METHOD FOR PRODUCING METAL ALUMINUM BY MOLTEN SALT ELECTROLYSIS OF ALUMINUM OXIDE

- CENTRAL SOUTH UNIVERSITY

A method for producing metal aluminum by molten salt electrolysis of aluminum oxide is provided. The method for producing metal aluminum by molten salt electrolysis of aluminum oxide 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 raw material is added to the anode chamber to obtain high-purity metal aluminum in the cathode chamber. The disclosure 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.

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Description
CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is the national phase entry of International Application No. PCT/CN2022/088925, filed on Apr. 25, 2022, which is based upon and claims priority to Chinese Patent Application No. 202110499895.8, filed on May 8, 2021, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The disclosure belongs to the technical field of aluminum electrolysis, and specifically relates to a method for producing metal aluminum by molten salt electrolysis of aluminum oxide.

BACKGROUND

As an important light metal, aluminum is widely used in transportation, equipment, packaging, building materials, electric wires, etc. In 2020, China's primary aluminum (electrolytic aluminum) production was 37.08 million tons, ranking first among the top ten nonferrous metals.

The current method for producing metal aluminum still uses the traditional Hall-Heroult molten salt electrolysis process. In this process, by using a pre-baked anode electrolytic cell including carbon anode, cryolite molten salt electrolyte and carbon cathode, an aluminum oxide raw material is electrolyzed at 900 to 960° C. to obtain primary aluminum, and at the same time, the carbon anode is continuously consumed to produce CO2-based gas. This method, although widely used, still faces a number of problems: (i) The carbon anode is consumed in large quantities, and needs to be replaced periodically, which affects the production efficiency. Moreover, the produced gas mixture containing CO2, CO, SO2 and fluorocarbon may pollute the environment. (ii) The ordinary carbon cathode has poor wettability to the aluminum liquid, which not only increases the cell voltage, but also produces a large amount of waste cathode carbon blocks containing toxic substances after damage. (iii) Relatively high requirements for chemical components and physical properties of the aluminum oxide raw material make the upstream aluminum oxide industry have to face the pressure from deep desilication, and it is difficult to economically utilize bauxite and aluminum-containing secondary resources with poor quality. (iv) The purity of primary aluminum as the electrolysis product is only 99.00 to 99.85%, and impurity elements such as Si and Fe seriously affect the performance and application of the primary aluminum. (v) The electrolysis requires a large energy consumption. The power consumption per ton of aluminum is about 13,000 kW·h, with a power efficiency of only about 50%. If refined aluminum is produced by electrorefining primary aluminum, this requires more than 12,000 kW·h/t-Al.

SUMMARY Technical Problem

In order to solve the above problems, researchers have proposed numerous solutions, including use of low-temperature electrolyte, use of inert anodes, and use of wettable cathodes.

The purpose of the use of low-temperature electrolyte is to save energy and reduce consumption by reducing the electrolysis temperature. The low-temperature electrolyte includes: sodium cryolite (Na3AlF6) systems with low cryolite ratio, lithium cryolite (Li3AlF6) systems, potassium cryolite (K3AlF6) systems and mixed cryolite systems. The sodium cryolite systems with low cryolite ratio or lithium cryolite systems may significantly reduce the solubility and dissolution rate of aluminum oxide, and the undissolved aluminum oxide raw material may easily settle to the bottom of the cathode aluminum liquid (the density of the aluminum oxide raw material is greater than that of the cathode aluminum liquid), forming a cathode crust, which will cause disturbance and disorder in the electrolysis process and affect the normal operation of the electrolysis process. The potassium cryolite systems have a good dissolution effect on aluminum oxide, but K+ may severely corrode and damage the cathode carbon block at the bottom of the electrolytic cell, resulting in damage or reduced life of the electrolytic cell, so potassium salt is generally not allowed to be added to the molten salt electrolyte. As a result, the mainstream electrolyte for aluminum electrolysis is still the sodium cryolite system with a working temperature of 900° C. or more.

The use of inert anodes has the advantages of no greenhouse gas emission and no need for frequent electrode replacement. Inert anode materials have been widely studied. However, the inert anodes are prone to corrosion and damage in a working environment with high temperature (>900° C.) and fluoride molten salt, and the produced impurity ions may easily enter the primary aluminum, causing product contamination. Therefore, the inert anodes are still not used on a large scale in industry. The wettable cathodes typically use a composite of TiB2 and graphite, but the use of the wettable cathodes still faces some problems such as high consumption per cell, high cost, and high proneness of coming-off and floating-up of surface TiB2.

In summary, these current methods for producing metal aluminum in the traditional electrolytic cell by using the industrial staple product aluminum oxide as the raw material not only strictly require complete dissolution of aluminum oxide in the electrolyte, but also make it difficult to ensure the purity of the metal aluminum product. In addition, the problems of poor adaptability, high operating requirements and high cost also make it difficult to apply and promote the above methods industrially.

Technical Solution

An object of the disclosure is to provide a method for producing metal aluminum by molten salt electrolysis of aluminum oxide.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, by using an electrolytic cell divided into an anode chamber and a cathode chamber, after the electrolytic cell is powered on to operate, an aluminum oxide material is added to the anode chamber to obtain a metal aluminum product in the cathode chamber.

The anode chamber and the cathode chamber are configured to physically separate an anolyte from a catholyte, the anode chamber is provided with an anode, and the cathode chamber is provided with a cathode.

The bottom of the electrolytic cell is filled with an alloy medium, and the alloy medium respectively contacts the anolyte and the catholyte and is configured to form electrochemical reaction interfaces of aluminum ions/aluminum atoms and serve as the transfer medium of the aluminum atoms.

The overall process of molten salt electrolysis of aluminum oxide includes: in the anode chamber, the aluminum oxide raw material is added to the anolyte, an oxidation reaction occurs at the anode while a gas is evolved, and aluminum ions (dissolved and/or undissolved) in the anode chamber are reduced to aluminum atoms at the interface between the anolyte and the alloy medium that enter the liquid alloy medium. In the cathode chamber, aluminum atoms in the alloy medium lose electrons at the interface between the catholyte and the alloy medium to form aluminum ions that enter the catholyte, and the aluminum ions in the catholyte are reduced to aluminum atoms to form a metal aluminum liquid that floats on the catholyte.

When the electrolytic cell is powered on to operate, an anode current density is controlled at 0.1 to 1.5 A/cm2 and a temperature is controlled at 700 to 950° C. under normal working conditions. The specific working temperature depends on specific components of the anolyte, the catholyte or the liquid alloy, but it is important to ensure that the working temperature is higher than a liquidus temperature of the anolyte or the catholyte, and a solidifying point of the alloy medium. In the industrial application stage, the working temperatures of the cathode chamber and the anode chamber may be the same or different, which can be achieved by different heat dissipation/heat generation conditions, such as adjustment of a distance between the electrode and the alloy medium or forced heat dissipation, or the anode chamber and cathode chamber are not adjacent to each other in space.

The aluminum oxide raw material used in the disclosure may be smelter grade aluminum oxide (referring to the industry standard “YS/T 803-2012 Smelter grade alumina”), or aluminum oxide with excessive content of impurity such as silicon or iron, or aluminum oxide with unsatisfactory physical properties (e.g., specific surface area and particle size distribution) according to the standard “Smelter grade alumina”, or secondary aluminum-containing resources such as aluminum ash, aluminum slag, high-aluminum fly ash and waste aluminum oxide may be used/incorporated.

The Al content in the metal aluminum product is ≥99.90 wt %, and contents of metal impurities achieve the requirements for the refined aluminum or high-purity aluminum product.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the anolyte is a fluoride system or a chloride system.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the anolyte is a fluoride system containing 60 to 90 wt % of cryolite, 5 to 30 wt % of AlF3, 1 to 10 wt % of Al2O3 and 0 to 15 wt % of additive. The cryolite is one or more of Na3AlF6, Li3AlF6 and K3AlF6, and the additive is one or more of LiF, NaF, KF, CaF2, MgF2, BaF2 and NaCl.

According to common knowledge in the art, an electrolyte containing AlF3 and MeF (Me=Li, Na, K) in a mole ratio of 1:3 and an electrolyte containing Me3AlF6 (Me=Li, Na, K) are equivalent and interchangeable. The above components and compositions are only a common expression, and there are many other expressions. For example, the mass fraction may be converted to the corresponding mole fraction. If the Me3AlF6 (Me=Li, Na, K) component is replaced with the two components AlF3 and MeF (Me=Li, Na, K), then the electrolyte includes AlF3, MeF (Me=Li, Na, K), Al2O3 and an additive.

The anolyte of the fluoride system contains the cryolite (Me3AlF6, Me=Li, Na, K) component, and thus has a certain solubility to the aluminum oxide raw material. By adding AlF3 and other fluorides or chlorides, the liquidus temperature of the electrolyte can be reduced, and the physical and chemical properties such as conductivity can be adjusted. When the aluminum oxide raw material is added to the fluoride system, the aluminum oxide undergoes a dissolution reaction and produces dissolved aluminum-containing ions (e.g., AlF4−, AlOF54−, and other ions containing aluminum, uniformly denoted as Al3+) and oxygen-containing ions (e.g., AlOF54−, Al2OF106−, and other ions containing oxygen, uniformly denoted as O2−). Under the action of the electric field, the oxygen-containing ions in the anode chamber undergo an oxidation reaction on the anode, and O2 or COx (x=1 or 2) gas is evolved; and aluminum-containing ions undergo a reduction reaction at the interface between the anolyte and the alloy medium, and aluminum atoms are produced and enter the alloy medium. The equations are:


O2−−2e−+1/xC→1/xCOx↑(x=1 or 2)  graphite anode:


O2−−2e−→0.5O2↑  or inert anode:


Al3++3e−→Al(alloy medium)  interface:

The solid aluminum oxide raw material at the interface between the alloy medium and the anolyte (the density of alloy medium is greater than that of aluminum oxide raw material) may continue to be dissolved in the anolyte and supply the aluminum-containing ions continuously consumed at the interface, so as to reduce concentration polarization and avoid side reactions, or the aluminum oxide raw material may directly undergo a reduction reaction at the interface, to ensure that the aluminum-containing ions in the anode chamber are continuously reduced to aluminum atoms that enter the alloy medium. The interface reaction is:


Al2O3+6e−→2Al(alloy medium)+3O2−

As can be seen from above, even the undissolved or saturated aluminum oxide raw material can still participate in the reaction at the interface, that is, the method and the electrolytic cell used have broken through the limitation that requires aluminum oxide to be completely dissolved in the electrolyte. Therefore, the fluoride anolyte may use not only the conventional sodium cryolite systems, but also the sodium cryolite systems with low cryolite ratio, the lithium cryolite systems and the mixed systems thereof, which have slightly lower solubility for aluminum oxide but can achieve the purpose of low-temperature electrolysis. Of course, the fluoride anolyte may also use the potassium cryolite system or a cryolite system containing potassium salt, which can achieve the purpose of low-temperature electrolysis and have the advantages of relatively good solubility for aluminum oxide and no damage to the cathode carbon block by K+.

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

    • a conventional sodium cryolite system, containing 80 to 90 wt % of Na3AlF6, 5 to 15 wt % of AlF3, 2 to 10 wt % of Al2O3, and 3 to 10 wt % of one or more additives selected from CaF2, MgF2, LiF, KF and NaCl;
    • a low-molecular-ratio sodium cryolite system I, containing 60 to 85 wt % of Na3AlF6, 10 to 25 wt % of AlF3, 1 to 10 wt % of Al2O3, and 1 to 15 wt % of one or more additives selected from CaF2, MgF2, LiF and KF;
    • a low-molecular-ratio sodium cryolite system II, containing 50 to 75 wt % of Na3AlF6, 20 to 35 wt % of AlF3, 1 to 8 wt % of Al2O3, and not more than 10 wt % of one or more additives selected from CaF2, MgF2, LiF and KF;
    • a potassium cryolite system, containing 60 to 90 wt % of K3AlF6, 6 to 30 wt % of AlF3, 1 to 10 wt % of Al2O3, and not more than 10 wt % of one or more additives selected from CaF2, MgF2, NaF and LiF;
    • a sodium-lithium cryolite system, containing 50 to 70 wt % of Na3AlF6, 5 to 45 wt % of Li3AlF6, 5 to 25 wt % of AlF3, 1 to 8 wt % of Al2O3, and not more than 10 wt % of one or more additives selected from CaF2, MgF2 and KF;
    • a sodium-potassium cryolite system I, containing 50 to 80 wt % of Na3AlF6, 5 to 30 wt % of K3AlF6, 10 to 30 wt % of AlF3, 1 to 10 wt % of Al2O3, and not more than 10 wt % of one or more additives selected from LiF, CaF2 and MgF2; and
    • a sodium-potassium cryolite system II, containing 30 to 50 wt % of Na3AlF6, 20 to 50 wt % of K3AlF6, 10 to 30 wt % of AlF3, 1 to 10 wt % of Al2O3, 1 to 10 wt % of LiF, and not more than 5 wt % of CaF2 or/and MgF2.

The above systems have their own characteristics. Taking the sodium-potassium cryolite II as an example, this system contains more K3AlF6, which can increase the solubility of the aluminum oxide raw material and can work together with the added AlF3 to promote the reduction of the liquidus temperature of the anolyte so as to achieve the purpose of low-temperature electrolysis; and the LiF helps in improving the conductivity of the electrolyte. In contrast, this electrolyte containing a large amount of potassium salt is rarely used in traditional electrolytic cells, otherwise the penetration and damage of K+ to the cathode carbon block on the cell bottom will seriously shorten the service life of the electrolytic cell.

Further, in order to adjust the physical and chemical properties such as conductivity and liquidus temperature of the fluoride system anolyte, chlorides of alkali metals and alkaline earth metals may be added to the system, but the total addition amount of chlorides should be not more than 5 wt %, otherwise the stability of the electrolyte will be affected.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the anolyte is a chloride system; and the chloride system is CaCl2, or the chloride system includes CaCl2 and one or more of LiCl, NaCl, KCl, BaCl2, CaF2 and LiF and a mole percentage of the CaCl2 in the chloride system is not lower than 50%.

The above chloride system anolyte has very low solubility for the aluminum oxide raw material, but has a certain solubility for O2−. When the aluminum oxide raw material is added to the chloride system anolyte, under the action of the electric field, the solid aluminum oxide raw material directly undergoes a reduction reaction at the interface between the anolyte and the alloy medium. The aluminum ions are reduced to aluminum atoms that enter the alloy medium, and the dissociated O2− is dissolved in the anolyte and migrates to the anode, and then undergoes an oxidation reaction on the anode surface. The equations are:


Al2O3+6e−→2Al(alloy medium)+3O2−  interface:


O2−−2e−+1/xC→1/xCOx↑(x=1 or 2)  graphite anode:


O2−−2e−→0.5O2↑  or inert anode:

Further, in order to adjust the physical and chemical properties of the chloride system anolyte, fluorides of alkali metals, fluorides of alkaline earth metals, fluorides of aluminum, and oxides of alkali metals and oxides of alkaline earth metals may be added to the chloride system. A carbon conductive agent or metal powder may also be mixed with the aluminum oxide raw material and the aluminum oxide raw material may be molded and sintered so as to improve the electrochemical reactivity of the aluminum oxide raw material at the interface.

In the anode chamber, impurities in the aluminum oxide raw material will have different electrochemical behaviors due to their different precipitation potentials. The impurities such as Li, Ca and Na, which are more active than Al, will be enriched in the anolyte, and the impurities such as Fe, Si, Mn and Ti, which are more inert than Al, will be reduced and enriched in the alloy medium.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the catholyte is a pure fluoride system or a fluoride-chloride system.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the catholyte is a pure fluoride system containing 20 to 40 wt % of BaF2, 30 to 50 wt % of AlF3, 15 to 40 wt % of NaF and not more than 20 wt % of additive, and the additive is one or more of CaF2, LiF, Li3AlF6 and MgF2.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the catholyte is a fluoride-chloride system containing 50 to 70 wt % of BaCl2, 15 to 30 wt % of AlF3, 10 to 30 wt % of NaF and 0 to 15 wt % of additive, and the additive is one or more of LiF, Li3AlF6, CaF2, MgF2, NaCl, LiCl, CaCl2 and MgCl2.

In the cathode chamber, the aluminum atoms in the alloy medium lose electrons at the interface between the alloy medium and the catholyte, and the generated Al3+ (Al3+ denotes all aluminum-containing ions such as AlF4−) enters the catholyte. The Al3+ in the catholyte is reduced into aluminum atoms at the interface between the cathode or metal aluminum liquid and the catholyte, which enter the liquid metal aluminum product. The equations are:


Al(alloy medium)−3e−→Al3+  interface:


Al3++3e−→Al(metal aluminum liquid)  cathode:

The impurities Fe, Si and Mn in the alloy medium have less active electrochemical properties than Al, and thus, do not undergo the oxidation reaction but remain in the liquid alloy medium, thereby having little influence on the cathode product metal aluminum. However, as the electrolysis proceeds, the impurities Fe and Si in the alloy medium are continuously enriched and the concentrations increase. At this time, the liquid alloy medium needs to be extracted for purification, and the purified liquid alloy returns to the electrolytic cell to continue working. For example, the extracted alloy medium may be subjected to condensation to firstly crystallize and precipitate the high-melting-point Fe-containing intermediate metal phase or elemental silicon, or the alloy medium may be used as an anode and oxidized by electrolysis to precipitate Al, Fe, and Si thereof.

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the anode is carbon anode or inert anode.

The inert anode includes ceramic materials (e.g., SnO2 and doped SnI2, NiFe2O4, CaTiO3, CaRuO3, CaRuxTi1-xO3 and ITO), metal materials (e.g., a Cu—Al alloy, an Ni—Fe alloy or an Ni—Fe—Cu alloy), cermet composites (e.g., Cu—NiFe2O4, Cu—NiO—NiFe2O4, Ni—NiO—NiFe2O4, Cu—Ni—NiO—NiFe2O4 and Ni—CaRuxTi1-xO3).

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the cathode is one or a composite of more of graphite, aluminum and inert wettable cathode material (e.g., TiB2, TiB2/C or ZrB2).

In the method for producing metal aluminum by molten salt electrolysis of aluminum oxide according to specific implementations of the disclosure, the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In and Sb, preferably an Al—Cu alloy, in which an Al content is 40 to 75 wt %; and the alloy medium remains in a liquid state during normal electrolysis, and has the density greater than that of the anolyte and the catholyte.

The composition of the alloy medium may be determined from an alloy phase diagram according to the specific working temperature. For example, an Al—Cu alloy having an Al content of 40 to 75 wt % has a melting point of lower than 700° C., so the electrolysis operation at any temperature ranging from 700 to 950° C. can use the Al—Cu alloy medium with this composition.

When a fluoride system or chloride system anolyte with relatively poor solubility for the aluminum oxide raw material is used or when the addition speed is relatively high, the content of Al in the alloy medium may be appropriately reduced to ensure the density of the alloy medium to be greater than the density of the aluminum oxide raw material.

Beneficial Effects

    • (1) Low requirements for raw material and high purity of product: The electrolytic cell used by the method has the functions of purification and impurity removal. Based on different polarization potentials of different elements, among the impurities in the aluminum oxide raw material, the elements (e.g., Li, Na, K, Ca and Mg) that are more active than aluminum will be enriched in the electrolyte, and the elements (e.g., Si, Ti, Fe, V and Mn) that are more inert than aluminum will be enriched in the alloy medium. It is difficulty for all these impurities to enter the metal aluminum product, so that the purity of the metal aluminum product can be ensured (aluminum content ≥99.90 wt %), and the ranges of contents of impurities, especially silicon and iron, in the aluminum oxide raw material can be widened properly.
    • (2) High adaptability and capability of continuous production: The method uses the industrial staple product aluminum oxide as the raw material to directly and continuously produce the high-purity metal aluminum product, thereby avoiding the current process of obtaining refined aluminum by electrolyzing aluminum oxide first and then refining primary aluminum, shortening the production time and saving the production cost. In addition, the bottom of the electrolytic cell is filled with the alloy medium containing heavy metals and aluminum. Even the excessively added or partially supersaturated aluminum oxide raw material will continue to participate in the dissolution or electrochemical reaction at the interface between the liquid alloy medium and the anolyte, thereby improving the operation adaptability of the electrolytic cell and lowering the requirements for the solubility for the aluminum oxide raw material.
    • (3) Wide choice of electrolytes: The method avoids the requirements of the industrial electrolyte for the solubility of the aluminum oxide raw material, and also avoids the use of the K+-sensitive cathode carbon block at the cell bottom. Therefore, the conventional sodium cryolite system electrolyte and many low-temperature electrolytes (including the sodium cryolite systems with low cryolite ratio, the lithium cryolite systems, the potassium cryolite systems and the mixed systems thereof) may be used, and moreover, the chloride systems may also be used as the anolyte. The use of the low-temperature electrolyte helps in energy saving and consumption reduction in the aluminum electrolysis industry.
    • (4) Capability of use of inert anode: After the fluoride system or chloride system low-temperature electrolyte is used, the strong corrosion of fluorides to the inert anode at high temperature is avoided, and thus the service life of the inert anode is prolonged. The impurity elements produced by the inert anode being corroded will remain in the electrolyte or alloy medium and cannot enter the metal aluminum product easily, thereby ensuring the product purity. In addition, the clean and harmless O2 is produced on the surface of the inert anode, and there is no greenhouse gas CO2 or toxic and harmful gases produced.
    • (5) Energy saving and environment friendliness: Continuous production of high-purity metal aluminum can be realized only in one electrolytic cell, thereby improving the space utilization and accordingly reducing the heat loss. In combination with the low-temperature electrolyte and the inert anode, the electric energy can be further saved and the electric energy efficiency can be improved. Moreover, there are no emissions of exhaust gas such as greenhouse gas or toxic gas, or production of waste slag such as waste cathode carbon blocks.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the examples of the disclosure or the technical solutions in the prior art, the accompanying drawings used in the description of the examples or the prior art will be briefly described below. Apparently, the accompanying drawings in the following description are only some examples of the disclosure, and those skilled in the art can obtain other drawings according to these drawings without any creative work.

The FIGURE is a schematic sectional view of an electrolytic cell according to the disclosure.

In the FIGURE, 1—anode, 2—electrolytic cell body, 3—anolyte, 4—alloy medium, 5—catholyte, 6—metal aluminum product, 7—cathode.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to make the objectives, technical solutions and advantages of the disclosure clearer, the technical solutions of the disclosure will be described in detail below. Apparently, the described examples are merely some, rather than all the examples of the disclosure. All other embodiments obtained by those of ordinary skill in the art based on the examples of the disclosure without creative work are within the protection scope of the disclosure.

A method for producing metal aluminum by molten salt electrolysis of aluminum oxide in the disclosure is operated at a temperature of 700 to 950° C. in a powered environment, and the anode current density is 0.1-1.5 A/cm2. Aluminum oxide raw material is added to the anolyte of anode chamber, the anode loses electrons and evolves gas, and aluminum ions (dissolved and/or non-dissolved) are reduced and enter a liquid alloy medium. At the same time, in the cathode chamber, aluminum atoms in the liquid alloy medium lose electrons at the interface to form aluminum ions that enter a catholyte, and aluminum ions in the catholyte are reduced into metal aluminum atoms that enter the aluminum liquid floating on the catholyte.

The addition speed of the aluminum oxide raw material is calculated and determined based on the current intensity and the current efficiency according to Faraday's law.

In the disclosure, the anolyte and the catholyte are physically separated by the electrolytic cell, and both the anolyte and the catholyte contact the alloy medium, so that the interface reaction of aluminum ions/aluminum atoms and migration of aluminum atoms are completed by means of the alloy medium. Therefore, in order to effectively separate the catholyte from the anolyte, the structure of the electrolytic cell is shown in the FIGURE.

The electrolytic cell body 2 is divided into an anode chamber and a cathode chamber in space. The anode chamber is filled with the anolyte 3, and the anode 1 is inserted into the anolyte 3. The cathode chamber is filled with the catholyte 5, and the cathode 7 is inserted into the catholyte 5 or the liquid metal aluminum product 6. The bottom of the electrolytic cell is filled with the alloy medium 4 that respectively contacts the anolyte 3 and the catholyte 5 but does not contact the anode 1 or the cathode 7.

In addition to the electrolytic cell as shown in the FIGURE, the structure of the electrolytic cell may also be designed into various forms, for example, the electrolytic cell may be a U-shaped electrolytic cell. In addition, the electrolytic cell may be in a variety of shapes. For example, the electrolytic cell may be not limited to a round bottom, but can also be a trapezoidal bottom or a flat bottom.

Any aluminum electrolytic cell that can realize physical separation between the anolyte and the catholyte and the mediation function of the alloy medium can be applied to the method of the disclosure.

In large-scale application, the electrolytic cells may be operated and connected in series or in parallel.

Example 1

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al alloy in which an Al content was 60 wt %. Both the anode and the cathode were graphite rods.

The anolyte included: 80.3 wt % Na3AlF6+12.2 wt % AlF3+2.5 wt % Al2O3+3.0 wt % CaF2+1.0 wt % MgF2+1.0 wt % LiF.

The catholyte included: 35.0 wt % BaF2+30.0 wt % AlF3+30.0 wt % NaF+5.0 wt % CaF2.

The electrolytic cell was heated to 940° C. and held for 2 h. The electrolytic cell was powered on while controlling an anode current density at 1.2 A/cm2. After the electrolysis was started, an aluminum oxide raw material containing 95.5 wt % of Al2O3 was added regularly, and a total electrolysis time was 10 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.980%.

Example 2

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al—Zn alloy in which an Al content was 60 wt % and a Zn content was 5 wt %. The anode was a graphite rod, and the cathode was a TiB2-coated graphite.

The anolyte included: 50.0 wt % Na3AlF6+30.0 wt % Li3AlF6+13.5 wt % AlF3+2.0 wt % Al2O3+3.0 wt % CaF2+1.5 wt % MgF2.

The catholyte included: 65.0 wt % BaCl2+20.0 wt % AlF3+13.0 wt % NaF+2.0 wt % NaCl. The electrolytic cell was heated to 860° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 0.6 A/cm2. After the electrolysis was started, an aluminum oxide raw material containing 91.2 wt % of Al2O3 was added regularly, and a total electrolysis time was 10 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.970%.

Example 3

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al alloy in which an Al content was 50 wt %. The anode was a graphite rod, and the cathode was a TiB2-coated graphite.

The anolyte was CaCl2.

The catholyte included: 20.0 wt % BaF2+35.0 wt % AlF3+30.0 wt % NaF+15.0 wt % CaF2.

The electrolytic cell was heated to 850° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 0.7 A/cm2. Before and after the electrolysis was started, an aluminum oxide raw material containing 98.8 wt % of Al2O3 was added regularly, and a total electrolysis time was 10 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.989%.

Example 4

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al alloy in which an Al content was 62 wt %. The anode was inert anode made of a 15 wt % Fe-70 wt % Cu-35 wt % Ni alloy material, and the cathode was graphite.

The anolyte included: 70.0 wt % K3AlF6+21.5 wt % AlF3+3.5 wt % Al2O3+4.0 wt % LiF+1.0 wt % CaF2.

The catholyte included: 55.0 wt % BaCl2+27.0 wt % AlF3+16.0 wt % NaF+2.0 wt % CaF2.

The electrolytic cell was heated to 870° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 0.8 A/cm2. After the electrolysis was started, an aluminum oxide raw material containing 98.7 wt % of Al2O3 was added regularly, and a total electrolysis time was 10 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.995%.

Example 5

The bottom of the electrolytic cell was filled with a pre-alloyed Sn—Al alloy in which an Al content was 20 wt %. The anode was a graphite anode, and the cathode was a TiB2-coated graphite.

The anolyte included CaCl2) and LiCl in a mole ratio of 1:1.

The catholyte included: 60.0 wt % BaCl2+20.0 wt % AlF3+15.0 wt % NaF+5.0 wt % LiCl.

The electrolytic cell was heated to 800° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 0.4 A/cm2. Before and after the electrolysis was started, an aluminum oxide raw material containing 97.2 wt % of Al2O3 was added regularly, and a total electrolysis time was 15 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.986%.

Example 6

The bottom of the electrolytic cell was filled with a pre-alloyed In—Al alloy in which an Al content was 10 wt %. Both the anode and the cathode were graphite rods.

The anolyte included: 65.0 wt % Na3AlF6+20.5 wt % AlF3+3.5 wt % Al2O3+8.0 wt % KF+3.0 wt % CaF2.

The catholyte included: 25.0 wt % BaF2+36.0 wt % AlF3+27.0 wt % NaF+10.0 wt % CaF2+2.0 wt % Li3AlF6.

The electrolytic cell was heated to 900° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 0.5 A/cm2. After the electrolysis was started, an aluminum oxide raw material containing 98.9 wt % of Al2O3 was added regularly, and a total electrolysis time was 12 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.994%.

Example 7

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al alloy in which an Al content was 45 wt %. The anode was inert anode made of a CaRuO3 ceramic material, and the cathode was a TiB2/C composite.

The anolyte included CaCl2), NaCl and CaO in a mole ratio of 50:48:2.

The catholyte included: 60.0 wt % BaCl2+23.0 wt % AlF3+17.0 wt % NaF.

The electrolytic cell was heated to 840° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 0.2 A/cm2. Before and after the electrolysis was started, an aluminum oxide raw material containing 94.6 wt % of Al2O3 was added regularly, and a total electrolysis time was 20 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.975%.

Example 8

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al alloy in which an Al content was 70 wt %. The anode was inert anode made of a NiFe2O4-18 wt % NiO-17 wt % Cu cermet composites, and the cathode was a graphite rod.

The anolyte included: 42.3 wt % Na3AlF6+28.2 wt % K3AlF6+23.0 wt % AlF3+2.5 wt % Al2O3+4.0 wt % LiF.

The catholyte included: 22.0 wt % BaF2+46.0 wt % AlF3+26.0 wt % NaF+4.0 wt % CaF2+2.0 wt % LiF.

The electrolytic cell was heated to 880° C. and held for 2 h. A direct current was applied such that an anode current density was controlled at 1.0 A/cm2. After the electrolysis was started, a sand-like aluminum oxide raw material containing 99.1 wt % of Al2O3 was added regularly, and a total electrolysis time was 10 h.

After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.999%.

Example 9

This comparative example was different from Example 4 in that: on the basis of the anolyte including 70.0 wt % K3AlF6+21.5 wt % AlF3+3.5 wt % Al2O3+4.0 wt % LiF+1.0 wt % CaF2, 10 wt % of Al2O3 was also added, and the alloy medium was a Cu—Al alloy in which an Al content was 50 wt %. The other conditions were the same. After the electrolysis was completed, an Al content in the cathode product metal aluminum was measured as 99.991%, and moreover, there was still some undissolved aluminum oxide raw material on the alloy medium.

It could be inferred that even the aluminum oxide raw material that was added too fast or too much could stay on the alloy medium and continue to participate in the dissolution/electrochemical reaction, so as to maintain the continuous operation of the electrolysis process, and the purity of the obtained metal aluminum product was still relatively high.

Comparative Example 1

This comparative example was different from Example 1 in that: the bottom of the electrolytic cell was not filled with the alloy medium. The electrolyte was the same as the anolyte in Example 1, and the other conditions were the same. After the electrolysis was completed, the product metal aluminum was taken out of the electrolytic cell, and an Al content in the product metal aluminum was measured as 97.1%.

This indicated that in the absence of the alloy medium, there was no separation and impurity removal effect based on the electrochemical reaction at the alloy medium/electrolyte interface, and impurity elements Si and Fe directly entered the metal aluminum product, causing a reduction in product purity/grade.

Comparative Example 2

This comparative example was different from Example 4 in that: the bottom of the electrolytic cell was not filled with the alloy medium, and on the basis of the electrolyte including 70.0 wt % K3AlF6+21.5 wt % AlF3+3.5 wt % Al2O3+4.0 wt % LiF+1.0 wt % CaF2, 10 wt % of Al2O3 was also added (since there is no separation of the alloy medium, there was only one electrolyte in the electrolytic cell). The other conditions were the same. After the end of electrolysis, the product metal aluminum in the electrolytic cell was taken out, of which the Al content was measured as 97.8%, containing Fe, Si, Cu, Ni and other impurity elements. Some undissolved aluminum oxide raw material was also detected on the cell bottom.

This indicated that the aluminum oxide raw material that was added too fast or too much could hardly participate in the dissolution/electrochemical reaction effectively, thus forming the precipitate on the cell bottom. In addition, without the separation and impurity removal effect based on the electrochemical reaction at the alloy medium/electrolyte interface, the impurity element Si in the aluminum oxide raw material, and elements such as Cu, Ni and Fe generated after the surface of the inert anode was corroded directly entered the metal aluminum product, causing a reduction in the purity of the metal aluminum product.

Comparative Example 3

The bottom of the electrolytic cell was filled with metal aluminum with an Al content of 99.8%. Both the anode and the cathode were graphite rods. Both the anolyte and the catholyte included: 60.0 wt % Na3AlF6+12.0 wt % AlF3+5.0 wt % NaF+2.0 wt % CaF2+1.0 wt % MgF2+10.0 wt % NaCl+10.0 wt % KCl. The electrolytic cell was placed in an atmosphere filled with dry argon. The electrolytic cell was heated to 940° C. in a temperature programmed way, and held for 2 h. The electrolytic cell was powered on while controlling an anode current density at 0.8 A/cm2. After the electrolysis was started, a smelter grade aluminum oxide raw material containing 98.8 wt % of Al2O3 was added regularly. The addition speed was calculated and determined based on the current intensity and the current efficiency according to Faraday's law, and a total electrolysis time was 10 h. During the electrolysis process, irritating chlorine gas was found, and the electrolysis voltage fluctuated relatively widely. After the electrolysis was completed, no metal aluminum product was detected on the surface of the catholyte in the cathode chamber, and there was still unreacted aluminum oxide raw material under the metal aluminum on the bottom of the electrolytic cell, forming a precipitate/crust on the cell bottom.

Comparative Example 4

The bottom of the electrolytic cell was filled with metal aluminum with an Al content of 99.8%. Both the anode and the cathode were graphite rods. Both the anolyte and the catholyte included: 60.0 wt % Na3AlF6+12.0 wt % AlF3+5.0 wt % NaF+2.0 wt % CaF2+1.0 wt % MgF2+10.0 wt % NaCl+10.0 wt % KCl. Moreover, high-purity aluminum with an Al content of 99.999% was placed on the surface of the catholyte. The electrolytic cell was placed in an atmosphere filled with dry argon. The electrolytic cell was heated to 940° C. in a temperature programmed way, and held for 2 h. During the process, it was found that the melted high-purity aluminum liquid could not stably float on the surface of the catholyte, but automatically settled to the cell bottom, that is, the electrochemical system shown in the FIGURE could not be formed.

Comparative Example 5

The bottom of the electrolytic cell was filled with metal aluminum with an Al content of 99.8%. Both the anode and the cathode were graphite rods. The anolyte included: 60.0 wt % Na3AlF6+12.0 wt % AlF3+5.0 wt % NaF+2.0 wt % CaF2+1.0 wt % MgF2+10.0 wt % NaCl+10.0 wt % KCl. The catholyte included: 35.0 wt % BaF2+30.0 wt % AlF3+30.0 wt % NaF+5.0 wt % CaF2. The electrolytic cell was placed in an atmosphere filled with dry argon. The electrolytic cell was heated to 940° C. in a temperature programmed way, and held for 2 h. During the process, it was found that the metal aluminum liquid on the bottom of the electrolytic cell automatically floated up to the surface of the catholyte, that is, the electrochemical system shown in the FIGURE could not be formed.

Comparative Example 6

The corundum crucible was filled with molten salts including 35.0 wt % BaF2+30.0 wt % AlF3+30.0 wt % NaF+5.0 wt % CaF2, and an excessive smelter grade aluminum oxide raw material containing 98.8 wt % of Al2O3 was added and held at 940° C. for 2 h such that the aluminum oxide raw material was sufficiently dissolved. Then, the molten salts in which saturated Al2O3 was dissolved were taken out by decantation, and the undissolved aluminum oxide raw material and the residual molten salts are left in the corundum crucible.

The bottom of the electrolytic cell was filled with a pre-alloyed Cu—Al alloy in which an Al content was 60 wt %. Both the anode and the cathode were graphite rods. The above molten salts in which saturated Al2O3 was dissolved were used as the anolyte, and the catholyte was 35.0 wt % BaF2+30.0 wt % AlF3+3 0.0 wt % NaF+5.0 wt % CaF2. The electrolytic cell was placed in an atmosphere filled with dry argon. The electrolytic cell was heated to 940° C. in a temperature programmed way, and held for 2 h. The electrolytic cell was powered on such that an anode current density was controlled at 0.8 A/cm2. During the electrolysis process, the voltage fluctuated up and down and then increased sharply in the later period, and the electrolysis stopped in an instant.

The reason for the above phenomenon may be that relatively more Al2O3 dissolved in the anolyte at the beginning of electrolysis could maintain the normal operation of the electrolysis, but both the electrolyte resistance and the cell voltage were relatively high. As the electrolysis proceeded, Al2O3 in the anolyte was consumed continuously, and the electrolyte resistance decreased. However, the concentration polarization led to the occurrence of the anode effect, and the voltage fluctuated and increased sharply in the later period.

In addition, the anolyte contained a large amount of barium salt. The addition of the barium salt greatly decreased the solubility of the aluminum oxide, such that the anolyte needed to be taken out frequently and the aluminum oxide raw material needed to be transferred and dissolved frequently, causing low production efficiency.

The foregoing descriptions are merely specific implementations of the disclosure, but are not intended to limit the protection scope of the disclosure. Any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in the disclosure shall fall within the protection scope of the disclosure. Therefore, the protection scope of the disclosure shall be subject to the protection scope of the claims.

Claims

1. A method for producing metal aluminum by a molten salt electrolysis of aluminum oxide, comprising:

using an electrolytic cell divided into an anode chamber and a cathode chamber, after the electrolytic cell is powered on to operate, adding an aluminum oxide raw material to the anode chamber to obtain a metal aluminum product in the cathode chamber, wherein
the anode chamber and the cathode chamber are configured to physically separate an anolyte from a catholyte, the anode chamber is provided with an anode, and the cathode chamber is provided with a cathode; and
a bottom of the electrolytic cell is filled with an alloy medium, and the alloy medium respectively contacts the anolyte and the catholyte and is configured to form electrochemical reaction interfaces of aluminum ions/aluminum atoms and serve as a transfer medium of the aluminum atoms.

2. (canceled)

3. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the anolyte is a fluoride system containing 60 to 90 wt % of cryolite, 5 to 30 wt % of AlF3, 1 to 10 wt % of Al2O3, and 0 to 15 wt % of an additive; and the cryolite is one or more of Na3AlF6, Li3AlF6, and K3AlF6, and the additive is one or more of LiF, NaF, KF, CaF2, MgF2, BaF2, and NaCl.

4. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the anolyte is a chloride system; and the chloride system is CaCl2), or the chloride system comprises CaCl2) and one or more of NaCl, KCl, BaCl2, CaF2, LiCl, and CaO and a mole percentage of the CaCl2) in the chloride system is not lower than 50%.

5. (canceled)

6. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the catholyte is a pure fluoride system containing 20 to 40 wt % of BaF2, 30 to 50 wt % of AlF3, 15 to 40 wt % of NaF, and 0 to 20 wt % of an additive; and the additive is one or more of CaF2, LiF, Li3AlF6, and MgF2.

7. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the catholyte is a fluoride-chloride system containing 50 to 70 wt % of BaCl2, 15 to 30 wt % of AlF3, 10 to 30 wt % of NaF, and 0 to 15 wt % of an additive; and the additive is one or more of LiF, Li3AlF6, CaF2, MgF2, NaCl, LiCl, CaCl2, and MgCl2.

8. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the anode is a carbon anode or an inert anode.

9. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the cathode is one or a composite of graphite, aluminum, and an inert wettable cathode material.

10. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 1, wherein the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In, Bi, and Sb; and

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

11. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 10, wherein the alloy medium is an Al—Cu alloy, and an Al content in the alloy medium is 40 to 75 wt %.

12. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 3, wherein the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In, Bi, and Sb; and

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

13. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 4, wherein the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In, Bi, and Sb; and

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

14. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 6, and the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In, Bi, and Sb; and

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

15. The method for producing the metal aluminum by the molten salt electrolysis of the aluminum oxide according to claim 7, and the alloy medium is an alloy formed by Al and one or more of Cu, Sn, Zn, Ga, In, Bi, and Sb; and

the alloy medium remains a liquid state during a normal electrolysis and has a density greater than a density of the anolyte or a density of the catholyte.
Patent History
Publication number: 20240141529
Type: Application
Filed: Apr 25, 2022
Publication Date: May 2, 2024
Applicant: CENTRAL SOUTH UNIVERSITY (Changsha)
Inventors: Zhongwei ZHAO (Changsha), Fenglong SUN (Changsha), Yuntao LEI (Changsha)
Application Number: 18/288,406
Classifications
International Classification: C25C 3/18 (20060101); C22C 9/01 (20060101); C22C 13/00 (20060101); C22C 21/12 (20060101); C22C 21/18 (20060101); C22C 28/00 (20060101); C25C 3/08 (20060101);