EP2935637A1

EP2935637A1 - Method for synthesising a rare earth element by means of a redox reaction

Info

Publication number: EP2935637A1
Application number: EP14729358.3A
Authority: EP
Inventors: Karl Bernhard Friedrich; Marc Hanebuth; Alexander Tremel; Hanno Vogel
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2013-06-24
Filing date: 2014-06-12
Publication date: 2015-10-28
Also published as: DE102013211946A1; US20160115569A1; WO2014206748A1

Abstract

The invention relates to a method for synthesising a rare earth element by means of a redox reaction, the educt side of the reaction comprising a rare earth compound, in which the rare earth element is present in a positive oxidation state, and hydrogen. The invention is characterised in that the redox reaction takes place in two stages, wherein first a hydration reaction takes place between an elementary rare earth element and hydrogen to form a rare earth hydride and then a reaction takes place between the rare earth compound and the rare earth hydride, an elementary rare earth element and at the same time a hydrogen-containing compound being produced as the product of the reaction.

Description

description

Process for the preparation of a rare earth element by a redox reaction

The invention relates to a process for the preparation of a rare earth element by a redox reaction according to claim 1.

Rare earth elements, which are also called lanthanides in chemistry, are needed in many electronic devices and in the production of magnets. For example, the rare earth element neodymium is an important component of permanent magnets used in wind generators. The treatment and separation of rare earth elements is basically chemically complex, since the rare earth elements in nature very finely distributed, socialized (especially with each other) and occur in low concentrations. Frequently, the rare earth elements are present in phosphatic compounds, in particular in the crystal structure of monazite or xenotime or as minor constituents in apatite, which in turn occur finely distributed in deposits which may also contain iron. A partial step of this elaborate recovery process of rare earth elements in pure form is an electrolysis process in which preferably chlorides or fluorides of the rare earth element are used in molten form as the electrolyte. By applying a voltage between immersed graphite anode and inert tungsten cathode, the rare earth oxides dissolved in the electrolyte are converted to metal and C0 / C0 ₂ . At the carbon anode but also arise perfluorocarbons, such as CF ₄ or C ₂ F ₆ , which have a multiple of the greenhouse potential of C0 ₂ . Furthermore, in the presence of water, the highly toxic hydrofluoric acid can arise. All of these unwanted products that are produced during the electrolysis must be removed again by complex cleaning and neutralization processes, which considerably increases the overall process costs. The object of the invention is to provide a method for the preparation of rare earth elements in elemental form, which is cheaper and more environmentally friendly compared to the melt electrolysis used in the prior art.

The object is achieved in a method for the preparation of a rare earth element by a redox reaction according to claim 1. In the inventive method is located on the reactant side of the reaction, a rare earth compound, wherein the rare earth element is present in this compound in a positive oxidation number. Furthermore, hydrogen is present on the educt side of the redox reaction. The invention is characterized in that the redox reaction proceeds in two stages, first a hydrogenation reaction between an elementary rare earth element and hydrogen to form a rare earth hydride, followed by a reaction between the rare earth compound in which the rare earth element present therein has a positive oxidation number and the rare earth hydride, wherein the product of this reaction is an elemental rare earth element and at the same time a hydrogen-containing compound.

In principle, it is expedient for the rare earth element in the rare earth compound to be bonded to a halide or an oxide. The following reaction equations occur schematically:

3 SEH ₂ + 2 SEC1 ₃ - 5 SE + 6 HCl (equation 1)

This reaction is the overall reaction, and initially according to the invention, a reaction between hydrogen and the rare earth element is also to take place, whereby a rare earth hydride is formed according to the following equation:

2 SE + x H ₂ 2 SEH _: (equation 2) Equation 1 is a so-called synproportionation in which a pure element is formed from 2 compounds containing this element, this once being oxidized and once reduced, such a synproportionation is a special case of a redox reaction. The rare earth element has a negative oxidation number in Equation 1 in its hydride form, in its form as chloride (chloride being exemplified here) having a positive oxidation number (+ III). The rare earth element in the hydride is thereby oxidized, in the chloride it is reduced, whereby finally after the reaction elemental rare earth metal is present.

Both equations generally run separately, which can be done in situ, or in extreme cases, it may even be that no hydride is present in solid form in the reaction mixture, but that hydride is formed in situ in the reaction mixture. This is the case if the hydrogen activity in the gas phase is correspondingly high enough. The term in situ can thus have two different meanings, either a strict sequential reaction, in which case the reaction according to equation 2 first takes place and thus the hydride is first formed, which is then reacted with the chloride as reducing agent according to equation 1 , On the other hand, in situ it can also mean that the hydride is also a short-lived intermediate, which reacts quasi immediately and is reacted according to equation 1. Regardless of which variant is chosen, the hydrogen partial pressure (especially at high reaction temperatures) must be sufficiently high for a hydride to form at all. Otherwise, the equilibrium according to Equation 2 would be completely on the side of the educts, which, however, would also prevent an overall reaction according to Equation 1 in addition to the formation of a hydride. Therefore, it is useful to form gaseous substances such as after Example of equation 1, to remove the hydrochloride by suitable measures quickly from the reaction.

The term rare earth elements in particular the so-called lanthanides, including lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium and lutetium understood, but there are because of their chemical similarities in this case here also counted the yttrium and scandium. Rare earths are in turn compounds of rare earth elements, in particular their oxides, which do not include rare earth phosphates. Rare Earth elements in elemental form can be present in pure form or in mixtures or alloys of various rare earth elements

As already mentioned, the chloride mentioned in equation 1 is purely exemplary of a compound of the rare earth elements in which the rare earth element has a positive oxidation number. In principle, halides, in particular chlorides, bromides, iodides or fluorides and oxides, may be suitable for this purpose. For the hydrogenation reaction, it is expedient that the pressure prevailing in the reaction atmosphere is greater than 10 bar, in particular more than 40 bar. This is especially true at a reaction temperature of more than

800 ° C, most preferably at more than 1000 ° C instead.

In principle, it is expedient that the hydrogen-containing compound, according to the example of equation 1, the hydrochloride, on the product side of the redox reaction is gaseous and that the partial pressure of this hydrogen-containing compound is reduced as rapidly as possible, for example, by a rapid suction and subsequent cooling in a cold trap is feasible. It may be expedient to use a blower that removes the gaseous reactants of the redox reaction as quickly as possible from the reaction.

This reduces the total amount of hydrogen needed during the reaction. Further embodiments and further features of the invention will be explained in more detail with reference to the following figures. Features with the same name in different embodiments are provided with the same reference numerals. These are purely exemplary representations, which do not represent a restriction of the scope of protection per se. Showing:

Figure 1 is a schematic chain of process steps for obtaining rare earth elements from an ore;

Figure 2 is a schematic representation of a process with a Synproportionierung;

Figure 3 is a schematic representation of a method of Figure 2 with hydrogen recovery.

First, Figure 1 shows schematically the recovery process of rare earth metals, as exemplified by the mineral

Monazit is common, without being exhaustive, explained. The mineral monazite is a phosphate in which the metal ions often occur in the form of rare earth metals, in particular cerium, neodymium, lanthanum or praseodymium. Here, within a particle, it is not a homogeneous composition of rare earth metals, but in the crystal structure the lattice sites of the cations are occupied by different rare earth metals in different concentrations.

The starting raw materials containing the monazite mineral are first ground very finely and treated in a flotation plant 2 so that the monazite separates as well as possible from the other mineral constituents. The monazite is dried and treated according to the prior art in an oven, such as a rotary kiln 4, after prior mixing with sulfuric acid. Here, the phosphates are converted into sulfates. This process in the rotary kiln is found Temperatures up to 650 ° C instead. The conversion of phosphate to sulfate is useful because the rare earth sulfates are significantly more soluble in water than the phosphates of the rare earth metals.

The sulfuric acid-containing solution of rare earth sulfates is neutralized after treatment in the rotary kiln 4 and a subsequent leaching step in a neutralizer 6, i. the pH is increased by the addition of a basic substance, whereby unwanted substances are precipitated and separated so that an aqueous rare earth sulphate solution is present in the remaining liquid.

This resulting solution of a rare earth compound (sulfate, nitrate, chloride or the like) is usually in so-called mixer-settler devices 8 of a liquid / liquid extraction, ie a separation subjected. In this case, the solution is prepared by mixing an extraction medium dissolved in organic solvents such as kerosene, possibly with further additives, so that the rare earth cations, which have slightly different ion diameters for the same charge, reach different concentrations either in the aqueous part of the solution or in the organic solution Enrich part of the solution. Here, the organic phase and the aqueous phase of the mixture are alternately mixed and separated again in a multi-stage separation process, so that certain rare earth ions, depending on the extractant in the organic phase, increasingly concentrated until finally these ions are present in sufficient purity in one phase. This may require up to 200 separation steps per element.

The rare earth metals thus separated are subsequently precipitated in a precipitation device 10 by addition of a carbonate or oxalate, so that the corresponding rare earth carbonate or oxalate accumulates at the bottom of the precipitation device 10. This is in turn in a calcination, for example in a continuous furnace 12, through which a hot air stream is passed, calcined. Thus, after this process step, a discrete rare earth oxide is present. This discrete rare earth oxide may optionally be converted into a lower melting salt, e.g. For example, be converted into an iodide, a chloride or fluoride and in turn be supplied in molten form an electrolysis process, wherein elemental rare earth metal is deposited on a cathode of the electrolysis device. However, this process step is technically very complicated and also energy-intensive. For this reason, an alternative method for representing elemental rare earth elements including hydrogen is proposed here.

FIGS. 2 and 3 schematically show an example of a device which is suitable for carrying out a method according to the invention. FIG. 2 shows a schematic illustration of a reactor 24 which is fundamentally pressure-tight, which is illustrated by the seals 30. These seals 30 should be resistant to high temperatures, they may for example consist of graphite. The pressure-tight sealed reactor 24 has a feed line 26, which can optionally be regulated by a valve 28. As a result, in particular hydrogen gas is introduced into the reactor 24. In a crucible 34 are the Reaktionsedukte 36 or after completion of the reaction, the reaction products. The reactor is heated schematically by a heater 32, which is indicated here in the form of a heating coil. Above the crucible 34 is a gas discharge 38 which is arranged as large as possible, bell-like above the crucible 34, so that the reaction gas according to equation 1, in this example Hydrochloride, as far as possible the reaction can be withdrawn, so that the respective present partial pressure of Hydrochloride is kept low. This withdrawn reaction gas is cooled in a cold trap 40. This is also shown schematically here, in particular a cryogenic gene cold trap, for example, shown with liquid nitrogen. In principle, the partial pressure of the product, ie of the hydrochloride or of a corresponding compound which is obtained in the redox reaction on the product side, adsorptive, for example by molecular sieves, or absorptively, with HCl formed, for example, by liquid ammonia or an aqueous ammonia solution to be lowered.

The device described in FIG. 2 is used, in particular, to allow one of the two reactions according to Equation 1 and Equation 2 to occur in situ as near as possible in real time for an external observer almost simultaneously, in which case no solid hydride is introduced for the reaction. Among other things, it is advantageous to lower the pressure after a certain period of time and after a large part of the conversion has been achieved in accordance with reaction equation 1, and possibly even apply a vacuum. In this case, a remaining product gas, possibly also dissolved hydrogen, can be removed from the solid or the melt.

The conversion of the SECI3 according to equation 1 to the rare earth metal should proceed as completely as possible according to the equation described in order to produce a pure product. In actual practice, however, it is often the case under given circumstances that the reaction equations are not always completely on the right side, so that possibly reduced metal has yet to be purified by chlorides and hydrides. By lowering the pressure in the reactor and elevated temperature, the hydrides decompose and hydrogen can, as already described, be removed. The remaining chlorides are more expensive to remove. Due to the relatively low melting and boiling point of the rare earth chlorides, separation is also possible by high temperatures. The temperature is increased so much that the chlorides liquefy, but the metal remains solid and so both phases can be separated. Alternatively, the temperature can be raised to the boiling point of the chloride, which is preferably done with simultaneous lowering of the process pressure, so that the chlorides are distilled from the liquid metal phase. On the basis of Figure 3, in which a schematic diagram of a device is provided which is suitable for implementing the method according to the invention, in particular an advantageous process gas guide for the inventive method will be exemplified. The reactor 24, shown schematically as a box in FIG. 3, essentially corresponds to the reactor 24 in FIG. 2. The gas mixture, which consists of a mixture of HCl and hydrogen, is passed out of the reactor 24 as described above for example, a heat exchanger 44, in which this product gas mixture is cooled as HCl and H2. There are still two more heat exchange processes 45 and 46 until the product gas HCl is passed in mixture with H2 in the already described cold trap 40. In the cold trap 40, the gaseous hydrochloride precipitates at the reaction temperature, leaving the hydrogen or another carrier gas in the line. The precipitated hydrochloride is not shown here, as it is derived from the cold trap 40. The now separated hydrogen or another carrier gas, optionally also an inert gas such as argon, is now preheated by heat exchangers 46 and 44 through a blower 42 and fed back to the reactor 24. In the reactor 24, during the reaction described in accordance with Equations 1 and 2, a significantly increased hydrogen partial pressure prevails, which is maintained by the fact that hydrogen is always supplied to the reactor through the feed line 26. However, the reaction product hydrochloride, or optionally another hydrogen compound, depending on the starting material used, should have the lowest possible partial pressure, so that the reaction proceeds as completely as possible according to Equation 1 and always lies on the right side of the reaction equation. For this reason, as already described, the gas removal via the gas discharge 38 extracts this product gas as comprehensively as possible. It goes without saying also the carrier gas and the hydrogen also sucked with. The hydrogen pressure is maintained by renewing new hydrogen introduced via the supply line 26.

By condensing the hydrochloride in the cold trap 40 and reintroducing the hydrogen into the reactor 24, a disproportionate consumption of hydrogen can be avoided. In addition, the described heat exchangers allow effective heat and cold recovery, so that the overall process is very positive in terms of energy. Compared to the electrolytic melts used in the prior art, significantly less greenhouse gases are produced by the process described; furthermore, the recovered hydrochloride can be sold as a hydrochloric acid in a profitable manner.

Claims

claims

1. A process for the preparation of an elementary rare earth element by a redox reaction, wherein the reactant side of the reaction comprises a rare earth compound in which the rare earth element is present in a positive oxidation number and hydrogen, characterized in that the redox reaction proceeds in two stages, wherein initially a hydrogenation reaction between an elemental rare earth element and hydrogen is added to a rare earth hydride and then a reaction between the rare earth compound having a positive oxidation number and the rare earth hydride, wherein the product of the reaction, an elemental rare earth element and simultaneously a hydrogen-containing compound is generated.

2. The method according to claim 1, characterized in that the rare earth compound in which the rare earth element has a positive oxidation number, a halide or an oxide.

3. The method according to claim 1 or 2, characterized in that at least the hydrogenation reaction takes place at a pressure of more than 10 bar, in particular more than 40 bar.

4. The method according to any one of the preceding claims, characterized in that the redox reaction takes place at a temperature of more than 800 ° C, in particular at more than 1000 ° C.

5. The method according to any one of the preceding claims, characterized in that the hydrogen-containing compound on the product side of the redox reaction is gaseous and the partial pressure of this hydrogen-containing compound is lowered.

6. The method according to any one of the preceding claims, characterized in that the hydrogen-containing compound is removed by a blower from a reactor.