Recovery, purification and elaboration of rare earth elements for NiMH battery recycling – REPUTER

The scientific and technical approach aimed to overcome the barriers to recycling Ni-MH batteries by first using simple and approved technologies (such as separation by grinding / sieving, solvent extraction, oxide conversion, or molten salt electrolysis) and by adapting and optimizing them to this battery technology, thanks in particular to the knowledge acquired by the partners through other projects and to the active collaboration between manufacturers and research centers. Breakthrough technologies have also been studied, making it possible to better exploit REE through the synthesis of materials with promising applications, particularly in the field of catalysis. Finally, a quantification of the technical, economic, and environmental aspects of the process was carried out.
The scientific program of the project was structured into five main tasks:
- Recovery of REE-rich fractions from Ni-MH batteries via thermal and physical processes followed by hydrometallurgical treatment;
- Separation of REE from other elements present in the batteries by solvent extraction;
- Conversion to REO for the purpose of preparing oxides or cermets;
- Preparation of metallic REE by pyrometallurgy in fluoride or molten chloride medium;
- Life cycle analysis and technical and economic study of the process.

The process chosen combines steps of thermal and physical treatment of the batteries in order to produce a powder (called black mass) containing these REE; hydrometallurgy in order to extract the elements of interest and finally the conversion of REE into oxides, metallo-ceramic hybrid materials (cermet) or even alloys of metallic REE. A first recovery path involves dissolving the black mass in an acid medium, followed either by selective precipitation of the REE or by a solvent extraction step leading to a concentrated solution of high purity grouped REE. Experimental data acquired by solvent extraction allowed modeling the chemical equilibria and proposing a process flowsheet. The resulting purified REE are then converted into oxides and then reduced to the form of a metal alloy by molten salt electrolysis. Another innovative way of recovery uses the patented WAR process which makes it possible to produce cermet particles with controlled morphology, starting from the total dissolution of the black mass, without additional steps for REE separation. These particles can be of great interest to the automotive or petrochemical sectors due to their potential catalytic performance. The final stage of the project made it possible to quantify the environmental and economic aspects of a process resulting from these technical developments. This quantification will allow industrial partners to assess the viability of the studied process as well as the prospects for improvement.

The synergy between the various partners made it possible to envision new promising prospects.
The work undertaken in the grinding, screening and hydrometallurgical treatment of NiMH batteries opens up several perspectives, in connection with the other axis of the project. The extraction of TR in a nitric acid medium by TODGA was validated and a process flowsheet was calculated using the PAREX + code. The recovery of Co and Ni through a commercial extractant system could equally be studied before or after the recovery of TRs.
A particularly innovative recovery route consists of dissolving the black mass in order to produce leachate rich in rare earths and nickel. Then, the use of a patented process allows producing cermet particles with controlled morphology, without additional separation steps. The characteristics of these cermets make them potentially attractive in catalysis.
Concerning the extraction of Ni by liquid cadmium, proposed as a direct route for recovering nickel from black mass, tests on a larger scale, and in particular Cd distillation tests, will make it possible to calculate the mass balances more precisely and thus confirm the interest of this direct recovery route, without going through hydrometallurgical stages.
Following the technical and economic analysis of the process steps considered to be the most mature, the prospects lie in a drastic simplification of the process, with an elementary hydrometallurgical component aimed at obtaining rare earth concentrate followed by a high added value cermet, in addition to the valorization of the major nickel and cobalt flow that was not examined during this study.

The scientific production of the project was threefold:
- A set of fundamental results, disseminated in numerous international conferences and scientific seminars; they mainly relate to solvent extraction studies of REE, their conversion into cermet, as well as the production of metallic REE by molten salt electrolysis;
- Two families of patents claiming a new class of REE extractants from acid solutions, as well as an analytical technique for the production of REE in the metallic state;
- Finally, numerous technical reports accessible for the time being only to project partners, but which may ultimately be more widely disseminated, summarizing the main part of the scientific work carried out within the framework of the project, as well as standardization reports, in particular for harmonizing the technical and economic calculations and life cycle analysis between the involved partners.

The REPUTER project aims at the development of an efficient, closed-loop and eco-conceived rare earth recycling and separation process from end-of-life rechargeable nickel-metal hydride (Ni-MH) batteries, starting from battery collection down to the formulation of rare earths as pure oxides or metals ready to be used in various industrial applications. Rare earth elements (REE) have become essential for our modern economy, being considered today as the most critical raw materials group with the highest supply risk. Despite this situation, the recovery of REE from Ni-MH batteries is almost non-existent (less than 1% of the REE were recycled in 2011), most of the rare earths present ending up diluted in the slags and their reuse value consequently reduced. This situation is often due to an inefficient collection and sorting process and of various technological difficulties related to REE recovery, extraction, separation and conversion to metals. Therefore, a large effort is needed for overcoming these difficulties and improving the recycling rates, in line with the goals of the EU’s Energy Roadmap 2050. In the same time, recycling activities need to be complemented with new efficient and robust environmentally-friendly separation technologies and with an expertise in the conversion of rare earth oxides into metals or alloys.

The objectives of this proposal are to:
(i) Reinforce through common objectives the expertise and complementary competences gained in France in hydrometallurgy (spent nuclear fuel reprocessing) and in pyrometallurgy (aluminum, sodium, zirconium industry);
(ii) Remove the scientific and technical barriers currently affecting the development of REE recycling, particularly by innovating in terms of dedicated hydrometallurgical and pyrometallurgical process efficiency and compactness;
(iii) Bring experimental data and evaluate the possibility to reach a sufficient purity (> 99.5%) of the recycled rare earths at a 10 to 100 gram scale in order to use these purified oxides or metals for industrial applications (catalytic materials, magnets and new batteries);
(iv) Evaluate the impact of the recycling process using a life cycle analysis and a technical-economic study, allowing an extrapolation of the process to higher flows and helping the potentially interested industrial companies making an informed decision about the possible commercialization of the process.

The work plan is structured into six major tasks (including project coordination). The first step covers the efficient recovery and sorting of REE-rich fractions from end-of-life Ni-MH batteries, via mechanic and thermal operations, followed by acid leaching. The second task will address the optimized extraction and separation of REE from Nickel and other transition elements present in batteries, using hydrometallurgy (liquid-liquid solvent extraction) leading to pure REE in solution. The solvent formulation will be optimized, particularly by designing and studying new selective extractant molecules allowing an efficient intra-REE separation in the presence of transition metals. The conversion of separated light REE (such as La and Ce) into oxides will be carried out in a third task, with the aim of developing ceramic oxide materials with interesting catalytic properties for further valorisation. The forth task is dedicated to the development of pyrometallurgical technologies for the conversion of RE oxides (particularly Nd and Pr) into high purity RE metals. Different types of pyrometallurgical processes mainly based on molten salt electrolysis (alkaline or alkaline-earth chlorides and fluorides) will be studied and optimized in order to propose a robust solution and reach the purity requirements for specific applications (for the NdFeB magnet industry in particular). The last task is dedicated to a life cycle analysis and technical-economic study of the processes.