US20160102411A1

US20160102411A1 - Device for reducing a metal ion from a salt melt

Info

Publication number: US20160102411A1
Application number: US14/770,060
Authority: US
Inventors: Bernd 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: 2016-04-14
Also published as: DE102013211922A1; EP2935657A1; WO2014206746A1

Abstract

Between an anode and a cathode, a salt melt containing a metal ion is separated from the anode by a gap across which an electric arc can be formed. The metal ion is deposited on the anode and subsequently removed.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage of International Application No. PCT/EP2014/062216, filed Jun. 12, 2014 and claims the benefit thereof. The International Application claims the benefit of German Application No. 10 2013 211 922.4 filed Jun. 24, 2013, both applications are incorporated by reference herein in their entirety.

BACKGROUND

Described below is an apparatus for reducing a metal ion from a salt melt.
Rare earth elements, which are also referred to as lanthanides in chemistry, are required in many electronic components and in the production of magnets. 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 separate constituents in apatite, which are again finely distributed in deposits, which can also contain iron. A substep of this complicated process for obtaining rare earth elements in pure form is an electrolysis process in which chlorides or fluorides of the rare earth element in molten form may be used as electrolyte. Application of a voltage between immersed graphite anode and inert tungsten cathode results in the rare earth oxides dissolved in the electrolyte being converted into metal and CO/CO₂. However, perfluorocarbons such as CF₄or C₂F₆, which frequently have the greenhouse potential of CO₂, are also formed at the carbon anode. Furthermore, highly toxic hydrofluoric acid can be formed in the presence of water. All these undesirable products which are formed in the electrolysis have to be got rid of again by complicated purification and neutralization processes, which considerably increases the total process costs. Similar problems occur in principle in the electrolysis of salt melts using graphite electrodes, for which reason application to the preparation of rare earth elements can be considered to be illustrative.

SUMMARY

Described below is an apparatus which provides for the reduction of metal ions from metal-containing melts, in which there is a lower emission of damaging greenhouse gases compared to the prior art.
The apparatus for reducing a metal ion in a salt melt has an anode and a cathode. The apparatus is wherein a gap for formation of an electric arc is present between the anode and the salt melt. The metal ion may be a rare earth metal ion which is frequently prepared by electrolysis of salt melts. However, the apparatus is not restricted to the use of rare earth metal ions. Furthermore, the salt melt also contains oxygen ions which is due to the rare earth metal ion originally being present in solid form in the form of an oxide. An oxide is for the present purposes also subsumed under the term salt.
Compared to a known electric arc melting pot, the apparatus described has the difference that the electric arc is present across a gap between the anode and the surface of the salt melt. This in turn means, in contrast to the prior art in which graphite electrodes for the reduction of rare earth ions are dipped into the melt, that no carbon compounds which would form compounds with the anions, i.e., halide ions or oxygen ions, are formed. Thus, no carbon halides which are damaging particularly in terms of the greenhouse effect are formed. Furthermore, no hydrogen fluoride, i.e., no hydrofluoric acid, which is likewise highly toxic is formed in the case of this apparatus.
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 as rare earth elements in this case because of their chemical similarities. Rare earths are in turn compounds of rare earth elements, in particular the oxides thereof, but no rare earth phosphates are included here.
It has been found to be advantageous for the anode to be formed of a chemically inert material having good conductivity, for example copper, which if necessary is cooled from the inside. This avoids any compound between anions which are oxidized to the corresponding elements in the region of the electric arc and the material of the anode. It has been found to be particularly advantageous for the salt melt to contain oxygen ions, particularly instead of halide ions. The oxidation of the oxygen ions forms pure oxygen which is discharged as O₂via the offgas.
In an advantageous embodiment, an electrolysis vessel which serves to accommodate the salt melt is provided. This electrolysis vessel or the vessel wall thereof is in direct electrical contact with the cathode. In principle, electrically conductive constituents of the electrolysis vessel can likewise serve as cathode. This means that in an electrolysis operation, the positively charged cations, i.e., the metal ions, in particular rare earth metal ions, are deposited on the vessel wall and as a result of their high specific gravity settle at the bottom of the electrolysis vessel. This in turn leads to the elemental rare earth metal constituents, whether in solid or liquid form, being in electrical contact with the vessel wall and thus with the cathode and in turn acting as cathode. At the phase interface between the particles already precipitated as elemental metal and the salt melt, ever more metal atoms are deposited, so that a phase of pure metal is present in the lower region of the electrolysis vessel and can be separated off after the electrolysis process.
A plasma may be present above the salt melt, i.e., in the region of a hollow space above the salt melt, in which the anode is also arranged. For the present purposes, a plasma is an ionized gas, for example an ionized noble gas. As plasma gas, a mixture of argon and nitrogen may be used. This gas is also referred to as inert gas since it undergoes a chemical reaction neither with the salt melt nor with the material of the anode. In a further advantageous embodiment, the salt melt includes not only the oxide of the metal to be reduced, i.e., generally the rare earth metal, but also further oxides. These are oxides of metals which are more stable in respect of the electrolysis than the rare earth metal oxide and at the same time reduce the melting point of the salt melt. In principle, other salts can also be employed for reducing the melting point as long as these are sufficiently stable, in particular in respect of their anions, for no damaging halides to be formed at the anode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a sequence chart using schematic drawings of a process for extraction of rare earth metals from an ore; and

FIG. 2 is a schematic block diagram of the electrolysis of a salt melt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments, 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, for example, customary for the mineral monazite, is shown schematically in FIG. 1, without making any claims 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 in respect 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.
The starting raw materials containing the monazite mineral are firstly milled very finely and treated in a flotation plant 2 in such a way that the monazite is separated as well as possible from the other mineral constituents. The monazite is dried and, according to the related art, admixed with sulfuric acid and then treated in a furnace, for example a rotary tube furnace 4. Here, the phosphates are converted into sulfates. This process in the rotary tube furnace takes place at temperatures 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 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, resulting in 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 possible 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 at different concentrations either in the aqueous part of the solution or in the organic part of the solution. The organic phase and the aqueous phase of the mixture are here alternately mixed and separated again in a multistage separation process, so that particular rare earth ions become, depending on the extractant in the organic phase, ever more concentrated until these ions are present in sufficient purity in one phase. Up to 200 separation operations per element can be necessary here.
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 respective 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 in a tunnel kiln 12, through which a stream of hot air is passed. After this process, a discrete rare earth oxide is thus present.
This discrete rare earth oxide is continuously added to a molten electrolyte in the electrolysis plant 16. The electrolyte is mainly formed of the corresponding rare earth fluoride. The oxide compound dissociates into rare earth cations and oxygen anions in this electrolyte. The rare earth cations are reduced to elemental metal at the cathode and are collected in a collection vessel underneath the cathode. The oxygen ions react with the carbon of the anode to form CO/CO2, but fluorine ions also form compounds with the carbon of the anode and leave the electrolysis bath together in gaseous form.
The rare earth oxide can optionally be converted into a lower-melting salt, e.g. an iodide, a chloride or fluoride, before introduction into the electrolysis process and then be introduced in molten form into an electrolysis process, with elemental rare earth metal depositing at a cathode of the electrolysis apparatus.
The metal 20 obtained in liquid form is pumped out from the collection vessel underneath the cathode and cast to produce ingots.
FIG. 2 illustrates an advantageous embodiment of an electrolysis apparatus. This is a schematic depiction of an electrolysis apparatus. The apparatus has an anode 26 and a cathode 28. A salt melt 24 is accommodated in an electrolysis vessel 34. This salt melt 24 can be heated either by a resistance heating element (not shown here) or by an electric arc 32 which generates a plasma 33. A combination of a plurality of heating methods is also possible. A gap 30 is provided between the anode 26 and a surface 42 of the salt melt 24 and an electric arc 32 is present in this gap when a voltage is applied. This electric arc 32 leads to inert gas, in particular a mixture of argon and nitrogen, which is introduced via an inert gas feed line 36 being ionized and being present in the form of a plasma 33 above the surface 42. In a plasma space 44, in which the plasma 43 is present and which is largely sealed off from an atmosphere, a positive charge prevails. The negative charges of the salt melt 24, in particular oxygen ions, migrate to the surface 42 of the salt melt, also referred to as electrolyte, and are oxidized there to atomic oxygen at the boundary between the salt melt, i.e., the electrolyte, and the plasma. This means that the electrolyte should be conductive for rare earth ions, oxygen ions and also electrons. The atomic oxygen forms O2 molecules outside the plasma space 44 and leaves the plasma space through the offgas outlet 38.
The anode is a material which is self evidently firstly electrically conductive but on the other hand is inert to all reactants in the electrolysis system. For this purpose, the anode has to have internal water cooling so that it does not melt at the high plasma temperatures. It is possible to use, for example, copper as material here. However, the anode does not consist of carbon since carbon together with the oxidized elements, in particular with the oxygen but also with certain halides if they are present in the salt melt, tends to form gases which cause great damage to the atmosphere, in particular are strong greenhouse gases.
In contrast to the anode arranged above the salt melt, the cathode is electrically conductively connected to a vessel wall 40 of the electrolysis vessel underneath the salt melt. In principle, the vessel wall 40 can also be formed of an electrically conductive material and thus directly form the cathode 28. In this case, it would be advantageous for upper regions of the vessel wall or of the electrolysis vessel 34 to be electrically insulated from lower regions. As an alternative, it is also possible to make the electrolysis vessel of a refractory material which in its lower region has a cutout into which a metallic or other conductive cathode 28 is inserted. On application of an appropriate voltage, elemental metal which has formerly been present in the form of metal ions in the salt melt 24 is deposited at the electrically conductive cathode 28. The surface of the cathode 28 is thus covered very promptly by elemental metal, but this is likewise electrically conductive and thus builds up a fresh electrically conductive surface at which further ions can again be reduced. The electrolysis is stopped when there is no longer any voltage or when the salt melt 24 is present in chemical equilibrium and no further electrolysis takes place. Depending on the temperature in the electrolysis vessel, i.e., depending on the melting point of the electrolyte 24 or salt melt 24 used, and depending on the melting point of the metal being deposited, the latter can be present either in solid form or in liquid form at the cathode 28 in the lower region of the electrolysis vessel 34. Accordingly, the deposited metal, i.e., the rare earth metal 20, can be drained off when it is present in liquid form or can be taken out in pure, solid form after solidification of the salt melt 24.
A substantial advantage of the apparatus is firstly that there is a spacing between the anode 26 and the electrolyte 24 or the salt melt 24, i.e., the materials of the electrode do not come into direct contact with the salt melt 24 but are instead connected to one another in energy terms only indirectly via the electric are 32. A further important point is that, compared to known electric arc processes, the polarity is reversed so that the anode is positioned above the salt melt and the electric arc 32 prevails between the anode and the salt melt. This in turn leads to the now elemental, oxidized anions, which are generally present in gaseous form, rising upward and being able to escape from the apparatus via the plasma space 44 and the offgas outlet 38. Furthermore, it is possible as a result of this arrangement for the elemental metal to be isolated as material value to settle on the bottom of the apparatus at the cathode 28. Thus, a high measure of purity of the deposited metal 20 can also be achieved here.
A further advantage is to select the material of the salt melt 24 in such a way that very few halides and a large amount of oxygen ions are present, so that no damaging halogen compounds or elemental halogens occur in the oxidation of the anions. However, since the halogen compounds are not compounds with carbon, salts can also be present in the form of halides in the salt melt 24 when this serves to lower the melting point of the salt melt 24. Overall, production of CO2 is prevented and any after-treatment of the offgas becomes significantly simpler and less costly. This serves to make the ecologically problematical process for extraction of rare earth metals or other metals cheaper and more ecologically friendly.
A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of 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, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).

Claims

1-10. (canceled)

11. An apparatus for reducing a metal ion in a salt melt, comprising:

a cathode; and

an anode disposed above the salt melt with a gap for formation of an electric arc therebetween.

12. The apparatus as claimed in claim 11, wherein the salt melt comprises oxygen ions.

13. The apparatus as claimed in claim 11, wherein the salt melt comprises a rare earth metal ion.

14. The apparatus as claimed in claim 11, wherein the anode is inert toward materials in the salt melt.

15. The apparatus as claimed in claim 11,

further comprising an electrolysis vessel accommodating the salt melt; and

wherein the cathode is electrically connected to a wall of the electrolysis vessel.

16. The apparatus as claimed in claim 15, wherein the cathode is arranged at a bottom of the electrolysis vessel.

17. The apparatus as claimed in claim 11, wherein a plasma prevails above the salt melt.

18. The apparatus as claimed in claim 17, wherein an inert gas which forms the plasma is present above the salt melt.

19. The apparatus as claimed in claim 11,

further comprising an inert gas feed line and an offgas outlet, and

wherein the salt melt has a surface separated from air surrounding the apparatus.

20. The apparatus as claimed in claim 11, wherein the salt melt comprises an oxide of the metal ion to be reduced and additional oxides.