JPS6343475B2 - - Google Patents

Abstract

Alumina reduction cells are used to prepare aluminium metal by the electrolytic reduction of alumina dissolved in molten cryolite. The molten aluminium formed collects at the bottom of the cell. When the cell floor serves as cathode and is of carbon, the molten aluminium does not wet the carbon making it necessary to increase the inter-electrode distance and reducing efficiency. The invention provides a cell wherein a floor cathode comprises a carbon substrate having an adherent surface layer of electrodeposited titanium diboride which is readily wettable by molten aluminium and which makes possible considerable savings in the expense of running the cell. The titanium diboride layer can be formed in situ in the cell or can be formed outside the cell on carbonaceous elements which are subsequently installed in the cell.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved alumina reduction cell and method of manufacturing the same.

Metallic aluminum is produced by reducing alumina using electrolysis. Conventional alumina reduction cells usually have a refractory lining of carbon and consist of a vessel containing alumina dissolved in molten cryolite as the molten electrolyte. The cell or vessel floor typically consists of carbonaceous material. The carbonaceous material provides thermal insulation and serves as the cathode portion. At least one anode is disposed within the container spaced apart from the cathode. Aluminum is formed by electrolytic reduction of alumina with the flow of current between the anode and cathode. The density of the molten aluminum formed is higher than the cryolite electrolyte and the molten aluminum is collected as a metal pool on the floor of the cell. Molten metal aluminum is drained from the cell such that the pool of metal aluminum that forms on the cell floor is not too deep. It is quite obvious that the molten aluminum on the floor of the cell cannot contact the anode or cell leakage will occur.

Molten aluminum does not necessarily have a good affinity with carbonaceous materials, and it is not easy to uniformly wet the surface of the latter. This is most easily demonstrated by contacting a drop of molten aluminum with the untreated surface of a carbonaceous substrate. On the substrate the aluminum will form grains or globules and will not spread over the surface of the carbonaceous substrate. Operational problems arise because the molten metal aluminum wets rapidly and does not spread over the floor of the alumina reduction cell. The deeper the molten aluminum layer at the bottom of the cell, the greater the distance between the electrodes needs to be. The greater the distance between the electrodes, the less efficiently the cell operates and the greater the cell's power requirements. Additionally, the layer of molten aluminum formed on the untreated carbonaceous cell bed is thick enough to generate thermal or magnetic currents, making it somewhat unstable and prone to disturbances. It is. Waves therefore form within the layer which contact the anode and cause the cell to leak current. The thickness of the layer of molten aluminum at the bottom of the cell can vary from cell to cell, but is typically in the range of 3 to 8 centimeters.

Proposals have been made to vary the bed of an alumina reduction cell. For example, it is known that titanium carbide and zirconium carbide have a high affinity with molten aluminum, have good electrical conductivity, and have low solubility in molten aluminum. U.S. Pat. No. 3,471,380 (Bullough - October 7, 1969) describes a method for improving the performance of alumina reduction cells by forming a coating of useful metal carbides, particularly titanium or zirconium carbides, on the carbon cathode surface of the cell. has been done. The coating is produced by adding a refractory metal or its compound to the electrolyte within the cell. As a result, a metal carbide coating forms on the carbon cathode surface during cell operation. However, the sensitivity of refractory metal carbides to thermal shock does not make them ideal coatings for carbon substrates in alumina reduction cells.

US Defense Patent No. T993002 (Kaplan - April 1, 1980) discloses an apparatus for contacting molten aluminum with a titanium diboride surface at the bottom of an alumina reduction cell. The titanium diboride surface is equipped with refractory tiles that are fixed to a carbonaceous substrate. The tiles can be wetted with fused alumina and are specified to be chemically inert under the conditions of the electrolytic process. Titanium diboride is less sensitive to thermal shock than titanium carbide, and it is possible to store considerable energy on the surface of titanium diboride during operation of an alumina reduction cell; The proposed tiles are quite thick and therefore titanium diboride, a very expensive material, is necessarily wasted. Problems also arise from the need to bond the tiles to the carbonaceous substrate. It is believed that the method of the defense patent is even more serious than the problem of apparent vulcanization and is very expensive in terms of the tiles required. This is because the manufacturing and installation of the tiles incurs significant costs over and above the cost that could be saved due to the reduced energy consumption of the alumina reduction process.

U.S. Patent No. 3,697,390 (McCawley et al., 1972)
Oct. 10) and U.S. Pat. No. 3,827,954 (McCawley et al. Aug. 6, 1974) plate titanium, zirconium, or haunium boride on a substrate by electrodeposition from a molten borate bath. Forming is disclosed. The anode to be used includes the desired boride metal or boride itself. There is no mention in either patent of depositing a metal boride coating on a carbonaceous substrate and that the cathodes used in the examples are all metals, such as molybdenum, nickel, or "Inconel." Japanese Patent Publication No. 49-67844 (Toyota - July 1, 1974)
(Japan) discloses a method of coating ferrous metal or its alloys with a layer of titanium diboride by electrodeposition from a molten bath of borate containing molten titanium. It is stated that the carbon content of the metal or alloy cathode used in this method must be less than 0.1 percent unless a titanium carbide layer is formed.

As mentioned above, it has been recognized in the prior art that it would be desirable to provide an alumina reduction cell with a cathode or cathode coating that has good affinity for molten aluminum, and titanium diboride has been suggested in this connection; It has been thought that a titanium diboride layer cannot be formed by electrodeposition on a carbonaceous substrate without forming titanium.
It is therefore surprising that we have found that a titanium diboride layer can be formed on a carbonaceous substrate by electrodeposition.

It is therefore an object of the present invention to provide an alumina reduction cell consisting of a container with a refractory lining and at least one anode arranged within said container. A feature of the cell is that at least the floor portion of the vessel acts as a cathode and that the cathode includes a cathode substrate having a sticky surface area of electrodeposited titanium diboride. What is adhesiveness here?
This refers to the property that the surface layer adheres closely to the carbon substrate and is difficult to peel off.

The alumina reduction cells of the present invention can be fabricated by in-situ electrodeposition of a titanium diboride layer on the carbonaceous cathode within the cell, or by depositing titanium diboride on the surface of at least one of the carbonaceous blocks or elements from outside the cell. It can be fabricated by electrodepositing a layer of titanium oxide and then mounting blocks or elements within the cell to provide a coated cathode surface. In the latter case the coated blocks or elements are placed on the floor of the cell and fixed, for example with pitches.

A tacky surface layer of titanium diboride is provided according to the invention on a carbonaceous cell bed or on a component carbonaceous block or element containing a source of boron and having titanium or a compound thereof dissolved therein. Formed by electrodeposition from molten electrolyte.
The carbonaceous cell bed or block or element serves as the cathode and ensures the electrodeposition of titanium diboride thereon, which then contours the surface of the cathode.
forming a fairly sticky surface layer. Carbon is preferred as the anode, as it has been found that higher quality electrodeposits are formed with carbon anodes. However, it is also possible to use a consumable titanium anode, which is dissolved in the anode to provide a titanium value with a molten electrolyte. When using a titanium anode, it is not necessary to separately dissolve the titanium source in the molten electrolyte. However, if the titanium source is dissolved in the electrolyte, for example when using a carbon anode, it is preferred to use titanium dioxide or titanate as such source. In particular, it is preferred to use an electrolyte containing 2-10% by weight of titanium dioxide as the titanium source. The molten electrolyte must contain a boron source. It is also preferred to use anhydrous borates, particularly sodium tetraborate (borax) or potassium tetraborate, as such a boron source. Generally, the dissolved electrolyte must be sufficiently conductive to provide a sticky electrodeposit of titanium diboride on the carbonaceous cathode, and sufficiently fluid to be readily removed from the electrolytic cell. If the titanium diboride electrodeposit is formed in situ in an alumina reduction cell, the electrolyte must be removed and the cell cleaned.

The electrolysis conditions for electrodeposition of titanium diboride are not particularly critical, but the cell voltage should not exceed 2 volts to form a good quality electrodeposit. At a voltage of 2 volts or more, the electrodeposited material becomes powdery and the adhesiveness tends to deteriorate. The preferred voltage is 1.2 to
It is 1.8 volts. The current density can vary over a wide range, but suitable values are between 5 and 100 m.a./cm ² .
The temperature is of course chosen such that the electrolyte melts and has the desired conductivity. A suitable temperature is 900-1000°C. If necessary, a fluxing agent may be added to the electrolyte to assist in operating at the desired temperature. The electrolyte is preferably agitated to aid in the formation of a good quality electrodeposit. Conveniently, the stirring is caused by a rotating anode. The electrolysis time is highly dependent on the desired thickness for the surface layer of titanium diboride. Prolonged electrolysis and replenishment of electrolyte as necessary will result in thicker deposits. Electrodeposition can be repeated to form successive layers if desired.

The titanium diboride layer can be electrodeposited directly onto the untreated carbonaceous cathode, or an interlayer of titanium carbide can be pre-electrodeposited if desired.

The surface layer of titanium diboride on the carbonaceous cathode of the alumina reduction cell of the present invention has a high affinity for molten aluminum due to the above-mentioned advantages, and the metallic sodium that may be formed during alumina reduction penetrates into the interior of the cathode. reduce the phenomenon. Cathode destruction occurs when sodium penetrates the carbonaceous cathode.

The present invention will be described with reference to the following examples, with particular emphasis on the process of forming adhesive titanium diboride, which is the central idea of the present invention. Once the adhesive surface layer of titanium diboride is formed on the carbon substrate, the step of constructing the alumina reduction cell so that it becomes the container floor is a technique commonly used in the industry, so special supplementary information is required. There's nothing to do.

Example 1 Electrolysis of an electrolyte containing 5% by weight of TiO ₂ and 95% by weight of K ₂ B ₄ O ₇ was carried out using a graphite electrode. The anode was used as a stirrer. electrolysis
950℃, 1.3-1.8 volts and current density 36-56m.a./
cm ² for 4 hours.

After electrolysis, the remaining electrolyte is removed and the cathode is washed with water. X-ray diffraction confirmed the presence of titanium diboride, and optical microscopy confirmed the layer thickness to be approximately 50 microns. Wetness tests confirmed that it is easily compatible with molten aluminum.

Example 2 An electrolyte containing 5% by weight of TiO ₂ and 95% by weight of Na ₂ B ₄ O ₇ was electrolyzed using a carbon cathode and a graphite anode at 950°C, 1.3 volts, and a current density of 60-100 m.
^a./cm2 for 3.5 hours. After electrolysis, the cathode was washed to remove the electrolyte. X-ray diffraction and optical microscopy tests confirmed the presence of sticky titanium diboride.

Example 3 Electrolysis of Na ₂ B ₄ O ₇ was continued for 5 hours at 950° C., 1.5 volts, and a current density of 30 m.a./cm ² using a graphite cathode and a rotating titanium anode. After electrolysis, the cathode is cleaned to remove the electrolyte. X-ray diffraction and optical microscopy confirmed the presence of a sticky titanium diboride layer.

Example 4 Electrolysis of an electrolyte containing 2% by weight of TiO ₂ and 98% by weight of K ₂ B ₄ O ₇ was carried out using a graphite cathode and a titanium anode at 950°C, 1.2 volts, and a current density of 5 m.a./
^cm2 for 5 hours. After electrolysis, the remaining electrolyte was removed. X-ray diffraction and optical microscopy confirmed that the graphite cathode was coated with a layer of titanium diboride with an estimated thickness of 20 microns.

Example 5 Electrolysis of an electrolyte containing K ₂ B ₄ O ₇ was carried out at 950°C using a graphite cathode and a titanium anode.
It lasted for 4.5 hours at 1.5 volts and a current density of 35 m.a./cm ² . After electrolysis, the electrolyte was washed away. X-ray diffraction and optical microscopy confirmed the presence of a titanium diboride layer on the surface of the graphite cathode with an estimated thickness of about 50 microns.

Claims (1)

[Scope of Claims] 1. An alumina reduction cell consisting of a container with a refractory lining and at least one anode disposed in the container, with at least the bottom of the container acting as a cathode, the cathode being electrodeposited. An alumina reduction cell characterized in that it consists of a carbon substrate with a sticky surface layer of titanium diboride. 2. An alumina reduction cell consisting of a vessel with a refractory lining and at least one anode disposed within said vessel, at least a portion of the vessel floor being formed of carbon and acting as a cathode, containing and dissolving a boron source therein. Production of an alumina reduction cell characterized in that a molten electrolyte containing titanium or its compounds is electrolyzed at a voltage not exceeding 2 volts to form a sticky surface layer of titanium diboride on said cathode by electrodeposition. Method. 3 Electrolyzing a sticky surface layer of titanium diboride on the surface of at least one of the plurality of carbonaceous blocks at a voltage not exceeding 2 volts from a molten electrolyte containing a boron source and having titanium or its compounds dissolved therein. and then attaching the resulting carbonaceous block to the floor of a vessel in which an alumina reduction cell is to be formed, the alumina reduction characterized in that it is configured such that at least one surface of said block acts as a cathode surface of the cell. Cell manufacturing method.