EP0184515A1 - 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 shorn salts of rare earths or their alloys and to a device for carrying out this process.

In the description which follows, 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, electrolysis in a shorn 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 Z of pure neodymium in the case of a molten bath consisting of neodymium chloride and potassium chloride.

A first objective of the invention is therefore the preparation of a rare earth 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 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.

To this end, the process according to the invention for the preparation by electrolysis in a bath of molten salts of a rare earth or of an alloy of a rare earth with at least one metal chosen from the group of rare earths and transition metals is characterized in that a bath is used comprising at least one chloride of a rare earth, at least one alkali or alkaline-earth chloride and at least one alkaline or alkaline-earth fluoride.

According to a particularly preferred embodiment of the invention, the bath comprises at least as alkali metal lithium;

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 methods, characterized in that it comprises a tank, the mowing of which is extended by a withdrawal zone constituted by a pipe of internal internal cross section to that of the tank.

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

The method 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 tig. 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 accompanying drawings for which:

• - the tig. 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 earths. 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 earths, for example Nd-La, Nd-Ce, Nd-Pr or else alloys between one or more rare earths and a metal chosen from the group of transition metals. It is possible to use as transition metals all metals having a point of melting higher than the temperature of the molten salt bath at the time of electroJyse, a 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 be prepared in particular: 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 earths, 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 which is used for electrolysis comprises, in addition to the rare earth chloride (s), one or more alkali or alkaline earth chlorides and one or more alkali or alkaline earth fluorides.

It has proved advantageous to use lithium as the alkali metal. The presence of lithium halide in the bath has the effect of significantly increasing the yield.

Finally, according to the preferred embodiment of the invention, the bath comprises 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 / U% by weight, it is more particularly between 15 and 45%.

Furthermore, the proportion by weight between the alkali or alkaline earth chloride (s) on the one hand and the tluoride (s) on the other hand preferably varies between 1.5: 1 and 3: 1.

Finally, it is advantageous to use a bath having a content of one or more tluorides of at least 15 X.

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 7UU 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 according to the type of product prepared.

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

In the case of the preparation of an alloy of a rare earth with a transition metal, the cathode will be a consumable cathode made up of this transition metal or of the same rare earth-transition metal alloy as that which one is looking for. 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. The anodic current densities (Dca) are generally between 50 and 250 A.dm

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 + 1/2 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 inter distance polar was 65 mm for Examples 1 to 4. For each electrolysis, the metal produced was recovered after cooling in the crucible. The compositions of the baths are given in X by weight.

The metal yield indicated denotes the ratio of the rare earth metal obtained compared to the metal corresponding to the rare earth chloride (TRC1 ₃ ) introduced.

EXAMPLE 1 - Obtaining metal neodymium.

Electrolysis of 800 g of a molten mixture of composition NdCl ₃ : 13.3% is carried out at 850 ° C. and on a tungsten cathode (⌀ = 4 mm); LiCl: 62.0%; LiF: 24.7%. This electrolysis was carried out with a current of intensity I = 8.5 A, which corresponded to a cathodic current density dcc = 690A.dm, and to an anodic current density dca = at 60A.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-ter alloy with a low iron content.

800 g of bath with a composition very close to that corresponding to the mixture electrolyzed in Example 1, NdCl ₃ 13%, LiCl 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 (110A.dm ^-2 ); the dca was 25UA.dm ^-2 . After an electrolysis of 1 hour 20 minutes, 48 g of metal were recovered (metal yield of 8U, 4%), containing at least 89% of Nd, 8.7% of iron and 0.1% of lithium

EXAMPLE 3 - Obtaining a neodymium-ter alloy.

The mass and composition of the electrolysis bath and the temperature were identical to those considered in Example 2 (the neodymium chloride containing 7.5 X of 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. The value of the dca was 135 A.dm. After 2 hours and 3U minutes of electrolysis, 50 g of metal were obtained (metal yield of 84%) containing at least 85% of Nd, 12 X 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 TRCl ₃ if the metal yield was 100 X.

EXAMPLE 4 Obtaining pure lanthanum.

We start from a bath of the following composition: LaCl ₃ 13 X, LiCl 62%, LiF 25 X. We use a cathode constituted by a tungsten bar, the dcc is 690 A.dm, the dca is 60 A.dm . 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: LaCl ₃ 25 X, LiCl 53%, LiF 22 X. A cathode constituted by an iron bar is used, _2 2 the dcc is 165 A.dm, the dca is 215 A.dm and the interpolar distance of 60 mm. The temperature is 840 ° C. After t = 1.5 to an alloy comprising 92% of La and 7 X of Fe is obtained. The metal yield is 34 X.

EXAMPLE 6 Obtaining a neodymium-lanthanum alloy.

We start with a bath having the following composition: NdCl ₃ 26 X, LaCl ₃ 9%, LiCl 46 X, LiF 19%. The cathode is a tungsten bar, the dcc is 276 A.dm ^-2 , the dca is 235 A.dm 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 X.

EXAMPLE 7 Obtaining a neodymium alloy, lanthanum, iron.

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

The cathode is an iron bar, the d.c.c. is 100 A.dm, the d.c.a. of 142 A.dm and the interpolar distance of 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: PrCl ₃ 25%, LiCl 53 X, LiF 22%. The cathode is of the same type as in Example 7, the dcc is 100 A.dm, the dca is 140 A.dm 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 Obtaining pure lanthanum.

A bath of the following composition is used: LaCl ₃ 30%, KCl 38.6%, LiCl 31.4%. The anode is made of graphite, the cathode is made of lanthanum with a steel contact. The dcc is 55 A.dm ^-2 , the dca is 130 A.dm ^-2 , the interpolar distance is 40 mm. The temperature of the bath is 690 ° C. After 3 hours 31 minutes, 67 g of metal are obtained, the metal yield is 56%.

EXAMPLE 10 -

This example concerns the preparation of neodymium-ter 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 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 11 -

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

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

The anode is made of graphite, the cathode of iron. The bath temperature _2 is 940 ° C, the d.c.a. of 89 A.dm, the d.c.c. of _2 250 A.dm, the interpolar distance of 46 mm. After t = 0.94 to, an alloy of composition Gd 86%, Fe 14 X 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 by its mowing. 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 mower 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 b, 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 preferred to use for the parts 13 and 14 means of electric heating by contact or by radiation and with regard to the part 14 by induction optionally and for the zone 11 very flexible heating means for example of the fuel oil burner type or 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 shearing, in particular gray shearing with lamellar or spheroidal graphite. It is also possible to use shearing 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 pure rare earth, consumable cathode in transition metal or in rare earth-transition metal alloys 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 claimed 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 anode 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 inner ends of the anode (s).

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 electrolysis falls at the bottom of the cell 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 1.

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 inside the bath melting temperature and the metal or alloy is allowed to settle in part 13, zones 1U, 11 and 13 being heated. Zone 11 is then very quickly cooled and a salt plug is formed there. Then quickly heats the external part 14 of the zone 12 to pour the product collected at 13. A then lets the parts 13 and 14 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 within the framework of the claimed protection.

Claims (18)

1 - Process for the preparation by electrolysis in a bath of molten salts of a rare earth or of an alloy of a rare earth with at least one metal chosen from the group of rare earths and transition metals, characterized in that using a bath comprising at least one chloride of a rare earth, at least one alkali or alkaline earth chloride and at least one alkali or alkaline earth fluoride. 2 - Method according to claim 1 for the preparation of the above alloy, characterized in that one employs a cathode made of a transition metal or an alloy of a rare earth and a transition metal. 3 - Method according to claim 1 or 2 characterized in that a bath is used comprising at least as lithium alkali metal. 4 - Method according to one of claims 1 to 3 characterized in that a bath is used comprising at least one lithium chloride and one lithium fluoride. 5 - Method according to one of the preceding claims, characterized in that a bath is used in which the proportion by weight between the alkali or alkaline-earth chloride (s) and the fluoride (s) varies between 1.5: 1 and 3: 1. b - Method according to one of the preceding claims, characterized in that a bath having a content of one or more fluorides of at least 15% is used. / - Method according to claim 1 or one of claims 3 to 6, characterized in that one uses a bath comprising neodymium chloride. 8 - Method according to one of claims 2 to 6, characterized in that one uses a bath comprising neodymium chloride and in that the transition metal chosen is iron. 9 - Method according to one of the preceding claims, characterized in that one uses a bath having a content of one or more rare earth chloride between 10 and 70% by weight. JO - Method according to one of claims / to 9, characterized in that the electrolysis is carried out at a temperature between 650 ° C and 1100 ° C. 11 - Method according to any one of the preceding claims, characterized in that ettectue electrolysis under conditions such that the partial pressure of chlorine is at least 1.01.10 ⁴ Pa (0.1 atmosphere). 12 - Cell for electrolysis in molten salt bath, usable in particular for the implementation of the method according to any one of the preceding claims, characterized in that it comprises a tank whose bottom is extended by a draw-off zone constituted by a pipe with an internal cross section smaller than that of the tank. 13 - Cell according to claim 12, characterized in that the aforementioned pipe opens in the center of the bottom of the tank. 14 - Cell according to one of claims 12 or 13, characterized in that it is provided with a cylindrical cathode. 15 - Cell according to one of claims 12 to 14, characterized in that the anode is constituted by one or more cylinders arranged around the cathode. 16 - Cell according to one of claims 12 to 14, characterized in that the anode is constituted by a crown centered around the cathode. 17 - Cell according to one of claims 12 to 16, characterized in that the cathode is arranged vertical to the above-mentioned pipe. 18 - Cell according to one of claims 12 to 17, characterized in that the aforementioned draw-off pipe is provided with two longitudinally spaced valves delimiting a decantation space. 19 - Cell according to one of claims 12 to 17, characterized in that the abovementioned draw-off pipe comprises a part having a smaller cross section than that of the rest of the zone.

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