GB2062682A

GB2062682A - Protecting Electrodes of an Electrolytic Cell from Thermal Shock

Info

Publication number: GB2062682A
Application number: GB8035179A
Authority: GB
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1980-10-31
Filing date: 1980-10-31
Publication date: 1981-05-28

Abstract

In operating a cell for production of metal at elevated temperature having a cathode of the type including at least one hollow body defining a cavity, the cathode is prevented from cracking as its temperature is raised by the provision of a metal plug having a thermal conductivity greater than that of the hollow body in the cavity during the start up of the cell, the metal plug having a melting point approximate to the operating temperature. For aluminum electrolysis at about 950 DEG C., a cavity in each cathode 11 is filled with a metal plug 18, preferably a solid plug of 33.2% Cu-Al alloy. A portion of each cathode protruding into a chamber of the cell is provided with a sleeve 23 of insulating material, preferably expanded fibrous kaolin. A heat dispersing means 22 encircles each cathode radially outwardly thereof. A preferred form of the heat dispersing means is a stainless steel jacket located externally of the insulating material sleeve. After the cell is heated, the chamber is filled with a bath, and electrolysis is established. <IMAGE>

Description

SPECIFICATION Electrolytic Cell The present invention relates to thermal shock protection during initial heat up stages for electrodes of an electrolytic cell operating at an elevated temperature. Electrodes of the type requiring thermal shock protection by the method and apparatus of the present invention are of the type described in U.S. Patent 4,071,420. In a preferred embodiment, the cathodes of this patent are hollow cylindrical bodies of sintered TiB2 protruding out of a metal pad toward the anode. While the cathodes of U.S.Patent 4,071,420 are highly effective for the production of aluminum after being heated to an elevated operating temperature, it has been found in practice that such cathodes tend to be brittle and subject to cracks and breakage if not protected from thermal shock in the initial heat up or start up stages by the method and apparatus of the present invention.

A form of thermal shock protection for TiB2 electrodes during start up of an electrolytic cell is shown in British Patent 1,046,705. In Figure 5 and on pages 2-4 of the specification, there is disclosed an insulating layer around each refractory metal element comprising a one-half inch thick layer of powdered alumina. In a preferred embodiment, the insulating layer is retained in an aluminum container that melts away at cell operating temperatures. However, this British Patent fails to teach the desirability of providing a metal heat dispersing means having a melting point at least approximating the operating temperature of the cell, as disclosed and claimed herein.The refractory metal electrodes shown in the British Patent are solid rather than hollow so that the need for a heat conductor means inserted within a cavity in a hollow body electrode during start up is not suggested in the patent.

It is a principal object of the present invention to provide an electrolytic cell for production of a metal at an elevated operating temperature in which means are provided to protect the electrodes of the cell during initial heat up from thermal shock and accompanying cracks and breakage of the electrodes.

In accordance with the invention there is provided an electrolytic cell for production of metal at an elevated operating temperature, comprising an anode; a cathode spaced from the anode and including at least one hollow body, said hollow body defining a cavity open at a distal end closest the anode and closed at a proximal end opposed to the distal end; and heat conductor means in said cavity during start up of the cell comprising of the cell comprising a metal plug having a thermal conductivity greater than that of the hollow body and a melting point less than the melting point of aluminum.

In a preferred embodiment wherein the cell is used for electrolytic production of aluminum, the heat conductor means comprises a solid plug of 33.2% Cu-Al alloy having a melting point of about 5470C.

Two other forms of protection from thermal shock may also be used in preferred embodiment of the invention. When the cell is heated by an auxiliary heat source and the electrode includes a portion protruding into a chamber in the cell, the protruding portion may be encircled by a heat dispersing means proximate the electrode. The heat dispersing means comprises a metal jacket having a melting point at least approximating the operating temperature of the cell. In a preferred embodiment, the metal jacket is stainiess steel having a melting point of about 1 4400C., is highly resistant to oxidation at temperatures of up to at least 1 4000C. and is soluble in the cell chamber media after operating temperature is reached and the cell is placed into operation.

When the cell is heated by an auxiliary heat source and the electrode includes a portion protruding into a chamber in the cell, a heat insulating means may be interposed between the protruding portion of the electrode and the heat source. The insulating means is soluble in a molten bath poured into the cell chamber after operating temperature has been reached. In a preferred embodiment, the heat insulating means is a sleeve of expanded fibrous kaolin circumscribing the exposed portion of a cathode.

The preferred insulating means comprises a onehalf inch thickness of material having thermal conductivity less than about 0.7 BTU/(hr) (ft2) (OF/in).

Referring to the drawings: Figure 1 is a fragmentary perspective view of a portion of an electrolytic cell for the production of aluminum constructed in accordance with the present invention, Figure 2 is a top elevational view of the electrolytic cell of Figure 1, and Figure 3 is a fragmentary cross-sectional view taken along the line 3-3 of Figure 2, wherein the cell is heated to its operating temperature.

A preferred embodiment of an electrolytic cell for production of aluminum having cathodes 11 protected from thermal shock in accordance with the present invention is illustrated in Figure 1. The portion of the cell shown in Figure 1 is at room temperature prior to initiating start up and before any molten bath is poured into the cell chamber 12. For clarity of illustration, the carbon anode 13 is shown elevated several inches above the cathodes 11. In Figure 2, a rectangle 1 3a represents the shadow of the anode 1 3. Portions of the electrolytic cell not shown are similar to the cell described and illustrated in U.S. Patent 4,071,420.

Referring now to Figures 1-3, each cathode 11 comprise a hollow body or cylindrical sintered TiB2 tube 14 having a height of four inches, an internal diameter of three inches, and a wall thickness of one-half inch. Six round holes accepting the TiB2 tubes 14 were cut in a carbon cathode block 1 5 at a depth of 2-1/2 inches using a core drill bit of appropriate size. Proximal portions of the TiB2 tubes 14 were then cemented into each annular depression and leveled at a height of 1-1/2 inches above the upper surface portion 15a of the cathode block 15. Persons skilled in the art will understand that the dimensions stated above may be varied without departing from the present invention. A suitable cement 1 6 is sold under the trade designation C34 by Union Carbide Corporation.This cement includes particles of carbon in a resin base and provides an electrically conductive bond between the cathode block 15 and TiB2 tubes 14.

As shown in Figures 1 and 3, each cathode 11 includes a proximal portion cemented to the cathode block 1 5 and a distal or protruding portion extending toward the anode 13. The cathodes 11 are formed from hollow cylindrical bodies extending into the chamber 12 in the direction of the anode 13. When cemented to the cathode block 15, each hollow body defines a cavity 1 7 open at a distal end closest the anode 13 and closed at a proximal end opposite the distal end.

While the present invention is described with reference to a single preferred embodiment wherein TiB2 tubes 14 are provided with thermal shock protection, the cathodes 11 may be formed from other refractory hard substances. The borides, nitrides and carbides of titanium and zirconium, for example, are suitable. The start up method of the present invention may also be used for protecting from thermal shock anodes made of various other refractory materials including electrically conductive oxides of divalent and trivalent metals inert to the contents of the cell at its operating temperature. Examples of such oxides are CoCr2O4, Tire204, CoY2O4, N iCr204 and NiCo2O4.

The cavity 17 in each TiB2 tube 14 is filled by a heat conductor means, preferably a solid metal plug 18. The heat conductor means has a thermal conductivity greater than that of the TiB2 tube or hollow body 14, thereby minimizing temperature differences between various locations on each hollow body. In a preferred embodiment the metal plug 1 8 comprises a cylinder of an alloy of aluminum containing 33.2% copper. This alloy has a melting point of 5470C. Other alloys of aluminum having melting points in the range of about 450 to 5500C. are also suitable. Alloys of zine, tin and magnesium with aluminum may be used instead of the Cu-AI alloy described above.

Alloys that wet TiB2 are preferred. The plug 1 8 should preferably have a melting point less than the melting temperature of aluminum (6600 C.) in order to protect the cathodes 11 from thermal shock during final stages of heating. It is, of course, desirable that the plug 1 8 have a boiling point greater than the operating temperature of the cell.

The metal plug 18 is inserted as a solid cylinder into the cavity 1 7 prior to starting up the cell. Each TiB2 tube 14 is shaped to define a cylindrical cavity 17 having an inner diameter slightly greater than the outer diameter of the metal plug 1 8. The tube 14 therefore does not burst upon heating, even though metal of the plug 1 8 has a greater coefficient of thermal expansion than TiB2.

In the preferred embodiment of Figures 1-3, the cell is provided during start up with several carbon resistor blocks 21 extending between an upper surface portion 1 5a of the cathode block 15 and the anode 13. Each resistor block 21 has transverse dimensions of two inches by two inches and has a height of 2-3/4 inches. Each of these blocks 21 acts as an auxiliary radiant heat source to supply radiant heat to the chamber 12 during the final stages of cell heat up. A current of at least five amps per square inch is passed through the blocks 21, thereby heating their outer surfaces to temperatures of about 20000C. It has been gound that when unprotected TiB2 tubes 14 are exposed to radiant heat at temperatures of this magnitude, the tubes 14 may break under the stress of thermal shock.

The TiB2 tubes 14 are shown as protected from thermal shock by encircling each tube 14 with a heat dispersing means or metal jacket 22. Each jacket 22 is at least about 1/8 inch thick, has a diameter one inch greater than the corresponding tubes 14 and is 1/2 inch higher than the tubes 14.

Mild steel and stainless steel have been tried for the heat dispersing means 22, but stainless steel is superior because of its higher melting point and resistance to high temperature oxidation. The heat dispersing means should have a melting point at least approximating the operating temperature of the cell. When the cell is used for electrolytic production of aluminum, the heat dispersing means preferably has a melting point of greater than about 1 4000C. The heat dispersing means should also be resistant to oxidation and structurally rigid at temperatures of about 14000C.

Additional thermal shock protection is provided by interposing a heat insulating means between the resistor blocks 21 and TiB2 tubes 14. Each heat insulating means preferably comprises a generally cylindrical sleeve 23 of expanded fibrous kaolin insulating material. As shown in Figure 3, the sleeve 23 has a thickness of about 1/2 inch, filling the gap between the metal jacket 22 and hollow TiB2 tube 14. One preferred insulating material is sold under the trade designation Kaowool Unifelt 3000 by Babcock Wilcox. Other insulating materials may be used provided they have melting points of greater than about 1400 to 1 5000C., are chemically stable at temperatures of at least 20000C. and are soluble in the cell contents or media at the operating temperature of the cell. In a preferred electrolytic cell for production of aluminum, the cell chamber contains a molten bath at its operating temperature that is predominantly cryolite, and the kaolin sleeve dissolves in the molten bath.

When other insulating materials are substituted for kaolin, the insulating effect should preferably be equivalent to a 1/2 inch thickness of material having a thermal conductivity less than about 0.7 BTU/(hr) (ft2) (OF/in). This preferred thickness may be reduced if a better insulator is used, but a greater thickness of material is desirable when thermal conductivity of the heat insulating means is increased. When the heat insulating means is expanded kaolin, a lesser thickness of as little as 1/4 inch may be quite beneficial.

In a preferred cell start up performed in accordance with the present invention, 240 hollow TiB2 tubes 14 were cemented to a cathode block 1 5 and protected from thermal shock, as shown in Figures 1 and 2. Several hundred pounds of charcoal briquettes (not shown) were distributed around the TiB2 tubes 14 in accordance with the preheating method disclosed in U.S. Patent 4,146,444. The anodes 1 3 were then placed in the cell, resting on the resistor blocks 21. The sides, ends and middle of the cell were filled with charcoal adjacent the anodes 13 to a depth of about five to six inches. The briquettes were soaked with kerosene and ignited, with air being pumped into the cell through ducts (not shown) to enhance burning.

The ignited charcoal briquettes act as an auxiliary source of radiant heat in the initial stages of cell start up. The cell temperature rose to about 550 to 6000C. at a rate of about 50 C/hr.

After the cell reached a temperature of about 6000C., DC current was passed from the anodes 1 3 through the resistor blocks 21 and cathode block 1 5. When the cell temperature reached about 9500C., the spent charcoal was removed, the cell was filled with solid and liquid bath having the following nominal composition in weight precent: 79% cryolite, 11% AIR3, 6% CaF2 and 4% Awl203, and electrolysis was started. After all of the bath was melted, the resistor blocks 21 were removed, and the anode 13 were lowered to within about 3/8 inch of the TiB2 tubes 1 4.

In the first few days of electrolysis, aluminum tapped from the cell has significant levels of Cu, Fe and Si impurities resulting from dissolution of the thermal shock protection elements of the cell.

However, within 1 8 days the cell was producing 99.9% pure aluminum. Inspection of the cell after five weeks revealed that only one TiB2 tube has been chipped and another TiB2 tubes was broken.

Claims

1. An electrolytic cell for production of metal at an elevated operating temperature, comprising an anode; a cathode spaced from the anode and including at least one hollow body, said hollow body defining a cavity open at a distal end closest the anode and closed at a proximal end opposed to the distal end; and heat conductor means in said cavity during start up of the cell comprising a metal plug having a thermal conductivity greater than that of the hollow body and a melting point less than the melting point of aluminum.

2. An electrolytic cell according to claim 1, which includes a chamber for containing a molten bath electrically connecting the anode and cathode, said hollow body including a portion protruding into the chamber; and heat dispering means encircling said portion during start up of the cell, said heat dispersing means comprising a metal jacket spaced radially outwardiy of said portion and proximate thereto.

3. An electrolytic cell according to claim 2, wherein the metal jacket has a melting point at least approximating the operating temperature of the cell.

4. An electrolytic cell according to claim 3, wherein the metal jacket has a melting point of greater than about 14000C.

5. An electrolytic cell according to any one of claims 2 to 4, wherein the metal jacket is soluble in a molten bath contained in the chamber, at the operating temperature of the cell.

6. An electrolytic cell according to any one of claims 2 to 5, which includes a heat insulating means comprising a sleeve of an insulating material encircling the protruding portion of the hollow body adjacent thereto.

7. An electrolytic cell of claim 6, wherein the insulating material is soluble in a molten bath contained in the chamber, at the operating temperature of the cell.

8. An electrolytic cell according to any one of the preceding claims, wherein the hollow body comprises a refractory hard substance which is a boride, nitride or carbide of titanium or zirconium.

9. An electrolytic cell according to any one of the preceding claims, wherein the metal plug constitutes an alloy of aluminum.

1 0. An electrolytic cell according to claim 9, wherein the metal plug constitutes an aluminumcopper alloy.

11. An electrolytic cell for the production of metal at an elevated operating temperature substantially as hereinbefore described and is illustrated in the accompanying drawings.

12. A method for starting up an electrolytic cell for production of metal at an elevated operating temperature, heating a cell according to any one of the preceding claims, to its operating temperature.