RENEWABLE MAGNESIUM REMOVING AGENT AND ITS USE IN PREPARATION OF LOW-MAGNESIUM LITHIUM-RICH BRINE

Abstract

A renewable magnesium removing agent and its use in a preparation of a low-magnesium lithium-rich brine are provided. The magnesium removing agent includes a magnesium phosphate double salt of an alkali metal or ammonium. A regeneration of the magnesium removing agent is realized by adding the magnesium removing agent into Mg.sup.2+-containing chloride salt solution, wherein Mg.sup.2+in the chloride salt solution and the magnesium removing agent are subjected to a magnesium removing reaction to form a solid-phase reaction product and carrying out a solid-liquid separation on an obtained mixed reaction product after the magnesium removing reaction is ended to separate the solid-phase material comprising a magnesium phosphate hydrate and then separating out a chlorine salt of the alkali metal or the ammonium from a remaining liquid-phase material, and finally carrying out a regeneration reaction on the magnesium phosphate hydrate and the chlorine salt of the alkali metal or the ammonium.

Claims

1. A use a magnesium phosphate double salt of an alkali metal or ammonium as a magnesium removing agent, wherein the magnesium phosphate double salt comprises a combination of one or more of ammonium magnesium phosphate, potassium magnesium phosphate and sodium magnesium phosphate, and the use comprises: at least adding the magnesium phosphate double salt into a chloride salt solution containing an Mg.sup.2+ concentration of ≥10 g/L, wherein at least a portion of Mg.sup.2+ and the magnesium phosphate double salt are subjected to a magnesium removing reaction to form a solid-phase reaction product; carrying out a solid-liquid separation on an obtained mixed reaction product after the magnesium removing reaction is ended to separate a solid-phase material, wherein the solid-phase material comprises the solid-phase reaction product, wherein the solid-phase reaction product comprises a magnesium phosphate hydrate, then separating a chlorine salt of the alkali metal or the ammonium from a remaining liquid-phase material; and carrying out a regeneration reaction on the solid-phase reaction product and the chlorine salt of the alkali metal or the ammonium, wherein the alkali metal is selected from Na and/or K, to achieve a regeneration of the magnesium phosphate double salt.

2-4. (canceled)

5. The use according to claim 1, wherein the magnesium phosphate double salt and a crystal form control agent are cooperatively used, wherein an amount of the crystal form control agent is 5%-30% of a total mass of the magnesium phosphate double salt and the crystal form control agent, and the crystal form control agent comprises MgHPO.sub.4.3H.sub.2O, Mg.sub.3(PO.sub.4).sub.2.8H.sub.2O or Mg.sub.3(PO.sub.4).sub.2.22H.sub.2O.

6. The use according to claim 1, wherein a temperature of the magnesium removing reaction is 30-90° C.

7. The use according to claim 1, wherein a weight ratio of the magnesium phosphate double salt to the chloride salt solution is 0.1:1-1:1.

8. (canceled)

9. (canceled)

10. The use according to claim 1, wherein the solid-phase material further comprises an unreacted magnesium phosphate double salt.

11. (canceled)

12. The use according to claim 1, specifically comprising: carrying out the regeneration reaction on the solid-phase reaction product and a saturated solution of the chlorine salt of the alkali metal or the ammonium.

13. The use according to claim 1, wherein a temperature of the regeneration reaction is 0-25° C.

14. A method for preparing a low-magnesium lithium-rich brine, comprising: providing a first solution, wherein the first solution is a chloride salt solution, and the first solution at least contains Mg.sup.+ and Li.sup.+; adding a magnesium removing agent into the first solution, wherein the magnesium removing agent comprises a magnesium phosphate double salt of an alkali metal or ammonium, the magnesium phosphate double salt comprises a combination of one or more of ammonium magnesium phosphate, potassium magnesium phosphate, sodium potassium magnesium phosphate and sodium ammonium magnesium phosphate so that at least a portion of Mg.sup.2+ and the magnesium phosphate double salt undergo a magnesium removing reaction to form a solid-phase reaction product; carrying out a solid-liquid separation on an obtained mixed reaction product after the magnesium removing reaction is ended; obtaining a solid-phase material and a liquid-phase material after the solid-liquid separation is finished, wherein the solid-phase material comprises the solid-phase reaction product, wherein the solid-phase reaction product comprises a magnesium phosphate hydrate, and the liquid-phase material is a second solution; separating the chlorine salt of the alkali metal or the ammonium from the second solution; and carrying out a regeneration reaction on the solid-phase reaction product and the chlorine salt of the alkali metal or the ammonium, wherein the alkali metal is selected from Na and/or K, to achieve a regeneration of the magnesium phosphate double salt.

15. (canceled)

16. (canceled)

17. The method according to claim 14, comprising: cooperatively using the magnesium phosphate double salt and a crystal form control agent, wherein an amount of the crystal form control agent is 5%-30% of a total mass of the magnesium phosphate double salt and the crystal form control agent, and the crystal form control agent comprises MgHPO.sub.4.3H.sub.2O, Mg.sub.3(PO.sub.4).sub.2.8H.sub.2O or Mg.sub.3(PO.sub.4).sub.2.22H.sub.2O.

18. (canceled)

19. The method according to claim 14, wherein a temperature of the regeneration reaction is 0-25° C.

20. The method according to claim 14, further comprising: evaporating a separated second solution after the solid-liquid separation is finished so that the chlorine salt of the alkali metal or the ammonium is precipitated out and a third solution is obtained; and continuing to evaporate the third solution so that a carnallite in the third solution is precipitated out and a fourth solution is obtained to achieve a removal of magnesium and an enrichment of lithium.

21. The method according to claim 20, further comprising: after the solid-liquid separation is finished, washing a separated solid-phase material once or many times using a washing liquid and collecting a used washing liquid and mixing with the second solution, and then evaporating the used washing liquid to obtain the third solution; wherein the washing liquid comprises freshwater or diluted brine.

22. The method according to claim 21, wherein a weight ratio of the washing liquid to the solid-phase material is 5:1-1:1, and a washing temperature is 0-90° C.

23. The method according to claim 20, further comprising: adding the magnesium phosphate double salt of the alkali metal or the ammonium in the fourth solution so that at least the portion of Mg.sup.2+ and the magnesium phosphate double salt undergo the magnesium removing reaction to form the solid-phase reaction product; and carrying out the solid-liquid separation on the obtained mixed reaction product after the magnesium removing reaction is ended so as to obtain the solid-phase material and a fifth solution, to further achieve the removal of the magnesium and the enrichment of the lithium.

24. The method according to claim 14, wherein a temperature of the magnesium removing reaction is 30-90° C.

25. The method according to claim 14, wherein a weight ratio of the magnesium phosphate double salt to the chloride salt solution is 0.1:1-1:1.

26. The method according to claim 14, wherein in the chloride salt solution, a concentration of Mg.sup.2+ is ≥10 g/L, a concentration of Li.sup.+ is ≥0.3 g/L, and a concentration of Ca.sup.2+ is 1 g/L.

27. The method according to claim 23, further comprising: circularly carrying out the removal of the magnesium and the enrichment of the lithium with the fifth solution as the first solution until the low-magnesium lithium-rich brine is obtained, wherein in the low-magnesium lithium-rich brine, a concentration of Mg.sup.2+ is 15-4 g/L, a concentration of Li.sup.+ is 10-30 g/L, and a mass ratio of the magnesium to the lithium is 0.3-3:1.

28. The method according to claim 14, further comprising: pretreating an original brine of a salt lake to form the first solution.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] For making the objective, technical solution and advantages of the present application more clear, the present application will be described in detail by combining with embodiments. It should be understood that embodiments described here are only for explaining the present application but not intended to limit the present application.

[0037] An aspect of the present application firstly provides use of magnesium phosphate double salt of alkali metal or ammonium as a magnesium removing agent.

[0038] Further, the magnesium removing agent is renewable.

[0039] Further, the alkali metal comprises potassium (K) and/or sodium (Na). The magnesium phosphate double salt comprises but is not limited to a combination of any one or more of ammonium magnesium phosphate, potassium magnesium phosphate, sodium potassium magnesium phosphate and ammonium magnesium phosphate, or solid solution of these compounds.

[0040] In some embodiments, the use comprises: adding the magnesium phosphate double salt into Mg.sup.2+-containing chloride salt solution so that at least a portion of Mg.sup.2+ in the solution and the magnesium phosphate double salt undergo magnesium removing reaction to form a solid-phase reaction product.

[0041] Preferably, the magnesium phosphate double salt and a crystal form control agent (for example MgHPO.sub.4.3H.sub.2O, Mg.sub.3(PO.sub.4).sub.2.8H.sub.2O or Mg.sub.3(PO.sub.4).sub.2.22H.sub.2O) are cooperatively used after being mixed. The crystal form control agent is 5 wt %˜30 wt % of magnesium phosphate double salt and crystal form control agent. The crystal form control agent can improve the filtering performance of a magnesium phosphate solid-phase reaction product generated in magnesium removing reaction.

[0042] Further, the use can also comprises: carrying out solid-liquid separation on the obtained mixed reaction product after the magnesium removing reaction is ended, wherein the separated solid-phase material comprises the solid-phase reaction product which comprises magnesium phosphate hydrate, and then separating out chlorine salt of alkali metal or ammonium from the remaining liquid-phase material; and

[0043] The solid-phase reaction product and the chlorine salt of alkali metal or ammonium are subjected to regeneration reaction, thereby realizing the regeneration of the magnesium phosphate double salt.

[0044] Wherein, through the magnesium removing reaction, the obtained products comprise magnesium phosphate hydrate as the solid-phase reaction product, and chlorine salt of alkali metal or ammonium, wherein the chlorine salt of alkali metal or ammonium is preserved in the solution.

[0045] Further, the solid-phase material can also be a mixture of magnesium phosphate hydrate and magnesium phosphate double salt.

[0046] Another aspect of an embodiment of the present application provides a method for removing magnesium from chloride salt solution, comprising: adding a magnesium removing agent into Mg.sup.2+-containing chloride salt solution, the magnesium removing agent comprises magnesium phosphate double salt of alkali metal or ammonium so that at least a portion of Mg.sup.2+ in the solution and the magnesium phosphate double salt undergo magnesium removing reaction to form a solid-phase reaction product.

[0047] In the specification, for magnesium phosphate double salt of alkali metal or ammonium or chlorine salt of ammonium, unless otherwise noted, the alkali metal elements contained therein are all selected from Na and/or K.

[0048] In the method according to an embodiment of the present application, the magnesium phosphate double salt at least has a function of the magnesium removing agent, which can be selected from a combination of one or more of ammonium magnesium phosphate, potassium magnesium phosphate, sodium potassium magnesium phosphate and sodium ammonium magnesium phosphate.

[0049] Further, the temperature of the magnesium removing reaction is 30˜90° C.

[0050] Further, a weight ratio of the magnesium phosphate double salt to the chloride salt solution is preferably 0.1:1˜1:1.

[0051] Further, the concentration of Mg.sup.2+ in the chloride salt solution is preferably larger than or equal to 10 g/L.

[0052] Further, in some embodiments, the method can also comprises: carrying out solid-liquid separation on the obtained mixed reaction product after the magnesium removing reaction is ended, wherein the separated solid-phase material comprises the solid-phase reaction product comprising magnesium phosphate hydrate (for example tetrahydrate, octahydrate and 22 hydrate of Mg.sub.3(PO.sub.4).sub.2), and then separating out the chlorine salt (for example, sodium chloride, potassium chloride and ammonium chloride) of alkali metal or ammonium from the remaining liquid-phase material.

[0053] Further, the solid-phase material also comprises the unreacted magnesium phosphate double salt.

[0054] More further, in the above embodiment, the method can also comprise: carrying out regeneration reaction on the solid-phase reaction product and the chloride salt of alkali metal or ammonium, thereby achieving the regeneration of the magnesium phosphate double salt.

[0055] The mechanism of the regeneration reaction can refer to the following formula:

NH.sub.4(K, Na)Cl+Mg.sub.3(PO.sub.4).sub.2.xH.sub.2O.fwdarw.NH.sub.4(K, Na)MgPO.sub.4.xH.sub.2O

[0056] Preferably, the method specifically comprises: carrying out regeneration reaction on the solid-phase reaction product and the saturated solution of the chlorine salt of alkali metal or ammonium.

[0057] Wherein, the temperature of the regeneration reaction is preferably 0˜25° C.

[0058] In the above embodiment, the used solid-liquid separation manner can be selected from but not limited to filtration, centrifugation, precipitation and the like, so that the solid-phase material is step by step or continuously taken out from the liquid-phase material.

[0059] In some embodiments of the present application, a method for enriching lithium and meanwhile removing magnesium from chloride salt solution comprises:

[0060] providing a chloride salt solution, wherein the concentration of Mg.sup.2+ is ≥10 g/L, the concentration of Li.sup.+ is ≥0.3 g/L, and the concentration of Ca.sup.2+ is ≤1 g/L;

[0061] adding magnesium phosphate double salt of alkali metal or ammonium to the chloride salt solution in a dosage as a magnesium removing agent, at least partially reacting the magnesium removing agent with Mg.sup.2+ in the solution to form a magnesium removing solid product (magnesium phosphate hydrate) so that Mg.sup.2+ in the solution enters the solid phase, and meanwhile the alkali metal or ammonium ions in the magnesium phosphate double salt enter the solution, wherein the amount of the added magnesium phosphate double salt is set in the following manner: after the magnesium phosphate double salt is added to react with Mg.sup.2+ in the solution, and the concentration of Mg.sup.2+ in the remaining solution is ≥4 g/L.

[0062] Another aspect of the embodiment of the present application also provides a method for preparing low-magnesium lithium-rich brine, comprising:

[0063] providing a first solution, the first solution being chloride salt solution, and the first solution at least containing Mg.sup.2+ and Li.sup.+;

[0064] adding a magnesium removing agent into the first solution, the magnesium removing agent comprising magnesium phosphate double salt of alkali metal or ammonium, so that at least a portion of Mg.sup.2+ and the magnesium phosphate double salt undergo magnesium removing reaction to form a solid-phase reaction product; and

[0065] carrying out solid-liquid separation on the obtained mixed reaction product after the magnesium removing reaction is ended.

[0066] In some embodiments, the method also comprises:

[0067] obtaining a solid-phase material and a liquid-phase material after the solid-liquid separation is finished, wherein the solid-phase material comprises the solid-phase reaction product which comprises magnesium phosphate hydrate, and the liquid-phase material is a second solution; and

[0068] separating the chlorine salt of alkali metal or ammonium from the second solution; and

[0069] carrying out regeneration reaction on the solid-phase reaction product and the chlorine salt of alkali metal or ammonium, thereby achieving the regeneration of the magnesium phosphate double salt.

[0070] Wherein, the magnesium phosphate double salt can be selected from but is not limited to a combination of any one or more of ammonium magnesium phosphate, potassium magnesium phosphate, sodium potassium magnesium phosphate and sodium ammonium magnesium phosphate, or solid solution thereof.

[0071] Preferably, the magnesium removing agent can also be a mixture of the magnesium phosphate double salt and a crystal form control agent, wherein the content of the crystal form control agent is 5 wt %˜30 wt %. The crystal form control agent comprises MgHPO.sub.4.3H.sub.2O, Mg.sub.3(PO.sub.4).sub.2.8H.sub.2O or Mg.sub.3(PO.sub.4).sub.2.22H.sub.2O, but are not limited thereto.

[0072] In some embodiments, the method can specifically comprises: carrying out regeneration reaction on the solid-phase reaction product and the saturated solution of the chloride salt of the alkali metal or ammonium. Further, the temperature of the regeneration reaction is 0˜25° C.

[0073] In some embodiments, the method can also comprises:

[0074] evaporating the separated second solution after the solid-liquid separation is finished so that the chlorine salt of alkali metal or ammonium therein is precipitated out and a third solution is obtained; and

[0075] continuing to evaporate the third solution so that the carnallite in the solution is precipitated out and a fourth solution is obtained, thereby achieving removal of magnesium and enrichment of lithium.

[0076] Further, the method also comprises: after the solid-liquid separation is finished, washing the separated solid-phase material once or many times using washing liquid and collecting the used washing liquid and mixing with the second solution, and then evaporating to obtain the third solution; wherein the washing liquid comprises freshwater or diluted brine. Preferably, a weight ratio of the washing liquid to the solid-phase material is 5:1˜1:1, and the washing temperature is 0˜90° C.

[0077] More further, the method also comprises:

[0078] adding the magnesium phosphate double salt of alkali metal or ammonium in the fourth solution so that at least a portion of Mg.sup.2+ and the magnesium phosphate double salt undergo magnesium removing reaction to form a solid-phase reaction product; and

[0079] carrying out solid-liquid separation on the obtained mixed reaction product after the reaction is ended so as to obtain a solid-phase material and a fifth solution, further achieving removal of magnesium and enrichment of lithium.

[0080] In the above embodiment of the present application, the temperature of the magnesium removing reaction is preferably 30˜90° C.

[0081] In the above embodiment of the present application, a weight ratio of the magnesium phosphate double salt to the chloride salt solution is 0.1:1˜1:1.

[0082] In the above embodiment of the present application, preferably, in the chloride salt solution, the concentration of Mg.sup.2+ is ≥10 g/L, the concentration of Li.sup.+ is ≥0.3 g/L, and the concentration of Ca.sup.2+ is ≤1 g/L.

[0083] More further, the method also comprises: cyclically carrying out the removal of magnesium and enrichment of lithium with the fifth solution as the first solution until low-magnesium lithium-rich brine is obtained, wherein in the low-magnesium lithium-rich brine, the concentration of Mg.sup.2+ is 15˜4 g/L, the concentration of Li.sup.+ is 10˜30 g/L, and a mass ratio of magnesium to lithium is 0.3˜3:1.

[0084] In the above embodiment of the present application, various processes can be repeated, namely, the cycle of magnesium removal-evaporation-magnesium removal can be carried out for many times, and in the chloride salt solution, the concentration of Mg.sup.2+ is reduced to 15˜4 g/L, the concentration of Li.sup.+ is increased to 10˜30 g/L, and meanwhile a mass ratio of magnesium to lithium is 0.3˜3, namely, low-magnesium lithium-rich bribe is obtained.

[0085] In some embodiments of the present application, the method also comprises: pretreating the original brine of a salt lake to form the first solution.

[0086] For example, the original brine of the salt lake is evaporated and concentrated until the concentration of Mg.sup.2+ is 10 g/L˜70 g/L and the concentration of Li.sup.+ is 0.5 g/L˜6 g/L to obtain the raw brine, and then the soluble calcium salt is added into the raw brine to remove sulfate ions and enrich magnesium ions to obtain the first solution. Wherein, the soluble calcium salt can be selected from calcium chloride or calcium hydroxide solution or the like, and is not limited to thereto.

[0087] In the above embodiment of the present application, the brine can be evaporated and concentrated by evaporation of salt field and other manners. Of course, the same objects can be achieved by using bribe concentration technologies such as forced evaporation process, electrodialysis or reverse osmosis.

[0088] For example, a more typical implementation mode of the embodiment of the present application may include the following steps:

[0089] (1) a high-magnesium brine A.sub.0 (i.e., the first solution mentioned above) is provided, and the concentration of Mg.sup.2+ in the high-magnesium brine A.sub.0 is greater than or equal to 15 g/L. Sufficient or excessive magnesium phosphate double salt (such as ammonium magnesium phosphate, NH.sub.4MPO.sub.4.6H.sub.2O) is added into the high-magnesium brine A.sub.0 to be reacted as a magnesium removing agent for reaction, and the solid-liquid separation is carried out after the reaction at the appropriate temperature for a set time to obtain magnesium removal brine A.sub.1 and magnesium precipitate B1, the concentration of Mg.sup.2+ in the magnesium removal brine A.sub.1 is 4 g/L˜15 g/L, and the concentration of Li.sup.+ is within the range of 0.3 g/L˜4 g/L.

[0090] The above high-magnesium brine A.sub.0 can be prepared from the original brine of salt lake through pre-treatment processes such as evaporation concentration, sulfate radical removal, and the process operation and process conditions of these pretreatment can be known to those skilled in the art.

[0091] The magnesium removing agent being ammonium magnesium phosphate is taken as an example. After ammonium magnesium phosphate is added into high-magnesium brine A.sub.0, the slurry is obtained when the weight ratio of solid to liquid is 0.1˜1:1. The slurry is reacted at 30° C. 90° C. for 1-12 h, and then solid-liquid separation is carried out to obtain magnesium removal brine A.sub.1 (namely, the above second solution) and magnesium precipitate B1. The obtained magnesium precipitate B1 is magnesium phosphate octahydrate, magnesium phosphate 22 hydrate, a mixture of magnesium phosphate octahydrate, magnesium phosphate 22 hydrate or and a mixture of magnesium phosphate octahydrate, magnesium phosphate 22 hydrate and magnesium ammonium phosphate.

[0092] (2) The magnesium precipitate B1 obtained in step (1) is washed with washing solution such as water or diluted brine, the solid-liquid weight ratio when washing is within the range of 1-4:1, the washing time is 0.5-3 h, and the used washing solution A.sub.2 and magnesium precipitate B11 are obtained by solid-liquid separation.

[0093] (3) The used washing solution A.sub.2 is mixed with the magnesium removal brine A.sub.1 obtained in step (1) to obtain brine A.sub.3. The brine A.sub.3 is evaporated and concentrated until the chlorine salt of alkali metal or ammonium in the brine is precipitated out. After solid-liquid separation, low-magnesium lithium-rich brine A.sub.4 and chlorine salt of solid alkali metal or ammonium are obtained (if the magnesium removing agent is sodium magnesium phosphate and magnesium ammonium phosphate, solid sodium chloride and solid ammonium chloride will be obtained here).

[0094] For example, brine A.sub.3 can be evaporated to a concentration range of 40 g/L˜50 g/L of Mg.sup.2+, and a large amount of solid ammonium chloride is precipitated out in this process. The residual brine after ammonium chloride is precipitated out is brine A.sub.31 (namely, the above third solution). The ammonium carnallite is precipitated out when the brine A.sub.31 is continued to be evaporated to a concentration range of 80 g/L˜100 g/L of Mg.sup.2+ The residual brine after ammonium carnallite is precipitated out is brine A.sub.32 (namely, the above fourth solution). The ratio of Mg to Li in the brine A.sub.32 is 1/2˜2/3 of the ratio of Mg to Li in A.sub.3. The brine A.sub.32 is diluted, when the concentration of magnesium ions is within the concentration range of magnesium ions in the raw brine A, the brine A.sub.32 is returned back to the first step and mixed with the raw brine A to prepare high-magnesium brine A.sub.0.

[0095] (4) The chlorine salt of solid alkali metal or ammonium obtained in step (3) is prepared into a solution, and mixed with the magnesium washing precipitate B11 obtained in step (2) to obtain magnesium phosphate double salt; the magnesium phosphate double salt obtained in this step is used as the magnesium removing agent in step (1), and the steps (1) to (4) are cycled to realize preparation of low-magnesium lithium-rich brine A.sub.4 with a magnesium removing agent regeneration method.

[0096] For example, the obtained solid ammonium chloride can be prepared into saturated ammonium chloride solution, magnesium precipitate B11 is added into saturated ammonium chloride solution for reaction, a reaction temperature is 5-30° C., stirring is carried out for 0.5-3 h, and solid-liquid separation is conducted to obtain ammonium magnesium phosphate and regeneration solution A.sub.5; the regeneration solution A.sub.5 is circularly introduced into brine A.sub.3 of step (3) for evaporation and concentration until the ratio of magnesium to lithium in the obtained low-magnesium lithium-rich brine A.sub.4 is less than 2 and the concentration of Li.sup.+ is whitin the range of 10 g/L˜30 g/L, the comprehensive recovery of lithium in the whole preparation process can be controlled to be 50%˜90%.

[0097] In order to make the purpose, technical solution and advantages of the embodiment of the present application more clearer, the technical solution in the embodiment of the present application will be described clearly and completely below. If the specific conditions are not specified in the embodiment, the conventional conditions or the conditions recommended by the manufacturer shall be followed. The reagents or instruments used without manufacturers are conventional products that can be purchased on the market.

[0098] Example 1: 500 g of mixed aqueous solution simultaneously containing magnesium chloride and lithium chloride was taken, in which the concentration of magnesium ions is 40 g/L and the concentration of lithium ions is 1.2 g/L. The magnesium precipitating agent potassium magnesium phosphate hexahydrate was added in four times for total 400 g. After reacting at 50 ° C. for 4 h, solid-liquid separation was conducted to obtain 450 g of magnesium precipitation solid product and 450 g of product solution, wherein the concentration of magnesium ions was reduced to 6.5 g/L, the concentration of lithium ions was 0.8 g/L, and the concentration of potassium ions was 110 g/L.

[0099] Example 2: 500 g of mixed aqueous solution simultaneously containing magnesium chloride and lithium chloride was taken, in which the concentration of magnesium ion is 40 g/L and the concentration of lithium ions was 1.2 g/L, 400 g of magnesium ammonium phosphate hexahydrate was added in four times for total 400 g. After reacting at 80° C. for 4 h, the solid-liquid separation was conducted to obtain 440 g of magnesium precipitation solid product and 460 g of product solution, wherein the concentration of magnesium ions was reduced to 7.1 g/L and the concentration of lithium ions was 1.0 g/L.

[0100] Example 3: 500 g of mixed aqueous solution simultaneously containing magnesium chloride and lithium chloride was taken, in which the concentration of magnesium ions was 40 g/L and the concentration of lithium ions was 1.2 g/L. 500 g of magnesium potassium phosphate tetrahydrate was added in four times for total 500 g. After reacting at 30° C. for 4 h, solid-liquid separation was conducted to obtain 448 g of magnesium precipitation solid products and 550 g product solution. The concentrations of magnesium ions was reduced to 5.9 g/L and the concentrations of lithium ions was 0.83 g/L.

[0101] Example 4: 500 g of mixed aqueous solution simultaneously containing magnesium chloride and lithium chloride was taken, in which the concentration of magnesium ions was 40 g/L, the concentration of lithium ions was 1.2 g/L, magnesium precipitation agent sodium magnesium ammonium tetrahydrate was added in four times for total 500 g. After reacting at 90° C. for 4 h, solid-liquid separation was conducted to obtain 622 g of magnesium precipitation solid product and 470 g of product solution, wherein the concentration of magnesium ions was reduced to 10.2 g/L, and the concentration of lithium ions was 0.76 g/L.

[0102] Example 5: 330 g of water in 450 g of the product solution obtained in example 1 was evaporated at 20° C. to obtain 74 g of potassium chloride solid, and 41 g of brine was remained, wherein the concentration of magnesium ions was 57 g/L, the concentration of lithium ions was 7 g/L and the concentration of potassium ions was 17 g/L.

[0103] Example 6: 440 g of magnesium precipitation solid product obtained in example 2 was added into 500 g of saturated ammonium chloride regeneration solution to react at 20° C., so as to realize the regeneration of the magnesium precipitant. 420 g of magnesium ammonium phosphate was yielded and 516 g of regeneration solution was remained.

[0104] Example 7: 4000 g of the product solution obtained from example 1 was repeatedly used. After the treatment process described in example 5, and the remaining brine was 400 g. 420 g of regenerated magnesium precipitant obtained from example 6 was added into the remaining brine to be strongly stirred and reacted for 2 h at 70° C., solid-liquid separation was conducted to obtain 450 g of magnesium precipitation solid product and remain 355 g of brine. In the remaining brine, the concentration of magnesium ions was 17 g/L, the concentration of lithium ions was 6.4 g/L, and the concentration of ammonium ions was 69 g/L.

[0105] Example 8: 130 g of water in 355 g of remaining brine obtained in Example 7 was evaporated at 25° C. to precipitate out 53 g of ammonium chloride, and then 50 g of water was continued to be evaporated to precipitate out 55 g of ammonium carnallite, and 60 g of remaining brine was obtained in which the concentration of magnesium ions was 34 g/L, the concentration of lithium ions was 30 g/L, and the weight ratio of magnesium to lithium was 1.13.

[0106] Example 9: under the same conditions as that in example 1, when the crystal form control agent MgHPO.sub.4.3H.sub.2O whose weight was 10% of the weight of the magnesium precipitation agent was added, the entrainment rate of mother liquor in the magnesium precipitation solid product was reduced from more than 40 wt % without the addition of crystal form control agent to 16 wt %.

[0107] In addition, under the same conditions as in example 1, the crystal form control agent Mg.sub.3 (PO.sub.4).sub.2.8H.sub.2O or Mg.sub.3 (PO.sub.4).sub.2.22H.sub.2O was added, and the amount of the crystal form control agent is 5% and 30% of the total mass of the magnesium precipitation agent and the crystal form control agent, and the entrainment rates of mother liquor in the magnesium precipitation solid product were reduced to 20% and 15% respectively.

[0108] Example 10: 450 g of magnesium precipitation solid product obtained in example 1 was washed using 300 g water for three times to obtain 354 g of washing solution in which

[0109] in three times to obtain 354 g of washing solution, in which the concentration of magnesium ions was 18 g/L and the concentration of lithium ions was 0.55 g/L.

[0110] Example 11: the brine beach of a salt lake was dried to form old brine A, which is composed of 1.27 g/L Li.sup.+, 3.163 g/L Na.sup.+, 1.218 g/L K.sup.+, 77.115 g/L Mg.sup.2+, 8.98 g/L SO.sub.4.sup.2−, 3.83 g/L B and 224.52 g/L 250 g of old brine A was diluted with 500 g of fresh water, and then 300 g of magnesium ammonium phosphate hexahydrate was added. After reaction at 50° C. for 2 hours, solid-liquid separation was conducted to obtain 430 g of magnesium precipitation solid products and 616 g of product brine B, wherein the concentration of magnesium ions was reduced to 9 g/L, the concentration of lithium ions was 0.35 g/L, and the concentration of ammonium ions was 33 g/L. At this time, the weight ratio of Mg to Li in brine was decreased from 62 to 25.

[0111] Example 12: 400 g of water in product brine B in example 11 was evaporated at room temperature until the concentration of magnesium ions was 26.5 g/L, then 90 g of magnesium precipitation agent magnesium ammonium phosphate hexahydrate was added to react at 60° C. for 2 h, and then solid-liquid separation was conducted to obtain 132 g of magnesium precipitation solid product and 175 g of product brine C, wherein the concentration of magnesium ions was reduced to 8.7 g/L and the concentration of lithium ions was 1 g/L. At this time, the weight ratio of Mg to Li in brine was reduced from 25 to 9.

[0112] Example 13: 80 g of water in brine C obtained in example 12 was evaporated at room temperature until the concentration of magnesium ions was 31 g/L, 41 g of a mixture of ammonium chloride and ammonium carnallite was precipitated out, and then 22 g of magnesium precipitation agent magnesium ammonium phosphate hexahydrate was added to react at 60° C. for 2 h, and then solid-liquid separation was conducted to obtain 35 g of magnesium precipitation solid product and 35 g of brine D, wherein the concentration of magnesium ions was reduced to 10 g/L, and the concentration of lithium ions was 6 g/L. At this time, the weight ratio of Mg to Li in brine was decreased from 9 to 1.6.

[0113] Example 14: the magnesium precipitation products (590 g in total) obtained from examples 11 to 13 were stirred with 800 g of NH.sub.4Cl saturated solution at room temperature for 3 h, and then the solid-liquid separation was conducted to obtain 500 g of regenerated magnesium precipitation agent. Then, 300 g of fresh water was used to wash the regenerated magnesium precipitation agent for five times, and 430 g of dry magnesium precipitation agent was obtained after filter pressing.

[0114] In addition, the inventor of the present invention also conducted experiments with other raw materials and process conditions listed in the specification with reference to examples 1 to 14. The results show that by virtue of the method provided by the embodiment of the present application, preparation of low-magnesium lithium-rich brine can be efficiently and low-cost realized.

[0115] It should be understood that the above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.