RU2840274C1 - Method of electrolytic refining of aluminium or alloys thereof from metal impurities (embodiments) - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: inventions relate to non-ferrous metallurgy and can be used for refining aluminium melt or its alloys from metal impurities. Methods include placing a melt of contaminated aluminium or an aluminium alloy and an electrolyte containing a melt of a mixture of salts of alkali or alkali-earth metals and aluminium salts, in device for electrolytic refining, DC supply and electrolytic refining. Refining process is carried out with cathode current density in range from 0.5 A/cm² to 21 A/cm² and in temperature range from 780 °C to 920 °C. Electrolyte in molten state has density higher or lower than density of molten cathode aluminium or its alloy.

EFFECT: higher efficiency, replenishment and correction of electrolyte composition.

10 cl, 4 dwg, 2 ex

Description

Field of technology

The invention relates to non-ferrous metallurgy and can be used for refining molten aluminum or its alloys from metallic impurities.

State of the art

Cleaning aluminum from unwanted impurities is an important technology that allows obtaining high-purity aluminum. Recently, an important trend has emerged towards the need to create a technology that allows working with various types of raw materials, including various aluminum scrap. One of the most important areas of this work is the creation and improvement of electrochemical cleaning technologies. The cleaning technology was first proposed in patent US 673364 and was a three-layer electrolyzer in which the role of the anode metal was performed by a contaminated aluminum alloy. A significant disadvantage of this method is the high energy costs of the cleaning process, which leads to a high cost price of purified aluminum.

Taking into account the ever-stricter environmental initiatives and international legislation, the creation of energy-efficient technologies for aluminum purification is an important and urgent task.

A method is known (H. Chang, V. de Nora and J.A. Sekhar. "Materials Used in the Production of Aluminum by the Eroult-Hall Method." - Publ. Krasnoyarsk. state University, Krasnoyarsk, 1998) of three-layer electrochemical refining of metals, according to which aluminum, subject to refining from undesirable metallic impurities, such as iron, silicon, copper, nickel and others, is specially weighted by adding copper (about 30%) and such an alloy is polarized anodically during electrolysis. The electrolyte is a chloride-fluoride melt. The refining temperature is 800 °C. In this case, pure aluminum is lighter than the electrolyte and, being a cathode, floats to the surface of the electrolyte. The diagram of such an electrolyzer is shown in Fig. 1, where:

1. On-board units

2. Coal block hearth

3. Thermal insulation

4. Anode current lead

5. Aluminum-copper anode alloy

6. Chloride-fluoride electrolyte

7. Refined aluminum

8. Cathode current lead

9. Lid

10. Taphole for pouring aluminum.

When direct current is passed through the anode, conditions are created for the preferential dissolution of aluminum and more electronegative metals, while less electronegative metals accumulate in the anode alloy, which is periodically cleaned from intermetallic impurities of iron, silicon, copper, etc. that crystallize during the accumulation. In the cathode process, conditions for the electrochemical extraction of more electronegative metals are not achieved, thus, only aluminum is extracted. The disadvantage of the known electrolyzer is that, in order to exclude accidental contamination of the cathode metal with particles of the anode alloy, a large interelectrode distance of 15 cm is maintained, which results in a high voltage on the electrolyzer (about 5.5 V) and, as a result, a high specific energy consumption, even with a current output of η = 0.98, amounting to more than 18 kW h / kg Al. Another disadvantage of the method is the need to use copper to make the anode alloy heavier.

Also known is a method and device for electrochemical refining (patent RU 2558316, published 27.07.2015), in which the technical result is achieved by using vibroacoustic, and/or ultrasonic, and/or electromagnetic, and/or MHD effects on the following components of the electrolyzer: membrane (on one side and/or on both sides), electrolyte, primary unrefined metal, refined metal, interfaces between the unrefined metal, membrane and refined metal. The disadvantage of this method is the impossibility of replenishing and monitoring the composition of the electrolyte, due to which the electrolyzer has a short service life, limited by the buffer capacity of the electrolyte for more electronegative metals, located only in the pores of the diaphragm.

The closest to the claimed device and method for refining aluminum is a prototype, where the technical solution is based on the fact that the primary metal to be purified is immersed in a container made of a material permeable to the electrolyte, but not permeable to the metal, placed in the electrolyte, and the purified metal is formed on the bottom of the electrolyzer or on the surface of a separate cathode (patent US 4115215, published 19.09.1978). The disadvantage of this method is the predominance of the cathode surface area over the anode surface area, thus the overall productivity of the device is limited by the anode current density, which determines the quality of the resulting metal.

Disclosure of invention

The technical problem of the invention is creation of a method intended for electrolytic refining of aluminum with increased productivity of the working volume, reduced energy consumption per unit of production and the possibility of carrying out long-term electrolysis by adding and adjusting the composition of the electrolyte, as well as the possibility of changing the diaphragm if necessary.

The technical problem is solved in that, according to one of the variants in the method of electrolytic refining of aluminum and/or its alloys from impurities, including placing a melt of contaminated aluminum and an electrolyte containing a melt of a mixture of alkali or alkaline earth metal salts and an aluminum salt in a device for electrolytic refining, supplying direct current and carrying out electrolytic refining, according to the claimed invention, the refining process is carried out with a cathode current density in the range from 0.5 to 21 A/ ^cm2, in the temperature range from 780 to 920°C, and the electrolyte in the molten state has a density lower than the density of cathode aluminum (5).

In a particular case, a method in which a device is used for electrolytic refining of aluminum and/or its alloys from metallic impurities, comprising a container with a lid (12) lined with refractory materials, for placing therein molten anodic aluminum (8) with an anodic current lead (9), an electrolyte (7) and molten cathodic aluminum (5), a cathode with a current lead (4) and a collection tank (11) for cathodic aluminum (5), at least one porous diaphragm (6), which is made in the form of a closed removable container installed by means of a fastener (10) on the lid (12) and filled with an electrolyte (7), permeable to the electrolyte and impermeable to molten aluminum or aluminum alloy with impurities and molten cathodic aluminum (5), into which a cathode with a current lead (4) is loaded and which is immersed in molten anodic aluminum (9), wherein the cathode c the current lead (4) is secured inside the container of the porous diaphragm (6) in a vertical position, and drainage channels (13) are made in the bottom part of the cathode, located at the level of the electrolyte melt mirror to enable the removal of cathode aluminum (5) from the container of the porous diaphragm (6) into the collection tank (11).

According to the second variant, in the method of electrolytic refining of aluminum and/or its alloys from impurities, including placing a melt of contaminated aluminum and an electrolyte containing a melt of a mixture of alkali or alkaline earth metal salts and an aluminum salt in a device for electrolytic refining, supplying direct current and carrying out electrolytic refining, according to the claimed invention, the refining process is carried out with a cathode current density in the range from 0.5 to 21 A/ ^cm2, in the temperature range from 780 to 920°C, and the electrolyte in the molten state has a density greater than the density of cathode aluminum (5).

In a particular case, a method in which a device is used for electrolytic refining of aluminum and/or its alloys from metallic impurities, comprising a container with a lid (12) lined with refractory materials, for placing therein molten anodic aluminum (8) with an anodic current lead (9), an electrolyte (7) and molten cathodic aluminum (5), a cathode with a current lead (4) and a collection tank (11) for cathodic aluminum (5), at least one porous diaphragm (6), which is made in the form of a closed removable container installed by means of a fastener (10) on the lid (12) and filled with an electrolyte (7), permeable to the electrolyte and impermeable to molten aluminum or aluminum alloy with impurities and molten cathodic aluminum (5), into which a cathode with a current lead (4) is loaded and which is immersed in molten anodic aluminum (9), wherein the cathode c the current lead (4) is secured inside the container of the porous diaphragm (6) in a vertical position, and in the walls of the cathode (4) or in the fastening (10) and cover (12) drainage channels (13) are made, located at the level of the electrolyte melt mirror to enable the removal of cathode aluminum (5) from the container of the porous diaphragm (6) into the collection tank (11).

The vertical cathode (4) can be made of aluminum-wettable ceramics, such as TiB ₂ or ZrB ₂ .

The porous diaphragm (6) can be made of carbon materials or inorganic fibrous materials or of oxide ceramics, for example, of Al ₂ O ₃ or MgO.

Drainage channels (13) in the walls of the cathode (4) or in the bottom part of the cathode (4) can be made at an angle of 45 to 90 degrees relative to the vertical wall of the cathode.

The drainage channel (13), made in the fastening (10) of the diaphragm (6) can be connected to the collection tank (11) by means of a guide channel placed in the cover (12), which can be made of an aluminum-wetted material or lined with an aluminum-wetted material to enable the cathode aluminum (5) to be drained by gravity.

Brief description of the drawings

The method in which the device is used is shown in Fig. 2, Fig. 3 and Fig. 4 and comprises: side blocks 1, bottom blocks 2, thermal insulation 3, vertical cathode with current lead 4, cathode aluminum 5, removable porous diaphragm 6, chloride-fluoride or fluoride electrolyte 7, anode aluminum 8, anode current lead 9, fastening 10 of porous diaphragm 6, drainage channels 13 located in the walls of the cathode or fastening 10, aluminum evacuation device with a collecting tank of cathode aluminum 11, device covers 12. The upper surface of the cathode and anodic aluminum can be protected from oxidation in air with salts and/or argon, or vacuum. Removable porous diaphragm 6 can be made of electrically conductive or non-conductive material.

Fig. 2 shows an electrolyzer with one diaphragm, which provides for the use of a chloride-fluoride or fluoride electrolyte with a density greater than that of the cathode aluminum. Due to this, the cathode aluminum 5 is collected on the melt mirror in the upper part of the diaphragm, from where it is removed using the aluminum accumulation and evacuation device 11.

The electrolyzer with paired diaphragms (Fig. 3) provides for the use of a chloride-fluoride or fluoride electrolyte with a density lower than that of the cathode aluminum. Due to this, the cathode aluminum 5 is collected in a receiver in the lower part of the diaphragm, from where it is removed using an aluminum accumulation and evacuation device 11. The upper surface of the electrolyte and the anodic aluminum can be protected from oxidation in the air by salts and/or argon, or vacuum. In this case, a principle of electrochemical cleaning of aluminum similar to that of the device (Fig. 2) is used, due to which the same technical result is achieved as when using an electrolyte with a density greater than that of the cathode aluminum.

Fig. 4 shows an electrolyzer with one diaphragm, which provides for the use of a chloride-fluoride or fluoride electrolyte with a density greater than the density of the cathode aluminum. Due to this, the cathode aluminum 5 is collected on the melt mirror in the upper part of the diaphragm, from where it is removed through a drainage channel 13, located in the fastening of the diaphragm 10 with the help of a device for accumulating and evacuating cathode aluminum 11 into a collection tank.

The process involves electrolytic transfer of aluminum ions from the anode surface limited by a porous diaphragm through the electrolyte melt to the cathode surface. In this case, such conditions of anodic dissolution and cathodic deposition are maintained (electrolyte temperature in the range of 780 to 920°C, current density in the range of 0.5 to 21 A/ ^cm2 ), at which mainly less electronegative metals than aluminum do not dissolve on the anode and more electronegative metals than aluminum do not deposit on the cathode. Oxide ceramics, carbon materials, and inorganic fibers are used as the diaphragm material. The diaphragm material is selected in such a way that it does not allow the melt being purified and the cathode metal to penetrate through it due to the wetting effect and capillary forces. Due to the possibility of forming a container with electrolyte of a known shape in the liquid metal melt and the absence of the risk of short circuit between the metal layers, it is possible to carry out the process in a vertical arrangement with a precisely regulated value of the interpolar distance, which allows precise control of the temperature regime and ensures reduced power consumption with a significant increase in the specific productivity of the melt volume. There is also no need to weight the anode melt with copper. The availability of the electrolyte melt mirror allows, if necessary, to supplement, control and adjust its composition.

A comparative analysis with the prototype allows us to conclude that the claimed method differs from the known one by the purpose of the diaphragm to create a container with electrolyte in the melt of the anode metal, while in the prototype the diaphragm is filled with the anode metal and loaded into the electrolyte. In addition, the cathode is located vertically, which allows increasing the specific productivity while reducing heat loss, performing precise adjustment of the MPR, providing easy access to the electrolyte for the purpose of its control, adjustment and replenishment, and also making the cathode an easily replaceable element during maintenance of the installation.

Implementation of the invention

Example 1.A removable porous diaphragm (6), secured by means of a fastener (10) on the refining device and filled with a chloride-fluoride or fluoride dry electrolyte (7), with a density in the molten state greater than the density of the cathode aluminum (5), in the range of 3.3÷3.6 g/cm, is immersed in the pre-melted anodic aluminum (8).³, due to which the cathode aluminum (5) rises to the surface of the melt. After the electrolyte has melted, a vertical cathode with a current lead (4) and drainage channels at the level of the electrolyte melt mirror is placed in a removable porous diaphragm (6), and an anode current lead (9) is lowered into the anode aluminum and direct current is turned on. The refining process is carried out with a cathode current density in the range from 0.5 to 21 A/cm², providing the release of cathode aluminum (5), and a temperature range from 780 to 920°C. The purified alloy is removed from the working area of the diaphragm by means of an accumulation and evacuation device (11) with a collecting tank of cathode aluminum through a drainage channel (13) made in the vertical cathode (4) in a periodic mode. The removal of intermetallics in the aluminum or its alloy being purified is carried out periodically using a hand tool, with the help of which solid compounds are mechanically separated and removed from the container with the anode metal as they accumulate. As it is consumed, a fresh portion of the anode alloy is added or loaded into the container with the anode aluminum (9) through the lid.

The following salt systems can be used as electrolytes:

- NaF-AlF ₃ , K.O.=1.0÷1.8;

- 5÷50% by weight NaCl-NaF-AlF _3, K.O. = 1.0÷2.8;

- 15÷45% by weight BaF ₂ -NaF-AlF3 _, K.O. = 1.0÷2.8;

- 35÷60% by weight BaCl2 _- NaF-AlF3 _, K.O. =1.0÷2.8.

The electrolyte level is maintained constant and, if necessary, a fresh portion of the salt mixture is added or loaded. A portion of the electrolyte can also be taken from the diaphragm to conduct physical and chemical analyses of its composition and determine the need for adjustments for individual components. The electrolysis temperature depends on the choice of a specific salt system and ranges from 780 to 920°C. The cathode current density also depends on the choice of electrolyte and ranges from 0.5 to 21 A/ ^cm2 .

The diaphragm material can be oxide ceramics, carbon materials, or inorganic fibrous materials.

The claimed technical solution can be implemented in industrial production and purification of aluminum or its alloys.

Example 2. Two removable porous diaphragms (6), secured by means of a fastener (10) on a refining device and filled with a chloride-fluoride or fluoride dry electrolyte (7) with a density in the molten state in the range of 1.6÷2.2 g/ ^cm3 , are immersed in pre-melted anodic aluminum (8), due to which the resulting cathode aluminum (5) is lowered to the bottom of the diaphragm. After melting the electrolyte, a vertical cathode with a current lead (4) and drainage channels in the bottom part of the cathode is placed in the removable porous diaphragm (6), and an anode current lead (9) is lowered into the anodic aluminum and direct current is turned on. The refining process is carried out with a cathode current density in the range of 0.5 to 21 A/ ^cm2 and a temperature range of 780 to 920°C. The purified alloy is removed from the working area of the diaphragm via a drainage channel (13) in a periodic mode using an aluminum evacuation device with a cathode aluminum collecting tank (11). Intermetallics in the aluminum or its alloy being purified are removed periodically using a hand tool, with the help of which solid compounds are mechanically separated and removed from the container with the anode metal as they accumulate. As the anode aluminum (9) is consumed, a fresh portion of the anode alloy is added or loaded through the lid.

Claims (10)

1. A method for electrolytic refining of aluminum or its alloys from metallic impurities, comprising placing molten aluminum or aluminum alloy containing metallic impurities and an electrolyte containing a molten mixture of alkali or alkaline earth metal salts and aluminum salts in a device for electrolytic refining, supplying direct current and carrying out electrolytic refining, characterized in that the refining process is carried out with a cathode current density in the range from 0.5 to 21 A/ ^cm2 , in the temperature range from 780 to 920°C, and the electrolyte in the molten state has a density greater than the density of the molten cathode aluminum or aluminum alloy (5). 2. The method according to claim 1, in which a device for electrolytic refining of aluminum or its alloys from metallic impurities is used, comprising a container with a lid (12) lined with refractory materials, for placing in it molten anodic aluminum or aluminum alloy (8) with an anodic current lead (9), an electrolyte (7) and molten cathodic aluminum or aluminum alloy (5), a cathode with a current lead (4) and a device for accumulating and evacuating (11) molten cathodic aluminum or aluminum alloy (5) into a collection tank, at least one porous diaphragm (6), which is made in the form of a closed removable container installed by means of a fastener (10) on the lid (12) and filled with electrolyte (7), permeable to the electrolyte and impermeable to molten anodic aluminum or aluminum alloy (8) with impurities and molten cathodic aluminum or aluminum alloy (5), into which a cathode with a current lead (4) is loaded and which is immersed in molten anode aluminum or aluminum alloy (8), wherein the cathode with a current lead (4) is secured inside the container of the porous diaphragm (6) in a vertical position, and in the walls of the cathode with a current lead (4) or in the fastening (10) and cover (12) drainage channels (13) are made, located at the level of the molten electrolyte mirror to enable the removal of molten cathode aluminum or aluminum alloy (5) from the container of the porous diaphragm (6) into a collection tank. 3. A method for electrolytic refining of aluminum or its alloys from metallic impurities, comprising placing molten aluminum or aluminum alloy containing metallic impurities and an electrolyte containing a molten mixture of alkali or alkaline earth metal salts and aluminum salts in a device for electrolytic refining, supplying direct current and carrying out electrolytic refining, characterized in that the refining process is carried out with a cathode current density in the range from 0.5 to 21 A/ ^cm2 , in the temperature range from 780 to 920°C, and the electrolyte in the molten state has a density lower than the density of the molten cathode aluminum or aluminum alloy (5). 4. The method according to claim 3, in which a device for electrolytic refining of aluminum or its alloys from metallic impurities is used, comprising a container with a lid (12) lined with refractory materials, for placing in it molten anodic aluminum or aluminum alloy (8) with an anodic current lead (9), an electrolyte (7) and molten cathodic aluminum or aluminum alloy (5), a cathode with a current lead (4) and a device for accumulating and evacuating (11) molten cathodic aluminum or aluminum alloy (5) into a collection tank, at least one porous diaphragm (6), which is made in the form of a closed removable container installed by means of a fastener (10) on the lid (12) and filled with electrolyte (7), permeable to the electrolyte and impermeable to molten anodic aluminum or aluminum alloy (8) with impurities and molten cathodic aluminum or aluminum alloy (5), into which a cathode with a current lead (4) is loaded and which is immersed in molten anode aluminum or aluminum alloy (8), wherein the cathode with a current lead (4) is fixed inside the container of the porous diaphragm (6) in a vertical position, and in the bottom part of the cathode with a current lead (4) drainage channels (13) are made, located at the level of the molten electrolyte mirror to enable the removal of molten cathode aluminum or aluminum alloy (5) from the container of the porous diaphragm (6) into a collection tank. 5. The method according to item 4, in which the drainage channels (13) in the bottom part of the cathode with the current lead (4) are made at an angle of 45 to 90 degrees relative to the vertical wall of the cathode. 6. The method according to item 2 or 4, in which the cathode with the current lead (4) is made of aluminum-wettable ceramics, for example TiB ₂ or ZrB ₂ . 7. The method according to claim 2 or 4, wherein the porous diaphragm (6) is made of carbon materials, inorganic fibrous materials or oxide ceramics, for example Al ₂ O ₃ or MgO. 8. The method according to item 2, in which the drainage channels (13) in the walls of the cathode with the current lead (4) are made at an angle of 45 to 90 degrees relative to the vertical wall of the cathode. 9. The method according to claim 2, in which the drainage channel (13), formed in the fastening (10) of the diaphragm (6), is connected to the collection tank by means of a guide channel located in the cover (12). 10. The method according to claim 9, wherein the guide channel in the cover (12) is made of an aluminum-wetted material or is lined with an aluminum-wetted material to enable the removal of molten cathode aluminum or aluminum alloy (5) by gravity.