CA1185789A - Apparatus for extracting metals from metal carbides - Google Patents

Abstract

APPARATUS FOR EXTRACTING METALS FROM METAL CARBIDES

Abstract An apparatus for extracting metals from metal carbides is described. It comprises a vertical reactor including a self-supporting graphite retort having a plurality of oblique wall passages, said retort being adjacent to the hollow central passageway of said vertical reactor, said reactor having an unheated outer wall and including an exit port for removing extracted molten metal at the lower end of said central passageway, said outlet being formed by said unheated outer wall.

Description

-~:~85~8~
The present invention relates to an apparatus for extract-ing metals from metal carbides, and is a divisional of Canadian application Serial No. 366,101, filed December 3, 1980.
The above parent application relates to a method and appar-atus for the thermal production o~ a group of metal carbides and/or metals wherein a mixture of metal oxide and carbon is reduced to metal carbide and the metal then extracted from the metal carbide. Carbon or graphite is used to encapsulate the agglomerates of the metal oxide and carbon. The carbon capsule lQ or jacket serves as a housing for the agglomerate during the reducing reaction. The amount of carbon required for reduction of the metal oxide and formation of the carbide is provided in the agglomerate. The group of metals to which the invention pertains include the carbide-forming (generall~
considered hard-to-reduce) metallic elements aluminum, boron, ~ilicon, titanium, zirconium, tantalum, niobi~m, molybdenum tungsten and uranium.
It is a particular object of the invention to produce aluminum in the form of a pure metal, in a two-stage process, 2Q ~ia the intermediate product aluminum carbide, but to generally reduce the oxides of the other metals mentioned above only to the stage of stable carbides.
The thermal processes for producing the above-men~ioned carbides and metals generally differ quite substantially from one another and are usually tailored to the particular metal concerned. The method according to the present invention permits the production of these metal carbides and at least partiai extraction of the metals therefrom according to a common principle~ Using aluminum as an example, the basic 3a problems o~ carbothermic reduction of metal oxides that are hard to reduce, as well as details of the method according to the invention, will be described in greater detail.

~35~

The known single stage methods for the thermal dir-ect reduction of aluminum oxide with carbon have thus far not been proven to be of value in commercial practice ~ue to chemical, physical and procedural problems. In particular, it has not been possible to de~elop a continuous industrial process.
The chemi c21 problems are primarily due to the fact that, in accordance with thermodynamics, the reduction of aluminum oxide with carbon will first lead over the intermedi-ate stage of oxycarbide to the formation of aluminum carbide ~3578~

1 (A14C3). Aluminum carbide formation i5 basically favored over aluminum formation. Through a reduced supply of carbon during the reductîon process, formation of aluminum carbide can be suppressed and the proportion of aluminum increased.
However, with a decrease in the quantity of carbon present, the competing reactiori leading ~o the formation of volatile aluminum suboxide (A120)is increasingly favoredO Thust the increasing volatilization of the aluminum oxide used in the process must be reckoned with, if aluminum formation is to be increased in the known processes.
The course o reduction in the prior art processes is furthermore hindered because the preferentially formed aluminum carbide clissolves both in aluminum oxide and aluminum.
Although the dissolved aluminum carbide can be further converted lS with aluminum oxicle to aluminum, under practical conditions thls reaction does; not proceed exclusively in the direction of aluminum.
Another aggravating cir~umstance is the relatively narrow temperatur~ range of 2050 to 2150 degrees centigrade that must be maintained for the aluminum oxide reaction to achieve a favorable yield in aluminum. Beginning at about 2000 degrees centigrade~, an undesirable vaporization of the aluminum is noticed. For t:he formation of aluminum carbide according to the invention the preferred reduction temperatures lie between 1950 and 2050 degrees centigrade. At these temperatures and with a sufficient carbon supply, the vaporization losses through aluminum suboxide and aluminum can be kept to a minimum.
The required narrow temperature range of around 2000 degrees centigrade poses a serious technical problem for the reduction process for producing aluminum carbide.
The physical difficulties of carbothermic aluminum production are that the molten aluminum oxide (melting point ~L~L8S~

1 around 2050 deyrees centigrade) is specifically heavier than the liquid aluminum. A molten mixture of aluminum oxide, aluminum carbide and carbon (which mixture is present in greater quantity than the tappable aluminum) collects at the bottom of the reclucing furnace. Also, graphi~zed carbon which can form during the reduc~ion process has abou~ the same density as li~uidl aluminum~ An enrichment of aluminum car~ide leads to hard-to-melt compositions ~as aluminum carbide consti-tutes a solid phase) and with higher concentrations in aluminum oxide or aluminum increases the melting points of the binary mixtures.
The problems of a carbothermic aluminum oxide reduc-tion shown here to be of a chemical and physical nature naturall3 lead to considerable procedural dificulties. Because of the required high reduction temperatures electric arc furnaces were principally used ~.or industrial aluminum oxide reduction, as is customary in elect:rometallurgy. Only in small-scale ~ests were direc~ly or indir~ctly heated resistance furnaces used; the heating elements consistin~ of carbon or graphite. Neither electric arc furnalces nor other electrical heating systems used hitherto have proven successful for the carbothermic production of aluminum.
In addition to single-stage reduction of aluminum oxide to aluminum, two-stage processes have also been proposed wherein a mixture of aluminum and aluminum carbide that can be tapped in the li~uid state is produced first in an arc furnace, an~ then the aluminum separated from the Al-A14C3 melt by liquation in the presence of a flux or obtained via an aluminum subchloride distillation. According to another proposed method, the aluminum-containing melt products are mechanically prepared by hot milling and the aluminum then separated by straining The control of the reduction process in the arc 357~9 furnace and the c~xide-free tapping of the very hot and relatively light Al-A14C3 melt present enormous d if f i cul ties.
A three stage process for thermal aluminum production has also been proposed in which aluminum carbide is pro-duced as an intermedia~e product. Wi~h a limited amount of carbon the aluminum oxide is reduced to a gaseous mixture of A12O, Al vapor and CO. The mixture of A12O
and Al vapor (distilled out of the reducing furnace) is then converted with an excess of carbon to aluminum carbide. In a third stage, the aluminu~ carbide is decom-posed to aluminum and carbon at temperatures of 2000 degrees centigrade and pressures of 20 to 50 torr. The aluminum is condlensed as a liquid or solid. The drawbacks o~ this three-st:age process lie in the fact that the thermal decomposition of aluminum carbide under vacuum is extremely dif~icult to bring about~ Additional difficul-tie~ complicate the complete reaction of A12O with the carbon to form A14C3 and the reverse reaction of Al vapor with CO.
An overview of the known carbide processes of thermic aluminum production is given by E. Herrmann in the journal "Aluminium" 1961, No. 4, pages 215-218.
It is an object of the parent invention to avoid the drawbacks of the prior art processes and to provide a method and apparatus for the continuous production of metal carbide and elemental metal. According to the parent invention, this object is achieved by admixing a metal oxide and carbon; forming said metal oxide and carbon mixture into a plurality of agglomerates, the amount of 57~

carbon in each of the agglomerates being sufficient for - reducing said metal oxide to a carbide of said metal;
encasing each of said agglomerates within a shell comprising at least one material selected from the group consisting of carbon and graph-ite; placing said agglomerate containinq shells in a tightly packed formation; coking said shells; and reducin~ the metal oxide in said agglomerates to form metal carbide by electrical resistance heating of said tightly packed shell ~ormation, said shells remaining as a housin~ for said agglomerates and serving as an electrical resistance element for heating the agglomerates during said reducing reaction.
The preferred form of the invention relates to the preparation of aluminum carbide and aluminum.
The first important step of the process invo].ves the proper selection of charging material and its preparation. A fine-particle aluminum oxide is mixed with a carbon carrier of good binding property. The particulate aluminum oxide material preferred for use in the invention is of the type commonly used for feeding the electro~
lytic cell of a smelting operation. PreEerably, the oxide particles have an average particle size of about 50 micromete~s and a maximum particle size of 180 micrometers (This means all of the oxide particles pass an 80 mesh ~Tyler) sieve.) .~.s the carbon carrier, it is possible to use carbon binders such as tars, pitches, resins or especially the so-called extracts Erom solvent and pressure extraction of bituminous coals.

These materials can be used alone or mixed with powdered carbon or coke particles.

The green mixture of aluminum oxide and carbon binder is agglomerated into small briquettes. Agglomer-ation in the sense of the present text refers to processes for making ~ine-particle materials into lumps, as described in the journal "Chemie-Ingenieur-Technik" 51 (1979), No. 4, Pages 266-188. One agglomeration technique suitable for use in the present invention consists of mixing the alumina, carbon powder and a pitch binder at elevated temperatures in a mixer or kneader and compacting the green mix to form shaped briquettes on a roll type briquetting press. Preferred briquettes are, for example, isometric cylinders, balls or pillow-shaped briquettes which can be formed in large numbers on ringroll mills or bench presses The size of the briquettes is between about 1 and 10 cm in diameter (~or ball or cylinder shaped articles), or 1-10 cm height or thickness (for briquettes in rectangular or fla~tened shapes)~ The prefer~ed briquette dimensional ranye is between about 3 and 6 mm (diameter, height, or thickne~s depending on shape). The compac~ed and formed mixture of aluminum oxide and carbon binder is then 2a surrounded with a shell of exclusively carbon. This shell is developed from a likewise plastically ormable mass, as prepared for example from petroleum coke powder and pitch.
The briquette of the charging material can be compared with the structure of a hazelnutv the mixture of aluminum oxide and carbon binder serving as the core and low-ash coke dust and pitch serving as the shell. This carbon shell can be pressed around or rolled on, as in pelletizing.
The green briquettes are then subjected ~o a coking process. In this process, the pitch inside the shell and the carbon binder inside the core are transformed into solid carbon or coke. The coked pitch inside the shell has to fulfill the task of a good binder coke since high strength requirements are made on the carbon shell. The - 6a -g~L85~
carbon shell must withstand cer~ain compression, impact and abrasion stresses in the subsequent stages of the process. The wall thickness of the shell is exactly dimensioned so that the outer cover of the core mixture will be equal to the stresses to which the briquette is subjected. The exact wall thickness that is selected is varied and depencls on the nature of the stresses, impact and compression t:hat is present in the subsequent process steps, the thickness being greater with increasing stress -lQ or abrasion. The coked carbon shell s~rves the additional tasks of conducting th~ electric current and as an electri-cal resistance el,ement for heating of the agglomerate.
Generally sp~aking the carbon or graphite capsule or jacket serves as a housing for the agglomerate during the reducing reactiorl. The amount of carbon required for reduction oE the metal oxide and formation of the carbide is provided in the agglomerate.
Xn the proce~s, the briquettes are heated by electrical resistance heating. According to the invention, in a compact mass of briquettes (i.e. one in which a mass of bri~uettes are in contact with one another) the electrical current flows mai,nly from briquette to briquette over ~he carbon shells. In the reduction o~ the aluminum oxide with carbon to aluminum carbide the carbon shell remains as a sturdy casing. The carbon shell of the bri~uettes insures that the electrical resistance conditions and heat supply remain largely uninfluenced by changes in the core during reduction. The reducing gas leaves the core via the naturally existislg porous channels in the carbon shell.
The carbon shell also serves as a small transporting vessel for the aluminum carbide produced by the reaction.
The strength of the core mixture of oxide and coke is i7~l~
of minor importanre. The coking residue of the carbon binder in the core mixture serves as reducing carbon for the aluminum oxide and is sufficient for the reduction to aluminum carbide. In the coked core mixture, for example, the coke content and/or the reducing carbon amounts to about 30-35%.
The invention claimed herein is related to the invention discussed above and relates to an apparatus for extracting metals from metal carbides. It comprises a vertical reactor including a self-supporting graphite retort having a plurality of obliquP wall passages, said retort being adjacent to the hollow central passageway of said vertical reactor, said reactor having an unheated outer wall and including an exit port for removing extracted molten metal at the lower end of said central passageway, said outlet being ~ormed by said unheated outer wall.
The inven~ion will be further explained with reference to the drawing in which, Figure 1 depicts a cross sectional view of a ball shaped briquette used as a charging material in ~he parent invention;
Figure 2 depicts a cross sectional view of a combined calcining and reduction furnace according to the parent invention;
Figure 3 illustrates a cross sectional view of an extraction reactor according to the present invention; and Figure 1 shows a cross sectional view of a ball-shaped briquette of ~he charging material. In this drawing, a denotes the carbon shell and b denotes the core mixture of aluminum oxide and carbon. Prior to being placed in the reducing furnace, the green briquettes must be coked or cal-cined. The calcining takes place either separately from the ~185~

1 reducing furnace in shaf~, tunnel or rotary tubular furnaces up to temperatures of about 800-lO00 degrees centigrade or directly in a pre!liminary stage of ~he reducing furnace. Since the briquettes must all be heated to the reducing temperature S a combined furnace unit for both calcining and reducing will save energy.
Figure 2 illustrates a combined calcining and reduc-ing furnace. The briquettes are hea~ed and calcined in the indirectly heated (e.g., by means of gas) shaft section I of the furnace. The reduction of the aluminum oxide takes place by direct electrical resistance heating in furnace section II.
The reduced material is removed in furnace section III.
The charging material from the briquettes is intro-duced via a hopper l and entry port 2 into the vertical muffle space 3.
The inside wall 7 o the vertical muffle 3 is constructed of silica bricks or fire clay bricks having an alumina content greater than 60%. The flues 8 have .
exterior refractory walls 9. The furnace has an exterior shell lO, preferably of steel. During the descent and heating of the charging material in the furnace, the volatile pitch components from the shells and the binder of ~he core mixture are expelled if green briquettes are used. Together with the reducing gas from furnace section II, which mainly consists of carbon monoxide, the volatile components leave muffle space 3 through skylights ll. A part of the gaseous mixture of volatile components and reducing gas is led into flues 8 and burned with preheated air, which is conveyed via channels 13. The hot waste gases leave shaft section I via channel 12. From here, the hot waste gases are moved to a heat exchanger (recuperator3 for preheating the air. The second part of the gaseous mixture is drawn off via :Line 14 and burned, e.g., in a boiler plant g 1 or otherwise disp~sed o~ or recovered. The distribution of the gas streams into flues 8 and line 14 is regulated by valves 15. The heat of combustion of the gaseous mixture of reducing gas and volatile c:omponen~s of the charge is considerably greater than is required for the external heating of the muffle. ~t ~he lower end of the muffle space 3, the briquettes reach a temperatu,re o about 1300 degrees centigrade.
From muffle space 3, the precalcined, solid briquettes of the charge move into the reducing space 16 of furnace section II. The electric current for heating of the charge is conveyed through side electrodes 20/21 and central electrode 23/24. The side electrodes ~0/21 are positioned in electrically non-conductive tubes 22. Jrhe electrodes 20 and 24 consist of electrographite and are screwed onto water-cooled shaft~ 21 and 24. In the zone of the electrodes, the current, whether alternating or direc~ current, flows over the bulk formed by the briquettes. The current is ad~usted so that temperatures of 1950 to 2050 Idegrees centigrade are reached there and the core mixture of briquettes is converted to aluminum carbide.
Furnace section II is internally lined with carbon bricks 17.
A heat insulating layer 18 of ceramic refractory material is disposed betweerl the carbon brick structure 17 and the water-cooled outer jac:ket 19.
The core-reduced, aluminum carbide-containing briq-uettes are removed while hot via the reractoty-lined channels 25 in furnace section IXI and then via vibration chutes 27.
The channels 25 are lined with a ceramic material 26 such as mullite or alumina bricks. The discharge temperature lies at about 1500-1600 degrees centigrade. In place of vibration chutes, rotary tables or screw conveyors can be used. The removal and transfer of the fully reduced charging mater;al to containers or directly to an interconnected extraction reactor 78~

1 (e.g., as in ~igure 3) is effected under the exclusion of air.
The prc)cess of carbide production described above for aluminum can similarly be employed for o~her carbide-form-ing metals that are hard to reduce such as, for example, boron, silicon, titanium, zirconium, tantalum, niobium, molybdenum, tungsten or uranium for which similar difficulties arise in arc furnace reductioln as in the reduction of Al2O3. The advantage of the process according to the invention lies primarily in the fact that the carbides of the aforenamed metals can be ob~ained in continuous operationO The processes known hitherto flor obtaining these metal carbides operate in a discontinuous manner. It is likewise possible to obtain high-melting borides, e.g., titanium boride or zirconium boride, directly from the respective oxides in a continuous operation.
The aluminum carbide obtained from the carbothermic aluminum oxide reduction in carbon capsules constitutes an intermediate product feom which the aluminum is to be extracted.
The extraction of the aluminum takes place with gaseous alum~
inum fluoride, AlF3, at temperatures above 1100 degrees cent~
igrade. (AlF3 sublimes at about llO0 degrees centigrade).
It is known that aluminum fluoride reacts at high temperatures with aluminum and forms gaseous aluminum sùbfluoride, AlF, which with tempera~ure decrease disproportionates again to aluminum and aluminum fluoride. The transport reaction via the aluminum subfluoride can be used for the extraction of aluminum from its carbide. Temperatures of 1500 to 1600 degrees centigrade are required for the reaction of aluminum carbide with aluminum fluoride to form aluminum subfluoride.
The process according to the invention provides for maintaining a temperature difference, in a closed reactor space, between the reaction sites for formation and dispropor-7~

1 tionation of alumin~m subfluoride, which will allow an automatic continuous process of aluminum extraction.
The extraction reactor and its manner of operation will now be explained more fully with reference to Figure 3.
The extraction reactor consists of a central, cylindrical reaction space 30 and an annular space 31. The central reac~
tion space 30 is formed by a thick-walled graphite tube 32 with oblique windows 33. Graphite tube 32, because of its large size, is composed of individual rings that are inserted into one another~ At the top and bot~om, graphite tube 32 ends in connection rings 34 made of graphite into which current-carrying bolts 35 are screwed. The contact pressure between graphite tube 32 and connection rings 34 is assured by pressure springs 36. The graphite tube 32 is heated in a resistance lS process with electric current. At its outer circumference, the reactor is lined with A12O3 -containing bricks 37. The bricks 37 contain a high ~uantity (more than 70~) of alumina and may be at least mullite or at maximum pure corundum bricks.
The aluminum carbide-containing material is intro duced via a sealable chamber 38 into the central reaction space 30 at an entry temperature of o~er 1100 degxees centigrade.
The carbon material remaininq after the aluminum extraction is discharged through cooling space 44 and then via the dis-charge tray 39 with scraper 40. The central reaction space 30 is also charged with aluminum fluoride when starting the reactor with the aluminum carbide~containing material. The interior temperatures of about 1500 - 1600 degrees centigrade within reaction space 30 allow the aluminum fluoride to vap-orize. The aluminum fluoride condenses in a layer 41 on re-actor wall 37. The aluminum fluoride layer 41 brings about an additional heat insulation, its thickness increasing with ~57 !3~

1 a sufficient AlF3 supply until the temperature on the inner surface of layer 41 rises to above 1100 degrees centigrade and no further AlF3 is condensed. There is thus formed an AlF3 atmosphere inside the reactor. In ~he central reaction space 30, the AlF3 reacts at te~peratures be~ween 1500 and 1600 degrees centigrade with the aluminum carbide to form AlF which then diffuses through to the cooler reactor wall 37 and to the condensed AlF3 layer 41 respec~ively, where it disproportionates again to aluminum and gaseous AlF3~ In this way, aluminum is constantly transpor~ed rom the inner space of ~he graphite tube through windows 33 and annular space 31 to the surrounding wall 41/37. The al~minum deposited there descends down the wall, collects at the bottom 42 ~f annular space 31 and is drawn off discontinuously through a taphole 43 or continuously through a siphon (not shown in ~he Figure).
The temperature of the charging material to be ex-tracted must be higher in the transition zone from chamber 38 to central reaction space 30, than the condensation temp-erature at the surface of layer 41 so that no aluminum de-posits on the charge. The charging material is introduced into the extraction reactor through an aperture 46 and a slide closure 45 of refractory material which is opened dur-ing charging. This charging device can also be of a differ-ent design, e.g., a bell or flap closure~ The discharge temperature of the cerbon material remaining after the extrac-tion should likewise be higher than the surface temperature of layer 41. The carbon residues in the upper section of cool-ing space 44 provide an effective heat insulation in downward direction and thus prevent disproportionation of AlF there.
The extraction reactor can have a circular or rect-angular cross section. Also several extraction units can be . ~
~8~

1 combined into a set.
As ment,ioned before, the aluminum oxide-carbon mix-ture is enclosed with a sturdy carbon shell and the A12O3-C
core reduced to aluminum carbide in a reducing furnace (Figure

2). Prior to enl:ry into the extraction reactor, the çarbon shell is preferably broken up or blasted to allow the AlF3 easier access to the aluminum carbide. Basically, the carbon material discharqed from the extraction reactor is returned to the process cycle, i.e., used again for preparing A12O3-carbon mixture O.t encasing in carbon.
In a fulrther preferred embodimen~ of the invention, one may also use prefabricated carbon or graphite vessels for the aluminum oxide carbon binder mixture. It is no~ necessary that the carbon containers enclose the A12O3-C mixture all lS around. Pre~erred container configurations are cylindrical pots or sleeves t:hat are open on one or both sides. Prefabric-ated carbon and graphite containers are conveniently used repeatedly in the! process cycle until they must be replaced due to we,ar or breakage.
The use of prefabricated containers is more fully explained in the ~ollowing example:
Using an extruder device, pipes are extruded from a suitable carbon material ~e.g., the same base material used for the shells or casinqs of the agglomerated particles -petroleum coke filler material and electrode pitch) and then baked at 1200 degrees centigrade in an annular baking furnace or fully graphitized. The carbon tubes are cut into uniform section, i.e., sleeves or rings. The prefabricated carbon sleeves are fillecl with a plastic mîxture of aluminum oxide, petroleum coke powder and a tar base binder material. To ~ive the mixture inside the carbon sleeve a better grip the sleeves can have inward directed teeth formed along with the 1 extrusions. The filled sleeves serve as charging ma~erial both f~r the reducing furnace according to Figure 2 and the extraction reactor according to Figure 3. In shaft section I of the reducing furnace~ the A12O3-C binder mass is coked, and in the reducing section II the is reduced to aluminum carbide. ~rhe carbon sleeve is the supporting shell as well as the resistance heating element for the A12O33 C
mass inside it. In the shaft section, the A12O3-C binder mixtures loses, for example, 5-20% of its weight and shrinks as a result. During the reduction to aluminum carbide, the coked A12O3-C mass undergc~es a further loss in weight of about 55%. Here too, a sligh~ shrinkage occurs. The brown to brown-black aluminuml carbide forms a porous, spongy~ compres-sible structure. In the extraction reactor, the hollow spaces between the supporting sleeves of carbon make possible an e~fective di~fusion exchange, i.e., transpor~ation of gaseous AlF3 to the aluminu~n carbide and carrying away of gaseous AlF.
A slight addition of calcium fluoride and/or mag-nesium fluoride to l:he aluminum fluoride in the extraction ~0 reactor of up to about 5 weight % has proven to be effective for promoting coagulation of separated aluminum droplets.
The following example provides further details on the thermal extraction of aluminum by the process o~ invention.
100 parts of powdery alumina with a maximum part-!5 icle size of 100 micrometers is thoroughly mixed with 24 partsof pulverized petroleum coke and 46 parts of electrode pitch having a softening point of 70 degrees centigrade. The mixing procedure is carried out at a temperature of about 180 degrees centi~rade in a heated mixerO The green mixture, still in 0 the warm state, is pr.essed into short cylindrical graphite tubes on a die press. A specific pressure of 5 N/mm2 is applied for compacting the mixture in the tube. The outer ~5~

1 diameter of the tubes is 50 mm, the wall thickness 4 mm and ~he t~be leng~h 100 mm. One tube contains about 110 cubic centi-meters of the green mix corresponding to a quantity of approxi-mately 200 grams.
The graphite tubes which are filled with the mix-ture of alumina, petroleum coke powder, and pitch are charged into the muffle 3 ~f a reduction furnace as illustrated in Figure 2. The circular muffle has an inside diameter of 1.5 m and a height of 8 meters. It is heated by means of ga~ from the outside. The tubes move slowly down the muffle and are heated up to about 1000 degrees centigrade at the lower out-let of the muffle.
During the hea~ing-up operation, the pitch binder i9 coked, giving a coke-residue of approximately 65%. Thus, 30 parts by weight of coke remain in the mixture from the original 46 par~s of pitch, or in other words, the coked mix-ture is composed of about approximately 65 par~s by w~eight of alumina and 35 parts by weight of carbon.
In the lower section II of the reduction furnace the graphite tubes are brought to a temperature in the range of 1950 to 2000 degrees centigrade. An electric current of 50 kA at a voltage of 60 V is passed through the packing of the graphite tubes serving as a resistor material~ The alumina-carbon mixture in the graphite tubes is converted to aluminum carbide. The average residence time of the graphite ~ubes in the reduction furnace fro~ charging to discharging amounts to about 12 hours.
After the carbide-bearing graphite tubes are dis-charged from the rleduction furnace, they are transferred to an extracting reactor as in Figure III. A cooling down of the reactor feed below 1200 de~rees centigrade is avoided.
For this reason, t'he carbide bearing tubes are conveyed in 57~3~

closed vibrating tubular carriers to the feeding device of the reactor. The central graphite muffle 30 in the reactor provides the heat necessary for the extraction process. The internal diameter of the graphite muffle is 1.2 meters; its height is 6 meters. Extraction proceeds faster than reduction. Thus, the residence time of the carbide-containiny graphite tubes in the graphite muffle is only 3.5 to 4 hours. The extraction is a self-maintain-ing circulating process between two temperatures. The temperature in the graphite muffle is kept close to 1500 degrees centigrade, the surface temperature of the outside wall which is covered with aluminum fluoride slightly above 1100 degrees centrigrade. Aluminum is produced at a rate of approximately 300 kg per hour and is tapped at the bottom of the rea,ctor at a temperature of 1100 degrees centigrade. The carbon which is set free by the decom-position o~ the carbide is removed from the graphite tubes and recycled. Also~ the graphite tubes are used again as con~ainers for a new cycle.
The aforementioned metal carbides of boron, silicon, titanium, zirconium, tantalum, niobium, molybdenum, tungsten or uranium are hard materials, that are widely used in industry. For ~he production of hard metals, carbides of titanium, tantalum and tungsten are preferably used. Titanium c:arbide, TiC, is for example produced according to the process of the p~esent invention in that a mixture of titanium dioxide, carbon black and pitch is prepared so that when coked, 3 mols carbon exist per mol TiO2. The mass of titanium dioxide, carbon blaclc and pitch is pressed into graphite sleeves and the so obtained briquettes charged into the calcining and reducing furnace according to the invention. The reduction to ~ 17 -57~

1 titanium carbide takes place at temperatures of around 2000 to 2500 degrees centigrade. Maintaining of narrowly limited temperature ranqes is here not Qf the same importance as in the preparation of aluminum carbide. The reduction of titan-ium dioxide can, for example, be coupled with that of tungstentrioxide to directly obtain co-carbides of TiC and WC, as needed for hard metal production.
It is also possible to introduce hydrogen or nitro-gen into the reducing ~urnace from the direc~ion of the dis-charge end of the process either to improve by means of hydro-gen the reduction conditions or to form carbonitrides in the case of nitrogen.
I wish therefore to be limited not by the foregoing description of a preferred embodiment of the invention but~
on the contrary, solely by the claims granted to me.

~0

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for extracting metals from metal carbides comprising a vertical reactor including a self-supporting graphite retort having a plurality of oblique wall passages, said retort being adjacent to the hollow central passageway of said vertical reactor, said reactor having an unheated outer wall and including an exit port for removing extracted molten metal at the lower end of said central passageway, said outlet being formed by said unheated outer wall.

2. Apparatus according to claim 1 wherein said vertical retort comprises a thick wall graphite tube with oblique windows communicating with said central passageway.

3. Apparatus according to claim 2 wherein said retort is interconnected with a plurality of current-carrying electrodes.

4. Apparatus according to claim 3 wherein said graphite tube comprises a plurality of individual interconnected graphite rings.

5. Apparatus for extracting metals from metal carbides comprising a reactor comprising an outer wall, an annular reaction space formed by a graphite tube within said outer wall, said graphite tube being disposed about the generally circular hollow core of said reactor and having passage ways communicating with said hollow core space; said passage ways being slanted downwardly at an oblique angle toward said hollow core, and an exit passage way from within said hollow tube to the exterior of said reactor for discharging molten aluminum metal.

6. Apparatus as defined in claim 5 wherein the reactor includes a lining of bricks consisting of more than 70% by weight of alumina.