WO2018114925A1

WO2018114925A1 - Electrochemical production of rare earth alloys and metals comprising a submerged liquid anode

Info

Publication number: WO2018114925A1
Application number: PCT/EP2017/083504
Authority: WO
Inventors: Arne Petter Ratvik; Ana Maria Martinez CUELLAR; Karen Sende OSEN; Bjarte Arne Øye; ASBJøRN SOLHEIM
Original assignee: Sintef Tto As
Priority date: 2016-12-21
Filing date: 2017-12-19
Publication date: 2018-06-28
Also published as: EP3339480A1; EP3339480B1

Abstract

The present invention discloses an electrochemical method of extracting rare earth (RE) elements from alloys containing RE elements in an electrolysis cell comprising an anode compartment located below a submerged liquid anode connected to a positive electric lead, wherein a submerged cathode connected to a negative electric lead is collecting the extracted RE element(s), the anode and cathode being separated by a fluoride based liquid electrolyte.

Description

Electrochemical production of rare earth alloys and metals comprising a

submerged liquid anode

The work leading to this invention has received funding from the European Union's Horizon 2020 and Innovation program under Grant Agreement No.

680507.

FIELD OF THE INVENTION The present invention is related to electrochemical extraction of rare earth alloys and metals from alloys containing rare earth elements, e.g. from end of life products, in a one-step electrochemical extraction method comprising a

submerged liquid anode with high capacity for dissolving alloys containing rare earth element(s), especially rare earth element(s) alloyed with at least one transition group element in the periodic table.

BACKGROUND OF THE INVENTION

Rare earth alloys and metals are important ingredients in modern electronic components like semiconductors and display screens, but also in products like permanent magnets, renewable energy, etc. The Peoples Republic of China is dominating the production of rare earth elements, and for example, in 2011 the Chinese production covered 97 % of the world market. In addition, the geological availability and distribution of rear earth elements are distributed unevenly around the world and it is therefore an international interest in developing alternative second sources of rare earth materials mitigating problems related to price fluctuations and reliable and sustainable delivery of rare earth alloys and metals. This situation has triggered development of methods and systems for recovering rare earth alloys and metals from scrap metals and permanent magnets. A general overview of prior art techniques providing recycling of permanent magnets can be found in the article "Technique for recovering rare-earth metals from spent sintered Nd-Fe-B magnets without external heating" by R. Sasai and N. Shimamura, Journal of Asian Ceramic Societies, 4 (2016) 155-158. A common technique in prior art when recycling used permanent magnets and scrap metals comprises a step of oxidizing the permanent magnet and scrap metal materials followed by purifying and separation steps before electrowinning of the rare earth oxide material(s) dissolved in a molten salt.

An example of prior art is CN 103409649B disclosing a metallothermic process for the production of rare earth element(s), the method comprising addition of rare earth salt(s) to a liquid molten salt in contact with a liquid metal alloy, the alloy comprising liquid aluminium and lithium, lithium being reduced from the molten salt in an electrochemical process with a carbon anode prior to

introducing the rare earth salt(s) to the molten salt, the lithium in the liquid aluminium alloy gives a metallothermic reduction of the rare earth(s) in the molten salt.

Prior art electrochemical refining of metals is applied both in aqueous and molten salt electrochemical processes. The electrochemical refining is commonly used for metals containing impurities and rarely employed for alloys. The refining can be achieved by using a solid metal anode with minor amounts of impurities or a liquid alloy anode in which the metal with (minor) impurities are dissolved . Using direct current, the metal in the anode, subject to refining, is transported through the electrolyte and deposited as a pure metal on the cathode, either in liquid or solid form. More noble impurities remain in the anode or anode compartment while less noble elements accumulate in the electrolyte.

An example of prior art for refining of a metal is the three-layer refining of liquid aluminium dissolved in a copper containing alloy invented by Hoopes and patented in 1925. The refining process takes place in a vertically arranged molten salt cell, wherein relatively pure aluminium from the Hall-Heroult process for electrowinning of aluminium are dissolved in the copper alloy at the bottom of the cell, and the refined high purity aluminium is deposited as a liquid aluminium cathode floating on the top of the electrolyte. Another example of prior art is J. Lucas, P. Lucas, T. Le Mercier, A. Rollat and W. Davenport, in "Rare Earths. Science, Technology, Production and Use", Elsevier 2015. A further prior art reference is S. Pang, S. Yan, Z. Li, D. Chen, L. Xu and B.

Zhao, "Development on Molten Salt Electrolytic Methods and Technology for Preparing Rare Earth Metals and Alloys in China", Chinese Journal of Rare Metals, 35(3) (2011) 440-450. Recovering rare earth alloys and metals from a specific electrochemical process is also subject to cost/benefit assessments in addition to environmental considerations. The costs of recovered rare earth alloys and metals have to be on a level accepted by end users of the recovered materials. Therefore, there is a need of a cost effective and direct method for recovering rare earth alloys not subject to traditional electrochemical refining to remove inherent impurities.

An aspect of the present invention is to reduce the number of process steps, and at the same time maximise output of RE element(s), by recycling of RE elements contained in alloys with non-RE elements, e.g. permanent magnets alloyed with at least one transition element.

The present invention is based on a submerged liquid alloy anode system with a high capacity for dissolving commonly used RE alloys subject to recycling .

OBJECT OF THE INVENTION

In particular, it may be seen as an object of the present invention to provide a method of recycling RE containing permanent magnets and/or scrap metals alloyed with RE elements, providing a submerged liquid alloy anode containing dissolved RE alloyed metals to be recycled in an electrochemical process in one step. Commonly, these alloys contain less than 40 % RE elements where the prior art methods proposed for electrochemical refining relates to removing inherent impurities, typically in the range of less than 1 %, to yield RE metals of high purity. It is a further object of the present invention to provide an alternative to the prior art method for extracting rare earth elements from alloys. SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of extracting RE elements alloyed with at least one transition element, exemplified by permanent magnets and/or magnet swarf, in an electrochemical process with a fluoride based molten salt electrolyte, comprising the steps of:

- arranging an electrolysis cell comprising a submerged liquid alloy anode with dissolved RE alloy(s), contained in an anode compartment, connected to a positive electric lead, and a submerged cathode connected to a negative electric lead for recovering the RE element(s), the anode and cathode being separated by a fluoride based liquid electrolyte,

- the liquid anode comprises Al and Si in quantities providing a liquid domain with the alloy(s) with RE element(s) to be electrochemically extracted,

- the composition of Al and Si is further selected to provide a low melting temperature region being able to dissolve high quantities of transition element(s), contained in the RE(s) magnet alloys.

DESCRIPTION OF THE FIGURES

The method and system according to the present invention will now be described in more detail with reference to the accompanying figures. The accompanying figures illustrates an example of embodiment of the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 illustrates some aspects of the present invention.

Figure 2 illustrates further aspects of the present invention. Figure 3 illustrates an example of embodiment of the present invention. DETAILED DESCRIPTION OF AN EMBODIMENT

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. The mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention . Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous. The known technology used in China relies on an electrolytic process using a vertically arranged cell comprising consumable carbon anodes and molybdenum or tungsten as inert or iron as consumable cathode materials. The RE is deposited in a liquid form at a temperature around 1050 °C. In case an iron cathode is used, the deposited RE element usually dissolve iron to form a liquid RE-iron alloy. The electrolyte usually consists of an equimolar REF3- L1 F mixture, and the RE2O3 (rare earth oxide) raw material is fed batch wise or continuously to the top of the electrolyte where it dissolves into the electrolyte as oxyfluoride species. A parameter of interest when designing an electrolysis cell is the kinetics of the electrodes. It is believed that liquid anodes perform better in this respect than an anode consisting of solid chunks of alloys when refining RE elements.

An example of a prior art liquid anode is the referenced Hoopes method. Hoopes method is designed to produce high purity metals where the starting point is a relatively pure metal with inherent impurities from extraction process,

exemplified by refining of aluminium with a purity well above 99 % produced in the conventional Hall-Heroult process, thus reducing accumulation of more noble impurities in the anode and less noble elements contaminating the electrolyte. Applying Hoopes three layer methodology in electrochemical extraction of RE element from alloys, e.g . used permanent magnets, will not be possible due to the high specific densities of the RE alloys and alloying elements and the fact that applicable fluoride electrolytes with an intermediate density between the density of the extracted RE element(s) and recycled alloys containing RE element(s) does not exist.

The present invention offers a different cell design with a submerged liquid alloy anode with a high solubility for RE alloys as schematically illustrated in Figure 1. The RE containing alloy is placed in the anode compartment 10 from where the RE element(s) present in the raw material (for example Nd, Dy, Pr) will be anodically dissolved in the form of ions, which will be discharged at the cathode 11 as metal(s) 12. The recovery of REE(s) from the RE alloy material(s) can be extracted, and a valuable RE product can be obtained in one single

electrochemical step.

It is not desirable to work at a high temperature providing a liquid state of the RE alloy(s), e.g . permanent magnets, as the melting point of these alloys are above 1400 °C. The high temperature is a challenge since the high temperature enhance corrosion of the cell materials and evaporation from the electrolyte. Therefore, according to an aspect of the present invention, the desired working temperature is below 1100 °C. Providing a lower melting point, when recycling permanent magnets and/or scrap RE alloys comprising Fe, can be achieved by adding low melting point materials known to achieve such an effect. For example, with respect to a permanent magnet comprising Nd, it is known that Cu forms low melting phases with Nd, but not with Fe. However, using Al the inventors has demonstrated that Al forms low melting areas with Nd and Fe in the Al rich regions. Calculating a ternary phase diagram of Al-Nd-Fe can be achieved with commercially available thermodynamic computer programs like FactSage as known in prior art. The same demonstration has been performed with other RE elements with similar results. However, there are further aspects to take into consideration when arranging a liquid alloy anode forming a multinary liquid alloy system that fulfils all requirements necessary for an electrochemical process providing necessary efficiency and output of recovered RE element(s) from RE alloy(s) comprising significant amounts of transition metal(s), e.g. Fe.

Examples of further requirements are:

• Anode forming a liquid metal phase comprising at least the RE.

• Providing a melting point of the anode alloy comprising the dissolved RE to be 1100 °C or lower.

• Low vapour pressure at the working temperature, for example at 1050 °C.

• Low cost, non-toxic and abundant materials.

It is further known that Si forms low melting point alloys with several elements like Cu and Al . When calculating and verifying the calculations in a laboratory test of a phase diagram of Fe-AI-Si, it is possible to observe that the phase diagram reveals a large liquid region with high contents of Fe in the three component phase diagram with Al and Si existing below a temperature of 1100 C. Further, it is also evident from such calculations and laboratory verifications that the liquid content of Fe at 1050 °C varies from 20 wt% without Si present to about 50 wt% when Si is present. Further, Al-Si alloys are commercially available, which is an important aspect when considering

commercial applications of a liquid anode according to the present invention. Figure 2 illustrates examples of how liquidus curves of the quaternary system of Al-Si-Fe-Nd can be obtained. The same type of illustration is valid for other RE elements. The cross sections of the AI-Si-Fe-RE system from contact lines between the AISi corner and the Fe-RE (for example Fe-Nd) side of the triangle in Figure 2 will provide liquidus curves enabling a prediction of melting points of the anode composition when the electrolysis proceeds and the content of RE element(s) decreases. It can also be used to determine the maximum amount of the transition element(s), e.g . Fe, that can be added with the alloy before the liquid anode has to be replenished or renewed . Higher concentration of Si will for example increase the amount of Fe-RE in the liquid phase at 1050 °C when there is a rich Fe composition. If the concentration of RE is high the opposite is observed . However, verification of this aspect of the present invention confirms a liquid phase with more than 30 wt% of permanent magnet material and scrap metal comprising Fe and RE at 1050 °C.

Table 1 below illustrates the aspects of some possible anode-alloy compositions when recycling a permanent magnet comprising Fe and RE. Table 1 :

The liquid anode is forming a multinary liquid alloy system having a large liquid domain for the RE alloys, with a high capacity for containing transition elements used in RE alloys. Further, the multinary liquid alloy system is provided with metal elements being more noble than the RE element(s) to be extracted in the electrochemical process.

According to an example of embodiment of the present, a method of extracting rare earth (RE) elements from alloys containing transition element(s), permanent magnets and/or magnet swarf, in an electrochemical process with a fluoride based molten salt electrolyte, comprises the steps of:

Further, the transition element(s) is Fe.

Further, the liquid anode comprises added Al and Si in quantities providing a liquid domain below 1100 °C with high solubility for alloys containing RE elements, e.g . permanent magnets.

Further, the added specific amounts of respectively Al and Si elements is forming a multinary liquid alloy system with the RE alloys, having a working temperature of the electrochemical cell below 1100 °C. Further, the step of dissolving recycled RE alloys, e.g. permanent magnet materials, may provide a liquid anode comprising AI-Si-Fe-RE-B.

Further, the permanent magnets may be Nd based permanent magnets. Further, the RE containing permanent magnets and/or scrap metals may be delivered into the liquid anode compartment from a feeding chamber.

Figure 3 illustrates an example of principles of a functional cell according to the present invention. A tube or canal 20 provides transport of waste material to be recycled into the molten alloy being part of the liquid anode 21 residing in a compartment. An electric lead 22 is connected to a positive electric pole 23 of the power supply. The electric lead 22 is connected to a finger like electrode configuration being arranged inside the molten alloy. The cathode 24 is connected to the negative pole of the power supply and at the bottom of the cell below the cathode a compartment is arranged receiving cathode products in case the cathode product is a liquid metal or alloy.

Claims

Claims:

1. A method of extracting rare earth (RE) elements from alloys containing transition element(s), permanent magnets and/or magnet swarf, in an electrochemical process with a fluoride based molten salt electrolyte, comprising the steps of:

2. The method according to claim 1, wherein the transition element(s) is Fe.

3. The method according to claim 1, wherein the composition of the liquid anode allows a working temperature of the electrochemical cell below 1100 °C.

4. The method according to any claim 1-3, wherein the step of dissolving permanent magnetic material is providing a liquid alloy anode comprising AI-Si-Fe-RE-B.

5. The method according to any claim 1-4, wherein the permanent magnets are Nd based permanent magnets.

6. The method according to claim 1, wherein anode feed materials are

delivered into the liquid anode compartment from a feeding chamber.

7. An electrochemical production cell arranged to support a method according to any claims 1-6.