U.S. patent application number 14/770062 was filed with the patent office on 2016-04-28 for method for synthesizing a rare earth element by redox reaction.
This patent application is currently assigned to SIEMENS AKTIENGESELLSCHAFT. The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Bernd FRIEDRICH, Marc HANEBUTH, Alexander TREMEL, Hanno VOGEL.
Application Number | 20160115569 14/770062 |
Document ID | / |
Family ID | 50928126 |
Filed Date | 2016-04-28 |
United States Patent
Application |
20160115569 |
Kind Code |
A1 |
FRIEDRICH; Bernd ; et
al. |
April 28, 2016 |
METHOD FOR SYNTHESIZING A RARE EARTH ELEMENT BY REDOX REACTION
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.
Inventors: |
FRIEDRICH; Bernd; (Aachen,
DE) ; HANEBUTH; Marc; (Nuremburg, DE) ;
TREMEL; Alexander; (Erlangen, DE) ; VOGEL; Hanno;
(Monheim, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
Munchen |
|
DE |
|
|
Assignee: |
SIEMENS AKTIENGESELLSCHAFT
Munich
DE
|
Family ID: |
50928126 |
Appl. No.: |
14/770062 |
Filed: |
June 12, 2014 |
PCT Filed: |
June 12, 2014 |
PCT NO: |
PCT/EP2014/062218 |
371 Date: |
August 24, 2015 |
Current U.S.
Class: |
75/717 |
Current CPC
Class: |
C22B 5/02 20130101; C22B
59/00 20130101; C22B 5/12 20130101 |
International
Class: |
C22B 59/00 20060101
C22B059/00; C22B 5/02 20060101 C22B005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 24, 2013 |
DE |
102013211946.1 |
Claims
1-6. (canceled)
7. A method of preparing an elemental rare earth element,
comprising: concurrently producing a product elemental rare earth
element and a hydrogen-containing compound as products of a redox
reaction, a starting material side of the redox reaction including
hydrogen and a rare earth compound having a rare earth element in a
positive oxidation number, and the redox reaction including, as
reaction stages, a hydrogenation reaction between a reactant
elemental rare earth element and the hydrogen to form a rare earth
hydride, and, subsequently, a reaction between the rare earth
compound and the rare earth hydride.
8. The method as claimed in claim 7, wherein the rare earth
compound is a halide or an oxide.
9. The method as claimed in claim 7, wherein at least the
hydrogenation reaction takes place at a pressure of more than 10
bar.
10. The method as claimed in claim 9, wherein the
hydrogen-containing compound produced by the redox reaction is
gaseous, and wherein the method further comprises lowering a
partial pressure of the hydrogen-containing compound.
11. The process as claimed in claim 10, further comprising
discharging the hydrogen-containing compound from the reactor by a
blower.
12. The process as claimed in claim 9, further comprising
discharging the hydrogen-containing compound from the reactor by a
blower.
13. The method as claimed in claim 9, wherein the rare earth
compound is a halide or an oxide.
14. The method as claimed in claim 13, wherein the redox reaction
is performed at a temperature of more than 800.degree. C.
15. The method as claimed in claim 14, wherein the
hydrogen-containing compound produced by the redox reaction is
gaseous, and wherein the method further comprises lowering a
partial pressure of the hydrogen-containing compound.
16. The process as claimed in claim 15, further comprising
discharging the hydrogen-containing compound from the reactor by a
blower.
17. The method as claimed in claim 16, wherein the pressure is more
than 40 bar.
18. The method as claimed in claim 17, wherein the temperature is
more than 1000.degree. C.
19. The method as claimed in claim 9, wherein the pressure is more
than 40 bar.
20. The method as claimed in claim 9, wherein the redox reaction is
performed at a temperature of more than 800.degree. C.
21. The method as claimed in claim 20, wherein the
hydrogen-containing compound produced by the redox reaction is
gaseous, and wherein the method further comprises lowering a
partial pressure of the hydrogen-containing compound.
22. The process as claimed in claim 21, further comprising
discharging the hydrogen-containing compound from the reactor by a
blower.
23. The method as claimed in claim 7, wherein the redox reaction is
performed at a temperature of more than 800.degree. C.
24. The method as claimed in claim 23, wherein the temperature is
more than 1000.degree. C.
25. The method as claimed in claim 7, wherein the
hydrogen-containing compound produced by the redox reaction is
gaseous, and wherein the method further comprises lowering a
partial pressure of the hydrogen-containing compound.
26. The process as claimed in claim 7, further comprising
discharging the hydrogen-containing compound from the reactor by a
blower.
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.sub.2.
However, perfluorocarbons such as CF.sub.4 or C.sub.2F.sub.6, which
have many times the greenhouse potential of CO.sub.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:
3REH.sub.2+2RECl.sub.3.fwdarw.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:
2RE+xH.sub.2.revreaction.2REH.sub.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.degree. C., and may take
place at more than 1000.degree. 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.degree. 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.sub.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.sub.2 is cooled. Two
further heat exchange processes 45 and 46 are carried out until the
product gas HCl in admixture with H.sub.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).
* * * * *