A preparation method of anhydrous magnesium salt

Abstract

The present invention discloses a method for preparing anhydrous magnesium salt, comprising the steps of: subjecting anhydrous magnesium formate and an ammonium salt composed of anions of the anhydrous magnesium salt to be produced and ammonium ions to a solid-phase reaction or a liquid-phase reaction at a predetermined temperature to produce anhydrous magnesium salt, formic acid, and ammonia; removing volatilized formic acid and ammonia and ammonium formate synthesized by cooling the formic acid and ammonia, and obtaining the anhydrous magnesium salt. The method of the present invention is safe and reliable and can produce anhydrous magnesium salt of extremely high quality.

Description

Technical Field

[0001] The present invention relates to the field of preparation of inorganic salts, and in particular to a method for preparing anhydrous magnesium salt. Background Art

[0002] The existing technology primarily uses the Pidgeon process to produce metallic magnesium. This process uses high-grade dolomite as its primary raw material, which results in high energy consumption, large carbon emissions, and environmental pollution. Compared to the Pidgeon process, the electrolytic process offers advantages such as low energy consumption (the latter is approximately 2.86 times the former), continuous production, a high degree of mechanization and automation, and high production performance. Anhydrous magnesium salts, such as anhydrous magnesium chloride, have a wide range of uses. In addition to being used as pharmaceutical intermediates, catalyst supports, and desiccants, they are primarily used as the basic raw material for electrolytic magnesium.

[0003] One of the key issues in the electrolytic production of magnesium metal is the availability of high-quality, low-cost raw materials for anhydrous magnesium salts such as anhydrous magnesium chloride. Therefore, inexpensive bischofite (MgCl2·6H2O), potassium carnallite (KCl·MgCl2·6H2O), and ammonium carnallite (NH4Cl·MgCl2·6H2O) from salt lake brines can be selected as raw materials. Anhydrous magnesium chloride is prepared by dehydration, and then electrolysis is used to produce magnesium metal. Existing technical routes for the preparation of anhydrous magnesium chloride using the above raw materials mainly include gas shielded heating dehydration, organic solvent distillation and molecular sieve dehydration, magnesium chloride double salt and complex decomposition dehydration, and high-temperature chlorination. However, these dehydration methods all have their own disadvantages. For example, the gas shielded heating dehydration method requires a large amount of hydrogen chloride gas, which consumes a lot of energy and causes severe equipment corrosion at high temperatures. The organic solvent distillation and molecular sieve dehydration methods involve the decomposition of organic reagents and magnesium chloride, and the anhydrous magnesium chloride product contains a large amount of carbon and magnesium oxide, making it difficult to use as an electrolysis raw material. The magnesium chloride double salt and complex decomposition dehydration method mainly synthesizes magnesium chloride hexaammine, which is then decomposed to prepare anhydrous magnesium chloride. This method uses high temperatures above 400°C for deammonification. Since ammonia mixes with air at high temperatures, there is a risk of explosion and questionable safety. In addition, if hydrated magnesium chloride is used as the starting material for the synthesis of magnesium chloride hexaammine, the chlorine produced by electrolytic magnesium production cannot be recycled. If magnesium hydroxide or oxide is used as the raw material, highly corrosive hydrochloric acid and ammonia are required in the preparation process, increasing the harm to the environment. The Chinese patent application "A method for preparing anhydrous magnesium chloride" (application number: CN202111573672) discloses for the first time a method for preparing anhydrous magnesium chloride using anhydrous magnesium formate and anhydrous hydrogen chloride in an anhydrous organic solvent, and producing formic acid as a by-product. However, this method uses anhydrous hydrogen chloride as a raw material, and there are certain safety risks and equipment corrosion problems. Therefore, finding a safer and more reliable method for preparing anhydrous magnesium chloride has always been an urgent problem to be solved in this field. Summary of the Invention

[0004] In order to make up for the deficiencies of the above-mentioned prior art, the present invention provides a method for preparing anhydrous magnesium salt.

[0005] The technical problem of the present invention is solved by the following technical solutions:

[0006] A method for preparing anhydrous magnesium salt comprises the following steps:

[0007] Anhydrous magnesium formate and an ammonium salt composed of anions and ammonium ions of the anhydrous magnesium salt to be generated are subjected to a solid phase reaction or a liquid phase reaction at a predetermined temperature to generate anhydrous magnesium salt, formic acid and ammonia gas, and the volatilized formic acid and ammonia gas and the ammonium formate synthesized by cooling the formic acid and the ammonia gas are removed to obtain anhydrous magnesium salt.

[0008] Preferably, the anhydrous magnesium formate and the ammonium salt are prepared according to a theoretical molar ratio or in a molar excess of the ammonium salt.

[0009] Preferably, the liquid phase reaction of anhydrous magnesium formate and an ammonium salt consisting of an anion of anhydrous magnesium salt to be generated and ammonium ions at a predetermined temperature comprises:

[0010] S1, stirring the ultrafine powder of the anhydrous magnesium formate and the ammonium salt in a solvent to form a suspension, and performing a double decomposition reaction at 120° C.-150° C. to generate anhydrous magnesium salt, formic acid and ammonia gas;

[0011] S2. During step S1, under the purge of high-temperature protective gas, gaseous formic acid and ammonia are removed under negative pressure to obtain a magnesium salt suspension suspended in the solvent;

[0012] S3, post-treating the magnesium salt suspension to obtain anhydrous magnesium salt solid.

[0013] Preferably, the solid phase reaction of anhydrous magnesium formate and an ammonium salt composed of anions of the anhydrous magnesium salt to be generated and ammonium ions at a predetermined temperature comprises:

[0014] A1. Directly subjecting the ultrafine powders of the anhydrous magnesium formate and the ammonium salt to a solid phase reaction at 120-180° C. to generate anhydrous magnesium salt, formic acid and ammonia gas;

[0015] A2. During step A1, the gaseous formic acid and ammonia generated in step A1 are removed with a high-temperature protective gas, and ammonium formate synthesized by cooling the formic acid and ammonia is removed to obtain an anhydrous magnesium salt solid.

[0016] Preferably, the solvent is at least one of a high-boiling-point organic solvent and an ionic liquid; wherein the boiling point of the high-boiling-point organic solvent is above 150°C, the melting point of the ionic liquid is not higher than 90°C, and the decomposition temperature is above 200°C; preferably, the high-boiling-point organic solvent is at least one of DMF, formamide, and phenoxyethanol; preferably, the ionic liquid is at least one of tributylmethylammonium bistrifluoromethanesulfonimide and octyl-3-methylimidazole tetrafluoroborate.

[0017] Preferably, before carrying out the solid phase reaction or the liquid phase reaction, the following pre-reaction is further included:

[0018] anhydrous magnesium formate and an ammonium salt composed of an anion of anhydrous magnesium salt to be generated and ammonium ions are added to anhydrous formic acid according to the theoretical molar ratio of double decomposition reaction, the reaction temperature is raised to the boiling point of anhydrous formic acid under protective gas protection, and the mixture is stirred thoroughly for 1-4 hours while maintaining condensation and reflux. After the condensation and reflux are stopped, the formic acid is removed to obtain a solid mixture comprising magnesium formate, ammonium chloride, magnesium chloride and ammonium formate;

[0019] The solid mixture is subjected to the solid-phase reaction or the liquid-phase reaction to generate anhydrous magnesium salt, formic acid and ammonia gas, and the volatilized formic acid and ammonia gas and the ammonium formate synthesized by cooling the formic acid and the ammonia gas are removed to obtain anhydrous magnesium salt.

[0020] Preferably, the method further comprises the step of preparing anhydrous magnesium formate:

[0021] (1) magnesium chloride, magnesium sulfate, or magnesium nitrate containing crystal water is subjected to double decomposition reaction with ammonium formate in a solvent to generate a mixed salt precipitate of magnesium formate dihydrate and ammonium chloride, ammonium sulfate, or ammonium nitrate;

[0022] (2) Separating the mixed salt by recrystallization separation technology to obtain magnesium formate dihydrate and solid crystalline powder of ammonium chloride or ammonium sulfate or ammonium nitrate, and drying and dehydrating at above 105° C. to obtain anhydrous magnesium formate.

[0023] Preferably, the method further comprises: recovering the generated formic acid and ammonia to form ammonium formate, and then using the ammonium formate in the step of preparing anhydrous magnesium formate.

[0024] Preferably, the anhydrous magnesium salt is one of anhydrous magnesium chloride, anhydrous magnesium sulfate and anhydrous magnesium nitrate, and correspondingly, the ammonium salt is one of ammonium chloride, ammonium sulfate and ammonium nitrate.

[0025] The beneficial effects of the present invention compared with the prior art include: the present invention provides a new method for preparing anhydrous magnesium salt, wherein anhydrous magnesium formate and an ammonium salt are subjected to a solid-phase reaction or a liquid-phase reaction at a predetermined temperature to generate anhydrous magnesium salt, formic acid and ammonia gas, and the volatilized formic acid and ammonia gas and the ammonium formate synthesized by cooling are removed to obtain anhydrous magnesium salt. The method of the present invention is safe and reliable and can obtain anhydrous magnesium salt of extremely high quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 1 is a schematic diagram of the equipment and process for preparing anhydrous magnesium chloride in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] The present invention will be further described below in conjunction with preferred embodiments. It should be noted that, unless there is a conflict, the embodiments and features in the embodiments of the present application can be combined with each other.

[0028] In the process of preparing anhydrous magnesium chloride, how to remove the crystallization water of magnesium chloride is very critical, the method for removing the crystallization water of magnesium chloride mainly includes direct dehydration method (directly removing water molecules for magnesium chloride) and indirect dehydration method (such as magnesium chloride double salt and complex decomposition dehydration method is to first convert magnesium chloride into coordination compound, and then carry out high-temperature deamination; such as anhydrous magnesium chloride is synthesized by reacting anhydrous hydrogen chloride with magnesium formate), but the above-mentioned method for synthesizing anhydrous magnesium chloride by reacting anhydrous hydrogen chloride with magnesium formate has certain safety risks and equipment corrosion risks, therefore, it is necessary to find a safer and more reliable method, anhydrous magnesium formate is converted into anhydrous magnesium chloride, anhydrous magnesium sulfate and other inorganic magnesium salts again, to meet the downstream industries such as electrolytic magnesium and pharmaceutical industry to anhydrous magnesium chloride, anhydrous magnesium sulfate and anhydrous magnesium nitrate and other anhydrous magnesium salts harsh quality requirements. The anhydrous magnesium nitrate prepared by the present invention can be used for producing concentrated nitric acid after dehydrating dilute nitric acid.

[0029] The preferred embodiment of the present invention relies on the existing indirect dehydration method, that is, a magnesium salt with crystal water (such as magnesium chloride hexahydrate, magnesium sulfate heptahydrate, magnesium nitrate hexahydrate) can be first converted into magnesium formate dihydrate, and then the crystal water of the magnesium formate is removed to obtain anhydrous magnesium formate, and anhydrous magnesium formate is used as a raw material to prepare anhydrous magnesium chloride. Unlike the previously disclosed method, the present invention reacts anhydrous magnesium formate with an ammonium salt composed of an anion of the anhydrous magnesium salt to be generated and an ammonium ion to synthesize magnesium salt, formic acid and ammonia gas, and removes the volatilized formic acid and ammonia gas and the ammonium formate synthesized by cooling the formic acid and the ammonia gas to obtain anhydrous magnesium salt.

[0030] The indirect dehydration method in the preferred embodiment of the present invention is used to prepare anhydrous magnesium salts such as anhydrous magnesium chloride. The process conditions are mild and controllable. Compared with the direct dehydration method (such as directly removing the crystal water from the magnesium chloride raw material with crystal water), the process difficulty is smaller, the dehydration effect is good, and the cost is low, the equipment requirements are low, the investment is small, and it is easy to achieve capacity expansion.

[0031] Chemicals in this document, unless otherwise specified as containing water or water, are considered anhydrous. "Anhydrous" generally refers to a water content of less than 0.5%, and "%" refers to percentage by weight unless otherwise specified. "Room temperature" refers to a temperature within the range of 15-45°C.

[0032] A specific embodiment of the present invention provides a method for preparing anhydrous magnesium salt, comprising the following steps: subjecting anhydrous magnesium formate and an ammonium salt composed of anions of the anhydrous magnesium salt to be generated and ammonium ions to a solid-phase reaction or a liquid-phase reaction at a predetermined temperature to generate anhydrous magnesium salt, formic acid, and ammonia; and removing the volatilized formic acid and ammonia and ammonium formate synthesized by cooling the formic acid and ammonia to obtain the anhydrous magnesium salt.

[0033] In a preferred embodiment, the anhydrous magnesium formate and the ammonium salt are prepared according to a theoretical molar ratio or in a molar excess of the ammonium salt. Preferably, if the anhydrous magnesium formate and the ammonium salt are prepared in a molar excess of the ammonium salt, the excess ammonium salt can be removed by thermal decomposition (for example, calcination at above 300° C.) after the reaction is completed.

[0034] In a preferred embodiment, the liquid phase reaction of anhydrous magnesium formate and an ammonium salt composed of an anion of anhydrous magnesium salt to be generated and ammonium ions at a predetermined temperature comprises:

[0035] S1. The ultrafine powders of the anhydrous magnesium formate and the ammonium salt are stirred into a suspension in a solvent, and a double decomposition reaction is performed at 120° C. to 150° C. (the reaction temperature is lower than the boiling point of the solvent) to generate anhydrous magnesium salt, formic acid and ammonia gas; preferably, the average particle size of the anhydrous magnesium formate and the ammonium salt powders is less than 10 microns, and the smaller the particle size, the better.

[0036] S2. During step S1, under a high temperature (preferably 120° C.-150° C.) protective gas purge, the volatilized gaseous formic acid and ammonia are removed at a negative pressure (preferably a vacuum degree between 0 and -0.1 MPa) to obtain a magnesium salt suspension suspended in the solvent; by adopting a high temperature protective gas purge adapted to the reaction temperature of step S1, the generated gaseous formic acid and ammonia can be taken away to promote the continued progress of the reaction.

[0037] S3, post-treating the magnesium salt suspension to obtain anhydrous magnesium salt solid.

[0038] For example, the post-treatment in step S3 is preferably: filtering and washing the magnesium salt suspension, removing the solvent, and drying (if there is ammonium formate and / or excess ammonium salt synthesized by cooling formic acid and ammonia, the drying can be carried out at a high temperature so that the ammonium formate and / or ammonium salt decomposes) to obtain an anhydrous magnesium salt solid, or directly heating the magnesium salt suspension to above the boiling point of the solvent, evaporating the high boiling point solvent to obtain an anhydrous magnesium salt solid, and naturally cooling under air-tight conditions to obtain an anhydrous magnesium salt.

[0039] In a preferred embodiment, the solid phase reaction of anhydrous magnesium formate and an ammonium salt composed of anions of anhydrous magnesium salt to be generated and ammonium ions at a predetermined temperature comprises:

[0040] A1. Directly subjecting the ultrafine powders of the anhydrous magnesium formate and the ammonium salt to a solid-phase reaction at 120-180° C. to generate anhydrous magnesium salt, formic acid, and ammonia gas; preferably, the average particle size of the anhydrous magnesium formate and the ammonium salt powders is less than 10 μm, and the smaller the particle size, the better.

[0041] A2. During step A1, a high-temperature protective gas (preferably 120° C. to 180° C.) is used to remove the gaseous formic acid and ammonia generated in step A1, and ammonium formate synthesized by cooling the formic acid and ammonia is removed to obtain an anhydrous magnesium salt solid. By using a high-temperature protective gas purge suitable for the reaction temperature of step A1, the generated gaseous formic acid and ammonia can be removed to promote the continued progress of the reaction.

[0042] In a preferred embodiment, the sealed container for the solid-phase reaction can be fixed or rotated or reciprocated. Preferably, the solid-phase reaction is carried out in a container that rotates or reciprocates to promote mixing between solid-phase ultrafine powders and surface renewal between powder particles. In a preferred embodiment, the solvent is at least one of a high-boiling point organic solvent and an ionic liquid; wherein the boiling point of the high-boiling point organic solvent is above 150°C, the melting point of the ionic liquid is not higher than 90°C, and the decomposition temperature is above 200°C; preferably, the high-boiling point organic solvent is at least one of DMF, formamide, and phenoxyethanol; preferably, the ionic liquid is at least one of tributylmethylammonium bistrifluoromethanesulfonimide (TBMA-TFSI, CAS: 405514-94-5) and octyl-3-methylimidazolium tetrafluoroborate (OMIM BF4, CAS: 244193-52-0).

[0043] In a preferred embodiment, before carrying out the solid-phase reaction or the liquid-phase reaction, the following pre-reaction is further included: anhydrous magnesium formate and an ammonium salt composed of anions of the anhydrous magnesium salt to be generated and ammonium ions are added to anhydrous formic acid according to the theoretical molar ratio of the double decomposition reaction; the reaction temperature is raised to the boiling point of anhydrous formic acid under protective gas protection and maintained under condensation and reflux, and then fully stirred for 1-4 hours. After condensation and reflux are stopped, formic acid is removed to obtain a solid mixture comprising magnesium formate, ammonium chloride, magnesium chloride and ammonium formate; the solid mixture is subjected to the solid-phase reaction or the liquid-phase reaction to generate anhydrous magnesium salt, formic acid and ammonia; the volatilized formic acid and ammonia and the ammonium formate synthesized by cooling the formic acid and the ammonia are removed to obtain anhydrous magnesium salt.

[0044] In a preferred embodiment, the ammonium formate synthesized by cooling the formic acid and ammonia can be removed by heating and roasting (for example, roasting at a temperature above 180° C.).

[0045] In a preferred embodiment, the process further comprises the following steps:

[0046] (1) magnesium chloride, magnesium sulfate, or magnesium nitrate containing crystal water is subjected to double decomposition reaction with ammonium formate in a solvent to generate a mixed salt precipitate of magnesium formate dihydrate and ammonium chloride, ammonium sulfate, or ammonium nitrate;

[0047] (2) Separating the mixed salt by recrystallization separation technology to obtain magnesium formate dihydrate and solid crystalline powder of ammonium chloride or ammonium sulfate or ammonium nitrate, and drying and dehydrating at above 105° C. to obtain anhydrous magnesium formate.

[0048] In a preferred embodiment, the process further comprises: recovering the generated formic acid and ammonia to form ammonium formate, and then using the ammonium formate in the step of preparing anhydrous magnesium formate. The reaction formula is as follows:

[0049] NH3↑+HCOOH↑=NH4HCO2(After volatilization, it reacts with cooling to synthesize)

[0050] In a preferred embodiment, the anhydrous magnesium salt is one of anhydrous magnesium chloride, anhydrous magnesium sulfate and anhydrous magnesium nitrate, and correspondingly, the ammonium salt is one of ammonium chloride, ammonium sulfate and ammonium nitrate.

[0051] When the anhydrous magnesium salt is anhydrous magnesium chloride, the reaction formula is as follows:

[0052] Mg(HCO2)2+2NH4Cl=MgCl2↓+2NH3↑+2HCOOH↑

[0053] When the anhydrous magnesium salt is anhydrous magnesium sulfate, the reaction formula is as follows:

[0054] Mg(HCO2)2+(NH4)2SO4=MgSO4↓+2NH3↑+2HCOOH↑

[0055] When the anhydrous magnesium salt is anhydrous magnesium nitrate, the reaction formula is as follows:

[0056] Mg(HCO2)2+2NH4NO3=Mg(NO3)2↓+2NH3↑+2HCOOH↑

[0057] The present invention is described in detail below through more specific examples.

[0058] Example 1

[0059] Taking bischofite (MgCl2·6H2O) as the starting material to prepare anhydrous magnesium chloride as an example, the technical solution of Example 1 mainly includes the following five steps:

[0060] 1. Dissolve 320.3g of magnesium chloride hexahydrate in 1000ml of anhydrous methanol solvent and filter to remove sodium salts, potassium salts and other impurities that are insoluble in organic solvents and other insoluble impurities;

[0061] 2. In the system of step 1, about 198.7 g of ammonium formate was added according to a molar ratio of magnesium chloride hexahydrate to ammonium formate of 1:2, and the mixture was fully stirred. At room temperature, magnesium chloride hexahydrate and ammonium formate underwent a double decomposition reaction to generate magnesium formate dihydrate and ammonium chloride (mixed salt precipitation). Part of the crystal water was converted into free water. A dry gas such as nitrogen was used for gas stripping and dehydration to remove the free water in the solvent system. The gas stripping temperature was set at 60°C.

[0062] The reaction formula is as follows:

[0063] MgCl2·6H2O+2NH4HCO2=Mg(HCO2)2·2H2O↓+2NH4Cl↓+4H2O↑

[0064] 3. After collecting the mixed salt precipitate by filtration, the magnesium formate dihydrate crystals and the ammonium chloride crystals are separated by recrystallization technology, and the two are dissolved in water to prepare a solution. After evaporation and concentration to near saturated solubility, the solution is dried by air flow spray at a drying temperature exceeding 105° C. to obtain anhydrous magnesium formate ultrafine powder and ammonium chloride ultrafine powder, respectively;

[0065] 4. Mix anhydrous magnesium formate (180.14 g) and ammonium chloride (168.55 g) in a molar ratio of 1:2 and grind them into micron-sized fine powder.

[0066] 5. The mixed powder obtained in step 4 is added to DMF and stirred to form a suspension. The mixture is then heated to 120°C-150°C in a kettle or tubular reactor to cause a double decomposition reaction between the two to produce anhydrous magnesium chloride, formic acid, and ammonia. Under a nitrogen purge, the volatilized formic acid and ammonia are removed under negative pressure, and the ammonium formate synthesized by cooling the formic acid and ammonia is removed to obtain a suspension of magnesium chloride in DMF. The suspension is filtered, washed, and then the solvent is removed and dried to obtain approximately 150g of anhydrous magnesium chloride product. The formic acid and ammonia removed by the nitrogen are cooled, washed with anhydrous methanol to produce ammonium formate, and then returned to step 2 for recycling.

[0067] like Figure 1 As shown, for example, step 5 is specifically as follows: the system (anhydrous magnesium formate + ammonium chloride + DMF) in the metathesis reactor 1 is heated to 120-150° C. and then subjected to a metathesis reaction to obtain a suspension containing anhydrous magnesium chloride (which may inevitably contain ammonia and formic acid to synthesize ammonium formate when cooled), and the suspension is filtered, washed, dried, and calcined after passing through a post-processor 5 to obtain an anhydrous magnesium chloride solid product. At the same time, nitrogen is introduced into the metathesis reactor 1 to carry the ammonia and formic acid generated by the reaction to the absorption tower 2, where they are absorbed by the sprayed solvent (such as anhydrous ethanol) and reacted with cooling to form an ammonium formate solution; after the ammonium formate solution is desolvated and crystallized in the evaporative crystallizer 3, it enters the post-processor 6 to complete filtration, washing, and drying to obtain an ammonium formate powder product, and the solvent volatilized by heat in the evaporative crystallizer 3 passes through the condenser 4, condenses into a liquid, and is cooled and circulated as an absorption solvent for the spray liquid at the top of the absorption tower 2.

[0068] The anhydrous magnesium chloride prepared by the above method has extremely low water content and high purity, and can meet the quality requirements of electrolytic magnesium and the like for anhydrous magnesium chloride.

[0069] Table 1: Test results of anhydrous magnesium chloride obtained in this embodiment 1

[0070]

[0071]

[0072] Note: “——” in the above table indicates that there are no relevant standards.

[0073] Example 2

[0074] The first four steps of this embodiment 2 are the same as steps 1 to 4 of embodiment 1;

[0075] 5. After uniformly mixing the obtained 180.14 g of anhydrous magnesium formate and 168.55 g of ammonium chloride solid powder, milling for about 30 minutes, the resulting mixture was added to 1000 ml of anhydrous formic acid, maintaining stirring at a medium speed, gradually heating the material temperature to the boiling of the solvent, and maintaining condensation and reflux for one hour, stopping the condensation and reflux, and evaporating the solvent and then condensing and collecting for standby use, and vacuuming the remaining solid to remove all residual solvent to obtain a solid mixture (which comprises magnesium chloride and ammonium formate generated by magnesium formate and ammonium chloride in anhydrous formic acid, and unreacted magnesium formate and ammonium chloride).

[0076] 6. The solid mixture obtained in step 5 (comprising magnesium formate, ammonium chloride, magnesium chloride and ammonium formate) was added to a sealed container containing 1000 ml of ionic liquid (tributylmethylammonium bistrifluoromethanesulfonimide). The ammonium formate was dissolved in the ionic liquid, while the magnesium chloride was insoluble. While maintaining stirring, the temperature of the reaction mass was gradually raised to between 120-150° C., and nitrogen was slowly introduced to promote the following reaction between magnesium formate and ammonium chloride:

[0077] Mg(HCO2)2+2NH4Cl=MgCl2↓+2NH4HCO2

[0078] NH4HCO2=NH3↑+HCOOH↑

[0079] The ammonia and formic acid volatilized by the above reaction will be carried away by the circulating nitrogen. At this time, the nitrogen outlet is switched to a cooling absorption tower, and another low-temperature DMF is used to spray and absorb the nitrogen and the ammonia and formic acid it carries. The following reaction occurs to obtain an ammonium formate-DMF solution:

[0080] NH3+HCOOH=NH4HCO2

[0081] 7. The DMF absorption liquid containing ammonium formate can react with magnesium chloride hexahydrate again after natural cooling in the absorption liquid storage tank to achieve recycling;

[0082] 8. The suspension containing magnesium chloride solids obtained in step 6 is collected by precision filtration and then placed in a circulating mother liquor storage tank. The filter cake is dissolved in anhydrous methanol and then concentrated and crystallized by heating to obtain anhydrous magnesium chloride product. The filtrate can be returned to step 1 to dissolve bischofite. The results of sampling and testing are shown in the following table:

[0083] Table 2: Test results of anhydrous magnesium chloride obtained in this embodiment 2

[0084]

[0085]

[0086] Note: “——” in the above table indicates that there are no relevant standards.

[0087] The anhydrous magnesium chloride prepared by the above method has extremely low water content and high purity, and can meet the quality requirements of electrolytic magnesium and the like for anhydrous magnesium chloride.

[0088] Example 3

[0089] The difference from Example 2 is that step 6 is replaced by step 6.3, that is, no solvent is used, a solid phase reaction is directly adopted, ammonia and formic acid generated by the solid reaction are taken away by high temperature nitrogen at 120°C-180°C at a flow rate of 10ml / s, and the formic acid and ammonia in the gas phase are drawn out, and after cooling, they are sprayed and absorbed with low temperature anhydrous ethanol in an absorption tower to obtain an ammonium formate solution. After about 4 hours, it is detected that the nitrogen outlet in the solid phase reactor no longer contains ammonia, then the reaction is terminated, and the product remaining in the solid phase reactor is anhydrous magnesium chloride. After sampling and testing, the results are shown in the following table:

[0090] Table 3: Test results of anhydrous magnesium chloride obtained in this embodiment 3

[0091]

[0092]

[0093] Note: “——” in the above table indicates that there are no relevant standards.

[0094] Example 4

[0095] Taking magnesium sulfate heptahydrate (MgSO4·7H2O) as the starting material to prepare anhydrous magnesium sulfate as an example, the technical solution of Example 4 mainly includes the following five steps:

[0096] 1. Dissolve 328g of magnesium sulfate heptahydrate in 1000ml of anhydrous methanol solvent and filter to remove sodium salts, potassium salts and other insoluble impurities that are insoluble in organic solvents;

[0097] 2. In the system of step 1, about 168 g of ammonium formate was added according to a molar ratio of magnesium sulfate heptahydrate to ammonium formate of 1:2, and the mixture was fully stirred. At room temperature, magnesium sulfate heptahydrate and ammonium formate underwent a double decomposition reaction to generate magnesium formate dihydrate and ammonium sulfate (mixed salt precipitation). Part of the crystal water was converted into free water. A dry gas such as nitrogen was used for gas stripping and dehydration to remove the free water in the solvent system. The gas stripping temperature was set at 60°C.

[0098] The reaction formula is as follows:

[0099] MgSO4·7H2O+2NH4HCO2=Mg(HCO2)2·2H2O↓+(NH4)2SO4↓+5H2O↑

[0100] 3. After collecting the mixed salt precipitate by filtration, the magnesium formate dihydrate crystals and ammonium sulfate crystals are separated by recrystallization technology, and the two are dissolved in water to prepare a solution. After evaporation and concentration to near saturated solubility, air flow spray drying is adopted at a drying temperature exceeding 105° C. to obtain anhydrous magnesium formate ultrafine powder and ammonium sulfate ultrafine powder, respectively;

[0101] 4. Mix anhydrous magnesium formate (152 g) and ammonium sulfate (175.6 g) in a 1:1 molar ratio and grind them into micron-sized fine powder.

[0102] 5. The mixed powder obtained in step 4 is added to DMF, stirred and mixed to form a suspension, and then heated to 120°C-150°C in a kettle or tubular reactor to cause a double decomposition reaction between the two to generate anhydrous magnesium sulfate, formic acid and ammonia. Under nitrogen purge, the volatilized formic acid and ammonia are removed under negative pressure to obtain a suspension of magnesium sulfate suspended in DMF. After filtering and washing the suspension, the solvent is removed and dried to obtain about 160g of anhydrous magnesium sulfate product. The formic acid and ammonia carried away by the nitrogen are cooled, washed with anhydrous methanol and returned to step 2 for recycling. After sampling and testing, the results are shown in the table below:

[0103] Table 4: Test results of anhydrous magnesium sulfate obtained in this embodiment 4

[0104]

[0105]

[0106] Note: “——” in the above table indicates that there are no relevant standards.

[0107] Example 5

[0108] Taking magnesium nitrate hexahydrate Mg(NO3)2·6H2O as the starting material to prepare anhydrous magnesium nitrate as an example, the technical solution of Example 5 mainly includes the following five steps:

[0109] 1. Dissolve 311.2g of magnesium nitrate hexahydrate in 1000ml of anhydrous methanol solvent and filter to remove sodium salts, potassium salts and other impurities that are insoluble in organic solvents and other insoluble impurities;

[0110] 2. In the system of step 1, about 153.1 g of ammonium formate was added according to a molar ratio of magnesium nitrate hexahydrate to ammonium formate of 1:2, and the mixture was fully stirred. At room temperature, magnesium nitrate hexahydrate and ammonium formate underwent a double decomposition reaction to generate magnesium formate dihydrate and ammonium nitrate (mixed salt precipitation). Part of the crystal water was converted into free water. Gas stripping and dehydration were performed with a dry gas such as nitrogen to remove the free water in the solvent system. The gas stripping temperature was set at 60°C.

[0111] The reaction formula is as follows:

[0112] Mg(NO3)2·6H2O+2NH4HCO2=Mg(HCO2)2·2H2O↓+2NH4NO3↓+4H2O↑

[0113] 3. After collecting the mixed salt precipitate by filtration, the magnesium formate dihydrate crystals and the ammonium nitrate crystals are separated by recrystallization technology. After separation, the two are dissolved in water to prepare a solution, and then evaporated and concentrated to near saturated solubility, and then air flow spray drying is adopted at a drying temperature exceeding 105° C. to obtain anhydrous magnesium formate ultrafine powder and ammonium nitrate ultrafine powder respectively;

[0114] 4. Mix anhydrous magnesium formate (138.77 g) and ammonium nitrate (194.29 g) in a molar ratio of 1:2 and grind them into micron-sized powder.

[0115] 5. The mixed powder obtained in step 4 is added to DMF, stirred and mixed to form a suspension, and then heated to 120°C-150°C in a kettle or tubular reactor to cause a double decomposition reaction between the two to generate anhydrous magnesium nitrate, formic acid and ammonia. Under nitrogen purge, the volatilized formic acid and ammonia are removed under negative pressure to obtain a suspension of magnesium nitrate suspended in DMF. After filtering and washing the suspension, the solvent is removed to obtain about 180g of anhydrous magnesium nitrate product. The formic acid and ammonia carried away by the nitrogen are cooled, washed and absorbed by anhydrous methanol, and then returned to step 2 for recycling. After sampling and testing, the results are shown in the table below:

[0116] Table 5: Test results of anhydrous magnesium nitrate obtained in this embodiment 5

[0117]

[0118]

[0119] Note: “——” in the above table indicates that there are no relevant standards.

[0120] The specific embodiment of the present invention discloses for the first time a new method for preparing anhydrous magnesium salts. Specifically, anhydrous magnesium salts such as anhydrous magnesium chloride (anhydrous magnesium sulfate, anhydrous magnesium nitrate) can be obtained from carnallite, bischofite or other magnesium salts containing crystal water as initial raw materials through an anhydrous magnesium formate route and a two-step metathesis reaction, and ammonium formate is produced as a by-product, which can be recycled. The obtained anhydrous magnesium salt has the advantages of stable and reliable quality, low production cost, and easy expansion to achieve large-scale production.

[0121] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. Those skilled in the art will recognize that several equivalent substitutions or obvious variations can be made without departing from the scope of the present invention, and that any equivalent performance or application should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for preparing anhydrous magnesium salt, characterized in that, The steps include: Anhydrous magnesium formate and an ammonium salt composed of anions and ammonium ions of the anhydrous magnesium salt to be generated are subjected to a solid-phase reaction or a liquid-phase reaction at a predetermined temperature to generate anhydrous magnesium salt, formic acid and ammonia gas, and the volatilized formic acid and ammonia gas and the ammonium formate synthesized by cooling the formic acid and ammonia gas are removed to obtain anhydrous magnesium salt; wherein: The liquid phase reaction comprises: S1. Stirring the ultrafine powders of the anhydrous magnesium formate and the ammonium salt into a suspension in a solvent, and performing a double decomposition reaction at 120°C-150°C to generate anhydrous magnesium salt, formic acid and ammonia; the solvent is at least one of a high-boiling-point organic solvent and an ionic liquid; wherein the boiling point of the high-boiling-point organic solvent is above 150°C, the melting point of the ionic liquid is not higher than 90°C, and the decomposition temperature is above 200°C; S2. During step S1, under the purge of high-temperature protective gas, gaseous formic acid and ammonia are removed under negative pressure to obtain a magnesium salt suspension suspended in the solvent; S3, post-treating the magnesium salt suspension to obtain anhydrous magnesium salt solid; The solid phase reaction comprises: A1, the ultrafine powder of the anhydrous magnesium formate and the ammonium salt is directly subjected to a solid phase reaction at 120-180 ° C to generate anhydrous magnesium salt, formic acid and ammonia; A2. During step A1, using a high-temperature protective gas to remove the gaseous formic acid and ammonia generated in step A1, and removing the ammonium formate synthesized by cooling the formic acid and the ammonia, to obtain an anhydrous magnesium salt solid; Before carrying out the solid phase reaction or the liquid phase reaction, the following pre-reaction is further included: adding anhydrous magnesium formate and an ammonium salt composed of an anion of the anhydrous magnesium salt to be generated and an ammonium ion into anhydrous formic acid according to the theoretical molar ratio of the double decomposition reaction, raising the reaction temperature to the boiling point of anhydrous formic acid under protective gas protection, maintaining condensation and reflux, and fully stirring for 1-4 hours, then stopping condensation and reflux, and removing formic acid to obtain a solid mixture comprising magnesium formate, the ammonium salt, the magnesium salt, and ammonium formate; The solid mixture is subjected to the solid-phase reaction or the liquid-phase reaction to generate anhydrous magnesium salt, formic acid and ammonia gas, and the volatilized formic acid and ammonia gas and the ammonium formate synthesized by cooling the formic acid and the ammonia gas are removed to obtain anhydrous magnesium salt.

2. The preparation method of anhydrous magnesium salt according to claim 1, wherein The anhydrous magnesium formate and the ammonium salt are mixed according to a theoretical molar ratio or in a manner such that the ammonium salt has an excess molar amount.

3. The preparation method of anhydrous magnesium salt according to claim 1, wherein The high boiling point organic solvent is at least one of DMF, formamide, and phenoxyethanol; and the ionic liquid is at least one of tributylmethylammonium bistrifluoromethanesulfonimide and octyl-3-methylimidazolium tetrafluoroborate.

4. The preparation method of anhydrous magnesium salt according to claim 1, wherein Also includes the step of preparing anhydrous magnesium formate: (1) magnesium chloride, magnesium sulfate or magnesium nitrate containing crystal water is subjected to double decomposition reaction with ammonium formate in a solvent to generate a mixed salt precipitate of magnesium formate dihydrate and ammonium chloride, ammonium sulfate or ammonium nitrate; (2) Separating the mixed salt by recrystallization separation technology to obtain magnesium formate dihydrate and solid crystalline powder of ammonium chloride or ammonium sulfate or ammonium nitrate, and drying and dehydrating at above 105°C to obtain anhydrous magnesium formate.

5. The method for preparing anhydrous magnesium salt according to claim 4, wherein Also includes: The generated formic acid and ammonia are recovered to form ammonium formate, which is then used in the step of preparing anhydrous magnesium formate.

6. The method for preparing anhydrous magnesium salt according to claim 1, wherein The anhydrous magnesium salt is one of anhydrous magnesium chloride, anhydrous magnesium sulfate and anhydrous magnesium nitrate. Correspondingly, the ammonium salt is one of ammonium chloride, ammonium sulfate and ammonium nitrate.

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