassium-rich brine and undissolved sodium chloride salt.

[0012] In the above-mentioned evaporation and concentration process, the potassium-rich brine obtained in the dissolution process is evaporated, crystallized and separated in the evaporation and concentration process to obtain sodium-potassium saturated brine and sodium chloride, wherein the evaporation end point of the evaporation and concentration process is the co-saturation point of sodium and potassium.

[0013] In the above-mentioned vacuum potassium extraction process, the sodium-potassium saturated brine obtained in the evaporation and concentration process is subjected to three-stage vacuum flash evaporation to obtain potassium chloride product and potassium-extracted brine, wherein the vacuum degree of the last stage of the three-stage vacuum flash evaporation is 2KPaA to 4KPaA.

[0014] The second technical solution of the present invention is achieved through the following measures: a device for implementing a method for extracting lithium from salt lake brine to produce by-product potassium fertilizer, comprising a nanofiltration unit, a potassium extraction unit and an evaporation unit, the inlet of the nanofiltration unit is fixedly connected to a salt lake brine delivery pipeline, the first outlet of the nanofiltration unit and the first inlet of the potassium extraction unit are fixedly connected to a purified unsaturated brine delivery pipeline, the second outlet of the nanofiltration unit is fixedly connected to a nanofiltration concentrated water delivery pipeline, the first outlet of the potassium extraction unit and the inlet of the evaporation unit are fixedly connected to a potassium extraction brine delivery pipeline, the second outlet of the potassium extraction unit is fixedly connected to a potassium chloride product output pipeline, the first outlet of the evaporation unit and the second inlet of the potassium extraction unit are fixedly connected to a sodium-potassium mixed salt delivery pipeline, and the second outlet of the evaporation unit is fixedly connected to a high-lithium brine output pipeline.

[0015] The following is a further optimization and / or improvement of the second technical solution of the above invention:

[0016] The above-mentioned potassium extraction unit includes a dissolving device, an evaporation and concentration device and a vacuum potassium extraction device. A purified unsaturated brine conveying pipeline is fixedly connected between the first inlet of the dissolving device and the first outlet of the nanofiltration unit, a sodium-potassium mixed salt conveying pipeline is fixedly connected between the second inlet of the dissolving device and the first outlet of the evaporation unit, a dissolving output pipeline is fixedly connected between the outlet of the dissolving device and the inlet of the evaporation and concentration device, an evaporation and concentration output pipeline is fixedly connected between the first outlet of the evaporation and concentration device and the inlet of the vacuum potassium extraction device, a sodium chloride output pipeline is fixedly connected to the second outlet of the evaporation and concentration device, a potassium brine conveying pipeline is fixedly connected between the first outlet of the vacuum potassium extraction device and the inlet of the evaporation unit, and a potassium chloride product output pipeline is fixedly connected to the second outlet of the vacuum potassium extraction device.

[0017] The above-mentioned dissolving device includes a brine preheater, a slurry mixing tank, a first dissolving tank and a second dissolving tank. The first liquid inlet on the upper part of the brine preheater is fixedly connected to a purified unsaturated brine delivery pipeline. The first liquid outlet on the lower part of the brine preheater and the first liquid inlet on the top of the slurry mixing tank are fixedly connected to a purified unsaturated brine heating output pipeline. The second liquid inlet on the upper part of the brine preheater is fixedly connected to a steam pipeline. The second liquid outlet on the lower part of the brine preheater is fixedly connected to a condensate pipeline. The second inlet on the top of the slurry mixing tank is fixedly connected to a sodium-potassium mixed salt delivery pipeline. The lower part of the slurry mixing tank is fixedly connected to the first liquid inlet on the top of the brine preheater. A mixed slurry conveying pipeline is fixedly connected between the outlet and the first inlet at the top of the first dissolving tank, a first desalted water input pipeline is fixedly connected to the second inlet at the top of the first dissolving tank, an overflow pipeline is fixedly connected between the upper outlet of the first dissolving tank and the upper inlet of the second dissolving tank, a second desalted water input pipeline is fixedly connected between the first desalted water input pipeline and the top inlet of the second dissolving tank, a dissolution output pipeline is fixedly connected to the lower outlet of the second dissolving tank, and a dissolution output second pipeline is fixedly connected between the lower outlet of the first dissolving tank and the dissolution output pipeline.

[0018] A slurry mixing pump is fixedly installed on the mixed slurry delivery pipeline, and a potassium-rich brine pump is fixedly installed on the dissolution output pipeline between the second dissolution output pipeline and the outlet of the dissolution output pipeline.

[0019] Therefore, the present invention utilizes the principle of by-producing potassium chloride in the process of producing lithium carbonate products from salt lake resources, thereby achieving the purpose of producing potash fertilizer products. It has the advantages of high potassium extraction efficiency, good environmental affinity, low energy consumption and low investment. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Attachment Figure 1 This is a schematic block diagram of the process flow of Example 6 of the present invention.

[0021] Attachment Figure 2 This is a schematic block diagram of the process flow of the potassium extraction unit in Example 7 of the present invention.

[0022] Attachment Figure 3 This is a schematic diagram of the process flow of the dissolving device in Example 8 of the present invention.

[0023] The codes in the accompanying drawings are: 1 is a brine preheater, 2 is a slurry mixing tank, 3 is a first dissolving tank, 4 is a second dissolving tank, 5 is a purified unsaturated brine transmission pipeline, 6 is a purified unsaturated brine heating output pipeline, 7 is a steam pipeline, 8 is a condensate pipeline, 9 is a sodium-potassium mixed salt transmission pipeline, 10 is a mixed slurry transmission pipeline, 11 is a first desalted water input pipeline, 12 is an overflow pipeline, 13 is a second desalted water input pipeline, 14 is a dissolving output pipeline, 15 is a second dissolving output pipeline, 16 is a slurry mixing pump, 17 is a potassium-rich brine pump, 18 is a salt lake brine transmission pipeline, 19 is a nanofiltration concentrated water transmission pipeline, 20 is a potassium extraction brine transmission pipeline, 21 is a potassium chloride product output pipeline, 22 is a high lithium brine output pipeline, 23 is an evaporation concentration output pipeline, and 24 is a sodium chloride output pipeline. DETAILED DESCRIPTION

[0024] The present invention is not limited to the following embodiments, and specific embodiments can be determined based on the technical solutions of the present invention and actual conditions. Unless otherwise specified, the various chemical reagents and chemicals mentioned in the present invention are all chemical reagents and chemicals commonly used in the prior art. In the present invention, unless otherwise specified, the equipment and devices used are all equipment and devices commonly used in the art.

[0025] The present invention will be further described below in conjunction with the embodiments:

[0026] Example 1: The method for producing by-product potassium fertilizer in the lithium extraction production of salt lake brine is carried out according to the following method: in the first step, the brine from the salt lake is first subjected to a nanofiltration unit to remove divalent impurity ions therein to obtain purified unsaturated brine and nanofiltration concentrated water, the nanofiltration concentrated water is returned to the salt lake for recycling, and the purified unsaturated brine is sent to the potassium extraction unit; in the second step, the purified unsaturated brine is subjected to dissolution, evaporation concentration and vacuum potassium extraction in the potassium extraction unit to obtain potassium chloride product and potassium-extracted brine; in the third step, the potassium-extracted brine is sent to an evaporation unit, and after evaporation, crystallization and separation, high-lithium brine and sodium-potassium mixed salt are obtained, the high-lithium brine is sent to the downstream process, and the sodium-potassium mixed salt is returned to the potassium extraction unit to be recovered to obtain the potassium chloride product.

[0027] In the above embodiment, the sodium-potassium mixed salt is returned to the potassium extraction unit to recycle potassium chloride. The nanofiltration unit should be placed before the potash fertilizer unit to remove divalent ions in the brine and produce purified unsaturated brine.

[0028] The above embodiment proves one of the beneficial effects of the present invention: by using salt lake resources to produce lithium carbonate products in the process of by-product potassium chloride, the comprehensive utilization level of salt lakes is improved, and the economic benefits are greatly improved. At the same time, a membrane method is first used to remove impurities, and then potassium chloride is produced by physical methods such as evaporation and separation. No impurity ions other than salt lakes are introduced during the process, and no salt lake pollution is caused. The environmental affinity is good, and the sodium and potassium solid salts are cyclically dissolved. After nanofiltration water production, the potassium element recovery rate can reach 95.0% to 99.0%, which is higher than other potassium extraction schemes. The main components of the salt lake brine include 1.0g / L of Li + , 90g / L of Na + , 24.5g / L K + , 21g / L SO4 2- and 24.5g / L CO3 2- .

[0029] The present invention sends salt lake brine into a nanofiltration unit to intercept divalent ions to obtain the purified SO4 in unsaturated brine. 2- Concentration and CO3 2- The concentration is almost negligible, Li + The concentration is about 1.1g / L.

[0030] Example 2: As an optimization of the above example, in the second step, the potassium extraction unit includes a dissolution process, an evaporation concentration process and a vacuum potassium extraction process.

[0031] The above embodiment demonstrates the second beneficial effect of the present invention: by providing a dissolution process before potassium chloride crystallization, the sodium chloride and potassium mixed salt produced by evaporation in the latter stage of the dissolution of potassium chloride crystallization is used to adjust the brine composition of the solution, thereby avoiding the result of an increase in the concentration of other ions in the brine, such as lithium ions, caused by direct evaporation and concentration. The increase in the concentration of other ions will reduce the content of potassium chloride when the brine reaches saturation. On the one hand, the scale of the evaporation and concentration process and the vacuum potassium extraction process in the subsequent potash fertilizer unit is reduced. On the other hand, the content of other ions such as lithium ions entering the vacuum potassium extraction process is reduced, thereby avoiding the influence of other ions on the sodium-potassium phase diagram, and the operation is more stable.

[0032] Example 3: As an optimization of the above embodiment, in the dissolution process, the purified unsaturated brine is heated to a temperature of 70°C to 80°C by the brine preheater 1, and then mixed with the sodium-potassium mixed salt obtained by the evaporation unit and dissolved to obtain potassium-rich brine and undissolved sodium chloride salt.

[0033] The above embodiment demonstrates the third beneficial effect of the present invention: using purified unsaturated brine to dissolve sodium-potassium mixed salt greatly improves the dissolution time and saves the investment cost of large-scale hot dissolution tanks and thickeners.

[0034] Example 4: As an optimization of the above embodiment, in the evaporation and concentration process, the potassium-rich brine obtained in the dissolution process is evaporated, crystallized, and separated in the evaporation and concentration process to obtain sodium-potassium saturated brine and sodium chloride, wherein the evaporation end point of the evaporation and concentration process is the co-saturation point of sodium and potassium.

[0035] Purification of unsaturated brine is first dissolved in the dissolution process, and the returned sodium potassium mixed salt is separated into sodium chloride solid through evaporation and concentration process, and the Na in the concentrate is + With K + The ion concentration ratio is adjusted to a co-saturated state and sent to the vacuum potassium extraction process.

[0036] Example 5: As an optimization of the above embodiment, in the vacuum potassium extraction process, the sodium-potassium saturated brine obtained in the evaporation and concentration process is subjected to three-stage vacuum flash evaporation to obtain potassium chloride product and potassium-extracted brine, wherein the final vacuum degree of the three-stage vacuum flash evaporation is 2KPaA to 4KPaA.

[0037] The above examples demonstrate the fourth beneficial effect of the present invention: through the three-stage vacuum flash evaporation of the vacuum potassium extraction process, the purity of the obtained potassium chloride product is above 98%, and by circulating the sodium-potassium mixed salt, the product recovery rate after nanofiltration can reach above 95%; at the same time, compared with the existing process, the present invention does not use cooling crystallization, and the flash steam is directly used to heat the raw brine before nanofiltration. The entire device does not require an external low-temperature cooling source, and energy consumption is greatly reduced.

[0038] The potassium brine in the present invention is concentrated by the evaporation unit, and Li +The concentration is about 25g / L to 30g / L, and high-lithium brine can be obtained.

[0039] Example 6: Figure 1 As shown, the device for implementing the method of extracting lithium from salt lake brine and producing by-product potassium fertilizer includes a nanofiltration unit, a potassium extraction unit and an evaporation unit. The inlet of the nanofiltration unit is fixedly connected to a salt lake brine conveying pipeline 18, the first outlet of the nanofiltration unit and the first inlet of the potassium extraction unit are fixedly connected to a purified unsaturated brine conveying pipeline 5, the second outlet of the nanofiltration unit is fixedly connected to a nanofiltration concentrated water conveying pipeline 19, the first outlet of the potassium extraction unit and the inlet of the evaporation unit are fixedly connected to a potassium extraction brine conveying pipeline 20, the second outlet of the potassium extraction unit is fixedly connected to a potassium chloride product output pipeline 21, the first outlet of the evaporation unit and the second inlet of the potassium extraction unit are fixedly connected to a sodium-potassium mixed salt conveying pipeline 9, and the second outlet of the evaporation unit is fixedly connected to a high-lithium brine output pipeline 22.

[0040] Example 7: Figure 2 As shown, as an optimization of the above embodiment, the potassium extraction unit includes a dissolving device, an evaporation and concentration device and a vacuum potassium extraction device. A purified unsaturated brine conveying pipeline 5 is fixedly connected between the first inlet of the dissolving device and the first outlet of the nanofiltration unit, a sodium-potassium mixed salt conveying pipeline 9 is fixedly connected between the second inlet of the dissolving device and the first outlet of the evaporation unit, a dissolving output pipeline 14 is fixedly connected between the outlet of the dissolving device and the inlet of the evaporation and concentration device, an evaporation and concentration output pipeline 23 is fixedly connected between the first outlet of the evaporation and concentration device and the inlet of the vacuum potassium extraction device, a sodium chloride output pipeline 24 is fixedly connected to the second outlet of the evaporation and concentration device, a potassium extraction brine conveying pipeline 20 is fixedly connected between the first outlet of the vacuum potassium extraction device and the inlet of the evaporation unit, and a potassium chloride product output pipeline 21 is fixedly connected to the second outlet of the vacuum potassium extraction device.

[0041] Example 8: Figure 3As shown, as an optimization of the above embodiment, the dissolving device includes a brine preheater 1, a slurry mixing tank 2, a first dissolving tank 3 and a second dissolving tank 4. The first liquid inlet on the upper part of the brine preheater 1 is fixedly connected to a purified unsaturated brine delivery pipeline 5, and the first liquid outlet at the lower part of the brine preheater 1 and the first liquid inlet at the top of the slurry mixing tank 2 are fixedly connected to a purified unsaturated brine heating output pipeline 6. The second liquid inlet on the upper part of the brine preheater 1 is fixedly connected to a steam pipeline 7, and the second liquid outlet at the lower part of the brine preheater 1 is fixedly connected to a condensate pipeline 8. The second inlet on the top of the slurry mixing tank 2 is fixedly connected to a sodium-potassium mixed salt delivery pipeline 9. The slurry mixing tank A mixed slurry delivery pipeline 10 is fixedly connected between the lower outlet of the first dissolving tank 2 and the first inlet at the top of the first dissolving tank 3. A first desalted water input pipeline 11 is fixedly connected to the second inlet at the top of the first dissolving tank 3. An overflow pipeline 12 is fixedly connected between the upper outlet of the first dissolving tank 3 and the upper inlet of the second dissolving tank 4. A second desalted water input pipeline 13 is fixedly connected between the first desalted water input pipeline 11 and the top inlet of the second dissolving tank 4. A dissolving output pipeline 14 is fixedly connected to the lower outlet of the second dissolving tank 4. A dissolving output second pipeline 15 is fixedly connected between the lower outlet of the first dissolving tank 3 and the dissolving output pipeline 14.

[0042] As needed, a brine preheater 1 is provided to increase the temperature of the purified unsaturated brine, thereby increasing the dissolution rate of the sodium-potassium mixed salt on the one hand and improving the solubility of the sodium-potassium mixed salt in the brine on the other hand.

[0043] The first dissolution tank 3 and the second dissolution tank 4 ensure sufficient time to fully dissolve the sodium-potassium mixed salt. At the same time, the overflow pipeline 12 is connected in the middle to ensure that the potassium-rich brine transported to the subsequent process does not contain undissolved solid salts. The desalted water input pipeline is convenient for flushing in the event of an accident blockage. At the same time, two or more overflow pipelines 12 can be provided as needed.

[0044] As needed, agitators may be provided in the slurry mixing tank 2 , the first dissolving tank 3 and the second dissolving tank 4 .

[0045] Example 9: Figure 3 As shown, as an optimization of the above embodiment, a slurry mixing pump 16 is fixedly installed on the mixed slurry delivery pipeline 10, and a potassium-rich brine pump 17 is fixedly installed on the dissolution output pipeline 14 between the dissolution output second pipeline 15 and the outlet of the dissolution output pipeline 14.

[0046] As needed, each device and pipeline of the apparatus for implementing the method for extracting lithium from salt lake brine and producing by-product potassium fertilizer can be fixedly installed with valves, thermometers, liquid level gauges, pressure gauges, and automatic control units that are well known in the art and enable normal operation.

[0047] On the basis of the same potassium chloride production capacity, the present invention has the following advantages: on the one hand, since the sodium-potassium saturated brine of the present invention has a higher potassium chloride content, it is easy to crystallize, the evaporation unit is smaller in scale, the investment is more economical, and the energy consumption is also lower; on the other hand, the content of other ions except sodium chloride and potassium chloride is very low, which has little effect on the crystallization phase diagram; therefore, the equilibrium point during potassium chloride crystallization is clearer and the operation is more stable.

[0048] In summary, the present invention utilizes the principle of producing potassium chloride as a by-product in the process of producing lithium carbonate products from salt lake resources, thereby achieving the purpose of producing potash fertilizer products. It has the advantages of high potassium extraction efficiency, good environmental affinity, low energy consumption and low investment.

[0049] The above technical features constitute the embodiments of the present invention, which have strong adaptability and implementation effect. Non-essential technical features can be added or removed according to actual needs to meet the requirements of different situations.

Claims

1. A method for extracting lithium from salt lake brine to produce by-product potash fertilizer, characterized in that The process is as follows: in the first step, the brine from the salt lake is first filtered through a nanofiltration unit to remove divalent impurity ions therein to obtain purified unsaturated brine and nanofiltration concentrated water, the nanofiltration concentrated water is returned to the salt lake for recycling, and the purified unsaturated brine is sent to the potassium extraction unit; in the second step, the purified unsaturated brine is dissolved, evaporated and concentrated, and vacuum potassium is extracted in the potassium extraction unit to obtain potassium chloride product and potassium-extracted brine, wherein the potassium extraction unit includes a dissolution process, an evaporation and concentration process, and a vacuum potassium extraction process. In the evaporation and concentration process, the potassium-rich brine obtained in the dissolution process is evaporated, crystallized, and vacuum potassium extracted in the evaporation and concentration process. After separation, sodium-potassium saturated brine and sodium chloride are obtained, wherein the evaporation end point of the evaporation and concentration process is the co-saturation point of sodium and potassium. In the vacuum potassium extraction process, the sodium-potassium saturated brine obtained in the evaporation and concentration process is subjected to three-stage vacuum flash evaporation to obtain potassium chloride product and potassium extraction brine, wherein the final vacuum degree of the three-stage vacuum flash evaporation is 2KPaA to 4KPaA; in the third step, the potassium extraction brine is sent to the evaporation unit, and after evaporation, crystallization and separation, high-lithium brine and sodium-potassium mixed salt are obtained. The high-lithium brine is sent to the downstream process, and the sodium-potassium mixed salt is returned to the potassium extraction unit to be recovered to obtain the potassium chloride product.

2. The method for producing by-product potash fertilizer in lithium extraction from salt lake brine according to claim 1, characterized in that In the dissolution process, the purified unsaturated brine is heated to 70°C to 80°C in a brine preheater, and then mixed with the sodium-potassium mixed salt obtained from the evaporation unit and dissolved to obtain potassium-rich brine and undissolved sodium chloride salt.

3. A device for implementing the method for extracting lithium from salt lake brine to produce by-product potash fertilizer according to claim 1 or 2, characterized in that It includes a nanofiltration unit, a potassium extraction unit and an evaporation unit. The inlet of the nanofiltration unit is fixedly connected to a salt lake brine delivery pipeline. The first outlet of the nanofiltration unit and the first inlet of the potassium extraction unit are fixedly connected to a purified unsaturated brine delivery pipeline. The second outlet of the nanofiltration unit is fixedly connected to a nanofiltration concentrated water delivery pipeline. The first outlet of the potassium extraction unit and the inlet of the evaporation unit are fixedly connected to a potassium extraction brine delivery pipeline. The second outlet of the potassium extraction unit is fixedly connected to a potassium chloride product output pipeline. The first outlet of the evaporation unit and the second inlet of the potassium extraction unit are fixedly connected to a sodium-potassium mixed salt delivery pipeline. The second outlet of the evaporation unit is fixedly connected to a high-lithium brine output pipeline. The potassium extraction unit includes a dissolving device, an evaporation and concentration device and a vacuum potassium extraction device, a purified unsaturated brine delivery pipeline is fixedly connected between the first inlet of the dissolving device and the first outlet of the nanofiltration unit, a sodium-potassium mixed salt delivery pipeline is fixedly connected between the second inlet of the dissolving device and the first outlet of the evaporation unit, a dissolving output pipeline is fixedly connected between the outlet of the dissolving device and the inlet of the evaporation and concentration device, an evaporation and concentration output pipeline is fixedly connected between the first outlet of the evaporation and concentration device and the inlet of the vacuum potassium extraction device, a sodium chloride output pipeline is fixedly connected to the second outlet of the evaporation and concentration device, a potassium brine delivery pipeline is fixedly connected between the first outlet of the vacuum potassium extraction device and the inlet of the evaporation unit, and a potassium chloride product output pipeline is fixedly connected to the second outlet of the vacuum potassium extraction device.

4. The device according to claim 3, characterized in that The dissolving device includes a brine preheater, a slurry mixing tank, a first dissolving tank and a second dissolving tank. The first liquid inlet on the upper part of the brine preheater is fixedly connected to a purified unsaturated brine delivery pipeline. The first liquid outlet on the lower part of the brine preheater and the first liquid inlet on the top of the slurry mixing tank are fixedly connected to a purified unsaturated brine heating output pipeline. The second liquid inlet on the upper part of the brine preheater is fixedly connected to a steam pipeline. The second liquid outlet on the lower part of the brine preheater is fixedly connected to a condensate pipeline. The second inlet on the top of the slurry mixing tank is fixedly connected to a sodium-potassium mixed salt delivery pipeline. The outlet on the lower part of the slurry mixing tank is fixedly connected to a sodium-potassium mixed salt delivery pipeline. A mixed slurry conveying pipeline is fixedly connected between the outlet and the first inlet at the top of the first dissolving tank, a first desalted water input pipeline is fixedly connected to the second inlet at the top of the first dissolving tank, an overflow pipeline is fixedly connected between the upper outlet of the first dissolving tank and the upper inlet of the second dissolving tank, a second desalted water input pipeline is fixedly connected between the first desalted water input pipeline and the top inlet of the second dissolving tank, a dissolving output pipeline is fixedly connected to the lower outlet of the second dissolving tank, and a dissolving output second pipeline is fixedly connected between the lower outlet of the first dissolving tank and the dissolving output pipeline.

5. The device according to claim 4, characterized in that A slurry mixing pump is fixedly installed on the mixed slurry delivery pipeline, and a potassium-rich brine pump is fixedly installed on the dissolution output pipeline between the second dissolution output pipeline and the outlet of the dissolution output pipeline.

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