A method for co-producing lithium and potassium in magnesium sulfate salt lakes

Abstract

The present application discloses a method for the co-production of lithium and potassium in magnesium sulfate salt lakes, and relates to the technical field of potassium and lithium extraction from salt lakes. The method comprises the following steps: naturally evaporating the brine of the magnesium sulfate salt lake until it is saturated with potassium chloride, separating the solid salt, mixing the remaining brine with lithium extraction brine, naturally evaporating the brine, separating the solid salt, obtaining old brine, and subjecting the old brine to a lithium extraction process to obtain a lithium salt product and a lithium extraction tail brine A. The present application returns the lithium extraction brine, which is mainly composed of magnesium chloride and is produced during the lithium salt production process, to the salt field and mixes it with potassium chloride saturated brine, thereby changing the precipitation path of the potassium salt ore and skipping the precipitation stage of soft potassium magnesium sulfate. This increases the yield of potassium salt and carnallite salt, while preventing the lithium in the brine from precipitating as lithium sulfate ore in the carnallite salt pool, thereby increasing the lithium salt yield. The method is characterized by simple operation, high efficiency, and readily available raw materials, and has practical significance for increasing the yield of potassium and lithium in the process of salt lake resource development.

Description

Technical Field

[0001] The present application relates to the technical field of potassium and lithium extraction from salt lakes, and in particular to a method for co-producing lithium and potassium from magnesium sulfate-type salt lakes. Background Art

[0002] The current method of using magnesium sulfate type lithium-containing brine to produce potassium and lithium products is as follows: first, the original brine of the salt lake is dried to obtain potassium-containing mixed salt (including sodium chloride, potassium chloride and carnallite), and the potassium-containing mixed salt is made into ore pulp, flotation or reverse flotation, decomposition crystallization, screening and dehalogenation to obtain crude potassium, and then the crude potassium is washed and dehalogenated again to obtain refined potassium, that is, potassium chloride with higher purity. After the brine is dried to precipitate carnallite, the remaining liquid phase contains a large amount of magnesium chloride, enriched and concentrated lithium chloride and a small amount of sodium chloride and potassium chloride, which is usually called lithium-containing old brine. This lithium-containing old brine is due to K + 、Na + The ion content is low, and the +1-valent lithium ions can be separated from the +2-valent magnesium ions by electrodialysis membrane or nanofiltration membrane separation method to obtain a lithium-rich solution. The lithium-rich solution is then evaporated, concentrated, impurities removed, and lithium precipitated to obtain crude lithium carbonate. The crude lithium carbonate is then washed, dried, and demagnetized to obtain battery-grade lithium carbonate.

[0003] For some magnesium sulfate-containing lithium brines, a mixed salt of leonite, magnesium sulfate, and potassium chloride will precipitate during the evaporation process. This mixed salt results in a low yield when recovering potassium, which is inferior to sylvite and carnallite. Moreover, when the initial lithium concentration in the brine is high, lithium sulfate (Li2SO4·H2O) solid will precipitate during the evaporation process. This part of lithium will be mixed with other salts and difficult to recover, resulting in waste.

[0004] Therefore, in order to solve the above problems, it is necessary to develop a method that can comprehensively recover potassium and lithium resources in this type of salt lake brine, reduce impurity generation, and improve the yield of lithium potassium products. Summary of the Invention

[0005] The present application provides a method for the co-production of lithium and potassium in magnesium sulfate type salt lakes, which changes the salt precipitation route of the original magnesium sulfate type salt lake brine by adding brine, avoids the precipitation stage of soft potassium sulfate and lithium sulfate, and improves the output and purity of potassium salt and lithium salt products.

[0006] In order to solve the above technical problems, the present application provides a method for co-producing lithium and potassium in magnesium sulfate salt lakes, comprising the following steps:

[0007] The magnesium sulfate salt lake brine is naturally evaporated to potassium chloride saturation, and the solid salt is separated. The remaining brine and lithium extraction brine are evenly mixed and naturally evaporated to magnesium chloride saturation, and the solid salt is separated to obtain old brine. The old brine is subjected to a lithium extraction process to obtain a lithium salt product and lithium extraction tail brine A;

[0008] The mass percentage of magnesium ions in the lithium extraction brine is greater than 4.5%, the mass percentage of chloride ions is greater than 13%, the mass percentage of lithium ions is less than 0.07%, and the mass percentage of sulfate ions is less than 2%.

[0009] Wherein, the composition of the magnesium sulfate salt lake brine is Na + , K + Mg 2+ / / Cl - 、SO4 2- -H2O is located in the potassium chloride region in the phase diagram of the five-component water-salt system.

[0010] The magnesium sulfate salt lake brine of the present application returns the lithium extraction brine generated in the lithium salt production process to the salt field and mixes it with potassium chloride saturated brine. The lithium extraction brine mainly contains magnesium chloride, which changes the precipitation path of the potassium salt ore and skips the precipitation stage of soft potassium magnesium sulfate, retaining potassium in the subsequent carnallite salt precipitation stage, thereby increasing the output of potassium salt and carnallite salt, that is, increasing the output of high-quality potassium ore raw materials, and also improving the purity of potassium mixed salt, making the potassium ore precipitated in the salt field more conducive to the subsequent potassium chloride production; at the same time, it avoids the lithium in the brine from precipitating in the carnallite salt pool in the form of lithium sulfate ore, thereby improving the enrichment of lithium in the old brine, becoming a lithium extraction raw material, and increasing the lithium salt output. The method has the characteristics of simple operation, high efficiency, and easy availability of raw materials, and has practical significance for increasing the output of potassium and lithium in the process of salt lake resource development.

[0011] In another embodiment, the following steps are included:

[0012] When the magnesium sulfate salt lake brine is naturally evaporated to saturation with potassium chloride, the solid salt is separated, and the remaining brine and lithium extraction brine are evenly mixed according to a mass ratio, and naturally evaporated to saturation with epsom salt, the solid salt is separated, and the remaining brine is further naturally evaporated to saturation with carnallite, and the solid salt is separated, and the remaining brine is further naturally evaporated to saturation with magnesium chloride, and the solid salt is separated to obtain old brine, and the old brine is subjected to a lithium extraction process to obtain a lithium salt product and lithium extraction tail brine A.

[0013] In another embodiment, the lithium extraction brine is lithium extraction tail brine A and / or lithium extraction tail brine B, and the preparation method of the lithium extraction tail brine B comprises the following steps:

[0014] The magnesium sulfate type salt lake brine is naturally evaporated to magnesium chloride saturation or when the mass fraction of magnesium is greater than 8.5%, the solid salt is separated to obtain old brine, and the old brine is subjected to a lithium extraction process to obtain a lithium salt product and lithium extraction tail brine B.

[0015] In another embodiment, the method for preparing the lithium-extracted tail halide B comprises the following steps:

[0016] The magnesium sulfate salt lake brine is naturally evaporated to potassium chloride saturation, and solid salt is separated. The remaining brine is further evaporated to kainsonite saturation, and solid salt is separated. The remaining brine is further evaporated to epsomite saturation, and solid salt is separated. The remaining brine is further naturally evaporated to carnallite saturation, and solid salt is separated. The remaining brine is further evaporated to magnesium chloride saturation or the mass fraction of magnesium is greater than 8.5%, and solid salt is separated to obtain old brine. The old brine is subjected to a lithium extraction process to obtain a lithium salt product and lithium extraction tail brine B.

[0017] In another embodiment, the lithium extraction process is a selective electrodialysis lithium extraction technology, a nanofiltration membrane lithium-magnesium separation technology, or an electrochemical deintercalation lithium extraction technology.

[0018] In another embodiment, the magnesium sulfate salt lake brine Mg 2+ / SO4 2- The mass ratio is greater than 0.5, SO4 2- / Li + The mass ratio is less than 40.

[0019] In another embodiment, the magnesium sulfate salt lake brine includes 0.07% L + 、6.992%Na + 1.399%K + 、1.641%Mg 2+ , 1.762% SO4 2- 、16.005% Cl - , 0.073% B and other trace elements whose mass percentages are all below 0.04%.

[0020] In another embodiment, the mass fraction content of sodium and potassium ions in the old brine is 0.5-2 g / L.

[0021] In another embodiment, when the lithium extraction tail brine B is used as the lithium extraction brine, the mass ratio of the magnesium sulfate type salt lake brine used in preparing the lithium extraction tail brine A and the lithium extraction tail brine B is 1:1.

[0022] In another embodiment, when the lithium extraction tail brine A is used as lithium extraction brine, the mass ratio of the remaining brine mixed with the lithium extraction brine and the magnesium sulfate type salt lake brine used to prepare the lithium extraction tail brine A is 1:1.

[0023] In another embodiment, the lithium extraction brine is 100% by mass of lithium extraction tail brine A or lithium extraction tail brine B.

[0024] In another embodiment, the magnesium sulfate salt lake brine is naturally evaporated to saturation with potassium chloride, the solid salt is separated, and the remaining brine and the lithium extraction brine are evenly mixed in a mass ratio of (1-8):1.

[0025] In another embodiment, the solid salt containing potassium is used in the production of potassium chloride.

[0026] Compared with the prior art, the embodiments of the present invention have the following beneficial effects:

[0027] 1. The magnesium sulfate salt lake brine of the present application returns the old lithium brine generated in the lithium salt production process to the salt field and mixes it with potassium chloride saturated brine. The old lithium brine is mainly composed of magnesium chloride, which changes the precipitation path of potassium salt ore. The precipitation stage of soft potassium magnesium sulfate can be skipped, and potassium is retained in the subsequent carnallite salt precipitation stage, so that the output of potassium salt and carnallite salt is increased, that is, the output of high-quality potassium ore raw materials is increased, and the purity of potassium mixed salt is also improved, so that the potassium ore precipitated in the salt field is more conducive to the subsequent production of potassium chloride.

[0028] 2. The present application avoids the precipitation of lithium in the brine in the form of lithium sulfate ore in the carnallite salt pool by mixing the old brine rich in magnesium chloride with brine saturated with potassium chloride, thereby improving the enrichment of lithium in the old brine. The brine is then used as a raw material for lithium extraction, thereby increasing the output of lithium salt. This method has the characteristics of simple operation, high efficiency, and easy availability of raw materials. It has practical significance for increasing the output of potassium and lithium in the process of salt lake resource development. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 :For this application, magnesium sulfate salt lake brine is at 25℃Na + , K + Mg 2+ / / Cl - 、SO4 2- -H2O five-element water-salt system phase diagram salt precipitation route (Note: P0 is the original brine composition point; P1 is the brine composition point after the first brine addition; L0 is the salt precipitation route of the original brine without brine addition; L1 is the salt precipitation route of the brine after the first brine addition);

[0030] Figure 2 : A partially enlarged view of the salt precipitation route of the magnesium sulfate type salt lake brine of this application (Note: P0 is the original brine composition point; P1 is the brine composition point after the first brine addition; L0 is the salt precipitation route of the original brine without brine addition; L1 is the salt precipitation route of the brine after the first brine addition);

[0031] Figure 3 : This is the process flow of a method for co-producing lithium and potassium in a magnesium sulfate salt lake in Comparative Example 1 of this application;

[0032] Figure 4 : This is the process flow of a method for co-producing lithium and potassium in a magnesium sulfate salt lake in an embodiment of the present application. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0034] Comparative Example 1

[0035] A method for co-producing lithium and potassium in magnesium sulfate salt lakes, using brine from the Uyuni Salt Lake in Bolivia as raw material. The composition of the brine is shown in Table 1 below. 2+ / SO4 2- The mass ratio is 0.93, SO4 2- / Li + The mass ratio is 25.2, which belongs to the magnesium sulfate type salt lake brine system. The salt precipitation route of the brine is as follows: Figure 1-2 As shown in P0 and L0, the initial composition point of brine is located in the potassium chloride phase region. The process flow is as follows Figure 3 As shown, the following steps are included:

[0036] (1) Take 200kg of magnesium sulfate salt lake brine and evaporate it naturally in sodium salt pool, potassium salt pool, potassium mixed salt pool A, potassium mixed salt pool B and carnallite pool respectively. Figure 1 and attached Figure 3 Various salts were precipitated along the route. The composition of various salts in different salt pools is shown in Table 2 below. The specific evaporation process is as follows:

[0037] ① Spread the original brine in the sodium salt pool under natural sunlight to precipitate sodium chloride salt until the brine is saturated with potassium chloride, and then separate the solid salt;

[0038] ② The remaining brine continues to evaporate in the potash salt pool to precipitate potash salt until the brine is saturated with soft kainsonite (MgSO4·K2SO4·6H2O), and the solid salt is separated;

[0039] ③ The remaining brine continues to evaporate in the potassium mixed salt pool A to precipitate potassium mixed salt A until the brine is saturated with epsom salt (MgSO4·7H2O). The solid salt is then separated and the soft potassium magnesium sulfate component in the solid salt is detected, which accounts for 41.4% by mass.

[0040] ④ The remaining brine continues to evaporate in the potassium mixed salt pool B to precipitate potassium mixed salt B until the brine is saturated with carnallite (KCl·MgCl2·6H2O), and the solid salt is separated;

[0041] ⑤ The remaining brine continues to evaporate in the carnallite pool to precipitate carnallite salt until the brine is saturated with magnesium chloride, and the solid salt is separated. 0.17 kg of lithium sulfate is precipitated in the carnallite pool to obtain old brine;

[0042] (2) The potassium salt obtained in step (1) is used for the production of potassium chloride. The composition of the old brine is shown in Table 3. The obtained old brine is subjected to a lithium extraction process. The lithium extraction process is selective electrodialysis to obtain a lithium chloride solution and a lithium extraction tail brine. The lithium chloride solution is used for further production of lithium-related products. The composition of the lithium extraction tail brine is shown in Table 3. The lithium extraction old brine is used for the next batch of evaporation and brine addition.

[0043] Table 1 - Composition and content of magnesium sulfate salt lake brine

[0044] Components <![CDATA[Li + ]]> <![CDATA[Na + ]]> <![CDATA[K + ]]> <![CDATA[Mg 2+ ]]> <![CDATA[SO4 2- ]]> <![CDATA[Cl - ]]> B Ion content (%) 0.070 6.992 1.399 1.641 1.762 16.005 0.073

[0045] Table 2 - Composition of salt precipitated from brine in different salt ponds during step (1) of comparative example 1

[0046]

[0047] Table 3 - Composition of old brine and lithium extraction old brine in step (2) of comparative example 1

[0048]

[0049] Example 1

[0050] A method for co-producing lithium and potassium in magnesium sulfate salt lakes, using brine from the Uyuni Salt Lake in Bolivia as raw material. The composition of the brine is shown in Table 1. 2+ / SO4 2- The mass ratio is 0.93, SO4 2- / Li + The mass ratio is 25.2, which belongs to the magnesium sulfate type salt lake brine system. The salt precipitation route of the brine is as follows: Figure 1-2 As shown in P1 and L1, the initial composition point of brine is located in the potassium chloride phase region. The process flow is as follows Figure 4 As shown, the following steps are included:

[0051] (1) Take 200kg of magnesium sulfate salt lake brine, and the brine is as follows Figure 1 and attached Figure 4 Various salts were precipitated by natural evaporation in a sodium salt pool until potassium chloride was saturated and about to precipitate, and the solid salt was separated. At this time, the mass of the brine was 76.1 kg. The 35.51 kg lithium extraction tail brine obtained in step (2) of the comparative example 1 was mixed with the brine and the brine was added. Natural evaporation was continued until the old brine was obtained. The composition of various salts in different salt pools is shown in Table 4 below. The specific evaporation process is as follows:

[0052] ① Spread the original brine in the sodium salt pool under natural sunlight to precipitate sodium chloride salt until the brine is saturated with potassium chloride, and then separate the solid salt;

[0053] ② The remaining brine and the lithium extraction tail brine obtained in step (2) of the comparative example 1 are mixed evenly, and the mixture is evaporated in a sylvite pool to precipitate sylvite until the brine is saturated with sylvitonium salt (MgSO4·7H2O), and the solid salt is separated and the solid salt is detected to be free of soft potassium magnesium sulfate components;

[0054] ③ The remaining brine continues to evaporate in the potassium mixed salt pool to precipitate potassium mixed salt until the brine is saturated with carnallite (KCl·MgCl2·6H2O), and then the solid salt is separated and tested to see if there is no soft potassium magnesium sulfate component in the solid salt;

[0055] ④ The remaining brine continues to evaporate in the carnallite pool to precipitate carnallite salt until the brine is saturated with magnesium chloride, and the solid salt is separated to obtain old brine. It is detected that there is no lithium sulfate component in the solid salt;

[0056] (2) The potassium salt obtained in step (1) is used for the production of potassium chloride, and the obtained old brine is subjected to a lithium extraction process, wherein the lithium extraction process is selective electrodialysis, and a lithium chloride solution and a lithium extraction tail brine are obtained respectively, wherein the lithium chloride solution is used for further production of lithium-related products, and the lithium extraction old brine is used for the brine addition in the next batch of evaporation process.

[0057] Table 4 - Composition of salt precipitated from brine in different salt ponds during brine evaporation in step (1) of Example 1

[0058]

[0059] Table 5 - Composition of old brine and lithium extraction old brine in step (2) of Example 1

[0060]

[0061] It can be seen from the analysis of the salt precipitation process of Example 1 and Comparative Example 1 in combination with Table 2 and Table 4 that, compared with the comparative example 1, the evaporation is not carried out by the brine operation, and the precipitation path of the potassium salt mine is changed by brine in Example 1 of the present application, so that the output of potassium salt, potassium mixed salt and carnallite salt is increased, and the precipitation stage of the potassium mixed salt A of the comparative example 1 is skipped, and the precipitation of soft potassium magnesium sulfate is avoided. Instead, potassium is retained in the subsequent carnallite stage, so that lithium sulfate is no longer precipitated in the carnallite pool, while improving the purity of the potassium salt, ensuring that all lithium can enter the old brine, thereby improving the enrichment of lithium.

[0062] Example 2

[0063] A method for co-producing lithium and potassium in magnesium sulfate salt lakes, using brine from the Uyuni Salt Lake in Bolivia as raw material. The composition of the brine is shown in Table 1. 2+ / SO4 2- The mass ratio is 0.93, SO4 2- / Li + The mass ratio is 25.2, which belongs to the magnesium sulfate type salt lake brine system. The process flow is as follows Figure 4 As shown, the following steps are included:

[0064] (1) Take 200kg of magnesium sulfate salt lake brine, and the brine is as follows Figure 1 and attached Figure 4 Various salts were precipitated by natural evaporation in a sodium salt pool until potassium chloride was saturated and about to precipitate, and the solid salt was separated. At this time, the mass of the brine was 76.1 kg. The 51 kg lithium extraction tail brine obtained in step (2) of Example 1 was mixed with the brine and the brine was added. Natural evaporation was continued until the old brine was obtained. The composition of various salts in different salt pools is shown in Table 6 below. The specific evaporation process is as follows:

[0065] ① Spread the original brine in the sodium salt pool under natural sunlight to precipitate sodium chloride salt until the brine is saturated with potassium chloride, and then separate the solid salt;

[0066] ② The remaining brine and the lithium extraction tail brine obtained in step (2) of Example 1 are mixed evenly, and the mixture is evaporated in a sylvite pool to precipitate sylvite until the brine is saturated with sylvitonium salt (MgSO4·7H2O), and the solid salt is separated and the solid salt is detected to be free of soft potassium magnesium sulfate components;

[0067] ③ The remaining brine continues to evaporate in the potassium mixed salt pool to precipitate potassium mixed salt until the brine is saturated with carnallite (KCl·MgCl2·6H2O), and then the solid salt is separated and tested to see if there is no soft potassium magnesium sulfate component in the solid salt;

[0068] ④ The remaining brine continues to evaporate in the carnallite pool to precipitate carnallite salt until the brine is saturated with magnesium chloride, and the solid salt is separated to obtain 20.2 kg of old brine. It is detected that there is no lithium sulfate component in the solid salt;

[0069] (2) The potassium salt obtained in step (1) is used for the production of potassium chloride, and the obtained old brine is subjected to a lithium extraction process, wherein the lithium extraction process is selective electrodialysis, and a lithium chloride solution and a lithium extraction tail brine are obtained respectively, wherein the lithium chloride solution is used for further production of lithium-related products, and the lithium extraction old brine is used for the brine addition in the next batch of evaporation process.

[0070] Table 6 - Composition of salt precipitated from brine in different salt ponds during step (1) of Example 2

[0071]

[0072] When magnesium sulfate type salt lake brine is used to produce potassium and lithium by brine addition, depending on the composition of the original brine, the soft leonite and even leonite phase regions can be skipped to avoid the precipitation of soft leonite and even leonite. For example, after the brine of Example 1 is subjected to three cycles of brine addition, after the sylvite is completely precipitated, the brine is very close to the saturation point of carnallite. Thereafter, a potash ore with carnallite as the main component will be precipitated, which is a high-quality potash ore raw material.

[0073] Comparative Example 2

[0074] A method for co-producing lithium and potassium in magnesium sulfate salt lakes, using brine from the Uyuni Salt Lake in Bolivia as raw material. The composition of the brine is shown in Table 1. 2+ / SO4 2- The mass ratio is 0.93, SO4 2- / Li + The mass ratio is 25.2, which belongs to the magnesium sulfate type salt lake brine system, including the following steps:

[0075] (1) Take 200kg of magnesium sulfate salt lake brine, and the brine is as follows Figure 1 Various salts were precipitated by natural evaporation in a sodium salt pool until potassium chloride was saturated and about to precipitate, and then the solid salt was separated. At this time, the mass of the brine was 76.1 kg. The 19.4 kg old brine obtained in step (1) of comparative example 1 was mixed with the brine and the brine was added. Natural evaporation was continued until the old brine was obtained. The composition of various salts in different salt pools is shown in Table 7 below. The specific evaporation process is as follows:

[0076] ① Spread the original brine in the sodium salt pool under natural sunlight to precipitate sodium chloride salt until the brine is saturated with potassium chloride, and then separate the solid salt;

[0077] ② The remaining brine and the old brine obtained in step (1) of the comparative example 1 are mixed evenly, and the mixture is evaporated in a sylvite pool to precipitate sylvite until the brine is saturated with sylvitonium (MgSO4·7H2O), and the solid salt is separated and the solid salt is detected to be free of the soft potassium magnesium sulfate component;

[0078] ③ The remaining brine continues to evaporate in the potassium mixed salt pool to precipitate potassium mixed salt until the brine is saturated with carnallite (KCl·MgCl2·6H2O), and then the solid salt is separated and tested to see if there is no soft potassium magnesium sulfate component in the solid salt;

[0079] ④ The remaining brine continues to evaporate in the carnallite pool to precipitate carnallite salt until the brine is saturated with magnesium chloride, and the solid salt is separated. 0.33 kg of lithium sulfate is precipitated in the carnallite pool, and 13.8 kg of old brine is obtained;

[0080] (2) The potassium salt obtained in step (1) is used for the production of potassium chloride, and the obtained old brine is subjected to a lithium extraction process, wherein the lithium extraction process is selective electrodialysis, and a lithium chloride solution and a lithium extraction tail brine are obtained respectively, wherein the lithium chloride solution is used for further production of lithium-related products, and the lithium extraction old brine is used for the brine addition in the next batch of evaporation process.

[0081] Table 7 - Composition of salt precipitated from brine in different salt ponds during step (1) of comparative example 2

[0082]

[0083] In combination with the salt precipitation process analysis of Example 1 and Comparative Example 2 in Table 4 and Table 7, it can be seen that when Comparative Example 2 adopts the old brine without lithium to directly add brine, the output of potassium salt mine is reduced by 9%. It also skips the precipitation stage of potassium mixed salt A, and the output of potassium mixed salt mine is increased by 6%. However, this is a mine that is relatively difficult to handle for potassium salt; the output of carnallite mine is also reduced, that is, high-quality potash raw materials have all been reduced. Because the old brine added does not add lithium, 0.33kg of lithium sulfate ore is precipitated in the carnallite pool, which is 94% higher than the 0.17kg when the brine is not added in Comparative Example 1. This part of lithium solid ore will be difficult to recycle.

[0084] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of the present application in detail. It should be understood that the above description is only a specific embodiment of the present application and is not intended to limit the scope of protection of the present application.

Claims

1. A method for co-producing lithium and potassium in magnesium sulfate salt lakes, characterized in that: The following steps are involved: The magnesium sulfate salt lake brine is naturally evaporated to potassium chloride saturation, and solid salt is separated. The remaining brine and lithium extraction brine are evenly mixed according to a mass ratio, and naturally evaporated to epsom salt saturation, and solid salt is separated. The remaining brine is further naturally evaporated to carnallite saturation, and solid salt is separated. The remaining brine is further naturally evaporated to magnesium chloride saturation, and solid salt is separated to obtain old brine. The old brine is subjected to a lithium extraction process to obtain a lithium salt product and lithium extraction tail brine A; The mass percentage of magnesium ions in the lithium extraction brine is greater than 4.5%, the mass percentage of chloride ions is greater than 13%, the mass percentage of lithium ions is less than 0.07%, and the mass percentage of sulfate ions is less than 2%. Wherein, the composition of the magnesium sulfate salt lake brine is Na + , K + Mg 2+ / / Cl - 、SO4 2- -H2O is located in the potassium chloride region in the phase diagram of the five-component water-salt system.

2. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 1, wherein: The lithium extraction brine is lithium extraction tail brine A and / or lithium extraction tail brine B, and the preparation method of the lithium extraction tail brine B comprises the following steps: The magnesium sulfate type salt lake brine is naturally evaporated to magnesium chloride saturation or when the mass fraction of magnesium is greater than 8.5%, the solid salt is separated to obtain old brine, and the old brine is subjected to a lithium extraction process to obtain a lithium salt product and lithium extraction tail brine B.

3. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 2, wherein: The preparation method of the lithium-extracted tail halogen B comprises the following steps: The magnesium sulfate salt lake brine is naturally evaporated to potassium chloride saturation, and solid salt is separated. The remaining brine is further evaporated to saturation of philippine, and solid salt is separated. The remaining brine is further evaporated to saturation of philippine, and solid salt is separated. The remaining brine is further evaporated to carnallite saturation, and solid salt is separated. The remaining brine is further evaporated to magnesium chloride saturation or the mass fraction of magnesium is greater than 8.5%, and solid salt is separated to obtain old brine. The old brine is subjected to a lithium extraction process to obtain a lithium salt product and a lithium extraction tail brine B.

4. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 1, wherein: The lithium extraction process is a selective electrodialysis lithium extraction technology, a nanofiltration membrane lithium-magnesium separation technology or an electrochemical deintercalation lithium extraction technology.

5. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 1, wherein: The magnesium sulfate salt lake brine Mg 2+ / SO4 2- The mass ratio is greater than 0.5, SO4 2- / Li + The mass ratio is less than 40.

6. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 1, wherein: The mass fraction contents of sodium and potassium ions in the old brine are both 0.5-2 g / L.

7. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 2, characterized in that: When the lithium extraction tail brine B is used as the lithium extraction brine, the mass ratio of the magnesium sulfate type salt lake brine used in preparing the lithium extraction tail brine A and the lithium extraction tail brine B is 1:

1.

8. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 2, wherein: When the lithium extraction tail brine A is used as lithium extraction brine, the mass ratio of the remaining brine mixed with the lithium extraction brine and the magnesium sulfate type salt lake brine used to prepare the lithium extraction tail brine A is 1:

1.

9. The method for co-producing lithium and potassium in a magnesium sulfate type salt lake according to claim 2, wherein: The lithium extraction brine is 100% by mass of lithium extraction tail brine A or lithium extraction tail brine B.

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