Method for preparing anhydrous magnesium chloride through three-stage dehydration

Abstract

The invention provides a method for preparing anhydrous magnesium chloride through three-stage dehydration, which comprises the following steps: step 1, cooling and crystallizing a magnesium chloride solution through a granulation tower to obtain magnesium chloride solid particles containing 4.8-5.0 mol of water; step 2, carrying out hot air drying on the magnesium chloride particles by adopting a two-stage vertical fluidized bed, and reducing the water content to 2-1.8 mol H2O / mol MgCl2; and step 3, finally, electrically heating the crucible furnace, introducing HCl gas, carrying out molten salt dehydration on MgCl2 containing 2-1.8 mol of water, controlling the temperature in the furnace to be 950 + / -10 DEG C and the partial pressure of HCl to be greater than or equal to 0.05 MPa, and drying MgCl2 by adopting a continuous feeding and indirect discharging mode to obtain the anhydrous magnesium chloride melt with the water content of 0.02-0.15% and the MgO content of less than or equal to 0.2%. The method has the advantages of high dehydration efficiency, high product purity, simplified process, reduced energy consumption and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of anhydrous magnesium chloride, and particularly relates to a method for preparing anhydrous magnesium chloride by three-stage dehydration.

Background Art

[0002] In traditional metal magnesium electrolysis processes, there are many technical bottlenecks in the magnesium chloride dehydration process. For example, directly using an HCl fluidized bed drying system for drying and dehydration has problems such as large investment and complex control systems. In the HCl drying stage, it is difficult to control the temperature field, there are situations such as tower plate blockage and large fluctuations in the pressure of the HCl compressor. Using the method of organic solvent distillation and molecular sieve dehydration, magnesium chloride hydrate is easily soluble in polar organic solvents, and an anhydrous alcohol solution of magnesium chloride can be obtained by distillation and molecular sieve adsorption. However, further distillation to remove the organic solvent has problems such as the decomposition of the organic reagent and magnesium chloride, and the anhydrous magnesium chloride product contains a large amount of carbon and magnesium oxide and cannot be used as an electrolysis raw material. The obtained anhydrous magnesium chloride solution needs to be further separated to prepare anhydrous magnesium chloride, and the process is complex.

[0003] Therefore, although existing technologies have tried to optimize the process through equipment transformation, there are still problems such as complex processes, high investment costs, and insufficient drying efficiency. Therefore, there is an urgent need for a preparation process of anhydrous magnesium chloride that can simplify the process, reduce energy consumption, and dehydrate efficiently.

Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing anhydrous magnesium chloride by three-stage dehydration, which has the advantages of efficient dehydration, high product purity, simplified process, and reduced energy consumption.

[0005] The present invention is implemented as follows:

[0006] A method for preparing anhydrous magnesium chloride by three-stage dehydration, the steps of the method are as follows:

[0007] Step 1, the magnesium chloride solution is cooled and crystallized through a granulation tower to obtain magnesium chloride solid particles containing 4.8 - 5.0 mol of water;

[0008] Step 2, the magnesium chloride particles are dried with hot air by using a two-stage vertical fluidized bed to reduce the water content to 2 - 1.8 mol H2O / mol MgCl2;

[0009] Step 3, finally, through an electric heating crucible furnace and introducing HCl gas, molten salt dehydration is carried out on MgCl2 containing 2 - 1.8 mol of water, controlling the temperature in the furnace to be 950 ± 10 °C, the HCl partial pressure ≥ 0.05 MPa, and drying MgCl2 in a continuous feeding and indirect discharging mode to obtain an anhydrous magnesium chloride melt with a water content of 0.02% - 0.15% and an MgO content ≤ 0.2%.

[0010] Furthermore, the inlet air volume of the granulation tower is 503000 ± 500 m 3 / h, the wind speed is 0.3 ± 0.02 m / s, and the inlet cooling air temperature ranges from 19 ± 2 °C.

[0011] Furthermore, the drying temperature of the first-stage fluidized bed is controlled at 135 ± 5 °C, and the drying temperature of the second-stage fluidized bed is controlled at 178 ± 5 °C.

[0012] Furthermore, the electric heating crucible furnace is directly connected to the electrolytic cell, and the anhydrous magnesium chloride melt treated in step 3 is siphon-transported to the electrolytic cell through a vacuum magnesium chloride ladle.

[0013] The present invention has the following advantages:

[0014] The present invention adopts a three-stage process to synergistically optimize and dehydrate in stages with high precision, high efficiency and energy saving. By cooling crystallization (step 1), fluidized bed classification drying (step 2) and molten salt dehydration (step 3), the water content is gradually reduced, avoiding excessive energy consumption or material decomposition caused by one-time high-temperature dehydration. In the molten salt dehydration stage, HCl gas (partial pressure ≥ 0.05 MPa) is introduced to effectively inhibit the hydrolysis of magnesium chloride to generate MgO, ensuring that the MgO content in the final product is ≤ 0.2%. The temperature of the electric heating crucible furnace is controlled at 950 ± 10 °C, which not only ensures complete dehydration (water content 0.02% - 0.15%), but also avoids excessive volatilization or decomposition of MgCl2, greatly inhibiting hydrolysis and improving the product purity. The prepared high-purity anhydrous magnesium chloride melt can be directly used for the production of electrolytic magnesium without additional treatment, reducing unnecessary process flows, while also reducing the operating cost of the electrolytic cell and extending the equipment life.

[0015] In summary, through innovative designs such as segmented dehydration, precise temperature control and molten salt dehydration, the present invention prepares anhydrous magnesium chloride with a water content of 0.02% - 0.15% and an MgO content of ≤ 0.2%, which can be directly used for the production of electrolytic magnesium, simplifies the process flow, reduces energy consumption and production costs, solves the problems of easy hydrolysis, high energy consumption, insufficient product purity and complex and difficult-to-control processes in the traditional magnesium chloride dehydration process, and has the advantages of high efficiency, energy saving, environmental protection and industrial feasibility.

Specific Embodiments

[0016] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention.

[0017] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installation", "connection", and "linkage" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0018] The present invention relates to a method for preparing anhydrous magnesium chloride by three-stage dehydration, and the steps of the method are as follows:

[0019] Step 1: The magnesium chloride solution is cooled and crystallized through a granulation tower to obtain magnesium chloride solid particles containing 4.8 - 5.0 mol of water.

[0020] Step 2: The magnesium chloride particles are dried with hot air by using a two-stage vertical fluidized bed to reduce the water content to 2 - 1.8 mol H2O / mol MgCl2.

[0021] Step 3: Finally, an electric heating crucible furnace is used and HCl gas is introduced to perform molten salt dehydration on MgCl2 containing 2 - 1.8 mol of water. The temperature in the furnace is controlled at 950 ± 10 °C, and the HCl partial pressure ≥ 0.05 MPa. The mode of continuous feeding and indirect discharging is adopted to dry MgCl2, and anhydrous magnesium chloride melt with a water content of 0.02% - 0.15% and an MgO content ≤ 0.2% is obtained. During the molten salt dehydration stage, HCl gas (partial pressure ≥ 0.05 MPa) is introduced to effectively inhibit the hydrolysis of magnesium chloride to generate MgO, ensuring that the MgO content in the final product ≤ 0.2%, meeting the requirements of the electrolysis process for low magnesium oxide. The temperature of the electric heating crucible furnace is controlled at 950 ± 10 °C, which not only ensures complete dehydration (water content 0.02% - 0.15%) but also avoids excessive volatilization or decomposition of MgCl2.

[0022] The present invention gradually reduces the water content through cooling crystallization (Step 1), fluidized bed classification drying (Step 2), and molten salt dehydration (Step 3), avoiding excessive energy consumption or material decomposition caused by one-time high-temperature dehydration. The three-stage process is synergistically optimized, with precise dehydration in stages, high efficiency, and energy conservation.

[0023] The inlet air volume of the granulation tower is 503000 ± 500 m 3 / h, the wind speed is 0.3 ± 0.02 m / s, and the inlet cooling air temperature is taken as 19 ± 2 °C.

[0024] The drying temperature of the first-stage fluidized bed is controlled at 135 ± 5 °C, and the drying temperature of the second-stage fluidized bed is controlled at 178 ± 5 °C. The first-stage (135 ± 5 °C) and second-stage (178 ± 5 °C) fluidized beds are used for staged drying, which not only prevents thermal decomposition of magnesium chloride caused by local overheating but also gradually reduces the moisture content to 1.8 - 2 mol, laying a foundation for subsequent high-temperature dehydration.

[0025] The electric heating crucible furnace is directly connected to the electrolytic cell, and the anhydrous magnesium chloride melt treated in step 3 is siphon-transported to the electrolytic cell through a vacuum magnesium chloride ladle. The direct connection between the electric heating crucible furnace and the electrolytic cell enables the dehydrated melt to be siphon-transported to the electrolytic cell through a vacuum ladle, avoiding intermediate cooling and secondary melting, reducing energy consumption and the risk of moisture absorption. At the same time, the high-purity anhydrous magnesium chloride melt obtained by dehydration can be directly used in the production of electrolytic magnesium, reducing the operating cost of the electrolytic cell, extending the equipment life, and saving the process.

[0026] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention. For those not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are followed. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be purchased commercially.

[0027] Embodiment

[0028] Step 1, cooling crystallization in the granulation tower: The magnesium chloride solution is concentrated to 48% - 51%. The magnesium chloride brine enters the granulation centrifuge, and the magnesium chloride brine forms droplets and is sprayed from the centrifuge into the granulation tower in the form of droplets. The droplet-shaped magnesium chloride brine falls in the granulation tower, makes countercurrent contact with the ambient air cooling air flow introduced through the louver system, and makes co-current contact with the air flow at the top of the tower, thereby controlling the particles to fall in an umbrella-shaped trajectory and the temperature of the air discharged from the top of the tower. As the droplet-shaped brine cools and crystallizes, solid particles (magnesium chloride solid particles containing 4.8 - 5.0 mol of water) are formed. A discharge hopper is provided at the bottom of the granulation tower to receive the granulated wet particles. The particles come out from the bottom of the granulation tower and are transported to a set of vibrating screens through a belt and bucket conveyor system. The large particles are removed by screening. The large particles are redissolved and granulated again. The remaining particles are sent to the wet material bin and then to the air fluidized bed dryer. Three sets of tail gas scrubbing systems are provided at the top of the granulation tower to scrub the tail gas to remove the chlorides therein, and the produced solution is used for hydrochloric acid scrubbing.

[0029] Step 2: Fluidized bed drying: The main purpose of the air fluidized bed dryer is to partially dry the MgCl₂ particles, from approximately 4.8 - 5.0 mol H₂O / mol MgCl₂ to approximately 1.8 - 2 mol H₂O / mol MgCl₂. The hydrated magnesium chloride particles in the wet particle storage bin are added to the air fluidized bed dryer through the feed pipe. The particles are added to the top tray of the two-stage vertical fluidized bed using hot air. The hot air, as the heating / drying medium, is added through the bottom of each stage. The electric heater of the air fluidized bed dryer heats the compressed air and supplies it to the gill feeder in the top tray, so that the wet particles are evenly spread on the surface of the fluidized bed. For ease of control, each air fluidized bed drying system selects two-stage fluidized beds, with a total of 2 sets. The drying temperature of the first-stage fluidized bed is controlled at approximately 135 °C, and the drying temperature of the second-stage fluidized bed is controlled at approximately 178 °C. Through system regulation, part of the secondary air is returned to the first-stage fluidized bed for use. The dried tail gas passes through a three-stage scrubbing tower of HCl scrubbing, fine scrubbing, and spraying to ensure that the tail gas meets the emission standards, and the produced acidic solution is returned to the hydrochloric acid system for use as a scrubbing liquid. The dried 1.8 - 2 mol H₂O / mol MgCl₂ is pneumatically transported to the molten salt drying system.

[0030] Step 3: Molten salt drying: The molten salt drying system is to set up an electrically heated crucible furnace, control the drying temperature in the furnace at 950 °C and the HCl partial pressure above 0.05 MPa, and dry magnesium chloride in a continuous feeding and indirect discharging mode, gradually removing the water molecules in MgCl₂ to obtain anhydrous magnesium chloride melt with a water content of 0.02% - 0.15% and an MgO content ≤ 0.2%. The vacuum magnesium chloride ladle is used to siphon the magnesium chloride melt from the crucible furnace, and in special cases, argon gas pressure is used for transportation. Then it is added to the electrolytic cell for the production of metallic magnesium. The crucible furnace discharges slag regularly according to the production situation. The crucible furnace slag is mainly composed of MgO and can be returned to the leaching process.

[0031] The parameters of each equipment are as follows:

[0032] (1) Granulation tower

[0033] In the first-phase project, 276.64 t / h of magnesium chloride solution needs to be processed;

[0034] The temperature of the 50% magnesium chloride solution is 186 °C;

[0035] The temperature of the crystallized product after cooling ≤ 60 °C;

[0036] The temperature of the cooling air at the inlet of the granulation tower is taken as 19 °C; (The annual average temperature at the project location is 6.1 °C, and the annual average temperature in summer is 19 °C)

[0037] The required cooling air volume is 2013767 m 3 / h;

[0038] The inlet air volume of the granulation tower is 503441 m 3 / h;

[0039] The cross-sectional wind speed of the granulation tower is taken as 0.3 m / s;

[0040] The calculated diameter of the empty tower is 24.37 m, and it is rounded to 24 m;

[0041] (2) Complete set of air-dried fluidized bed device

[0042] The amount of magnesium chloride to be processed in the first-phase project is 276.64 t / h;

[0043] Number of tower plates: 2 layers

[0044] The first layer of tower plate:

[0045] Surface area: 44 m 2

[0046] Fluidization velocity: 1.4 m / s

[0047] The second layer of tower plate:

[0048] Surface area: 32 m 2

[0049] Fluidization velocity: 0.97 m / s

[0050] Temperature of the granular material infeed: 60 °C

[0051] Temperature of the fluidized bed discharge: 178 °C

[0052] Hot air volume for drying is 1166958 m 3 / h. According to the fluidization velocity of 1.4 m / s, the area of the fluidized bed is 231.54 m 2 .

[0053] In summary, the present invention adopts a three-stage process for collaborative optimization and phased precise dehydration, which is highly efficient and energy-saving. By gradually reducing the water content through cooling crystallization (step 1), fluidized bed classification drying (step 2) and molten salt dehydration (step 3), it avoids excessive energy consumption or material decomposition caused by high-temperature dehydration at one time. In the molten salt dehydration stage, HCl gas (partial pressure ≥ 0.05 MPa) is introduced, which effectively inhibits the hydrolysis of magnesium chloride to generate MgO, ensuring that the MgO content in the final product is ≤ 0.2%. The temperature of the electric heating crucible furnace is controlled at 950 ± 10 °C, which not only ensures thorough dehydration (water content 0.02% - 0.15%), but also avoids excessive volatilization or decomposition of MgCl2, can greatly inhibit hydrolysis and improve the product purity; the prepared high-purity anhydrous magnesium chloride melt can be directly used for electrolytic magnesium production without additional treatment, reducing unnecessary process flows, while also reducing the operating cost of the electrolytic cell and extending the equipment life.

[0054] In summary, through innovative designs such as segmented dehydration, precise temperature control, and molten salt dehydration, the anhydrous magnesium chloride prepared by the present invention has a water content of 0.02% - 0.15% and an MgO content of ≤0.2%, and can be directly used in the production of electrolytic magnesium, simplifying the process flow, reducing energy consumption and production costs, solving the problems of easy hydrolysis, high energy consumption, insufficient product purity, and complex and difficult-to-control processes in the traditional magnesium chloride dehydration process, and having the advantages of high efficiency, energy conservation, environmental protection, and industrial feasibility.

[0055] Although the specific embodiments of the present invention have been described above, those skilled in the art should understand that the specific embodiments we described are illustrative rather than used to limit the scope of the present invention. Equivalent modifications and changes made by those skilled in the art in accordance with the spirit of the present invention should be covered by the scope protected by the claims of the present invention.

Claims

1. A method for preparing anhydrous magnesium chloride by three-stage dehydration, characterized in that: The method steps are as follows: Step 1, the magnesium chloride solution is cooled and crystallized through a granulation tower to obtain magnesium chloride solid particles containing 4.8-5.0 mol of water; Step 2: Using a two-stage vertical fluidized bed to dry the magnesium chloride particles with hot air to reduce the water content to 2-1.8 mol H2O / mol MgCl2; Step 3: Finally, the MgCl2 containing 2-1.8 mol of water is dehydrated by an electrically heated crucible furnace and HCl gas is introduced, the temperature in the furnace is controlled to be 950±10°C, the HCl partial pressure is ≥0.05MPa, and the MgCl2 is dried in a continuous feeding and indirect discharge mode to obtain an anhydrous magnesium chloride melt with a moisture content of 0.02%-0.15% and a MgO content of ≤0.2%.

2. A method for preparing anhydrous magnesium chloride by three-stage dehydration according to claim 1, characterized in that: The inlet air volume of the granulation tower is 503000±500m 3 / h, wind speed is 0.3±0.02m / s, and inlet cooling air temperature is 19±2℃.

3. A method for preparing anhydrous magnesium chloride by three-stage dehydration according to claim 1, characterized in that: The drying temperature of the primary fluidized bed is controlled at 135±5°C, and the drying temperature of the secondary fluidized bed is controlled at 178±5°C.

4. A method for preparing anhydrous magnesium chloride by three-stage dehydration according to claim 1, characterized in that: The electric heating crucible furnace is directly connected to the electrolytic cell, and the anhydrous magnesium chloride melt treated in step 3 is siphoned to the electrolytic cell through a vacuum magnesium chloride ladle.