GB2290081A - Destruction of spent aluminium smelting carbon electrodes in molten salt - Google Patents

Abstract

A process for destruction of spent aluminum smelting carbon anodes and carbon block cathodes which are contaminated with cryolite, Na3AlF6, comprises introducing such spent carbon electrodes into a molten salt bath, e.g. molten sodium carbonate, at elevated temperature, e.g. about 900 DEG C, together with air or oxygen, and catalytically oxidizing and destroying the spent carbon anodes in such molten bath, and recovering an off-gas containing chiefly CO2 and retaining spent salts including sodium fluoride and sodium aluminate in the molten salt bath. The molten salt bath also destroys any small amount of NaCN which may also be present in the cryolite contaminant, and converts such NaCN to nitrogen and CO2, which are released in the off-gas. <IMAGE>

Description

ALUMINUM SMELTING ELECTRODE DESTRUCTION WITH MOLTEN SALT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the destruction of waste carbon electrodes from aluminum smelting operations, and particularly relates to a molten salt process for destroying spent carbon anodes and carbon block cathodes which are contaminated with alkali halide salt from the electrolytic precess for smelting aluminum.

2. Description of the Prior Art In the Hall/Heroult process for smelting alumnum, aluminum oxide, Al2O3, is dissolved in cryolite, Na3AlF6, and is electrolyzed in a furnace containing a fused cryolite D-th having a carbon block cathode bottom and hard carbon anodes suspended from above and contacting the fused cryolite bath. Melted aluminum deposits on the carbon block cathodes and the carbon anodes are lowered as they wear ir-co the fue cryolite bath.

Both the spent carbon anodes and carbon block cathodes are contaminated with alkali halide salts and particularly the cryolite. The cryolite impregnates iiito the carbon anodes and cathodes and the carbon wears out when it loses its dimensional stability where it thins sufficiently. The carbon electrodes become contaminated with alkali salts, predominantly fluoride in the cryolite.

Disposal of such spent electrodes is expensive and land burial thereof is politically unacceptable.

Molten salt oxidation (MSO) has been demonstrated to be a suitable technology for destroying hazardous organic waste. The technology achieves this destruction by the catalytic oxidation of the organic compounds by molten salt such as sodium carbonate at elevated temperatures.

U. S. Patent o. 3,708,270 discloses a process for pyrolyzing carbonaceous materials such as solid wastes by use of a molten salt comprising an alkali metal carbonate and from about 1 to 25 wt % of an alkali metal sulfide.

U. S. Patent No. 3,916,617 discloses a process for the partial oxidation and complete gasification cf a carbonaceous material such as coal to produce a combustible gas, which utilizes a molten salt comprising an alkali metal carbonate and about 1 to 25 wt % o an alkali metal sulfide.

SUMMARY OF THE INVENTION It has ben found that spent carbon: ictr'des including spent carbon anodes and carbon block cathodes, from aluminum smelting and contaminated with alkali fluoride salts, particularly cryolite, can be safely destroyed by introducing such spent carbon electrodes containing particularly cryolite contaminant salt, into a molten alkali carbonate bath such as sodium carbonate at elevated temperature of about 9000 to about 10000C. The carbon electrodes are catalytically oxidized and destroyed, forming an off-gas containing CO2 and retaining spent contaminant salt, e.g. sodium fluoride and sodium aluminate from the cryolite contaminant, in the molten alkali carbonate bath.

A small amount of sodium chloride can be added to the alkali carbonate, e.g. sodium carbonate, bath to provide excess chloride ion for reaction with the alkali metal, i.e.

sodium, in the Na3AlF6, or cryolite. Further, a small amount of alkaline earth metal oxide, particularly calcium oxide, can be added to the alkali carbonate or sodium carbonate molten bath to prezerentially form calcium fluoride with the fluorine in the cryolite. Addition of such salts to the alkali carbonate molten bath, reduces the teraperature of operation of the r,c)lten salt bath.

OBJECTS OF THE INVENTION It is accordingly olle object of the present invention to provide an improved simple and safe method for the destruction of aluminum smelting electrodes.

Another object is the provision of an efficient process for the destruction of spent carbon anodes and carbon block cathodes contaminated with alkali halide salts, in a molten salt bath and converting substantially all of the carbon in such electrodes to CO2, while retaining contaminant alkali halide salts in the bath.

Yet another object is to provide a process of the above type employing the molten sodium carbonate bath at elevated temperatures.

Other objects and advantages will appear hereinafter.

DESCRIPTION OF THE DRAWING The sole figure of the drawing illustrates a system for feeding chunks of spent aluminum smelting carbon electrodes to a molten salt bath for destruction therein according to the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS The nair. concept of the present invention is to remove the cryolite from the aluminum smelting carbon electrodes, including the anodes and cathodes, and simultaneously to destroy and convert the carbon electrodes by oxidation to benign compounds.Broadly, this is accomplished according to the invention by a process which comprises feeding spent carbon electrodes containing Na3AlF6 contaminant salt, preferably in the form of chunks from said electrodes, into a molten alkali carbonate bath, feeding oxygen into the molten salt bath, and catalytically oxidizing and destroying the carbon of such electrodes, recovering an off-gas containing CO2, and retaining spent contaminant salt, chiefly in the form of sodium fluoride and sodium aluminate, in the molten alkali carbonate bath.

Materials which are destroyed according to the invention are the spent carbon anodes and carbon block cathodes from electrolytic aluminum smelting operations, and which are contaminated with the cryolite 4a3AiF6, medium employed in the aluminum smelting operation. Such cryolite contaminant also an contain 2 small amount of sodium cyanide. Although such sodium cyanide contaminant may be present in amounts less than 0.1t of the amount or cryolite salt, it is important to destroy such poisonous material, toyethCr with the carbon of the electrodes.The carbon anodes and carbon block cathodes are generally ground into suitably sized chunks before being fed to the molten salt bath.

The molten salt oxidation is carried out in the presence of air or oxygen. Sufficient oxygen is employed to convert all of the carbon of the aluminum smelting anodes and cathodes to carbon. It is preferred to employ an excess amount of oxygen so as to convert substantially all of the carbon in the waste electrodes to CO2 with relatively little CO present. If at least 20% excess oxygen is employed, the product gas will contain chiefly CO2 w th substantially no CO being present in the off-gas.

The molten salt bath comprises substantially alkali metal carbonate such as sodium carbonate, potassium carbonate or lithium carbonate or mixtures thereof. Where sodium carbonate is employed as molten salt, temperature of reaction ranges from about GCO to about 10000C. However, the temperature of the molten salt bath can be reduced by employing mixtures such as a mixture of 50% sodium carbonate arid 50% potassium carbonate, b weight. Also a mixture of alkali metal carbonate such as sodium carbonate and a halide salt such as sodium chloride, potassium chloride or calcium chloride, containing about 10% te out 90% of halide salt, e.g. a mixture of 90% sodium carbonate and 10% sodium chloride, can be employed.Use of such mixtures will reduce the temperature in the molten salt bath, e.g. to a range of say 6000 to about 8000C.

Also, a small amount of alkaline earth metal oxide, e.g. about 2 to about 20% of calcium oxide by weight of the total molten salt bath, can be added thereto to provide calcium ion for preferential formation of calcium fluoride from the fluorine in the Na3AlF6 contaminant salt.

Since a small amount of sodium cyanide is also usually present in the sodium aluminum fluoride electrolyzing medium of the aluminum smelting process, e.g.

from about 0.05 to about 0.5% by weight of the sodium aluminum fluoride, the molten salt oxidation has the additional function of oxidizing the sodium cyanide to nitrogen and CO2, which are also present in the off-gas.

The molten salt is employed as a catalyst in the reaction and does not enter into the reaction. Thus, the molten salt bath, in addition to serving as a catalytic reaction medium for destroying the carbon and sodium cyanide, also serves the function of retaining spent salts, and vclunle -educiny the carbon waste. lhe off-gases CO2 and nitrogen are relatively benign. Operation with excess air or oxygen prevents CO formation and yields CO Referring to the Figure of the drawing, the feed system 10 illustrates one system designed to transport carbon waste from the spent carbon anodes and carbon block cathodes from aluminum smelting operations, to a molten salt bath 12 contained in an electric furnace 14.

For this purpose, a feed hopper 16 is provided for receiving chunks of such carbon anodes and cathodes containing spent cryolite contaminant and sodium cyanide.

In preferred practice, a vibrator device indicated at 22 is provided on the feed hopper 16 to facilitate movement of the carbon electrode-chunks 18 out of the feed hopper and into an inclined chute 24 communicatiny with the bottom of the feed hopper 16. The vibrator device 22 can be any well known type of vibrator such as a pneumatic or electromechanical vibrator. Incremental passage of the waste chunks 18 from feed hopper 16 into the inclined chute 4 is controlled by means of a control valve 26.

The downwardly inclined chute 24 terminates at its lower end in a lock hopper 28 having a double lock v e avrangeraent as indicated at 30. The inclined chute 24 is preferably vibrated by means r a vibrator at 32 similar to vibrator 22, to facilitate transport of the carbon chunks 18 downwardly through the inclined chte 24. By means of the lock hopper 28, the feed chunks 18 introduced therein from the chute 24 can be dropped in increments via valve 30 into the molten salt bath 12 in furnace 14.

The molten salt bath 12 into which the carbon waste chunks 18 are dropped from the lock hopper 28, is contained in an alumina tube 34 located within a furnace liner 36, e.g. of Inconel, within the electric furnace. The molten salt bath 12 contains an alkali metal carbonate such as sodium carbonate, at a temperature of 900" to 10000C.

The carbon feed chunks 18 are fed incrementally from lock hopper 28 and down through an alumina downcomer 37 into the molten salt bath 12 in tube 34 for submerged reactive contact of the feed chunks 18 with the molten salt and decomposition or oxidation of the waste feed chunks form essentially CO2 and nitrogen. For carrying out the decomposition or oxidation of the carbon chunks 18 and NaCN in the cryolite contaminant in the salt bath 12, process air is introduced at 40 and passed downwardly through a narrow tube 42 provided within tube 34 and into the molten salt bath 12.The product gas formed in the reaction, a mixture of CO2 and t22 passes upwardly through an annulus 44 betweeii the wall of tube 34 and dowllcomer 37 to the gas vent 38.

Spent contamin llt salt is retained in the sodium carbonate bath.

Other means such as conveying air or a screw conveyor can be used to feed the chunks or pieces of electrodes contaminated with cryolite below the surface of the molten alkali carbonate bath.

The following are examples of practice of the invention: EXAMPLE 1 A ground carbon anode stub from aluminum smelting operations, consisting of pieces 1/4" or less is fed into a bath of sodium carbonate at 10000C below the surface of the bath using conveying air. The air is added at 20% excess, assuring contplete conversion of the carbon to carbon dioxide. Residual cryolite in the anode is converted to sodium fluoride and sodium aluminate. Residual sodium cyanide, which is contained in the anode feed material, is converted to carbon dioxide and nitrogen, which are released with the off-gas. The sodium is retained in the bath as sodium carbonate or sodium fluoride.

Minus 1/4" sized spent carbon anode stubs from aluminum smelting operations are fed with a screw conveyor below the surface of a molten bath of sodium carbonate with 10E calcium oxide added to tlie bath. The bath is maintained at 10000C and 20% excess air is added at the bottom of the bath to vigourously sparge the bath. The gas velocity is approximately 2 feet per second as it passes upward through the bath. The calcium combines with fluorine from the residual cryolite forming calcium fluoride. The carbon anode material is oxidized to carbon dioxide. Sodium cyanide which is added to the bath with the anode material is converted to carbon dioxide and nitrogen gases. The sodium in the cyanide is retained in the bath.

EXAMPLE 3 Sized spent carbon anode material from aluminum smelting operations is fed with conveying air in 20% excess to assure complete conbustion of the carbon to carbon dioxide in a 9000C bath of sodium carbonate with 10% sodium chloride added to suppress the melting point of the bath, thereby giving an increased operating margin. The sodium chloride does not enter into the reaction with the anode, cryolite or cyanide contaminant, and is retained in the melt. Residual crlite in the anode material is convert to sodium fluoride and sodium alun;inate, Psidul sodium cyanide contained in the anode material is converted to CO2 and N,, which are released in the off-gas. The carbon ir! the anode material is converted completely to CO2 discharged in the off-gas.

From the foregoing, it is seen that the invention provides a simple and efficient process for destroying aluminum smelting carbon anodes and cathodes which are contaminated with the cryolite salt medium of the aluminum smelting process and also usually sodium cyanide, by subjecting such carbonaceous material in the form of chunks to catalytic oxidation in a molten salt bath, e.g. sodium carbonate, to convert such waste to an off-gas containing chiefly CO2, and retaining the spent contaminant salt in the molten bath, and at the same time destroying any sodium cyanide present in the initial cryolite contaminant salt.

It is to be understood that what has been described is merely illustrative of the principles of the invention and that numerous arrangements ill accordance with this invention may be devised by one skilled in the art without departing from the spirit and scope thereof.

Claims (15)

CLAIMS:

1. A process for destruction of spent carbon electrodes from aluminum smelting, which comprises introducing said spent carbon electrodes containing Na3AlF6 contaminant into a molten alkali carbonate bath, introducing oxygen and catalytically oxidizing and destroying the carbon of said electrodes, removing an off-gas containing CO2, and retaining spent contaminant salt in the molten alkali carbonate bath.

2. A process according to claim 1, in which said Na3AlF6 contaminant also contains a small amount of sodium cyanide, and said process including also oxidizing said sodium cyanide to nitrogen and CO2 contained in said off-gas.

3. A process according to claim 1 or 2, in which said spent contaminant salt includes sodium fluoride and sodium aluminate.

4. A process according to any of claims 1 to 3, in which said molten alkali carbonate bath is substantially sodium carbonate.

5. A process according to any of claims 1 to 3, in which said molten alkali carbonate bath is sodium carbonate and includes sodium chloride.

6. A process according to any of claims 1 to 3, in which said molten alkali carbonate bath is sodium carbonate and includes a small amount of calcium oxide.

7. A process according to any of claims 1 to 3, in which said alkali metal carbonate is sodium carbonate and includes about 10 to about 90% of sodium chloride by weight of the total molten salt bath.

8. A process according to any of claims 1 to 3, in which said alkali metal carbonate is sodium carbonate and includes about 2 to about 20% of calcium oxide by weight of the total molten salt bath, to provide calcium ion for preferential formation of calcium fluoride from the fluorine in said Na3AlF6 contaminant.

9. A process according to any of claims 1 to 8, employing an excess amount of oxygen so as to convert substantially all of the carbon in said electrodes to carbon dioxide.

10. A process according to any of claims 1 to 9, wherein said electrodes from aluminum smelting are carbon anodes and/or carbon block cathodes.

11. A process for destruction of spent carbon electrodes from aluminum smelting, which comprises introducing into a molten sodium carbonate bath at a temperature of about 9000 to about 10000C chunks of spent carbon anodes and carbon block cathodes from aluminum smelting and containing Na3AlF6 contaminant and a small amount of NaCN, introducing air into said molten sodium carbonate bath, and catalytically oxidizing and destroying said anodes and said cathodes, and destroying said NaCN, and converting said Na3AlF6 contaminant to sodium fluoride and sodium aluminate spent salts, removing an off-gas containing CO2 and N2, and retaining said spent salts in the molten sodium carbonate bath.

12. A process according to claim 11, said molten sodium carbonate bath including about 10% to about 90% of sodium chloride by weight of the total molten salt bath and lowering the temperature of said molten sodium carbonate bath.

13. A process according to claim 11 or 12, said molten sodium carbonate bath including about 2 to about 20% of calcium oxide by weight of the total molten salt bath to provide calcium ion for preferential formation of calcium fluoride from the fluorine in said Na3AlF6 contaminant, and also lowering the temperature of said molten sodium carbonate bath.

14. A process according to any of claims 11 to 13, employing an amount of air providing at least 20% excess oxygen so as to convert said chunks of spent carbon anodes and spent carbon cathodes chiefly to CO2.

15. A process for destruction of spent carbon electrodes from aluminum smelting, substantially as hereinbefore described with reference to any of the Examples and/or the accompanying drawing.