EP4671416A1

EP4671416A1 - METHOD AND DEVICE FOR THE ELECTROREFINING OF ALUMINUM IN ELECTROLYSIS CELLS (EMPLOYMENTS)

Info

Publication number: EP4671416A1
Application number: EP24760688.2A
Authority: EP
Inventors: Evgenij Valer'evich ZHELEZNOV; Poman Ivanovich KRAJDENKO; Viktor Khrist'yanovich MANN; Dmitrij Konstantinovich Ryabov; Aleksandr Yur'evich TELESHEV
Original assignee: Obshchestvo S Ogranichennoj Otvetstvennost'yu "institut Legkikh Materialov I Tekhnologij"
Current assignee: Obshchestvo S Ogranichennoj Otvetstvennost'yu "institut Legkikh Materialov I Tekhnologij"
Priority date: 2023-02-20
Filing date: 2024-02-16
Publication date: 2025-12-31
Also published as: WO2024177534A1; EP4671416A8; CN120712382A

Abstract

The invention relates to non-ferrous metal industry and can be used for refining aluminium alloys from metallic impurities. The device comprises no less than one porous removable diaphragm, permeable to bath and impermeable to molten aluminium, filled with bath, wherein cathode is vertically mounted and immersed in molten aluminium with metallic impurities, placed in a vessel with an anode current lead. The method involves placing a melt of contaminated aluminium and a bath of salts of alkali or alkaline-earth metals and aluminium salt into an electrolytic refining device and performing electrolytic refining at a cathodic current density from 0.5 to 21 A/cm2 a temperature ranging from 780 °C to 920 °C. Technical effect: increased capacity, with the ability to adjust the bath composition.

Description

Technical Field

The invention relates to non-ferrous metal industry and can be used for refining molten aluminium or its alloys to remove metallic impurities.

Prior Art

The purification of aluminium from undesirable impurities is a crucial technology to produce high purity aluminium. The need for a technology to handle different types of raw materials including a variety of aluminium scrap has recently emerged as a significant trend. Among the most crucial areas of this work is creation and improvement of electrochemical purification technologies. The purification technology was first proposed in patent US 673364 and comprised a three-layer cell with a contaminated aluminium alloy as an anode metal. This method has a significant drawback in terms of high energy costs of the purification, thus resulting in a high cost of purified aluminium.
In light of ever-tightening environmental initiatives and international legislation, developing energy-efficient aluminium refining technologies is an important and urgent task.
A method is known (Zhang, H., De Nora, V., Sekhar, J. A. Materials Used in the Hall-Heroult Cell for Aluminum Production - Krasnoyarsk State University, Krasnoyarsk, 1998 / original text: Zhang, H., De Nora, V., Sekhar, J. A. Materials Used in the Hall-Heroult Cell for Aluminum Production, 1994) of three-layer electrochemical refining of metals whereby aluminium to be refined from undesirable metallic impurities such as iron, silicon, copper, nickel and others is specially weighted by adding copper (about 30%) and such alloy is anodically polarised during electrolytic reduction. The bath is a chloride-fluoride melt. Refining temperature is 800 °C. In this case, pure aluminium is lighter than the bath and, being the cathode, rises to the bath surface. A schematic diagram of such a cell is shown in Fig. 1, where:

1.Side blocks
2.Carbon bottom block
3.Thermal insulation
4.Anode current lead
5.Aluminium-copper anode alloy
6.Chloride-fluoride bath
7.Refined aluminium
8.Cathode current lead
9.Cover
10.Tap hole for casting aluminium

When direct current is applied, the anode provides conditions for preferential dissolution of aluminium and more electronegative metals, and less electronegative metals accumulate in the anode alloy to be periodically purified from intermetallides of iron, silicon, copper, etc. that crystallise as impurities accumulate. In the cathodic process, conditions for the electrochemical separation of more electronegative metals are not achieved, thus only aluminium is separated. The drawback of the known cell is the need to maintain a large interelectrode distance of 15 cm in order to exclude accidental contamination of the cathode metal with particles of anode alloy, thus resulting in a high voltage on the cell (about 5.5 V) and, consequently, a high specific energy consumption, even at current output η=0.98, more than 18 kWh/kg Al. Another disadvantage of the method is the necessity of using copper to weight the anode alloy.
There are also known a method and device for electrochemical refining (patent RU 2558316, published on July 27, 2015 ) wherein the technical effect is achieved by applying vibroacoustic, and/or ultrasonic, and/or electromagnetic, and/or MHD impact on the following components of the cell: membrane (on one side and/or both sides), bath, primary crude metal, refined metal, interface surfaces between the crude metal, membrane and refined metal. The drawback of this method is the inability to replenish and control the bath composition that results in the cell having a low life cycle, limited by the bath buffer capacity for more electronegative metals, located only in diaphragm pores.
The closest to the claimed device and method for refining aluminium is the prototype where technical solution is based on submersion of the primary metal to be refined in a vessel made of a material permeable to the bath but not permeable to the metal, placed in the bath, and the refined metal is formed on the cell bottom or on the individual cathode surface (patent US 4115215, published on September 19, 1978 ). The drawback of this method is the predominance of the cathode surface area over the anode surface area, thus the overall device capacity is limited by the anode current density determining the quality of the obtained metal.

Invention Disclosure

The technical problem of the invention is to provide a method and device for electrolytic refining of aluminium with increased working volume capacity, reduced energy consumption per product unit and the ability to perform long-term electrolytic reduction by replenishing and adjusting the bath composition, as well as the ability to replace the diaphragm if necessary.
The technical problem is solved by the fact that, according to one of the options, the device for electrolytic refining of aluminium and/or its alloys from metal impurities, having a covered vessel lined with refractory materials, for placing therein molten anode aluminium with anode current lead, bath and molten cathode aluminium, a cathode with a current lead and a collecting tank for cathode aluminium, no less than one porous diaphragm, according to the claimed invention, the porous diaphragm of the device is made in the form of a closed removable vessel, mounted by means of attachment on a cover and filled with bath, permeable to bath and impermeable to molten aluminium or aluminium alloy with impurities and molten cathode aluminium, wherein a cathode with a current lead is loaded and immersed in molten anode aluminium. At that, the cathode with a current lead is vertically fixed inside the porous diaphragm vessel, and the bottom part of the cathode has drainage channels located at the level of cathode aluminium melt for its removal from the porous diaphragm vessel into the collecting tank.
In a second option, a device for electrolytic refining of aluminium and/or its alloys from metallic impurities, having a covered vessel lined with refractory materials for accommodating therein molten anode aluminium with anode current lead, bath and molten cathode aluminium, the bath and molten cathode aluminium, a cathode with a current lead and a collecting tank for cathode aluminium, no less than one porous diaphragm, according to the claimed invention, the porous diaphragm of the device is made in the form of a closed removable vessel, mounted by means of attachment on the cover and filled with bath, permeable to bath and impermeable to molten aluminium or aluminium alloy with impurities and molten cathode aluminium, wherein the cathode with current lead is loaded and immersed in molten anode aluminium. At that, the cathode with a current lead is vertically fixed inside the porous diaphragm vessel, and the cathode walls or in the attachment and cover contain drainage channels located at the level of the melt mirror to enable removal of cathode aluminium from the porous diaphragm vessel into the collecting tank.
Vertical cathode may be made of an aluminium-wettable ceramic, such as TiB₂ or ZrB₂.
Porous diaphragm may be made of carbon materials or inorganic fibrous materials or oxide ceramics such as Al₂O₃ or MgO;
The drainage channels in the cathode walls or in the cathode bottom may be at an angle of 45 to 90 degrees relative to the vertical cathode wall.
The drainage channel made in the diaphragm attachment may be connected to the collecting tank by a guide channel placed in the cover, and this channel may be made of aluminium-wetted material or lined with aluminium-wetted material to allow cathode aluminium to be drained by gravity.
The technical problem is also solved by using the device in the method of electrolytic refining of aluminium and/or its alloys from impurities, including placing the melt of contaminated aluminium and bath containing the melt of a mixture of salts of alkali or alkaline earth metals and aluminium salt in the device for electrolytic refining, leading DC current and performing electrolytic refining, according to the claimed invention. At that, refining is performed with cathodic current density from 0.5 to 21 A/cm², at temperatures ranging from 780 °C to 920 °C, and the molten bath has a density less or greater than the density of cathode aluminium (5).

Brief Description of Drawings

The claimed device is shown in Fig. 2, Fig. 3 and Fig. 4 and contains: side blocks 1, bottom blocks 2, thermal insulation 3, vertical cathode with a current lead 4, cathode aluminium 5, removable porous diaphragm 6, chloride-fluoride or fluoride bath 7, anode aluminium 8, anode current lead 9, attachment 10 of porous diaphragm 6, drainage channels 13 located in the cathode walls or in the attachment 10, aluminium evacuation device with a collecting tank for cathode aluminium 11, device covers 12. The top surface of cathode aluminium and anode aluminium may be protected from oxidation in air by salts and/or argon, or by vacuum. The removable porous diaphragm 6 may be made of electrically conductive or non-electrically conductive material.
Fig. 2 shows a single diaphragm cell providing for the use of a chloride-fluoride or fluoride bath with a density greater than that of cathode aluminium. Cathode aluminium 5 is thereby collected on the melt mirror at the diaphragm top, wherefrom it is removed by the aluminium accumulation and evacuation device 11.
The paired diaphragm cell (Fig. 3) provides for the use of a chloride-fluoride or fluoride bath with a density less than that of cathode aluminium. Cathode aluminium 5 is thereby collected in a receiver at the bottom of the diaphragm, from where it is removed by the aluminium accumulation and evacuation device 11. The top surface of the bath and the anode aluminium may be protected from oxidation in air by salts and/or argon, or by vacuum. In this case, a principle similar to the device (Fig. 2) is used for the electrochemical purification of aluminium, achieving the same technical effect as when using a bath with a density greater than that of cathode aluminium.
Fig. 4 shows a single diaphragm cell providing for the use of a chloride-fluoride or fluoride bath with a density greater than that of cathode aluminium. Cathode aluminium 5 is thereby collected on the melt mirror in the top part of the diaphragm, from where it is removed through a drainage channel 13 placed in the diaphragm attachment 10 by the cathode aluminium accumulation and evacuation device 11 into the collecting tank.
The process involves the electrolytic transfer of aluminium ions from the anode surface bounded by a porous diaphragm through the bath melt to the cathode surface. At that, such conditions of anodic dissolution and cathodic deposition maintained (bath temperature from 780 °C to 920 °C, current density from 0.5 to 21 A/cm²) provide predominantly no dissolution of less electronegative metals at the anode than aluminium and no deposition of more electronegative metals at the cathode than aluminium. Oxide ceramics, carbon materials and inorganic fibres are used as the diaphragm material. The diaphragm material is selected so as to prevent the melt to be purified and the cathode metal from penetrating through it due to wetting effects and capillary forces. The ability to form a vessel with a bath of known shape in the liquid metal melt and the absence of danger of short circuit between the metal layers make it possible to perform the process vertically with a precisely adjustable value of the interpole distance, thus allowing precise control of the temperature conditions and providing reduced power consumption with a significant increase in the specific capacity of the melt volume. There is also no need to add weight to the anode melt with copper. The accessibility of the bath melt mirror allows for the addition, monitoring and adjustment of its composition if necessary.
Comparative analysis with the prototype shows that the claimed method differs from the known method by using the diaphragm to create a vessel with the bath in the anode metal melt, while in the prototype, the diaphragm is filled with anode metal and loaded into the bath. Additionally, the cathode is positioned vertically, thus increasing specific capacity while reducing heat loss, allows precise adjustment of the anode-cathode distance, provides easy access to the bath for monitoring, adjustment and replenishment, and makes the cathode an easily replaceable component during unit maintenance.

Embodiment of the Invention

Example 1. A removable porous diaphragm (6) is immersed in the pre-melted anode aluminium (8), fixed by attachment (10) on the refining device and filled with chloride-fluoride or fluoride dry bath (7), with a density in the molten state greater than that of cathode aluminium (5), ranging from 3.3 to 3.6 g/cm³, causing cathode aluminium (5) to rise to the melt surface. After the bath melts, a vertical cathode with a current lead (4) and drainage channels at the level of the bath melt mirror is placed in the removable porous diaphragm (6), and the anode current lead (9) is lowered into the anode aluminium, and direct current is applied. Refining is performed with a cathodic current density ranging from 0.5 to 21 A/cm², ensuring the release of cathode aluminium (5), and a temperature ranging from 780 °C to 920 °C. The purified alloy is removed on the batch basis from the working area of the diaphragm through the drainage channel (13) in the vertical cathode (4) by the accumulation and evacuation device (11) with the cathode aluminium collecting tank. Intermetallics in the aluminium or its alloy being purified are periodically removed using a hand tool that mechanically separates and removes solid compounds from the anode metal vessel as they accumulate. As it is consumed, a fresh batch of anode alloy is added or loaded into the anode aluminium vessel (9) through the cover.
The following salt systems can be used as baths:

NaF-AlF₃, K.O.=1.0÷1.8;
5÷30% w/w. NaCl-NaF-AlF₃, K.O. =1.0÷2.8;
15÷45% w/w. BaF₂-NaF-AlF₃, K.O. =1.0÷2.8;
35÷60% w/w. BaCl₂-NaF-AlF₃, K.O. =1.0÷2.8.

The bath level is maintained constant, and a top-up is carried out or a fresh batch of salt mixture is added if necessary. A bath batch can also be taken from the diaphragm for physicochemical analyses of its composition and to determine if adjustments to individual components are necessary. The temperature of electrolytic reduction depends on the choice of a particular salt system and ranges from 780 °C to 920 °C. The cathodic current density also depends on the choice of bath and ranges from 0.5 to 21 A/cm².
The diaphragm material may be as follows:

oxide ceramics;
carbon materials;
inorganic fibrous materials.

The claimed technical solution may be implemented in the process of industrial production and purification of aluminium or its alloys.
Example 2. Two removable porous diaphragms (6), fixed by attachment (10) on the refining device and filled with chloride-fluoride or fluoride dry bath (7), with a density in the molten state ranging from 1.6 to 2.2 g/cm³, are immersed in the pre-melted anode aluminium (8), causing the formed cathode aluminium (5) to sink to the diaphragm bottom. After the bath melts, a vertical cathode with a current lead (4) and drainage channels at the cathode bottom is placed in the removable porous diaphragm (6), and an anode current lead (9) is lowered into the anode aluminium, and direct current is applied. Refining is performed with a cathode current density ranging from 0.5 to 21 A/cm² and a temperature ranging from 780 °C to 920 °C. The purified alloy is removed on the batch basis from the working area of the diaphragm through the drainage channel (13) using the aluminium evacuation device with the cathode aluminium collecting tank (11). Intermetallics in the aluminium or its alloy being purified are periodically removed using a hand tool that mechanically separates and removes solid compounds from the anode metal vessel as they accumulate. As it is consumed, a fresh batch of anode alloy is added or loaded into the anode aluminium vessel (9) through the cover.

Claims

A device for electrolytic refining of aluminium and/or its alloys from metal impurities, having a cover vessel (12) lined with refractory materials to hold molten anode aluminium (8) with an anode lead (9), bath (7), and molten cathode aluminium (5), a cathode with a current lead (4), and a collecting tank (11) for cathode aluminium (5), no less than one porous diaphragm (6), characterised in that the porous diaphragm (6) is a closed removable vessel, mounted by attachment (10) on the cover (12) and filled with bath (7), permeable to bath but impermeable to molten aluminium or aluminium alloy with impurities and molten cathode aluminium (5), wherein the cathode with the current lead (4) is loaded and immersed in molten anode aluminium (9), with the cathode and current lead (4) vertically fixed inside the porous diaphragm vessel (6), and drainage channels (13) in the bottom part of the cathode are located at the level of molten cathode aluminium (5) to enable its removal from the porous diaphragm vessel (6) into the collecting tank (11).
A device according to claim 1, characterised in that the cathode (4) is made of an aluminium-wettable ceramic, such as TiB₂ or ZrB₂.
A device according to claim 1, characterised in that the porous diaphragm (6) is made of carbon materials.
A device according to claim 1, characterised in that the porous diaphragm (6) is made of inorganic fibrous materials.
A device according to claim 1, characterised in that the porous diaphragm (6) is made of oxide ceramics, such as Al₂O₃ or MgO.
A device according to claim 1, characterised in that the drainage channels (13) in the bottom part of the cathode (4) are angled from 45 to 90 degrees relative to the vertical cathode wall.
A method for electrolytic refining of aluminium and/or its alloys from impurities, involving placing the melt of contaminated aluminium and bath containing a melt of a mixture of salts of alkali or alkaline-earth metals and aluminium salt in a device for electrolytic refining, supplying direct current, and conducting electrolytic refining, characterised by the use of the device according to any of claims 1-6, with the refining process performed with a cathodic current density ranging from 0.5 to 21 A/cm², and a temperature ranging from 780 °C to 920 °C, and the molten bath has a density less than that of cathode aluminium (5).
A device for electrolytic refining of aluminium and/or its alloys from metal impurities, having a cover vessel (12) lined with refractory materials for accommodating therein molten anode aluminium (8) with an anode lead (9), bath (7) and molten cathode aluminium (5), a cathode with a current lead (4) and a collecting tank (11) for cathode aluminium (5), no less than one porous diaphragm (6), differing in that the porous diaphragm (6) is designed as a closed removable vessel mounted by means of an attachment (10) on a cover (12) and filled with bath (7), permeable to bath and impermeable to molten aluminium or aluminium alloy with impurities and molten cathode aluminium (5), wherein the cathode with current lead (4) is loaded and immersed in molten anode aluminium (9), wherein the current-supported cathode (4) is vertically fixed inside the porous diaphragm vessel (6), and in the cathode walls (4) or in the attachment (10) and cover (12) there are drainage channels (13) located at the level of the bath melt mirror to enable removal of cathode aluminium (5) from the porous diaphragm vessel (6) into the collecting tank (11).
A device according to claim 8, characterised in that the cathode (4) is made of an aluminium-wettable ceramic, such as TiB₂ or ZrB₂.
A device according to claim 8, characterised in that the porous diaphragm (6) is made of carbon materials.
A device according to claim 8, characterised in that the porous diaphragm (6) is made of inorganic fibrous materials.
A device according to claim 8, characterised in that the porous diaphragm (6) is made of oxide ceramics, such as Al₂O₃ or MgO.
A device according to claim 8, characterised in that the drainage channels (13) in the cathode walls (4) are angled between 45 and 90 degrees relative to the vertical cathode wall.
A device according to claim 8, characterised in that the drainage channel in the diaphragm attachment (10) is connected to the collecting tank (11) via a guide channel located in the cover (12).
A device according to claim 8, characterised in that the guide channel in the cover (12) is made of or lined with aluminium-wetted material to allow cathode aluminium (5) to be drained by gravity.
A method for the electrolytic refining of aluminium and/or its alloys from impurities, involving placing a melt of contaminated aluminium and bath containing a melt of a mixture of salts of alkali or alkaline-earth metals and aluminium salt into a device for electrolytic refining, supplying direct current, and performing electrolytic refining, characterised by using a device according to any of claims 8-15, with the refining process performed at a cathodic current density ranging from 0.5 to 21 A/cm², and a temperature ranging from 780 °C to 920 °C, and the molten bath having a density greater than that of cathode aluminium (5).