Abstract:
A method for synthesising a rare earth element by a redox reaction. The starting material side of the reaction for synthesising the rare earth element includes a rare earth compound, in which the rare earth element is present in a positive oxidation state, and hydrogen. The redox reaction takes place in two stages. First, a hydration reaction takes place between an elementary rare earth element and hydrogen to form a rare earth hydride. Then, a reaction takes place between the rare earth compound and the rare earth hydride. An elementary rare earth element and a hydrogen-containing compound are produced at the same time as the product of the reaction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to International Application No. PCT/EP2014/062218 filed on Jun. 12, 2014 and German Application No. 10 2013 211 946.1 filed on Jun. 24, 2013, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    Described below is a process for preparing a rare earth element by a redox reaction. 
         [0003]    Rare earth elements, which are also referred to as lanthanides in chemistry, are required in many electronic components and in the production of magnets. Thus, for example, the rare earth element neodymium is an important constituent of permanent magnets which are used in wind generators. The work-up and separation of rare earth elements is in principle chemically complicated since the rare earth elements occur in nature in very finely distributed and associated (especially with one another) form and in low concentrations. The rare earth elements are frequently present in phosphate compounds, in particular in the crystal structure of monazite or xenotime or as secondary constituents in apatite, which in turn occur finely distributed in deposits which can also contain iron. A part of this complicated process for isolating rare earth elements in pure form is an electrolysis process in which chlorides or fluorides of the rare earth elements in molten form are preferably used as electrolyte. Application of a voltage between an immersed graphite anode and an inert tungsten cathode results in the rare earth oxides dissolved in the electrolyte being converted into metal and CO/CO 2 . However, perfluorocarbons such as CF 4  or C 2 F 6 , which have many times the greenhouse potential of CO 2 , are also formed at the carbon anode. Furthermore, the highly toxic hydrofluoric acid can be formed in the presence of water. All these undesirable products which are formed during the electrolysis have to be gotten rid of again by complicated purification and neutralization processes, which considerably increases the total process costs. 
       SUMMARY 
       [0004]    Described below is a process for preparing rare earth elements in elemental form, which compared to the melt flux electrolysis employed in the prior art is cheaper and more environmentally friendly. 
         [0005]    In the process, a rare earth compound in which the rare earth element is present in a positive oxidation number is present on the starting material side of the reaction. Furthermore, hydrogen is present on the starting material side of the redox reaction. The redox reaction proceeds in two stages, with a hydrogenation reaction between an elemental rare earth element and hydrogen to form a rare earth hydride occurs first and a reaction between the rare earth compound in which the rare earth element ion present therein has a positive oxidation number and the rare earth hydride subsequently taking place, where the product of this reaction is an elemental rare earth element and at the same time a hydrogen-containing compound. 
         [0006]    Basically, it is advantageous for the rare earth element to be bound to a halide or an oxide in the rare earth compound. Here, the following reactions schematically take place: 
         [0000]      3REH 2 +2RECl 3 →5RE+6HCl   (eq. 1)
 
         [0007]    This reaction is the overall reaction and occurs upon a reaction between hydrogen and the rare earth element to form a rare earth hydride according to the following equation: 
         [0000]      2RE+xH 2 ⇄2REH x    (eq. 2)
 
         [0008]    The equation 1 is a so-called synproportionation in which a pure element is formed from 2 compounds containing this element, with this being oxidized in one case and reduced in the other case. Such a synproportionation is a special case of a redox reaction. The rare earth element in its hydridic form in equation 1 has a negative oxidation number, and in its form as chloride (with chloride being mentioned here as an example) has a positive oxidation number (+III). The rare earth element in the hydride is oxidized, while in the chloride it is reduced and elemental rare earth metal is ultimately present after the reaction has occurred. 
         [0009]    The two equations generally proceed separately, with this being able to take place effectively in situ; in the extreme case, it can even be that no hydride in solid form is present in the reaction mixture but instead hydride is formed in situ in the reaction mixture. This is the case when the hydrogen activity in the gas phase is sufficiently high. The term in situ can thus have two different meanings: either it is a strict sequential reaction, with the reaction according to equation 2 taking place first and the hydride thus being formed first and subsequently reacting as reducing agent with the chloride according to equation 1, or the term in situ can also mean that the hydride is a short-lived intermediate which effectively immediately reacts further and is reacted according to equation 1. 
         [0010]    Regardless of which variant is selected, the hydrogen partial pressure (in particular at high reaction temperatures) has to be sufficiently high for a hydride to be able to be formed at all. Otherwise, the equilibrium according to equation 2 would lie completely on the side of the starting materials, but this would not only prevent the formation of a hydride but at the same time also prevent an overall reaction according to equation 1. It is therefore advantageous for gaseous substances formed, like the hydrogen chloride according to the example of equation 1, to be removed quickly from the reaction site by suitable measures. 
         [0011]    The term rare earth elements refers, in particular, to the lanthanides, including, inter alia, lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium and lutetium, but yttrium and scandium are also counted here because of their chemical similarity in this case. Rare earths are in turn compounds of rare earth elements, in particular the oxides thereof, with rare earth phosphates not being included here. Rare earth elements in elemental form can be present in pure form or in mixtures or alloys of various rare earth elements. 
         [0012]    As indicated above, the chloride mentioned in equation 1 is purely an example of a compound of the rare earth elements in which the rare earth element has a positive oxidation number. Halides, in particular chlorides, bromides, iodides or fluorides, and also oxides can in principle be advantageous for this purpose. It is advantageous for the hydrogenation reaction for the pressure prevailing in the reaction atmosphere to be greater than 10 bar, in particular more than 40 bar. This takes place, in particular, at a reaction temperature of more than 800° C., and may take place at more than 1000° C. 
         [0013]    It is in principle advantageous for the hydrogen-containing compound, in the example of equation 1 hydrogen chloride, to be in gaseous form on the product side of the redox reaction and for the partial pressure of this hydrogen-containing compound to be reduced very promptly, which can be achieved, for example, by rapid drawing off and subsequent cooling in a cold trap. It can be advantageous here to use a blower which removes the gaseous reactants of the redox reaction from the reaction very quickly. In this way, the total consumption of hydrogen required during the reaction is reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of embodiments, taken in conjunction with the accompanying drawings of which: 
           [0015]      FIG. 1  is a sequence chart using schematic drawings of a process for extraction of rare earth elements from an ore; 
           [0016]      FIG. 2  is a schematic cross section view of a device for carrying out the process of  FIG. 1  with a synproportionation; and 
           [0017]      FIG. 3  is a schematic block diagram of the process as per  FIG. 2  with hydrogen recovery. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0019]    Firstly, the process for extraction of rare earth metals, as is customary, for example, for the mineral monazite, will be illustrated schematically with the aid of  FIG. 1 , without making any claim as to completeness. The mineral monazite is a phosphate in which the metal ions frequently occur in the form of rare earth metals, in particular cerium, neodymium, lanthanum or praseodymium. Here, there is not a homogeneous composition of rare earth metals within a particle, but instead the lattice sites of the cations in the crystal structure are occupied by various rare earth metals in different concentrations. 
         [0020]    The starting raw materials containing the monazite mineral are firstly milled very finely and treated in a floatation plant  2  in such a way that the monazite is separated very well from the other mineral constituents. The monazite is dried and, according to the prior art, treated in a furnace, for example a rotary tube furnace  4 , after prior mixing with sulfuric acid. The phosphates are converted into sulfates here. This process in the rotary tube furnace takes place at temperatures of up to 650° C. The conversion of phosphate into sulfate is advantageous since the rare earth sulfates are significantly more readily soluble in water than the phosphates of the rare earth metals. 
         [0021]    The sulfuric acid-containing solution of rare earth sulfates is, after the treatment in the rotary tube furnace  4  and a subsequent leaching, neutralized in a neutralization apparatus  6 , i.e. the pH is increased by addition of a basic substance, with undesirable substances being precipitated and separated off so that an aqueous rare earth sulfate solution is present in the remaining liquid. 
         [0022]    This resulting solution of a rare earth compound (sulfate, nitrate, chloride or the like) is usually subjected to a liquid/liquid extraction, i.e. a separation, in mixer-settler apparatuses  8 . Here, the solution is treated by mixing with an extractant dissolved in organic solvents such as kerosene including possibly further additives in such a way that the rare earth cations which in the case of the same charge have slightly different ion diameters accumulate in different concentrations either in the aqueous part of the solution or in the organic part of the solution. Here, the organic phase and the aqueous phase of the mixture are alternately mixed and separated again in a multistage separation process, so that particular rare earth ions are present, depending on the extractant in the organic phase, in ever greater concentrations until these ions are present in sufficient purity in one phase. Here, up to 200 separation operations per element can be necessary. 
         [0023]    The rare earth metals which have been separated in this way are subsequently precipitated by addition of a carbonate or oxalate in a process in a precipitation apparatus  10 , so that the corresponding rare earth carbonate or oxalate accumulates at the bottom of the precipitation apparatus  10 . This is in turn calcined in a calcination apparatus, for example a tunnel kiln  12 , through which a hot air stream is passed. A discrete rare earth oxide is thus present after this process. 
         [0024]    This discrete rare earth oxide can optionally be converted into a low-melting salt, e.g. into an iodide, a chloride or a fluoride, and in turn fed in molten form to an electrolysis process in which elemental rare earth metal deposits at a cathode of the electrolysis apparatus. However, this process is technically very complicated and likewise energy-intensive. For this reason, an alternative process for preparing elemental rare earth elements which involves hydrogen is proposed. 
         [0025]      FIGS. 2 and 3  schematically show an example of an apparatus which is suitable for carrying out the process described herein.  FIG. 2  shows a schematic depiction of a reactor  24  which is essentially pressure-tight, which is indicated by the seals  30 . These seals  30  should be high-temperature-resistant and can, for example, be formed of graphite. The reactor  24  which has been closed in a pressure-tight manner has a feed line  26  which can optionally be regulated by a valve  28 . Through this, hydrogen gas, in particular, is introduced into the reactor  24 . The reaction starting materials  36  or, after the reaction is complete, the reaction products are present in a crucible  34 . The reactor is, schematically, heated by a heating device  32  which is indicated here in the form of a heating coil. Above the crucible  34 , there is a gas offtake  38  which may be arranged over a large area, in a bell-like manner over the crucible  34  so that the reaction gas according to equation 1, in this example hydrogen chloride, can be taken off over the surface of the reaction, so that the partial pressure of hydrogen chloride prevailing in each case is kept low. This reaction gas which has been drawn off is cooled in a cold trap  40 . This is likewise shown schematically here; in particular, a cryogenic cold trap, for example containing liquid nitrogen, is shown here. The partial pressure of the product, i.e. the hydrogen chloride or a corresponding compound which is formed on the product side in the redox reaction, can in principle be decreased adsorptively, for example by molecular sieves, or absorptively by passing the HCl formed through, for example, liquid ammonia or an aqueous ammonia solution. 
         [0026]    The apparatus illustrated in  FIG. 2  serves, in particular, to allow one of the two reactions according to equation 1 and equation 2 to proceed virtually simultaneously in situ, in a manner that may be close in time after one another for an external observer; in this case, no solid hydride is initially charged for the reaction. It is, inter alia, advantageous to reduce the pressure and possibly even apply a vacuum after a certain period of time and after a major part of the conversion according to reaction equation 1 has been achieved. Here, a product gas remaining, possibly also dissolved hydrogen, can be removed from the solid or from the melt. 
         [0027]    The conversion of the RECl 3  into the rare earth metal according to equation 1 should proceed very completely according to the equation described in order to produce a pure product. However, in the practical reaction it is more often the case under given circumstances that the reaction equations do not always lie completely on the right-hand side, so that reduced metal may still have to be freed of chlorides and hydrides. Lowering the pressure in the reactor and increasing the temperature leads to decomposition of the hydrides and hydrogen can, as described above, be drawn off. The remaining chlorides are more difficult to remove. Due to the relatively low melting and boiling point of the rare earth chlorides, they can, however, likewise be separated off by high temperatures. Here, the temperature is increased until the chlorides liquefy but the metal remains in solid form and the two phases can thus be separated. As an alternative, the temperature can be increased up to the boiling point of the chloride, which may be effected with simultaneous lowering of the process pressure, so that the chlorides are distilled from the liquid metal phase. 
         [0028]    A process gas flow for the process will be illustrated by way of example with the aid of  FIG. 3  which shows a schematic diagram of an apparatus which is suitable for implementing the process according to embodiments of the invention. The reactor  24 , schematically shown as a box in  FIG. 3 , corresponds essentially to the reactor  24  in  FIG. 2 . The gas mixture, which includes a mixture of HCl and hydrogen, is discharged from the reactor  24  as described above; this is followed, for example, by a heat exchanger  44  in which this product gas mixture of HCl and H 2  is cooled. Two further heat exchange processes  45  and  46  are carried out until the product gas HCl in admixture with H 2  is introduced into the above-described cold trap  40 . In the cold trap  40 , the hydrogen chloride which is gaseous at reaction temperature is condensed and the hydrogen or another carrier gas remains in the line. The condensed hydrogen chloride is not shown here since it is discharged from the cold trap  40 . The now separated hydrogen or another carrier gas, optionally an inert gas such as argon, is now, driven by a blower  42 , preheated via heat exchangers  46  and  44  and fed back into the reactor  24 . A significantly increased hydrogen partial pressure, which is maintained by hydrogen being continually fed into the reactor through the feed line  26 , prevails in the reactor  24  during the above-described reaction according to equations 1 and 2. However, the reaction product hydrogen chloride, or optionally another hydrogen compound depending on the starting material used, should have a very low partial pressure so that the reaction according to equation 1 proceeds virtually to completion and always lies on the right-hand side of the reaction equation. 
         [0029]    For this reason, as described above, this product gas is drawn off very comprehensively via the gas offtake  38 . Here, the carrier gas and the hydrogen are of course likewise drawn off, too. The hydrogen partial pressure is maintained by fresh hydrogen being introduced via the feed line  26 . 
         [0030]    As a result of the condensation of the hydrogen chloride in the cold trap  40  and the reintroduction of the hydrogen into the reactor  24 , a disproportionately high consumption of hydrogen can be avoided. In addition, effective heat and cold recovery can be effected by the heat exchangers described, so that the overall process proceeds very positively from an energy point of view. Compared to the electrolyte melts used in the prior art, the process described produces significantly smaller amounts of greenhouse gases, and the recovered hydrogen chloride can also be sold as a profit as hydrochloric acid. 
         [0031]    The invention has been described in detail with particular reference to embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).