A kind of preparation method of anhydrous magnesium chloride

Abstract

The present invention discloses a method for preparing anhydrous magnesium chloride, comprising the following steps: (1) subjecting anhydrous magnesium formate powder to a gas-solid reaction with a mixed gas of anhydrous hydrogen chloride and chlorine or a liquid-solid reaction in an anhydrous organic solvent to obtain a crude anhydrous magnesium chloride product; (2) subjecting the crude anhydrous magnesium chloride product to post-treatment to obtain anhydrous magnesium chloride. The present invention prepares anhydrous magnesium chloride by reacting a mixed gas of anhydrous hydrogen chloride and chlorine with anhydrous magnesium formate powder. Under the condition that anhydrous hydrogen chloride and chlorine are present at the same time, the reaction activity of anhydrous magnesium formate can be significantly improved, thereby improving the conversion rate of magnesium formate. The anhydrous magnesium chloride product produced by the present invention has a lower water content and can better meet the stringent requirements of the electrolytic magnesium industry on the quality of anhydrous magnesium chloride products.

Description

Technical Field

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

[0002] China has extremely rich resources of magnesium chloride hydrate (mainly magnesium chloride hydrate represented by carnallite and bischofite). The magnesium chloride resources in the brine of Lop Nur Salt Lake and the Chaka Salt Lake in Qinghai alone exceed 5 billion tons.

[0003] Anhydrous magnesium chloride has a wide range of uses. In addition to being used as a pharmaceutical intermediate, catalyst carrier, desiccant, etc., it is mainly used as the basic raw material for electrolytic magnesium. At present, almost all of my country uses the Pidgeon process to prepare magnesium metal. The main raw material of this method is high-grade dolomite. There are disadvantages such as high energy consumption and environmental pollution in the process of preparing magnesium metal. Our country has clearly pointed out that the development of the Pidgeon process to produce magnesium metal is not encouraged, but the electrolytic process should be developed. Compared with the Pidgeon process, the electrolytic process has the advantages of low energy consumption (the latter is about 2.86 times that of the former), continuous production, high degree of mechanization and automation, and high production indicators. At present, more than 80% of magnesium metal abroad is produced by electrolysis. Therefore, choosing cheap bischofite (MgCl2·6H2O) or carnallite (KCl·MgCl2·6H2O) in salt lake brine as raw materials, preparing anhydrous magnesium chloride by dehydration, and then using electrolysis to produce magnesium metal will be the fundamental way out for the development of magnesium metallurgy in my country.

[0004] One of the key issues in the electrolytic preparation of magnesium metal is to provide high-quality, low-cost anhydrous magnesium chloride raw materials. Vigorously developing the electrolytic magnesium industry and preparing high-purity anhydrous magnesium chloride at low cost is a key step in the electrolytic preparation of magnesium. In the existing technical routes for the preparation of anhydrous magnesium chloride, there are gas protection heating dehydration method, organic solvent distillation and molecular sieve dehydration method, magnesium chloride double salt and complex decomposition dehydration method, high-temperature chlorination and other processes. Among them, the gas protection heating dehydration method requires a large amount of hydrogen chloride gas, which consumes a lot of energy and causes serious equipment corrosion at high temperatures; the organic solvent distillation and molecular sieve dehydration method has the disadvantage of decomposing organic reagents and magnesium chloride, and the anhydrous magnesium chloride product contains a large amount of carbon and magnesium oxide, which cannot be used as an electrolytic raw material; the magnesium chloride double salt and complex decomposition dehydration method mainly synthesizes hexammoniated magnesium chloride, and then decomposes to prepare anhydrous magnesium chloride. If this method uses hydrated magnesium chloride as a raw material, the chlorine produced by electrolytic magnesium production cannot be recycled. If magnesium hydroxide or oxide is used as a raw material, both highly corrosive hydrochloric acid and ammonia are used in the preparation process, which increases the harm to the environment.

[0005] Chinese invention patent application CN114014346A first disclosed a method for preparing anhydrous magnesium chloride from anhydrous magnesium formate, comprising the following steps: reacting anhydrous magnesium formate powder with anhydrous hydrogen chloride gas in an anhydrous organic solvent to generate anhydrous magnesium chloride and formic acid, removing the anhydrous organic solvent and formic acid to obtain anhydrous magnesium chloride. The method provides a method for synthesizing anhydrous magnesium chloride based on an anhydrous organic solvent, with mild reaction conditions. Compared with the traditional dehydration method of hexahydrate magnesium chloride, it has the characteristics of low investment and low energy consumption. However, the method still has shortcomings, mainly including: 1. Due to the use of organic solvents, the steps of solvent self-dehydration and product and solvent separation are added, and a series of problems such as the fire safety level of the building need to be considered for industrial equipment and equipment workshops; 2. The anhydrous magnesium chloride for electrolysis has extremely strict requirements on the water content (required to be less than 0.1%). Since the anhydrous magnesium chloride obtained by the above method is fine powder, has a large specific surface area and is easy to absorb moisture, it is difficult to ensure that the moisture content of the product meets the standard; 3. The main use of anhydrous magnesium chloride is electrolytic magnesium. The large amount of chlorine produced by electrolytic magnesium is difficult to be completely consumed by downstream industries, resulting in a backlog. The problem of "chlorine imbalance" (i.e., a large amount of excess chlorine produced by electrolytic magnesium production) has become a problem restricting the development of the electrolytic magnesium industry. Existing methods are not helpful in solving the "chlorine imbalance" dilemma of the electrolytic magnesium industry.

[0006] It should be noted that the information disclosed in the above background technology section is only used for understanding the background of the present invention, and therefore may include information that does not constitute the prior art known to ordinary technicians in the field. Summary of the invention

[0007] In order to make up for the deficiencies of the above-mentioned prior art, the present invention provides a method for preparing anhydrous magnesium chloride.

[0008] The present invention adopts the following technical solution:

[0009] A method for preparing anhydrous magnesium chloride comprises the following steps:

[0010] (1) subjecting anhydrous magnesium formate powder to a gas-solid reaction with a mixed gas of anhydrous hydrogen chloride and chlorine or to a liquid-solid reaction in an anhydrous organic solvent to obtain a crude anhydrous magnesium chloride product;

[0011] (2) subjecting the crude anhydrous magnesium chloride product to post-treatment to obtain anhydrous magnesium chloride.

[0012] The present invention has the following advantages or beneficial effects:

[0013] The present invention prepares anhydrous magnesium chloride by reacting a mixed gas of anhydrous hydrogen chloride and chlorine with anhydrous magnesium formate powder. Under the condition that anhydrous hydrogen chloride and chlorine exist simultaneously, the reaction activity of anhydrous magnesium formate can be significantly improved, thereby improving the conversion rate of magnesium formate, and the conversion rate of magnesium formate exceeds 99%. The anhydrous magnesium chloride product produced by the present invention has a lower water content and can better meet the stringent requirements of the electrolytic magnesium industry on the quality of anhydrous magnesium chloride products.

[0014] In a further technical solution, anhydrous magnesium chloride can be prepared by a gas-solid reaction in a continuous gas-solid reaction device or an intermittent gas-solid reaction container, or by a liquid-solid reaction in an anhydrous organic solvent.

[0015] In a further technical solution, the tail gas generated in step (1) is condensed to room temperature, the by-product formic acid is recovered, and the anhydrous hydrogen chloride and carbon dioxide that are not condensed at room temperature are compressed to liquefy, and then the anhydrous hydrogen chloride and carbon dioxide are liquefied and separated by distillation, and recovered separately, and the recovered anhydrous hydrogen chloride can be recycled in step (1). Since the anhydrous hydrogen chloride can be recycled, there is no need to purchase anhydrous hydrogen chloride from outside, thereby reducing the cost of raw materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The present invention is a flow chart of preparing anhydrous magnesium chloride according to a specific embodiment of the present invention.

[0017] Figure 2 This is a schematic diagram of the gas-solid reaction equipment for preparing anhydrous magnesium chloride in Example 1 of the present invention. DETAILED DESCRIPTION

[0018] The following is a detailed description of the embodiments of the present invention. It should be emphasized that the following description is only exemplary and is not intended to limit the scope and application of the present invention. In the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0019] The term "anhydrous" as used herein, unless otherwise specified, generally refers to a water content of less than 0.5 wt %; "%" refers to mass percentage "wt %" unless otherwise specified; the term "room temperature" or "normal temperature" as used herein refers to a temperature within the range of 15-40°C.

[0020] like Figure 1 As shown, a specific embodiment of the present invention provides a method for preparing anhydrous magnesium chloride, comprising the following steps: (1) subjecting anhydrous magnesium formate powder to a gas-solid reaction with a mixed gas of anhydrous hydrogen chloride and chlorine or reacting in an anhydrous organic solvent to obtain a crude anhydrous magnesium chloride product; and (2) subjecting the crude anhydrous magnesium chloride product to post-treatment to obtain anhydrous magnesium chloride.

[0021] In step (1), the main chemical reaction equation is as follows:

[0022] Mg(HCO2)2+2Cl2=MgCl2+2HCl+2CO2

[0023] Mg(HCO2)2+2HCl=MgCl2+2HCOOH

[0024] HCOOH+Cl2=CO2+2HCl

[0025] Preferably, the chlorine in the mixed gas of anhydrous hydrogen chloride and chlorine can be derived from the chlorine produced by electrolytic magnesium, so as to solve the "chlorine imbalance" dilemma caused by a large amount of excess chlorine produced by electrolytic magnesium production.

[0026] In some preferred embodiments, in step (1), the gas-solid reaction of anhydrous magnesium formate powder with a mixed gas of anhydrous hydrogen chloride and chlorine is carried out in a continuous gas-solid reaction device, such as Figure 2 As shown, the continuous gas-solid reaction equipment includes a mixer 1 and a multi-stage gas-solid reaction unit, each of which includes a column reactor 2, a cyclone separator 3, a bag dust collector 4, a feeder 5 and a hopper 6, the feeder 5 is connected to the upper part of the column reactor 2, and is used to load the column reactor 2 with magnesium formate water powder, the cyclone separator 3 is connected to the discharge port of the column reactor 2, the gas outlet of the cyclone separator 3 is connected to the bag dust collector 4, and the cyclone The discharge port of the separator 3 and the discharge port of the bag dust collector 4 are respectively connected to the hopper 6. Through the coordinated cooperation of the cyclone separator 3 and the bag dust collector 4, the crude anhydrous magnesium chloride product and the gas can be better separated to collect the crude anhydrous magnesium chloride product from the hopper 6; the gas outlet of the bag dust collector in the previous stage of the gas-solid reaction unit is connected to the lower part of the column reactor of the next stage of the gas-solid reaction unit, and the mixer 1 is connected to the lower part of the column reactor 2 of each stage of the gas-solid reaction unit; the gas-solid reaction comprises the following steps:

[0027] S1, filling anhydrous magnesium formate powder into a column reactor 2 to form an anhydrous magnesium formate filler layer;

[0028] S2, preheating the mixed gas of anhydrous hydrogen chloride and chlorine to 50-100° C., passing through the mixer 1 from the bottom of each column reactor 2, passing through the anhydrous magnesium formate filler layer from bottom to top, controlling the temperature of the discharge port of the column reactor 2 at a predetermined temperature, and performing a gas-solid reaction to obtain anhydrous magnesium chloride crude product; preferably, the mixed gas is passed into the column reactor 2 at a flow rate of 1.0 to 3.0 cm / s;

[0029] S3, after the magnesium formate is completely converted, the anhydrous magnesium chloride crude product is collected from the hopper 6, and anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine are added to the column reactor 2 to carry out a continuous reaction.

[0030] Preferably, between steps S2 and S3, the step of judging whether the anhydrous magnesium formate in the column reactor has been reacted is also included: the mass fraction of chlorine and carbon dioxide in the mixed gas in the outlet gas flow of each column reactor is continuously monitored, and when the change of the mass fraction of the two in the mixed gas tends to be constant within a predetermined time (for example, more than 30 minutes), it can be considered that the anhydrous magnesium formate in the column reactor has been completely converted. Herein, "complete conversion of magnesium formate" means that the conversion rate of magnesium formate is more than 99%.

[0031] Preferably, before and after the anhydrous magnesium formate powder is loaded, the column reactor 2 (such as Figure 2 As shown, nitrogen is introduced from the bottom of the column reactor 2 to purge it) to remove the residual air in the column reactor 2 to ensure the drying of the anhydrous magnesium formate powder.

[0032] In some preferred embodiments, the preparation method further comprises the following steps: (3) condensing the tail gas produced in step (1) to room temperature, recovering the by-product formic acid, and compressing the anhydrous hydrogen chloride and carbon dioxide that are not condensed at room temperature (for example, by pressurizing them to 3 MPa or more with a compressor) to liquefy them, and then liquefying and separating the anhydrous hydrogen chloride and carbon dioxide by distillation, and recovering them separately. Part of the recovered anhydrous hydrogen chloride can be returned to the front end (i.e., in step (1)), mixed with chlorine, and recycled as a mixed gas, thereby reducing the production cost of anhydrous magnesium chloride.

[0033] In a preferred embodiment, from the perspective of ease of industrial production, further Figure 2As shown, the gas-solid reaction equipment also includes an exhaust gas recovery device to recover the exhaust gas, and the exhaust gas recovery device includes an induced draft fan 7, a first condenser 8, a formic acid storage tank 9, a compressor 10, a reboiler 11, a distillation tower 12, a second condenser 13, a condensate storage tank 14, a carbon dioxide liquefied gas storage tank 15 and a hydrogen chloride liquefied gas storage tank 16. Specifically, the first condenser 8 is connected to the bag filter 4 of the last-stage gas-solid reaction unit through the induced draft fan 7, the formic acid storage tank 9 and the compressor 10 are connected to the first condenser 8 respectively, the compressor 10 is connected to the distillation tower 12, the top of the distillation tower 12 is connected to the carbon dioxide liquefied gas storage tank 15 through the second condenser 13 and the condensate storage tank 14 in turn, the condensate storage tank 14 is also connected to the upper part of the distillation tower 12, the bottom of the distillation tower 12 is connected to the hydrogen chloride liquefied gas storage tank 16 and the reboiler 11 respectively, and the reboiler 11 is connected to the lower part of the distillation tower 12. When anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine are subjected to a gas-solid reaction in a continuous gas-solid reaction device, the specific process of recovering the tail gas generated in step (1) is as follows: Figure 2 As shown, the tail gas at the outlet of the bag filter 4 of the last-stage gas-solid reaction unit is introduced into the first condenser 8 through the induced draft fan 7 and condensed to room temperature, and the by-product formic acid enters the formic acid storage tank 9. The anhydrous hydrogen chloride and carbon dioxide that are not condensable at room temperature are compressed to liquefaction by the compressor 10 and enter the distillation tower 12 from the middle part of the distillation tower 12. After the anhydrous hydrogen chloride and carbon dioxide are liquefied in the distillation tower 12, the carbon dioxide is condensed from the top of the distillation tower 12 through the second condenser 13 and enters the condensate storage tank 14. The carbon dioxide liquefied gas in the condensate storage tank 14 is partially refluxed into the distillation tower 12, and partially discharged into the carbon dioxide liquefied gas storage tank 15. Hydrogen chloride is discharged from the bottom of the distillation tower 12, and part of the hydrogen chloride liquefied gas enters the hydrogen chloride liquefied gas storage tank 16 (the recovered anhydrous hydrogen chloride can be recycled in step (1)), and part of the hydrogen chloride liquefied gas returns to the distillation tower 12 through the reboiler 11.

[0034] By controlling the flow rate of the mixed gas at the lower gas inlet of the column reactor 2 in each stage of the gas-solid reaction unit, the ratio of hydrogen chloride to chlorine and the preheating temperature of the mixer, the gas flow temperature at the upper outlet of each stage of the column reactor 2 (for example, the outlet at the top or upper side of the column reactor 2) is controlled not to exceed 200°C, and preferably controlled within the range of 110-160°C.

[0035] Preferably, a microporous ceramic support plate 21 is provided at the bottom of the column reactor 2 to ensure that the anhydrous magnesium formate powder does not leak through, and the mixed gas can pass through the microporous ceramic support plate 21 and the anhydrous magnesium formate filler layer from bottom to top and then react with the anhydrous magnesium formate powder.

[0036] Preferably, in step S1, the height of the anhydrous magnesium formate filler layer is 300 to 500 mm, and anhydrous hydrogen chloride and chlorine (preferably chlorine generated by electrolytic magnesium) are pre-mixed and preheated to 50-100° C., and flow-through contact is generated with anhydrous magnesium formate powder from the bottom of the column reactor through a microporous ceramic support plate, and a gas-solid reaction in a fixed bed or fluidized bed is achieved at different gas flow rates (for example, when the bed height (i.e., the height of the anhydrous magnesium formate filler layer) of the material is naturally stacked at 300 to 500 mm, the gas flow rate is 0-1.0 cm / s for a fixed bed mode, and the gas flow rate is 1.0 to 3.0 cm / s for a fluidized bed mode, and the gas flow rate exceeds 3.0 cm / s if the material is taken out). Preferably, a gas-solid reaction in a fluidized bed mode is used, that is, the gas flow rate of the mixed gas of anhydrous hydrogen chloride and chlorine introduced from the bottom of the column reactor in step S2 is preferably 1.0 to 3.0 cm / s.

[0037] Preferably, the mixer 1 is connected to the lower part of the column reactor 2 through an air inlet valve 17, and a nitrogen valve 18 is also provided at the lower part of the column reactor 2. In step S3, when collecting the crude anhydrous magnesium chloride product, the air inlet valve 17 of the column reactor where the reaction is completed can be closed, and the nitrogen valve 18 can be opened to perform nitrogen purge. The residual gas in the column reactor section is first purged and replaced with nitrogen, and then the gas flow rate is increased to blow out the powdered product in the column reactor section, which is captured by the cyclone separator 3 and collected by the hopper.

[0038] In some preferred embodiments, in step S2, the mixed gas of anhydrous hydrogen chloride and chlorine is preheated to 90-99°C.

[0039] In some preferred embodiments, in step S2, by controlling at least one of the flow rate of the mixed gas, the feed temperature, and the mass fraction of chlorine in the mixed gas, such as lowering the feed temperature, lowering the mass fraction of chlorine in the mixed gas, etc., the temperature of the discharge port of the column reactor is controlled below 200°C, and more preferably, the temperature of the discharge port of the column reactor is controlled within the range of 110-160°C.

[0040] In other preferred embodiments, in step (1), anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine are subjected to a liquid-solid reaction in an anhydrous organic solvent, comprising the following steps: dispersing anhydrous magnesium formate powder in an anhydrous organic solvent, introducing a mixed gas of anhydrous hydrogen chloride and chlorine, and reacting at 0-3 MPa to obtain a crude anhydrous magnesium chloride product.

[0041] Preferably, in the liquid-solid reaction of step (1), the anhydrous organic solvent is at least one of carbon tetrachloride, methanol and ethanol. The use of the anhydrous organic solvent can better absorb the mixed gas composed of chlorine and anhydrous hydrogen chloride.

[0042] Preferably, in the liquid-solid reaction of step (1), the reaction temperature can be above 40°C, more preferably, the reaction temperature is above 40°C and below the boiling point of the anhydrous organic solvent, or the reaction temperature is above the boiling point of the anhydrous organic solvent and the reaction is carried out by condensation reflux.

[0043] In some preferred embodiments, in the mixed gas of anhydrous hydrogen chloride and chlorine, the mass fraction of chlorine is 0.5%-99.5%, and the remainder is anhydrous hydrogen chloride.

[0044] In some preferred embodiments, in the mixed gas of anhydrous hydrogen chloride and chlorine, the mass fraction of chlorine is 30%-70%, and the balance is anhydrous hydrogen chloride. By limiting the mass fraction of chlorine to 30%-70%, the reaction speed can be increased, the reaction time can be shortened, and the conversion rate per unit time can be increased.

[0045] In some further preferred embodiments, the gas-solid reaction of the anhydrous magnesium formate powder with the mixed gas of anhydrous hydrogen chloride and chlorine in step (1) is carried out in an intermittent gas-solid reaction vessel, wherein the intermittent gas-solid reaction vessel comprises a closed pressure reactor, and the closed pressure reactor can rotate around a central axis; the gas-solid reaction comprises the following steps:

[0046] A1, after removing the gas inside the closed pressure reactor to form a negative pressure, anhydrous magnesium formate powder is added;

[0047] A2. After the addition is completed, the gas inside the closed pressure reactor is exhausted again so that the vacuum degree inside the closed pressure reactor is between -0.09MPa and -0.1MPa;

[0048] A3, injecting a mixed gas of anhydrous hydrogen chloride and chlorine into the closed pressure reactor at room temperature so that the internal pressure of the reaction vessel is between 1 and 2 MPa;

[0049] A4, rotating the closed pressure reactor around the central axis and maintaining the temperature in the closed pressure reactor at 100-120° C., rotating the closed pressure reactor for more than 8 hours, then reducing the temperature in the closed pressure reactor to below 40° C., and stopping the rotation of the closed pressure reactor;

[0050] A5, extracting the gas in the closed pressure reactor and replacing all the gas in the closed pressure reactor with nitrogen;

[0051] A6. Open the discharge port of the closed pressure reactor to obtain a crude anhydrous magnesium chloride product from the discharge port.

[0052] In some preferred embodiments, the anhydrous magnesium formate powder is prepared by one of the following methods:

[0053] Method 1: Evaporating and concentrating the magnesium formate aqueous solution to prepare a saturated aqueous solution, filtering and removing impurities, removing free water by spray drying to obtain magnesium formate dihydrate powder, and then drying by airflow (such as air and / or nitrogen airflow) at 120-200° C. to remove crystal water and obtain anhydrous magnesium formate powder;

[0054] Preferably, in method one, precision filtration means that the cutoff size of the filter medium is between 5 nanometers and 0.5 microns.

[0055] Preferably, in method one, the particle size of the anhydrous magnesium formate powder can be controlled by adjusting the nozzle aperture (e.g., nozzle diameters of 0.5 mm, 0.7 mm, 0.75 mm, 1.0 mm, 1.5 mm, 2.0 mm, etc.) and / or pressure parameters (the nozzle pressure is preferably between 2-20 MPa) during the spray drying process, and the average particle size of the anhydrous magnesium formate powder is preferably between 5-20 μm, so that it has a high specific surface area. Alternatively, in method one, the particle size of the anhydrous magnesium formate powder is not controlled by the nozzle aperture and / or pressure parameters during the spray drying process, and after spray drying, the anhydrous magnesium formate powder is milled to a suitable particle size (e.g., an average particle size between 5-20 μm) by milling or the like, so that the anhydrous magnesium formate powder has a high specific surface area.

[0056] Method 2: drying the magnesium formate dihydrate powder in an air flow at 120-200° C. to remove crystal water and obtain anhydrous magnesium formate powder; preferably, after removing the crystal water, grinding is performed to obtain anhydrous magnesium formate powder with an average particle size of 5-20 μm.

[0057] Method 3: adding a saturated aqueous solution of magnesium formate to anhydrous methanol and / or anhydrous ethanol (preferably anhydrous methanol), stirring to form a suspension (preferably, stirring at a speed of ≥150 rpm during the stirring process); evaporating the solvent, refluxing the anhydrous methanol after rectification, and removing water (free water) in the solvent; after most of the water in the solvent is removed, adding a solvent with a boiling point of ≥110° C. (such as formamide), continuing to evaporate the solvent, and removing anhydrous methanol and / or anhydrous ethanol; maintaining the temperature at 150-200° C., continuing to evaporate under negative pressure, and removing magnesium formate crystal water; filtering to obtain anhydrous magnesium formate, washing with an anhydrous organic solvent having a boiling point lower than 100° C. (preferably at least one of carbon tetrachloride, methanol and ethanol) to remove the residual solvent having a boiling point ≥ 110° C., and vacuum removing the anhydrous organic solvent having a boiling point lower than 100° C. to obtain anhydrous magnesium formate powder, preferably, the anhydrous magnesium formate powder has an average particle size of 5-20 μm. If the average particle size of the anhydrous magnesium formate powder is not within the range of 5-20 μm, the anhydrous magnesium formate powder can also be ground into a particle size within the range of 5-20 μm by grinding or the like.

[0058] In some preferred embodiments, the step (2) comprises the following steps:

[0059] A. The crude anhydrous magnesium chloride product is heated to 714° C. to 720° C. to melt and is kept warm and allowed to stand for clarification, so that the anhydrous magnesium chloride and the unreacted magnesium formate are separated by stratification; preferably, the crude anhydrous magnesium chloride product is placed in a hot melt tank or a crucible and heated to 714° C. to 720° C. to melt, and is kept warm and allowed to stand for more than 24 hours.

[0060] B. Collect the clear magnesium chloride molten salt at the bottom while it is hot (714° C. to 720° C.) to obtain anhydrous magnesium chloride. When storing the anhydrous magnesium chloride, the obtained anhydrous magnesium chloride can be naturally cooled to room temperature in a closed container isolated from air, and then protected with nitrogen to avoid moisture reversion.

[0061] By means of high-temperature melting, natural cooling to room temperature in a closed container isolated from air, and protection with nitrogen, the anhydrous magnesium chloride is prevented from absorbing moisture and returning to moisture due to environmental factors (especially moisture in the air).

[0062] The specific embodiments of the present invention are further described below.

[0063] Example 1

[0064] The method for preparing anhydrous magnesium chloride using bischofite (MgCl2·6H2O) as the initial raw material comprises the following steps:

[0065] S0, grind 1000g of bischofite into fine powder, pass it through a 100-mesh vibrating sieve, dissolve it with 2L of hot water, filter it accurately to remove impurities, and then evaporate and concentrate the filtrate. The volatilized steam is condensed in a heat exchanger with circulating water at normal temperature, and the evaporation and dehydration are continued under negative pressure until the temperature rises to 115°C, and under a relatively large vacuum degree (relative vacuum degree is between -75KPa and -100KPa), observe and confirm that no steam condensate is removed, then stop heating, break the vacuum with nitrogen, add 100g of high-purity active magnesium oxide (light-burned powder, MgO purity greater than 99%), stir evenly, filter while hot to remove insoluble matter, and obtain a clear and transparent filtrate;

[0066] S1. Add 620.4 g of high-purity (purity>99%, analytically pure) ammonium formate to the filtrate obtained in step S0, maintain the temperature of the material in the reactor at 100°C, continue stirring for 1-2 hours (such as 1.5 hours) to completely dissolve the ammonium formate, raise the temperature to 110-120°C again, and evaporate the water under negative pressure (relative vacuum degree is 0 to -100KPa) until no steam and condensed water are discharged. Double decomposition reaction occurs at this stage. Since the solubility of magnesium formate is less than that of magnesium chloride, ammonium formate and ammonium chloride, it precipitates first. Stop heating, transfer the material to a crystallization tank and cool it naturally for 24 hours, and filter it with a centrifuge to obtain a mixed salt composed of ammonium chloride and magnesium formate (dihydrate);

[0067] S2, using recrystallization technology to separate the generated magnesium formate (dihydrate) and ammonium chloride, and then drying them separately (including but not limited to oven drying and airflow drying. Wherein, the drying conditions of magnesium formate dihydrate are: normal pressure, 125°C; the drying conditions of ammonium chloride are: normal pressure, 105°C) to obtain anhydrous magnesium formate powder and ammonium chloride crystals; wherein, the purity of ammonium chloride crystals> 99%, can be sold as a by-product; anhydrous magnesium formate powder enters step S3, in this example, 525g ammonium chloride and 560g anhydrous magnesium formate are obtained. Adopting the above steps S0-S2, anhydrous magnesium formate powder is obtained, which is used as the raw material for preparing anhydrous magnesium chloride in the following steps.

[0068] like Figure 2As shown, it is a schematic diagram of the following gas-solid reaction equipment for preparing anhydrous magnesium chloride from anhydrous magnesium formate, which includes a mixer 1, a multi-stage gas-solid reaction unit (in this example, a two-stage gas-solid reaction unit), an air inlet valve 17, a nitrogen valve 18 and an exhaust gas recovery device, the exhaust gas recovery device includes an induced draft fan 7, a first condenser 8, a formic acid storage tank 9, a compressor 10, a reboiler 11, a distillation tower 12, a second condenser 13, a condensate storage tank 14, a carbon dioxide liquefied gas storage tank 15, and a hydrogen chloride liquefied gas storage tank 16, wherein each stage of the gas-solid reaction unit includes a column reactor 2, a cyclone separator 3, a bag filter 4, a feeder 5 and a hopper 6, a microporous ceramic support plate 21 is provided at the bottom of the column reactor 2, a nitrogen valve 18 is provided at the lower part of the column reactor 2, and the feeder 5 It is connected to the upper part of the column reactor 2 and is used to load the column reactor 2 with magnesium formate powder. The cyclone separator 3 is connected to the discharge port of the column reactor 2. The gas outlet of the cyclone separator 3 is connected to the bag-type dust collector 4. The discharge port of the cyclone separator 3 and the discharge port of the bag-type dust collector 4 are respectively connected to the hopper 6. Through the coordinated cooperation of the cyclone separator 3 and the bag-type dust collector 4, the anhydrous magnesium chloride crude product and the gas can be better separated to collect the anhydrous magnesium chloride crude product from the hopper 6; the gas outlet of the bag-type dust collector in the gas-solid reaction unit of the previous stage is connected to the lower part of the column reactor of the gas-solid reaction unit of the next stage, and the mixer 1 is connected to the lower part of the column reactor 2 of the gas-solid reaction unit of each stage through the air inlet valve 17. The first condenser 8 is connected to the bag filter 4 of the last-stage gas-solid reaction unit through the induced draft fan 7, the formic acid storage tank 9 and the compressor 10 are respectively connected to the first condenser 8, the compressor 10 is connected to the distillation tower 12, the top of the distillation tower 12 is connected to the carbon dioxide liquefied gas storage tank 15 through the second condenser 13 and the condensate storage tank 14 in turn, the condensate storage tank 14 is also connected to the upper part of the distillation tower 12, the bottom of the distillation tower 12 is respectively connected to the hydrogen chloride liquefied gas storage tank 16 and the reboiler 11, and the reboiler 11 is connected to the lower part of the distillation tower 12.

[0069] S3, the anhydrous magnesium formate obtained in step S2 is ground into a fine powder with an average particle size of 20 microns, and loaded into each column reactor 2. The air in the column reactor 2 is first purged with dry nitrogen, and then the air inlet valve 17 of the mixer 1 is slowly opened. A mixed gas of anhydrous hydrogen chloride and chlorine (mass ratio is 1:1) is introduced from the bottom of the column reactor 2, and flows from bottom to top through a microporous ceramic support plate 21 and anhydrous magnesium formate filler layer (in this example, the height of the filler layer is 300mm). By heating the mixed gas, the initial temperature of the gas entering the bottom of the column reactor 2 is controlled at 95°C, and the gas flow rate is controlled at 2cm / S. Since each level of gas-solid reaction unit is the same, the remaining gas coming out of the gas-solid reaction unit of the previous level continues to enter the gas-solid reaction unit of the next level. The gas composition (such as chlorine and carbon dioxide) at the top outlet of each column reactor 2 is detected by a sampler (not shown). When the gas composition at the outlet of a column reactor 2 remains constant for a long time (such as more than 30 minutes), it can be judged that the reaction in the column reactor 2 has been completed;

[0070] S4, closing the air inlet valve 17 of the column reactor 2 where the reaction has been completed, opening the nitrogen valve 18, blowing nitrogen from the bottom of the column reactor 2, and gradually increasing the air flow rate until all the powdered solids therein are blown out, and after separation by the cyclone separator 3 and the bag filter 4, the reaction product anhydrous magnesium chloride is collected by the hopper 6;

[0071] S5, collecting enough anhydrous magnesium chloride from the hopper in step S4, isolating it from air, loading it into a crucible and heating it to 715°C to melt it, and standing it for a long time (>24H) to clarify it, so that the anhydrous magnesium chloride and the unreacted magnesium formate can be separated by layers. The clear magnesium chloride molten salt at the bottom layer is collected while hot, and anhydrous magnesium chloride that meets the product quality requirements can be obtained. In this example, 463g of anhydrous magnesium chloride is obtained, and the conversion rate of magnesium formate exceeds 99% (in this example, the conversion rate of magnesium formate is 99.3%);

[0072] S6. After the anhydrous magnesium chloride obtained in step S5 is naturally cooled to room temperature under nitrogen protection and air isolation, sampling is performed for detection and analysis. The detection results obtained are shown in Table 1 below.

[0073] S7, condense the tail gas generated in step S4 to room temperature, recover the by-product formic acid, and compress the anhydrous hydrogen chloride and carbon dioxide that are not condensed at room temperature (for example, pressurize them to more than 3MPa by a compressor) to liquefy them, and then liquefy and separate the anhydrous hydrogen chloride and carbon dioxide by distillation, and recover them separately. Figure 2As shown, specifically, the tail gas at the outlet of the bag filter 4 of the last-stage gas-solid reaction unit is introduced into the first condenser 8 through the induced draft fan 7 and condensed to room temperature, the by-product formic acid enters the formic acid storage tank 9, the anhydrous hydrogen chloride and carbon dioxide that are not condensable at room temperature are compressed to liquefaction by the compressor 10 and enter the distillation tower 12 from the middle of the distillation tower 12, after the anhydrous hydrogen chloride and carbon dioxide are liquefied separately in the distillation tower 12, the carbon dioxide is condensed from the top of the distillation tower 12 through the second condenser 13 and enters the condensate storage tank 14, the carbon dioxide liquefied gas in the condensate storage tank 14 is partially refluxed into the distillation tower 12, and partially discharged into the carbon dioxide liquefied gas storage tank 15, the hydrogen chloride is discharged from the bottom of the distillation tower 12, partially enters the hydrogen chloride liquefied gas storage tank 16 (the recovered anhydrous hydrogen chloride can be recycled in step S4), and partially returns to the distillation tower 12 through the reboiler 11.

[0074] Table 1: Test results of anhydrous magnesium chloride obtained in this embodiment 1

[0075]

[0076]

[0077] Note: (1) The “content %”, “appearance”, “pH”, “infrared correlation %”, “residue on ignition %” and “solubility” in the above table refer to the test methods and instruments listed in GB / T 8453-87; (2) “——” in the above table means there is no relevant standard.

[0078] The gas-solid reaction of Example 1 has the following advantages: 1. The effect of preparing anhydrous magnesium formate into anhydrous magnesium chloride can be achieved without consuming solvent, which reduces the fire safety level requirements for the buildings to which the production equipment and plant facilities belong, and reduces the material cost caused by solvent loss; 2. Continuous production can be achieved, and it is easy to scale up; 3. Chlorine can be directly consumed in the reaction, and the hydrogen chloride produced by the reaction can be recycled and sold to the outside, which significantly reduces the raw material cost and product production cost; 4. The by-product anhydrous formic acid product has excellent product quality and outstanding economic benefits; 5. The equipment anti-corrosion difficulty is small (the corrosiveness of anhydrous hydrogen chloride is much lower than that of aqueous hydrogen chloride).

[0079] In the process of exploring the preparation of anhydrous magnesium chloride, the inventor unexpectedly discovered that the reaction between anhydrous magnesium formate powder and anhydrous hydrogen chloride or chlorine gas alone in the absence of solvent is very slow, and the conversion rate of magnesium formate is not high. However, when these two gases are present at the same time, the reaction activity of anhydrous magnesium formate can be significantly improved, and the conversion rate of magnesium formate can be improved. The inventor speculates that this may be related to the micro-explosion of the solid surface caused by solid surface adsorption and / or gas-solid reaction, which causes the reaction interface to be continuously updated. In this regard, for example Figure 2The inventor conducted the following comparative experiments on the device shown:

[0080] 1. Reaction of chlorine with magnesium formate: At room temperature, it is found that it hardly reacts; above 100°C, there is a slow reaction, and the conversion rate in 24 hours is between 5% and 10%. For example, in the comparative example made by the inventor, at 105°C, the reaction lasts for 24 hours, and the conversion rate of magnesium formate is 6.8%.

[0081] 2. Reacting anhydrous hydrogen chloride with magnesium formate: At room temperature, the reaction rate of anhydrous hydrogen chloride with magnesium formate mainly depends on the mass transfer rate, and the conversion rate exceeds 90% in 3-5 hours. For example, in the comparative example made by the inventor, the conversion rate of magnesium formate was 92% at 40° C. and 4 hours of reaction.

[0082] 3. At room temperature, anhydrous hydrogen chloride and chlorine are mixed in a mass ratio of 1:1, and the conversion rate exceeds 95% in 1-2 hours; increasing the temperature is conducive to accelerating the reaction rate. When the initial temperature of the gas entering the bottom of the column reactor is controlled above 90°C, it can be completely converted in about 30 minutes (under the premise of excess gas, magnesium formate is completely converted, and the conversion rate is ≥99%).

[0083] Example 2

[0084] The method for preparing anhydrous magnesium chloride using ammonium carnallite (MgCl2·NH4Cl·6H2O) as the initial raw material comprises the following steps:

[0085] S0. Grind 1000g of ammonium carnallite into fine powder, pass through a 100-mesh vibrating sieve, put it into a glass reactor, and then put in 2L of anhydrous formamide solvent, maintain medium-speed stirring, and heat to 160°C to make the ammonium carnallite fully dissolved, then evaporate and remove most of the water, condense and recover the water-containing formamide solvent, and add anhydrous formamide to the reactor in an appropriate amount. After removing part of the water from the ammonium carnallite, it becomes low-water ammonium carnallite (MgCl2·NH4Cl·nH2O, where n=1-2);

[0086] S1. Stop heating, maintain stirring, and after the material is naturally cooled to 95°C, add 380.63g of high-purity (purity>99%, analytically pure) ammonium formate into the glass reactor, stir for 4 hours to complete the double decomposition reaction, and obtain magnesium formate and ammonium chloride. Naturally cool to room temperature, filter and wash the precipitated mixed salt of magnesium formate and ammonium chloride, and then recrystallize the mixed salt with pure water to separate magnesium formate and ammonium chloride from the mixed salt;

[0087] S2, using recrystallization technology to separate the magnesium formate (dihydrate) and ammonium chloride and dry them separately (including but not limited to oven drying and airflow drying. Wherein, the drying conditions of magnesium formate dihydrate are: normal pressure, 120°C-200°C; the drying conditions of ammonium chloride are: normal pressure, 100°C-150°C) to obtain anhydrous magnesium formate powder and ammonium chloride crystals (about 789.3g); wherein the purity of the ammonium chloride crystals is>99%, and can be sold as a by-product; the anhydrous magnesium formate powder enters step S3;

[0088] S3, grind 345g of anhydrous magnesium formate obtained in S2 into fine powder, add it to a stainless steel reactor, put into anhydrous solvent carbon tetrachloride 1 to 1.5L (in this example, add 1.5L), pass nitrogen to replace the gas in the reactor, turn on the agitator, stir for about two hours, so that the magnesium formate is evenly dispersed in the solvent, then close the exhaust valve, pass a mixed gas of anhydrous hydrogen chloride and chlorine (mass ratio is 1:1) into the closed reactor, gradually increase the pressure, finally make the pressure in the reactor controlled at 3MPa and then gradually stop adding gas, control the reaction temperature at 50°C, stir for 3-4 hours, observe the pressure in the reactor, if the pressure slowly decreases and maintains a fixed value for a long time (such as more than 30 minutes), then judge that the reaction is complete;

[0089] S4, slowly open the vent valve to discharge the unreacted gas, and then purge with nitrogen for 15-30 minutes. Then heat until the solvent boils, remove the solvent by vacuum condensation, and obtain the reaction product anhydrous magnesium chloride;

[0090] S5. After collecting the anhydrous magnesium chloride obtained in step S4 by purging with nitrogen, isolate it from the air, put it into a crucible and heat it to 715°C to melt it, and let it stand for a long time (>24 hours) to clarify it, so that the anhydrous magnesium chloride and the unreacted magnesium formate can be separated by layers. The clear magnesium chloride molten salt at the bottom layer is collected while hot, and anhydrous magnesium chloride that meets the product quality requirements can be obtained. In this example, 286g of anhydrous magnesium chloride is obtained, and the conversion rate of magnesium formate is 99.5%;

[0091] S6. After the anhydrous magnesium chloride obtained in step S5 is naturally cooled to room temperature under the condition of nitrogen protection and air isolation, sampling is performed for detection and analysis. The detection results obtained are shown in Table 2 below:

[0092] Table 2: Test results of anhydrous magnesium chloride obtained in this embodiment 2

[0093]

[0094]

[0095] Note: (1) The “content %”, “appearance”, “pH”, “infrared correlation %”, “residue on ignition %” and “solubility” in the above table refer to the test methods and instruments listed in GB / T 8453-87; (2) “——” in the above table means there is no relevant standard.

[0096] The anhydrous magnesium chloride product produced by the present invention has a lower water content and can better meet the stringent requirements of the electrolytic magnesium industry on the quality of anhydrous magnesium chloride products.

[0097] Example 3

[0098] In this embodiment, anhydrous magnesium formate powder is used as a starting material, and the method for preparing anhydrous magnesium chloride comprises the following steps:

[0099] 1. Anhydrous magnesium formate powder is subjected to an intermittent gas-solid reaction with a mixed gas of anhydrous hydrogen chloride and chlorine, specifically in an intermittent gas-solid reaction vessel, wherein the intermittent gas-solid reaction vessel comprises a closed pressure reactor which can rotate around a central axis. In this example, a 3000L closed pressure vessel (hereinafter referred to as "reaction vessel") is used, which can withstand negative pressure and 10MPa air pressure. In this example, the discharge port of the reaction vessel is also connected to a hopper, and the inner wall of the reaction vessel is also provided with a steam coil to heat the interior of the reaction vessel and a circulating cooling water coil to remove the heat of the reaction vessel and cool it down. The specific steps are as follows:

[0100] A1. Use a vacuum pump to extract the gas in the reaction container. After forming a negative pressure, pump 10.5 kg of anhydrous magnesium formate powder in the feeding bucket into the reaction container.

[0101] After A2 and anhydrous magnesium formate were added, the gas in the reaction vessel was evacuated again with a vacuum pump until the vacuum degree in the reaction vessel was between -0.09 MPa and -0.1 MPa, and then vacuuming was stopped;

[0102] A3. Open the gas inlet valve and inject a mixed gas consisting of anhydrous hydrogen chloride and chlorine in a mass ratio of 2:1 into the reaction vessel at room temperature (25°C in this example) so that the internal gas pressure of the reaction vessel is between 1 and 2 MPa. In this example, the gas inlet valve is closed after the pressure reaches 1.5 MPa.

[0103] A4. Turn on the rotation button to make the reaction vessel start to rotate around the central axis, open the valve of the steam coil, and gradually increase the air temperature and material temperature in the reaction vessel by heating. Since it is an exothermic reaction, as the gas-solid reaction proceeds, the temperature in the reaction vessel will further increase. Through the control of the circulating cooling water coil, the circulating water removes the heat generated by the reaction and automatically adjusts the circulating water flow rate to keep the temperature in the closed pressure reactor between 100-120°C, and maintain the rotation of the reaction vessel for more than 8 hours. During the rotation process, the anhydrous magnesium formate powder is constantly in contact with the gas in the container to promote the reaction. After the reaction is completed, the gas temperature in the reactor is reduced to below 40°C, and the rotation of the reaction vessel is stopped.

[0104] A5. Turn on the vacuum pump to extract the gas in the reactor, and use an absorption tower containing sodium hydroxide aqueous solution to absorb the reaction tail gas through elution and then empty it;

[0105] A6. Turn off the vacuum pump, break the vacuum with nitrogen, close the nitrogen tube, and evacuate again. Repeat this process until all the gas in the reaction vessel is replaced with nitrogen.

[0106] A7. Open the discharge port of the reaction vessel and load the generated 8.74 kg of crude anhydrous magnesium chloride into a packaging barrel through a hopper. The conversion rate of magnesium formate is close to 100%.

[0107] 2. The obtained crude anhydrous magnesium chloride product is post-treated (the post-treatment process is the same as step S4 of Example 1) to obtain anhydrous magnesium chloride.

[0108] The obtained anhydrous magnesium chloride was tested, and the result was the same as that in Table 1 of Example 1.

[0109] The above content is a further detailed description of the present invention in combination with specific / preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, without departing from the concept of the present invention, they can also make several substitutions or modifications to these described embodiments, and these substitutions or modifications should be regarded as belonging to the protection scope of the present invention. In the description of this specification, the description of the reference terms "an embodiment", "some embodiments", "preferred embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in combination with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily target the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In the absence of contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples. Although the embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and modifications can be made herein without departing from the scope of protection of the patent application.

Claims

1. A method for preparing anhydrous magnesium chloride, characterized in that, The steps include: (1) subjecting anhydrous magnesium formate powder to a gas-solid reaction with a mixed gas of anhydrous hydrogen chloride and chlorine or to a liquid-solid reaction in an anhydrous organic solvent to obtain an anhydrous magnesium chloride crude product, wherein the main chemical reaction equation is as follows: Mg(HCO2)2+2Cl2=MgCl2+2HCl+2CO2 Mg(HCO2)2+2HCl=MgCl2+2HCOOH HCOOH+Cl2=CO2+2HCl; (2) subjecting the crude anhydrous magnesium chloride product to post-treatment to obtain anhydrous magnesium chloride, comprising the following steps: A. Heating the crude anhydrous magnesium chloride product to 714°C to 720°C to melt and keeping the temperature to stand for clarification, so that the anhydrous magnesium chloride and the unreacted magnesium formate are separated by layers; B. Collect the clear magnesium chloride molten salt at the bottom while it is hot to obtain anhydrous magnesium chloride.

2. The preparation method according to claim 1, characterized in that: In step (1), the gas-solid reaction of anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine is carried out in a continuous gas-solid reaction device, the continuous gas-solid reaction device comprises a mixer and a multi-stage gas-solid reaction unit, each stage of the multi-stage gas-solid reaction unit comprises a column reactor, a cyclone separator, a bag dust collector, a feeder and a hopper, the feeder is connected to the upper part of the column reactor, the cyclone separator is connected to the discharge port of the column reactor, the gas outlet of the cyclone separator is connected to the bag dust collector, the discharge port of the cyclone separator and the discharge port of the bag dust collector are respectively connected to the hopper; the gas outlet of the bag dust collector in the gas-solid reaction unit of the previous stage is connected to the lower part of the column reactor of the gas-solid reaction unit of the next stage, and the mixer is connected to the lower part of the column reactor of each stage of the gas-solid reaction unit; the gas-solid reaction comprises the following steps: S1, filling anhydrous magnesium formate powder into a column reactor to form an anhydrous magnesium formate filler layer; S2, preheating the mixed gas of anhydrous hydrogen chloride and chlorine to 50-100° C., introducing it from the bottom of the column reactor, passing through the anhydrous magnesium formate filler layer from bottom to top, controlling the temperature of the discharge port of the column reactor at a predetermined temperature, and performing a gas-solid reaction to obtain anhydrous magnesium chloride crude product; S3. After all the magnesium formate is converted, the crude anhydrous magnesium chloride product is collected from the hopper, and anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine are added to the column reactor to carry out a continuous reaction.

3. The preparation method according to claim 2, characterized in that: In step S2, the mixed gas of anhydrous hydrogen chloride and chlorine is preheated to 90-99° C.; and / or In step S2, the temperature of the discharge port of the column reactor is controlled below 200° C. by controlling at least one of the flow rate of the mixed gas, the feed temperature, and the mass fraction of chlorine in the mixed gas.

4. The preparation method according to claim 2, characterized in that: In step S1, the height of the anhydrous magnesium formate filler layer is 300-500 mm; in step S2, the gas flow velocity of the mixed gas of anhydrous hydrogen chloride and chlorine introduced from the bottom of the column reactor is 1.0-3.0 cm / s.

5. The preparation method according to claim 2, characterized in that: Between steps S2 and S3, the method further includes a step of determining whether all the anhydrous magnesium formate in the column reactor has been converted: continuously monitoring the mass fractions of chlorine and carbon dioxide in the mixed gas in the outlet gas flow of each stage of the column reactor, and when the changes in the mass fractions of chlorine and carbon dioxide in the mixed gas tend to be constant within a predetermined time, it is considered that all the anhydrous magnesium formate in the column reactor has been converted.

6. The preparation method according to claim 3, characterized in that: In step S2, the temperature of the discharge port of the column reactor is controlled within the range of 110-160°C.

7. The preparation method according to claim 5, characterized in that: The predetermined time is more than 30 minutes.

8. The preparation method according to claim 1, characterized in that: In step (1), the gas-solid reaction of anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine is carried out in an intermittent gas-solid reaction container, wherein the intermittent gas-solid reaction container comprises a closed pressure reactor, and the closed pressure reactor rotates around a central axis; the gas-solid reaction comprises the following steps: A1, after removing the gas inside the closed pressure reactor to form a negative pressure, anhydrous magnesium formate powder is added; A2. After the addition is completed, the gas inside the closed pressure reactor is exhausted again so that the vacuum degree inside the closed pressure reactor is between -0.09MPa and -0.1MPa; A3, injecting a mixed gas of anhydrous hydrogen chloride and chlorine into the closed pressure reactor at room temperature so that the internal pressure of the reaction vessel is between 1 and 2 MPa; A4, rotating the closed pressure reactor around the central axis and maintaining the temperature in the closed pressure reactor at 100-120° C., rotating the closed pressure reactor for more than 8 hours, then reducing the temperature in the closed pressure reactor to below 40° C., and stopping the rotation of the closed pressure reactor; A5, extracting the gas in the closed pressure reactor and replacing all the gas in the closed pressure reactor with nitrogen; A6. Open the discharge port of the closed pressure reactor to obtain a crude anhydrous magnesium chloride product from the discharge port.

9. The preparation method according to claim 1, characterized in that: In step (1), anhydrous magnesium formate powder and a mixed gas of anhydrous hydrogen chloride and chlorine are subjected to a liquid-solid reaction in an anhydrous organic solvent, comprising the following steps: dispersing anhydrous magnesium formate powder in an anhydrous organic solvent, introducing a mixed gas of anhydrous hydrogen chloride and chlorine, and reacting at 0-3 MPa to obtain an anhydrous magnesium chloride crude product.

10. The preparation method according to claim 9, characterized in that: In step (1), the anhydrous organic solvent is at least one of carbon tetrachloride, methanol and ethanol.

11. The preparation method according to any one of claims 1 to 10, characterized in that: In the mixed gas of anhydrous hydrogen chloride and chlorine, the mass fraction of chlorine is 0.5%-99.5%, and the remainder is anhydrous hydrogen chloride.

12. The preparation method according to claim 11, characterized in that: In the mixed gas of anhydrous hydrogen chloride and chlorine, the mass fraction of chlorine is 30%-70%.

13. The preparation method according to any one of claims 1 to 10, characterized in that: The anhydrous magnesium formate powder is prepared by one of the following methods: Method 1: Evaporate and concentrate the magnesium formate aqueous solution to prepare a saturated aqueous solution, filter and remove impurities, remove free water by spray drying to obtain magnesium formate dihydrate powder, and then dry it in an air flow at 120-200°C to remove crystal water to obtain anhydrous magnesium formate powder; Method 2: Drying the magnesium formate dihydrate powder in an air flow at 120-200°C to remove crystal water to obtain anhydrous magnesium formate powder; Method 3: adding a saturated aqueous solution of magnesium formate to anhydrous methanol and / or anhydrous ethanol, stirring to form a suspension; evaporating the anhydrous methanol and / or anhydrous ethanol, refluxing the anhydrous methanol and / or anhydrous ethanol after rectification, and removing water from the anhydrous methanol and / or anhydrous ethanol; after most of the water in the solvent is removed, adding a solvent with a boiling point ≥110°C, continuing to evaporate the solvent, and removing the anhydrous methanol and / or anhydrous ethanol; maintaining the temperature at 150-200°C, continuing to evaporate under negative pressure, and removing crystal water of magnesium formate; filtering to obtain anhydrous magnesium formate, washing with an anhydrous organic solvent with a boiling point lower than 100°C to remove the solvent residue with a boiling point ≥110°C, and vacuum removing the anhydrous organic solvent with a boiling point lower than 100°C to obtain anhydrous magnesium formate powder.

14. The preparation method according to any one of claims 1 to 10, characterized in that: The average particle size of the anhydrous magnesium formate powder is between 5 and 20 μm.

15. The preparation method according to any one of claims 1 to 10, characterized in that: The following steps are also included: (3) The tail gas generated in step (1) is condensed to room temperature, the by-product formic acid is recovered, and the anhydrous hydrogen chloride and carbon dioxide that are not condensed at room temperature are compressed to liquefy, and then the anhydrous hydrogen chloride and carbon dioxide are liquefied and separated by distillation, and recovered separately.

16. The preparation method according to claim 15, characterized in that: The recovered anhydrous hydrogen chloride is recycled in step (1).

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