WO2022237511A1 - Method for producing metal aluminum by molten salt electrolysis of aluminum oxide - Google Patents

Abstract

The present invention belongs to the technical field of aluminum electrolysis, and in particular relates to a method for producing metal aluminum by molten salt electrolysis of aluminum oxide. The method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to the present invention uses an electrolytic cell. The electrolytic cell is divided into an anode chamber and a cathode chamber, and is filled with melts such as anolyte, catholyte and an alloy medium. The electrolytic cell is powered on to operate and an aluminum oxide material is added to obtain high-purity metal aluminum in the cathode chamber. The present invention provides an aluminum electrolysis method having the advantages of strong electrolysis operation adaptability, large selectivity of electrolysis materials and raw materials, being energy saving and environmentally friendly, and being capable of directly producing refined aluminum or high-purity aluminum.

Description

Method for producing metal aluminum by molten salt electrolytic alumina technical field

The invention belongs to the technical field of aluminum electrolysis, and in particular relates to a method for producing metallic aluminum by molten salt electrolysis of alumina.

Background technique

As an important light metal, aluminum is widely used in transportation, equipment, packaging, building materials, wires and other fields. In 2020, China's primary aluminum (electrolytic aluminum) output will be 37.08 million tons, ranking first among the top ten non-ferrous metals.

The current method of producing metal aluminum still adopts the traditional Hall-Heroult (Hall-Heroult) molten salt electrolysis process. The electrolysis equipment is a prebaked anode electrolytic cell composed of a carbon anode, a cryolite molten salt electrolyte, and a carbon cathode. , with alumina as raw material, primary aluminum is obtained by electrolysis at 900-960°C, while the carbon anode is continuously consumed and _CO2 -based gas is produced. Although this method has been widely used, it still faces many problems: ①The carbon anode consumes a lot, and the anode needs to be replaced every certain period, which affects the production efficiency, and the produced carbon dioxide contains CO ₂ and CO, SO ₂ , carbon The mixed gas of fluorine compounds pollutes the environment; ②Ordinary carbon cathodes have poor wettability to molten aluminum, which not only increases the cell voltage, but also produces a large number of waste cathode carbon blocks containing toxic substances after damage; ③The raw material alumina The chemical composition and physical properties of bauxite have high requirements, causing the upstream alumina industry to face the pressure of deep desiliconization, and it is difficult to economically utilize poor-quality bauxite and aluminum-containing secondary resources; ④ The purity of the primary aluminum of the electrolysis product is only 99.00~99.85%, Si, Fe and other impurity elements seriously affect the performance and application of primary aluminum; The production of refined aluminum by electrolytic refining requires more than 12000kW·h/t-Al of energy consumption.

technical problem

In order to solve some of the above problems, researchers have proposed many solutions, including low-temperature electrolytes, inert anodes, wettable cathodes, etc.

The purpose of using low-temperature electrolyte is to save energy and reduce consumption by reducing the electrolysis temperature, which includes: low molecular ratio sodium cryolite (Na ₃ AlF ₆ ) system, lithium cryolite (Li ₃ AlF ₆ ) system, potassium cryolite (K ₃ AlF ₆ ) system and mixed cryolite system, in which low molecular ratio sodium cryolite system or lithium cryolite system will lead to a significant decrease in the solubility and dissolution rate of alumina, while undissolved alumina raw materials are easy to sink into the lower part of the cathode aluminum liquid (The density of alumina raw material is greater than the density of cathode aluminum liquid), forming a cathode crust, causing disturbance and disorder in the electrolysis process, and affecting the normal progress of the electrolysis process. Although the potassium cryolite system has a good dissolution effect on alumina, K ⁺ has a serious corrosion and damage effect on the cathode carbon block at the bottom of the electrolytic cell, resulting in damage to the electrolytic cell or shortening the service life, so potassium salt is generally not allowed to be added into the molten salt electrolyte. Therefore, the current mainstream electrolyte for aluminum electrolysis still uses the sodium cryolite system, and the working temperature is above 900°C.

The use of inert anodes has the advantages of no greenhouse gas emissions and no need for frequent electrode changes. Inert anode materials have been extensively studied, but due to high temperature (>900 ° C) and fluoride molten salt working environment, inert anodes are prone to corrosion And damage, and the impurity ions produced are easy to enter the primary aluminum, causing product pollution, so the industry has not yet used inert anodes on a large scale. Wettable cathodes usually use a composite material of TiB ₂ and graphite, but they also face problems such as large amount of single cell, high cost, and TiB ₂ on the surface is easy to fall off and float up.

In short, the current method of electrolytically obtaining metal aluminum by using the industrial bulk product alumina as raw material in the traditional electrolytic cell not only strictly requires that the alumina be completely dissolved in the electrolyte, but also the purity of the metal aluminum product is difficult to guarantee. In addition, some of the above-mentioned methods are difficult to be applied in industrial applications due to problems such as poor adaptability, high operation requirements, and high cost.

technical solution

The object of the present invention is to provide a method for producing metal aluminum by electrolytic alumina with molten salt.

According to the method for producing metallic aluminum by molten salt electrolytic alumina in a specific embodiment of the present invention, the method is implemented by using an electrolytic cell, and the electrolytic cell is divided into an anode chamber and a cathode chamber. Alumina feedstock, in which a metallic aluminum product is obtained in said cathode compartment, wherein,

The anode chamber and the cathode chamber are used to physically separate the anode electrolyte from the cathode electrolyte, and the anode chamber is provided with an anode, and the cathode chamber is provided with a cathode;

The bottom of the electrolytic cell is also filled with an alloy medium, and the alloy medium is in contact with the anode electrolyte and the cathode electrolyte respectively, and is used for building an electrochemical reaction interface of aluminum ions/aluminum atoms and as a transmission medium for aluminum atoms.

The overall process of molten salt electrolytic alumina is as follows: in the anode chamber, alumina raw materials are added to the anode electrolyte, oxidation reaction occurs on the anode and gas is precipitated, and aluminum ions (dissolved and/or non-dissolved) in the anode chamber are deposited in the anode electrolyte The interface with the alloy medium is reduced to aluminum atoms and enters the liquid alloy medium; in the cathode chamber, the aluminum atoms in the alloy medium discharge at the interface between the cathode electrolyte and the alloy medium to form aluminum ions and enter the cathode electrolyte. Aluminum ions are reduced to aluminum atoms, forming metallic aluminum liquid, which floats on the catholyte.

When the electrolyzer is electrified and running, the anode current density is controlled at 0.1-1.5A/cm ² and the temperature is controlled at 700-950°C under normal working conditions. The specific working temperature depends on the specific composition of the anolyte, catholyte or liquid alloy, but it must be ensured that the working temperature is higher than the initial crystallization temperature of the anolyte or catholyte and the freezing point of the alloy medium. In the industrial application stage, the working temperature of the cathode chamber and the anode chamber can be the same or different, which can be achieved by different heat dissipation/heat generation conditions, such as adjusting the distance between the electrode and the alloy medium, forced heat dissipation, or the anode chamber and the cathode chamber Not adjacent in space.

The alumina raw material used in the present invention can be metallurgical grade alumina (refer to the industry standard "YS/T 803-2012 Metallurgical grade alumina"), or alumina with excessive impurities such as silicon or iron, or use physical properties (such as specific surface area, particle size distribution) alumina that does not meet the requirements of metallurgical grade, or use/mix some secondary aluminum-containing resources such as aluminum ash, aluminum slag, high-aluminum fly ash, waste alumina, etc.

In the metal aluminum product, the Al content is ≥99.90wt%, and the metal impurity content meets the product requirements of refined aluminum or high-purity aluminum.

According to the method for producing metallic aluminum by molten salt electrolysis of alumina according to a specific embodiment of the present invention, the anode electrolyte is a fluoride system or a chloride system.

According to the method for producing metal aluminum by molten salt electrolytic alumina in a specific embodiment of the present invention, the anode electrolyte is a fluoride system containing 60-90wt% cryolite, 5-30wt% AlF ₃ , 1-10wt% Al ₂ O ₃ , 0~15wt% additives, wherein, cryolite is one or more of Na ₃ AlF ₆ , Li ₃ AlF ₆ , K ₃ AlF ₆ , and the additives are LiF, NaF, KF, CaF ₂ , MgF ₂ , BaF _2. One or more of NaCl.

According to common knowledge in the field, the electrolyte containing 1:3 (molar ratio) of AlF ₃ , MeF (Me=Li, Na, K) and Me ₃ AlF ₆ (Me=Li, Na, K) is equivalent, Interchangeable. The above ingredients and composition are just a common expression, and there are many other expressions, for example, the mass fraction can be converted into the corresponding mole fraction; AlF ₃ , MeF (Me=Li, Na, K) two groups Substituting the Me ₃ AlF ₆ (Me=Li, Na, K) component, the electrolyte is composed of AlF ₃ , MeF (Me=Li, Na, K), Al ₂ O ₃ and additives.

The fluoride system anolyte has a certain solubility to the alumina raw material because it contains cryolite (Me ₃ AlF ₆ , Me=Li, Na, K), and the addition of AlF ₃ and other fluorine salts or chlorides can reduce the The physical and chemical properties such as the initial crystal temperature of the electrolyte and the electrical conductivity are adjusted. When the alumina raw material is added to the fluoride system, the alumina will undergo a dissolution reaction and generate dissolved aluminum-containing ions (such as AlF ₄ ⁻ , AlOF ₅ ^{4 −} and other aluminum-containing ions, which are collectively represented by Al ³⁺ ) and oxygen-containing ions (such as AlOF ₅ ^{4 −} , Al ₂ OF ₁₀ ^{6 −} and other oxygen-containing ions, which are collectively represented by O ^{2 −} ). Under the action of the electric field, the oxygen-containing ions in the anode chamber undergo oxidation reaction on the anode, and precipitate O ₂ or CO _x (x=1 or 2) gas, while the aluminum-containing ions are reduced at the interface between the anode electrolyte and the alloy medium React, generate aluminum atoms and enter into the alloy medium, the reaction formula is:

Graphite anode: O ^{2 −} −2e ⁻ +1/xC→1/xCO _x ↑ (x=1 or 2)

or inert anode: O ^{2 −} −2e ⁻ →0.5O ₂ ↑

Interface: Al ³⁺ +3e ⁻ →Al (alloy medium)

The solid alumina raw material at the interface between the alloy medium and the anode electrolyte (the density of the alloy medium is greater than that of the alumina raw material) can continue to dissolve in the anode electrolyte and supplement the aluminum ions that are continuously consumed at the interface to reduce concentration polarization And avoid the occurrence of side reactions, or directly carry out the reduction reaction at the interface to ensure that the aluminum ions in the anode chamber are continuously reduced to aluminum atoms and enter the alloy medium. The interface reaction is:

Al ₂ O ₃ +6e ⁻ →2Al (alloy medium)+3O ^{2 −}

According to the above principles, it can be seen that even undissolved or saturated alumina raw materials can still participate in the reaction at the interface, that is, the method and the electrolytic cell used have broken through the limitation that alumina should be completely dissolved in the electrolyte. Therefore, the fluoride anolyte can not only use the conventional sodium cryolite system, but also the low molecular ratio sodium cryolite system, lithium cryolite system and their mixed systems, which have slightly lower solubility for alumina but can reach low temperature For the purpose of electrolysis, of course, potassium cryolite system or potassium salt-containing cryolite system can also be used. They can not only achieve the purpose of low-temperature electrolysis, but also have good solubility for alumina, and there is no K ⁺ on the cathode carbon. Block damage.

Specifically, the fluoride system includes but is not limited to:

Conventional sodium cryolite system, containing 80~90wt% Na ₃ AlF ₆ , 5~15wt% AlF ₃ , 2~10t% Al ₂ O ₃ , 3~10wt% CaF ₂ , MgF ₂ , LiF, KF, NaCl one or more additives in

Low molecular ratio sodium cryolite system I, containing 60-85wt% Na ₃ AlF ₆ , 10-25wt% AlF ₃ , 1-10wt% Al ₂ O ₃ , 1-15wt% CaF ₂ , MgF ₂ , LiF, One or more additives in KF;

Low molecular ratio sodium cryolite system II, containing 50~75wt% Na ₃ AlF ₆ , 20~35wt% AlF ₃ , 1~8wt% Al ₂ O ₃ , and the content is not more than 10wt% of CaF ₂ , MgF ₂ , LiF, KF one or more additives in

Potassium cryolite system, containing 60-90wt% K ₃ AlF ₆ , 6-30wt% AlF ₃ , 1-10wt% Al ₂ O ₃ , and one of CaF ₂ , MgF ₂ , NaF and LiF with a content not greater than 10wt% or multiple additives;

Sodium-lithium cryolite system, containing 50-70wt% Na ₃ AlF ₆ , 5-45wt% Li ₃ AlF ₆ , 5-25wt% AlF ₃ , 1-8wt% Al ₂ O ₃ , CaF ₂ , One or more additives in MgF ₂ , KF;

Sodium potassium cryolite system I, containing 50-80wt% Na ₃ AlF ₆ , 5-30wt% K ₃ AlF ₆ , 10-30wt% AlF ₃ , 1-10wt% Al ₂ O ₃ , LiF with a content not greater than 10wt%, One or more additives in CaF ₂ and MgF ₂ ;

Sodium potassium cryolite system II, containing 30~50wt% Na ₃ AlF ₆ , 20~50wt% K ₃ AlF ₆ , 10~30wt% AlF ₃ , 1~10wt% Al ₂ O ₃ , 1~10wt% LiF, the content is not More than 5 wt% of CaF ₂ or/and MgF ₂ .

The systems listed above have their own characteristics. Take sodium potassium cryolite system II as an example. This system contains more K ₃ AlF ₆ components, which can improve the solubility of alumina raw materials, and together with the added AlF ₃ promote The reduction of the primary crystal temperature of the anode electrolyte can achieve the purpose of low-temperature electrolysis, while LiF is beneficial to improve the conductivity of the electrolyte. In contrast, this kind of electrolyte containing a large amount of potassium salt is not easily used in traditional electrolyzers, otherwise the penetration and damage of K ⁺ to the cathode carbon block at the bottom of the tank will seriously shorten the service life of the electrolyzer.

Further, in order to adjust the physical and chemical properties such as electrical conductivity and initial crystal temperature of the anolyte of the fluoride system, chlorides of alkali metals and alkaline earth metals can also be added to the system, but the total amount of chlorides added is not more than 5wt% , otherwise it will affect the stability of the electrolyte.

According to the method for producing metallic aluminum by molten salt electrolytic alumina in a specific embodiment of the present invention, the anode electrolyte is a chloride system, and the chloride system is CaCl ₂ , or CaCl ₂ and LiCl, NaCl, KCl, BaCl ₂ , CaF _2. One or more compositions in LiF, and the molar percentage of CaCl ₂ in it is not less than 50%;

The above-mentioned chloride system anolyte has very low solubility to alumina raw material, but has certain solubility to O ^{2 −} . When the alumina raw material is added to the anode electrolyte of the chloride system, under the action of an electric field, the solid alumina raw material directly undergoes a reduction reaction at the interface between the anode electrolyte and the alloy medium, and the aluminum ions in it are reduced to aluminum atoms, And into the alloy medium, the dissociated O ^{2 −} dissolves in the anode electrolyte and migrates to the anode, and then an oxidation reaction occurs on the surface of the anode. The reaction formula is:

Interface: Al ₂ O ₃ +6e ⁻ →2Al (alloy medium)+3O ^{2 −}

Graphite anode: O ^{2 −} −2e ⁻ +1/xC→1/xCO _x ↑ (x=1 or 2)

or inert anode: O ^{2 −} −2e ⁻ →0.5O ₂ ↑

Further, in order to adjust the physical and chemical properties of the chloride system anolyte, it is also possible to add alkali metal fluorides, alkaline earth metal fluorides, aluminum fluorides, and alkali metal oxides to the chloride system. , Oxides of alkaline earth metals. It is also possible to mix carbonaceous conductive agent or metal powder into the alumina raw material, shape and sinter the alumina raw material, so as to improve the electrochemical reactivity of the alumina raw material at the interface.

In the anode chamber, the impurities in the alumina raw material will have different electrochemical behaviors due to the difference in precipitation potential. Among them, impurities such as Li, Ca, and Na, which are more active than Al, will be enriched in the anode electrolyte, and impurities that are more inert than Al Fe, Si, Mn, Ti and other impurities will be reduced and enriched in the alloy medium.

According to the method for producing metallic aluminum by molten salt electrolysis of alumina according to a specific embodiment of the present invention, the catholyte is a pure fluoride system or a fluoride chloride system.

According to the method for producing metallic aluminum by molten salt electrolytic alumina in a specific embodiment of the present invention, the catholyte is a pure fluoride system, and the pure fluoride system contains 20-40wt% BaF ₂ , 30-50wt% AlF ₃ , 15 ~40wt% NaF and an additive with a content not greater than 20wt%, and the additive is one or more of CaF ₂ , LiF, Li ₃ AlF ₆ , and MgF ₂ .

According to the method for producing metal aluminum by molten salt electrolytic alumina in a specific embodiment of the present invention, the catholyte is a fluoride chloride system containing 50-70wt% BaCl ₂ , 15-30wt% AlF ₃ , 10-30wt% NaF and 0 ~15wt% additive, the additive is one or more of LiF, Li ₃ AlF ₆ , CaF ₂ , MgF ₂ , NaCl, LiCl, CaCl ₂ , MgCl ₂ .

In the cathode chamber, the aluminum atoms in the alloy medium discharge at the interface between the alloy medium and the catholyte, and the generated Al ³⁺ (Al ³⁺ represents the ions of all aluminum-containing elements such as AlF ₄ ⁻ ) enters the catholyte, and the catholyte The Al ³⁺ in the cathode is reduced to aluminum atoms at the interface between the cathode or the metal aluminum liquid and the catholyte, and enters into the liquid metal aluminum product. The reaction formula is:

Interface: Al (alloy medium)−3e ⁻ →Al ³⁺

Cathode: Al ³⁺ +3e ⁻ →Al (metal aluminum liquid)

However, impurities such as Fe, Si, and Mn in the alloy medium are not as active as Al in electrochemical properties, so they do not undergo oxidation reactions and continue to remain in the liquid alloy medium, thus having little impact on the cathode product metal aluminum. However, as the electrolysis proceeds, impurities such as Fe and Si in the alloy medium are continuously enriched and the concentration is continuously increased. At this time, the liquid alloy medium needs to be extracted for purification, and the purified liquid alloy is returned to the electrolytic cell to continue working. For example, the extracted alloy medium can be preferentially crystallized by coagulation method to precipitate iron-containing intermediate metal phase or elemental silicon with high melting point, or use the alloy medium as an anode to oxidize and precipitate Al, Fe, Si and other elements in it by electrolytic method.

According to the method for producing metallic aluminum by molten salt electrolysis of alumina according to a specific embodiment of the present invention, the anode is a carbon anode or an inert anode.

Among them, the inert anode includes ceramic materials (such as SnO ₂ and doped SnO ₂ , NiFe ₂ O ₄ , CaTiO _{3 , CaRuO 3 , CaRux} Ti _{1 − x} O ₃ , ITO), metal materials (such as Cu-Al alloy, Ni -Fe alloy, Ni-Fe-Cu alloy), cermet composite materials (such as Cu-NiFe ₂ O ₄ , Cu-NiO-NiFe ₂ O ₄ , Ni-NiO-NiFe ₂ O ₄ , Cu-Ni-NiO-NiFe ₂ O ₄ , Ni-CaRu _x Ti _{1 − x} O ₃ ).

According to the method for producing metallic aluminum by molten salt electrolysis of alumina according to a specific embodiment of the present invention, the cathode is one or more of graphite, aluminum, and inert wettable cathode materials (such as TiB ₂ , TiB ₂ /C, ZrB ₂ ). complex.

According to the method for producing metallic aluminum by molten salt electrolytic alumina according to a specific embodiment of the present invention, the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In, and Sb, preferably Al - Cu alloy, wherein the Al content is 40-75wt%; the alloy medium remains in a liquid state during normal electrolysis, and has a density greater than that of the anolyte and the catholyte.

The composition of the alloy medium can be determined from the alloy phase diagram according to the specific working temperature. For example, for the Al-Cu alloy with an Al content of 40-75wt%, its melting point is below 700°C, so any temperature between 700-950°C The Al-Cu alloy medium of this composition can be used in the electrolysis operation below.

When using a fluoride system or chloride system anode electrolyte with poor solubility for alumina raw materials, or when the feeding speed is fast, the content of Al in the alloy medium can be appropriately reduced to ensure that the density of the alloy medium is greater than that of the alumina raw material density.

Beneficial effect

(1) The raw material requirement is low, and the product purity is high. The electrolytic cell adopted in the method has the function of purifying and removing impurities. Based on the electrode potential difference of different elements, the impurities in the alumina raw material, among which elements more active than aluminum (such as Li, Na, K, Ca, Mg) will be enriched in the electrolyte, while elements more inert than aluminum ( For example, Si, Ti, Fe, V, Mn) will be enriched in the alloy medium, and it is difficult for them to enter the metal aluminum product, which not only ensures the purity of the metal aluminum product (aluminum content ≥ 99.90wt%), but also The requirements for the content of impurities in alumina raw materials can be appropriately relaxed, especially silicon and iron.

(2) Strong adaptability and continuous production. This method uses the industrial bulk product alumina as raw material to directly and continuously produce high-purity metal aluminum products, avoiding the existing process of first electrolyzing alumina and then refining raw aluminum to obtain refined aluminum, shortening production time and saving production costs . In addition, the bottom layer of the electrolytic cell is a liquid alloy medium containing heavy metals and aluminum. Even if excessively added or partially supersaturated alumina raw materials will continue to participate in dissolution or electrochemical reactions at the interface between the liquid alloy medium and the anode electrolyte, thereby improving This not only improves the operational adaptability of the electrolytic cell, but also relaxes the requirements for the solubility of alumina raw materials.

(3) Wide choice of electrolytes. This method avoids the requirements of industrial electrolytes on the solubility of alumina raw materials, and also avoids the use of cathode carbon blocks at the bottom of the tank that are sensitive to K ⁺ , so conventional sodium cryolite system electrolytes and various low-temperature electrolytes (including low molecular ratio The sodium cryolite system, lithium cryolite system, potassium cryolite system and their mixed systems), and the chloride system can also be used as the anode electrolyte. The use of low-temperature electrolyte is conducive to energy saving and consumption reduction in the aluminum electrolysis industry.

(4) An inert anode can be used. After adopting the low-temperature electrolyte of the fluoride system or the chloride system, the strong corrosion of the inert anode by the fluoride salt at high temperature is avoided, and the service life of the inert anode is prolonged. The impurity elements produced by the corrosion of the inert anode will also remain in the electrolyte or alloy medium, and it is difficult to enter the metal aluminum product, so the purity of the product can be guaranteed. In addition, clean and harmless O ₂ is produced on the surface of the inert anode, and no greenhouse gas CO ₂ or toxic and harmful gases are produced.

(5) Energy saving and environmental protection. The continuous production of high-purity metal aluminum can be realized in only one electrolytic tank, and the space utilization rate is high, which reduces the loss of heat accordingly. Combined with the use of low-temperature electrolytes and inert anodes, it can further save electric energy and improve electric energy efficiency, and there is no emission of greenhouse gases or toxic gases, and no waste residues such as waste cathode carbon blocks.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. Those skilled in the art can also obtain other drawings based on these drawings without creative work.

Fig. 1 is the sectional schematic view of electrolyzer of the present invention;

In the figure, 1-anode, 2-electrolyzer body, 3-anode electrolyte, 4-alloy medium, 5-cathode electrolyte, 6-metal aluminum product, 7-cathode.

Embodiments of the present invention

In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be described in detail below. Apparently, the described embodiments are only some of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other implementations obtained by persons of ordinary skill in the art without making creative efforts fall within the protection scope of the present invention.

The method for producing metallic aluminum by molten salt electrolytic alumina of the present invention is to conduct electrified operation at a temperature of 700-950° C., and the anode current density is 0.1-1.5 A/cm ² . The alumina raw material is added to the anode electrolyte in the anode chamber, the anode is discharged and gas is precipitated, and the aluminum ions (dissolved and/or non-dissolved) are reduced and enter the liquid alloy medium; at the same time, in the cathode chamber, the liquid The aluminum atoms in the alloy medium discharge at the interface to form aluminum ions and enter the catholyte. The aluminum ions in the catholyte are reduced to metal aluminum atoms and enter the aluminum liquid floating above the catholyte.

The feed rate of the alumina raw material is calculated and determined by Faraday's law according to the current intensity and current efficiency.

In the present invention, the anode electrolyte and the cathode electrolyte are physically separated by the electrolytic cell, and both the anode electrolyte and the cathode electrolyte are in contact with the alloy medium, thereby completing the interface reaction of aluminum ions/aluminum atoms and the migration of aluminum atoms by means of the alloy medium. Therefore, in order to effectively realize the separation of the cathode electrolyte and the anode electrolyte, the structure of the electrolytic cell is shown in Figure 1 .

The electrolytic cell body 2 is spatially divided into an anode chamber and a cathode chamber. The anode compartment contains the anode electrolyte 3, the anode 1 is inserted into the anode electrolyte 3, the cathode compartment contains the catholyte 5, the cathode 7 is inserted into the catholyte 5 or the liquid metal aluminum product 6, and the bottom of the electrolytic cell contains an alloy medium 4, respectively Anolyte 3 is in contact with catholyte 5 , but not with anode 1 or cathode 7 .

In addition to the electrolytic cell shown in Figure 1, the structure of the electrolytic cell can also be designed in various forms, such as a U-shaped electrolytic cell. In addition, the shape of the electrolytic cell can be varied. For example, the bottom of the electrolytic cell is not limited to a round bottom, and can also be Trapezoidal bottom, flat bottom.

An aluminum electrolytic cell capable of realizing the physical separation of the anode electrolyte and the cathode electrolyte and the conduction of the alloy medium can be applied to the method of the present invention.

In large-scale applications, the electrolytic cells can be connected in series or in parallel.

Example 1

The bottom of the electrolytic cell contains a pre-alloyed Cu-Al alloy, in which the Al content is 60wt%, and the anode and cathode are graphite rods.

The composition of the anolyte is: 80.3wt% Na ₃ AlF ₆ + 12.2wt% AlF ₃ + 2.5wt% Al ₂ O ₃ + 3.0wt% CaF ₂ + 1.0wt% MgF ₂ + 1.0wt% LiF,

The catholyte composition is: 35.0wt% BaF ₂ +30.0wt% AlF ₃ +30.0wt% NaF+5.0wt% CaF ₂ .

Raise the temperature of the electrolytic cell to 940°C, keep it warm for 2 hours, and control the anode current density at 1.2A/cm ² by energizing it. After the electrolysis starts, regularly add alumina raw materials containing 95.5wt% Al ₂ O ₃ , and the total electrolysis time is 10 hours.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.980%.

Example 2

The bottom of the electrolytic cell contains a pre-alloyed Cu-Al-Zn alloy, in which the contents of Al and Zn are 60wt% and 5wt%, respectively, the anode is a graphite rod, and the cathode is _TiB2 coated graphite.

The composition of the anolyte is: 50.0wt% Na ₃ AlF ₆ +30.0wt% Li ₃ AlF ₆ +13.5wt% AlF ₃ +2.0wt% Al ₂ O ₃ +3.0wt% CaF ₂ +1.5wt% MgF ₂ ,

The catholyte composition is: 65.0wt% BaCl ₂ +20.0wt% AlF ₃ +13.0wt% NaF+2.0wt% NaCl.

Raise the temperature of the electrolytic cell to 860°C, keep it warm for 2 hours, and pass in direct current to control the anode current density at 0.6A/cm ² . After the electrolysis starts, regularly add alumina raw materials containing 91.2wt% Al ₂ O ₃ , and the total electrolysis time is 10h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.970%.

Example 3

The bottom of the electrolytic cell contains pre-alloyed Cu-Al alloy with 50wt% Al content, the anode is a graphite rod, and the cathode is TiB2 _- coated graphite.

The anolyte is CaCl ₂ ,

The catholyte composition is: 20.0wt% BaF ₂ +35.0wt% AlF ₃ +30.0wt%NaF+15.0wt%CaF ₂ .

Raise the temperature of the electrolytic cell to 850°C, keep it warm for 2 hours, pass in direct current to control the anode current density at 0.7A/cm ² , and regularly add alumina raw materials with an Al ₂ O ₃ content of 98.8wt% before and after the start of electrolysis , The total electrolysis time is 10h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.989%.

Example 4

The bottom of the electrolytic cell contains pre-alloyed Cu-Al alloy, in which the content of Al is respectively 62wt%. The anode is an inert anode of 15wt%Fe-70wt%Cu-35wt%Ni alloy material, and the cathode is graphite.

The composition of the anolyte is: 70.0wt% K ₃ AlF ₆ + 21.5wt% AlF ₃ + 3.5wt% Al ₂ O ₃ + 4.0wt% LiF + 1.0wt% CaF ₂ ,

The catholyte composition is: 55.0wt% BaCl ₂ +27.0wt% AlF ₃ +16.0wt% NaF+2.0wt% CaF ₂ .

Raise the temperature of the electrolytic cell to 870°C, keep it warm for 2 hours, and pass in direct current to control the anode current density at 0.8A/cm ² . After the electrolysis starts, the alumina raw material containing 98.7wt% Al ₂ O ₃ is added regularly. The total electrolysis time is 10h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.995%.

Example 5

The bottom of the electrolytic cell contains a pre-alloyed Sn-Al alloy with an Al content of 20wt%, the anode is a graphite anode, and the cathode is TiB2 _- coated graphite.

The anolyte composition is CaCl ₂ -LiCl with a molar ratio of 1:1,

The catholyte composition is: 60.0wt% BaCl ₂ +20.0wt% AlF ₃ +15.0wt% NaF+5.0wt% LiCl.

Raise the temperature of the electrolytic cell to 800°C, keep it warm for 2 hours, pass in direct current to control the anode current density at 0.4A/cm ² , add alumina raw materials containing 97.2wt% Al ₂ O ₃ regularly before and after the start of electrolysis, The total electrolysis time is 15h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.986%.

Example 6

The bottom of the electrolytic cell is filled with pre-alloyed In-Al alloy, wherein the content of Al is 10wt%, and the anode and cathode are graphite rods.

The composition of the anolyte is: 65.0wt% Na ₃ AlF ₆ +20.5wt% AlF ₃ +3.5wt% Al ₂ O ₃ +8.0wt% KF+3.0wt% CaF ₂ ,

The catholyte composition is: 25.0wt% BaF ₂ +36.0wt% AlF ₃ +27.0wt%NaF+10.0wt%CaF ₂ +2.0wt% Li ₃ AlF ₆ .

Raise the temperature of the electrolytic cell to 900°C, keep it warm for 2 hours, and pass in direct current to control the anode current density at 0.5A/cm ² . After the electrolysis starts, the alumina raw material containing 98.9wt% Al ₂ O ₃ is added regularly. The total electrolysis time is 12h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.994%.

Example 7

The bottom of the electrolytic cell contains pre-alloyed Cu-Al alloy, in which the content of Al is 45wt%, the anode is an inert anode of CaRuO ₃ ceramic material, and the cathode is a TiB ₂ /C composite material.

The anolyte composition is CaCl ₂ -NaCl-CaO with a molar ratio of 50:48:2,

The catholyte composition is: 60.0wt% BaCl ₂ +23.0wt% AlF ₃ +17.0wt% NaF.

Raise the temperature of the electrolytic cell to 840°C, keep it warm for 2 hours, and pass in direct current to control the anode current density at 0.2A/cm ² . Before and after electrolysis, alumina raw materials containing 94.6wt% Al ₂ O ₃ are regularly added. The total electrolysis time is 20h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.975%.

Example 8

The bottom of the electrolytic cell contains a pre-alloyed Cu-Al alloy, in which the Al content is 70wt%, the anode is an inert anode of NiFe ₂ O ₄ -18wt%NiO-17wt%Cu cermet composite material, and the cathode is a graphite rod.

The composition of the anolyte is: 42.3wt% Na ₃ AlF ₆ + 28.2wt% K ₃ AlF ₆ + 23.0wt% AlF ₃ + 2.5wt% Al ₂ O ₃ + 4.0wt% LiF,

The catholyte composition is: 22.0wt% BaF ₂ +46.0wt% AlF ₃ +26.0wt% NaF+4.0wt%CaF ₂ +2.0wt% LiF.

Raise the temperature _of the electrolytic cell to 880°C, keep it warm for ₂ hours, and pass in direct current to control the anode current density at 1.0A/cm ² . The time is 10h.

After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.999%.

Example 9

The difference between this comparative example and Example 4 is: in the composition of the anolyte of 70.0wt% K ₃ AlF ₆ +21.5wt% AlF ₃ +3.5wt% Al ₂ O ₃ +4.0wt%LiF+1.0wt% CaF ₂ On the basis, 10wt% of Al ₂ O ₃ is added, the alloy medium is a Cu-Al alloy with an Al content of 50wt%, and other conditions are the same. After the electrolysis, the Al content in the cathode product metal aluminum was determined to be 99.991%, and there were still some undissolved alumina raw materials on the alloy medium.

It can be inferred that even if the alumina raw material is added too quickly or in excess, it can still stay on the alloy medium and continue to participate in the dissolution/electrochemical reaction, maintaining the continuous operation of the electrolysis process, and the purity of the obtained metal aluminum product is still high.

comparative example 1

The difference between this comparative example and Example 1 lies in that: the inner bottom of the electrolytic cell does not hold the alloy medium. The electrolyte is the same as the anolyte in Example 1, and other conditions are the same. After the electrolysis, the product metal aluminum in the electrolytic cell was taken out, wherein the Al content was determined to be 97.1%.

It shows that in the absence of alloy medium, the separation and removal effect based on the electrochemical reaction of the alloy medium/electrolyte interface is lost, and impurity elements such as Si and Fe directly enter the metal aluminum product, resulting in a decrease in product purity/grade.

comparative example 2

The difference between this comparative example and Example 4 is that there is no alloy medium in the bottom of the electrolytic cell, and the composition is 70.0wt% K ₃ AlF ₆ +21.5wt%AlF ₃ +3.5wt% Al ₂ O ₃ +4.0wt%LiF On the basis of the electrolyte of +1.0wt% CaF ₂ , add 10wt% Al ₂ O ₃ (because there is no separation effect of the alloy medium, so there is only the above electrolyte in the electrolytic cell), other conditions are the same. After the electrolysis, the product metal aluminum in the electrolytic tank was taken out, and the Al content was determined to be 97.8%, containing Fe, Si, Cu, Ni and other impurity elements, and some undissolved alumina raw materials were found at the bottom of the tank.

It shows that the alumina raw material added too quickly or in excess is difficult to effectively participate in the dissolution/electrochemical reaction, so a precipitate is formed at the bottom of the tank. In addition, the separation and removal effect based on the electrochemical reaction of the alloy medium/electrolyte interface is lost. The impurity element Si in the alumina raw material, Cu, Ni, Fe and other elements produced after the surface of the inert anode is corroded directly enter the metal aluminum product. In this process, the purity of the metal aluminum product is reduced.

comparative example 3

The bottom of the electrolytic tank is filled with aluminum with an Al content of 99.8%, and the anode and cathode are graphite rods. Both the anolyte and catholyte are: 60.0wt% Na ₃ AlF ₆ +12.0wt% AlF ₃ +5.0wt% NaF+2.0wt% CaF ₂ +1.0wt% MgF ₂ +10.0wt% NaCl+10.0wt% KCl, the The electrolytic cell is placed in an atmosphere filled with dry argon, the temperature is programmed to rise to 940°C, and the temperature is kept for ₂ hours. _The anode current density is controlled at 0.8A/cm ² by electrification. After the electrolysis starts, the metallurgical Grade alumina raw material, the feeding speed is calculated and determined by Faraday's law according to the current intensity and current efficiency, the total electrolysis time is 10h, and irritating chlorine gas is found to be generated during the electrolysis process, and the electrolysis voltage fluctuates greatly. After the electrolysis, no metal aluminum product was found on the surface of the cathode electrolyte in the cathode chamber, and there was still unreacted alumina raw material under the metal aluminum at the bottom of the electrolytic cell, forming a precipitate/crust at the bottom of the cell.

comparative example 4

The bottom of the electrolytic tank is filled with aluminum with an Al content of 99.8%, and the anode and cathode are graphite rods. Both the anolyte and catholyte are: 60.0wt% _Na3AlF6 + 12.0wt% _AlF3 + _5.0wt % _NaF + 2.0wt% _CaF2 + 1.0wt% MgF2 + 10.0wt% NaCl + 10.0wt% KCl, and Place high-purity aluminum with an Al content of 99.999% on the surface of the catholyte, place the electrolytic cell in an atmosphere filled with dry argon, program the temperature to 940°C, and keep it warm for 2 hours. on the surface of the cathode electrolyte, but automatically settles to the bottom of the tank, that is, the electrochemical system as shown in Figure 1 cannot be formed.

comparative example 5

The bottom of the electrolytic tank is filled with aluminum with an Al content of 99.8%, and the anode and cathode are graphite rods. The anolyte composition is: 60.0wt% Na ₃ AlF ₆ +12.0wt% AlF ₃ +5.0wt% NaF+2.0wt% CaF ₂ +1.0wt% MgF ₂ +10.0wt% NaCl+10.0wt% KCl, and the catholyte composition is : 35.0wt% BaF ₂ +30.0wt% AlF ₃ +30.0wt% NaF+5.0wt% CaF ₂ , place the electrolytic cell in an atmosphere filled with dry argon, program the temperature to 940°C, and keep it warm for 2 hours, during which electrolysis is found The metal aluminum liquid at the bottom of the tank automatically floats to the surface of the catholyte, that is, the electrochemical system as shown in Figure 1 cannot be formed.

comparative example 6

The corundum crucible contains a molten salt with a composition of 35.0wt% BaF ₂ +30.0wt% AlF ₃ +30.0wt% NaF+5.0wt% CaF ₂ , and an excess of metallurgical grade oxide containing 98.8wt% Al ₂ O ₃ The aluminum raw material is kept at 940°C for 2 hours to fully dissolve, and then the molten salt dissolved in saturated Al ₂ O ₃ is taken out by decantation, while the undissolved alumina raw material and residual molten salt remain in the corundum crucible.

The bottom of the electrolytic cell contains a pre-alloyed Cu-Al alloy with an Al content of 60wt%. Both the anode and cathode are graphite rods. _The above molten salt dissolved with saturated _Al2O3 serves as the anolyte and the catholyte is 35.0wt%. BaF ₂ +30.0 wt% AlF ₃ +30.0 wt% NaF+5.0 wt% CaF ₂ . Place the electrolytic cell in an atmosphere filled with dry argon, program the temperature up to 940°C, and keep it warm for 2 hours, then electrify to control the anode current density at 0.8A/cm ² . .

The reason for the above phenomenon may be that there is relatively more Al ₂ O ₃ dissolved in the anode electrolyte at the beginning of electrolysis, which can maintain the normal operation of electrolysis, but the electrolyte resistance is relatively large, and the cell voltage is also relatively large. As the electrolysis proceeds, the Al ₂ O ₃ in the anode electrolyte is continuously consumed, and the electrolyte resistance becomes smaller, but the concentration polarization leads to the occurrence of the anode effect, and the voltage fluctuates and increases sharply in the later stage.

In addition, the anode electrolyte contains a large amount of barium salt, and the addition of barium salt will greatly reduce the solubility of alumina, so that the anode electrolyte needs to be frequently taken out, transported and dissolved, and the production efficiency is low.

The above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Anyone skilled in the art can easily think of changes or substitutions within the technical scope disclosed in the present invention. Should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (10)

1. A method for producing metal aluminum by molten salt electrolysis of alumina, characterized in that,

   The method is implemented using an electrolytic cell, the electrolytic cell is divided into an anode chamber and a cathode chamber, under the condition of energized operation, the alumina raw material is put into the anode chamber, and a metal aluminum product is obtained in the cathode chamber, wherein,

   The anode chamber and the cathode chamber are used to physically separate the anode electrolyte from the cathode electrolyte, and the anode chamber is provided with an anode, and the cathode chamber is provided with a cathode;

   The bottom of the electrolytic cell is also filled with an alloy medium, and the alloy medium is in contact with the anode electrolyte and the cathode electrolyte respectively, and is used for building an electrochemical reaction interface of aluminum ions/aluminum atoms and as a transmission medium for aluminum atoms.
2. The method for producing metallic aluminum by molten salt electrolysis of alumina according to claim 1, characterized in that the anode electrolyte is a fluoride system or a chloride system.
3. The method for producing metallic aluminum by molten salt electrolytic alumina according to claim 1 or 2, characterized in that the anolyte is a fluoride system, and the fluoride system contains 60-90wt% cryolite, 5-30wt% AlF ₃ , 1-10wt% Al ₂ O ₃ , 0-15wt% additive; the cryolite is one or more of Na ₃ AlF ₆ , Li ₃ AlF ₆ , K ₃ AlF ₆ , and the additive is One or more of LiF, NaF, KF, CaF ₂ , MgF ₂ , BaF ₂ , NaCl.
4. The method for producing metallic aluminum by molten salt electrolytic alumina according to claim 1 or 2, wherein the anolyte is a chloride system, the chloride system is CaCl ₂ , or the chloride system is composed of CaCl ₂ and one or more of NaCl, KCl, BaCl ₂ , CaF ₂ , LiCl, CaO, and the molar percentage of CaCl ₂ in the chloride system is not less than 50%.
5. The method for producing metallic aluminum by molten salt electrolysis of alumina according to claim 1, characterized in that the catholyte is a pure fluoride system or a fluoride chloride mixed system.
6. The method for producing metallic aluminum by molten salt electrolytic alumina according to claim 1 or 5, characterized in that the catholyte is a pure fluoride system, and the pure fluoride system contains 20-40wt% BaF ₂ , 30-40wt% 50wt% AlF ₃ , 15-40wt% NaF and 0-20wt% additive; the additive is one or more of CaF ₂ , LiF, Li ₃ AlF ₆ , MgF ₂ .
7. The method for producing metallic aluminum by molten salt electrolysis of alumina according to claim 1 or 5, characterized in that the catholyte is a fluoride chloride system, and the fluoride chloride system contains 50-70 wt% BaCl ₂ , 15-70 wt% 30wt% AlF ₃ , 10-30wt% NaF and 0-15wt% additive; the additive is one or more of LiF, Li ₃ AlF ₆ , CaF ₂ , MgF ₂ , NaCl, LiCl, CaCl ₂ , MgCl ₂ .
8. The method for producing metallic aluminum by molten salt electrolysis of alumina according to claim 1, wherein the anode is a carbon anode or an inert anode.
9. The method for producing metallic aluminum by molten salt electrolysis of alumina according to claim 1, wherein the cathode is one or more composites of graphite, aluminum, and inert wettable cathode materials.
10. The method for producing metallic aluminum by molten salt electrolytic alumina according to any one of claims 1 to 7, wherein the alloy medium is one of Al and Cu, Sn, Zn, Ga, In, Bi, Sb One or more alloys formed, preferably Al-Cu alloy, wherein the Al content is 40-75wt%;

    The alloy medium remains liquid during normal electrolysis and has a density greater than that of the anolyte or the catholyte.