CA2140823A1 - Aluminum smelting electrode destruction with molten salt - Google Patents
Aluminum smelting electrode destruction with molten saltInfo
- Publication number
- CA2140823A1 CA2140823A1 CA002140823A CA2140823A CA2140823A1 CA 2140823 A1 CA2140823 A1 CA 2140823A1 CA 002140823 A CA002140823 A CA 002140823A CA 2140823 A CA2140823 A CA 2140823A CA 2140823 A1 CA2140823 A1 CA 2140823A1
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- Prior art keywords
- bath
- molten
- carbon
- sodium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B4/00—Electrothermal treatment of ores or metallurgical products for obtaining metals or alloys
-
- A—HUMAN NECESSITIES
- A62—LIFE-SAVING; FIRE-FIGHTING
- A62D—CHEMICAL MEANS FOR EXTINGUISHING FIRES OR FOR COMBATING OR PROTECTING AGAINST HARMFUL CHEMICAL AGENTS; CHEMICAL MATERIALS FOR USE IN BREATHING APPARATUS
- A62D3/00—Processes for making harmful chemical substances harmless or less harmful, by effecting a chemical change in the substances
- A62D3/30—Processes for making harmful chemical substances harmless or less harmful, by effecting a chemical change in the substances by reacting with chemical agents
- A62D3/32—Processes for making harmful chemical substances harmless or less harmful, by effecting a chemical change in the substances by reacting with chemical agents by treatment in molten chemical reagent, e.g. salts or metals
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23G—CREMATION FURNACES; CONSUMING WASTE PRODUCTS BY COMBUSTION
- F23G7/00—Incinerators or other apparatus for consuming industrial waste, e.g. chemicals
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- A—HUMAN NECESSITIES
- A62—LIFE-SAVING; FIRE-FIGHTING
- A62D—CHEMICAL MEANS FOR EXTINGUISHING FIRES OR FOR COMBATING OR PROTECTING AGAINST HARMFUL CHEMICAL AGENTS; CHEMICAL MATERIALS FOR USE IN BREATHING APPARATUS
- A62D2101/00—Harmful chemical substances made harmless, or less harmful, by effecting chemical change
- A62D2101/40—Inorganic substances
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- A—HUMAN NECESSITIES
- A62—LIFE-SAVING; FIRE-FIGHTING
- A62D—CHEMICAL MEANS FOR EXTINGUISHING FIRES OR FOR COMBATING OR PROTECTING AGAINST HARMFUL CHEMICAL AGENTS; CHEMICAL MATERIALS FOR USE IN BREATHING APPARATUS
- A62D2101/00—Harmful chemical substances made harmless, or less harmful, by effecting chemical change
- A62D2101/40—Inorganic substances
- A62D2101/45—Inorganic substances containing nitrogen or phosphorus
-
- A—HUMAN NECESSITIES
- A62—LIFE-SAVING; FIRE-FIGHTING
- A62D—CHEMICAL MEANS FOR EXTINGUISHING FIRES OR FOR COMBATING OR PROTECTING AGAINST HARMFUL CHEMICAL AGENTS; CHEMICAL MATERIALS FOR USE IN BREATHING APPARATUS
- A62D2101/00—Harmful chemical substances made harmless, or less harmful, by effecting chemical change
- A62D2101/40—Inorganic substances
- A62D2101/49—Inorganic substances containing halogen
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- Mechanical Engineering (AREA)
- Environmental & Geological Engineering (AREA)
- General Chemical & Material Sciences (AREA)
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- Chemical Kinetics & Catalysis (AREA)
- Toxicology (AREA)
- General Health & Medical Sciences (AREA)
- General Engineering & Computer Science (AREA)
- Life Sciences & Earth Sciences (AREA)
- Health & Medical Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Geology (AREA)
- Manufacturing & Machinery (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Electrolytic Production Of Metals (AREA)
- Processing Of Solid Wastes (AREA)
Abstract
A process for destruction of spent aluminum smelting carbon anodes and carbon block cathodes which are contaminated with cryolite, Na3AlF6, comprising introducing such spent carbon electrodes into a molten salt bath, e.g.
molten sodium carbonate, at elevated temperature, e.g. about 900°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.
molten sodium carbonate, at elevated temperature, e.g. about 900°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.
Description
2~408Z3 9 3R058 ALUMINUM SMELTING ELECTRODE DESTRUCTION
WITH MOLTEN SALT
Gary D. Schnittgrund 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 prccess for destroying spent carbon anodes and carbon block cathodes which are contaminated with alkali halide salt from the electrolytic process for smelting aluminum.
2. Description of the Prior Art In the ~Iall/Heroult process for smelting aluminum, aluminum oxide, A12O3, is dissolved in cryolite, Na3AlF6, and is electrolyzed in a furnace containing a fused cryolite ~ath having a carbon block cathode bottom and hard carbon ar,odes suspended from above and contacting the fused ~ -cryolite bath. ~elted aluminum deposits on the carbon block -~
cathodes and the carbon anodes are lowered as they wear into the ~used cryolite bath.
Both the spent ~arbon anodes and carbon block cathodes are contaminated with alkali halide salts and partlcularly tile cryolite. The cryolite impregnates into 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 de~lonstrated to be a suitable technology for destroying hazardous organic waste. The technology achieves this destruction by the catalytic oxidation of the orgallic compounds by molten salt such as sodium carbonate at ele~ated temperatures.
U. S. Patent ~o. 3,708,270 discloses a process for pyrolyzing carbonaceous materials such as solid wastes by use of a ~olten 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 partiai oxidation and complete gasification of--~a car~onaceous ~aterlal such as coal to produce a combustible - ~ ^
gas, which utilizes a m~lten salt comprisiny an alkali metal - carbonate and about 1 to 25 wt % or an alkali metal sulfide.
SUM~qARY OF THE INVENTION
It has been found that spent carbon electrodes in~luding ~ent carbon anodes and carbon block cathodes, ,~5 /
.. _ . ..
, -, ,. -: , , , ,- - - ::
from aluminum smelting and contAm;n~ted with alkali fluoride salts, particularly cryolite, can be safely destroyed by introducing such spent carbon electrodes cont~;n;ng particularly cryolite cont~m;n~nt salt, into a molten alkali carbonate bath such as sodium carbonate at elevated temperature of about 900 to about lO00C. The carbon - -electrodes are catalytically oxidized and destroyed, forming an off-gas containing CO2 and retaining spent cont~m;nAnt salt, e.g. sodium fluoride and sodium aluminate from the cryolite cont~m;n~nt, 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 o~ide, particularly calcium oxide, can be added to the alkali carbonate or sodium ~ carbonate molten bath to preferentially form calcium--- fluorid~ with the fluorine in the cryolite. ~ddition~-of such salts to the-alkali carbonate molten-bath, reduces the temperature of operation of the molten salt bath.
OBJl~CTS OF TIIE XNVENT ION
It is accordingly one object of the present ~5 "` Z14082~ ` 93R058 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 cont~m;n~ted with alkali halide salts, in a molten salt bath and converting substantially all of -the carbon in such electrodes to CO2, while ret~;n;ng cont~m;n~nt 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.
~ESCRIPTION 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. ~--DET~ILED DESCRIPTION OF THE INVENTION
AND P~EFE~RED E~IBODIMENTS
The main 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 cont~min~nt 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 carbQn of such electrodes, r~covering an off-gas containing CO2, and retaining spent contaminant salt, chiefly in the form of sodium _luoride and sodium aluminate, in the molten alkali carbonate bath.
Materials which are destroyed according to the inventiGn are the spent carbon anodes and carbon block cathodes from electrolytic aluminum smelting operations, and .whi.ch are contaminated with the cryolite, Na3AlF6, me~ium employed in the aluminum smelting-operation. Such cryolite contaminant also can contain a small amount of sodium cyanide. Although such sodium cyanide contaminant may be present in a~.ounts less than 0.1~ of the amount of cryolite salt, it i~ important to destroy such poisonou~ material, toyether with the carbon of the el.ectrodes. The carbon : .
. -. , ~ -,, - .
-. X140823 - -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 with 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 900 to about 1000C. However, the temperature of the molten salt bath can be reduced by employing mixtures such as a mixture of 50~ sodium carbonate and 50% potassium carbonate, by weight. Also a mixture ol alkali metal carbonate such as sodium carbonate and a halide salt such as sodium chloride, potassium chloride or calcium chloride, containing about 10% to about 90~ o~ halide salt, e.g a mixture of 90% sodium carbonate and 10% sodium ~rl .. . . .. ..
. ~
chloride, can be employed. Use of such mixtures will reduce the temperature in the molten salt bath, e.g. to a range of say 600 to about 800C.
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 cont~;n~nt 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, alsG serves the function of retaining spent salts, and volu~e reducing the carbon waste. The off-gases CO2 and nitrogen are relatively benign. Operation with excess air or oxygen prevents CO formation and yiel~s CO2.
. . . . . .. . .. . . . .. . .. . . . . . . ... ... ..
2~08Z~
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 communicating with the kottom 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 -24 is controlled by means of a control valve 26.
The downwardly inclined~chute 24 terminates at its slower end in a lock hopper 28 having a double lock vaSLve ~o arrangement as indicated at 30. The inclined chuie 24 is preferably vibrated by means of a vibrator at 32 similar to vibrator 22, to facilitate transport of the carbon chunks 18 downwardly through the inclined chute 24. By means of the . .
- 21408~
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 ~ath 12 contains an alkali metal carbonate such as sodium carbonate, at a temperature of 900 to 1000C.
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 decornposition or oxidation of the waste feed chunks form essentially CG2 and nitrogen. For carrying out the decomposition or oxidation of the carbon chunks 18 and NaCN
in the cr~olite cont~min~nt 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 C2 and N2, passes upwardly through an annulus ~4 between the wall of tube 34 and downcomer 37 to the gas vent 38.
Spent contaminant salt is retained in the sodium carbonate ~ath.
.
2~40823- -Other means such as conveying air or a screw conveyor can be used to feed the chunks or pieces of electrodes cont~m;n~ted with cryolite below the surface of the molten alkali carbonate bath.
The following are examples of practice of the invention:
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 1000C below the surface of the bath using conveying air. The air is added at 20~ excess, assuring complete 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 witn-the off-gas. The sodium is retained in the-bath as sodium carbonate or sodium fluoride.
.
2 0 E~ALipLE 2 Minus 1/4'~ si~ed spent carbon anode stubs fror.l aluminum smelting operations are fed with a screw conveyor be1OW the surface of a molten hath of sodium carbonate with 10~ calcium oxide added to the bath. The bath is maintained 2l~0a23 at 1000C 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 materia-l 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.
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 900C bath of sodium carbonate with 10~ sodium chloride added to suppress the melting point of the bath, thereby giving an increased operating margin. The-s~dium ;
chloride does not enter into the reaction with the anode, - cryolite or cyanide contaminant, and is retained in the melt. Residual cryolite in the anode material is converted to scdium fluoride and sodium aluminate. Residual sodium cyanide contained in the anode material is converted to CO2 and N~, which ~re released in the off-gas. The carbon i~
21~0823 9 3R058 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 timè destroying any sodium cyanide present in the initial cryolite contaminant sa]t.
It is to be understood that what has been described is merely illustrative of the principles of the -invention and that numerous arrangements in accordanee with this invent1on may be devised by one skilled in the art without departing from the spirit and scope thereof.
. . .
WITH MOLTEN SALT
Gary D. Schnittgrund 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 prccess for destroying spent carbon anodes and carbon block cathodes which are contaminated with alkali halide salt from the electrolytic process for smelting aluminum.
2. Description of the Prior Art In the ~Iall/Heroult process for smelting aluminum, aluminum oxide, A12O3, is dissolved in cryolite, Na3AlF6, and is electrolyzed in a furnace containing a fused cryolite ~ath having a carbon block cathode bottom and hard carbon ar,odes suspended from above and contacting the fused ~ -cryolite bath. ~elted aluminum deposits on the carbon block -~
cathodes and the carbon anodes are lowered as they wear into the ~used cryolite bath.
Both the spent ~arbon anodes and carbon block cathodes are contaminated with alkali halide salts and partlcularly tile cryolite. The cryolite impregnates into 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 de~lonstrated to be a suitable technology for destroying hazardous organic waste. The technology achieves this destruction by the catalytic oxidation of the orgallic compounds by molten salt such as sodium carbonate at ele~ated temperatures.
U. S. Patent ~o. 3,708,270 discloses a process for pyrolyzing carbonaceous materials such as solid wastes by use of a ~olten 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 partiai oxidation and complete gasification of--~a car~onaceous ~aterlal such as coal to produce a combustible - ~ ^
gas, which utilizes a m~lten salt comprisiny an alkali metal - carbonate and about 1 to 25 wt % or an alkali metal sulfide.
SUM~qARY OF THE INVENTION
It has been found that spent carbon electrodes in~luding ~ent carbon anodes and carbon block cathodes, ,~5 /
.. _ . ..
, -, ,. -: , , , ,- - - ::
from aluminum smelting and contAm;n~ted with alkali fluoride salts, particularly cryolite, can be safely destroyed by introducing such spent carbon electrodes cont~;n;ng particularly cryolite cont~m;n~nt salt, into a molten alkali carbonate bath such as sodium carbonate at elevated temperature of about 900 to about lO00C. The carbon - -electrodes are catalytically oxidized and destroyed, forming an off-gas containing CO2 and retaining spent cont~m;nAnt salt, e.g. sodium fluoride and sodium aluminate from the cryolite cont~m;n~nt, 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 o~ide, particularly calcium oxide, can be added to the alkali carbonate or sodium ~ carbonate molten bath to preferentially form calcium--- fluorid~ with the fluorine in the cryolite. ~ddition~-of such salts to the-alkali carbonate molten-bath, reduces the temperature of operation of the molten salt bath.
OBJl~CTS OF TIIE XNVENT ION
It is accordingly one object of the present ~5 "` Z14082~ ` 93R058 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 cont~m;n~ted with alkali halide salts, in a molten salt bath and converting substantially all of -the carbon in such electrodes to CO2, while ret~;n;ng cont~m;n~nt 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.
~ESCRIPTION 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. ~--DET~ILED DESCRIPTION OF THE INVENTION
AND P~EFE~RED E~IBODIMENTS
The main 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 cont~min~nt 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 carbQn of such electrodes, r~covering an off-gas containing CO2, and retaining spent contaminant salt, chiefly in the form of sodium _luoride and sodium aluminate, in the molten alkali carbonate bath.
Materials which are destroyed according to the inventiGn are the spent carbon anodes and carbon block cathodes from electrolytic aluminum smelting operations, and .whi.ch are contaminated with the cryolite, Na3AlF6, me~ium employed in the aluminum smelting-operation. Such cryolite contaminant also can contain a small amount of sodium cyanide. Although such sodium cyanide contaminant may be present in a~.ounts less than 0.1~ of the amount of cryolite salt, it i~ important to destroy such poisonou~ material, toyether with the carbon of the el.ectrodes. The carbon : .
. -. , ~ -,, - .
-. X140823 - -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 with 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 900 to about 1000C. However, the temperature of the molten salt bath can be reduced by employing mixtures such as a mixture of 50~ sodium carbonate and 50% potassium carbonate, by weight. Also a mixture ol alkali metal carbonate such as sodium carbonate and a halide salt such as sodium chloride, potassium chloride or calcium chloride, containing about 10% to about 90~ o~ halide salt, e.g a mixture of 90% sodium carbonate and 10% sodium ~rl .. . . .. ..
. ~
chloride, can be employed. Use of such mixtures will reduce the temperature in the molten salt bath, e.g. to a range of say 600 to about 800C.
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 cont~;n~nt 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, alsG serves the function of retaining spent salts, and volu~e reducing the carbon waste. The off-gases CO2 and nitrogen are relatively benign. Operation with excess air or oxygen prevents CO formation and yiel~s CO2.
. . . . . .. . .. . . . .. . .. . . . . . . ... ... ..
2~08Z~
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 communicating with the kottom 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 -24 is controlled by means of a control valve 26.
The downwardly inclined~chute 24 terminates at its slower end in a lock hopper 28 having a double lock vaSLve ~o arrangement as indicated at 30. The inclined chuie 24 is preferably vibrated by means of a vibrator at 32 similar to vibrator 22, to facilitate transport of the carbon chunks 18 downwardly through the inclined chute 24. By means of the . .
- 21408~
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 ~ath 12 contains an alkali metal carbonate such as sodium carbonate, at a temperature of 900 to 1000C.
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 decornposition or oxidation of the waste feed chunks form essentially CG2 and nitrogen. For carrying out the decomposition or oxidation of the carbon chunks 18 and NaCN
in the cr~olite cont~min~nt 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 C2 and N2, passes upwardly through an annulus ~4 between the wall of tube 34 and downcomer 37 to the gas vent 38.
Spent contaminant salt is retained in the sodium carbonate ~ath.
.
2~40823- -Other means such as conveying air or a screw conveyor can be used to feed the chunks or pieces of electrodes cont~m;n~ted with cryolite below the surface of the molten alkali carbonate bath.
The following are examples of practice of the invention:
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 1000C below the surface of the bath using conveying air. The air is added at 20~ excess, assuring complete 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 witn-the off-gas. The sodium is retained in the-bath as sodium carbonate or sodium fluoride.
.
2 0 E~ALipLE 2 Minus 1/4'~ si~ed spent carbon anode stubs fror.l aluminum smelting operations are fed with a screw conveyor be1OW the surface of a molten hath of sodium carbonate with 10~ calcium oxide added to the bath. The bath is maintained 2l~0a23 at 1000C 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 materia-l 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.
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 900C bath of sodium carbonate with 10~ sodium chloride added to suppress the melting point of the bath, thereby giving an increased operating margin. The-s~dium ;
chloride does not enter into the reaction with the anode, - cryolite or cyanide contaminant, and is retained in the melt. Residual cryolite in the anode material is converted to scdium fluoride and sodium aluminate. Residual sodium cyanide contained in the anode material is converted to CO2 and N~, which ~re released in the off-gas. The carbon i~
21~0823 9 3R058 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 timè destroying any sodium cyanide present in the initial cryolite contaminant sa]t.
It is to be understood that what has been described is merely illustrative of the principles of the -invention and that numerous arrangements in accordanee with this invent1on may be devised by one skilled in the art without departing from the spirit and scope thereof.
. . .
Claims (14)
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 electodes, removing an off-gas containing CO2, and retaining spent contaminant salt in the molten alkali carbonate bath.
2. The process as defined in claim 1, said Na3AlF6 contaminant also containing a small amount of sodium cyanide, and including also oxidizing said sodium cyanide to nitrogen and CO2 contained in said off-gas.
3. The process as defined in claim 1, said spent contaminant salt including sodium fluoride and sodium aluminate.
4. The process as defined in claim 1, said molten alkali carbonate bath being substantially sodium carbonate.
5. The process as defined in claim 1, said molten alkali carbonate bath being sodium carbonate and including sodium chloride.
6. The process as defined in claim 1, said molten alkali carbonate bath being sodium carbonate and including a small amount of calcium oxide.
7. The process as defined in claim 1, said alkali metal carbonate being sodium carbonate and including about 10 to about 90% of sodium chloride by weight of the total molten salt bath.
8. The process as defined in claim 1, said alkali metal carbonate being sodium carbonate and 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.
9. The process as defined in claim 1, employing an excess amount of oxygen so as to convert substantially all of the carbon in said electrodes to carbon dioxide.
10. The process as defined in claim 1, wherein said electrodes from aluminum smelting are carbon anodes and 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 900° to about 1000°C 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. The process as defined in 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. The process as defined in claim 11, 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. The process as defined in claim 11, 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.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US25473294A | 1994-06-06 | 1994-06-06 | |
| US08/254,732 | 1994-06-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2140823A1 true CA2140823A1 (en) | 1995-12-07 |
Family
ID=22965377
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002140823A Abandoned CA2140823A1 (en) | 1994-06-06 | 1995-01-23 | Aluminum smelting electrode destruction with molten salt |
Country Status (5)
| Country | Link |
|---|---|
| KR (1) | KR960001149A (en) |
| CN (1) | CN1120015A (en) |
| BR (1) | BR9500384A (en) |
| CA (1) | CA2140823A1 (en) |
| GB (1) | GB2290081A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN114618859B (en) * | 2022-02-28 | 2023-06-06 | 武汉大学 | Method for recycling waste crystalline silicon solar panels |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3845190A (en) * | 1972-06-20 | 1974-10-29 | Rockwell International Corp | Disposal of organic pesticides |
| US4145396A (en) * | 1976-05-03 | 1979-03-20 | Rockwell International Corporation | Treatment of organic waste |
| EP0515727A1 (en) * | 1991-05-20 | 1992-12-02 | Rockwell International Corporation | Waste treatment system |
| US5385710A (en) * | 1993-12-21 | 1995-01-31 | Rockwell International Corporation | Diffusion resistant refractory for containment of fluorine-rich molten salt |
-
1995
- 1995-01-23 CA CA002140823A patent/CA2140823A1/en not_active Abandoned
- 1995-01-30 BR BR9500384A patent/BR9500384A/en not_active Application Discontinuation
- 1995-02-15 CN CN95101978A patent/CN1120015A/en active Pending
- 1995-02-16 KR KR1019950003176A patent/KR960001149A/en not_active Application Discontinuation
- 1995-03-10 GB GB9504853A patent/GB2290081A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| CN1120015A (en) | 1996-04-10 |
| KR960001149A (en) | 1996-01-25 |
| GB9504853D0 (en) | 1995-04-26 |
| GB2290081A (en) | 1995-12-13 |
| BR9500384A (en) | 1996-01-09 |
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