RU2499848C2 - Plasma-carbon production method of rare-earth metals, and device for its implementation - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method involves carbon thermal reduction of oxide compound of rare-earth metal in vacuum so that powder of rare-earth metal carbide, which is free from residues of oxygen impurity, is obtained. Then, it is cooled down and mixed with high-melting metal powder in the ratio that is sufficient for performance of exchange reactions between rare-earth metal carbide and high-melting metal, and mixture is heated with hot volumetric plasma discharge to the temperature of ≥1800°C. With that, evaporating rare-earth metal is collected on condensers and hard-alloy carbide of high-melting metal is obtained. The device includes a vacuum system, cathode and anode assemblies arranged concentrically in the chamber, and a steam line and a condenser-cooler, which are coaxial to them. With that, an internal electrode represents an anode of high-current vacuum plasma discharge burning in an annular discharge cavity formed with coaxial cylindrical electrodes. The anode is made from high-melting electrically conducting material in the form of a crucible having a capacity, and a thin-wall cathode enveloping it, outside which there located is a starting resistance heater, is also made from high-melting electrically conducting material, for example tungsten, tantalum or graphite.

EFFECT: improving extraction of rare-earth metal.

5 cl, 1 dwg

Description

The invention relates to metallurgy, in particular to the production of rare-earth metals of group III of the periodic system of elements, including scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), lutetium (Lu). The indicated group of rare-earth elements is united by a common chemical analogy of all twelve metals, high chemical activity and close boiling point (T _k > 2500 ° C).

From the existing level of technology there is a known method for producing the considered rare-earth metals from oxide compounds (see Metallurgy of rare metals. Zelikman AN, Krein O.E., Samsonov GV / M .: Metallurgy, 1978. 560 p. Page 529-540), consisting of the following operations:

1. Halogenation of oxides by gaseous fluorine, chlorine or solutions of hydrofluoric and hydrochloric acids. Filtration and drying of the obtained halide salts.

2. Mixing dry powders of halide salts with shavings of metal reducing agents, for example, Mg, Na or Ca.

3. Heating the resulting mixture to the temperatures of the onset of active metallothermal reduction of halide salts ( _heating T ≥1000 ° C) and holding the reacting mass at this temperature without air until all reactions cease.

4. Cooling the reacted mass and separating the reduced metal from the slag.

5. Double refining remelting of reduced metals under vacuum to remove them from slags and impurities of reducing metals.

6. Slag processing for the separation of rare earth metals.

The disadvantages of this method, the elimination of which is directed by our proposed invention, are:

1. Low yield of the products obtained, since during metallothermal processes up to 20% of the product remains in the slag.

2. Industrial hazards and environmental pollution by fluorine, chlorine or acids during the halogenation of oxides.

3. The high cost of production, since reducing agents are obtained by electrolysis of molten fluorides and chlorides; while the environment is polluted by fluorine or chlorine; the need for double refining to produce pure metals.

4. The presence in reducing metals, although insignificant, but nevertheless inevitable impurities, which during reduction go into the resulting rare-earth metals.

The problem to which the claimed technical solution is directed is to create a device and develop a method for producing pure rare-earth metals by carbon thermal reduction of their oxide compounds to obtain an intermediate in the form of rare-earth metal carbides that do not contain residues of unreduced oxides.

This problem is solved due to the fact that when implementing the inventive method for producing rare earth metals Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, But, Er, Lu, boiling at a temperature of more than 2500 ° C, by carbon thermal reduction their oxide compounds to produce carbides of the listed metals that do not contain oxygen; subsequently, for distillation of the reduced rare-earth metals from carbides and obtaining these metals in pure form, carbide powders after cooling are mixed with refractory metal powders in a ratio necessary and sufficient for exchange reactions between rare-earth carbides and refractory metals to occur and heated to ≥1800 ° С for exchange reactions to produce carbides of refractory metals and evaporation of the resulting pure rare-earth metals, followed by condensation of ditch on capacitors; as a result, two commodity products are obtained in one electrothermal operation: pure rare-earth metal and refractory metal carbide, and metal powders, for example, tungsten metal (W) powders, are used as refractory metals to carry out exchange reactions; the process is carried out in a device containing a vacuum chamber with water-cooled outer walls, a vacuum system, cathode and anode nodes placed concentrically in the chamber, a steam pipe coaxial with them, and a condenser-cooler used to collect evaporated metals; according to the invention, the internal electrode is the anode of a high-current vacuum plasma discharge burning in an annular discharge cavity formed by coaxial cylindrical electrodes, the anode made of a refractory electrically conductive material in the form of a glass (crucible), in which the entire process charge is placed, and the thin-walled cathode surrounding it is made also from refractory electrically conductive material, for example, tungsten, tantalum or graphite, heaters are installed to start the electric furnace around the cathode whether resistance.

The technical result provided by the above set of features of the claimed method and device is to increase the extraction of the main product from the starting materials, the creation of non-waste production with minimal environmental pollution, obtaining pure and ultrapure metals, reducing the cost of obtaining rare earth metals.

The technical result is achieved by the fact that pure rare-earth metals are obtained by carbon thermal reduction of their oxide compounds to obtain an intermediate in the form of carbides of rare-earth metals that do not contain residues of unreduced oxides, the intermediate is mixed with a powder of refractory metals to ensure an exchange reaction:

$$Me{C}_2+2W=Me\uparrow +2WC,(\mathrm{one})$$

after which the rare-earth metal evaporates, is transferred through the steam line to the condenser installation area and condenses, and the carbide of the refractory metal (for example, W) remains in powder form in the crucible of the electric furnace. Extraction of valuable rare-earth metals increases from 80% by known technology to 95% or more using the proposed method for producing products. The metal production cycle is reduced, slag processing operations to extract rare earth metals, and the use of toxic materials are excluded.

The technical result is also achieved by the fact that the claimed device for implementing the method has a cathode-anode assembly providing heating of the crucible-anode due to a high-current vacuum plasma discharge burning in an annular cavity formed by cylindrical electrodes, the inner electrode is made in the form of a glass (crucible), which is placed the processed feed mixture: MeC ₂ + W; the mixture is heated to a temperature of T≥1800 ° C and a substitution reaction is carried out.

The proposed plasma-carbon method for producing the above metals consists of the following operations:

1. Mixing powdered rare earth oxides with a carbon-containing component, for example, acetylene black. The amount of the carbon-containing component is taken from the calculation of stoichiometry for carrying out carbon thermal reduction of the metal oxide and binding of the reduced metal to the most durable carbide, for example, neodymium, i.e. double neodymium carbide (NdC ₂ ). The ratio of the starting components for the plasma-carbon process for producing metallic neodymium is calculated by the reaction:

$$N{d}_2{O}_3+7C=2Nd{C}_2+3CO\uparrow .(2)$$

Eleven of the above metals are chemical analogues of neodymium, included in group III of the periodic system of elements. The reaction and formulas given above for neodymium will be identical for all the others - only with the inevitable replacement of the chemical designations of the metals themselves and the corresponding atomic constants.

2. Heating the mixture without air access to a temperature of more than 2000 ° C with evacuation of carbon oxides released during reduction and obtaining pure carbides powders. The bonding of the reduced metals with carbon is the necessary condition, the fulfillment of which, along with a high heating temperature and the use of vacuum, ensures the success of a new technical solution for the production of rare-earth metals, that is, ultimately obtaining oxygen-free substances, excluding environmentally harmful metallothermic reduction operations. A positive result is provided due to two circumstances of a physico-chemical nature. Firstly, an excess of any reagent (in this case, carbon, see reaction (2)) contributes to the processes in the direction of consumption of this reagent and the release of CO. But this purely quantitative regularity of the "acting masses" is greatly enhanced if an excess of carbon binds the reduced metal to a strong compound, for example, NdC ₂ . In this case, the formation of strong carbides by reactions of the type (2) ensures ultimate success, since only in this case it is possible to keep the metals being reduced in the condensed phase and prevent their evaporation at temperatures necessary for the reduction to take place, freeing the condensed system from oxygen.

3. Cooling the obtained powders of rare-earth metal carbides and mixing them with powders of pure refractory metals in a ratio that, when heated to a temperature of T≥1800 ° C, the exchange reaction proceeds using the example of neodymium according to the type of reaction (I):

$$Nd{C}_2+2W=Nd\uparrow +2WC.(3)$$

The exchange reaction (3) was recorded using a refractory tungsten metal, since the products obtained (Nd condensate and WC) are solid products that are widely used in industry: neodymium as the basis of new magnetic materials (Nd-FB), tungsten carbide as the basis hard alloys, which are widely used in the machining of metals. Obtaining them simultaneously in one technological operation is an important sign of a significant difference that determines the advantages of a new technical solution.

The technical nature of the proposed technical solution is illustrated by the drawing, which shows a device that allows you to implement the proposed plasma-carbon method for producing rare earth metals.

A vacuum electric furnace with a volume plasma discharge contains a capacitor 1, in which an accumulating condensate 2, a vacuum chamber 3, a steam pipe 4, a screen insulation 5, a cathode 6, a crucible anode 7, a discharge cavity 8, a cathode holder 10, an anode holder 11, an insulating charge 12, are formed current leads 13, insulator-seal 14, starting resistance heaters 15. The electric furnace is equipped with a typical vacuum system that provides operating pressure in the vacuum chamber up to 1 · 10 ^-3 mm RT.article

The crucible anode 7 is made of graphite, tantalum or tungsten and is installed along the axis of the furnace. A bulk or tableted charge 9 is loaded into the crucible. A cathode 6 made of graphite, tantalum or tungsten is installed coaxially with the crucible-anode with a gap of 10-15 mm. Outside the cathode, graphite rod heaters are located around the perimeter, heating the cathode to a temperature of 1900 ° C, at which a volumetric plasma discharge is ignited between the cathode and the crucible anode. After ignition of the discharge, the resistance heaters are turned off. Coaxially considered system outside the resistance heaters are installed reflective screens made of graphite, tantalum, tungsten and heat-resistant steel. The electric furnace condenser 7, mounted coaxially with the crucible anode 7, is cooled by water or air, providing a temperature below the condensation of rare earth vapor on the surface of the condensation.

By direct experiments it was found that the developed heating system has a high energy efficiency and that in the crucible anode where the charge is located, the temperatures necessary for the implementation of reaction (1) or (3) are achieved and maintained by a hot volume plasma discharge. Argon is supplied into the discharge cavity between the crucible anode and cathode, in the atmosphere of which a volumetric ring vacuum plasma discharge is lit when the electric circuit is supplied with direct current (voltage up to 60 V, operating current up to 7000 A). In this case, the charge located in the crucible anode is heated and reaction (3) proceeds in it with the release of the product (rare-earth metal) in the vapor phase. The metal vapor through the steam line 4 enters the cooled surface of the condenser, condenses into a solid phase and forms the resulting product. In the crucible anode, the second product of the technology remains in the solid phase - tungsten carbide.

The proposed method and device for its implementation have the following advantages:

1. A new method for producing rare-earth metals is a non-waste technology that excludes emissions of harmful components into the environment. Carbon monoxide released during the conversion of metal oxide to carbide (see reaction (2)) easily burns to carbon dioxide at the outlet of the furnaces.

2. The direct yield and extraction of rare earth metals from their oxides increases from 80% according to the known technology to 95% or more according to the proposed one.

3. The number of technological conversions is reduced and slag processing operations to extract rare earth metals from them are eliminated.

4. Due to the conjugate and joint production of two marketable products: condensates of rare-earth metals and carbides of refractory metals, the specific consumption of electricity and other operating costs that are characteristic of multistage methods for producing rare-earth metals are significantly reduced.

Claims (5)

1. A method of obtaining a rare earth metal boiling at a temperature of ≥2500 ° C from the series: Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu, including carbon thermal reduction of its oxide compound in vacuum with obtaining a rare earth metal carbide powder free of oxygen impurity residues, cooling the carbide powder, mixing it with a refractory metal powder in a ratio necessary and sufficient for exchange reactions between the rare earth carbide and the refractory metal, and heating the mixture with hot bulk plasma discharge to a temperature of ≥1800 ° C with the capture of the evaporating rare-earth metal on capacitors and obtaining carbide carbide of refractory metal. 2. The method according to claim 1, characterized in that tungsten metal powders are used as carbide-forming refractory metal. 3. A device for producing a rare-earth metal boiling at a temperature of ≥2500 ° C, from the series: Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, But, Er, Lu, containing a vacuum chamber with water-cooled outer walls , a vacuum system, cylindrical electrodes placed concentrically in the chamber, a steam line coaxial with them and a condenser-cooler used to collect REMs, the internal electrode being the anode of a high-current vacuum plasma discharge located in an annular discharge cavity formed by coaxial cylindrical electrodes, made of a refractory electrically conductive material in the form of a crucible having a capacity sufficient to accommodate a mixture of rare earth carbide and powder of a refractory metal, and the surrounding thin-walled electrode, which is a cathode, is also made of a refractory electrically conductive material, for example tungsten, tantalum or graphite. 4. The device according to claim 3, characterized in that a starting resistance heater is located outside the cathode. 5. The device according to claim 3 or 4, characterized in that it is equipped with a loading tray that allows you to feed the mixture into the crucible of the anode during the process.