Insulating lid for aluminum production cells

Abstract

An aluminum production cell includes an inert anode and an insulating lid comprising alumina and at least one metal fluoride. The insulating lid preferably comprises about 35-90 wt. % of a mixture of sodium fluoride and aluminum fluoride and about 10-65 wt. % alumina.

Description

FIELD OF THE INVENTION

[0002] The present invention relates to aluminum production cells and, more particularly, relates to insulating lids for aluminum production cells having inert anodes.

BACKGROUND INFORMATION

[0003] The energy requirements and cost efficiencies of aluminum smelting cells can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional consumable carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce essentially no CO2 or CF4 emissions.

[0004] The successful retrofit of conventional consumable anode Hall aluminum production cells with inert anodes requires extremely stable cell operation with respect to bath chemistry and heat balance. To achieve cell stability, several operating parameters must be controlled. The reduced capacity of inert anode cells to produce heat in comparison with conventional cells having consumable carbon anodes requires heat losses to be minimized in order to maintain an adequate heat balance and stable operation. The amount of alumina dissolved in the bath must be controlled within a very narrow range, e.g., about 7 to 8 percent, which is essential for low corrosion rates of the inert anodes and to prevent deposits of excess alumina on the cathode surface. At an operating voltage equivalent to a conventional carbon anode cell, an inert anode cell will generate less heat due to a high decomposition potential. Control of the cell temperature is crucial because the alumina saturation level of the bath is proportional to the bath temperature. Temperature control is also crucial to the formation and stability of the protection layer of frozen bath or ledge along the sidewalls of the cell. In a conventional Hall cell, this ledge prevents horizontal electrical currents between the anode and cathode as well as protecting the sidewall lining material from erosion. In a cell with inert anodes, this ledge may also protect the inert anodes from reduction by carbonaceous material of the cell lining.

[0005] A need exists for increased insulation in inert anode aluminum production cells in order to reduce heat losses from the cells. The present invention has been developed in view of the foregoing.

SUMMARY OF THE INVENTION

[0006] The present invention improves the operating stability of an inert anode aluminum production cell by controlling the heat loss through a top insulating cover. The top cover is the primary area that heat loss can be regulated during normal operation. Reduction in heat loss is preferably obtained by utilizing a multiple layer insulation system on top of the inert anodes. The initial layer of insulation preferably comprises fabricated blocks of a cryolite and alumina mixture. These blocks, which are resistant to attack from the bath fumes, may shield and support subsequent layers of high temperature insulation. To further protect the upper layers of insulation, a semi-gas-tight barrier may be created by filling voids between the blocks with loose refractory material, such as coarse tabular alumina choked by a smaller particle size alumina material. The heat balance of the cell may be maintained, e.g., by adding or removing layers of insulation from the top of the cell.

[0007] An additional application for the preformed insulating blocks of the present invention is during startup of an inert anode cell. The blocks may be used as an inner sidewall between a carbon cell lining and the anodes. This inert lining provides horizontal electrical insulation and protect the anodes from carbothermic reduction until a permanent cryolite ledge is formed.

[0008] An aspect of the present invention is to provide an aluminum production cell insulating lid assembly comprising at least one preformed refractory block comprising Al2O3 and at least one material selected from NaF, AlF3, CaF2 and MgF2.

[0009] Another aspect of the present invention is to provide an aluminum production cell including a cathode, at least one anode, and an insulating lid above the cathode and anode(s). The insulating lid comprises at least one preformed refractory block including Al2O3 and at least one material selected from NaF, AlF3, CaF2 and MgF2.

[0010] These and other aspects of the present invention will be more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a partially schematic sectional side view of an inert anode aluminum production cell including an insulating lid in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] FIG. 1 illustrates an insulating lid 10 of an aluminum production cell 11 in accordance with an embodiment of the present invention. The insulating lid 10 includes preformed refractory blocks 12 which are positioned above inert anodes 14. The inert anodes 14 may be made of a ceramic material as described in Dimilia et al. U.S. Ser. No. 10/291,874, filed Nov. 8, 2002 or in Weirauch et al. U.S. Ser. No. 10/291,874 filed Nov. 9, 2002; or a cermet material as described in Ray et al. U.S. Pat. No. 6,423,204 issued Jul. 23, 2002. The disclosures of the aforesaid patent applications and patent are incorporated herein by reference.

[0013] The preformed refractory blocks 12 preferably comprise from about 35 to about 90 weight percent of at least one metal fluoride selected from sodium fluoride, aluminum fluoride, and mixtures thereof and from about 10 to about 65 weight percent Al2O3. Compositions comprising mixtures of sodium fluoride and aluminum fluoride together with alumina are preferred. More preferably, the preformed refractory block comprises about 50 to 75 weight percent of a NaF/AlF3 mixture, and the Al2O3 comprises from about 25 to about 50 weight percent. The weight ratio of NaF:AlF3 preferably ranges from about 1:1 to about 2:1. A particularly preferred weight ratio of NaF:AlF3 is about 1.5:1. The NaF and AlF3 may be provided separately or together, e.g., in the form of synthetic cryolite. In addition to, or in place of, the NaF and AlF3, other materials, such as calcium aluminate, CaF2, and MgF2, may be used. For example, the refractory block material may comprise crushed bath held together with a suitable amount of binder, such as sodium aluminum tetrafluoride or alumina cement.

[0014] The preformed insulating blocks 12 may be formed by mixing, e.g., the Al2O3, NaF/AlF3 mixture and binder components in the desired weight ratios, followed by pressing and heating to a temperature sufficient to bind the block together, e.g., 1,000° C. The thickness and shape of the preformed insulating blocks 12 may be selected in order to provide sufficient structural integrity and heat insulating properties. For example, thicknesses of from less than 1 inch to greater than 1 foot may be used.

[0015] After the preformed refractory blocks 12 are positioned above the inert anodes 14, spaces around the preformed refractory blocks 12 may be packed with loose insulation 16, such as particles, fibers, and the like. For example, the loose insulation may comprise tabular alumina, crushed bath and/or alumina particles. As a particular example, the loose insulation may comprise from 75 to 90 weight percent ESP dust and from 10 to 25 weight percent crushed bath.

[0016] Additional layers of insulation 18 may be positioned above the preformed refractory blocks 12, as shown in FIG. 1. The additional insulation 18 may include alumina particles, ceramic fiber, fiberfrax, fiberglass, or any other suitable material.

[0017] In addition to their use as insulating lids, the preformed refractory blocks of the present invention may be used as sidewall liners of the cell. FIG. 1 illustrates a preformed sidewall block 20 positioned inside the sidewall 22 of the cell. The sidewall block 20 may be the same composition as the insulating blocks 12. In this case, the preformed sidewall insulation 20 may act as a sacrificial material which may be at least partially consumed during operation of the cell.

[0018] The aluminum production cell 11 also includes a chamber 30 containing a molten salt bath 32 comprising cryolite and dissolved alumina; and a cathode 34 spaced from the inert anodes 14. When an electric current passes between the inert anodes 14 and the cathode 34, aluminum 36 is produced at the cathode 34 and oxygen is produced at the anodes 14.

[0019] Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.

Claims

1. An aluminum production cell comprising at least one inert anode, a chamber containing a molten salt bath contacting said inert anode, a cathode spaced from said inert anode, and an insulating lid covering said chamber said insulating lid comprising a preformed refractory block containing about 10-65 wt. % alumina and about 35-90 wt. % of at least one metal fluoride selected from sodium fluoride, aluminum fluoride, and mixtures thereof.

2. The aluminum production cell of claim 1, wherein alumina comprises about 25-50 wt. % of the preformed refractory block.

3. The aluminum production cell of claim 1, wherein said insulating block comprises sodium fluoride and aluminum fluoride.

4. The aluminum production cell of claim 1, wherein said insulating block comprises sodium fluoride and aluminum fluoride in an NaF:AlF3 weight ratio of about 1:1 to about 2:1.

5. The aluminum production cell of claim 4, wherein the NaF:AlF3 weight ratio is about 1.5:1.

6. The aluminum production cell of claim 1, wherein said refractory block further comprises at least one metal fluoride selected from calcium fluoride and magnesium fluoride.

7. The aluminum production cell of claim 1, wherein said inert anode comprises a ceramic or cermet material.

8. The aluminum production cell of claim 1, wherein said inert anode comprises nickel oxide and iron oxide.

9. The aluminum production cell of claim 1, further comprising loose insulation around a portion of the preformed insulating block.

10. The aluminum production cell of claim 9, further comprising at least one additional layer of insulating material above the preformed insulating block.

11. The aluminum production cell of claim 1, wherein the preformed insulating block is positioned above at least one anode of said cell.

12. A process for producing aluminum in a cell comprising at least one inert anode, a chamber containing a molten salt bath containing alumina, a cathode spaced from said inert anode, and an insulating lid covering said chamber, said process comprising (a) passing an electric current between said anode and said cathode, thereby to produce aluminum at the cathode, and (b) conserving heat in said chamber by providing said insulating lid with a composition comprising about 10-65 wt. % alumina and about 35-90 wt. % of at least one metal fluoride.

13. The process of claim 12, wherein said insulating lid comprises at least one metal fluoride selected from sodium fluoride, aluminum fluoride, magnesium sluoride, calcium fluoride, and mixtures thereof.

14. The process of claim 12, wherein said insulating lid comprises a preformed refractory block comprising about 50-75 wt. % of a mixture of sodium fluoride and aluminum fluoride and about 25-50 wt. % alumina.

15. The process of claim 14, wherein said mixture comprises NaF and AlF3 in an NaF:AlF3 weight ratio of about 1:1 to about 2:1.