EP0184515B1 - Process for the electrolytic preparation of rare-earth elements or their alloys, and apparatus for carrying out the process - Google Patents

Abstract

1. A process for the preparation of a rare earth metal or an alloy of rare earth metal with at least one metal selected from the group consisting of rare earths and transition metals by electrolysis in a bath of molten salts, characterised by effecting on solid electrodes the electrolysis of at least one chloride of a rare earth in a bath of salts essentially comprising lithium chloride and lithium fluoride ; the cathode being a cathode of tungsten or formed by at least one rare earth metal when preparing a rare earth metal or an alloy of rare earth metals, the cathode being formed by a transition metal or an alloy of a rare earth metal and a transition metal when preparing an alloy of at least one rare earth metal and at least one transition metal.

Description

The present invention relates to a process for the electrolytic preparation in baths of molten salts of rare earth metals or their alloys and to a device for carrying out this process.

In the following description, the term “rare earths” is understood to mean any element belonging to the group formed by yttrium and the lanthanides except for samarium, europium, ytterbium and thullium.

Currently, the electrolysis in a molten medium of a rare earth chloride and in particular of neodymium poses problems by the very low yields obtained. This is due to the high solubility of the metal in the presence of its chloride. Such a process is described in the article by T. KURITA (Denku Kagaku, 1967, 35 (7) p. 496-501). There is reported a yield which does not reach 20% of pure neodymium in the case of a molten bath consisting of neodymium chloride and potassium chloride.

According to US 2,961,387, a process for refining a rare earth metal is known. Example 1 describes the electrolysis of a cerium chloride in a bath of LiCI-LiF salts, by means of a molten metal cathode made of a Ce-Cu eutectic alloy, which leads to the enrichment of this in cerium, then the alloy obtained is used as an anode in another electrolysis step, in order to obtain cerium of high purity by cathodic deposition.

We also studied in the article "Electrode processes during cerium electrolysis in chloride-fluoride melts" Chemical Abstracts 62, 7384h-7385a (1965) the electrolysis of cerium 3+ generated from an anode into cerium by anodic oxidation in presence of LiCI-KCI molten eutectic and possibly LiF.

Finally, it was mentioned in FR-430 116 obtaining cerium (or similar metals) by direct electrolysis of the rare earth salt in the presence of a light metal salt electro positive with respect to cerium.

A first objective of the invention is therefore the preparation of a rare earth metal by electrolytic means under conditions such that the process is applicable industrially.

A second objective of the invention is the development of a process for the preparation of rare earth metal alloys which is also industrially transposable.

Finally, another objective of the invention is the development of a device allowing in particular the implementation of the above methods.

For this purpose, the process according to the invention for the preparation by electrolysis in a bath of molten salts, of a rare earth metal or of an alloy of a rare earth metal with at least one metal chosen from the group of rare earths and transition metals is characterized in that one carries out on solid electrodes, the electrolysis of at least one chloride of a rare earth in a salt bath essentially comprising lithium chloride and lithium fluoride .

Furthermore, the invention also relates to a cell for electrolysis in a bath of molten salts which can be used in particular for the implementation of the above processes, characterized in that it comprises a tank whose bottom is extended by a withdrawal zone constituted by a pipe of internal cross section smaller than that of the tank: said pipe being provided with two longitudinally spaced valves delimiting a settling space or comprising a part (11) having a smaller cross section than that of the rest of the zone.

According to a particular embodiment of the invention the above-mentioned pipe opens at the center of the bottom of the tank.

The process of the invention makes it possible to obtain a metal of good purity or an alloy with a high rare earth content with high metal yields which can exceed 80% and this under industrially applicable conditions.

• - la fig. 1 est une vue en coupe schématique d'un premier mode de réalisation d'une cellule d'électrolyse selon l'invention.
• - la fig. 2 est une vue en coupe schématique d'un second mode de réalisation d'une cellule d'électrolyse selon l'invention.

Other characteristics and advantages of the invention will be better understood on reading the description which follows, and concrete but non-limiting examples of implementation of the method. Reference will also be made to the appended drawings for which:

• - fig. 1 is a schematic sectional view of a first embodiment of an electrolysis cell according to the invention.
• - fig. 2 is a schematic sectional view of a second embodiment of an electrolysis cell according to the invention.

As indicated above, the invention relates to the preparation of metals and alloys of rare earth metals. It applies more particularly to the preparation of neodymium metal from neodymium chloride, it in fact offers in the case of neodymium a particularly significant improvement in yield.

The invention also relates to the preparation of alloys and in particular the preparation of alloys based on neodymium.

The alloys capable of being prepared are the alloys between rare earth metals, for example Nd-La, Nd-Ce, Nd-Pr or else alloys between one or more rare earth metals and a metal chosen from the group of metals of transition. Can be used as transition metals, all metals having a melting point higher than the temperature of the molten salt bath at the time of electrolysis, temperature which for example can vary between 650 ° C and 1100 ° C. Examples of such metals include iron, cobalt, nickel and chromium. Thus, according to the invention, the following alloys can in particular be prepared: Nd-Fe, La-Fe, Nd-La-Fe, Pr- Fe.

According to the invention, one starts from the chloride of the rare earth which one seeks to prepare in the metal form. To obtain an alloy comprising several rare earth metals, one then starts from a bath comprising a mixture of the chlorides of each of these rare earths. These chlorides should preferably have a water content of less than 6% by weight.

The bath used for electrolysis also includes the rare earth chloride (s), at least one lithium fluoride and one lithium chloride. In this case, the best yields are generally obtained.

The proportion of the rare earth chloride (s) in the bath can vary between 10 and 70% by weight, it is more particularly between 15 and 45%.

Furthermore, the proportion by weight between lithium chloride on the one hand and lithium fluoride on the other hand preferably varies between 1.5: 1 and 3: 1.

Finally, it is advantageous to use a bath having a lithium fluoride content of at least 15%.

The temperature of the bath during electrolysis is fixed so as to be higher than the melting temperature of the electrolytic bath.

Generally, this temperature is between 650 ° C and 1100 ° C, especially between 700 and 900 ° C.

The conditions relating more particularly to electrolysis will be described below.

As regards the electrodes, a graphite anode is generally used. The nature of the cathode can vary depending on the type of product prepared.

When preparing a pure rare earth metal, a tungsten cathode is advantageously used. We can also use a cathode made of the same rare earth metal that we are trying to prepare. Cathodes of the same type will be used to prepare an alloy of rare earth metals with one another.

In the case of the preparation of an alloy of a rare earth metal with a transition metal, the cathode will be a consumable cathode made up of this transition metal or of the same rare earth metal-transition metal alloy as that which one seeks to obtain.

The voltage across the electrodes will generally be between 4 and 10 V.

The cathodic current densities (Dcc) can vary between 70 A.dm- ² and 700 A.dm ^-2 , more particularly between 100 and 250 A.dm ^-2 . The anodic current densities (Dca) are generally between 50 and 250 A.dm ^-2 .

Furthermore, it has been found that it is advantageous to conduct the electrolysis under conditions such that there is a partial pressure of chlorine in the gas phase which is at least 1.01 × 10 ⁴ Pa (0, 1 atmosphere). In such a case, there is a transformation of the oxychlorides present in the bath according to the reaction TROCl + Cl ₂ → TRCl ₃ + ½ O ₂ ; the oxychlorides have in fact been brought there as impurities with the rare earth chlorides. In this case, it is possible to use as starting materials rare earth chlorides which may have an oxychloride level of up to 25% by weight.

Examples will now be given. The tests described were carried out in alumina crucibles with graphite anodes with a diameter of 10 to 25 mm; the interpolar distance was 65 mm for examples 1 to 4. For each electrolysis, the metal produced was recovered after cooling in the crucible. The bath compositions are given in% by weight.

The metal yield indicated denotes the ratio of the rare earth metal obtained relative to the metal corresponding to the rare earth chloride (TRCI ₃ ) introduced.

Example 1 - Obtaining metal neodymium.

Electrolysis of 800 g of a molten mixture of composition NdC1 ₃ : 13.3% is carried out at 850 ° C. and on a tungsten cathode (0 = 4 mm); LiCI: 62.0%; LiF: 24.7%. This electrolysis was carried out with a current of intensity 1 = 8.5 A, which corresponded to a cathodic current density dcc = 690 A.dm- ² , and to an anodic current density dca = at 60 A.dm ² . The voltage across the electrodes was between 4.6 and 5.0 V. A 4 hour electrolysis provided, with a metal yield of 40%, 24.1 g of metal whose neodymium, lithium and tungsten contents were 98%, 0.07% and <1% respectively.

Example 2 - Obtaining a neodymium-iron alloy with a low iron content.

800 g of bath with a composition very close to that corresponding to the electrolyzed mixture in Example 1, NdCl ₃ 13%, LiCI 62%, LiF 25%, were used. The electrolysis was carried out at 730 ° C. on a cathode made up of 65 g of Nd / Fe alloy with 20% iron (alloy previously produced by calciothermy). The electrical contact was ensured by means of a steel rod. The intensity of the electrolysis current was high, 25 A, but corresponded only to a low dcc (11 OA.dm ² ); the dca was 250A.dm ² . After an electrolysis of 1 hour 20 minutes, 48 g of metal were recovered (metal yield of 80.4%), containing at least 89% of Nd, 8.7% of iron and 0.1% of lithium.

Example 3 - Obtaining a neodymium-iron alloy.

The mass and composition of the electrolysis bath and the temperature were identical to those considered in Example 2 (neodymium chloride containing 7.5% oxychloride and 2.7% water). The intensity of the electrolysis current was lower (13.5 A). At the cathode, consisting of an iron grid (consumable) and a steel contact, the dcc was 100 A.dm ² . The value of the dca was 135 A.dm ² . After 2 hours and 30 minutes of electrolysis, 50 g of metal were obtained (metal yield of 84%) containing at least 85% of Nd, 12% of iron and 0.7% of lithium.

In the examples which follow (4 to 8) the duration of electrolysis is given with respect to which is the time theoretically necessary for the reduction of the totality of TRC1 ₃ if the metal yield was 100%.

Example 4 - Obtaining pure lanthanum.

We start with a bath of the following composition: LaCl ₃ 13%, LiCI 62%, LiF 25%. A cathode constituted by a tungsten bar is used, the dcc is 690 A.dm ^-2 , the dca is 60 Adm ^-2 . The interpolar distance is 65 mm. The temperature is 800 ° C. After t = to, a metal is obtained whose lanthanum content is at least 95%, the metal yield is 33%.

Example 5 - Obtaining a lanthanum-iron alloy.

The bath has the following composition: LaC1 ₃ 25%, LiCI 53%, LiF 22%. A cathode constituted by an iron bar is used, the dcc is 165 A.dm ^-2 , the dca is 215 A.dm- ² and the interpolar distance is 60 mm. The temperature is 840 ° C. An alloy comprising 92% of La and 7% of Fe is obtained after t = 1.5 to an alloy. The metal yield is 34%.

Example 6 - Obtaining a neodymium-lanthanum alloy.

We start with a bath having the following composition: NdCl ₃ 26%, LaCl ₃ 9%, LiCI 46%, LiF 19%. The cathode is a tungsten bar, the dcc is 276 Adm ^-2 , the dca is 235 A.dm- ² and the interpolar distance is 63 mm. The temperature is 860 ° C. After an electrolysis time of t = 0.7 to, we obtain with a metal yield of 57%, an alloy of composition: Nd 81%, La 18%, with a lithium content of 0.1%.

Example 7 - Obtaining a neodymium, lanthanum, iron alloy.

The bath has the following composition: NdCl ₃ 15%, LaCl ₃ 10%, LiCI 53%, LiF 22%.

The cathode is an iron bar, the dcc is 100 A.dm ^-2 , the dca is 142 A.dm ² and the interpolar distance is 40 mm. The temperature is 750 ° C. After t = 1.5 to, an alloy of the following composition is obtained with a metal yield of 74%: Nd 55%, La 37%, Fe 9.2%, Li 0.5%.

Example 8 - Obtaining a praseodymium-iron alloy.

A bath is used with the following composition: PrC1 ₃ 25%, LiCI 53%, LiF 22%. The cathode is of the same type as in Example 7, the dcc is 100 A.dm- ² , the dca is 140 A.dm ² and the interpolar distance is 45 mm. The bath temperature is 750 ° C. After t = 1.5 to, an alloy of composition Pr 86%, Fe 12%, Li 0.5% is obtained. The metal yield is 60%.

Example 9

This example concerns the preparation of neodymium-iron alloys. The composition of the bath was varied. In all cases, the anode is made of graphite and the cathode is an iron rod.

• T désigne la température du bain en °C
• t désigne la durée de l'électrolyse par rapport à to défini ci-dessus
• Di désigne la distance interpolaire en mm
• R désigne le rendement métal tel que défini ci-dessus.
  

The results are reported in the table below.

• T denotes the bath temperature in ° C
• t denotes the duration of the electrolysis with respect to to defined above
• Di designates the interpolar distance in mm
• R denotes the metal yield as defined above.
  

Example 10

This example concerns the preparation of a gadolinium-iron alloy.

A bath of composition is used: Gd C1 ₃ 26%; LiCl 52.3%; LiF 21.7%.

The anode is made of graphite, the cathode of iron. The bath temperature is 940 ° C, the dca of 89 A.dm- ² , the dcc of 250 A.dm ^-2 , the interpolar distance of 46 mm. After t = 0.94 to, an alloy of composition Gd 86%, Fe 14% is obtained. The metal yield is 42%.

The device for implementing the method will now be described.

The cell according to the invention is designed so as to allow the metal or the alloy formed to be poured or drawn off through its bottom. It therefore includes in its lower part a withdrawal zone in which the product formed is collected by decantation, this zone having such a configuration or being provided with withdrawal means such that the metal or the alloy can be easily extracted.

Referring to the figures, one can see such a cell 1, consisting of an upper part 2, generally a cylindrical tank and a lower part in the form of a pipe 3 constituting the withdrawal zone. This area has an internal cross section smaller than that of the upper part and preferably it is constituted by a pipe extending in the vertical extension of the tank and advantageously having a cylindrical cross section. The pipe 3 preferably opens out at the center of the bottom 4 of the upper part.

To facilitate the pouring of the product, it is possible to provide a bottom 4 slightly inclined downwards, for example of the order of 10 °.

The upper part of the cell is provided with external heating means of the electric heating type by radiation, contact or induction or else heating by gas or oil burner.

Figures 1 and 2 illustrate two particular embodiments of the withdrawal zone.

In FIG. 1, the withdrawal zone or pipe 3 consists of three distinct zones, a connection zone 5, a central zone 6 and an external zone 7. The central zone 6 is separated from the other two by valves 8 and 9 fully open and remotely controlled. This zone 6 thus delimited forms a settling space. Zones 5 and 6 are each provided with heating means of the electric heating type for example.

With regard to the dimensions, it can be noted that the diameter and the height of the zone 6 is a function of the racking frequency. Generally, one can provide a height of zone 6 between 2.5 times and 6 times that of zone 5.

The draw-off zone 3 in FIG. 2 is also made up of three zones, a connection zone 10, a central zone 11, the particularity of which is to have a cross section smaller than that of the rest of zone 3 and in particular of the zone 10 and an external zone 12. This same zone 12 is itself divided into two parts: a part 13 adjacent to the zone 11 and an external part 14. These two parts are essentially distinguished from each other by the fact that they include independent heating means.

Zone 11 is also provided with heating means. It is preferable to use for parts 13 and 14 electrical heating means by contact or by radiation and with regard to part 14 by induction optionally and for zone 11 very flexible heating means, for example of the oil or gas burner type. gas.

The dimensions of the racking area are also a function of the racking frequency. Preferably, the diameters of the zones 10 and 12 are identical and are in a ratio of 2 to 4 approximately with that of the zone 11. With regard to the heights, it is possible to provide substantially similar heights for the zones 10 and 11 and part 14, that of part 3 being 3 to 5 times larger.

The entire electrolysis cell is made of a material capable of withstanding the temperature of the bath and corrosion due to the various products used. Mention may be made, as suitable material, of cast iron, in particular gray cast iron with lamellar or spheroidal graphite. It is also possible to use cast iron alloyed with chromium or nickel or preferably with molybdenum-silicon.

Different shapes and arrangement of electrodes can be used within the scope of the cell of the invention.

A graphite anode is generally used. With regard to the cathode, the nature thereof depends on the type of product prepared as has been seen above: tungsten for a pure rare earth metal, consumable cathode in transition metal or in rare earth metal-metal alloys transition for alloys.

Generally, a cylindrical cathode is used, placed vertically in the tank, preferably in the center. In particular in the case where the withdrawal zone of the cell is constituted by a cylindrical pipe, it is advantageous to have the cathode vertical to this pipe.

According to a preferred embodiment of the invention, the cathode is hollow and cylindrical. It is thus possible to provide the cell with rare earth chloride through the hollow central part of the cathode.

Finally it is also possible to use a horizontal cathode.

Different forms of anodes can be used:

As shown in FIG. 1, the anode can consist of one or more vertical cylinders 15 arranged around the cathode 16. Six cylinders 15 can be used, for example.

According to the embodiment of FIG. 2, the anode can also be constituted by a circular crown 17 centered around the cathode 16. Instead of a crown, crown sectors could also be used.

Finally, it should be noted that the electrodes are advantageously placed in the cell so that the lower end of the cathode is closer to the bottom of the tank than the lower ends of one or more anodes.

The operation of the devices described above will now be given.

The cell is continuously supplied with rare earth chloride through a hopper. In the case of a hollow cathode of the type described above, the chloride is introduced into the central part of this electrode.

The metal or alloy formed during the electrolysis falls to the bottom of the tank and is recovered in the withdrawal zone in a cyclic manner.

In the case of the device of FIG. 1, the valve 8 being open, the valve 9 closed, the metal or the alloy is allowed to completely fill the zone 6. Once the filling is completed, the valve 8 is closed and the valve 9 so as to allow the product to flow into the external zone 7.

With the device of Figure 2, we proceed as follows. Part 14 of zone 12 is kept cold, that is to say at a temperature below the melting temperature of the bath and the metal or the alloy is allowed to settle in part 13, the zones 10, 11 and 13 being heated. Zone 11 is then very quickly cooled and a salt plug is formed there. The external part 14 of the zone 12 is then rapidly heated to pour the product collected at 13. The parts 13 and 14 are then allowed to cool to form a new salt plug in the zone 12 and the zone 11 is gradually reheated.

Naturally, the device which has been described above can be applied to any type of electrolysis in a bath of molten salts in which a final product is collected by decantation. Its application is therefore not limited to the baths which have been described more particularly in the present description.

Of course, the invention is in no way limited to the embodiments described which have been given only by way of examples. In particular, it includes all the means constituting technical equivalents of the means described as well as their combinations if these are implemented as part of the claimed protection.

Claims (23)

1. A process for the preparation of a rare earth metal or an alloy of rare earth metal with at least one metal selected from the group consisting of rare earths and transition metals by electrolysis in a bath of molten salts, characterised by effecting on solid electrodes the electrolysis of at least one chloride of a rare earth in a bath of salts essentially comprising lithium chloride and lithium fluoride; the cathode being a cathode of tungsten or formed by at least one rare earth metal when preparing a rare earth metal or an alloy of rare earth metals, the cathode being formed by a transition metal or an alloy of a rare earth metal and a transition metal when preparing an alloy of at least one rare earth metal and at least one transition metal.

2. A process according to claim 1 characterised in that the anode used is a graphite anode.

3. A process accordinq to one of claims 1 and 2 characterised by using a bath comprising neodymium chloride.

4. A process according to one of claims 1 and 2 characterised by using a bath comprising a neodymium chloride and characterised in that the transition metal selected is iron.

5. A process according to claim 1 characterised by effecting on an anode of graphite and on a cathode of tungsten or of neodymium, electrolysis of a bath comprising neodymium chloride, lithium chloride and lithium fluoride.

6. A process according to claim 1 characterised by effecting on an anode of graphite and a con- summable iron cathode, the electrolysis of a bath comprising neodymium chloride, lithium chloride and lithium fluoride.

7. A process according to one of the preceding claims characterised b_'/ using a bath in which the proportion by weight as between the lithium chloride and the lithium fluoride varies between 1.5:1 and 3:1.

8. A process according to one of the preceding claims characterised by using a bath having a lithium fluoride content of at least 15%.

9. A process according to one of the preceding claims characterised by using a bath having a content of one or more rare earth chlorides of between 10 and 70% by weight.

10. A process according to one of the preceding claims characterised by effecting the electrolysis operation at a temperature of between 650 °C and 1100 °C.

11. A process according to any one of the preceding claims characterised by effecting electrolysis under conditions such that the partial chlorine pressure is at least 1.01.10⁴Pa (0.1 atmosphere).

12. A cell for electrolysis in a bath of molten salts, which can be used for carrying out the process according to any one of the preceding claims, comprising a tank provided with heating means, and solid electrodes disposed in said tank, characterised in that the bottom of the tank is prolonged vertically by a drain zone formed by a conduit of cross-section that is smaller than that of the tank, and that the anode is formed by a ring which is centered around the cathode.

13. A cell for electrolysis in a bath of molten salts, which can be used for carrying out the process according to any one of the preceding claims, comprising a tank provided with heating means, and solid electrodes disposed in said tank, characterised in that the bottom of the tank is prolonged vertically by a drain zone formed by a conduit of cross-section that is smaller than that of the tank, which zone opens at the bottom of the tank and forms a settlement space for the molten bath collected in said zone.

14. A cell according to one of claims 12 and 13 characterised in that said conduit opens at the centre of the bottom of the tank.

15. A cell according to one of claims 12 to 14 characterised in that it is provided with a cylindrical cathode.

16. A cell according to claim 15 characterised in that said cathode is hollow.

17. A cell according to one of claims 13 to 16 characterised in that the anode is formed by one or more cylinders disposed around the cathode.

18. A cell according to one of claims 13 to 16 characterised in that the anode is formed by a ring centered around the cathode.

19. A cell according to one of claims 12 to 18 characterised in that the cathode is disposed in vertical alignment with said conduit.

20. A cell according to one of claims 12 to 19 characterised in that said drain conduit is provided with two longitudinally spaced valves defining a settlement space.

21. A cell according to claim 20 characterised in that the connecting zone (5) and the central settlement zone (6) are provided with heating means.

22. A cell according to one of claims 12 to 21 characterised in that the drain zone is formed by a conduit of internal cross-section that is smaller than that of the tank and comprising a part (11) which is of smaller cross-section than that of the remainder of the zone.

23. A cell according to claim 22 characterised in that the central zone (11) and the parts (13) and (14) of the external zone are provided with heating means.

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