Process for the production of magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide

Abstract

This invention discloses a method for preparing magnesium / magnesium alloys by the thermal reduction of magnesium chloride with calcium carbide. Calcium carbide is added to a molten salt containing MgCl2 to initiate a reduction reaction, and the reduction products are separated by forced convection. This invention, based on the reduction of MgCl2 with calcium carbide, possesses the thermodynamic advantages of a spontaneous, exothermic reaction, successfully avoiding the low tolerance for MgO impurities present in traditional molten salt electrolysis processes, thereby reducing the high dehydration cost of MgCl2·6H2O. Simultaneously, the process completely eliminates Cl2 generation, eradicating Mg-Cl2 side reactions, equipment corrosion, and Cl2 disposal procedures, shortening the production process. Furthermore, the use of forced convection technology achieves efficient product separation and significantly enhances reaction kinetics. The resulting C can be further used in energy storage materials for high-value utilization. Compared with traditional MgCl2 electrolysis processes, this method significantly reduces unit energy consumption, CO2 emissions, and production costs, demonstrating substantial economic and environmental benefits and broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical process technology, specifically relating to a method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride from calcium carbide. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Salt lake brines are rich in resources such as potassium (K), sodium (Na), lithium (Li), and magnesium (Mg), making them valuable strategic resources. However, the extraction of K and Li resources currently faces severe challenges. Taking the Qarhan Salt Lake in Qinghai Province as an example, the production of 1 ton of KCl or Li₂CO₃ produces 10-12 tons of MgCl₂·6H₂O as a byproduct. The utilization rate of this byproduct magnesium salt is less than 1%, and large quantities are discarded. This discarded MgCl₂·6H₂O redissolves into the brine, altering its original chemical composition. This not only severely restricts the sustainable exploitation of K and Li resources but also causes soil salinization and groundwater pollution, resulting in "magnesium hazard" in salt lakes. Therefore, achieving efficient and clean utilization of MgCl₂·6H₂O is a key objective in promoting sustainable metallurgy and a circular economy strategy.

[0004] Currently, the main method for utilizing MgCl2·6H2O is to convert it into metallic Mg. This process mainly involves two steps: dehydration and electrolysis. The key to this process is dehydrating MgCl2·6H2O into high-purity anhydrous MgCl2. During the dehydration process, H2O with four longer bonds is more easily removed. Heating in air at approximately 361°C yields MgCl2·2H2O. However, further heating promotes hydrolysis, generating MgO, which can impair subsequent electrolysis. Once the MgO content in the molten salt electrolyte exceeds 0.2 wt.%, serious operational problems arise: First, MgO impurities deposit on the surface of the steel cathode, forming a passivation layer that severely degrades the wettability of Mg droplets on the steel cathode, thus inducing the formation of fish-roe-like Mg droplets (diameter <1 mm). These dispersed, fish-roe-like Mg droplets are easily further encapsulated by MgO, preventing them from coalescing. This leads to severe loss of metallic Mg. Firstly, excessive MgO will inevitably reduce current efficiency. Secondly, it will react with the graphite anode to generate CO and CO2, which will accelerate the consumption of the graphite anode and significantly shorten its lifespan. Furthermore, the emission of CO and CO2 gases will increase environmental pollution. Thirdly, MgO will significantly increase the viscosity of the molten salt electrolyte, which will not only reduce the ionic conductivity of the electrolyte but also hinder the effective circulation of the electrolyte. This will hinder the separation of Mg droplets, electrolyte, and Cl2 generated by the anode during the circulation process, resulting in obstructed aggregation and floating of Mg droplets. It will also prolong the time that Mg droplets remain in the electrolyte and are exposed to Cl2, increasing the probability of side reactions between Mg droplets and Cl2, and further reducing current efficiency. To suppress the hydrolysis of MgCl2·2H2O and reduce the introduction of MgO impurities, dehydration is typically performed in a highly corrosive HCl environment at 300-400°C. This method not only places stringent requirements on related equipment in the production process but also significantly increases production costs; in existing technologies, this dehydration step alone accounts for approximately 50% of the total process cost of molten salt electrolysis. Furthermore, the molten salt electrolysis process itself faces the challenge of Cl2 precipitation at the anode. The highly corrosive Cl2 not only exacerbates equipment corrosion and increases equipment costs but also needs to be reintroduced into the PVC production chain along with calcium carbide for further processing, thus extending the overall production cycle. Therefore, developing a novel MgCl2 conversion pathway that can simultaneously overcome the limitations of MgO impurities and the problem of Cl2 precipitation is of significant theoretical and practical value for achieving the efficient, low-cost, and clean conversion of MgCl2·6H2O. Summary of the Invention

[0005] This invention aims to at least partially solve one of the technical problems in related technologies. Therefore, the main objective of this invention is to provide a method for preparing magnesium / magnesium alloys by the thermal reduction of magnesium chloride from calcium carbide, aiming to solve problems such as low tolerance for MgO impurities and anodic Cl2 precipitation in existing MgCl2 utilization processes, so as to further achieve efficient utilization of MgCl2·6H2O.

[0006] The objective of this invention is achieved through the following technical solution: In a first aspect, the present invention provides a method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide, wherein calcium carbide is added to a molten salt containing MgCl2 to carry out a reduction reaction, and the reduction products are separated by forced convection.

[0007] This invention uses calcium carbide as a reducing agent to reduce anhydrous MgCl2 in molten salt to obtain reduction products Mg, C and CaCl2; The chemical reaction formula for its reduction reaction is: MgCl2 + CaC2 = Mg + 2C + CaCl2; and by applying a flow field to the reduction process to achieve forced convection, and by utilizing the density difference between the reduction products Mg, C and CaCl2, Mg can be effectively collected above the molten salt and C is collected at the bottom of the molten salt, thus achieving three-phase separation. In some specific embodiments, the reduction route has the thermodynamic advantage of spontaneous and exothermic reaction, successfully avoiding the problem of low tolerance for MgO impurities in traditional molten salt electrolysis processes, thereby reducing the high dehydration cost of MgCl2·6H2O; the mass fraction of MgO in the molten salt is 0.1-10%.

[0008] In some specific embodiments, the molten salt includes one or more of alkali metals, alkaline earth metal chlorides, and alkaline earth metal fluorides.

[0009] In the method described in this application, the reaction between anhydrous MgCl2 and calcium carbide is a thermodynamically spontaneous exothermic process. As the temperature increases, the thermodynamic driving force of the forward reaction decreases, so a lower temperature is more conducive to the reaction. In order to reduce the operating temperature, alkaline earth metal chlorides and alkaline earth metal fluorides (such as NaCl, KCl, KF) are introduced to lower the melting point of the molten salt.

[0010] In some specific embodiments, the alkaline earth metal chloride includes one or more of NaCl, KCl, and CaCl2; the alkaline earth metal fluoride includes one or more of NaF, KF, and MgF2.

[0011] In some specific embodiments, the mass fraction of CaC2 in the calcium carbide is 10-90%, and / or the particle size of the calcium carbide is 0.001-50 mm.

[0012] In some specific embodiments, the MgCl2 in the molten salt has a mass fraction of 5-80%, and the mass ratio of MgCl2 to calcium carbide is 5-20:1.

[0013] In some specific embodiments, the reduction reaction is carried out at 300-1500°C for 1-20 hours.

[0014] In some specific embodiments, the forced convection strategy involves dynamically purging the molten salt system with inert gas at a flow rate of 10-200 mL / min for 10-30 min.

[0015] In some specific implementations, the forced convection strategy is achieved by purging argon gas into the molten salt, including but not limited to purging gas through a corundum gas pipe inserted from the top of the molten salt; Furthermore, to prevent the metal Mg from being oxidized, the entire reaction process is carried out in a sealed reactor, and argon gas is continuously introduced into the reactor for protection. In some specific embodiments, one or more of Sn, Zn, Al, Pb, Cu, Ag, Ga, Sb, and Bi are added to the molten salt.

[0016] In some specific embodiments, after the reduction is completed and the crucible has cooled to room temperature, it is removed, and Mg / Mg alloy beads are collected from above the molten salt and C is collected from the bottom. After washing with HCl and drying, high-purity C is obtained. The cooling process is carried out under argon protection to avoid oxidation of the metallic Mg / Mg alloy.

[0017] Secondly, the present invention also provides a method for preparing metallic Mg / Mg alloys and high-purity C using the aforementioned method.

[0018] Compared with the prior art, the present invention has at least the following advantages: 1) The method for preparing magnesium / magnesium alloy by thermal reduction of magnesium chloride with calcium carbide provided by the present invention is a new process based on the reduction of MgCl2 with calcium carbide; the reduction route has the thermodynamic advantages of spontaneous and exothermic reaction, and successfully avoids the problem of low tolerance of MgO impurities in the traditional molten salt electrolysis process, thereby reducing the high dehydration cost of MgCl2·6H2O. 2) The process of this invention completely eliminates Cl2 generation, eradicates Mg-Cl2 side reaction, equipment corrosion, and Cl2 disposal and treatment processes, and shortens the production process; 3) This application achieves efficient product separation and significant enhancement of reaction kinetics by using forced convection technology. The obtained C can be further used in energy storage materials to achieve high-value utilization, such as zinc-ion battery negative electrode protective coatings and supercapacitors. Compared with the traditional MgCl2 electrolysis process, the unit energy consumption, CO2 emissions and production costs are greatly reduced, showing huge economic and environmental benefits and broad application prospects. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below.

[0020] Figure 1 Thermodynamic calculations of the reduction reaction in the method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide provided by the present invention; Figure 2 This is a product diagram without forced convection in the method for preparing magnesium / magnesium alloy by thermal reduction of magnesium chloride with calcium carbide in Example 1 of the present invention; Figure 3 This is a product diagram under forced convection in the method for preparing magnesium / magnesium alloy by thermal reduction of magnesium chloride with calcium carbide in Example 2 of the present invention; Figure 4 The images show actual Mg bodies after forced convection under different conditions in the method for preparing magnesium / magnesium alloy by thermal reduction of magnesium chloride with calcium carbide in Example 3 of this invention. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are merely descriptive and not limiting, and should not be construed as limiting the scope of protection of the present invention.

[0022] When a quantity, concentration, or other value or parameter is described as a range, preferred range, or preferred upper and lower limits, it should be understood that it is equivalent to specifically disclosing any range by combining any pair of upper or preferred values ​​with any lower or preferred values, regardless of whether the range is specifically disclosed. Unless otherwise stated, the numerical range values ​​listed herein include the endpoints of the range and all integers and fractions within that range.

[0023] Unless otherwise stated, all percentages, parts, ratios, etc. in this document are by weight.

[0024] The materials, methods, and embodiments described herein are exemplary and should not be construed as limiting unless otherwise stated.

[0025] In the following embodiments, the calcium carbide used has a CaC2 mass fraction of 10-90% and a particle size of 0.001-50 mm; The molten salt used contains 0.1-10% MgO by mass. The raw material calcium carbide was of high purity and was purchased from Shanghai Maclean's Reagent Co., Ltd.; the other raw materials were of AR grade analytical grade and were also purchased from Shanghai Maclean's Reagent Co., Ltd.

[0026] Example 1

[0027] This embodiment provides a method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide, which includes the following steps: 1) Mix 72g MgCl2, 33g NaCl, 42g KCl and 3g KF evenly and place them in an alumina crucible to obtain molten salt; 2) Under an inert atmosphere, place the corundum crucible containing molten salt from step 1) into the reactor, and simultaneously add 5g of calcium carbide to the molten salt. Maintain the reaction temperature at 700℃ for 6 hours. After the reaction is complete, only small, unaggregated magnesium beads are collected, such as... Figure 2 As shown.

[0028] Example 2

[0029] This embodiment provides a method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide, which includes the following steps: 1) Mix 72g MgCl2, 33g NaCl, 42g KCl and 3g KF evenly and place them in an alumina crucible to obtain molten salt; 2) Under an inert atmosphere, place the corundum crucible containing molten salt from step 1) into the reactor, and add 5g of calcium carbide to the molten salt. Control the reaction temperature at 700℃ and maintain the reaction for 6h. 3) After the reaction was completed, forced convection was achieved by dynamically purging the reactor with inert gas at a flow rate of 30 mL / min for 25 min using a two-hole purging method, thereby realizing the separation of the Mg-molten salt-C three-phase products. The distribution of the three-phase products is as follows: Figure 3 As shown.

[0030] Example 3

[0031] This embodiment provides a method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide, which includes the following steps: 1) Mix 72g MgCl2, 33g NaCl, 42g KCl and 3g KF evenly and place them in an alumina crucible to obtain molten salt; 2) Under an inert atmosphere, place the corundum crucible containing molten salt from step 1) into the reactor, and add 5g of calcium carbide to the molten salt. Control the reaction temperature at 700℃ and maintain the reaction for 6h. 3) After the reaction was completed, forced convection was achieved by dynamically purging the reactor with inert gas at flow rates of 50, 80, and 100 mL / min for 25 minutes using a two-hole purging method. This resulted in the separation of the Mg-molten salt-C three-phase mixture. The actual magnesium beads in the reaction products were as follows: Figure 4 As shown, the Mg reduction rate reached 89.3% at a purging rate of 100 mL / min.

[0032] Comparative Example 1 This embodiment provides a method for preparing magnesium by molten salt electrolysis of magnesium chloride, which includes the following steps: 1) After mixing 72g MgCl2, 33g NaCl, 42g KCl, 3g KF and 5g CaCl2 evenly, a mixed molten salt is obtained; 2) Place the mixed molten salt from step 1) in a corundum crucible, using a graphite anode and a stainless steel cathode, at a temperature of 700°C and a pressure of 0.4 A / cm. 2 Constant current electrolysis was performed for 6 hours at a current density of [value missing].

[0033] In this electrolysis process, the current efficiency is 86% after electrolysis at 700℃ for 6 hours, and the average cell voltage is 3.3924V. Based on this, the energy consumption for producing 1 ton of Mg is calculated to be 8698.1 kWh. The method for preparing magnesium / magnesium alloys by the thermal reduction of magnesium chloride with calcium carbide provided in this application does not require additional energy consumption during the entire reaction process, reduces CO2 emissions, and significantly lowers production costs, demonstrating huge economic and environmental benefits and broad application prospects.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide, characterized in that, In a molten salt containing MgCl2, calcium carbide is added to carry out a reduction reaction, and the reduction products are separated by forced convection.

2. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 1, characterized in that, The molten salt contains 0.1-10% MgO by mass.

3. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 2, characterized in that, The molten salt also includes one or more of alkali metals, alkaline earth metal chlorides, and alkaline earth metal fluorides.

4. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 3, characterized in that, The alkaline earth metal chloride includes one or more of NaCl, KCl, and CaCl2; the alkaline earth metal fluoride includes one or more of NaF, KF, and MgF2.

5. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 1 or 2, characterized in that, The calcium carbide contains 10-90% CaC2 by mass and / or has a particle size of 0.001-50 mm.

6. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 5, characterized in that, The MgCl2 in the molten salt has a mass fraction of 5-80%, and the mass ratio of MgCl2 to calcium carbide is 5-20:

1.

7. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 1, characterized in that, The conditions for the reduction reaction are: reduction at 300-1500℃ for 1-20 hours.

8. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 7, characterized in that, The forced convection strategy involves dynamically purging the molten salt system with inert gas at a flow rate of 10-200 mL / min for 10-30 min.

9. The method for preparing magnesium / magnesium alloys by thermal reduction of magnesium chloride with calcium carbide according to claim 1, characterized in that, It also includes adding one or more of Sn, Zn, Al, Pb, Cu, Ag, Ga, Sb, and Bi to the molten salt.

10. A method according to any one of claims 1-9 for preparing metallic Mg / Mg alloys and high-purity C.

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