1 Resource-related challenges in the context of circular economy and energy transition
The availability of metals and mineral resources is a critical factor in a healthy economy, especially one which is also striving to be a circular economy. For thermodynamic reasons, such an economy cannot be closed, i.e. in addition to secondary resources, primary resources always have to play a role. This is shown schematically in the lead figure. This illustrates how the Helmholtz Institute Freiberg for Resource Technology (HIF) (marked in blue) addresses the subject of the value chain from metallic and mineral raw materials through to the product, and subsequently to the end of the product‘s life cycle in the context of the circular economy.
Important aspects of this are the topics of exploration, processing and metallurgy, as well as the interdisciplinary topic of recycling, combined with a fundamental analytical understanding of raw materials and complex mathematical modeling in the field of geometallurgy. There is a clear trend that in both the primary raw materials (natural mineral phases in ores) and the secondary raw materials (synthetic phases in end-of-life products) the intergrowths, i.e. the size of the phases, are getting finer and finer. For the raw material processing, this presents the challenge of being able to separate the ultrafine particles in a resource-efficient and energy-efficient manner, so that the future is not only fine-sized, but also fine in the sense of “good”, based on the play on words used at the flotation conference in 2015 “The future will be fine” (Wei Sung Ng, University of Melbourne, @MEI Flotation‘15).
One process that has been successfully separating fine and ultrafine particles for more than 100 years is flotation , which in its essential characteristics was patented in Dresden in 1877 for the processing of graphite. For this reason, the HIF decided to place a particular focus on this aspect of minerals processing research, partly due to the fact that flotation research had almost come to a standstill in post-reunification Germany. In the following sections, the current topics being dealt with in this especially interesting field of research are discussed.
2 The status of flotation science
According to Heinrich Schubert‘s definition, flotation is a process engineering process that numbers among the heterocoagulation separation methods . It is the only mechanical separation process in which interfacial chemistry can influence the separation characteristic of wettability.
Currently, the leading international institutes focusing on flotation research are predominantly located in Australia, South Africa, Chile, Brazil, China, the USA and Canada. However, in Europe too, especially in Poland, France, Great Britain, Scandinavia and also Germany (Fig. 1), an increased activity in this field is taking place under the auspices of the European Research Framework Program Horizon 2020, and driven by the newly established Knowledge and Innovation Community (KIC), i.e. the EIT Raw Materials.
3 Topics of flotation research at the Helmholtz Institute Freiberg
Starting in April 2012, the HIF started developing the research infrastructure for flotation research almost from zero, even though two of the world‘s leading flotation researchers – Prof. Heinrich Schubert (1926 – 2018) up to 1990 and Dr. Hans Joachim Schulze (1938 – 2003) up to 2003 – had been active in Freiberg with their groups and had set essential accents. After six years of current progress, several main research areas can now be presented here.
3.1 Ultrafine particle flotation
Continuing the progress of findings regarding ultrafine particle flotation, M.Sc. Johanna Sygusch has been participating since the end of 2017 in the new HIF project MultiDimFlot, funded by the Deutsche Forschungsgemeinschaft (DFG), as part of the DFG Priority Program 2045 MehrDimPart, investigating the multidimensional separation of ultrafine particles < 10 μm according to wettability, size and shape. For this purpose, a new machine development is being employed - a combined turbulent rotor-stator machine that achieves froth fractionation in deep froths. This is embedded in a programme of fundamental investigations into the particle properties.
3.2 Complex polymetallic raw materials
As depicted in Fig. 2, this project involving the HIF, the TU Bergakademie Freiberg and the Freiberg company UVR-FIA GmbH, as well as the mining company Saxore and the RWTH Aachen, and also under participation of the EU joint project FAME, under the direction of the British mining consultancy Wardell Armstrong, is now being brought to a conclusion in Freiberg with pilot tests on a complex ore. In these pilot tests, flotation is an important multi-step process, since it involves not just finely-intergrown minerals, but additionally precious metals (especially tin, zinc and indium) that are bonded in both sulfidic and non-sulfidic minerals. In the final stage of the pilot studies, supervised by M.Sc. Edgar Schach, the HIF‘s new acquisition – a Flotation Mini Pilot Plant (Eriez) – which is unique in Europe, will be used.
3.3 Graphite flotation used in the recycling of lithium-ion batteries
In connection with the increasing importance of the field of mobile electronics (smartphones, tablets, etc.) and especially that of electromobility (bicycles, buses and cars) a great quantity of spent lithium-ion batteries (LIB) can be expected in future years. These products are truly polymetallic, complex and fine particulate secondary raw materials and contain strategically important metals such as lithium, cobalt, manganese and nickel, as well as large quantities of refined spherical graphite. Especially for the recycling of graphite prior to the generally pyrometallurgical utilization of the metallic components, there is still no efficient separation process, meaning that no recycling takes place.
In view of the fine to ultrafine particulate nature of the composites in the so-called black mass (fraction < 1 mm after crushing of the batteries) and owing to the surface properties of the composites, an appropriate field of study is the application of flotation for the fraction < 100 µm, which makes up half of the material. This research is aimed particularly at separating the metal-containing functional particles from the graphite, as depicted in Fig. 3. This still very new field of research, driven by the activities of M.Sc. Anna Vanderbruggen, has not only succeeded in proving that separation of these materials is feasible, but has also extended the complex raw material analysis developed for primary raw materials into this field of the very complex secondary stream of black mass, greatly improving our understanding of raw materials.
In the next three years, pioneering scientific publications as well as developments in recycling technology in cooperation with partners from German industry and science can be expected in this area.
3.4 Bio flotation
As part of the BMBF project “BS2, Bioflotation of Sulfides in Seawater”, M.Sc. Guillermo Luque Consuegra has been researching marine bacteria and their metabolites since 2017 for use as selective biodepressant for pyrite in the processing of Chilean Cu/Mo deposits using seawater. During this project, several bacteria have been identified that had previously not been investigated for this application.
3.5 Water management in the flotation process
In collaboration with the HIF Department for System Integrated Metal Production led by Prof. Markus Reuter and the Department for Modeling and Evaluation led by Prof. Gerald van den Boogaart, M.Sc. Bruno Michaux has been working on the integration of process water circuits in the simulation of flotation systems in ore processing plants . This is a highly topical field of flotation research. In collaboration with Outotec, the effectiveness of new modeling and simulation approaches has already been demonstrated, and currently the approach to large-scale industrial plants is being further developed and tested. This work will have the consequence that water as an important resource will be more sustainably used in the flotation process.
3.6 Basic research into mineral interfaces in flotation
It has already been possible to develop the CP-AFM so far that it can be used in principle as a screening tool for the in situ testing of new flotation reagents on the mineral of the relevant ore. Using this tool, we are able to undertake precise mapping of hydrophobic interactions in situ and with high spatial resolution. This has prompted great interest on the part of the specialty chemicals companies that are active in the flotation reagent market, since the development and testing of new reagent regimes has up to now been very expensive. In addition, the measurement results place us in the position of being able to improve the understanding of the hydrophobic properties of mineral surfaces.
Since 2017, the HIF has also been involved within the framework of the doctorate work of Dipl.-Ing. Hao Sheng Wu in fundamental interface-sensitive solid-state analysis in collaboration with the Department of Analytics at HIF and the HZDR Ion Beam Center. The aim of this research is to obtain better knowledge of cassiterite flotation and a better understanding of mineral surfaces in general.
Absolutely in the spirit of the English play on words “the future will be fine”, we see great challenges for the flotation process in the future but also enormous opportunities with the background of the energy transition and the circular economy. We are looking forward to being joined by many strong and courageous partners from industry and science, who will accompany us on this exciting journey.
Not only at the HIF, but at the HZDR as a whole, flotation research will continue to grow. At the latest, this became abundantly clear when the Department for Transport Processes at Interface Surfaces was established at the Institute of Fluid Dynamics of the HZDR under the direction of Prof. Dr. Kerstin Eckert. This department is researching extremely interesting topics in the hydrodynamics of the flotation process.
We hope that the process of flotation and its investigation will have an important international location in Freiberg and the surrounding area, just like it did in earlier years.