WO2014117987A1 - Process for reducing rare earth oxides to rare earth metals - Google Patents

Abstract

The invention relates to a process for reducing rare earth oxides to rare earth metals (34), which comprises the following steps: iodination of a rare earth oxide to form a rare earth diiodide, heating of the rare earth diiodide above a melting point to form a rare earth diiodide melt (21), electrolysis of the rare earth diiodide melt (21) with deposition of the rare earth metal (34) at an electrode (23).

Description

description

Process for the reduction of rare earth oxides to rare earth metals

The invention relates to a process for the reduction of rare earth oxides to rare earth metals according to claim 1.

Rare earth metals, which in chemistry are also called lanthanides, are required in many electronic components and in the production of magnets. For instance, the rare earth element neodymium an important ingredient ^¬ part of permanent magnets used in wind generators. The treatment and separation of rare earth metals is generally chemically complex, since the rare earth metals are distributed very finely in nature, socialized and occur in ge ^¬ wrestling concentrations. Frequently, the rare earth elements are present in phosphatic compounds, in particular in the crystal structure of the monazite, which in turn occurs finely distributed in deposits which may also contain iron. Another rare earth occurrence is MineralBastnäsit, which is a fluorocarbonate. There is, for example, the rare earth metal cerium in the form as

Cerfluorocarbonat ago. After a complex preparation process, a rare earth oxide is present in the extraction of rare earth metals from said minerals. It is important to reduce this rare earth ^oxide and convert it to the rare earth element in metallic form. It is usually done by an electrolysis step, wherein the commonly refractory rare earth oxide is converted into a salt that melts more easily, which in turn is electrolyzed at its melting temperature in the molten state, wherein the rare earth metal at an electrode, the Katho ^¬ de, deposits ,

In this process step is due to the high-melting salts used, which usually have a melting point of about 800 ° C, a lot of electrical energy to Heating these salts over the melting point expended. The object of the invention is to reduce the energy consumption in the extraction of rare earth metals in the electrolysis step compared to the prior art.

The solution of the problem consists in a method with the features of claim 1.

In the inventive method for the reduction of rare earth oxides to rare earth metals according to claim 1, a rare earth oxide is first iodinated. Here, the oxygen of the rare earth oxide is at least partially replaced by iodine, so that a rare earth iodide, for example, cerium iodide or

Neodymium iodide arises. This rare earth iodide is heated to a temperature above its melting point.

This produces a rare earth ^iodide melt, which is subjected to an electrolysis. In the electrolysis of

Rare earth oxide melt separates the rare earth metal on an electrode, the cathode. The advantage of this invention is that the iodides of rare earth metals have a much lower melting point than other salts of rare earth metals, with little energy required to heat a rare earth salt, thus making the overall process more energy efficient.

The term rare earth metals, in particular, the so-called lanthanides, including lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium and ytterbium understood, but there are due to their chemical similarities in this case here, the yttrium and scandium added to that.

The melting point of a rare earth iodide may be lowered as a mixture with another low melting iodide. In particular, the mixture of a rare earth iodide with another iodide in a eutectic composition leads to a very low melting point. In particular, the iodides of lithium or potassium have Liums are found to be advantageous, which form eutectics with certain rare earth metal iodides having a melting point between 200 ° C and 300 ° C. The term rare earth iodide melt therefore includes in particular a salt melt, in particular a melt of iodides (which may also include oxides) containing rare earth iodides in molten form. In a further advantageous embodiment of the invention, the Seltenerdiodidschmelze is heated in certain Zeitabstän ^¬ the well above the melting point of the metal out ^¬ heats, so that the rare earth metal, which has been deposited on the cathode, also melts and from the electrode, the cathode melted off becomes. In this way, a continuous electrolysis process is possible, which is accompanied only by occasional temperature increases of the melt. Alternatively or additionally, the electrodes with the deposited rare earth metal can also be drawn out of the melt at regular intervals and replaced immediately with fresh electrodes. The rare earth metal deposited on the electrode can then be removed from the electrode by a mechanical process or by a melt process. This is also a continuous or quasi-continuous process. Energy-intensive cooling and heating of the melt is avoided in both alternatives. Also, the use of a mechanical wiper is expedient, this regularly wipes the deposited crystals from the cathode in the ongoing electrolytic ^¬ sebetrieb, wel ^¬ che to the ground and can be removed there regularly.

In a further embodiment of the invention, during the electrolysis, the rare earth iodide melt is covered by an inert gas, the inert gas passing through the electrolyte. segregation of iodine gas accumulates. This resulting inert gas-iodine gas mixture is passed out of the electrolysis cell and cooled in a first cooling step. Furthermore, there is a cooling step, in which in a so-called cold trap the

Iodine gas is separated from the inert gas-iodine gas mixture. The inert gas freed from the iodine gas, which has a significantly lower temperature compared to the outlet temperature from the electrolysis cell, is in turn passed into a heat exchanger, which serves to cool the inert gas-iodine gas mixture exiting the electrolysis cell. In this way, the inert gas, which was after separation from the iodine gas at a lower temperature level than before the separation, heated again and can be redirected as a hot inert gas into the electrolytic cell without much loss of energy. The deposited iodine gas can also be reused to iodine the rare earth oxide. The energy required for the gas supply is minimized.

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. This is purely exemplary Dar ^¬ positions, which are not taken limiting the scope in itself. It shows: a schematic chain of process steps for the extraction of rare earth metals from a rock; and a schematic representation of the electrolysis of a rare earth iodide with heat recovery.

First of all, the extraction process of rare earth metals, as is customary for the mineral monazite, without any claim to completeness, will be explained schematically in FIG. The mineral monazite is a phosphate in which the metal ions are often in the form of rare earth metals, especially cerium, neo- dym, lanthanum or praseodymium occur. This is not even within a mineral grain is a homoge ^¬ ne composition of rare earth metals, but the lattice sites of the cations are occupied by various rare earth metals in different concen ^¬ nen in the crystal structure.

The basic raw materials that contain the Monazitmineral, ^¬ the first very finely ground and treated in a flotation 2 so that the monazite is separated as well as possible from the remaining mineral ^¬ metallic components. The monazite is dried and placed in a furnace such as a rotary tube furnace 4 ^¬ with hot sulfuric acid. In the process, the monazite minerals are dissolved, with the phosphate ions being replaced by the sulfate ions of the sulfuric acid. This process in the rotary kiln takes place at temperatures up to 600 ° C. The replacement of phosphate ions by sulfate ions is expedient, since the rare earth sulfates are much 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 neutralization device 6, ie, the pH is increased by adding a ba ^¬ sischen substance, so that in this liquid is an aqueous rare earth sulfate.

This rare earth sulfate solution is usually in so-called mixer-settler devices of a liquid / liquid extraction, ie a separation subjected. Here, possibly by mixing a dissolved in organic solvents such as Kero ^¬ sin extractant incl. Is further to ^¬ sentences processed so that the rare earth cations, having slightly different Ionendurch- diameter for the same load, at different concentrations in the aqueous part of the solution and accumulate in the organic part of the solution. In this process, the organic phase and the aqueous phase of the mixture are mixed in a multistage separation process. alternately mixed and separated again, so that certain rare earth ions in the individual phases, ie the organic phase or the aqueous phase, increasingly concentrated until finally these ions are present in sufficient purity in one phase. In this case, up to 200 separation steps may be necessary. The rare earth metals thus separated are subsequently precipitated in a precipitation device 10 by adding 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 calcined in a calcination, for example in a continuous furnace, through which a hot air stream is passed. Thus, after this process step, a discrete rare earth oxide is present.

Usually now takes place an electrolysis step in which a salt of the discretely present rare earth metal on the

Melting point is heated and is electrolyzed or reduced for example by calcium. The cations of the rare earth metal salt migrate to the cathode where they separate out as elementary pure rare earth metal.

It is expedient here, the rare earth oxide, which is generated in the oxidation dationsvorrichtung 12, first in a

Iodine iodine device 14, ie to convert it into an iodide. In this case, for example, hot hydrogen iodide gas is injected into the rare earth oxide, whereby the oxygen in the oxide is replaced by the iodine. The rare earth is now either in pure form or as Ge ^¬ mixed with another iodide or another, the

Melting point-lowering salt, heated in the electrolytic cell 16 to above its melting point, wherein a

Rare earth iodide melt 21 is formed. In the

Rare earth iodide melt electrodes 23 are introduced and these are provided with direct current. This results in the electrolysis of the rare earth oxide melt 21, wherein the rare earth metal is deposited at the cathode metal. The The ions are also oxidized and rise as iodine gas 36 from the rare earth iodide melt 21. There, they mix with an inert gas 22 to an inert-iodine gas mixture 26, which is discharged from the electrolytic cell 16, in turn, new inert gas 22 is introduced into the electrolytic cell 16.

At regular intervals, the electrode 23, the cathode with the deposited rare earth metal 34 from the

Seltenerdiodidschmelze 21 are pulled out, wherein finally the rare earth metal 34 is separated from the anode 23 example, ^¬ by a melting process or by a mechanical method. Alternatively, the elemental rare earth element may be stripped from the electrode.

The use of iodine salts of rare earth metals has the advantage that they have a low melting point over other salts of rare earth metals. For example, the neodymium iodide has a melting point of only 787 ° C. In order to further lower the melting point of the electrolyte, that is the rare earth salt, it is possible to use a mixture with another salt, in particular with another low-melting iodide, in turn, in particular a eutectic mixture with another salt. These other salts, in particular iodides, must be more stable in reduction than the rare earth iodides, so that there is no deposition of other metals in the electrolysis. Thus, the more stable lithium iodide with cesium iodide forms a eutectic at 217 ° C or with potassium iodide at 286 ° C a eutectic. Thus, it is readily possible to mix a salt mixture with SE-iodides again, which have a melting point of less than 500 ° C. The usage of

Iodide compounds is therefore useful since

Iodide compounds have lower binding energies than alternative salts of rare earth metals, so that a lower melt bath voltage for the electrolysis is needed. Here ^¬ by the energy consumption during the electrolysis process is again greatly reduced itself. Here, it is expedient if a liquid electrolyte game, a mixture such as lithium iodide to cesium iodide or lithium iodide is present at ^¬ with potassium iodide in liquid form into the electrolytic cell. Subsequently, the iodide of the rare earth metal is mixed in this liquid electrolyte in the electrolysis cell 16. The electrolysis takes place, in particular of the rare earth iodide, wherein, as described, the rare earth metal ion migrates to the cathode where it is reduced to the rare earth metal. This is a continuous process, the other salts or iodides described merely serve as an electrolyte, which preferably remains inert during the electrolysis process, only the continuously added mixed rare earth iodide is separated by the electrolysis as described.

In addition to the reduction process of the rare earth and iodine gas produced per ^¬ but 36 exiting the Seltenerdiodidschmelze 21 and with an inert gas 22, the

Rare earth iodide melt 21 covered, mixed. This inert gas 22 is introduced into the electrolytic cell 16 at a temperature which is preferably not lower than the melting temperature of the rare earth iodide melt 21. In reference to the figure 2 shows an advantageous return ^¬ recovery of Iodgases and an energetically favorable sto ^¬ reitung the inert gas will be explained in the 22nd The hot

Inert gas 22, which preferably has a comparable temperature as the melting temperature of the rare earth oxide melt 21, is introduced into the electrolytic cell 16. There it mixes, as described, with the iodine gas 36 to a

Inert gas / iodine gas mixture 26. This inert gas / iodine gas mixture 26 is in turn discharged from the electrolytic cell 16 and cooled in a heat exchange process using a heat exchanger 28. In a further heat exchange process un ^¬ ter use of a cold trap 30, the iodine gas is deposited from the already abge ^¬ cooled inert gas-iodine gas mixture 26, which thereby condensed accordingly. For this second heat exchange process, the inert gas, now as an inert gas 22 'be ^¬ draws, discharged and introduced via a blower 32 in the heat exchanger 28 ^¬ . In this case, the inert gas 22 ', which is now clearly cooled after the two heat exchange processes 28 and 30, serves as a coolant for the hot

Inert gas-iodine gas mixture 26 and is heated up again. After this heating process as a coolant in the heat exchanger 28, the inert gas is referred to as inert gas 22 '' and ge ^¬ optionally with the influx of fresh inert gas 22 is again introduced into the electrolytic cell 16. In this case, under circum stances ^¬ the heating of the inert gas 22 in a

Inertgasstrombeheizung 24 take place, and the energy that has been lost in the heat exchange processes 28 and 30 and in the lines to be returned.

Overall, about 80% of the energy required for the heating of the inert gas 22 can be recovered by this described arrangement, at the same time the iodine can be recovered without further losses and is reintroduced for iodination of the rare earth oxide in the iodine 14.

Claims

claims A process for reducing rare earth oxides to rare earth metals (34), comprising the steps of iodinating a rare earth oxide to a rare earth iodide, heating the rare earth iodide to a temperature above a melting temperature Rare earth iodide melt (21), electrolysis of Seltenerdiodidschmelze (21) by depositing the Seltenerdme ^¬ talls (34) to an electrode (23). 2. The method according to claim 1, characterized in that the rare earth iodide is mixed with another iodide. 3. The method according to claim 2, characterized in that the iodides are mixed in a eutectic composition. 4. The method according to claim 2 or 3, characterized in that the other iodide is lithium iodide or potassium iodide. 5. The method according to any one of the preceding claims, characterized in that during the electrolysis by a mechanical stripping the rare earth metal is stripped from the cathode. 6. The method according to any one of the preceding claims, characterized in that during the electrolysis Tem ^{¬ an} increase in temperature of the rare earth oxide melt (21) beyond the melting point of the electrode deposited on the rare earth Seltenerdme- (34) addition, wherein the rare earth metal (34) of the Electrode (23) is melted off. 7. The method according to any one of claims 1 to 4, characterized in that the electrode (23) with the deposited rare earth metal (34) of the rare earth iodide melt (21) is withdrawn and the rare earth metal (34) from the electrode (23) au ^¬ ßerhalb the Seltenerdiodidschmelze (21) is removed. 8. The method according to any one of the preceding claims, characterized in that the rare earth iodide melt (21) is covered by an inert gas (22) and in the inert gas (22) iodine accumulates, which is released during the electrolysis in a Elekt ^¬ rolysezelle (16) , followed by cooling of the inert gas-iodine mixture (26), wherein subsequently the iodine from the inert gas-iodine gas mixture (26) by means of a cold trap (30) from the inert gas (22) is separated, characterized in that a in the Cold trap (30) cooled and separated from the iodine inert gas stream (22 ^λ ) through a heat exchanger (28) is passed, which serves to cool the inert gas-iodine gas mixture (26) after exiting an electrolysis cell (16).