US2931719A - Process and apparatus for the production of metals by dissociation of their carbides - Google Patents

Description

April 1960 c. D. MENEGOZ ETAL 2,931,719

PROCESS AND APPARATUS FOR THE PRODUCTION OF METALS BY DISSOCIATION OF THEIR CARBIDES Filed Sept. 6, 1957 3 Sheets-Sheet 1 Fial . INVENTORS Charles Daniel Mene goz n Andre'dacques Galg Win69 ATTORNEY April 1960 c. D. MENEGOZ ETAL PROCESS AND APPARATUS FOR THE PRODUCTION OF METALS BY DISSOCIATION OF THEIR CARBIDES Filed Sept. 6, 1957 3 Sheets-Sheet 2 INVENTORS B a we w .w a 3 :7 D r d m n A a u a c. D. MENEdoz ET AL PROCESS AND APPARATUS FIOR THE PRODUCTION OF April 5, 1960 2,931,719

METALS BY DISSOCIATION OF THEIR CARBIDES I Filed Sept. '6, 1957 3 Sheets-Sheet 3 1! 10.lit!!!IIIIIIIIIIIIIIIIN\ INVENTORS Charla 5 Daniel Mene goz dA'ndre' Jacques Gatg BY %7%[ Md/W 9 ATTORNEY United States Patent PROCESS AND APPARATUS FOR THE PRODUC- TION 0F METALS BY DISSOCIATION OF THEIR CARBIDES Charles Daniel Mngoz and Andr Jacques 'Galy, Grenoble, France, assignors to Pechiney, Compagnic de Produits Chimiques et Electrometallurgiques, Paris France, a corporation of France ApplicationSeptember 6, 1957, Serial No. 682,447

Claims priority, application France September 28, 1956.

10 Claims. (Cl. 75-10) It is known that by heating metallic carbides at a high temperature and; in a vacuum, they are dissociated into metallic vapors--which can be collected in an appropriate condenser-and into carbon, generally, in the form of graphite which substantially retains the volume of the original carbide.

Calcium, in'particular, hasbeen produced from CaC heated by resistances placed inside or outside the carbide body; however, the efiiciency of such processes from the standpoint of energy consumption is slight, and, the electric energy consumption is high.

The present invention, which isthe result of applicants researches, relates to a process. and an apparatus for the. production of metals by the, dissociation of their car bides.

Applicants have established that partially. dissociated. metallic carbide grains conduct an electric current suffieiently well; to enable the required dissociationtem:

perature tobe attained. Accordingly, the process whichr is the object of the present invention consists in passing an electric current directly throughthe metallic carbide grains during their dissociation, using the resistance of the charge itself for the heating.

The carbideutilized for this purpose consists, for example, of grainsv or particles. 7 to 10 millimeters in size which remain discrete, i.e. individually distinct during heating; any agglomeration presentsa risk of preventing the liberation of metallicivapors. 7 Hence, an important feature of the invention 15 to en sure progressive heating of the carbide charge which,

isfirst subjected to a temperature which enables the dissociation to start; the high temperature necessary to remove'the last traces of metal from the carbon residues, is only attained at; a time when fusion and agglomeration 2,931,719 Patented. Ann. .,1 259 2 residuesbeneath the electrodes before introducing the succeeding charge. Such, tamping has also the advantage of facilitating the current fiow through the zone of maximum heating, and of preventing useless condensation In the caseof'manganese carbide, the temperature at the start of the dissociation is 1200" C. and the maximum temperature is 1350 C.

I In the case of aluminum carbide, the temperatures are 16 50? C. and 1900 C; respectively.

Care must betaken to avoidlocalized packing-inside thecharge undergoingdissociation, because such packing causesformation'of voids and,subsequently, the striking of an arc and superheating, leading to partial fusion of the carbide grains and agglomeration thereof; Hence; an importantfeature ofthe process is to -pack or-tampthe of metallic vapors inside the mass of cold, but compact, carbonaceous residues.

The invention also relates to an apparatus for the commercial manufacture of metals by thermal dissociation of their carbides. This apparatus is illustrated diagrammatically in the annexed drawings, in which Figure 1 is a plan view of a section taken on line I-I of Figure 2;

Figure; 2 is a view-in elevation taken on line II-II of Figure 1,, the metal being here condensed to the solid state;-

Figure 3 is a view similar to Figure 2'of a modification in which'the metal is condensed to the liquid state.

The apparatus has a generally cylindrical form; it is surrounded by a metal jacket, 1. The dissociation zone is; situated at the center; it comprises in its upper part the electrodes 2 symmetrically arranged about the charging tube 7, for the'carbide grains, which is positioned axially in the cylinder and is closed by a plug 6. Below the electrodes there is disposed, in the cylindrical vessel 17, (a reserve of) the packed carbonaceous residue and adischarge device, doors 4, fordischarging the residue into the vessel 5.

Between the conical surface of the carbide grains undergoing dissociation and the outlet of the charging tube 7, there is provided an empty space S, in which the metallic vapors are liberated (disengaged); this space communicates at-itsupper part and at its entire periphery with the condensation zone 10 concentric with the dis sociation zone, from which it is separated by' a heat insulator layer 13. The surfaces 8 and 9 which delimit" the empty space at its upper part, and its peripheral communication with the condensation zone, can be easily scraped from the outside through the openings 11' for the purpose of removing the concreted masses which condense thereon.

In order to drop a charge of fresh carbide, it is necessary to open valve 12 and to thereupon lower plunger 6, this being accomplished without breaking the vacuum, thetfeed hopper 3v serving asa lock chamber in known manner. Tostop the delivery, the valve 12 is operated first and the plunger 6 is raised afterwards. Hence, normally chargingtube 7 is empty.

The circulation of the material from top to bottom in the furnace takes place in a semi-continuous manner as follows: a certain quantity of fresh carbide is dropped from the upper hopper 3. When it is sufiiciently spent, the current is switched off, the lower door 4 is opened-to permit a corresponding quantity of residue to fall into thelarge storage vessels provided in the bottom of the furnace. It will be seen that there is no obstacle opposed to the downward sliding of the residue. If the residue cannot descend by itself, then, it is pushed with the electrodeswhich are movable vertically and, also, by means of the smallplunger 6. Thercupon, the e'lectrodes'are raised above the material, so that part of the more orless spent residueslides under the electrodes, eventually pushed by the small plunger 6.

Finally, the electrodes are lowered, taking up approximately their initial position, enabling thereby the current to flow again. It will be observed that the pressure exerted bythe electrodes upon the spent residue causesit to be packed.

According to the nature of the condensed metal, i.e. according to the desirability of condensing it to the liquid or solid state, different temperatures are established at the intermediate surfaces 8 and 9 and at the condenser 10. The thickness of: theheatinsulating-walls 13 arid 14 are suitably chosen with that end in view. The outlets 19 connect the condenser with the vacuum pump (not shown).

When the residue storage vessel 5 and the condenser are full, normal pressure is reestablished in the furnace. If the metal has been condensed in the solid state on a removable sheet 10', the latter is removed through the annular bottom door 18, situated directly under the condenser. If the metal has been condensed in the liquid state (i.e. in the case of aluminum), the door 18 is replaced by a known type of tapping device 18', as shown on Figure 3.

In Figure 3, the metal is condensed in the liquid state on the wall 20, which, when necessary, can be cleaned through the openings 21 provided at the top of the furnace. In the case of aluminum, wall 20, as well as the circular vessel 22 which is in contact with liquid aluminum, can be made of aluminum nitride; the thickness of the heat insulator 13 between the dissociation and condensation zones is reduced and, in contrast, the thickness of the peripheral heat insulator 14 is increased.

Three electrodes are shown in the figures; however, a larger number can be provided if the furnace requires it. It is important to take all necessary measures to avoid any risk of shunting the current, or of a short circuit between the electrodes and the wall 17 of the dissociation zone which is generally made of carbon. The same applies in the case of guiding sleeve 16 of the electrode.

The edge-to-edge distance between the electrodes 2 can be greater by about 30% than the distance between these electrodes and the carbon wall 17; experience shows that the amount of current passing through the wall is negligible under these circumstances.

A short circuit may occur at for two reasons:

(a) If the carbon electrodes 2 and their sleeves 16, generally also of carbon, are too cold (at a temperature below the dew point of the metal vapor) there takes place condensation of solid or liquid metal, and the current will pass therethrough.

(b) If, on the contrary, the temperature is too high (close to that prevailing in the sliding conical mass), the atmosphere at 15 is highly conductive (even when working at a reduced pressure) and the current will pass through there, as an arc, instead of passing through the residue.

The present applicants have established that the short circuit which should obviously be avoided (feared), can be prevented by observing the following precautions:

Care should be taken not to adopt too wide a distance between electrodes and sleeve for, otherwise, metallic vapor will enter the gap and condense higher up. Moreover, heat losses become excessive because radiation from thehot zone strikes higher up on cold regions (places). In the case of graphite electrodes 200 mm. diameter, there should be adopted a distance of 5 to 30 mm. and, preferably, 10 mm.

The temperature in the electrodes depends on the current density in the electrode, and on the distance from the water-cooled metallic head (not shown) through which the current enters.

By varying (manipulating) these two factors, it is possible to attain at the electrodes, in the region 15 where they emerge from the sleeve, a temperature which stabilizes at 100 or 200 above the dew point of the metallic vapor under consideration. An analogous temperature prevails in the parts close to the sleeves.

To prevent short circuits at 15, resort can be had to an induced current of a light inert gas streaming from top to bottom around the electrodes and which expels the conductive metallic vapor from this zone. This gas will mix with the vapor stream flowing to the condenser. When operating at a reduced pressure, the gas is drawn ofi by the vacuum pump.

In some cases, the use of carbon sleeves can be avoided. When dissociating aluminum carbide, the sleeves can be per hour.

made of aluminum nitride, a material which is an insulator at high temperatures and which is not acted upon at all by either aluminum vapor or liquid aluminum. This avoids any short circuits.

There are given below several examples of the application of the process and apparatus forming the object of the present invention; however, these are not given by way of limitation.

The examples relate to a three-phase furnace comprising three graphite electrodes 200 mm. in diameter, through which passes a current of 7000 amperes. The edge-to-edge distance between the electrodes ranges between 220 and 300 millimeters; there is thus obtained a potential difference of 30 to 50 volts between electrodes, thereby producing a power of 364 to 607 kilovoltamperes. In the following three examples, a distance of 300 millimeters has been adopted.

Example I This relates to the production of calcium by the dissociation of CaC in a high vacuum (0.01 mm. mercury), and at a rather high temperature (1500 C. at the surface of the cone of the carbide grains, and 1700 C. at the lower ends of the electrodes). The dew point of calcium at the indicated operating pressure is 600 C., hence, the metal condenses in the solid state upon the sheet 10' (Figure 2) at the rate of 50 to kilogs. The walls 8 and 9 are at about 1000 C., and the CaO and CaC concretions condense entirely thereon. When the volume of condensed metal amounts to 1000 liters, i.e. 1800 kilogs. calcium, the vacuum is broken by introducing an inert gas into the furnace, the annular door 18 is opened (Fig. 2), and the condensed metal is removed. The concretions deposited on surfaces 8 and 9 can then be scraped through the openings 11 and removed through door 18. carbonaceous residues contained in vessel 5 are also removed. Following toassembly, the vacuum is reestablished in the furnace and a new operation is started.

Example 2 This pertains to the production of manganese by dissociation of manganese carbide Mn C in a vacuum of 0.05 mm. mercury; however, the temperature is about 1200 C. at the surface of the cone of the carbide grains and about 1350 C. in the hottest zone. The dew point of manganese at the adopted pressure is 1050 C. Hence, it condenses in the solid state at the rate of 60 to kilogs. per hour. The surfaces 8 and 9 are at a temperature of about 1000 C. The heat insulating layer at the periphery of the furnace is substantially thicker than in the case of the production of calcium and, in contrast, the thickness of heat insulator between the: dissociation and condensation zones is smaller. The run is continued until the condensate occupies a volume of 500 liters, that is, 3500 kg. manganese.

Example 3 Relates to the production of aluminum by dissociation of its carbide Al C at a pressure of 0.5 mm. mercury and a temperature of about 1650" C. at the surface of the cone of thecarbide grains, and l900 C. at the lower end of the electrodes. The aluminum, being only slightly volatile, condenses on a rather hot wall (dew point 1400 C.); hence, it is collected in the liquid state (in the apparatus shown in Figure 3) at the rate of 45 to 75 kg. per hour.

While the process has been particularly described with reference to the production of Al, Ca and Mn, it is to be clearly understood that the process is also applicable to the production of other metals by the dissociation of their respective carbides, as lithium, beryllium, magnesium, barium, strontium, lanthanum, gallium, bispauth, uranium, etc.

Suitable inert gases for the purposes of the present invention are argon, helium, krypton, neon, xenon, nitrogen, hydrogen, hydrocarbons, etc. The three latterones may be used with metals which do not form nitrides or hydrides at the operating temperature, or in the cases where the formation of nitride or hydride is not inconvenient.

We claim:

1. In the process of producing metals by the thermal dissociation of their carbides in a vacuum into metallic vapor and a carbonaceous residue, and wherein the carbides are heated to the dissociation temperature by electrical means, the improvement in said process which comprises the step of: passing an electric current through a charge consisting of metallic carbides in particulate form, whereby the carbides are progressively heated to their initial dissociation temperatures but below the fusion point of the resultant residue, and the maximum temperature attained occurs in the exhausted carbonaceous residue substantially free of carbides and metal.

'2. Process according to claim 1, wherein calcium carbide is dissociated at a temperature within the. range of 1500-1700" C. at a pressure of about 0.01 mm. Hg, and the liberated calcium is recovered in the solid state.

3. Process according to claim 1, wherein manganese carbide is dissociated at a temperature within the range of 1200-1350 C. and a pressure of about 0.05 mm. Hg, and the liberated manganese is recovered in the solid state.

4. Process according to claim 1, wherein aluminum carbide is dissociated at a temperature within the range of 1650-1900 C. and a pressure of about 0.5 mm. Hg, and the liberated aluminum is recovered in the liquid state.

5. In the process of producing metals by the thermal dissociation of their carbides in a vacuum into metallic vapor and a carbonaceous residue, and wherein the carbides are heated to the dissociation temperature by electrical means, the improvement in said process which comprises the step of: passing an electric current through a charge of metallic carbides in particulate form, whereby the carbides are heated to their dissociation temperatures, said current being supplied by a plurality of vertical, symmetrically disposed electrodes delimiting a polygonal area, while the charge is supplied to the center of said area, and the electrodes have their lower free ends in contact with residue substantially free of carbides and metal.

6. Process according to claim 5, wherein the residue is tamped down prior to the introduction of a fresh charge.

7. A substantially continuous electrothermic process for producing metals, comprising the following steps: supplying by gravity to the upper end of a vertically disposed furnace a charge, in particulate form, of a carbide of the metal to be produced, said charge accumulating on top of a tamped carbonaceous residue formed as described hereafter: passing an inert gas through the furnace to displace the air therefrom; maintaining a pres sure below 1 mm. of Hg in the said furnace; passing a current through said charge whereby it is heated to the dissociation temperature of the carbide and decomposed into a vapor of the metal to be produced and a carbonaceous residue; condensing and recovering said vapor; tamping said residue and supplying on top thereof a fresh charge to be decomposed, and removing a portion of said residue from the furnace.

8. Apparatus for the electrothermic decomposition of carbides comprising, in combination: a furnace of generally cylindrical shape having disposed in the central portion thereof from the top downwards and in the order named, respectively, a charge supply tube, electrodes surrounding said tube, a dissociation zone, a residue collection zone, outlet means for said residue from said collection zone, and a vapor disengagement space disposed above the dissociation zone and connected at its upper part, and along its entire periphery, with a condensation zone.

9. Apparatus according to claim 8, wherein the condensation zone surrounds the dissociation zone and is separated therefrom by a heat insulating wall.

10. Apparatus according to claim 8, wherein the condensation zone is surrounded on its exterior with a heat insulating wall.

References Cited in the file of this patent UNITED STATES PATENTS 984,503 Arsem Feb. 14, 1911 996,474 Fink June 27, 1911 1,576,883 Weaver Mar. 16, 1926 2,122,419 'Hanawalt July 5, 1938 2,213,170 Peake et a1. Aug. 27, 1940 2,219,059 Suchy et a1. Oct. 22, 1940 2,400,000 Gardner May 7, 1946 2,550,684 Fouquet May 1, 1951 2,582,120 Hansgirg Jan. 8, 1952 2,724,644 Mathicu Nov. 22, 1955 2,769,705 Sem Nov. 6, 1956 FOREIGN PATENTS 550,732 Great Britain Ian. 21, 1943

Claims (1)

1. IN THE PROCESS OF PRODUCING METALS BY THE THERMAL DISSOCIATION OF THEIR CARBIDES IN A VACUUM INTO METALLIC VAPOR AND A CARBONACEOUS RESIDUE, AND WHEREIN THE CARBIDES ARE HEATED TO THE DISSOCIATION TEMPERATURE BY ELECTRICAL MEANS, THE IMPROVEMENT IN SAID PROCESS WHICH COMPRISES THE STEP OF: PASSING AN ELECTRIC CURRENT THROUGH A CHARGE CONSISTING OF METALLIC CARBIDES IN PARTICULATE FORM, WHEREBY THE CARBIDES ARE PROGRESSIVELY HEATED TO THEIR INITIAL DISSOCIATION TEMPERATURES BUT BELOW THE FUSION POINT OF THE RESULTANT RESIDUE, AND THE MAXIMUM TEMPERATURE ATTAINED OCCURS IN THE EXHAUSTED CARBONACEOUS RESIDUE SUBSTANTIALLY FREE OF CARBIDES AND METAL.

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