A Novel Pyrometallurgical Recycling Process for Lithium-Ion Batteries and Its Application to the Recycling of LCO and LFP

Article A Novel Pyrometallurgical Recycling Process for Lithium-Ion Batteries and Its Application to the Recycling of LCO and LFP

Alexandra Holzer * , Stefan Windisch-Kern , Christoph Ponak and Harald Raupenstrauch

Chair of Thermal Processing Technology, Montanuniversitaet Leoben, Franz-Josef-Strasse 18, 8700 Leoben, Austria; [email protected] (S.W.-K.); [email protected] (C.P.); [email protected] (H.R.) * Correspondence: [email protected]; Tel.: +43-3842-402-5803

Abstract: The bottleneck of recycling chains for spent lithium-ion batteries (LIBs) is the recovery of valuable metals from the black matter that remains after dismantling and deactivation in pre- treatment processes, which has to be treated in a subsequent step with pyrometallurgical and/or hydrometallurgical methods. In the course of this paper, investigations in a heating microscope were conducted to determine the high-temperature behavior of the cathode materials lithium cobalt oxide

(LCO—chem., LiCoO2) and lithium iron phosphate (LFP—chem., LiFePO4) from LIB with carbon addition. For the purpose of continuous process development of a novel pyrometallurgical recycling process and adaptation of this to the requirements of the LIB material, two different reactor designs

were examined. When treating LCO in an Al2O3 crucible, lithium could be removed at a rate of 76% via the gas stream, which is directly and purely available for further processing. In contrast, a removal rate of lithium of up to 97% was achieved in an MgO crucible. In addition, the basic capability of the concept for the treatment of LFP was investigated whereby a phosphorus removal rate of 64% with a simultaneous lithium removal rate of 68% was observed.

Keywords: lithium-ion batteries (LIBs); recycling; pyrometallurgy; critical raw materials; lithium removal; phosphorous removal; recovery of valuable metals Citation: Holzer, A.; Windisch-Kern, S.; Ponak, C.; Raupenstrauch, H. A Novel Pyrometallurgical Recycling Process for Lithium-Ion Batteries and 1. Introduction Its Application to the Recycling of LCO and LFP. Metals 2021, 11, 149. The development of lithium-ion batteries (LIBs) has experienced an enormous up- https://doi.org/10.3390/met11010149 swing in recent years, which is, in addition to portable devices, mainly due to the steadily increasing demand in the electric vehicle (EV) sector. According to forecasts, this trend Received: 11 December 2020 will continue in the coming years [1,2]. Further prognoses predict that sales of LIBs are Accepted: 11 January 2021 expected to increase from 160 GWh in 2018 to over 1.2 TWh in 2030 [1]. Their use in electri- Published: 14 January 2021 cal appliances, EVs and stationary storage is due to their advantages over other storage media, such as high energy density, long service life and high operating voltage [3,4]. Since Publisher’s Note: MDPI stays neu- consumed LIBs contain a large number of valuable metals, recycling has a considerable tral with regard to jurisdictional clai- environmental impact in view of the conservation of valuable resources [5]. In addition to ms in published maps and institutio- this idea of resource protection, waste reduction and the energy-efficient and economical nal affiliations. treatment of hazardous substances are also driving recycling efforts [6]. The timeliness and necessity of recycling LIBs is further underlined by the 2020 list of critical raw materials published by the European Commission. Among others, cobalt, lithium and phosphorus can be found [7]. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. A major challenge with regard to recycling is posed by the strongly fluctuating waste This article is an open access article stream. This is the product of the requirements of the countless applications for energy distributed under the terms and con- storage and the resulting multitude of electrode materials of LIBs [8]. In the respective ditions of the Creative Commons At- literature there is a variety of different recycling processes, which can basically be divided tribution (CC BY) license (https:// into preparation for recycling, pre-treatment and main processing, including pyro- and creativecommons.org/licenses/by/ hydro-metallurgy. In the first mentioned area, the processes of discharging and dismantling 4.0/). can be found [5]. The aim of the pre-treatment is to improve the recovery rate, to adapt

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the waste stream to the downstream process step and to reduce the energy consumption of the following pyro- or hydro-metallurgical process [6,9]. In Europe, there are several companies that already perform the preparation and pre-treatment of spent LIBs on a larger scale, like Accurec Recycling GmbH, Duesenfeld GmbH or Redux GmbH [10–12]. The latter starts the recycling process with collection and temporary storage, followed by manual sorting. As of this point in time there is still a considerable safety risk due to the residual charge of the LIBs. They are completely discharged, and the energy gained is fed back into the operating network. Subsequently, components such as electronics, cables, plastics, aluminum, and iron are dismantled and sorted. During the subsequent deactivation, the coating of the conductor foils is dissolved and the separator as well as the electrolyte are removed. During the mechanical treatment, the remaining components such as iron, aluminum, copper and the fine material (also called active material or black matter) of cathode and anode material are separated. The separation of the individual fractions is carried out with a magnetic separator, air separator and sieving [13]. The resulting black matter can be further treated in a pyro- or hydro-metallurgical process. In pyrometallurgical treatment of LIBs, the physiochemical transformation temperatures above 1400 ◦C are used to recover the valuable metals [14]. As a partial step in an overall process, pyrometallurgy is a suitable instrument for purifying the feed stream of substances undesirable for hydrometallurgy. Fluorine, chlorine, graphite, phosphorus, etc., pose a particular challenge to hydrometallurgy. Pyrometallurgical processes are generally robust against impurities and organic contaminants, because volatile components can be evaporated [5]. Graphite from the anode can be used as a reducing agent and burned in various processes in the presence of oxygen, thus helping to maintain the process temperature. Since the reaction kinetics in pyrometallurgical processes increase extremely due to the high temperatures, productivity is higher compared to hydrometallurgy [15]. Although the large number of research activities in recent years has focused on hydrometallurgy [9], there is significant scientific output in the field of pyrometallurgy, some of which is already being applied on an industrial scale. Several recent reports claim that large-scale pyrometallurgical processes have greater potentials in terms of sustainability than their hydrometallurgical counterparts [16–21]. Industrial scale processes are those that have more than 1000 t/a recycling capacity. In Europe, the companies Umicore, Ac- curec and Nickelhütte Aue should be mentioned here, and outside the EU, for example, SungEel, Kyoei Seiko and Dowa. The overall processes usually lead via a mechanical and/or thermal step to pyro- and hydro-metallurgy [5]. The pyrometallurgical step is typically based on shaft furnaces or electric arc furnaces for melting this feedstock [22]. A direct comparison of the recycling efficiency of the individual processes is often very difficult, since the reference basis of the values given is usually not given or only partially given. However, it can be stated that recycling routes which include a pyrometallurgical step have the highest overall recycling efficiency, in some cases exceeding 50% [5]. Since pyrometallurgical processes are operated at high temperatures, their energy requirements are correspondingly high. In addition, large quantities of waste gas are produced which have to be treated. A disadvantage of current pyrometallurgical processes is the slagging of lithium, the recovery of which in turn requires an enormous hydrometallurgical effort [9,23]. The economic efficiency of lithium recovery depends on the concentration in the slag. As a rule, in the co-processing of LIBs in metallurgical plants, the lithium is diluted to such an extent that recovery is not economically feasible [24]. In recent years, a number of advances have been made in the field of slag post-treatment. These research ventures on a non-industrial scale focus, for example, on the concentration of Li in the slag by selective addition of slag-forming agents during the pyrometallurgical process and subsequent hydrometallurgical treatment [25,26]. Recent progress has also been made in the area of early-stage lithium extraction. In this process, sulphate roasting treatment was used to convert the cathode material from NMC batteries into a water-soluble lithium sulphate (Li2SO4) and a water-insoluble oxide (NiCoMn-oxide) [5]. However, depending on the price of lithium, processes specially developed for LIB recycling may in future be Metals 2021, 11, 149 3 of 22

quite economical in terms of lithium recovery [24]. Various advantages and disadvantages also result from the different interconnection types of the overall process. For example, the primary energy consumption via pyrometallurgical routes is higher, but the resulting additional costs are more than compensated by lower operating costs in the hydrometallurgical step [5]. The recycling of P from LIBs is described in the literature in very few publications. Most of them are related to the hydrometallurgical process route, other processes deal with the regeneration of the cathode material [27]. Hydrometallurgical processes are highly selective and can therefore achieve high purities [15]. Leaching is the key process in hydrometallurgy to convert the metals to ions in a solution. This can be divided into bio leaching with metabolic excrements of microorganisms or fungi and chemical leaching with organic or inorganic acids [28–30]. Subsequently, the valuable metals are separated and recovered from the leaching solution. Since the structure of the leaching solution is complicated, it is usually necessary to use several different methods from the portfolio of solvent extraction, chemical precipitation and electrochemical deposition [28]. Hydrometallurgical methods result in extremely good recycling rates of up to 100% [28,31]. They also require a high level of equipment and a large number of process steps, which usually results in a correspondingly high volume of polluted wastewater. In order to operate the process economically, it is very important to separate and concentrate as many metals and impurities as possible in advance. For each additional metal, at least 1–2 additional process steps would be required, which is only economical if the metal value or quantity is correspondingly high [15]. Especially with regard to the raw materials contained in LIBs, which are included in the list of critical raw materials of the European Commission [7], and from an ecological point of view, a sustainable handling of spent LIBs is essential. According to Elwert et al. [32], recycling processes specialized in LIBs will gain more and more importance in the future. This is due to the increasing rate of return of spent LIBs to the waste stream, more regulations by the authorities and also decreasing amounts of valuable nickel and cobalt for direct use in nickel and cobalt producing plants. Furthermore, the growing market for LFP and the increasing interest in lithium recovery also plays a major role. Of particular importance in terms of regulation is the recently published European Commission proposal to revise EU Directive 2006/66/EC, which sets recovery rates of up to 70% for Li and 95% for other valuable metals such as Co, Ni and Cu by 2030 [33], which forces recyclers to increase recovery rates and their process efficiency. It can be summarized that there is a multitude of different recycling processes and methods, which are characterized by their positive properties in certain areas but also have individual disadvantages. In the field of pyrometallurgy, lithium slagging and in particular the absence of possibilities to recover from the slag with reasonable effort can be identified as a bottleneck. The novel pyrometallurgical recycling process presented in this paper is characterized by the recovery of an alloy with a simultaneous utilization of lithium and phosphorus via the gas flow. The following points provide a more detailed insight into the theoretical considerations and practical implementations for the most efficient recovery of valuable metals from LIBs using this process. Initially, appropriate analyses were carried out to better understand the behavior of cathode materials in high-temperature applications under reducing conditions. To determine the lithium removal rate without the presence of phosphorus, the cathode material lithium cobalt oxide (LCO) was examined in an experiment. In addition to the successive optimization of the reactor concept and adaptation to the waste stream from spent LIBs, another experiment with LCO in a modified setup was performed and compared to the previous one. To verify the basic suitability of the pyrometallurgical apparatus for the simultaneous removal of phosphorus and lithium via the gas flow, experiments were carried out with the cathode material lithium iron phosphate (LFP). Metals 2021, 11, 149 4 of 22

2. Process Concept and Methods 2.1. Used Materials In total, three different experiments were carried out with two types of feedstock. As Windisch-Kern et al. [34] have already described, experiments with black mass from a pre- processing step have already shown that lithium could be removed to a considerable extent. As this has raised additional questions, a detailed investigation of the pure cathode materials, i.e., LCO, lithium nickel manganese cobalt oxide (NMC—chem., LiNi0.33Mn0.33Co0.33O2), lithium nickel cobalt aluminum oxide (NCA—chem., LiNi0.8Co0.15Al0.05O2) and LFP, was indispensable. For the purpose of clarifying the questions dealt with in this paper, the materials LCO and LFP were used, which were produced by the Chinese company Gelon Energy Corporation. The appearance of this feedstock can be described as a ﬁne, black powder. Since this carbo-thermal process requires a reducing agent and the graphite bed in the reactor is only used for energy input, graphite powder from coke pellets of a steel mill is added. The graphite cubes with a side length of 2.5 cm come from electrodes of a steel mill and have an average purity of 99% with an electrical resistance of 4–8 µ Ωm and a density of 1.55–1.75 g cm−3.[35] The amount of graphite powder required for the reduction was determined by stoichiometry of the respective cathode material. For this purpose, the weighed mass of the cathode material was multiplied by the molar ratio of LiCoO2 or LiFePO4. After determining the moles O by multiplying the mass O by the relative atomic mass of O, the necessary mass of C was calculated by using the relative mass of C and assuming that a conversion to CO takes place in the reactor. The corresponding percentage C requirement is ﬁnally obtained by a rule of three of the masses of O and C. Table1 shows the composition of the input materials determined from their stoichiometric composition.

Table 1. Composition of the mixture of cathode material and graphite powder in wt.%.

Compound Li Co Fe P C LCO-C 5.67 48.17 - - 20.00 LFP-C 3.34 - 26.90 14.92 24.00

The products obtained from the experiments were examined by ICP-OES and ICP-MS by means of aqua regia digestion according to ÖNORM EN 13657:2002-12.

2.2. Material Specific Investigations Since the behavior of the individual cathode materials at high temperature applications is hardly or not at all described in the literature, detailed investigations were undertaken at the Chair of Thermal Processing Technology. These included analyses in a Hesse Instruments EM 201 with an HR18-1750/30 furnace heating microscope. The results should be used for planning the process control in the following experiments in the inductively heated reactor, which is presented in Section 2.3. Furthermore, a better understanding of the behavior of the cathode materials should be gained. To be able to simulate the planned process as detailed as possible, graphite powder was added to the cathode material. The addition of graphite powder was carried out to an extent of 10 wt.% under the assumption that C is converted to CO2 and transported away via the argon-purged atmosphere. The mixture of the corresponding cathode material and graphite powder was examined with at least one reproduction experiment. For better comparability a uniform heating rate was always set, which corresponds to the maximum possible with the heating microscope used. This is primarily to ensure the shortest possible residence time in the furnace chamber since interactions of LCO with the furnace material consisting of Al2O3 have been determined and damage to this should be prevented as far as possible. Up to a temperature of 1350 ◦C, a heating rate of 80 ◦C/min was selected, from 1350 to 1450 ◦C 50 ◦C/min and up to 1700 ◦C a heating rate of 10 ◦C/min with a holding time of 15 min at 1700 ◦C was dialed. To avoid oxidation with the ambient air, the reactor was flushed with argon at a flow rate Metals 2021, 11, x FOR PEER REVIEW 5 of 22 Metals 2021, 11, 149 5 of 22

argon at a flow rate of 2 L/min. A maximum furnace temperature of 1700 °C was chosen, of 2 L/min. A maximum furnace temperature of 1700 ◦C was chosen, which allows for an which allows for an approximate sample temperature of 1630 °C. approximate sample temperature of 1630 ◦C. Figure 1 illustrates the standardized sample preparation. The material is centrally Figure1 illustrates the standardized sample preparation. The material is centrally positioned in a cylinder with a diameter of 3.5 mm and a height of 2.5 mm on an Al2O3 positioned in a cylinder with a diameter of 3.5 mm and a height of 2.5 mm on an Al2O3 plateletplatelet withwith anan approximateapproximate weightweight ofof 0.10.1 g.g.

Figure 1. Structure of the sample on the white sample plate made of Al O before the trial. Figure 1. Structure of the sample on the white sample plate made of Al22O33 before the trial. 2.3. Reactor Concept 2.3. Reactor Concept The novel reactor concept, which was constructed at the Chair of Thermal Processing TechnologyThe novel of the reactor Montanuniversitaet concept, which Leoben, was constr is baseducted on at thethe inductiveChair of Thermal heating ofProcessing graphite piecesTechnology in a packed of the Montanuniversitaet bed reactor. The cornerstone Leoben, is based of knowledge on the inductive generation heating in thisof graph- field wasite pieces laid atin thea packed chair alreadybed reactor. in 2012, The cornerstone by the EU subsidizedof knowledge project generation RecoPhos in this for field the pyrometallurgicalwas laid at the chair treatment already of in sewage 2012, by sludge the EU ash subsidized for simultaneous project recoveryRecoPhos of for phosphorus the pyro- andmetallurgical the contained treatment valuable of sewage metals. sludge Its results ash arefor describedsimultaneous by Schönberg recovery of et phosphorus al. [36] and Samieiand the et contained al. [37]. Based valuable on this metals. and correspondingIts results are described follow-up by projects, Schönberg also inet al. the [36] field and of basicSamiei oxygen et al. [37]. furnace Based slag on (BOFS) this and treatment, correspon ading batch follow-up operated projects, post-lab-scale also in plant the field and of a pilot-scalebasic oxygen plant furnace as a continuous slag (BOFS) process treatment, have a been batch developed operated andpost-lab-scale built. This knowledgeplant and a advantagepilot-scale plant was used as a tocontinuous adapt the process mechanism have forbeen pyrometallurgical developed and built. recovery This ofknowledge valuable metalsadvantage from was processed used to LIB adapt material the mechanism in two ways. for Onpyrometallurgical the one hand, the recovery theoretical of valuable idea of themetals continuous from processed reactor, LIB which material should in be two conceptually ways. On the similar one hand, to the the set-up theoretical from research idea of workthe continuous in the field reactor, of sewage which sludge should ash andbe concep BOFStually utilization, similar is applied.to the set-up On the from other research hand, thework post-lab-scale in the field of setup sewage developed sludge ash in and the BOFS subject utilization, area mentioned is applied. above On canthe other initially hand, be usedthe post-lab-scale for first experiments setup developed without furtherin the subject adaptations. area mentioned In the long above term, can the realizationinitially be ofused larger for first scales experiments and corresponding without further throughput adapta oftions. recycled In the material long term, as a the continuous realization unit of islarger planned. scales Intensive and corresponding research activities throughput on a of small recycled scale material are indispensable as a continuous for the unit most is efficientplanned. implementation Intensive research of gradual activities scale-ups on a small to industrial scale are maturity.indispensable In view for ofthe the most process effi- developmentcient implementation as well asof thegradual knowledge scale-ups gained to industrial about the maturity. input material, In view the of the previously process mentioneddevelopment apparatus as well as in the batch knowledge operation, gained the so-called about the InduMelt input material, plant, is the used previously for this purpose.mentioned These apparatus two process in batch concepts operation, and theirthe so-called respective InduMelt challenges plant, and is developments used for this arepurpose. explained These in two detail process below. concepts and their respective challenges and developments are explained in detail below. 2.3.1. Continuous Reactor Concept 2.3.1.In Continuous order to treat Reactor the expected Concept future waste stream from used LIBs, a technology with correspondinglyIn order to treat high the throughput expected future rates iswaste required. stream The from currently used LIBs, pursued a technology approach with at thecorrespondingly Chair of Thermal high Processingthroughput Technology rates is required. is based The on currently a continuous pursued reactor approach concept at whichthe Chair currently of Thermal exists asProcessing a pilot plant Technology with a material is based throughput on a continuous rate of 10 reactor kg/h. Evenconcept if, according to initial findings from investigations of the LIB black matter, the design must which currently exists as a pilot plant with a material throughput rate of 10 kg/h. Even if, differ from that used for sewage sludge ash and BOFS, the basic principle remains the according to initial findings from investigations of the LIB black matter, the design must same. Ponak [38] describes the so-called InduRed reactor as an cylindrical arrangement of differ from that used for sewage sludge ash and BOFS, the basic principle remains the refractory materials filled with pieces of electrode graphite which allow a horizontal and same. Ponak [38] describes the so-called InduRed reactor as an cylindrical arrangement of radial homogeneous temperature distribution when heated by the induction coils, as seen refractory materials filled with pieces of electrode graphite which allow a horizontal and in Figure2. radial homogeneous temperature distribution when heated by the induction coils, as seen in Figure 2.

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Figure 2. Schematic illustrationillustration ofof thethe InduRed InduRed reactor reactor for for the the continuous continuous treatment treatment of of sewage sewage sludge sludgeashes and ashes BOFS and [ 39BOFS]. [39].

The process starts with the materialmaterial feed from a feed vessel above the reactor via a screw conveyor and a low volume of argon.argon. Inert gas purging is highly relevant at thisthis position, especially at higher temperatures, as the graphite bed is protected against oxidation by possible false air and, mainly, toto directdirect small ash or black matter particles directly onto the graphite surface. In the first first zone, the melting zone, the fine-grainedfine-grained materialmaterial inserted is heated to melting temperature withoutwithout reaching the reductionreduction temperature ofof the critical component phosphorus. The The resulting molten film film then movesmoves throughthrough thethe reactor to the reduction zone. In this zone th thee corresponding energy is induced so that the reduction temperature is reached close to the implemented gas flue.flue. At this point, it hashas to be mentioned that the graphite pieces areare not supposed to participate in the reductionreduction reactions and serve only as aa susceptorsusceptor material.material. Added carbon powder functions as aa reductant. Through this reaction process, phosphorusphosphorus is converted into the gaseous phase and can can be be removed removed directly directly from from the the reactor reactor via via the thegas gasflue flueby means by means of a negative of a negative pres- pressure-generatingsure-generating induced induced draft draft fan. fan.Downstre Downstreamam there there is a is post-c a post-combustionombustion chamber chamber in whichin which external external air airor oxygen or oxygen are areused used to convert to convert elemental elemental phosphorus phosphorus to P to2O P5. 2TheO5. sub- The sequentsubsequent hydrolysis hydrolysis finally finally enables enables the production the production of phosphoric of phosphoric acid. acid. The Theremaining remaining materialmaterial in the in thereactor reactor moves moves on to on the to discharge the discharge zone, zone, where where the third the third and last and coil last provides coil pro- videsenough enough energy energy that the that phosphorus-free the phosphorus-free materi materialal does not does reach not reachthe solidification the solidification temperaturetemperature and andfinally finally leaves leaves the thereactor reactor via viathe thereactor reactor floor. floor. The The resulting resulting material material can can be dividedbe divided into into a metal a metal and and a slag a slag fraction, fraction, whic which,h, however, however, are are not not ye yett separated separated from each other in the current expansion stage and are collected in a vessel vessel below below the the reactor reactor output. output. The advantages of this apparatus are manifold.manifold. In comparison with an electric arc furnace (EAF), no molten bath of metal is formed so that the P (g)–Fe (l) contact possibility furnace (EAF), no molten bath of metal is formed so that the P22 (g)–Fe (l) contact possibility and in further consequence the formation of of iron iron phosphide phosphide can can be be decreased decreased immensely. immensely. This fact is promoted by a thin molten film, which massively shortens the distance of mass This fact is promoted by a thin molten film, which massively shortens the distance of mass transport. In this case it is particularly important for the diffusion of P and its removal as transport. In this case it is particularly important for the diffusion of P and its removal as gas. The graphite pieces offer a large surface area for reactions and by coupling into the gas. The graphite pieces offer a large surface area for reactions and by coupling into the induction field, the heat for the endothermic reduction reactions is permanently provided induction field, the heat for the endothermic reduction reactions is permanently provided directly at the respective particle surface. Even if the energy demand is increased, the main directly at the respective particle surface. Even if the energy demand is increased, the main form of the reduction reaction is direct reduction, resulting in a lower carbon demand. [38] form of the reduction reaction is direct reduction, resulting in a lower carbon demand. In the course of the reaction processes in the reactor, a very low oxygen partial pressure [38] In the course of the reaction processes in the reactor, a very low oxygen partial pres- and a correspondingly high CO to CO2 ratio is established, which in turn promotes the sure and a correspondingly high CO to CO2 ratio is established, which in turn promotes reduction reactions [40]. the reduction reactions [40]. In order to use this process also for the waste stream from spent LIBs, the reactor In order to use this process also for the waste stream from spent LIBs, the reactor design and the corresponding post-treatment of the output streams must be adapted. The designinput material and the for corresponding the planned continuouspost-treatment process of the comes output from streams a pre-treatment must be adapted. plant, which The inputis a fine material fraction for as the low planned in Cu and continuous Al as possible process consisting comes from of a mixture a pre-treatment of cathode plant, and which is a fine fraction as low in Cu and Al as possible consisting of a mixture of cathode

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anode materials. After being fed into the pyrometallurgical reactor, the material should react according to the principle described above. The most important difference is that the idea of the treatment of this material is to remove not only phosphorus but also lithium from the reactor via the gas flow. An initial concept for the post-treatment of the liquid fraction, which leaves the reactor chamber via its bottom, provides for an oxygen inlet. Thus, in accordance with the different oxygen affinities, for example, the input stream of NMC, LCO, NCA and LFP should result in the purest possible CoNiFe alloy. Oxygen-affine elements such as Mn and Al, as well as the residues of P and Li that are not removed via the gas phase, are to be slagged. The resulting products can therefore either be sold on the market as raw materials as required, or further broken down into their constituent parts in further post-treatment steps, for example via the hydrometallurgical route. A further additional important step to be investigated is the post-treatment of the resulting gas fraction. In particular, it will be necessary to implement a corresponding process for the separation of Li and P and consequently to treat them further according to the resulting qualities. As the points just described show, a combination with other processes should be aimed for. With regard to the overall reactor design, an adaptation of the current development will be essential. This includes issues such as the optimal refractory material for the reactor wall or a possible need to expand the gas extraction system.

2.3.2. Batch Reactor Concept Based on the technology described in Section 2.3.1, the process design shall be adapted to the requirements of the black matter out of LIBs. For this purpose, in-depth tests were carried out for a better understanding of the material to be processed, which are partly described in Windisch-Kern et al. [34]. Since the scale of a continuous pilot plant for experiments of this kind would firstly be too complex and secondly would not correspond to the research status at the Chair of Thermal Processing Technology in the LIB field, the experiments were carried out on a post-laboratory scale. For this purpose, the reactor concept of the unit operating in batch mode was adopted from the developments in the field of sewage sludge ash and BOFS, as shown in Figure3a and hereinafter referred to as Design 1. The system behind it is similar to the continuous concept, with the difference that the material to be investigated is already in the reactor at the beginning of the experiment and there is no material output via the ground. Most of the material melted during the experiment is accumulated and collected at the bottom of the reactor or adheres to the cube surface as spherical formations. In addition, the gas outlet is also not subjected to negative pressure, so that the resulting gases leave the reactor without constraint. For the construction of the reactor, an Al2O3 ring with a diameter of 20 cm, Al2O3 mortar and refractory concrete were used. The graphite bed provides a cube surface of 1725 cm2 for the transfer of the induced heat. An insulation around the reactor has the function of the protection of the induction coil, to reduce the heat losses and to enable as good a separation as possible from ambient air. To enable a qualitative measurement of the exhaust gas flow, a gas scrubber was additionally installed at the outlet of the gas flue (Figure3b). This was realized with a bubbling frit in which the exhaust gas is enriched in a 2.5 molar H2SO4 solution. The temperature was measured on the outside of the reactor by two category S-thermocouples and inside the reactor by two category K-thermocouples. Due to the expected breakage of the second mentioned thermocouples, they are only used to find a correlation between the outside temperature and the inside temperature. Preliminary tests have shown that LCO, with its high cobalt content, is highly reactive to the crucible material of Al2O3. In addition, sampling proved to be particularly difficult because it was not possible to separate the black mass clearly from the mortar. This makes it almost impossible to close the mass balance in the future. Another disadvantageous fact of this reactor concept is that, due to its position, the highest induction of the current takes place in the upper part of the reactor. Because of the inevitable turbulence in the reactor during the experiment due to the gases, the material accumulates at the bottom of the Metals 2021, 11, x FOR PEER REVIEW 7 of 22

and anode materials. After being fed into the pyrometallurgical reactor, the material should react according to the principle described above. The most important difference is that the idea of the treatment of this material is to remove not only phosphorus but also lithium from the reactor via the gas flow. An initial concept for the post-treatment of the liquid fraction, which leaves the reactor chamber via its bottom, provides for an oxygen inlet. Thus, in accordance with the different oxygen affinities, for example, the input stream of NMC, LCO, NCA and LFP should result in the purest possible CoNiFe alloy. Oxygen-affine elements such as Mn and Al, as well as the residues of P and Li that are not removed via the gas phase, are to be slagged. The resulting products can therefore either be sold on the market as raw materials as required, or further broken down into their constituent parts in further post-treatment steps, for example via the hydrometallurgical route. A further additional important step to be investigated is the post-treatment of the resulting gas fraction. In particular, it will be necessary to implement a corresponding process for the separation of Li and P and consequently to treat them further according to the resulting qualities. As the points just described show, a combination with other processes should be aimed for. With regard to the overall reactor design, an adaptation of the current development will be essential. This includes issues such as the optimal refractory material for the reactor wall or a possible need to expand the gas extraction system.

2.3.2. Batch Reactor Concept Based on the technology described in Section 2.3.1, the process design shall be adapted to the requirements of the black matter out of LIBs. For this purpose, in-depth tests were carried out for a better understanding of the material to be processed, which are partly described in Windisch-Kern et al. [34]. Since the scale of a continuous pilot plant for experiments of this kind would firstly be too complex and secondly would not correspond to the research status at the Chair of Thermal Processing Technology in the LIB field, the experiments were carried out on a post-laboratory scale. For this purpose, the reactor concept of the unit operating in batch mode was adopted from the developments in the field of sewage sludge ash and BOFS, as shown in Figure 3a and hereinafter referred to as Design 1. The system behind it is similar to the continuous concept, with the difference that the material to be investigated is already in the reactor at the beginning of the experiment and there is no material output via the ground. Most of the material melted during the experiment is accumulated and collected at the bottom of the reactor or adheres Metals 2021, 11, 149 to the cube surface as spherical formations. In addition, the gas outlet is also not subjected8 of 22 Metals 2021, 11, x FOR PEER REVIEW 8 of 22 to negative pressure, so that the resulting gases leave the reactor without constraint. For the construction of the reactor, an Al2O3 ring with a diameter of 20 cm, Al2O3 mortar and refractory concrete were used. The graphite bed provides a cube surface of 1725 cm2 for Figure 3. (a) Schematic representationthereactor, transfer so the ofof energythethe origininduced supplyal InduMelt heat. position An plant insulation is (Design suboptimal. around1) [34]; To( bthe) take overall reactor into setup accounthas in the test function the operation. mentioned of the protectiondisadvantages, of the a induction new design coil, was to developed,reduce the whichheat losses is shown and to in enable Figure 4as and good hereinafter a separa- referred to as Design 2. tion asTo possible enable from a qualitative ambient measurementair. of the exhaust gas flow, a gas scrubber was additionally installed at the outlet of the gas flue (Figure 3b). This was realized with a bubbling frit in which the exhaust gas is enriched in a 2.5 molar H2SO4 solution. The temperature was measured on the outside of the reactor by two category S-thermocouples and inside the reactor by two category K-thermocouples. Due to the expected breakage of the second mentioned thermocouples, they are only used to find a correlation between the outside temperature and the inside temperature. Preliminary tests have shown that LCO, with its high cobalt content, is highly reactive to the crucible material of Al2O3. In addition, sampling proved to be particularly difficult because it was not possible to separate the black mass clearly from the mortar. This makes it almost impossible to close the mass balance in the future. Another disadvantageous fact of this reactor concept is that, due to its position, the highest induction of the current takes place in the upper part of the reactor. Because of the inevitable turbulence

in the(a )reactor during the experiment due to the gases,(b the) material accumulates at the bottom of the reactor, so the energy supply position is suboptimal. To take into account Figure 3. (a) Schematic representationthe mentioned of the originaldisadvantages, InduMelt a plant new (Design design 1)was [34 ];developed, (b) overall setupwhich in is test shown operation. in Figure 4 and hereinafter referred to as Design 2.

Figure 4. Schematic representation of the new reactor design (Design 2). Figure 4. Schematic representation of the new reactor design (Design 2).

ThisThis is is a a cylindricalcylindrical crucible with a a half-a half-arcrc bottom bottom made made of ofMgO. MgO. It was It was placed placed cen- centrallytrally on on a refractory a refractory concrete concrete structure structure in ina way a way that that only only the the lower lower part part of the of the MgO MgO cru- cruciblecible is iswithin within the the induction induction coil. Appropriate insulationinsulation mademade ofof refractory refractory matting matting shouldshould reduce reduce the the heat heat loss loss and and thus thus the the energy energy requirement requirement and and protect protect as as far far as as possible possible againstagainst the the ingress ingress of of false false air air from from the the environment. environment. To To be be able able to to make make a a qualitative qualitative statementstatement about about the the escaping escaping gas gas during during the the test, test, an an exhaust exhaust pipe pipe made made of of Al Al2O23Owas3 was again again implemented.implemented. The The temperature temperature was was measured measured by by a a category category S-thermocouple S-thermocouple from from below below andand in in the the reactor reactor by by two two category category K-thermocouples. K-thermocouples. ForFor a a direct direct comparison comparison of of the the different different reactor reactor concepts concepts of of the the InduMelt InduMelt plant, plant, the the samesame feedstock, feedstock, the the mixture mixture LCO-C LCO-C mentioned mentioned in in Section Section 2.1 2.1 with with a quantitya quantity of of 550 550 g, g, was was examinedexamined in in both both crucible crucible concepts. concepts. To ensureTo ensu thatre reproduciblethat reproducible initial initial conditions conditions prevailed pre- invailed both designs,in both designs, the charging the charging of the cubes of the and cubes the sample and the was sample also performedwas also performed uniformly uni- in allformly experiments. in all experiments. Thus, at the Thus, beginning, at the beginni 15 cubesng, were 15 cubes positioned were positioned in the reactor in the and reactor one thirdand ofone the third sample of the was sample charged was onto charged them. onto After them. positioning After positioning a K-thermocouple, a K-thermocouple, another 10another cubes were10 cubes performed were performed followed byfollowed addition by of addition another of third another of the third sample. of the This sample. was repeatedThis was a secondrepeated time a second to ﬁnally time ﬁll to the finally reactor fill with the reactor 11 cubes with after 11 positioning cubes afterthe positioning second

Metals 2021, 11, x FOR PEER REVIEW 9 of 22

the second K-thermocouple. This filling quantity also represents the maximum possible capacity of Design 1. The content of Design 2 is approximately 25% larger which, however, was not utilized due to the aforementioned comparability with Design 1. After the experiment, all components of the reactor are weighed. The adhesions to the graphite cubes are removed by light mechanical processing. These adhesions are consequently separated into fractions larger and smaller than 1 mm by means of a sieve tower, together with the remaining finer fraction that may be produced. With the aid of a magnet, these are further separated into magnetic and non-magnetic, with the former finally being assigned to metal and the latter referred to as slag. Larger pieces of metal are collected together after Metals 2021, 11, 149 checking with a magnet. The same is done with larger non-magnetic9 of 22 pieces, which are again referred to as slag. The individual fractions are finally weighed and analyzed. The main difference in sampling between Design 1 and Design 2 is the collection of the dif- K-thermocouple. This filling quantity also represents the maximum possible capacity of fusedDesign areas 1. The of content the ofgrout Design or 2 isreactor approximately adhesions, 25% larger which which, however,will be was discussed not in more detail in Sectionutilized 3.2.1. due to the aforementioned comparability with Design 1. After the experiment, all componentsThe aim of is the to reactor determine are weighed. the The interaction adhesions to between the graphite the cubes cathode are removed material respectively the by light mechanical processing. These adhesions are consequently separated into fractions reactionlarger and products smaller than of 1which mm by and means the of acorrespond sieve tower, togethering crucible with the remainingmaterial and to compare them withfiner each fraction other. that may On be produced.the other With hand, the aid the of a magnet,individual these are transfer further separated coefficients should provide into magnetic and non-magnetic, with the former finally being assigned to metal and the informationlatter referred on to as whether slag. Larger the pieces choice of metal of are the collected crucible together material after checking affects with the a recovery rates of the individualmagnet. The species. same is done with larger non-magnetic pieces, which are again referred to as slag. The individual fractions are finally weighed and analyzed. The main difference in samplingIn a third between trial, Design the 1 and basic Design suitability 2 is the collection of the of the overall diffused reactor areas of the concept grout for the treatment of LFPor reactorwith adhesions,the aim whichof removing will be discussed Li and in more P from detail inthe Section material 3.2.1. was investigated. For this purpose, DesignThe aim is to1 determinefrom Figure the interaction 3a was between selected the cathode again material in which respectively a quantity the of 394.5 g of the reaction products of which and the corresponding crucible material and to compare them mixturewith each LFP-C other. Onfrom the otherSection hand, 2.1 the individualwas charged transfer into coefficients the reactor. should provide information on whether the choice of the crucible material affects the recovery rates of the individual species. 3. ResultsIn a third and trial, Discussion the basic suitability of the overall reactor concept for the treatment 3.1.of High-Temperature LFP with the aim of removing Properties Li and of P the from Cathode the material Materials was investigated. Used For this purpose, Design 1 from Figure3a was selected again in which a quantity of 394.5 g of the mixtureIn a LFP-Cfirst fromstep Section to determine 2.1 was charged the into behavior the reactor. of cathode materials from LIBs at tempera- tures3. Results above and 1600 Discussion °C and under reducing conditions, experiments were performed in a heating3.1. High-Temperature microscope. Properties Figure of the 5a Cathode provides Materials a Usedpicture of the result of the experiment with the mixtureIn awith first step LCO. to determine A strong the behavior dark ofblue cathode colora materialstion from was LIBs observed at temperatures on the platelet. This may above 1600 ◦C and under reducing conditions, experiments were performed in a heating be microscope.due to a Figurereaction5a provides between a picture the of Al the2 resultO3 platelet of the experiment and cobalt with the to mixture form cobalt aluminate with its withtypical LCO. Ablue strong appearance. dark blue coloration [41] was The observed produc on thet of platelet. the melting This may be process due to under reducing con- a reaction between the Al2O3 platelet and cobalt to form cobalt aluminate with its typical ditionsblue appearance is a metal [41 ].structure, The product which of the melting can be process classified under reducing as strongly conditions magnetic is after examination witha metal a magnet. structure, This which magnetism can be classified could as strongly also magnetic be detected after examination in experiments with a with LFP, which is magnet. This magnetism could also be detected in experiments with LFP, which is shown shownin (b). in In contrast(b). In to contrast the experiment to the described experiment above, there described is no blue coloration,above, there but a is no blue coloration, butbrown a brown to reddish to appearance.reddish appearance.

(a) (b) FigureFigure 5. 5. ConditionCondition of of the the sample sample during during examination examination under the heating under microscope: the heating (a) after microscope: (a) after analysisanalysis for for LCO-C;LCO-C; (b) after(b) analysisafter analysis for LFP-C. for LFP-C. Figure6 displays the recording of the replication experiment LCO-C via optical measurementFigure 6 in displays the heating microscope.the recording In Figure 6of the the cross-sectional replication area ofexperiment the exper- LCO-C via optical measurement in the heating microscope. In Figure 6 the cross-sectional area of the experiments with LCO-C (black dotted line) and its repetition LCO-C-Re (green dotted line) over temperature during the experiment is shown. This cross-sectional area is the size of

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Metals 2021, 11, x FOR PEER REVIEW 10 of 22

iments with LCO-C (black dotted line) and its repetition LCO-C-Re (green dotted line) over temperature during the experiment is shown. This cross-sectional area is the size ofthe the sample sample cylinder cylinder detected detected by bythe the heating heating microscope microscope respectively respectively its change its change with with tem- temperatureperature increase. increase.

FigureFigure 6.6. Results of of the the heating heating microscope microscope of ofLCO-C: LCO-C: trend trend of the of thecross-sectional cross-sectional area areaof the of ex- the periments LCO-C and LCO-C-Re during heating and the median value of the both graphics. experiments LCO-C and LCO-C-Re during heating and the median value of the both graphics.

ItIt should should be be mentioned mentioned that that by by comparing comparing the the recorded recorded images images with with the the corresponding correspond- valuesing values of the of cross-sectional the cross-sectional area, area, faulty faulty measurements measurements caused caused by incorrect by incorrect detection detection of theof baselinethe baseline by the by heatingthe heatin microscopeg microscope were were removed removed from thefrom data the series. data series. Since theSince basic the behaviorbasic behavior at the individualat the individual temperatures temperatures is nearly is nearly identical identical and differs and differs only by only different by dif- cross-sectionalferent cross-sectional areas, a areas, mean a value mean was value determined, was determined, which represents which represents the red line.the red When line. lookingWhen looking at Figure at 6Figure, the first 6, the noteworthy first notewort surfacehy surface changes changes can be can observed be observed from from 675 ◦675C onwards.°C onwards. From From 675 ◦ 675C to °C 845 to◦ C845 a growth°C a growth of the cross-sectionalof the cross-sectional area was area detected, was detected, which subsequentlywhich subsequently decreased decreased again to 1054again◦C to with 1054 single °C with deflections single todeflections about 80% to of about the original 80% of area.the original Up to a area. temperature Up to a oftemperature 1127 ◦C an of increase 1127 °C in an magnification increase in magnification was detected, was which de- remainedtected, which relatively remained constant relatively with singleconstant deflections with single up deflections to 1380 ◦C. up From to 1380 this temperature°C. From this on,temperature the cross-sectional on, the cross-sectional area decreased area continuously decreased withcontinuously a smaller with slope a smaller in the range slope of in 1393the range◦C to 1507of 1393◦C °C and to a1507 significant °C and decreasea significant upto decrease 1525 ◦C. up Up to to1525 the °C. end Up there to the was end a furtherthere was decrease a further of the decrease cross-sectional of the cross-sectional area, which, area, however, which, when however, looking when at the looking single at imagesthe single from images the heating from the microscope, heating microscope can be traced, can back be traced to the continuousback to the distributioncontinuous dis- of thetribution molten of material the molten on the material platelet. on the platelet. FigureFigure7 7illustrates illustrates the the results results of of the the experiments experiments in in the the heating heating microscope microscope with with the the mixturemixture LFP-C. LFP-C. The The previously previously mentioned mentioned measurement measurement error error was was particularly particularly striking striking ◦ inin the the first first experiment experiment of of LFP-C LFP-C (black (black dotted dotted line line in in Figure Figure7 )7) in in the the range range from from 1163 1163 C°C ◦ toto about about 1400 1400 C.°C. Nevertheless, Nevertheless, a a corresponding corresponding trend trend could could be be determined determined by by correctly correctly measuringmeasuring individual individual values values in somein some cases, cases, which which in turn in turn could could be confirmed be confirmed by a repeated by a re- measurementpeated measurement of LFP-C-Re of LFP-C-Re (green dotted (green line dotte in Figured line7 in). Again,Figure the7). Again, mean value the mean is shown value inis theshown red curve.in the red curve. The results show that up to a temperature of approximately 920 ◦C the cross-sectional area first rises slightly and then falls back to just below 100% of the initial value. Afterwards, a pulsating enlargement of the surface takes place which decreases at about 1200 ◦C. After a further pulsating behavior between 1240 and 1310 ◦C the area is continuous again to remain relatively constant from about 1410 ◦C on.

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FigureFigure 7. 7.Results Results of of the the heating heating microscope microscope of of LFP-C: LFP-C: trend trend of of the the cross-sectional cross-sectional area area of of the the experi- exper- iments LFP-C and LFP-C-Re during heating and the median value of the both graphics. ments LFP-C and LFP-C-Re during heating and the median value of the both graphics.

3.2. InduMeltThe results Experiments: show that Process up to Developmenta temperature and of Suitability approximately of the Different920 °C the Reactor cross-sectional Concepts area first rises slightly and then falls back to just below 100% of the initial value. After- wards,The a main pulsating part ofenlargement the experimental of the investigationsurface takes ofplace LIB which cathode decreases materials at wasabout exam- 1200 ined°C. After in the a InduMelt further pulsating plant. For behavior this purpose, between three 1240 experiments and 1310 °C were the carried area is out,continuous which willagain be consideredto remain relatively separately constant below accordingfrom about to 1410 their °C purpose. on. According to the results of the analyses, as described in Section 3.1, the maximum necessary process temperatures for the3.2. tests InduMelt in the Experi InduMeltments: plant Process were Development set at 1525 ◦andC for Su LCO-Citability andof the 1400 Different◦C for Reactor LFP-C. This isConcepts due to the fact that the sample material should be completely liquid at this point in time accordingThe main to the part heating of the microscope. experimental investigation of LIB cathode materials was examined in the InduMelt plant. For this purpose, three experiments were carried out, which 3.2.1. Results from Experiments with LCO-C in Both Reactor Designs will be considered separately below according to their purpose. According to the results of theThe analyses, first experiment, as described which in Section is described 3.1, the maximum in detail below,necessary was process carried temperatures out in De- signfor the 1. Thetests entirein the experimentInduMelt plant lasted were nearly set at 8 1525 h. Figure °C for8 aLCO-C shows and the 1400 power °C for input LFP-C. of theThis induction is due to unit the andfact that the correspondingthe sample material temperatures should be over completely the test time.liquid Two at this type point S- thermocouplesin time according (S-TC to 1the and heating S-TC 2microscope. in Figure8a) were used on the reactor surface and two type K-thermocouples inside the reactor. The latter were initially installed at different heights3.2.1. Results in the reactor,from Experiments one in the areawith of LCO-C the first in cubeBoth layerReactor (K-TC Designs bottom) and one in the upper area (K-TC top). In contrast to the S-thermocouples, the measurement results of the The first experiment, which is described in detail below, was carried out in Design 1. K-thermocouples are subject to considerable fluctuations due to melting of their insulation, The entire experiment lasted nearly 8 h. Figure 8a shows the power input of the induction influences of the material in the reactor, etc. However, an approximate temperature spread unit and the corresponding temperatures over the test time. Two type S-thermocouples of the different thermocouple types of 500 ◦C could be determined up to the end. (S-TCDuring 1 and the S-TC process, 2 in Figure noticeable 8a) were anomalies used on were the documented. reactor surface Starting and two at0.95 type h K-ther- and a K-TCmocouples bottom inside temperature the reactor. of 450 The◦C latter a strong were formation initially installed of condensate at different in the exhaustheights pipein the toreactor, the gas one scrubber in the area was of observed. the first cube This la wasyer attributed(K-TC bottom) to the and drying one in of the the upper mortar. area The (K- temperatureTC top). In contrast spread of to the the S-thermocouples S-thermocouples, at the 1.75 measurement h (124 ◦C S-TC results 1) can of be the explained K-thermocou- by a slightples are realignment subject to of considerable S-TC 1. Especially fluctuations interesting due to was melting the continuously of their insulation, increasing influences white smokeof the frommaterial the exhaust in the reactor, pipe, which etc. startedHowever, at 5.10 an approximate h and 1180 ◦C temperature internal temperature spread of and the stoppeddifferent at thermocouple 1341 ◦C. This resultedtypes of 500 in a °C continuous could be whitedetermined deposit up in to the the exhaust end. pipe of the scrubber,During as shown the process, in Figure noticeable8b. After anomalies the white smokewere documented. formation stopped, Starting the at acid0.95 h in and the a scrubberK-TC bottom gradually temperature changed of from 450 transparent°C a strong toformation a slightly of yellow condensate liquid. in The the assumptionexhaust pipe thatto the the gas white scrubber deposits was are observed. Li or a correspondingThis was attributed compound to the coulddrying be of confirmed the mortar. after The analysistemperature of the spread liquid inof the scrubber,S-thermocouples which are at summarized1.75 h (124 °C in S-TC Table 1)2 .can be explained by a slight realignment of S-TC 1. Especially interesting was the continuously increasing white smoke from the exhaust pipe, which started at 5.10 h and 1180 °C internal temperature and stopped at 1341 °C. This resulted in a continuous white deposit in the exhaust

Metals 2021, 11, x FOR PEER REVIEW 12 of 22

pipe of the scrubber, as shown in Figure 8b. After the white smoke formation stopped, the acid in the scrubber gradually changed from transparent to a slightly yellow liquid. The Metals 2021, 11, 149 assumption that the white deposits are Li or a corresponding compound could be12 con- of 22 firmed after analysis of the liquid in the scrubber, which are summarized in Table 2.

(a) (b)

Figure 8. ExperimentalExperimental performance performance of of LCO-C LCO-C in in Design Design 1: 1:(a)( acomparison) comparison of power of power and and temperature temperature over over time; time; (b) de- (b) posits in the exhaust pipe of the gas scrubber. deposits in the exhaust pipe of the gas scrubber.

Table 2. Results from the gas scrubber respectively from its frit liquid after suction of the exhaust gasTable in 2.DesignResults 1 LCO-C from the in gasmg/L. scrubber respectively from its frit liquid after suction of the exhaust gas in Design 1 LCO-C in mg/L. Fraction Li Co FritFraction liquid 1650 Li 0.64 Co Frit liquid 1650 0.64 Sampling after the experiment revealed a total of 4 fractions, the results of which are shown in Table 3. Sampling after the experiment revealed a total of 4 fractions, the results of which are Tableshown 3. inAnalysis Table3 results. of the fractions of the experiment LCO-C in Design 1.

Table 3.FractionAnalysis results of the Weight fractions (g) of the experiment Li LCO-C(wt.%) in Design 1. Co (wt.%) Slag 3.20 5.62 0.12 Fraction Weight (g) Li (wt.%) Co (wt.%) Mortar 145.60 4.52 0.12 MetalSlag 251.70 3.20 0.01 5.62 100.00 0.12 Mortar 145.60 4.52 0.12 PowderMetal 251.7037.30 1.490.01 100.00 53.4 Powder 37.30 1.49 53.4 The fraction defined therein as slag could be identified as dark to light grey non- magnetic pieces smaller than 10 mm with minimal metallic inclusions, as illustrated in The fraction defined therein as slag could be identified as dark to light grey non- Figure 9a. The mortar shown represents the part into which the test material has diffused. magnetic pieces smaller than 10 mm with minimal metallic inclusions, as illustrated in This is optically visible by a dark discoloration of the originally white mortar. In Figure Figure9a. The mortar shown represents the part into which the test material has diffused. 9b the reactor is demonstrated from below after the concrete floor has been separated. This is optically visible by a dark discoloration of the originally white mortar. In Figure9b During sampling, care was taken to find the clearest possible separation between the the reactor is demonstrated from below after the concrete floor has been separated. During white mortar and the diffused areas, but this proved to be very difficult. The largest prod- sampling, care was taken to find the clearest possible separation between the white mortar uctand of the the diffused experiment areas, in terms but this of provedmass was to the be metal very difficult.fraction, which The largest could productbe obtained of the in piecesexperiment larger in than terms 10 mm. of mass The was metal the piece metal show fraction,n in Figure which 9c could serves be as obtained an example. in pieces The analysislarger than showed 10 mm. an The impressive metal piece purity shown of 100 in Figure% Co 9andc serves an impurity as an example. of only The 0.01% analysis Li. It shouldshowed be an noted impressive that there purity may of be 100% some Co variation and an impurity in sampling of only and 0.01% digestion Li. It shoulderrors, re- be sultingnoted that in the there overall may result be some not variation reaching in ex samplingactly 100%. and The digestion fourth fraction errors, resulting was a powder in the withoverall particles result notsmaller reaching than 1 exactly mm, as 100%. can be The seen fourth in Figure fraction 9d, waswhich a powderwas mostly with magnetic. particles Thissmaller property than 1 is mm, also as confirmed can be seen by in analyses Figure9 d,with which a cobalt was mostlycontent magnetic. of 53.4%. This property is also confirmed by analyses with a cobalt content of 53.4%.

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(a) (b) (c) (d) (a) (b) (c) (d) Figure 9. Products of the experiment LCO-C in Design 1: (a) slag; (b) ceramic ring and mortar seen from the bottom; (c) Figuremetal; 9. (d Products)Products powder. of of the the experiment experiment LCO-C LCO-C in in Design Design 1: 1:(a) ( aslag;) slag; (b) ( bceramic) ceramic ring ring and and mortar mortar seen seen from from the thebottom; bottom; (c) metal; (d) powder. (c) metal; (d) powder. Taking into account the respective weighed masses and the analysis results, the transfer coefficientsTakingTaking into into ofaccount account the individual the the respective respective elements weighed weighed of the masses fractions masses and andwerethe analysis the calculated. analysis results, In results, thedetail, trans- thethe fertransferanalyses coefficients coefﬁcients from ofICP-MS the of individual the and individual ICP-OES elements elementsof the of individual the of fractions the fractions fractions were were calculated. were calculated. converted In detail, In to detail, mass the analysesthepercent analyses andfrom frommultiplied ICP-MS ICP-MS and by and ICP-OESthe ICP-OES weighed of ofthe mass the individual individual at sampling. fractions fractions By adding were were converted convertedthe respective to to mass ele- percentment masses, andand multipliedmultiplied a total mass by by the theper weighed weighedelement mass couldmass at at sampling.be sampling.determined. By By adding Thisadding represents the the respective respective the elementamount ele- mentmasses,that wasmasses, a still total detectablea masstotal mass per in element perthe elementfractions could could in be the determined. be reactor determined. after This the This represents test. represents Afterwards, the amount the a amountcompar- that thatwasison was still of the detectablestill masses detectable in before the in fractions the and fractions after in thethe in reactorexperiment the reactor after theafterwas test. carriedtheAfterwards, test. out. Afterwards, The a difference comparison a compar- was of isontheassumed masses of the to beforemasses be a transfer and before after intoand the the experimentafter gas the flow experiment wasleaving carried the was out.reactor carried The during difference out. Thethe wasexperimentdifference assumed was or to a assumedbetransfer a transfer intoto be intothe a transfer theindividual gas into ﬂow solid the leaving gas fractions. flow the reactorle avingThe results during the reactor of the this experiment during calculation the orexperiment acan transfer be seen intoor ina transfertheFigure individual 10.into the solid individual fractions. solid The fractions. results of The this results calculation of this can calculation be seen in Figurecan be 10seen. in Figure 10.

FigureFigure 10.10. TransferTransfer coefficients coefficients of of the the elements elements into into the the individual individual fractions fractions in in % % of of the the experiment experiment in Figure 10. Transfer coefficients of the elements into the individual fractions in % of the experiment Designin Design 1. 1. in Design 1. AtAt thisthis pointpoint itit shouldshould bebe mentionedmentioned thatthat thethetransfer transfercoefficients coefficientsdetermined determined mustmust bebe seenseenAt this asas initial initialpoint guideitguide should valuesvalues be mentioned andand internalinternal that comparisoncomparison the transfer valuesvalues coefficients andand mustmust determined bebe confirmedconfirmed must beaccordinglyaccordingly seen as initial by by repeated repeated guide experimentsvalues experiments and internal in thein th optimum ecomparison optimum reactor reactorvalues setup. andsetup. Nevertheless, must Nevertheless, be confirmed the trend the accordinglywastrend also was observed also by observedrepeated in experiments inexperiments experiments with in NMC withthe optimumNMC and NCA, and reactor NCA, as described assetup. described inNevertheless, Windisch-Kern in Windisch- the trendetKern al. [ was34et ].al. Thealso [34]. resultobserved The ofresult the in transfer ofexperiments the transfer coefficients with coef ficientsNMC in Figure and in 10 FigureNCA, shows as10 a describedshows Li removal a Li in removal rate Windisch- of over rate Kern76%of over intoet al. 76% the [34]. gasinto The streamthe result gas from streamof the the transfer from cathode the coef materialcathodeficients used.material in Figure Based used. 10 on shows Based the thermokinetic a on Li theremoval thermoki- con-rate ofsiderationnetic over consideration 76% of into LCO the by ofgas Kwon LCO stream etby al. Kwon from [16] andtheet al. ca assuming [16]thode and material thatassuming most used. of that the Based most Li has on of leftthe the thethermoki- Li reactorhas left ◦ neticduringthe reactor consideration the phase during of ofthe white LCO phase smoke by of Kwon white (approximately et smoke al. [16] (approximately and 1160–1340 assumingC), 1160–1340that it can most be assumedof °C), the it Li can thathas be itleft as- is the reactor during the phase of white smoke (approximately 1160–1340 °C), it can be as- Lisumed2O. The that transfer it is Li of2O. over The 21% transfer Li into of theover mortar 21% Li can into be consideredthe mortar ascan an be undesirable considered result. as an sumedTheundesirable percentage that it result. is Li of2O. LiThe inThe percentage the transfer slag is of notof overLi negligible in 21%the sl Liag ininto is thenot the analysesnegligible mortar (Tablecan in thebe2 considered)analyses with 5.6%, (Table as but an 2) undesirableduewith to 5.6%, the small butresult. due quantity The to the percentage ofsmall slag quantity it isof insignificantLi in of the slag slag it for isis insignificantnot the negligible total consideration for in the the totalanalyses with consideration 0.6%. (Table The 2) withsmallwith 5.6%, 0.6%. amount but The due of small Li to in the amount the small metal ofquantity (0.1%) Li in the isof a slagmetal great it is(0.1%) result insignificant withis a great regard for result the to thetotal with purest consideration regard possible to the withmetal 0.6%. fraction. The Asmall further amount potential of Li forin the improvement metal (0.1%) can is bea great seen result when with considering regard to the the Li content of 1.8% in the powder, whereby this value can possibly be lowered with a longer

Metals 2021, 11, x FOR PEER REVIEW 14 of 22

Metals 2021, 11, 149 purest possible metal fraction. A further potential for improvement can be seen when14 ofcon- 22 sidering the Li content of 1.8% in the powder, whereby this value can possibly be lowered with a longer holding time of the final temperature. The result of Co can be interpreted as holdingextremely time promising. of the final Only temperature. 7.5% is found The in result the powder of Co can and be 95.2% interpreted in the asmetal, extremely which promising.can be directly Only transferred 7.5% is found for infurther the powder use in the and corresponding 95.2% in the metal, metal which industry. can beThe directly result- transferreding difference for to further 100% usecan inbe theexplained corresponding by the extremely metal industry. difficult The sampling, resulting especially difference the toidentification 100% can be of explained the individual by the extremelyfractions and difficult the subsequent sampling, especiallyweighing. the At identificationthis point, the ofproportion the individual in the fractionsslag and andmortar the can subsequent also be neglected weighing. with At less this than point, 0.1%. the The proportion compar- inison the of slag these and results mortar with can other also processes be neglected is difficult with lessat this than point 0.1%. because The the comparison composition of theseof the results input withmaterial other differs processes significantly is difficult from at this a real point waste because stream the of composition LIB. Nevertheless, of the inputby using material pure differscathode significantly material, without from a realimpuriti wastees stream such as of Al LIB. or Nevertheless,Cu, a value of bythe using theo- pureretically cathode maximum material, possible without removal impurities rate of such Li can as Al be or determined. Cu, a value Vest of the [15] theoretically describes a maximumLi2O transfer possible rate from removal the ratewaste of stream Li can be of determined.LIB of 40.5% Vest in their [15] process describes based a Li2 Oon transfer an elec- ratetric fromarc furnace. the waste Even stream though of LIB a direct of 40.5% comparison in their process with this based value on is an not electric possible, arc furnace. the gap Evenbetween though Vest’s a direct result comparison and the theoretically with this valuepossible is notvalue possible, in this themethod gap betweenshows an Vest’s enor- resultmous and potential. the theoretically possible value in this method shows an enormous potential. MuchMuch more more remarkable remarkable in in this this context context is theis the result result of Designof Design 2. The 2. The temperature temperature record rec- oford the of LCO-C the LCO-C experiment experiment in Design in Design 2, which 2, which can becan viewed be viewed in Figure in Figure 11 in 11 combination in combina- withtion thewith power the power input input over over time time with with the the same same naming naming as as in in the the previous previous experiment experiment describeddescribed above,above, showsshows thatthat thethe temperaturetemperature isis highesthighest inin thethe lowerlower partpart ofof thethe reactor.reactor. Thus,Thus, the the goal goal of of the the reactor reactor design design of of a a more more targeted targeted temperature temperature provision provision in in the the lower lower areaarea could could be be realized. realized.

FigureFigure 11. 11.Experimental Experimental performance performance of of LCO-C LCO-C in in Design Design 2, 2, comparison comparison of of power power and and temperature temperature over time. over time.

TheThe extreme extreme fluctuations fluctuations of of the the K-thermocouples K-thermocouples between between the testthe durationtest duration of approx- of ap- imatelyproximately 1 to 3 1 hto could 3 h could be explained be explained in retrospect in retrospect in suchin such a way a way that that after after the the insulation insulation aroundaround thethe thermocouplethermocouple wires had melted, melted, they they reconnected reconnected at at a ahigher higher point point in inthe the re- reactor.actor. The The initial initial theory theory could could be confirmed be confirmed when when the reactor the reactor was opened was opened after the after exper- the experiment,iment, because because the thermocouples the thermocouples could could be found be found in thein upper the upper part of part the ofreactor the reactor free of freecubes of and cubes no and longer no longerin the cube in the bed. cube Again, bed. a Again, white asmoke white formation smoke formation with the withsame the de- sameposits deposits in the exhaust in the exhaust pipe of pipe the of gas the scru gasbber scrubber could could be detected. be detected. This This phenomenon phenomenon oc- occurredcurred at atan an S-thermocouple S-thermocouple temperature temperature range range from from 1165 1165 to 1340 to 1340°C. Again,◦C. Again, as the assmoke the smokeintensity intensity decreased, decreased, a successive a successive discoloration discoloration of the acid of in the the acid scrubber in the from scrubber transparent from transparentto a light yellow to a light was yellowto be determined. was to be determined.The results of The the results acid analysis of the acidfrom analysis the gas scrub- from theber gas can scrubber be taken can from be takenTable from4. The Table high4 .valu Thee high of Li value confirms of Li the confirms impression, the impression, as already asassumed already in assumed the experiment in the experiment in Design in 1, Design that Li 1, can that be Li removed can be removed from the from reactor the reactor via the viagas the flow. gas flow.

Metals 2021, 11, x FOR PEER REVIEW 15 of 22

Table 4. Results from the frit liquid of the gas scrubber after suction of the exhaust gas in Design 2 LCO-C in mg/L.

Fraction Li Co Frit liquid 1230 1.2

Metals 2021, 11, 149 15 of 22 The results of the investigation of the test material LCO-C in Design 2 can be seen in Table 5. Essentially, the analysis differs from the experiment in Design 1 only in that there Table 4. Results from the frit liquid of the gas scrubber after suction of the exhaust gas in Design 2 LCO-Cwas no in mg/L. mortar due to the construction. When the MgO crucible was weighed after the experiment, it was found to be 81.2 g heavier than the initial weight. This could be at- Fraction Li Co tributedFrit to liquid adhesions on the crucible, 1230 which were mechanically 1.2 extracted as completely as possible. The result was a fine powder. Despite considerable mechanical effort, only 3.8 g couldThe results be removed of the investigation from the of the crucible test material without LCO-C indamage, Design 2 canwhich be seen will be referred to as cruci- inble Table adhesion5. Essentially, in the the analysisfollowing. differs from the experiment in Design 1 only in that there was no mortar due to the construction. When the MgO crucible was weighed after the experiment, it was found to be 81.2 g heavier than the initial weight. This could be attributedTable 5. toResults adhesions from on the the crucible, experiment which were LCO-C mechanically in Design extracted 2. as completely as possible. The result was a ﬁne powder. Despite considerable mechanical effort, only 3.8 g couldMaterial be removed from the crucible Weight without (g) damage, which will Li be (wt.%) referred to as Co (wt.%) crucible adhesion in the following. Slag 1.3 5.59 3.3 TableCrucible 5. Results fromadhesion the experiment LCO-C in Design 3.8 2. 6.29 27.2 MaterialMetal Weight (g)242.2 Li (wt.%) Co 0.12 (wt.%) 93.9 Slag 1.3 5.59 3.3 CruciblePowder adhesion 3.835.2 6.29 0.69 27.2 66.6 Metal 242.2 0.12 93.9 Powder 35.2 0.69 66.6 The fractions did not differ in their appearance from those in Figure 9a,c,d. Only the metalThe pieces fractions were did not larger, differ in theiras shown appearance in from Figure those 12. in Figure Analysis9a,c,d. Only of the the metal fraction revealed a metalpurity pieces of wereCo of larger, 93.9% as shown with in Figurea negligible 12. Analysis amount of the metal of fraction0.12% revealedLi. a purity of Co of 93.9% with a negligible amount of 0.12% Li.

Figure 12. 12.Metal Metal fraction fraction from the LCO-C from experiment the LCO-C in Design experiment 2. in Design 2. The transfer coefﬁcients, which are illustrated in Figure 13, were determined from the resultsThe in Table transfer5, taking intocoefficients, account the corresponding which are weight illust changesrated of in the Figure individual 13, were determined from fractions during the experiment. the Thisresults result in represents Table a5, unique taking selling into point account in the pyrometallurgical the corresponding processing weight of changes of the indi- cathodevidual material fractions from LIB.during Compared the toexperiment. Design 1, even though only 85.9% of the Co was transferred to an almost pure Co-metal phase, more than 97% of the Li were removed from the materialThis andresult the reactor represents via the gas a ﬂow.unique Thus, selling a nearly 21%point higher in Lithe removal pyrometallurgical rate processing of couldcathode be achieved material in Design from 2. LIB. Compared to Design 1, even though only 85.9% of the Co was transferred to an almost pure Co-metal phase, more than 97% of the Li were removed from the material and the reactor via the gas flow. Thus, a nearly 21% higher Li removal rate could be achieved in Design 2. Since the attribution of the Co not found to the gas phase is rather questionable to this extent and cannot be traced back exclusively to errors in sampling and analysis, a closer look at the results is necessary. In addition to the result display in Figure 13, another variant is possible. It is assumed that the difference of the weighed crucible adhesion (81.2 g) to the extracted amount (3.8 g) consists of the same composition as the extracted fraction. This results in a lithium removal rate of 81.7% with a Co value that was not found (i.e., attributed to the gas phase) of −3.12%. This variant cannot clarify the difference to 100%, but by combining the two methods, one obtains a range in which the transfer coefficients move.

Metals 2021, 11, 149 16 of 22 Metals 2021, 11, x FOR PEER REVIEW 16 of 22

Metals 2021, 11, x FOR PEER REVIEW 16 of 22

FigureFigure 13. 13.Transfer Transfer coefﬁcients coefficients of the of elements the elements into the into individual the indi fractionsvidual infractions % of the in experiment % of the experiment in Designin Design 2. 2.

SinceBesides the attributionthis result, of Design the Co 2 not also found turns to out the to gas be phase a better is rather choice questionable when considering to the thisinteractions extent and between cannot bethe traced sample back material exclusively and tothe errors reactor. in sampling As shown and in analysis,Figure 14a, a a mas- closer look at the results is necessary. In addition to the result display in Figure 13, another sive attack of the reactor wall was observed in Design 1 (Al2O3) with ring diameter reduc- variant is possible. It is assumed that the difference of the weighed crucible adhesion tions of up to 0.2 mm. On the other hand, Figure 14b shows the reactor in Design 2 (MgO) (81.2 g) to the extracted amount (3.8 g) consists of the same composition as the extracted fraction.after the This experiment. results in a From lithium the removal difference rate of betw 81.7%een with the a weighing Co value that before was notand found after the test, Figure 13. Transferit coefficientsis known of that the eltheements reactor into thewas indi overvidual 81 fractions g heavier in % afterwards. of the experiment The theory of adhesion to in Design 2. (i.e., attributed to the gas phase) of −3.12%. This variant cannot clarify the difference tothe 100%, reactor but bycan combining also be seen the twoin the methods, illustra onetion, obtains whose a rangeboundary in which is marked the transfer with the red Besides thiscoefﬁcientsarrow. result, Design move. 2 also turns out to be a better choice when considering the interactions betweenBesides the sample this material result, Design and the 2 alsoreactor. turns Asout shown to be in aFigure better 14a, choice a mas- when considering the interactions between the sample material and the reactor. As shown in Figure 14a, a sive attack of the reactor wall was observed in Design 1 (Al2O3) with ring diameter reductions of up to 0.2massive mm. On attack the other of the hand, reactor Figure wall 14b was shows observed the reactor in Design in Design 1 (Al2 2O (MgO)3) with ring diameter after the experiment.reductions From of the up difference to 0.2 mm. betw Oneen the the other weighing hand, Figure before 14 andb shows after the the test, reactor in Design 2 it is known that(MgO) the reactor afterthe was experiment. over 81 g heavier From the afterwards. difference betweenThe theory the of weighing adhesion before to and after the the reactor can test,also itbe is seen known in thatthe illustra the reactortion, waswhose over boundary 81 g heavier is marked afterwards. with Thethe theoryred of adhesion arrow. to the reactor can also be seen in the illustration, whose boundary is marked with the red arrow.

(a) (b) Figure 14. Visual appearance of the crucibles after the experiments: (a) traces of attack on the Al2O3 crucible wall in Design 1; (b) appearance of the MgO crucible after experiment 2 with obvious adhesions.

(Thisa) factor is particularly important for (long-termb) experiments in a continuous setup. If the sample material and the Al2O3-ceramics are in contact for a longer period of time, Figure 14. Visual appearance of the crucibles after the experiments: (a) traces of attack on the Figure 14. Visual appearance of the crucibles after the experiments: (a) traces of attack on the Al2O3 crucible wall in Design Al2O3 crucible wallthis in Design attack 1; would (b) appearance lead to ofa destructionthe MgO crucible of the after reactor. experiment In addition 2 with obvi- to the advantages already 1; (b) appearance of the MgO crucible after experiment 2 with obvious adhesions. ous adhesions. mentioned, Design 2 also features a much simpler construction and therefore easier sampling. A drawback of Design 2 is the higher energy input required, which can be seen in This factor theis particularly comparison important of the power for long-term curve of experiments Figures 8 and in a 11. continuous However, set- this can be solved by an up. If the sampleimproved material and positioning the Al2O3-ceramics of the induction are in contact coil. for Fu a rthermore,longer period the of time,consequences of adhesions this attack wouldin lead continuous to a destruction operation of the must reactor. be Ininvestigated. addition to the advantages already mentioned, Design 2 also features a much simpler construction and therefore easier sam- Even though the target temperatures were not reached in the experiments with pling. A drawback of Design 2 is the higher energy input required, which can be seen in the comparison ofLCO-C, the power it can curve still of be Figures assumed 8 and that 11. However, a sufficie thisntly can high be solvedtemperature by an was reached. Firstly, improved positioning of the induction coil. Furthermore, the consequences of adhesions in continuous operation must be investigated. Even though the target temperatures were not reached in the experiments with LCO-C, it can still be assumed that a sufficiently high temperature was reached. Firstly,

Metals 2021, 11, 149 17 of 22

This factor is particularly important for long-term experiments in a continuous setup. If the sample material and the Al2O3-ceramics are in contact for a longer period of time, this attack would lead to a destruction of the reactor. In addition to the advantages Metals 2021, 11, x FOR PEER REVIEWalready mentioned, Design 2 also features a much simpler construction and therefore easier 17 of 22 sampling. A drawback of Design 2 is the higher energy input required, which can be seen in the comparison of the power curve of Figures8 and 11. However, this can be solved by an improved positioning of the induction coil. Furthermore, the consequences of adhesions inthe continuous temperature operation in the must reactor be investigated. was measured in the space between the refractory mat and theEven graphite though cubes, the target as temperaturesmentioned wereabove, not whic reachedh implies in the experiments that the withtemperature LCO- in the cube C, it can still be assumed that a sufﬁciently high temperature was reached. Firstly, the bed must have been even higher due to the heat input in it. In addition, if the temperature temperature in the reactor was measured in the space between the refractory mat and the graphitewas too cubes, low, as the mentioned appearance above, of which the products implies that wo theuld temperature be different. in the An cube example bed of this is the mustmetal have from been Co, even as higher shown due in to Figures the heat input9c and in it.12, In whose addition, melting if the temperature point is known was to be 1495 °C. too low, the appearance of the products would be different. An example of this is the metal ◦ from3.2.2. Co, Results as shown from in Figures Experiments9c and 12, whosewith LFP-C melting point is known to be 1495 C. 3.2.2. ResultsThe test from with Experiments LFP-C was with carried LFP-C out in Design 1. Since the temperature measurement viaThe the test K-thermocouples with LFP-C was carried was outalready in Design faulty 1. Since from the the temperature beginning measurement of the experiment and a viarepair the K-thermocouples was no longer was possible already at faulty this point from the in beginningtime, only of the the experimentcurves of the and recordings from a repair was no longer possible at this point in time, only the curves of the recordings the S-thermocouples are visible in Figure 15. However, the delta value to the K-thermo- from the S-thermocouples are visible in Figure 15. However, the delta value to the K- thermocouplescouples should should be besimilar similar to to that that inin thethe LCO-C LCO-C experiment experiment in Design in Design 1, whereby 1, whereby approx- approximatelyimately 500 500°C ◦canC can be be added added to to the valuevalue of of the the S-thermocouples S-thermocouples to determine to determine the the internal internaltemperature temperature at higher at higher temperatures. temperatures.

FigureFigure 15. 15.Experimental Experimental performance performance of LFP-C of in DesignLFP-C 1,in comparison Design 1, of comparison power and temperature of power and temperature overover time. time.

During the heating process, smoke development was particularly noticeable at an outside temperatureDuring the of approximately heating process, 750 ◦C smoke (approximate development internal temperature was particularly of 1210 ◦C), noticeable at an whichoutside completely temperature ignited afterof approximately a short time. This 750 ﬂame, °C which (approximate is an indication internal of the temperature reac- of 1210 tion°C), of which phosphorus completely with oxygen ignited [42], couldafter ﬁnally a short be detectedtime. This constantly flame, up which to an outsideis an indication of the ◦ ◦ temperaturereaction of of phosphorus approximately with 860 Coxygen (approximate [42], internalcould finally temperature be detected of 1310 C)constantly as up to an shownoutside in Figure temperature 16a. During of thisapproximately time the acid in860 the °C gas (approximate scrubber changed internal its color temperature to a of 1310 brownish liquid. The results of the exhaust gas analysis can be taken from Table6. °C) as shown in Figure 16a. During this time the acid in the gas scrubber changed its color to a brownish liquid. The results of the exhaust gas analysis can be taken from Table 6.

Table 6. Results from the frit liquid of the gas scrubber after suction of the exhaust gas in Design 1 for LFP-C in mg/L.

Fraction Li Fe P Frit liquid 2.0 1.5 200

In the exhaust gas analysis only a small value for P and a very small amount of Li could be found, which is possibly due to the formation of a flame out of the exhaust pipe. In order to be able to make a statement about the efficiency of the reactor concept for the recycling of LFP, the results of the ICP-OES or ICP-MS must be examined more closely. A total of 5 fractions could be detected during sampling. The appearance of the fraction classified as non-magnetic slag (Slag 1) differed significantly from that in Figure 9a. Individual spheres with a diameter of up to 5 mm were detected, as shown in Figure 16b. In comparison with the appearance of the magnetic metal fraction in Figure 16d, the difficulty of clearly classifying the individual fractions is obvious. The analysis showed that

Metals 2021, 11, x FOR PEER REVIEW 18 of 22

the Fe content in the slag was even higher than in the material identified as metal. In addition, however, a significant amount of Li (3.19%) and P (15.9%) was also analyzed. Slag 2 in Table 7 is a non-magnetic powder with particles smaller than 1 mm the appearance of which is the same as in Figure 9d. The same applies to the magnetic material identified as powder. In this experiment, care was again taken during sampling to separate as much diffused areas of the sample material from the mortar. These brownish areas are also visible in Figure 16c, which shows the ceramic ring with the mortar after removal of the refractory concrete. Analysis of the metal displayed in Figure 16d shows only 51 wt.% Fe and over 8 wt.% P. This low Fe content suggests that no complete reduction has occurred. A further indication for the correctness of this assumption is the fact that during mechan- Metals 2021, 11, 149 ical processing of the fraction with a hammer, the spheres disintegrated into a powder18 of 22 already with a small amount of force.

(a) (b) (c) (d)

FigureFigure 16.16. ProductsProducts ofof thethe experimentexperiment LFP-CLFP-C inin DesignDesign 1:1: ((aa)) ﬂameflame formationformation inin exhaustexhaust gasgas ﬂow;flow; ((bb)) slag;slag; ((cc)) ceramicceramic ringring and mortar seen from the bottom; (d) metal. and mortar seen from the bottom; (d) metal.

Table 7. Results of the individual fractions after the experiment LFP-C in Design 1. Table 6. Results from the frit liquid of the gas scrubber after suction of the exhaust gas in Design 1 for LFP-CFraction in mg/L. Weight (g) Li (wt.%) Fe (wt.%) P (wt.%) Slag 1 36.1 3.19 53.60 15.90 Fraction Li Fe P Slag 2 16.3 4.95 0.61 7.89 MortarFrit liquid 66.0 2.00.90 1.53.29 2001.66 Metal 96.5 0.89 51.00 8.24 InPowder the exhaust gas analysis70.5 only a small 1.91 value for P and35.90 a very small amount8.43 of Li could be found, which is possibly due to the formation of a flame out of the exhaust pipe. In orderReferring to be able to the to respective make a statement masses of about the indi thevidual efficiency fractions, of the the reactor transfer concept coefficients for the recyclingcan be taken of LFP, from the Figure results 17. of the ICP-OES or ICP-MS must be examined more closely. AOf totalparticular of 5 fractions interest are could the beremoval detected rates during of Li sampling.(68.4%) and The P (64.5%) appearance over the of thegas fractionflow. The classified remaining as non-magnetic amount of Li slag here (Slag is distributed 1) differed significantlyrelatively evenly from among that in Figure the other9a . Individualfractions with spheres a slight with concentration a diameter of upon tothe 5 mmremaining were detected, powder. as As shown already in Figure mentioned, 16b. In it comparisonis reasonable with to assume the appearance that no ofcomplete the magnetic reduction metal of fraction Fe has in occurred. Figure 16 d,This the is difficulty also re- offlected clearly in classifyinga considerable the individualvalue of phosphorus fractions is in obvious. the metal The fraction. analysis showed that the Fe contentA direct in the comparison slag was even of the higher experiments than in the in materialDesign 1 identified shows that as the metal. Li removal In addition, rate however,for LFP-C aof significant 68.4% is 8% amount lower than of Li in (3.19%) the experiment and P (15.9%) with LCO-C. was also However, analyzed. the Slag parallel 2 in Tableremoval7 is of a non-magneticphosphorus of powder64.5% represents with particles a resp smallerectable result. than 1 It mm also the can appearance be seen that of in whichboth the is theLCO-C same test as inwithFigure a transfer9d . The coefficient same applies of 21.1% to the (Figure magnetic 10) materialfor Li and identified the LFP-C as powder.test with In6.1% this (Figure experiment, 17), a significant care was again amount taken of Li during was transferred sampling to to separate the mortar. as much If the diffusedresults of areas LCO-C of thein Design sample 2 materialare also included, from the it mortar. can be Theseassumed brownish that gasification areas are rates also visiblefor LFP-C in Figure are better 16c, whichin this showsconstruction the ceramic method. ring Looking with the at mortar the transfer after removal coefficients of the in refractoryFigure 17, concrete.a significant Analysis portion of of the the metal Fe (11.5%) displayed is attributed in Figure to 16 thed shows gas flow. only De 51 facto, wt.% this Fe and over 8 wt.% P. This low Fe content suggests that no complete reduction has occurred. A is the amount that was not recovered during sampling compared to the input amount. further indication for the correctness of this assumption is the fact that during mechanical The correctness or falsification of this classification and the above mentioned assumptions processing of the fraction with a hammer, the spheres disintegrated into a powder already must be investigated in further experiments. Particular attention must be paid to safety in with a small amount of force. the pyrometallurgical removal of phosphorus from LIB, consisting of the cathode material Table 7. Results of the individual fractions after the experiment LFP-C in Design 1.

Fraction Weight (g) Li (wt.%) Fe (wt.%) P (wt.%)

Slag 1 36.1 3.19 53.60 15.90 Slag 2 16.3 4.95 0.61 7.89 Mortar 66.0 0.90 3.29 1.66 Metal 96.5 0.89 51.00 8.24 Powder 70.5 1.91 35.90 8.43

Referring to the respective masses of the individual fractions, the transfer coefﬁcients can be taken from Figure 17. Metals 2021, 11, x FOR PEER REVIEW 19 of 22

LFP, in the exhaust gas post-processing. In addition to oxidation in the air, as shown in Metals 2021, 11, 149 19 of 22 Figure 16a, the high toxicity [43] is also of particular importance. These factors must be given special consideration in future developments of the reactor concept presented.

FigureFigure 17. 17.Transfer Transfer coefﬁcients coefficients of the of elements the elements into the into individual the indi fractionsvidual fractions in % of the in experiment % of the experiment in Designin Design 1. 1.

4. ConclusionsOf particular interest are the removal rates of Li (68.4%) and P (64.5%) over the gas flow. The remaining amount of Li here is distributed relatively evenly among the other fractionsWithin with athe slight scope concentration of this paper, on the the remaining suitability powder. of a Asnew already pyrometallurgical mentioned, it is recycling reasonableprocess associated to assume with that no materials complete from reduction LIBs of for Fe the has occurred.recovery Thisof valuable is also reflected metals was ininvestigated. a considerable With value the background of phosphorus of in a the continuo metal fraction.us adaptation of the reactor concept to the wasteA directstream comparison from spent of LIBs, the experiments two different in Design reactor 1 showsdesigns that were the Liused, removal in each rate of which forthe LFP-C cathode of 68.4% material is 8% LCO lower with than carbon in the experiment addition withwas LCO-C.examined However, for better the parallel comparability. removalIn a third of phosphorustrial, the basic of 64.5% capability represents of the a respectable technology result. for Itthe also treatment can be seen of that LFP in was also bothexamined. the LCO-C In addition, test with aknowledge transfer coefficient about the of 21.1%behavior (Figure of the 10) examined for Li and thecathode LFP-C materials test with 6.1% (Figure 17), a significant amount of Li was transferred to the mortar. If the used in high-temperature applications was investigated in an upstream step in a heating results of LCO-C in Design 2 are also included, it can be assumed that gasification rates formicroscope. LFP-C are better in this construction method. Looking at the transfer coefficients in FigureThe 17, atransfer significant coefficients portion of determined the Fe (11.5%) in is th attributede experiments to the gasof the flow. novel De facto, pyrometallurgi- this iscal the recycling amount that process was not serve recovered exclusively during as sampling a comparison compared of tothe the efficiency input amount. of the The presented correctnessreactor concepts or falsification or as a offirst this benchmark classification of and the the basic above suitability mentioned of assumptions the process must for the treat- bement investigated of LFP. in further experiments. Particular attention must be paid to safety in the pyrometallurgicalFrom the experiments removal of phosphorus in the heating from LIB,microscope consisting the of themaximu cathodem materialnecessary LFP, tempera- intures the exhaustfor the transformation gas post-processing. into In a additionmolten phase to oxidation could in be the determined. air, as shown This in Figure state of aggre- 16a, the high toxicity [43] is also of particular importance. These factors must be given gation is necessary in the long run to meet the requirements of the theoretically deter- special consideration in future developments of the reactor concept presented. mined principle of a continuous process. For the experiments with LCO a temperature of 4.1525 Conclusions °C and for LFP 1400 °C could be determined. WithinIn the thetrial scope LCO-C of this in Design paper, the 1, 95.2% suitability Co of of the a new original pyrometallurgical input fraction recycling was converted processinto the associated metal, with with a purity materials of Co from of 100%. LIBs for Due the to recovery the high of Li valuable content metalsin the mortar, was only investigated.76.4% of the With Li could the background be transferred of a continuous into the adaptation exhaust gas of the flow. reactor This concept contrasts to the with the wasteresult stream of the from experiment spent LIBs, with two LCO-C different in reactor Design designs 2. Its wereanalysis used, shows in each a of metal which purity of the93.9% cathode Co and material a remarkable LCO with lithium carbon additionremoval wasrate examinedin a range for from better 81.7% comparability. to 97.3%. In addi- In a third trial, the basic capability of the technology for the treatment of LFP was also tion to this impressive lithium removal rate, Design 2 has also proven to be the better examined. In addition, knowledge about the behavior of the examined cathode materials usedchoice in high-temperaturefor future use due applications to its interaction was investigated with the in feed an upstream material. step For in example, a heating massive microscope.interactions and attacks on the Al2O3 crucible have been detected in Design 1, whereas thereThe were transfer no physical coefficients damages determined with in the the MgO experiments crucible of thein Design novel pyrometallurgical 2. Although the danger recyclingof destruction process of serve the exclusively reactor wall as a comparisonduring long-term of the efficiency experiments of the presented in a future reactor continuous conceptsprocess orhas as been a first averted, benchmark the ofeffect the basicof the suitability detected of adhesions the process on for the the MgO treatment crucible still ofneeds LFP. to be investigated in further tests. However, the initially formulated goal of a more targeted heat supply in the lower part of the crucible was achieved by Design 2.

Metals 2021, 11, 149 20 of 22

From the experiments in the heating microscope the maximum necessary temperatures for the transformation into a molten phase could be determined. This state of aggregation is necessary in the long run to meet the requirements of the theoretically determined principle of a continuous process. For the experiments with LCO a temperature of 1525 ◦C and for LFP 1400 ◦C could be determined. In the trial LCO-C in Design 1, 95.2% Co of the original input fraction was converted into the metal, with a purity of Co of 100%. Due to the high Li content in the mortar, only 76.4% of the Li could be transferred into the exhaust gas flow. This contrasts with the result of the experiment with LCO-C in Design 2. Its analysis shows a metal purity of 93.9% Co and a remarkable lithium removal rate in a range from 81.7% to 97.3%. In addition to this impressive lithium removal rate, Design 2 has also proven to be the better choice for future use due to its interaction with the feed material. For example, massive interactions and attacks on the Al2O3 crucible have been detected in Design 1, whereas there were no physical damages with the MgO crucible in Design 2. Although the danger of destruction of the reactor wall during long-term experiments in a future continuous process has been averted, the effect of the detected adhesions on the MgO crucible still needs to be investigated in further tests. However, the initially formulated goal of a more targeted heat supply in the lower part of the crucible was achieved by Design 2. The experiment with LFP-C was performed in Design 1 and achieved a lithium removal rate of 68.4% with parallel phosphorus removal of 64.5%. Since the results of the LCO-C experiments showed that a higher lithium removal could be achieved when using Design 2, a repetition of the LFP-C experiment in this setup can be expected to result in a higher removal rate. In addition, since sampling in this experiment has proven to be particularly difficult due to the appearance of the fractions, and since detailed examination of the results has revealed questions that need to be clarified, such as the undetectable amount of Fe, further investigations are indispensable. Nevertheless, it can be summarized that Design 2 with its MgO crucible has proven to be a better choice with regard to its suitability for pyrometallurgical treatment of material from LIBs. This is due to its inertness to the sample material as well as the higher Li removal rate determined. In addition, it was found that the use of the technology is also suitable for the cathode material LFP and that considerable P and Li removal rates have already been achieved. However, in order to be able to treat the fluctuating waste stream from spent LIBs with an appropriate efficiency, in-depth investigations are needed beforehand to gain knowledge of the behavior of all common cathode materials. In order to increase efficiency, the fraction referred to as slag must also be subjected to more detailed investigations in the future, for example with an XRD analysis. From this, knowledge of the phases present is to be generated and the formation of these is to be suppressed with targeted measures. In the current development phase, this has not yet been the focus of research. Furthermore, it is necessary to identify the influence of additional fractions such as Cu and Al from conductor foils on the process. Compared to commercial techniques used today for recycling spent LIBs, the simultaneous recovery of lithium and phosphorus via the process presented in this paper is its most significant advantage. This first potential assessment for pyrometallurgical recovery of Li would also theoretically meet the requirements of the proposed amendment to the EU Directive 2006/66/EC of a Li recovery up to 70% by 2030. Aspects such as the economics, energy efficiency and environmental impact of this intermediate step in the overall recycling chain, as well as possible recovery rates of a waste stream of spent LIBs in the new reactor design, need to be determined in further studies.

Author Contributions: Conceptualization, A.H. and S.W.-K.; methodology, A.H.; investigation, A.H., S.W.-K. and C.P.; resources, A.H., S.W.-K. and C.P.; writing—original draft preparation, A.H.; writing—review and editing, A.H., S.W.-K., C.P. and H.R.; visualization, A.H.; supervision, C.P. and H.R.; project administration, C.P.; funding acquisition, H.R. All authors have read and agreed to the published version of the manuscript. Metals 2021, 11, 149 21 of 22

Funding: This research was funded by the Zukunftsfonds Steiermark with funds from the province of Styria, Austria, grant number GZ: ABT08-189002/2020 PN:1305. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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