CERMET INERT ANODE ASSEMBLY HEAT RADIATION SHIELD

Abstract

A method of protecting an inert anode assembly (16) operating in an electrolysis cell (10) for producing metal when an adjacent assembly (16′) is removed exposing remaining assemblies to low ambient temperatures (40) by utilizing heat radiation shields (24) which can circumscribe every inert anode assembly (16), where the shields (24) remain intact and in place in the cell (10) while operating in molten electrolyte (15) at about 850° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration, partly in section, showing replacement of an anode assembly 16′ in an electrolysis cell for making aluminum utilizing a molten electrolyte, with a still immersed adjacent anode assembly 16, both having an attached heat radiation shield; and

FIG. 2 is a graph of temperature drop of the anode vs. time, for anodes with radiation shields (Group 1) and without radiation shields (Group 2), as determined from both thermal measurements and simulations of anode assembly charge out processes.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to a method for protecting an inert anodes from thermal shock. Preferably, the inert anode is made of a cement or ceramic material. The present invention is further directed to a method for protecting a castable support for the anode from thermal shock

Referring now to FIG. 1, one type of operating electrolytic cell 10 for producing metal, such as aluminum is shown, and can include a carbon cathode floor 11 and sidewalls 12, 13 extending upwardly from the floor 11. The cell 10 will initially be described as the in place anode assembly 16, shown as the left assembly in FIG. 1. The sidewalls 12, 13 can be both covered by a solid crust 14. The floor 11 and sidewalls 12, 13 define a chamber above the molten cryolite bath 15 and aluminum deposit 17. A steel shell 18 supports the floor 11 and sidewalls 12, 13. A metal collector bar 19 carries current from the carbon cathode floor 11. The cell 10 includes several anodes 20 fastened by electrically conductive metal conductors 22 which can pass through a protective ceramic cover 28 and a layer of insulation 30. The conductors 22 are attached to a metallic distribution plate 32. The distribution plate is supported by a support beam 26 which can be used to raise or lower the anode assembly 16. The conductors 22, distribution plate 32, and support beam 26 together make up a support structure assembly for the anodes 20 and anode assembly 16. The ceramic cover 28 and insulation layer provide environmental and thermal protection.

The conductors 22 are made of any suitable material providing electrical conductivity to the anodes 20. The insulating layer 30 preferably includes one or more thermal insulating layers of any suitable composition. The protective cover 28 is made from a highly corrosion resistant ceramic material capable of being exposed to the severe environment above the molten bath 15. An electrically conductive metallic distribution plate 32 provides a current path between the support beam 26 and the conductors 22.

The inert anodes 20 are protected from thermal shock during removal of an adjacent anode assembly 16′ by heat radiation shields 24. The radiation shields preferably can be disposed a distance 25 above the bottom of the inert anode 20 as shown. The shields circumscribe at least two sides of the assembly and preferably, while not shown, surround the assembly and inert anodes 20 on all four sides. The distance 25 can range from 12 cm to 20 cm. The ambient atmosphere 40, is substantially cooler than the molten cryolite 15 by at least 800° C. As the anode assembly 16′ is removed, a major heat sink and radiation shield is lost and adjacent inert anodes are exposed to the ambient atmosphere 40 which can cause cooling of over 20° C. A change about 20° C. to 30° C. can provide sufficient thermal stress to initiate cracking of ceramic or cement inert anodes.

FIG. 2 illustrates a simulation of the change in anode surface temperature over time during change out, where series of curves shown as Group 2, show surface temperature changes without a radiation shield in ° C. vs. Group 1 with a radiation shield in ° C. As can be seen, Group 1 which includes shields made of a high alumina material having a thickness of 0.30 cm provided sufficient radiation protection from the ambient temperatures to limit the temperature change to about 20° C. to 30° C. Because the radiation shields must remain intact above the bath in order to protect the anodes from thermal shock, they must not dissolve in molten cryolite fumes.

The requirements for non-dissolvable, effective radiation shields which surround/circumscribe an anode assembly or plate to which the anode is attached in terms of ratio of shield compositions, porosity, thickness, thermal shock and the like are now described in detail. An effective radiation shield material must be resistant to chemical attack from fluoride fumes and occasional splashing of cryolite bath. It must also be able to withstand thermal shock encountered during anode insertion and movements of adjacent anodes. Simple or compound oxides of alumina with silica and calcia have been found to be both chemically and thermal shock resistant. Alumina content should be from 50 wt % to 95 wt % or more preferably 60 wt % to 85 wt %. Porosity must be low enough to afford good mechanical strength, but not so low as to negatively impact thermal shock resistance. Porosity should be in the range of 5 vol % to 30 vol %, or more preferably 10 vol %-25 vol %.

Thickness requirements are determined by strength and practical fabrication limitations. The minimum practical thickness which satisfies mechanical integrity and ease of fabrication should be used that is and from 0.3 cm to 4.0 cm is preferably in the range of 1.27 cm to 3.7 cm or more preferably 1.9 cm to 3.18.

Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Claims

1.-12. (canceled)

13. An anode assembly for use in an aluminum electrolysis cell, the anode assembly comprising: a plurality of anodes attached to a support structure assembly; anda heat radiation shield circumscribing at least two sides of the support structure assembly, wherein the heat radiation shield is spaced from the plurality of anodes, and wherein the bottom of the heat radiation shield extends below the bottom of the support structure assembly.

14. The anode assembly of claim 13, wherein the bottom of the heat radiation shield is above the bottom of the plurality of anodes.

15. The anode assembly of claim 14, wherein the distance from the bottom of the heat radiation shield to the bottom of the plurality of anodes is sufficient to allow a bottom portion of the plurality of anodes to be submerged in a molten cryolite bath of an aluminum electrolysis cell without submerging the heat radiation shield in the molten cryolite bath.

16. The anode assembly of claim 14, wherein the heat radiation shield is able to prevent a temperature drop within the anode assembly of more than 30° C. during insertion and removal of adjacent anode assemblies into and from the cryolite bath.

17. The anode assembly of claim 16, wherein the heat radiation shield comprises alumina and at least one of silica and calcia.

18. The anode assembly of claim 17, wherein the heat radiation shield comprises between 50 wt % and 95 wt % alumina.

19. The anode assembly of claim 13, wherein the heat radiation shield is resistant to chemical attack from fluoride fumes.

20. The anode assembly of claim 13, wherein the porosity of the heat radiation shield is from 5 vol. % to 30 vol. %.