Patent Application
20160115569
Described below is a process for preparing a rare earth element by a redox reaction.
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/CO2. However, perfluorocarbons such as CF4 or C2F6, which have many times the greenhouse potential of CO2, 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.
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.
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.
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:
3REH2+2RECl3→5RE+6HCl (eq. 1)
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:
2RE+xH2⇄2REHx (eq. 2)
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.
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.
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.
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.
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.
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.
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:
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.
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
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.
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.
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.
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.
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.
The apparatus illustrated in
The conversion of the RECl3 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.
A process gas flow for the process will be illustrated by way of example with the aid of
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.
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.
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).
| Number | Date | Country | Kind |
|---|---|---|---|
| 102013211946.1 | Jun 2013 | DE | national |
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/062218 | 6/12/2014 | WO | 00 |
